Journal of Petroleum Geology vol 21(2) April 1998 pp 187-211

THE DISTRIBUTION OF APTIAN SANDSTONES IN THE CENTRAL AND NORTHERN NORTH SEA

(UK SECTOR) - A LOWSTAND SYSTEMS TRACT PLAY

Part 2 distribution and exploration strategy

S Crittenden J M Cole and M J Kirk

In this paper we discu ss the distribution in the Central and Northern North Sea (UK sector) of the Late Aptian sandstones r~f reservoir potential which are assigned to members ofthe Sola Formation An exploration strategyfor the se sandstones is proposed based on sequence-stratigraphic and palaeogeographic models derived from the examination ofnumerous wells The sandstones were deposited by ma ss-flow processes as a consequence of a major tectonically-induced or enhanced Lat e Aptian fall in relative sea-level The distribution ofthese lowstandsandstones and thefacies developed was controlled by the structure and palaeogeography that existed in the study area both before and after this sea-level fall

The pattern offaulting in the study area during Sola Formation deposition is related to Jurassic and older tectonism which is associated with the development ofthe protoshyNorth Atlantic and the thermal subside nce ofthe North Sea rift system Faults controlled the areas ofsandstone provenance and also determined the routes by whi ch reworked sediments were transported into depocentres

We have mapped the distribution ofthese sandstones using both well and se ismic data Maps offault pattern s basinal and high areas and facies distributions have been generatedfor the top- Valhall Formation level (ie immediatelybeneath the Sola Formation) and also for the Sola Formation itself The se maps were used to mode the distribution ofAptian lowstand sandstones and prospects in the UK Central and Northern North Sea This modelling exercis e is ofrelevance f or the identification oflowstand sandstones with reservoir potential els ewhere on the NE Atlantic margin

Consultant Waye Cottage Cha gford Devon UK TQ13 8HN Geochem Group Ltd Chester Street Chester CH4 8RD Stratasearcli Consulting Inv erurie ASS 8XE

188 The distribution ofAptian sandstones Central Northern North Sea part 2

INTRODUCTION

Sandstones of Aptian age in the Central and Northern North Sea (UK sector) tend to be very localised in occurrence Here we discuss the distribution of Aptian sediments in this area focussing in particular on those sandstones which have reservoir potential The exploration strategy we propose is based on a sequence-stratigraphic model considered in Part I of this paper (referred to hereafter as Crittenden et al 1997) Data from numerous wells in the study area has been analysed Throughout we refer well depths to the Rig Kelly Bushing in metres although Imperial units are used for a number of completion logs held by the UK Dept of Trade and Industry (DTI)

Aptian sediments form part of the late syn-rift succession in the North Sea In the overall transgressive Early to mid-Cretaceous interval the Sola Formation represents a very significant Late Aptian regressive phase which we interpret to have had a mainly tectonic cause The Sola Formation lies above the Valhall Formation and beneath the Redby Formation (see fig I in Crittenden et al 1997) in the study area it comprises dark claystones and sandstones varying from a few metres thick to (exceptionally) over 350m in thickness where sand rich

Late Aptian lowstand sandstones with reservoir potential occur in a number of fields in the North Sea and are sealed by Sola Formation claystones These fields include Britannia Captain Saltire Andrew and Tiffany in the UK sector Agat in the Norwegian Northern North Sea (Norwegian Quadrant 35) and Victory west of Shetland (UK 2071) Hydrocarbon shows in Late Aptian sandstones have been recorded in numerous wells elsewhere in the North Sea and the NE Atlantic margin Aptian lowstand sandstone may have reservoir potential over a wider area than that discussed in this paper extending perhaps from the Haltenbank area (off mid-Norway) SW to the Porcupine Basin west of Ireland We have extended the search for Cretaceous lowstand sandstone reservoirs throughout the en-echelon basin suite on the NE Atlantic margin as part of a wider study and have examined a large number of wells tied to seismic profiles Of particular interest in this context is the recent report of the discovery of a substantial gas accumulation in thick Upper Cretaceous sandstones by a deep-water well drilled on the Nyk High off Mid-Norway

This paper is intended to contribute to the search for Aptian lowstand sandstone reservoirs in the NE Atlantic margin and the North Sea However we do not consider other important parameters such as seals traps source rocks maturation and migration

AN EXPLORATION STRATEGY FOR THE APTIAN SANDSTONES

Our discussion of Aptian sandstones with reservoir potential begins by considering the palaeotopography and palaeogeography that existed at the onset of and during Aptian times and by constructing appropriate palaeogeographical maps and depositional models shySimplified maps resulting from the study ofan extensive well and seismic data-base were published for the Lower Cretaceous of the southern and central Norwegian sectors of the North Sea by Hesjedal and Hamar (1983) Although these maps were not presented in a sequence-stratigraphic framework they served to emphasise the reservoir and sourceshy bull rock potential of the Lower Cretaceous in this area

Published maps for the North Sea (eg Cope et al 1992) indicate that the present-day distribution of Cretaceous sediments is determined both by the erosional margin of the interval and also by the limits of seismic resolution and the amount of data available For example Late Cretaceous and Tertiary erosion of the Jurassic and Lower Cretaceous succession defines the margins of the Fladen Ground Spur Therefore although Lower Cretaceous sediments may be absent from a particular area at the present day it does not of course follow that the area was a site of non-deposition during the Early Cretaceous

An understanding of the areal extent of marine deposition during the Early Aptian transgression is necessary in order to identify likely source areas for clastic sediments

---------

--- - - - - - - - - - - - - - - - - - - - - - - ----

189 S Crittenden et al

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bull EARLY APTIAN

TRANSGRESSIVE MAXIMUM

LEGEND

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Fig I Map of the Central and Northern North Sea showing the extent of sedimentation during the Early Aptian (th e V5 and V6 units of the Valhall Formation)

--

190

The distribution ofAptian sandstones Central Northern North Sea part 2

which were re-deposited during the Late Aptian regression These source areas comprise regions on the shelf and upper slope on which coarse clastics accumulated during the Early Aptian at the same time the V5 and V6 claystone units of the Val hall Formation were being deposited in sediment-starved basinal areas Fig I is a palaeogeographic map of the Central and Northern North Sea in the early Aptian and is based on well-log and seismic data From this map we can predict likely areas of shelf sand and carbonate deposition and also areas in which hemipelagic carbonate-rich claystones were deposited The shelfal deposits were reworked during the Late Aptian regression and together with eroded bedrock were redeposited as the sandstones assigned to members of the Sola Formation

The structural configuration of the study area at the end of the Early Aptian transgression is also important because fault patterns and fault-block configurations control sediment provenance transport and deposition Some of the fault patterns and structural lineaments may have been established during the Palaeozoic and may have had a controlling effect on Jurassic as well as Lower Cretaceous sedimentation For example the present- day coastline of Norway retlects deep-seated structural lineaments which are thought to have controlled Jurassic and Lower Cretaceous sediment transport routes from the Scandinavian Massif across the Norwegian shel f (Dore etal bull 1997) Deep-seated lineaments controlling sediment transp0l1 routes in the UK sector of the Central and Northern North Sea are less obvious However we propose that the Inner Moray Firth which is bounded to the west and north by the Helmsdale and Wick faults and by the Grampian uplands to the south intluenced the distribution of sediments derived from the Scottish Massif during the Lower Cretaceous

Fig 2 shows the extent of marine conditions in the Central and Northern North Sea following the Late Aptian regression and illu strates the distribution of Sola Formation claystones and sandstones Both Figs 1 and 2 are derived from more detailed maps we have constructed which integrate well and seismic data Viewed together Figs 1 and 2 indicate where the Val hall Formation is truncated by erosion where faults were active at this time and where depocentre margins are located These factors assist in predicting likely areas of Late Aptian mass-tlow deposition In addition these maps show areas of non-deposition and areas where the Valhall and Sola Formations were originally deposited but have subsequently been partially or completely removed

GENESIS OF LATE APTIAN SANDSTONES IN THE UK CENTRAL AND NORTHERN NORTH SEA

During the Early Aptian transgression the shallow shelves tlanking the North Sea Central and Viking Graben rift system were sites either of non-deposition and erosion or slow deposition resulting in condensed (sometimes incomplete) hemipelagic carbonate-rich sequences with local sandstones These sediments contain reworked fossils derived from Lower Cretaceous and older stratigraphic units During the Late Aptian regression these relatively unconsolidated sediments (particularly the shelfal sands) were reworked by slumping and gravity-tlow processes into deeper-water slope and basin environments The gravity flows were triggered primarily by tectonic activity causing uplift and shallowing of the basin margins and intra-basin highs and localised emergence and erosion Sedimentation continued during the ensuing Late Aptian sea-level lowstand and relative rise until the clastic source areas were submerged as the shorelines moved landward

The location ofsediment entry points into the grabens was controlled by the intersection offaults on the graben margins and by the graben-margin topography Forexample sediment supply may have occurred at transfer zones between en-echelon normal faults The redeposited sandstones rapidly change laterally into the relatively organic-rich marine claystones of the Sola Formation which constitutes a potential hydrocarbon source rock

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191 s Critte nden et al

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regress ion

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Fig 2 Map of the Central and Northern North Sea showing the extent of sed ime ntation during the Late Aptian (the Sola Formation)

192 The distribution ofAptian sandstones Central North ern North Sea part 2

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FORTH APPROACHES BASIN (PART)

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NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

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Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

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Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

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deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

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I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

188 The distribution ofAptian sandstones Central Northern North Sea part 2

INTRODUCTION

Sandstones of Aptian age in the Central and Northern North Sea (UK sector) tend to be very localised in occurrence Here we discuss the distribution of Aptian sediments in this area focussing in particular on those sandstones which have reservoir potential The exploration strategy we propose is based on a sequence-stratigraphic model considered in Part I of this paper (referred to hereafter as Crittenden et al 1997) Data from numerous wells in the study area has been analysed Throughout we refer well depths to the Rig Kelly Bushing in metres although Imperial units are used for a number of completion logs held by the UK Dept of Trade and Industry (DTI)

Aptian sediments form part of the late syn-rift succession in the North Sea In the overall transgressive Early to mid-Cretaceous interval the Sola Formation represents a very significant Late Aptian regressive phase which we interpret to have had a mainly tectonic cause The Sola Formation lies above the Valhall Formation and beneath the Redby Formation (see fig I in Crittenden et al 1997) in the study area it comprises dark claystones and sandstones varying from a few metres thick to (exceptionally) over 350m in thickness where sand rich

Late Aptian lowstand sandstones with reservoir potential occur in a number of fields in the North Sea and are sealed by Sola Formation claystones These fields include Britannia Captain Saltire Andrew and Tiffany in the UK sector Agat in the Norwegian Northern North Sea (Norwegian Quadrant 35) and Victory west of Shetland (UK 2071) Hydrocarbon shows in Late Aptian sandstones have been recorded in numerous wells elsewhere in the North Sea and the NE Atlantic margin Aptian lowstand sandstone may have reservoir potential over a wider area than that discussed in this paper extending perhaps from the Haltenbank area (off mid-Norway) SW to the Porcupine Basin west of Ireland We have extended the search for Cretaceous lowstand sandstone reservoirs throughout the en-echelon basin suite on the NE Atlantic margin as part of a wider study and have examined a large number of wells tied to seismic profiles Of particular interest in this context is the recent report of the discovery of a substantial gas accumulation in thick Upper Cretaceous sandstones by a deep-water well drilled on the Nyk High off Mid-Norway

This paper is intended to contribute to the search for Aptian lowstand sandstone reservoirs in the NE Atlantic margin and the North Sea However we do not consider other important parameters such as seals traps source rocks maturation and migration

AN EXPLORATION STRATEGY FOR THE APTIAN SANDSTONES

Our discussion of Aptian sandstones with reservoir potential begins by considering the palaeotopography and palaeogeography that existed at the onset of and during Aptian times and by constructing appropriate palaeogeographical maps and depositional models shySimplified maps resulting from the study ofan extensive well and seismic data-base were published for the Lower Cretaceous of the southern and central Norwegian sectors of the North Sea by Hesjedal and Hamar (1983) Although these maps were not presented in a sequence-stratigraphic framework they served to emphasise the reservoir and sourceshy bull rock potential of the Lower Cretaceous in this area

Published maps for the North Sea (eg Cope et al 1992) indicate that the present-day distribution of Cretaceous sediments is determined both by the erosional margin of the interval and also by the limits of seismic resolution and the amount of data available For example Late Cretaceous and Tertiary erosion of the Jurassic and Lower Cretaceous succession defines the margins of the Fladen Ground Spur Therefore although Lower Cretaceous sediments may be absent from a particular area at the present day it does not of course follow that the area was a site of non-deposition during the Early Cretaceous

An understanding of the areal extent of marine deposition during the Early Aptian transgression is necessary in order to identify likely source areas for clastic sediments

---------

--- - - - - - - - - - - - - - - - - - - - - - - ----

189 S Crittenden et al

3degE

208

bull EARLY APTIAN

TRANSGRESSIVE MAXIMUM

LEGEND

60N _~_vs-ve shy

2l7 middotICXrN1l

bull i

-I

_ 1lAl~ flrUJS

--- =e-~ shy-- -- ~h

bull Thin venee r 01 shallow shel f carb onates and ctasncs

26 27 bull 28

56N l- L -JL ---l ---l --J----shy

Fig I Map of the Central and Northern North Sea showing the extent of sedimentation during the Early Aptian (th e V5 and V6 units of the Valhall Formation)

--

190

The distribution ofAptian sandstones Central Northern North Sea part 2

which were re-deposited during the Late Aptian regression These source areas comprise regions on the shelf and upper slope on which coarse clastics accumulated during the Early Aptian at the same time the V5 and V6 claystone units of the Val hall Formation were being deposited in sediment-starved basinal areas Fig I is a palaeogeographic map of the Central and Northern North Sea in the early Aptian and is based on well-log and seismic data From this map we can predict likely areas of shelf sand and carbonate deposition and also areas in which hemipelagic carbonate-rich claystones were deposited The shelfal deposits were reworked during the Late Aptian regression and together with eroded bedrock were redeposited as the sandstones assigned to members of the Sola Formation

The structural configuration of the study area at the end of the Early Aptian transgression is also important because fault patterns and fault-block configurations control sediment provenance transport and deposition Some of the fault patterns and structural lineaments may have been established during the Palaeozoic and may have had a controlling effect on Jurassic as well as Lower Cretaceous sedimentation For example the present- day coastline of Norway retlects deep-seated structural lineaments which are thought to have controlled Jurassic and Lower Cretaceous sediment transport routes from the Scandinavian Massif across the Norwegian shel f (Dore etal bull 1997) Deep-seated lineaments controlling sediment transp0l1 routes in the UK sector of the Central and Northern North Sea are less obvious However we propose that the Inner Moray Firth which is bounded to the west and north by the Helmsdale and Wick faults and by the Grampian uplands to the south intluenced the distribution of sediments derived from the Scottish Massif during the Lower Cretaceous

Fig 2 shows the extent of marine conditions in the Central and Northern North Sea following the Late Aptian regression and illu strates the distribution of Sola Formation claystones and sandstones Both Figs 1 and 2 are derived from more detailed maps we have constructed which integrate well and seismic data Viewed together Figs 1 and 2 indicate where the Val hall Formation is truncated by erosion where faults were active at this time and where depocentre margins are located These factors assist in predicting likely areas of Late Aptian mass-tlow deposition In addition these maps show areas of non-deposition and areas where the Valhall and Sola Formations were originally deposited but have subsequently been partially or completely removed

GENESIS OF LATE APTIAN SANDSTONES IN THE UK CENTRAL AND NORTHERN NORTH SEA

During the Early Aptian transgression the shallow shelves tlanking the North Sea Central and Viking Graben rift system were sites either of non-deposition and erosion or slow deposition resulting in condensed (sometimes incomplete) hemipelagic carbonate-rich sequences with local sandstones These sediments contain reworked fossils derived from Lower Cretaceous and older stratigraphic units During the Late Aptian regression these relatively unconsolidated sediments (particularly the shelfal sands) were reworked by slumping and gravity-tlow processes into deeper-water slope and basin environments The gravity flows were triggered primarily by tectonic activity causing uplift and shallowing of the basin margins and intra-basin highs and localised emergence and erosion Sedimentation continued during the ensuing Late Aptian sea-level lowstand and relative rise until the clastic source areas were submerged as the shorelines moved landward

The location ofsediment entry points into the grabens was controlled by the intersection offaults on the graben margins and by the graben-margin topography Forexample sediment supply may have occurred at transfer zones between en-echelon normal faults The redeposited sandstones rapidly change laterally into the relatively organic-rich marine claystones of the Sola Formation which constitutes a potential hydrocarbon source rock

bull

bull

191 s Critte nden et al

208

1

LEGEND

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_ _IM FIWLT$ __ ~11cl

e-bull- in _-

60N LATE APTIAN REGRESSION

Sola FOfmallCJrl d uyrlone in cepoccerr es (70 middot 450)

59middotN r---Jgt=------+--+-r-r-+--+=-+--1Ift-=---r----~ Known Sola Sand M~~r

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bull 57 N r----j-----l---~--II___~=----=fi~-~--lth

Thin veneer of Sola Formation post Late Aplian

regress ion

26 bull 2827

56 N L __-L L ---l L --L=-~---

Fig 2 Map of the Central and Northern North Sea showing the extent of sed ime ntation during the Late Aptian (the Sola Formation)

192 The distribution ofAptian sandstones Central North ern North Sea part 2

LEGEND _ IlAlN f AUlS

- p~-u s cmiddot ~rfWIllI

bull 9Iudlaquot~IlI 1 _ - ~

19

FORTH APPROACHES BASIN (PART)

26 27 bull 28

NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

18 17 18 9 20

23-2 24- o

l degE

o 0

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0deg

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lOW

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7

26 27 28 29 30

o o

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Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

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and fWUlfCd1ft f~ noo a~

Knol SlI14 Snh lutll lkpu l l ~

1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

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c fG3amp8m --_

BASE NOT

STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

---------

--- - - - - - - - - - - - - - - - - - - - - - - ----

189 S Crittenden et al

3degE

208

bull EARLY APTIAN

TRANSGRESSIVE MAXIMUM

LEGEND

60N _~_vs-ve shy

2l7 middotICXrN1l

bull i

-I

_ 1lAl~ flrUJS

--- =e-~ shy-- -- ~h

bull Thin venee r 01 shallow shel f carb onates and ctasncs

26 27 bull 28

56N l- L -JL ---l ---l --J----shy

Fig I Map of the Central and Northern North Sea showing the extent of sedimentation during the Early Aptian (th e V5 and V6 units of the Valhall Formation)

--

190

The distribution ofAptian sandstones Central Northern North Sea part 2

which were re-deposited during the Late Aptian regression These source areas comprise regions on the shelf and upper slope on which coarse clastics accumulated during the Early Aptian at the same time the V5 and V6 claystone units of the Val hall Formation were being deposited in sediment-starved basinal areas Fig I is a palaeogeographic map of the Central and Northern North Sea in the early Aptian and is based on well-log and seismic data From this map we can predict likely areas of shelf sand and carbonate deposition and also areas in which hemipelagic carbonate-rich claystones were deposited The shelfal deposits were reworked during the Late Aptian regression and together with eroded bedrock were redeposited as the sandstones assigned to members of the Sola Formation

The structural configuration of the study area at the end of the Early Aptian transgression is also important because fault patterns and fault-block configurations control sediment provenance transport and deposition Some of the fault patterns and structural lineaments may have been established during the Palaeozoic and may have had a controlling effect on Jurassic as well as Lower Cretaceous sedimentation For example the present- day coastline of Norway retlects deep-seated structural lineaments which are thought to have controlled Jurassic and Lower Cretaceous sediment transport routes from the Scandinavian Massif across the Norwegian shel f (Dore etal bull 1997) Deep-seated lineaments controlling sediment transp0l1 routes in the UK sector of the Central and Northern North Sea are less obvious However we propose that the Inner Moray Firth which is bounded to the west and north by the Helmsdale and Wick faults and by the Grampian uplands to the south intluenced the distribution of sediments derived from the Scottish Massif during the Lower Cretaceous

Fig 2 shows the extent of marine conditions in the Central and Northern North Sea following the Late Aptian regression and illu strates the distribution of Sola Formation claystones and sandstones Both Figs 1 and 2 are derived from more detailed maps we have constructed which integrate well and seismic data Viewed together Figs 1 and 2 indicate where the Val hall Formation is truncated by erosion where faults were active at this time and where depocentre margins are located These factors assist in predicting likely areas of Late Aptian mass-tlow deposition In addition these maps show areas of non-deposition and areas where the Valhall and Sola Formations were originally deposited but have subsequently been partially or completely removed

GENESIS OF LATE APTIAN SANDSTONES IN THE UK CENTRAL AND NORTHERN NORTH SEA

During the Early Aptian transgression the shallow shelves tlanking the North Sea Central and Viking Graben rift system were sites either of non-deposition and erosion or slow deposition resulting in condensed (sometimes incomplete) hemipelagic carbonate-rich sequences with local sandstones These sediments contain reworked fossils derived from Lower Cretaceous and older stratigraphic units During the Late Aptian regression these relatively unconsolidated sediments (particularly the shelfal sands) were reworked by slumping and gravity-tlow processes into deeper-water slope and basin environments The gravity flows were triggered primarily by tectonic activity causing uplift and shallowing of the basin margins and intra-basin highs and localised emergence and erosion Sedimentation continued during the ensuing Late Aptian sea-level lowstand and relative rise until the clastic source areas were submerged as the shorelines moved landward

The location ofsediment entry points into the grabens was controlled by the intersection offaults on the graben margins and by the graben-margin topography Forexample sediment supply may have occurred at transfer zones between en-echelon normal faults The redeposited sandstones rapidly change laterally into the relatively organic-rich marine claystones of the Sola Formation which constitutes a potential hydrocarbon source rock

bull

bull

191 s Critte nden et al

208

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Sola FOfmallCJrl d uyrlone in cepoccerr es (70 middot 450)

59middotN r---Jgt=------+--+-r-r-+--+=-+--1Ift-=---r----~ Known Sola Sand M~~r

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regress ion

26 bull 2827

56 N L __-L L ---l L --L=-~---

Fig 2 Map of the Central and Northern North Sea showing the extent of sed ime ntation during the Late Aptian (the Sola Formation)

192 The distribution ofAptian sandstones Central North ern North Sea part 2

LEGEND _ IlAlN f AUlS

- p~-u s cmiddot ~rfWIllI

bull 9Iudlaquot~IlI 1 _ - ~

19

FORTH APPROACHES BASIN (PART)

26 27 bull 28

NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

18 17 18 9 20

23-2 24- o

l degE

o 0

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o 28

0deg

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04-1

lOW

11 12 13 14 15

7

26 27 28 29 30

o o

19

26

56degN L __-l-l---ll-lL -I -L-J-----

Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

SUJlJud vdl number ampIhidnC (II

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and fWUlfCd1ft f~ noo a~

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1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

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) 3m

2600m _shy

) 8m

shy shyOm

2703m

j

IHIigt 38mJ ~

27171

N1512-1 N1512middot2 DEPTHS THICKNESS DEPTHS THICKNESS

2OS4m 23m5m

2542 5m 19 5m --shy -shy Sm

22m 37m

2630m

2981171 2m

I Sm 5m

2O7Sm m

2667 171

~ r)

2Q87Sm 55171 3m

2903m 12m

11 I 275m

H 13m

r 2Ei ~5m

35m 3OO6m 2703m

N1512-3 DEPTHS THICKNESS

27_ shy mlm

2830m

) 31m

286 1171 ~r-

r) Om

28_ --shy

3m

286

1085171

nOm 200m

2908m

N1512-4 OEPTHS THICKNESS

-shy

~

272m

11m

2735m

25m

2750m

Om

2755m

2ltlm

2

N1616-1 DEPTHS nllCKNESS

-- fA-shyABSENT ABSENT

-- i~ ~-

~ l nl 2m IUm

cl784m shyYshy 11 c 18Q6m _

8m ctiCMm -_ shy

223m

elm3m m

c fG-3l4f11 3 171

c fG3amp8m --_

BASE NOT

STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

--

190

The distribution ofAptian sandstones Central Northern North Sea part 2

which were re-deposited during the Late Aptian regression These source areas comprise regions on the shelf and upper slope on which coarse clastics accumulated during the Early Aptian at the same time the V5 and V6 claystone units of the Val hall Formation were being deposited in sediment-starved basinal areas Fig I is a palaeogeographic map of the Central and Northern North Sea in the early Aptian and is based on well-log and seismic data From this map we can predict likely areas of shelf sand and carbonate deposition and also areas in which hemipelagic carbonate-rich claystones were deposited The shelfal deposits were reworked during the Late Aptian regression and together with eroded bedrock were redeposited as the sandstones assigned to members of the Sola Formation

The structural configuration of the study area at the end of the Early Aptian transgression is also important because fault patterns and fault-block configurations control sediment provenance transport and deposition Some of the fault patterns and structural lineaments may have been established during the Palaeozoic and may have had a controlling effect on Jurassic as well as Lower Cretaceous sedimentation For example the present- day coastline of Norway retlects deep-seated structural lineaments which are thought to have controlled Jurassic and Lower Cretaceous sediment transport routes from the Scandinavian Massif across the Norwegian shel f (Dore etal bull 1997) Deep-seated lineaments controlling sediment transp0l1 routes in the UK sector of the Central and Northern North Sea are less obvious However we propose that the Inner Moray Firth which is bounded to the west and north by the Helmsdale and Wick faults and by the Grampian uplands to the south intluenced the distribution of sediments derived from the Scottish Massif during the Lower Cretaceous

Fig 2 shows the extent of marine conditions in the Central and Northern North Sea following the Late Aptian regression and illu strates the distribution of Sola Formation claystones and sandstones Both Figs 1 and 2 are derived from more detailed maps we have constructed which integrate well and seismic data Viewed together Figs 1 and 2 indicate where the Val hall Formation is truncated by erosion where faults were active at this time and where depocentre margins are located These factors assist in predicting likely areas of Late Aptian mass-tlow deposition In addition these maps show areas of non-deposition and areas where the Valhall and Sola Formations were originally deposited but have subsequently been partially or completely removed

GENESIS OF LATE APTIAN SANDSTONES IN THE UK CENTRAL AND NORTHERN NORTH SEA

During the Early Aptian transgression the shallow shelves tlanking the North Sea Central and Viking Graben rift system were sites either of non-deposition and erosion or slow deposition resulting in condensed (sometimes incomplete) hemipelagic carbonate-rich sequences with local sandstones These sediments contain reworked fossils derived from Lower Cretaceous and older stratigraphic units During the Late Aptian regression these relatively unconsolidated sediments (particularly the shelfal sands) were reworked by slumping and gravity-tlow processes into deeper-water slope and basin environments The gravity flows were triggered primarily by tectonic activity causing uplift and shallowing of the basin margins and intra-basin highs and localised emergence and erosion Sedimentation continued during the ensuing Late Aptian sea-level lowstand and relative rise until the clastic source areas were submerged as the shorelines moved landward

The location ofsediment entry points into the grabens was controlled by the intersection offaults on the graben margins and by the graben-margin topography Forexample sediment supply may have occurred at transfer zones between en-echelon normal faults The redeposited sandstones rapidly change laterally into the relatively organic-rich marine claystones of the Sola Formation which constitutes a potential hydrocarbon source rock

bull

bull

191 s Critte nden et al

208

1

LEGEND

AQ URE2

B M e UCHAN tKIfl$l

G GLLH~ tfORST

A FI ANDRt W RlOOt EQW E NO Of WO~LO FAUL T

_ _IM FIWLT$ __ ~11cl

e-bull- in _-

60N LATE APTIAN REGRESSION

Sola FOfmallCJrl d uyrlone in cepoccerr es (70 middot 450)

59middotN r---Jgt=------+--+-r-r-+--+=-+--1Ift-=---r----~ Known Sola Sand M~~r

Sola FOrRa llCl1 ablcnlcroo-d

bull 57 N r----j-----l---~--II___~=----=fi~-~--lth

Thin veneer of Sola Formation post Late Aplian

regress ion

26 bull 2827

56 N L __-L L ---l L --L=-~---

Fig 2 Map of the Central and Northern North Sea showing the extent of sed ime ntation during the Late Aptian (the Sola Formation)

192 The distribution ofAptian sandstones Central North ern North Sea part 2

LEGEND _ IlAlN f AUlS

- p~-u s cmiddot ~rfWIllI

bull 9Iudlaquot~IlI 1 _ - ~

19

FORTH APPROACHES BASIN (PART)

26 27 bull 28

NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

18 17 18 9 20

23-2 24- o

l degE

o 0

12-1

o 28

0deg

7- 1

10-1 6middot 1 bull o 0

04-1

lOW

11 12 13 14 15

7

26 27 28 29 30

o o

19

26

56degN L __-l-l---ll-lL -I -L-J-----

Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

SUJlJud vdl number ampIhidnC (II

Sul CLaYlIDP and~

Arrrou DIlte hmu of lIlUMn nc dc[lllllln

and fWUlfCd1ft f~ noo a~

Knol SlI14 Snh lutll lkpu l l ~

1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

I-shyH3

HID I-shyH2

I-shyHI

A3

I--RODeY R2

I-shyA

53

-SO S2

-51

y -

vs -y

YAUWL -Y3

-YO

-Y

N1519-13 DEPTHS THICKNESS

Z655m _shy shy

3m

2677m

) 3m

2600m _shy

) 8m

shy shyOm

2703m

j

IHIigt 38mJ ~

27171

N1512-1 N1512middot2 DEPTHS THICKNESS DEPTHS THICKNESS

2OS4m 23m5m

2542 5m 19 5m --shy -shy Sm

22m 37m

2630m

2981171 2m

I Sm 5m

2O7Sm m

2667 171

~ r)

2Q87Sm 55171 3m

2903m 12m

11 I 275m

H 13m

r 2Ei ~5m

35m 3OO6m 2703m

N1512-3 DEPTHS THICKNESS

27_ shy mlm

2830m

) 31m

286 1171 ~r-

r) Om

28_ --shy

3m

286

1085171

nOm 200m

2908m

N1512-4 OEPTHS THICKNESS

-shy

~

272m

11m

2735m

25m

2750m

Om

2755m

2ltlm

2

N1616-1 DEPTHS nllCKNESS

-- fA-shyABSENT ABSENT

-- i~ ~-

~ l nl 2m IUm

cl784m shyYshy 11 c 18Q6m _

8m ctiCMm -_ shy

223m

elm3m m

c fG-3l4f11 3 171

c fG3amp8m --_

BASE NOT

STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

191 s Critte nden et al

208

1

LEGEND

AQ URE2

B M e UCHAN tKIfl$l

G GLLH~ tfORST

A FI ANDRt W RlOOt EQW E NO Of WO~LO FAUL T

_ _IM FIWLT$ __ ~11cl

e-bull- in _-

60N LATE APTIAN REGRESSION

Sola FOfmallCJrl d uyrlone in cepoccerr es (70 middot 450)

59middotN r---Jgt=------+--+-r-r-+--+=-+--1Ift-=---r----~ Known Sola Sand M~~r

Sola FOrRa llCl1 ablcnlcroo-d

bull 57 N r----j-----l---~--II___~=----=fi~-~--lth

Thin veneer of Sola Formation post Late Aplian

regress ion

26 bull 2827

56 N L __-L L ---l L --L=-~---

Fig 2 Map of the Central and Northern North Sea showing the extent of sed ime ntation during the Late Aptian (the Sola Formation)

192 The distribution ofAptian sandstones Central North ern North Sea part 2

LEGEND _ IlAlN f AUlS

- p~-u s cmiddot ~rfWIllI

bull 9Iudlaquot~IlI 1 _ - ~

19

FORTH APPROACHES BASIN (PART)

26 27 bull 28

NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

18 17 18 9 20

23-2 24- o

l degE

o 0

12-1

o 28

0deg

7- 1

10-1 6middot 1 bull o 0

04-1

lOW

11 12 13 14 15

7

26 27 28 29 30

o o

19

26

56degN L __-l-l---ll-lL -I -L-J-----

Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

SUJlJud vdl number ampIhidnC (II

Sul CLaYlIDP and~

Arrrou DIlte hmu of lIlUMn nc dc[lllllln

and fWUlfCd1ft f~ noo a~

Knol SlI14 Snh lutll lkpu l l ~

1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

I-shyH3

HID I-shyH2

I-shyHI

A3

I--RODeY R2

I-shyA

53

-SO S2

-51

y -

vs -y

YAUWL -Y3

-YO

-Y

N1519-13 DEPTHS THICKNESS

Z655m _shy shy

3m

2677m

) 3m

2600m _shy

) 8m

shy shyOm

2703m

j

IHIigt 38mJ ~

27171

N1512-1 N1512middot2 DEPTHS THICKNESS DEPTHS THICKNESS

2OS4m 23m5m

2542 5m 19 5m --shy -shy Sm

22m 37m

2630m

2981171 2m

I Sm 5m

2O7Sm m

2667 171

~ r)

2Q87Sm 55171 3m

2903m 12m

11 I 275m

H 13m

r 2Ei ~5m

35m 3OO6m 2703m

N1512-3 DEPTHS THICKNESS

27_ shy mlm

2830m

) 31m

286 1171 ~r-

r) Om

28_ --shy

3m

286

1085171

nOm 200m

2908m

N1512-4 OEPTHS THICKNESS

-shy

~

272m

11m

2735m

25m

2750m

Om

2755m

2ltlm

2

N1616-1 DEPTHS nllCKNESS

-- fA-shyABSENT ABSENT

-- i~ ~-

~ l nl 2m IUm

cl784m shyYshy 11 c 18Q6m _

8m ctiCMm -_ shy

223m

elm3m m

c fG-3l4f11 3 171

c fG3amp8m --_

BASE NOT

STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

192 The distribution ofAptian sandstones Central North ern North Sea part 2

LEGEND _ IlAlN f AUlS

- p~-u s cmiddot ~rfWIllI

bull 9Iudlaquot~IlI 1 _ - ~

19

FORTH APPROACHES BASIN (PART)

26 27 bull 28

NORTHERN MARGIN OF MID-NORTH SEA HIGH

Fig 3a Location map of the Central North Sea (UK Sector) showing structural elements and oilfields referred to in the text

DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION VSIV6 UNITS

The general distribution of Early Aptian sediments in the study area is shown in Fig bull I Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited in graben depocentres on seismic sections the se unit s appear to be conden sed For example the V6 (6m) and V5 (82m) units are present but thin at the top of the Valhall Formation in Well 63a-2z (located on Fig 3b) in the South Viking Graben In the majority of the well s studied in the Banff and Buchan Grabens however the Valhall Formation is thicker (eg 671-m thick in We1l203a-2 location on Fig 3b) including the V6 and V5 units The presence of the V6 unit in a large number of wells both in the Central and Viking Graben depocentres and on the flanking shelfal areas indicates that the Early Aptian transgression was widespread Shallow-water siliciclastic sediments were therefore deposited over a wide area

In the following section we discuss the occurrence of the V5 and V6 units in wells on the shelf areas flanking the depocentres In most instances these two units are too thin to be distinguished on seismic profiles hence well data is invaluable The areas considered are the East Shetland Platform Fladen Ground Spur Renee Ridge Buchan Horst-Glenn Horst Halibut HorstlHalibut Platform Western Platform Eastern flank of the Central and Viking Graben and Forties-Montrose High (Fig 3a)

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

18 17 18 9 20

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o 0

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Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

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ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

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and fWUlfCd1ft f~ noo a~

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Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

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I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

193 S Crittenden et al

6 7 8 9 10

21 22 23 24 25

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Fig 3b Location map of the Central North Sea showing wells referred to in the text

East Shetland Platform and Fladen Ground Spur The East Shetland Platform is a Mesozoic high which forms the western flank of the

Viking Graben and the East Shetland Basin (Fig 1) It includes the poorly-studied East Orkney Dutch Bank and Unst Basins which are intra-platformal areas of fault-con trolled subsidence in which Lower Cretaceous sediments are preserved (Fig 3a) The SE extremity of the East Shetland Platform is known as the Fladen Ground Spur In the Witch Ground Graben to the south and SW Lower Cretaceous strata thin markedly towards the flanks of the Spur covering thin Permo-Triassic strata and onlapping thicker Carboniferous units at the margin of the horst block The core of the Spur comprises Devonian Old Red Sandstones from which all the younger Palaeozoic sediments have been stripped Even during the Late Cretaceous transgression this palaeohigh was submerged beneath comparatively shallow waters and only accumulated a thin sequence of Upper Cretaceous sediments Thus Well 520-1 records Maastrichtian chalk unconformably overlying Devonian sediments In Wells 16121([-6 1612([-13 and 621-5 on the southern extremity of the Spur Upper Cretaceous chalks unconformably overly a thin Valhall Formation interval

We propose however that during the Early Aptian the Fladen Ground Spur comprised either an emergent landmass or more probably a shallow-shelf sea in which siliciclastics and carbonates were deposited A relati ve high-stand depositional model for the southern

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

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ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

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and fWUlfCd1ft f~ noo a~

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deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

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I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

194 The distribution ofAptian sandstones CentralI Northern North Sea part 2

Graben basinul area v Ith N condensed irnerval o f

hemipelagic claystones and marts (the V5 amp V6 units]

GRABEN INTERSECTION AREA + A A bull A ~fl ~ 2~~lh A

St( I - ln~ I ------------------

ValhoJll h llnl lllOJI VS V6 UlIl a TSTIII~1

Fig 4 Relative highstand depositional model for the Fladen Ground Spur during deposition of the Early Aptian V5 and V6 units (Valhall Formation)

part of this region is illustrated in Fig 4 The Fladen Ground Spur is a primary candidate for sediment sourcing during the Late Aptian lowstand Sediments may have been bull derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming the high itself as well as from the overlying Early Aptian shelfal sediments

The western margin of the Fladen Ground Spur extends from quadrant 14 through the northern and central parts ofquadrant 15 to the SW corner ofquadrant 16 Released well data (1518-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones are present along this flank of the Spur although this is probably a consequence of the well locations

The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and V6 sediments on the Piper Shelf as is seen in Wells 144-1152-11517-4 and 1517-6 Shelfal sands may also have accumulated on the Piper Shelf at this time

The Renee Ridge (Fig 5) The Renee Ridge forms the southern boundary of the Witch Ground Graben The northern

flank of the Ridge is according to seismic evidence onlapped across a series ofstepped fault blocks by a diminishing thickness of Lower Cretaceous sediments this also occurs on the southern flank abutting the Central Graben Indeed existing LowerCretaceous sediments

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

SUJlJud vdl number ampIhidnC (II

Sul CLaYlIDP and~

Arrrou DIlte hmu of lIlUMn nc dc[lllllln

and fWUlfCd1ft f~ noo a~

Knol SlI14 Snh lutll lkpu l l ~

1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

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STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

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BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

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KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

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LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

195 s Crittenden et ill

HIUBVT [fORST ---- -

JOm d raquo( IIJrn s~

ppn IIlo1Ccxtc r unl Calcdurum Sluth HaJINHGrarurc

SUJlJud vdl number ampIhidnC (II

Sul CLaYlIDP and~

Arrrou DIlte hmu of lIlUMn nc dc[lllllln

and fWUlfCd1ft f~ noo a~

Knol SlI14 Snh lutll lkpu l l ~

1fIP1_illllampc ckf(~lt innal OlIn

Fig 5 Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst area illustrating probable transportation routes for shallow-shelf sandstones into

deeper-water depocentres

bull almost overlap the Renee Ridge and may have done so during the Early Aptian transgression They were subsequently stripped off during the Late Aptian regression and other postshyEarly Cretaceous erosional events Wells examined on the Renee Ridge have recorded a thin veneer of Cromer Knoll Group sediments (eg Wells 1526-1 and 1527-2)

The Renee Ridge may have affected sediment deposition within the Witch Ground Graben by acting as a barrier to the transport further southwards of sediments derived from the Fladen Ground Spur However fault-controlled gulleys across the Renee Ridge may have acted as sediment conduits because some very thick Aptian sandstones are present in wells just to the south and SE in blocks 211 and 212 (see Fig 5)

The Ridge may have accumulated shallow-shelf sands which were reworked during the Late Aptian sea-level low and together with the products of erosion of the subcrop provided a minor proportion of the Late Aptian sandstones However the majority of the sands were probably derived from the Fladen Ground Spur to the NE and the Halibut Horst to the NW (Fig 5)

The Buchan Horst - Glenn Horst (Fig 5) This structure is a tilted and eroded generally east-west trending fault block composed

of Devonian and Carboniferous strata bounded by major faults to the north and south it is overlain unconformably by Lower Cretaceous to Recent rocks It can be traced from blocks 205 and 21 I eastwards into block 212 (Fig 5) Well data indicate that the Horst was completely submerged during the Early Aptian transgression It may have exerted some

l

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

I-shyH3

HID I-shyH2

I-shyHI

A3

I--RODeY R2

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53

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-51

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-Y

N1519-13 DEPTHS THICKNESS

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) 3m

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) 8m

shy shyOm

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N1512-1 N1512middot2 DEPTHS THICKNESS DEPTHS THICKNESS

2OS4m 23m5m

2542 5m 19 5m --shy -shy Sm

22m 37m

2630m

2981171 2m

I Sm 5m

2O7Sm m

2667 171

~ r)

2Q87Sm 55171 3m

2903m 12m

11 I 275m

H 13m

r 2Ei ~5m

35m 3OO6m 2703m

N1512-3 DEPTHS THICKNESS

27_ shy mlm

2830m

) 31m

286 1171 ~r-

r) Om

28_ --shy

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286

1085171

nOm 200m

2908m

N1512-4 OEPTHS THICKNESS

-shy

~

272m

11m

2735m

25m

2750m

Om

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N1616-1 DEPTHS nllCKNESS

-- fA-shyABSENT ABSENT

-- i~ ~-

~ l nl 2m IUm

cl784m shyYshy 11 c 18Q6m _

8m ctiCMm -_ shy

223m

elm3m m

c fG-3l4f11 3 171

c fG3amp8m --_

BASE NOT

STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

196 The distribution ofAptian sandstones Central Northern North Sea part 2

control on coarse- clastic deposition during the Late Aptian regression The V6 and V5 units of the Valhall Formation are present in Wells 2111-1 Ia-8 and 1a-13 on the horst flanks and are overlain by the Sola Formation In Well 211-6 in the centre of the horst the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here as Late Albian) immediately overlying the volcanics of the Raden Group Rattray Formation In Well 213a-4 (illustrated in fig 6 Crittenden et al 1997) the Sola Formation Sand Member contains feldspars which may have been derived from these volcanics

The Halibut Horst and Halibut Platform The Halibut Horst (Fig 5) and Halibut Platform (Fig 3a) are very shallow features

on which little or no Lower Cretaceous sediments are preserved However the fault complexes at their margins include downfaulted terraces on which Lower Cretaceous sediments are present including both shelfal and mass-flow sands (eg at the Captain field) We suggest that the area formed a shallow shelf during the Early Aptian and that the Halibut Horst was a positive area This horst acted as both an important source of sediments during the Late Aptian lowstand and also provided a topographic control on ~ sediment transport and deposition (Fig 5) That the Horst is prospective is illustrated by the occurrence of Late Aptian sands recorded in wells drilled in block 1324 in the faulted terraces on its southern flank

The Western Platform (Aberdeen 1 Forth Approaches Basin) Throughout the Mesozoic the Aberdeen Basin and Western Platform (parts of the

western margin of the Central Graben) formed an uplifted area on which a thin veneer of Lower Cretaceous sediments was deposited BGS borehole 8140 partially cored a Lower Cretaceous sequence of mudstones (Barremian to Albian) which in this vicinity is approximately 200-m thick (according to seismic data) Lott et al (1985) described the stratigraphy of the section The Early Aptian transgression is represented by planktonic foraminiferal floods which we assign to the V6 unit (our KL6 Biozone Zone 4 of Banner et al 1993 - see Crittenden et al 1997 p 19) The area undoubtedly accumulated shallow-shelf sands claystones and carbonates during the Early Aptian and provided material during the Late Aptian lowstand for erosion and transport into adjacent depocentres such as the Banff-Buchen Graben and the Central Graben Elsewhere on the Western Platform Lower Cretaceous sediments are either entirely absent as for example in Well 285-1 on the graben shoulder (where Upper Cretaceous chalks lie unconformably upon the Kimmeridge Clay Formation) or are present as a thin veneer as in Well 285-2 where a thin Sola Formation claystone interval lies upon the bull V6 unit of the Valhall Formation Wells 2812-1 and 297-2 are typical of the Western Platform A very thin veneer of Lower Cretaceous sediments is present in both wells above the Triassic in the former and above the Kimmeridge Clay Formation in the latter beneath the Upper Cretaceous Chalk

The Auk Horst (Fig 3) on the western shoulder of the Central Graben is interpreted to have been a high during the Early Cretaceous and only local deposits of marls (Wells 3016-1 and -2) conglomerates (Aptian-Albian) carbonate breccias (Neocomian) and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell 199 I) In Wells 3016-4 -5 and -9 Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones

To the SE the Auk Horst is offset to the east and forms the Argyll Ridge (Fig 3a) a NW-SE trending Palaeozoic high with the Chalk Group resting on Late Jurassic and older sediments However in Well 3024-4 approximately 13m ofAlbianlAptian marls lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the Duncan Sandstone Member In addition in Wells 3024-5 -6 and -2 the Lower Cretaceous interval is thin and rests directly upon the Permo-Triassic

197 S Crittenden et al

P1poundNUS

I-shyH3

HID I-shyH2

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A3

I--RODeY R2

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N1519-13 DEPTHS THICKNESS

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N1512-1 N1512middot2 DEPTHS THICKNESS DEPTHS THICKNESS

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N1512-3 DEPTHS THICKNESS

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STUDIED

I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

197 S Crittenden et al

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I Fig 6 The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the

eastern flank of the Central Graben (Li ne of section is marked on Fig 3a)

bull

The Eastern Flanks of the Central and Viking Graben The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor

Th e entire area is deepl y eroded as exempli fied by Well 22 10-1 here Upper Cretaceou s cha lks lie unconformably upon Triassic sedime ntary rocks Sediments may have been deposited on the high during the Early Aptian tran sgression and were removed by subse quent erosion For example the Maureen Sh el f is a fault-bounded terrace on the east flank of the Graben Intersection Area bordered by the Vestl and ArchJaeren High to the east and limited to the NW west and SW by the South Viking and SE Witch Ground Grabens It is referred to as the eastern part o f the Andrew Ridge by various authors (e g Bisewski 1990)

Well 1623-1 is located on the flank of the Maureen Sh elf on a terrace fault block which is downthrown to the west The well penetrated a thin Valhall Formation sec tion (a relatively condensed interval with a number of hiatuses and non- sequences) overlain by a thin and incomplete Sola Formation interv al no sandstones are present in thi s well Thi s would be expected in a high are a subjec t to non-dep ositi on andor subsequent eros ion during the Late Apti an regre ssion However the V5 and V6 units deposited during the Early Aptian transgression are present at the top of the Valhall Formati on

In well 1628-3 located on an isol ated high in the South Viking Graben adj acent to the Maureen Shelf UpperCretaceous chalks lie unconformably upon the Jurassic Wheth er sands tones and a complete Low er Cretaceou s interval were or were not deposited on the Maureen Shelf is not clea r as the area underwent pre - Sen oni an erosion which was associated with tectonic and halokinetic movem ents during the ove ra ll Late Creta ceou s tran sgression

However in wells in the Norw egian sector on the Vestland Arch (S leipner Terrace of the Utsira High) adjacent to the Viking Graben a thin highl y-condensed Early Apti an interval

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

198 The distribution ofAptian sandstones Central North ern North Sea pan 2

(V5 and V6 units) is present (Fig 6) and provides further evidence of the widespread nature of the transgression

Forties - Montrose High This high is 50-miles long and extends from block 211 0 in the north to block 2224

in the south It trends approximately NNW - SSE and is bounded by normal faults it was a dominant positive feature from the Late Palaeozoic to the Tertiary During the Early Aptian the area may have accumulated shallow shelf sediments (probably both carbonates and siliciclastics) derived both from within the basin and from elsewhere The basinal depocentres and terrace margins adjacent to the high are prospective for Sola Formation sandstones In addition there may be down-faulted areas on the high itself where Lower Cretaceous sediments are preserved (similar to the conditions which occur in the Unst Basin)

In the Forties field (blocks 21 10 and 226a) the oldest rocks penetrated are Middle Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation and younger sediments Where present the overlying Lower Cretaceous comprises a thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments ~ In Well 21110-1 Upper Cretaceous chalks lie unconformably upon Jurassic strata and a dark grey-brown sandstone is present at the base of the chalk interval above the Kimmeridge Clay Formation This sandstone may constitute an exploration target A similar base- Chalk transgressive sandstoneconglomerate ofEarly to midgt Cretaceous age is present in the West of Shetland Exploration area (authors observation) in the Magnus area of the Viking Graben and in the western part of the Halibut Horst In Wells 2211-1 and 2211-2 a thin Upper Cretaceous chalk interval lies unconformably on Triassic sedimentary rocks

In the south-central part of the Forties-Montrose High (blocks 2217 and 2218) Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls which lie unconformably upon the Triassic This thin section represents the extent of the Early Aptian and Albian transgressions (Wells 2217-T2 and 2218- 1)

THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND SANDSTONES

The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic high blocks (which to date have been the main target for oil exploration) but in the structurally- deeper hanging-wall and back-basin areas where little or no drilling for bull Lower Cretaceous targets has taken place In some areas deposition of material eroded during the Late Aptian regression from the crests of active fault blocks formed hanging-wall breccias and conglomerates

Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has resulted in what we interpret to be gulley- or canyon-fed fan systems Good examples are the sands in the Magnus Trough (authors observation) and in the Agat field area (Norway Quadrant 35) (authors observations and pers comm Brit Sauar) Other well-developed examples are located in the West of Shetland exploration area in wells adjacent to the Rona Ridge and the Shetland Platform Deposition of coarse clastics with reservoir potential took place in down-faulted basin margins and more importantly in transfer-fault zones During the Early Cretaceous as in the Late Jurassic salt movement and dissolution in the Central Graben complicated the processes of erosion and redeposition The principle examples of Aptian sandstones are located in the Moray Firth as discussed below

The Moray Firth Within the Moray Firth the Banff Basin and Buchan Graben form a corridor that

extends from quadrants 13 14 19 and 20 in the west through block 2112 to the northern

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

199 S Crittenden et aL

liR Hrv 1l1middotRSf( -rumiddot lt H

tI

IO-t1

it I W

Fig 7 Relative lowstand depositional model for the Fladen Ground Spur during deposition of the Late Aptian Sola Formation claystones and sandstones

part of quadrant 22 in the east The corridor extends to the Britannia field area in the southern Witch Ground Graben and the Graben Intersection Area - quadrants 1529 and 30 1626 27 and 28 and 221 and 2 (see Fig s 2 and 5)

bullReservoir sandstones of Aptian age are informally referred to on released composite

well log s as the Aasgard Sands and Captain Sands in the west on the western flanks of the Halibut Horst and as the Glenn Sands south of the Renee Ridge In the east south of the Fladen Ground Spur they are referred to as either the Bo sun Sands or the Kopervik Sands This profusion of informal names is confusing Johnson and Lou (1993) applied

a different but nevertheless consistent and formalised lithostratigraphical terminology to these sandstones and renamed the Sola Formation as the Carrack Formation

Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground Spur in the Witch Ground Graben and in the Graben Intersection Area to the south Thick Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig 7) Over 275m (gross) of sandstones are pre sent in Well 1627-2 and the se are almost certainly sourced from the Fladen Ground Spur Th e sandstones in Well 1626-3 are much reduced in thickness to only 55m gross but the thickness rapidly increases westwards into the southern part of quadrant 15 in the South Witch Ground Graben and 1125m (gross) are recorded in Well 1529a-2 Well 1628-6 encountered Lat e Aptian gasshybearing sandstones overlying a salt diapir In Well 1628-4 located on an intra-basinal high a very thin oil- bearing Aptian sandstone interval lies unconformably upon the Kimmeridge Clay Formation This marks the feather-edge of the Late Aptian sandstones which pinch-out to the east onto the Maureen Shelf (Andrew Ridge)

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

r-shy

200 The distribution oi Aptian sandstones Central Northern North Sea part 2

These sandstones were derived from the tectonically-active SW margin of the graben (the Renee Ridge) and were redeposited by gravity-driven processe s More importantly material was also shed from the tectonically-passive NE margin (Beach 1984) ie from the Fladen Ground Spur and from the Maureen Shelf and Jaeren High on the eastern flank of the Viking Graben during the Late Aptian regression Sandstones entered depocentres such as the Witch Ground Graben via (fault-controlled) gulleys This occurred at the same time as the deposition of condensed Sola Formation claystones (the Late Aptian-Early Albian S I S2 and S3 units) on shelfal areas and the deposition of thick claystone intervals in parts of the depocentres which did not receive coarse clastic material We suggest that some of the sandstones in the depocentres were subsequently reworked by submarine currents flowing parallel to the axi s of the graben and were redeposited as contourites

A lowstand depositional model for the Sola Formation sandstones in the Graben Intersection Area is illustrated in Fig 7 reference should also be made to Fig 5 of Crittenden et al (1997) The area immediately to the NE and north of the Forties High in the NE Central Graben is al so prospective for the development of Aptian sandstones For example the Sola Formation is 46-m thick in Well 228a-2 However the basal S I ~ unit is only 33-m thick and overlies the Early Aptian V6 unit of the Valhall Formation as indicated by an influx of tiny redorange-stained Praehedbergella sp assigned to the KL6 Biozone The thin S I interval may either represent condensation at the feather-edge of the depositional area to the north and NW or represent truncation associated with erosional scour In the nearby Well 223a-1 the Sola Formation claystones are 11- m thick and overlie 75m (gross) of Sola Formation sandstones The underlying Valhall Formation is truncated and the V6 and V5 units have not been identified Similarly in Well 2212-2 56m ofsandstones and claystones were drilled while the underlying Valhall Formation is truncated and the V6 and V5 units are missing No sandstones are present in the nearby Well 221-2a but 32m of Sola Formation claystones including greenshystained Praehedbergella sp characteristic of the KL5B Biozone within the S I unit overlie the V6 unit of the Valhall Formation The V6 unit in this well yielded a redl orange-stained Praehedbergella sp fauna which is characteri stic of the KL6 Biozone Th e underlying V5 unit yielded brown-stained Praehedbergella sp (HD9) characteristic of the KL7 Biozone

NW into the Witch Ground Graben in quadrant 14 in the region of the Tartan and Highlander fields Sola Formation sediments are much thicker but no Aptian sandstones have been recorded in wells for which data has been released However on the flanks of the Fladen Ground Spur Aptian sandstones may be expected in the form of mass-flow deposits and their erosive-based feeder channels forming good stratigraphic traps as the bull sandstones pinch-out up-dip Indeed the Upper Valhall sands tone (UVA of Casey and Romani 1992) in the Saltire field documented in Wells 1517-23 -21 and -22 (Casey and Romani ibid Casey et al 1993 ) agree with this model However their interpretation was based upon incompletely-cored material from the 1517-23 well

Th e present authors disagree with Casey et pound11s interpretation of the lithostratigraphy and age of the sandstones in Well 1517- 21 In that well we interpret the UVB sandstones to have been deposited in an erosive channel cut into the Valhall Formation cl aystones which has removed the Fischschiefer (the V5 unit) We sugges t that the Fischschiefer noted in the well by Casey et al has been wrongly identified and is a claystone drape or minor flooding surface within the Aptian sandstone package Thi s is consistent with the data considering the incomplete biostratigraphical coverage indicated by Casey et al (ibid) on their figures We suggest therefore that the Piper Shelf and the depositional area adjacent to it in the Witch Ground Graben is prospective for Late Aptian sandstone deposit s

Th e Halibut Horst and Halibut Shelfformed a major source area for sediments deposited in the Banff Basin and adjoining Buchan Graben (Fig 5) Within the axis of this basin in the region of the Ettrick field and to the south and west of it (ie in quadrant 20) Sola Formation

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

---- - - - -

201 S Crittenden et al

aB Islands - by granlIe bosses

~ sand prone lans sediment supply

bullbull deposKionalenvelope 01base01faunscarplan-slope aproncomplex ~ ~ shy - __ ~(CaledonianSouth Halibut Granite)

Fig 8 Schematic blrds-eye view of the 1426 - 13130 area during the Late Aptian seashylevel lowstand Clastic sediments are being reworked from shallow-water areas on the

Halibut Horst into adjacent basinal depocentres

sediments are over 122-m thick Most of the studied wells penetrated a great thickness of Sola Formation (135m in WeIl207a-) in some wells (eg 201-) the S I unit was

bull thin indicating condensation or erosion associated with nearby sandstone deposition Other wells had thin sandstone stringers (e g 203-1 -Za -3) and yet others adjacent to the northern margin of the basin recorded thick sandstones (eg 1426-1 1330- I and 13 30-3) A schematic depositional model for the 1426 and 1330 area is illustrated in Fig 8 Wells in the basin centre are claystone rich and Aptian sandstones are not recorded on any of the released wells we have examined

The Aptian sandstones in the Banff-Buchan Graben corridor to the south of the Halibut Horst form part ofan east-west orientated submarine fan - slope-apron complex consisting of small sandy debris-flowturbidite sand lobes with interchannel muds This complex was supplied with poorly-consolidated sediments from sources lying to the north From the available data we propose that these sandstone lobes did not extend far into the deeper parts of the basin to the south where only thick shale sequences (Sola Formation) were laid down However Aptian sandstones are probably present along the entire southern margin of the Halibut Horst as already explained

DEPOSITIONAL MODELS

Because the predictability ofthe Aptian sandstones has important economic implications the sandstones recovered by cores in the wells studied need to be carefully examined in

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

r

202 The distribution ofAptian sandstones Central North ern North Sea part 2

order to determine transport and depositional mechanisms A variety of gravity- driven processes (slides slumps debris flows turbidity currents) can transport sediment from the shelf edge and upper slope into deeper waters A consideration of the models which have been proposed for these gravity-driven processes is beyond the scope of this paper as is the differentiation of high-density turbidites and sandy debris flows which has recently received much discussion (Shanmugam et al 1994 1995 Shanmugam 1996)

We have studied numerous cores from Aptian claystones and sandstones and have identified a variety of sedimentological facies These facies have allowed us to interpret the probable mode ofdeposition and to suggest depositional models Our regional studies of Lower Cretaceous sedimentology (Fig 9) and palynofacies indicate two major depositional models for the Aptian sandstones of the Central and Northern North Sea

(i ) We interpret some of the Aptian sandstones particularly those in areas of more rapid subsidence to have entered the basin from a single major point source Thus sandshyrich borderland- type submarine fans (cf Link and Nilsen 1980) dominated by channelised mid-fan facies associations were formed on slope aprons This is exemplified by Well 213a-4 (Fig 5) (Crittenden et al 1997) _

(ii) We interpret other sandstones to have been deposited in inner fanslope areas shyThese were supplied by a number of moderately-sized channels incised into interchannel deposits (cf Chan and Dott 1983) rather than a single point source Locally small fan lobes were formed in the mid-fan area at the termination of both inner-fan and mid-fan channels Examples are the SW t1ank of the Halibut Horst (1330 and 1426 area see Figs 5 and 8)

Although evidence is absent from the cored sequences examined these deposits can be expected to grade rapidly into basin-plain sediments Submarine fan - slope-apron complexes comprise thick sand-rich laterally-continuous prisms and consist of a range of interbedded gravity deposits (sandy debris flows slumps and slides and turbidites)

Alternative depositional models for the sandstones in the corridor area such as a single large radial fan or an east- west oriented turbidite system have been considered However the facies associations observed comprise well-developed channelinterchannel units and thick channelised mid-fan deposits Associated outer-fan deposits are absent as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan progradation (Walker 1978 Stow 1986) A turbidite system prograding from the west along the corridor is considered to be unlikely despite the occurrence of more distal mid-fan type facies in wells in the east Coarser-grained and more feldspathic sandstones in Well 213a-4 in the east do not represent distal equivalents of the finer-grained less feldspathic sandstones observed in the west

We therefore favour a fan - slope-apron complex sourced from a northerly direction bull for the Late Aptian sandstones in the corridor area (Figs 5 and 8) The same depositional model is proposed for the sandstones in the Graben Intersection Area As in the Graben Intersection Area the basin-margins were ramped rather than composed of high-angle fault scarps and activity on the faults triggered slumping of sediment The absence of hanging-wall conglomerates and breccias indicates there was no major fault activity on the margins of the corridor However major lineaments such as the Wick Fault to the north (see fig 2 of Crittenden et al 1997) were expressed as fault scarps with which hanging-wall conglomerates and breccias have been found to be associated The southern part of the Buchan Graben may be sand-prone as the result of sands derived from the Peterhead Ridge to the south

The Central Graben We consider the Central Graben to be prospective for Late Aptian lowstand sandstones

and although the area is not discussed in detail a number of wells are now br iefly described

The absence of the S I unit from the base of the Sola Formation in some wells suggests that non-deposition condensation andor erosion has taken place This is consistent with

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

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BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

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JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

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LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

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SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

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TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

203 S Crittenden et al

Facies Grain Sizel Bed Boundaries Structuresl Interpretation Sorting Thickness Textures

1 CONGLOMERATE

Pebble 1middot2m Erosional bases

sharpor gradational tops

Matrix supported non-stratijed disorgainsed

Deposited from highdensity debris flows

2

BRECCIA Pebble 15m +

Erosivebases sharp tops

Clast supportedcrsorqarused Depositsof submarine taun

scarp avalanches

3 UNGRADED

SANDSTONES

Poorly to well sorted nne

- grained Several metres

Rare slightly erosive bases

sharp tops

(3A) Structureiess (3B) Bioturbated

(3C) Cross laminated convoluted

Deposits of high density turbiditycurrents

4 GRADED

SANDSTONES

Very fine to coarse poor to very well sorted

01 - 175m bu t

exceptionally over 10m

Slightly erosive bases I sharp

lOPS

Discont planar am dewatering I convoluted structures minorcross

laminae and bioturbation overallsequence

(4A) U fining (4B) U coarse (4C) No seq (40) Biolurb

Depositedfromhigh - low density turbidity currents

5 ARGILLACEOUS

MICACEOUS SANDSTONES

Very fine well sorted

15 - 50Cm Frequent

convolutebases sharp tops

Low - angle lam rare cross laminae I bioturbation

Deposited from low density turbidity currents

6 THIN BEDDED SANDSTONES

Siltstones mainly coarse silt

Siltstones dew ems

Sandstone slightlyerosive

Siltstonescrsconnnuous planar I lentic lam rarely

bioturb

Sandstonedeposited from low densityturbidity currents

WITH INTERBEDDED SILTSTONES

Sandstonesfine I very fine very

well sorted

Sandstones 01 - oam

bases I sharp tops

Sandstonesbioturb small rips discontparallel laminae

Siltstonesby combination of suspension and turbidite

processes

7 Mudsiltstones Mudsiltstones Mudlsillslonesparafl lam Mudsiltstones depositedby MUDSTONES I clay to coarse silt up to several Sandstone lentic lam bioturb combination 01 hemipelagicJ SILTSTONES metres slightlyerosive turbo processes

INTERBEDDED Sandstone very bases I sharp Sandstone drscont planar WITH THIN fine grainedvery Sandstone lt tops lam conv lam rare ripsI Sandstone from low density

SANDSTONES weil sorted a1m bioturbation turbidity currents

8 MUDSTONES I SILTSTONES

Clay to coarse silt scattered sand grainsin

places

Several metres to low tens ot

metres Typically sharp

Discontinuous and planar laminae bioturbatedin places

Mudssiltstones deposited by a combinationof hemipelagic

I turbiditic processes

9 Very finely GRADED

CARBONATES crystalline

micritic breccia 03 - a6m Slightlyerosive Massive or bioturbated

Depositsof gravellydebris flows and turbidity currents

WITH BRECCIAS clastsltScm

I 10 I GRADED

CARBONATES

Very finely crystalline

Variable 01 shy15m

Convolute I slightly erosive

BioturbatedI slump planes Deposited from turbidity

currents

11

UNGRADED CARBONATES

Very finely crystalline

015 - a8m Sharp I convolute Stromatoporoids I brecciation Platformmargin carbonate

build -up

12 Mud micrite Mud several CARBONATE MUDSTONES

WITH THIN Carbonate very

10s cms

Carbonate

Carbonate sharp convolute

Bioturbated slump planes Muds depositedfrom

suspension Carbonates possibly penplattcrrn oozes

CARBONATE BEDS finecrystalline ltO2m

Fig 9 Sedimentological facies identified from cored sequences of the Lower Cretaceous strata of the UKCS

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

204 The distribution ofAptian sands tones Cent ral Northern North Sea part 2

HIGHSTAND SYSTEMS TRACT - V61V5 UNITS

CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT RIDGE PLATFORM BASIN

Shelfalareaof accumulation of sands Very low rates of - silts carbonates amp claystones - ~__--- sedimentation in

deposition concentrated in updip platform-slope-basin area N portion of platform s

inactive fault at Top Valtlall

Relative hi h sea-Ievel- VfiiV5 units

ltc-gt-shy- shy--shy-= shy--

Top Valhall topography of the sea IIcor is very subdued a time of tectonic quiesce nce and sloping margin s rather than fault scarps The faull controlled basins had become inf illed as a consequence of the rising sea -level CUlminating in a maximum TSTHST in the Early Aptian

Fig 10 Schematic section across the Halibut Horst Halibut Platform and Banff Sub-basin during the Early Aptian transgression and accompanying deposition of the V6 unit

the proposed Late Aptian sea-level fall which elsewhere resulted in the deposition of gravity-flow deposits as discussed above

Aptian sandstones may be pre sent in basinal areas in the west (eg 2115) and east Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola Formation To the west of the Forties High the Sola Formation is 68-m thick in Well 211 7-1 and overlies the V6V5 units which are 16-m thick In the Kittiwake field the Sola Formation in Well 21118-6 is 117-m thick and overlies the V6V5 unit In wells drilled on the West Central Shelf adjacent to the Central Graben Lower Cretaceous strata are either very thin or absent For example in Well 2815-1 on the West Central Shelf no Lower Cretaceous sediments were recorded while in Well 2815a-2 the Sola Formation is thin and overlies the V6 unit However Wells 2923 -1 291 23h-2 2924- and 2925-1 on the shelfal are a flanking the gr aben recorded relatively thin Cromer Knoll Group sediments with a thin Sola Formation ov erlying the V6 unit In wells on the flanks and bull uplifted shoulders of the Central Graben Lower Cretaceous sediments are either ab sent due to non- deposition or eros ion or are very thin (rarely thick) Likewise in wells in the Gannet field area on the flanks of the Central Graben Lower Cretaceous sediments are either absent or are restricted to a thin veneer However in Well 2911b-l the Sol a Formation is 39-m thick and contains abundant reworked palynomorphs although we interpret it to be incomplete Th e sedime nts may be a clay-rich mass-flow or slump deposits associated with sands tones el sewhere

The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1 Well 2916a-J penetrated only 122m ofSola Formation above a truncated and incomplete Valhall Formation Wells 2914h-1 14h -2 and 14b-3 on the flanks of the Central Graben encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge Clay Formation and in the second and third cases a thin Cromer Knoll Group section (less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay Formation In Well 29119-1a Upper Cretaceous chalks overlie Triassic claystones and in Well 2920-1 Upp er Cr etaceous cha lks overl ie the Upper Jurassic Fulmar Sands

In Well 29118-1 however although the Lower Cretaceous interval is relatively thin a thin V6 unit representing the Early Aptian transgression is present Well 29119-2 is only a few miles

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

205 S Crittenden et al

LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES

CAITHNESS HALIBUT HALIBUT HORST SOUTH RIDGE PLATFORM HALIBUT

BASIN Incision and erosion Incision and erosion of horst

on shell with sediment with sediment deposited in

ESSJ Valhall Fm Vl-V4 IlI1il Late Aptian Sandstones

_V6N5 units ~ Sola Claystones (Carrack Fm)

deposited in basin

11 Valhall Fm Sandstones

basins to north and south

bullbullbullbullbullbull Reworked V6N5 units

Lowstand sea-level s

Fig 11 Schematic section across the Halibut Horst Halibut Platform and Banff Subshybasin during the Late Aptian regression and accompanying deposition of the Sola

Formation claystones and sandstones

to the NW of 2919-1a but here the Lower Cretaceous interval is relatively thick The Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick) which truncates the top of the Valhall Formation as the V6 and V5 units are missing

In Well 294a-1a a thick Lower Cretaceous interval is present including a thin Aptian sandstone (12-01 thick) in Well 295a-1 the Sola Formation is 66-01 thick with a very thin sandstone in the S I unit above the V6 and V5 units Well 2910-2 penetrated 118m of Sola Formation and the S I unit is sandy Well 295a-3 nearby has a condensed and incomplete Sola Formation above a thick V6 unit In Well 298b-1 there is a sandstone beneath the Redby Formation lying unconformably on the Triassic In Well 298b-2 the Sola Formation includes sandstone above the V6 unit In Well 299a-1 the Sola Formation is 945-01 thick and very sandy in the S I unit

East of the Argyll area in the Central Graben the Lower Cretaceous is quite thick in Norwegian waters as exemplified by Well 21 1-1 Well (UK) 3127-1 on the western flanks of the graben penetrated a relatively thin interval of Lower Cretaceous sediments lying on Triassic sediments Chalk of Early Santonian to Cenomanian age lies unconformably upon Late to Middle Albian Cromer Knoll Group Redby Formation claystones and sandstones These in turn lie above a thin succession of Albian - Aptian sandstones which in turn lie above a sequence of calcareous and sandy claystones assigned to the Val hall Formation (Early Aptian and older)

Viking Graben Little information is available on the prospectivity of the Lower Cretaceous interval

in the Viking Graben However hanging-wall and lowstand sandstones of Late Aptian age present a good exploration target here Sandstones ofAptian-to-Albian and Cenomanian ages are present in the Agat field (Gulbrandsen 1987 Shanmugam et al 1994) on the eastern flank of the graben in Norwegian waters

In the South and Central Viking Graben Boote and Gustav (1987) and Bisewski (1990) interpreted a Late Aptian sandy fan to be present in blocks 16121613 and 16 17 adjacent to the boundary-fault complex and suggested that it was sourced from the

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

206 The distribution ofAptian sandstones Central Northern North Sea part 2

~posilion oll~1iM

Fig 12 Schematic birds eye view of the East Shetland Basin during the Late Aptian Clastic sediments are being reworked from shallow-water areas on the East Shetland

Platform into the adjacent basin

Fladen Ground Spur For example Well 16117-Ra penetrated a moderate thickness of oilshy bullbearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al 1994) Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers (representing the feather-edge ofa fan system) In addition regressive sandstone horizons are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2 similar to the Agat field

In Well 16117-3 (Tiffany field) the thin Sola Formation lies above a 265-m thick sandstone (known as the Shirley Sand on the composite log) which in turn lies above the V6 unit In Well 16123-1 within the graben the Sola Formation is only 33-m thick without sands and overlies a thin Valhall Formation capped by a very thin V6V5 unit

In the North Brae area the Lower Cretaceous interval is thin over the Jurassic structure (wells 16I7a-7b and 7a-19) but can be seen on seismic profiles to thicken in block 162 adjacent to the Fladen Ground Spur boundary-fault complex The area is therefore prospective in that Lower Cretaceous sandstones derived from the Fladen Ground Spur during the Late Aptian lowstand may be present Well 9124b-2 penetrated a sandstone within the Sola Formation as does Well 16112a-4 further south

_________J

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

207 S Crittenden et al

EASTNORTH SHETLAND PLATFORM

N

t

retaceous

Fig 13 The distribution of Aptian sandstones (shaded) in the Magnus Trough

bull

East Shetland Basin This intracratonic basin (located on Fig I) developed in an extensional regime resulting

from Mesozoic rifting and plate separation between Greenland and Norway Both regional and local tectonic intluences together with eustatic() sea-level fluctuations controlled pattems ofsedimentation during the Early Cretaceous in this area Extension and subsidence in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in the East Shetland Basin and the formation of numerous fault-blocks which were rotated and tilted to the west The eastern margin of the basin comprises a ridge of tilted fault blocks which runs in a broad curve north from Alwyn SE (block 315) through Alwyn

North (block 39) and block 34 to the Brent and Statfjord fields A spur branches-off north of block 39 to the Gullfaks field Approximately parallel to this trend and situated about 25 km to the west a second ridge of tilted fault blocks runs through theAlwyn South (block 314) Ninian and Hutton Dunlin and Murchison fields

The majority of wells in the East Shetland Basin have been drilled on the crests of tilted Triassic and Jurassic fault blocks so that stratigraphic information for the Lower Cretaceous is very sparse In most wells a basal limestone and calcareous claystone interval (Mime Formation equivalent of the Norwegian sector) is present On highs this is equivalent to most of the Valhall Formation in basinal locations (which have been penetrated by relatively few wells) it is only representative of the basal part of the Valhall Formation (V I unit) the remainder being a relatively thick siliciclastic interval

The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is marked by a condensed interval in the East Shetland Basin where the Val hall Formation is carbonate-rich and encompasses many hiatuses and unconformities A major regression

b

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

208 The distribution ofAptian sandstones Central Northern North Sea part 2

occurred before deposition of the transgressive Late Aptian to Albian sediments which on lap the locally-eroded Jurassic-earliest Cretaceous topography

The inherited Late Jurassic north-south structural grain and renewed tectonic activity had profound effects on patterns of erosion sediment sourcing and sedimentation during the Early Cretaceous If sands were shed from platform areas and fault blocks during relative sea-level lowstands and times of tectonic uplift they may be expected to occur in the adjacent lows For example sands occur within some sub-basins in the East Shetland Basin the low adjacent to the East Shetland Platform boundary- fault system and in the North Viking Graben itself We suggest that if sandstones are present they will have been deposited by gravity-flow processes Coarser clastics (conglomerates and breccias) will be associated with footwall erosion

A birds eye view of the East Shetland Basin during the Middle Aptian is presented in Fig 12 and shows our speculative interpretation of the palaeotopography at this time We postulate that during the Late Aptian sediments were transported across and around the East Shetland Basin area and were deposited into the Viking Graben Some of these sediments are coarse siliciclastics Sediments may also have been transported into the Viking Graben from the east across the shelf in the Norwegian sector W

The Magnus Trough (Fig 13) Tectonic activity associated with the fault zones bounding the Magnus Trough and

related to proto-North Atlantic rifting resulted in erosion of the NW flank of the trough and also of the upthrown fault block forming the SE margin (just NW of the Magnus field) A considerable thickness of Lower Cretaceous sediments has been deposited in the southerly-dipping half graben These sediments are interpreted as mass-flow and turbidite deposits laid down in a fringe of coalescing fans These were supplied from the south (from the fault scarp) from the East Shetland Platform to the SW and from the Margareta Spur to the north and NW

Wells on the NW flank of the trough (2104-1 and 5-) did not encounter Lower Cretaceous sediments in the former well Upper Cretaceous mudstones of the Shetland Group lie unconformably on Permo-Triassic redbeds and in the latter well they lie on a thin Kimmeridge Clay Formation Wells in the trough however penetrated Lower Cretaceous sandstones in Well 21013-1 the thin Lower Cretaceous interval includes water-wet sandstones of Barremian-to-Aptian age The Lower Cretaceous sediments lie according to the OTI composite log unconformably upon Permo- Triassic redbeds In contrast Well 21015a-4 penetrated thick Lower Cretaceous sediments including Albian-Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone member) and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent)

We interpret the upper sandstone interval probably to be equivalent to the sandstone interval in 21013-1 The base of this sandstone is here interpreted to rest on the midshyAptian unconformity the Barremian age being interpreted from reworked fossils There is no well information for block 2116 in the southern part of the embayment although it is prospective for Lower Cretaceous sandstones

The thick sedimentary pile deposited during the Early Cretaceous in the Magnus Trough is interpreted to be a base-of-slope fan which developed on the downthrown side of the End of the World Fault Zone (Fig 13) This is very similar to the LowerCretaceous sequences recorded in (i) the Faeroe Trough (authors observations) (ii) the Haltenbank area off Mid-Norway (Hastings 1987 Shanmugam 1994 authors observations) (iii) the Moere Basin (iv) and theAgat area in the Norwegian sector of the North Viking Graben (Gulbrandsen 1987 authors observations) We have studied these areas extensively in terms of their Lower Cretaceous prospectivity but the results are outside the scope of this paper However to place this study in a more regional perspective Fig 14 illustrates our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

209 S Crittenden et al

WESTSHETLAHO BASINAONARtOGEFAE1K)Emiddot8HE11AHD TROUGH

Fig 14 Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of the West of Shetland - Faeroe Trough areas

Shetland exploration area Here extensive lowstand sandstones are associated with major lineaments including transform faults (eg the Judd Fault) which were active during the Cretaceous opening of the NE Atlantic Ocean

CONCLUSIONS

From this study of the Lower Cretaceous interval in the Central and Northern North Sea we conclude that deposition of the Sola Formation sandstones resulted from a regional Late Aptian regression These sandstones were deposited by mass-flow processes and represent a lowstand systems tract In complete sections they lie above the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation

Recognition of these lowstand deposits which are of reservoir potential requires careful mapping of the top of the Valhall Formation and of the areal extent of the Sola Formation It is particularly important to identify areas where the Valhall Formation has been truncated On seismic profiles erosional incisions in the top of the Valhall Formation and mounding of the overlying Sola Formation may indicate the presence of sand-rich sediments

The Late Aptian relative sea-level fall was associated with thermal subsidence together bull with the opening of the proto-North Atlantic which may have caused reactivation of basin-margin faults These movements together with the relative fall in sea-level account for the erosion and transport ofclastic material across shelfal areas and for the reworking of unconsolidated and unstable shelfslope sediments by gravity-driven processes into basin depocentres This process was discussed by Shanmugam et al (1994) In addition to unconsolidated Aptian sediments older Cretaceous and Jurassic sediments may also have been reworked by slumping from adjacent highs This is illustrated by Well 64071 5-1 off Mid-Norway where a slump ofVolgian Kimmeridge Clay Formation equivalent sediments (Spekk Formation) is found above Early Aptian sediments and below Late Aptian sediments

The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and conglomerates) within hanging-wall half-grabens is restricted to major faults which were active throughout the Early Cretaceous For example wells on the Halibut Shelf located in the hanging wall adjacent to the Wick Fault penetrate very coarse sandstones and

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

r 210 The distribution ofAptian sandstones Central Northern North Sea part 2

conglomerates as do wells in the hanging wall of the faulted NW margin of the Magnus Nose It is interesting to note that wells on the faulted margins of the Faeroe Trough and the West Shetland Basin also penetrated very coarse sandstones and conglomerates within the Aptian interval Coarse sandstones are also recorded in the Agat area which is adjacent to the basin-margin fault complex

We suggest that this methodology (ie integrating a consistently-interpreted well data-base and a sequence- stratigraphic model with seismic sections) could contribute to the search for other lowstand sandstones of Cretaceous age in the NE Atlantic margin Our studies of the NW European Continental Shelf indicate that lowstand sandstones are present within a number of stratigraphic intervals Valanginian and Hauterivian (Scapa Sandstone equivalent) Aptian Albian and Cenomanian (which in Norway are collectively assigned to the Agat Formation) Cenomanian and Turonian (intra-Lange Formation in Norway) latest Turonian to Coniacian (Lysing Formation in Norway) and Santonian and Campanian (Nise Formation ofNorway) All these sandstones are associated with major sea-level fluctuations All are of reservoir potential as evidenced by the oilshyand gasfields already discovered and by the numerous shows of these ages encountered in exploration wells Deep-water drilling in the NE Atlantic margin has confirmed the significance of thick Cretaceous (particularly Upper Cretaceous) strata as a target interval which is within the reach of the drill

ACKNOWLEDGEMENTS

The authors acknowledge discussions with numerous oil-company geologists engaged in the search for hydrocarbons in the North Sea In particular Sc acknowledges discussions with Professor Malcolm B Hart and Dr Ian Tunbridge (Plymouth University) Pedro Barbeito Wolfgang Witt Kristos Kapellos and Richard Ribis (allformerly ofShe II UK) Tony Dore (Statoil UK) and Steve Pinnock (Texaco UK) He also thanks Brit E Sauar Finn Livbjerg and John Gjelberg tNorsk Hydro Norway) for extensive and enthusiastic discussion on Cretaceous exploration in NW Europe Finally the authors acknowledge Professor K W Glennie (Aberdeen University amp Editorial Board) for constructive and critical Journal reviews and for useful suggestions which have improved the paper

Additional drafting was by Graffixx Consultancy (Henley-on-Thumes)

REFERENCES

BANNER F T COPESTAKE P and WHITE M R 1993 Barremian - Aptian Praehedbergellidae of the North Sea area a reconnaissance Bull Nat Hist Mus [Geol) 49(1) 1-30 ~

BISEWSKI H M 1990 Occurrence and depositional environment of the Lower Cretaceous sands in ~ the southern Witch Ground Graben In Tectonic Events Responsible for Britains Oil and Gas Reserves Hardman R P F and Brooks J (Eds) Geol So c Spec Publ 55325-338

BOOTE D R D and GUSTAV S H 1987 Evolvingdepositional systems within an active rift Witch Ground Graben North Sea In Brooks J and Glennie K W (Eds ) Petroleum Geology of NW EuropeProc 3rd Conference pp 819-833 Graham and Trotman

CASEY B J and ROMANI R S 1992 Reservoir Geology of the Saltire Field Witch Ground Graben North Sea BIII Centres Recli Explor -Prod E( Aquitaine 1612 235 -252

CASEY BJ ROMANI R S and SCHMITT R H 1993 Appraisal geology of the Saltire Field Witch Ground Graben North Sea In Parker J R (Ed) Petroleum geology of North West Europe Proceedings of the 4th Conference pp 507-517 Geological Society

CHAN MA and DOTT RH [983 Shelf and deep-sea sedimentation in Eocene forearc basin Western Oregon - fan or non-fan AAPC Bull 67 2 [00-2116

COPE Je W INGHAM JK and RAWSON PF 1992 Atlas of palaeogeography and lithofacies Geol Soc Memoir No 13

CRITTENDEN S COLE J M and HARLOW C J [991 The Early to Middle Cretaceous lithostratigraphy of the Central North Sea (UK Sector) Journ Petrol Geol 14(4)387-416

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966

211 S Crittenden et ot

CR ITTENDEN S COLE J M and KIRK M 1997 The di stribution of Aptian Sa nds to nes in the Ce ntra l and Northern North Sea (U K Sector) - A lowst and sys tems tra ct play Part I S tra tigraphy biostra tigraphic age determ ina tio n and ge nesis of the sa nds JOHn Petrol Geol 20(1 ) 3-25

DORE A G LU NDIN E R FIC HLER C and O LESEN 0 1997 Patterns of basement stru c ture and react ivati on a lo ng the NE Atlantic margin J OHrn Geol Soc 154 (I ) 85-92

GULBRANDSEN A 1987 Agot In Spe ncer A M c t a l (Eds) Geology of the No rwegian O il and Gas Fie lds pp 363-3 70 Graha m and Trot man

HE SJ EDAL A and HAMAR G P 1983 Lower Cretaceous stratigraphy and tect onics of the SSE Norwegian offsho re In J P H Kaassch iete r an d T J A Reij e rs (E ds) Petrol eum geo logy of the S E North Sea and adj acent ons ho re areas (T he Hagu e 1982) Geol en Mijnbou w 62 135-144

IN ESON J R 1989 Coarse-g rai ned submari ne fa n and s lo pe ap ron deposits in a Cretaceous backshyarc basin Antarcti ca Sedime ntology 36 793-819

JOHNSO N H and LOTT G K 1993 Cretaceous of the Centra l and Northern North Sea In Kn ox R W Omiddot B and Cordey W G (Eds) Lithost rati g raphic nomen clature of the U K North Sea Br iti sh Geological S urvey Nottingh am

KERLOG UE A C HERRY S bull DAVI ES H QUINE M AND SPOTII G 1994 The Tiffany and Toni oilfie lds Upper J urassic submarine fan reservoirs so uth Viking Graben UK Nor th Sea Petroleum Geoscienc e I 279-285

LINK M H and NILS EN TH 1980 T he Roc ks Sandston e an Eocen e sa nd- rich deep-sea fan deposit No rthern Santa Lucia Ran ge Californ ia J Ol rJI Sedim Pet rol SO 583-602

LOTI G K BALL K C and W IL KINSON I P 1985 M id-C ret ace ou s stra tig raphy of a co red bo reho le in the weste rn part of the Centra l No rth Sea Basin Proc Yorks Geol Soc 45 235-248

LOWE DR 1982 Sedime nt grav ity flows II Deposi t iona l mod els wi th specia l referen ce to the depos its of high den s ity turbidity curre nts Jo IIrn Se dim Pet rol 52 279-297

S HA NMUGAM G LEHTONEN L R ST RAU ME T SYVERTSEN S E HODGKI NS ON R J and S KIBE LI i M 1994 S lu mp and deb ris- flow do mi nat ed upper slope fac ies in the C re taceous o f the Norwegi an and North ern Nor th Sea (6 1-67deg N) Implicati ons fo r sand di st ribution AAPG BIII 78(6) 910-937

SH AN M UG AM G BLOCH R B MITCHELL S M BEAMISH G W 1 HODGKINSON R J DAM UTH J E ST RAU M E T SYVE RTSEN S E and S HIELDS K E 1995 Ba sin flo or fans in the North Sea seque nce s tra tigraphic mod el s IS se di me ntary fac ies AAPG Bull 79477-512

S HA NMUG AM G 1996 High de ns ity turbid ity cur re nts are they sandy debris flows Journ Sed Res 66 (I) 2 - 10

STOW DA V 1986 Deep clastic seas In Sedimentol og y Enviro nm ents and Facies Ed by HG Readin g Blackwe ll Sc ientific Publicat ion s

TREWIN N H and BRAMWELL M G 1991 T he Auk Fie ld Block 3016 UK North SeaIn UK O il and Gas Fie lds 25 Years Com me morative V olume Ab bot s I L ( Ed) Geol So c Lond Mem bull 14 227-236

T URN ER CC CONE N J M CONNELL ER and COO PER DM bull 1987 A depositional model for the South Bra e Oi lfie ld In Brooks J and G le nnie K W (Eds ) Petroleum Geology of North West Europe pp 853-864 Graham an d Trotma n

WALKER RGbull 1978 Deep water Sa nds tone Facies and Anc ient Submarine Fans Models for Exploration fo r S tra tigraphic T raps AA PG BIIII 62 932 -966