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Journal of Petroleum Geology, \'01. 20(1), January

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

(UK SECTORS): A LOWSTAND SYSTEMS TRACT "PLAY"

Part 1. Stratigraphy, biostratigraphic age determination, and genesis of the sandstones

S. Crittenden*, J. M. Cole**, and M. J. Kirk+

This paperdiscusses the lithostratigraphy, biostratigraphy and depositional sequence stratigraphy ofthe sandstones within the late Aptian Sola Formation in the Central and Northern North Sea (UK sector). The genesis and age ofthese sandstones are discussed and three depositional models are proposed. These sandstones have reservoir potential, and an exploration strategy is discussed in part two of this paper (referred to hereafter as Crittenden et al., in press).

• We interpret these sandstones to be the product ofmass-flow sedimentary processes

initiated by a tectonically-induced majorfall in relative sea-level during the late Aptian. Sediments, including sands. which accumulated Oil the shelfal and shelf/slope break areas ofthe North Sea during the early Aptian relative sea-level high were subsequently reworked and eroded; together with the products oferosion ofthe bedrock geology, they were transported down-slope into deeper-water basinal depocentres during the late Aptian.

INTRODUCTION

Sandstones are present at a variety of localities in the Northern and Central North Sea Basin and occur at several stratigraphical intervals within the Early to "middle" (mid-) Cretaceous. These sandstones tend to be very localised in occurrence in contrast to, for example, the more widespread sandstones of Jurassic age in the same area. The Early to mid-Cretaceous sandstones, which form part of the syn-rift and post-rift successions, are predominantly of "mass-now" origin. They are not usually very thick, except in certain

»Consultant. Waye Cottage, Chagford. Devon TQ i3 8HN. ** Geochem Group Ltd, Chester Street. Chester CH4 8RD. + Stratasearch Consulting, Inverurie, Aberdeen AB5i 8XE.

4 Distribution of Aptian Sandstones. Central/Northern North Sea: part 1

circumstances (for example, in the Witch Ground Graben, Magnus Trough and West of Shetland areas), and are therefore often difficult to detect on seismic sections.

This paper discusses the Aptian sediments , particularly the sandstones with reservoir potential, in the context of conventional stratigraphical (Crittenden et 01., 1991; in press) and sequence stratigraphy models in a tectonically-active area. This discussion and modelling exercise is important, because the trapping mechanisms of the potential hydrocarbon reservoirs comprise both structural and subtle stratigraphical components. An idealised composite well log for the Early to mid-Cretaceous interval in the area studied, which shows the presence of the late Aptian sand stones, is illustrated in Fig. 1. (The stratigraphic relationships of the late Aptian sandstones and claystones of the Sola Formation in three boreholes drilled in the Outer Moray Firth are illustrated in Figs. 6, 7 and 10).

This paper is based on the examination of data from over 400 wells (composite well logs, end-of-well reports, and biostratigraphical reports where available) and the biostratigraphical (palynology, palynofacies and micropalaeontology), lithostratigraphical and sedimentological (core-logging and petrography) analysis of ditch cuttings and core material.

The area discussed (Fig. 2) extends from the northern flank of the Mid-North Sea High to the Magnus Trough and includes the Britannia and Captain fields. At both these fields, hydrocarbons are produced from the Sola Formation sandstones. Other hydrocarbon accumulations have been recorded in the same interval from a number of wells in the Central North Sea. For example, wells 21/2-2, 2112-4 and 21/30-4, all of which were drilled more than ten years ago, have recorded gas/condensate in the Aptian sandstones. To date, the accumulation(s) remains undeveloped. However, it has been reported more recently that well 20/5c-6 on the northern flank of the Buchan Graben penetrated good quality reservoir sandstones of Aptian age, with shows of black oil. In our opinion, the Aptian sandstones of the Central North Sea are therefore an excellent exploration target, and may in the future be revealed to host very large accumulations of hydrocarbons.

Previous work A conventional stratigraphical framework (lithostratigraphical, biostratigraphical and

chronostratigraphical), together with initial suggestions for a sequence-stratigraphy model. was published by Crittenden et al ., (1991) (Fig. 1), and is followed in this paper.

The lithostratigraphy of the same interval was also recently reviewed by Johnson and Lott (1993) ; their lithostratigraphical subdivision is almost identical to that of the present Authors (op . c i t.. 1991). Johnson and Lott (1993), however, adopted a very conventional approach to lithostratigraphy, and did not use a sequence-stratigraphic approach to the analysis of strata. Hence, their scheme does not demonstrate the spatial and temporal relationships of the various rock units recognised.

SEQUENCE STRATIGRAPHY

The basin-wide siliciclastic and carbonate depositional patterns for the Early to mid­Cretaceous intervals of the North Sea can be explained by changes (which are partly a consequence of tectonic activity) in the patterns ofdepositional environment and relative sea-level. By combining biofacies trends (palaeoenvironment and palaeobathymetry), and lithological, well log, and age information, stratigraphers can identify changes in the trends that are a consequence of changes in the environment of deposition (Fig. 3 and 4). These correlatable changes in depositional environment associated with sea-level changes are the key to the interpretation of depositional sequence stratigraphy in well sections, and allow the prediction of areas where sandy sediments were generated and deposited.

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--6 Distribution of Aptian Sandstones. Central/Northern North Sell: part I

The principles of sequence stratigraphy have been explained in numerous texts (e.g. Emery and Myers, 1996), and are not repeated here. However, a few pertinent points are discussed in the following section.

Biozonal schemes provide a means for the explorationist to date and divide intervals of strata, recognise unconformities and hiatuses, identify condensed intervals and other important stratal surfaces. The depositional environment and palaeobathymetry can be determined from a detailed examination of the microfaunas and microfloras, together with foraminiferal biofacies and palynofacies analysis, and sedimentological information. Biofacies studies are an essential procedure for sequence stratigraphy. as they point the way to the recognition of systems tracts and chronostratigraphical surfaces.

Depositional Sequences

Depositional sequences have been defined as relatively conformable successions of genetically-related strata bounded at the top and bottom by unconformities, which are recognised on seismic sections and tied to well control (see Emery and Myers, 1996). The unconformities are defined as surfaces oferosion or non-deposition, and represent significant • time gaps. Typically, these features occur as angular unconformities around inner basin • margins. and pass basinward to correlative conformities in the depocentre (Hubbard. 1988).

Eustatic sea-level changes and isostatic adjustment of the Earth's crust are important controls on sequence architecture. as are accommodation changes. However, tectonism. both local and regional, is of equal importance. or in the Authors' opinion. in some circumstances of greater importance. All these factors have played a part in relative sea­level changes (as emphasised by Dolan, 1989) and sedimentation during the Early and mid-Cretaceous in the North Sea.

Sequence Boundaries Sequence boundaries develop during finite time intervals. Given the resolution of

biostratigraphical age dating and well samples. these intervals may be difficult to date accurately and are sometimes best expressed as an age range. This range takes account of the erosion associated with the boundary-forming event. Therefore. the determined age may be older than the onset of the event, and younger than the age of the resumption of deposition. This may be complicated by reworking of the fossils used for age determination. which is particularly notable in the late Aptian lowstand sandstones.

The Aptian sediments of the Central and Northern North Sea have been interpreted by the present Authors as a type I sequence, resulting from both tectonic activity and • eustatic changes.

Sequence Patterns

A type I sequence overlying a type I sequence boundary is created when the sea-level falls below the shelf break, and a lowstand systems tract is developed in the basin. In siliciclastic systems, this may consist of a slope environment dominated by sediments of mass-transport processes. with a lowstand fan deposited against, and at the toe of, the shelf slope (Fig . 5).

A lowstand systems tract develops when the shelf becomes subaerially exposed or covered by a very shallow sea; livers become incised, and the majority of sediment bypasses the shelf. The shelf is eroded and sediments are fed directly onto the slope and are transported downslope by slumping and other mass-transport processes. As relative sea-level rises again. the incised valleys will trap and fill with sediment. and if the sediment supply is sufficient deltas may form in the upper part of the canyons and feeder gulleys. Resedimentation will take place off the steep slope at the front of these deltas as debris flows and turbidites.

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I 8 Distribution of Aptian Sandstones. Central/Northern North Sea: part J

I THE CAUSES OF BASIN WIDE

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Fig. 3. The interaction of lithostratigraphy and biostratigraphy in the derivation of sequence stratigraphy.

As relative sea-level continues to rise, the supply of sediment cannot keep up with the rate ofaddition ofnew accommodation, and so a transgressive systems tract is established and the shelf is flooded to generate a transgressi ve surface; a package of onlapping strata is deposited. During the highstand systems tract, relative sea-level rise progressively slows and sediment aggradation and progradation takes place to give an oft1apping sediment package. Thus, in essence, a regression of the shoreline commences.

Condensed Sections •We consider the recognition of seismically-condensed sections, which are associated

with relati ve sea-level rises and which comprise hemipelagic orpelagic sediment deposited during a time ofpoor terrigenous sediment input; to be an essential procedure in sequence stratigraphy. Condensed sections can usually be identified by the occurrence ofmicrofaunaI and microfloral abundances and diversity peaks, and the condensation of biozones. Hence, a detailed faunal/floral data set presented graphically is essential. After identification, each condensed section is dated where possible by biostratigraphical data.

The condensed section depositional facies in a basinal setting comprises marine hemipelagic to pelagic sediments deposited at a relatively slow rate (due to comparative sediment starvation), in comparison to the overlying and underlying sediments. Condensed sections occur as follows:

(1) A primary condensed section is associated with the maximum flooding surface and with flooding onto the shelf. According to Vail and colleagues (jar a synopsis and discussion, see Emery and Myers, 1996), the maximum flooding surface is a downlap

9 S. Crittenden et al.

15/16-10 Biofacies Log

Scale 1:2000 Stratlgraphers: S. Crittenden. J. Cole Date : 24/9/91

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Fig. 4. An example of a biofacies log including the Aptian-Albian interval; well 15116-10 (in feet, and adapted from original composite log held at the Dept. of Trade and Industry).

10 Distribution of Aptian Sandstone s, Central/Northern North Sea: parI 1 r surface which separates the top of the backstepping tran sgressiv e sys tems tract from the base of the fore stepping highstand systems tract. The primary condensed section is commonly associated with authigenic minerals such as siderite, a mineral found commonly in the upper part of the Sola Formation (sensu Crittenden et 01., 1991 ), peaks in fossil abunda nces (particularly planktoni c foramin ifera, as is see n in the V5 and V6 units of the Valhall Formation ); and , in basins, with laminated shales .

If sedimentation rates are high. these major co nde nsed sec tions in basin al are as are characterised by thick hemipelagic calcareous claystones (for example the early Aptian V6 unit of the Valh all Formation ).

The major cond ens ed section has a ma ximum tlooding surface as its top boundary, which can be identified on a wire line log. Thi s is a co rre latable surface that co mmo nly represents the maximum shale point bet ween the fining-upward transgressive sys tems tract and the coarse ning-upw ard high stand sys tems tract in a silic iclas tic regime. In a she lfal situation , it is commonly represented by a high gamma-ray peak.

A good example of a maximum tlooding surface in a condensed section is the gamma­ray peak of the V5 unit (Figs. 6,7, 10). It will be seen later that the V5 and V6 units rep resent the greatest areal de velopment of the Earl y Aptian marine tran sgression in the Central and Northern North Sea. Another example of a tlooding surface is see n in the • Sola Formation S2 unit.

(2) In siliciclast ic regimes, minor condensed sections occur in the low stand systems tract at the toes of prograding co mplexes and top-of-slope fan s, at the base of the lowstand slope fa n/top of the basin-floor fan, and between channel overbank lobes and as hemipelagic cla ystone drapes within the low stand slope fan. They can also occ ur in the transgressive sys tems tract. Th ese minor co nde nsed sec tions are usually difficult to di stin gui sh from one another , and henc e are not co rrelatable over large di stanc es.

SEDIMENTATION PATTERNS DURING THE EARLY TO MID-CRETACEOUS

The concepts discussed above have allowed the co nstruction of a sequence-stratigraphic model for the Early to mid-Cretaceous sediments of the North Sea (C rittenden, unpublished research) . In this paper, we focu s on the Aptian to early Albian interval. However, in order to place the late Aptian sands tones in their stratigraphica l co ntext in the Central and Northern North Sea, the foll owing sy nops is is give n.

Palaeogeographical framework

Plate-tectonic reconstructions indicate that during the Early Cretaceous, the North Sea region was centred over appro ximately lat. 40 0N (Ziegler , 1990), and wa s part of a •tropical to sub-tropi cal archipelago of changing proportions. Thi s comprised numerous islands, shoals, sha llows, straits and bays, which encom passed the eastern UK and NW Europe. Thi s archipelago was itself part of a series of epi continental seas whi ch fringed the southern margin of the Boreal Ocean (ce ntred on the present-day Arcti c Ocean), and extended westward s over part s of North Am eri ca and eas twa rds over northern Europe and Siberia. Thi s shelf sea was separated from the Tethys Ocean in the so uth by a land­mass, of whi ch the London-B rabant-Belgium-Bohemian Massif was a part. Marine co nnec tions through this massif ex isted intermittently at various locations and times.

Th e Earl y to mid-Cretaceou s (Ryaza nian to Turonian ) has been interpreted in NW Europe as a period of gradual and susta ined eustatic sea-level rise (Cooper, 1977; Jenkyns, 1980 ), wh ich we assume to be of second-orde r magnitude. Regi onal and local tectonic activity has modified this trend (third-order and smaller-amplitude cycles), altho ugh tectonic activity, extensional faulting, block rot ation and erosion associated with graben ex tens ion in general diminishes throughout the Early Cretaceous.

II S. Crittenden ef 01.

TIME Proximal Discal

Fig. 5. A simplified sequence-stratigraphic depositional model for the sandstones of the Sola Formation..e

Regional structural setting Crustal extension in the North Sea during the Late Jurassic -r Early Cretaceous inv olved

approximately 70 km of stre tching (Ziegler. 1990). At this time, the North Atlantic was opening, and the area between the Galicia Bank (off Iberia) and Grand Banks (Canada) rifted during the late Barremian to ea rly Aptian. The Bay of Bis ca y area rifted during the Aptian, while rifting from Biscay to the Rockall Plateau occurred during the middle Albian.

An extensional tectonic regime exerts a profound influence on sedimentation, and has been discussed by numerou s authors (La mbiase, 1994). Crustal exten sion leads to normal faulting and the development of back-tilted and rotated fault blocks (e .g. the Central Graben and Viking Graben margins) as the basin develops. The uptilted crestal areas of the blocks are eroded, and the deri ved sediment is deposited on the lower slopes and troughs as fan s. In addition, given sufficient sediment supply from so urce areas, the lows act as sediment depocentres, and thick acc umulations of sediment build out into the basin s. Adjacent palaeohighs and slopes may show ev ide nce of thin and incomplete or co ndensed sequences, but may also be the sites ofshallow-marine she lf-sand and carbonate deposition during times of relative sea-level high (fo r example, the Fladen Ground Spur during the early Aptian).

Depositional Patterns In general during the Earl y to mid-Cretaceous, coarse sili cicl ast ic sedime nts were

deposited in the deeper basinal areas by mass-flow mechani sms during relative sea-l evel low stands wh en sediments bypassed the she lfal areas. Therefore, basinal silicicla stic sediment depositi on decreases during relative highstands of sea lev el, when river-borne sediments are trapped on she lf areas (e .g. during dep ositi on of the ea rly Aptian V6 unit of the Valhall Formation ). Earl y Cretaceou s sed ime nts accumulated during the latt er stages of rifting and during the post-rift stage in the North Atlantic margin and North Sea. In the Early Cretaceous, uplift of the flank s of the rift sys tem occurred to the we st, NW, SW and south of the study area. Consequently, the sedimentary cover of the massifs was eroded and deposited in adj acent depocentres (the West Shetland Basin and Faeroes Trough in the west , and the North Sea gra ben system to the ea st , SE and NE ).

Well ev ide nce indi cat es that the overall depositi on al facies regime throughout the Early to mid-Cretaceous in the North Sea area was characterised by a mixture ofsiliciclastics and carbonates. The diversit y of sedime ntar y regimes is re tlected by the variety of roc k

12 Distribution of Aptian SalldSIOIlCS. Central/Northern North Sell: part I

types. and the rapid facies changes which characterise the strata of this age in NW European basins (Kemper. 1973 . 1979). Despite this variety. there is a broad synchroneity ofdepositional variations throughout the area (Crittenden et al .• 1991). During deposition of the lower part of the Valhall Formation. the basin configuration retlected the preceding rift phase. and a number of depositional centres. intrabasin highs and shelfal areas were present. Some high areas were subaerially exposed and provided a source of clastic material. while others acted as submarine swells separating different foci of deposition . By the early Aptian. however. the basins had filled. the submarine relief had become reduced. tectonic activity was reduced. and the shelfal areas became flooded and coalesced. In addition. the adjacent land areas had been erosionally reduced and the climate had become arid; hence. the freshwater run-off into the basin had diminished.

The submarine topography in the region was onlapped and buried by Val hall Formation claystones and marls. Condensed. but sometimes relatively-thick,carbonate-rich sequences were deposited on submarine highs (these are of reservoir potential in some areas), and on the shelfal and terraced margins of the basins starved ofclastic sediments (e.g. the East Shetland Basin). Depending on the source ofsediment supply. the shelfal areas accumulated • marine clastics and carbonate-rich deposits at times of relative high sea-level during the Early Cretaceous. similar to shelfal areas existing at that time in southern England and Germany. These sediments were systematically reworked and redeposited by later episodes of transgression and regression.

Turbidite fans and base-of-slope aprons developed locally. and were sourced by the local erosion of exposed high and shelfal areas. They occur in areas adjacent to the Halibut Horst. the Renee Ridge. the Buchan Horst. the Forties - Montrose High and the flanks of the main graben system. such as the Fladen Ground Spur and Western Platform (Crittenden et al, 1991). A well-documented example in a growth-fault setting is the Scapa Sand Member of the Val hall Formation in the NW Witch Ground Graben (Boote and Gustav. 1987).

As a result of the tectonically-enhanced, late Aptian regression , a discrete sub-basin configuration was developed. The adjacent hinterland, shclful and slope areas were uplifted. exposed and eroded, faults were reactivated. and deposition of the late Aptian sands occurred by mass-transport processes in some of the basinal areas. The onset of deposition ofthe late Aptian sands is interpreted to be a function of tectonically-controlled (third-order) regression within the overall. Early to mid-Cretaceous (second-order) transgression. In "Vuilian" terminology. we suggest that these sands conform to a lowstand systems tract.

Throughout the late Aptian to Cenomanian transgressive phase (second-order). the discrete sub-basin configuration decreased as the intra-basin highs were buried. the • depocentres filled. and a relatively shallow but very extensive shelf sea developed. dominated by carbonate sedimentation.

Tectonostratigraphic Phases

On the basis of both relative sea-level changes and tectonic controls (including basin subsidence) on sedimentation. the Early to mid- Cretaceous interval in the study area can be subdivided into three depositional phases:

Phase I. Latest Rya zanian to Early Aptian: Valhall Formation

This phase was characterised by overall second-order relative rises in sea-level, which were modified by the results of shorter-term relative sea-level changes and local tectonic activity. These gave rise to third-order cycles of regression (i.e. the Scapa Sand Member and equivalents) and transgression associated with growth faults .

The sediments of the Val hall Formation comprise a generally onlapping interval consisting oflimestones, marls. claystones and interbedded clastics. The Valhall Formation

13 S. Crittenden et al.

CORE INTERP.

SEDIMENTOLOGY & PALYNOFACIES

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14 Distribution of Aptian Sands/ones, Central/Northern North. Sea : part I

culminates in the red-brown, hemipelagic marls of the V6 unit (Crittenden et al., 1991 ), (Figs. 6, 7 and 10), which represent the condensed section of a transgressive maximum, The top is interpreted as probably equivalent to sequence boundary 109.5 of Haq et al, (1987),

Phase 2, Aptian to Early Albian: Sola Formation (claystones and sandstones)

In the overall transgressive, Early to mid-Cretaceous interval, the Sola Formation (sensu Crittenden et al., 1991), including the sandstones, represents a very significant late Aptian regressive phase, which is interpreted principally to be the consequence of tectonics. In the Central North Sea, the initial regression and relative sea-level lowstand resulted in the localised emplacement of mass-flow units and tongues of coarse clastics into the slope and distal basinal areas in which the Sola Formation claystones were deposited.

Following the regression, an increase in the rate of transgression with minor regressive events progressed through to Albian times, and is associated with the waning activity of the extensional tectonic phase in the North Sea (Crittenden, 1987a,b). •

Phase 3, Middle Albian to intra-Turonian: transgressive Redby. Hidra and Plenus Marl Formations

The Redby Formation, of middle to late Albian age, marks the end of the tectonic phase (Austrian") and the beginning ofan interval ofsea-level lise and second-order transgression. It is characterised in the North Sea by extensive pelagic-carbonate deposition, and in general by an overall decrease in the presence of both coarse- and fine-grained siliciclastics, This was the result of the erosional peneplanation and progressive inundation of the intra­basin highs and margins which were the source of the sediments during periods of relatively low sea-level. During relative sea-level highs, coarser clastic material became trapped on the landward margins of the extensive shallow-shelf seas (eg . the "Greensands" of onshore southern UK, Holland and Germany), The more distal shelf areas became the sites of deposition of fine-grained, carbonate-rich, argillaceous material (eg. the Gault Clay of southern UK, and the Osning Sandstone and Flammenmergel of Germany), Carbonate platforms developed in some areas, such as Hunstanton (the late Albian Red Rock). In other areas such as the Agat area of the Norwegian North Sea, sandy fans developed on the flanks of narrow shelfal areas,

The overall depositional style is reflected in the foraminiferal faunas, which became more widespread and dominated by planktonic genera and by, for example, the proliferation ofcomplex chambered lituolids (Carter and Hart, 1977; Crittenden, 1988), The inundation of the intra-basin highs and basin-margins resulted in the establishment of a very extensive • and relatively shallow shelfal sea, which was connected to the Tethys in the south, the Boreal Ocean to the north and the proto-North Atlantic to the west. The hinterland wa s of low relief and arid, with little rainfall; hence, there was reduced run-off into the basin,

The overall trend throughout thi s interval was one of transgression. Minor changes in lithology and faunas, together with the pre sence of scour surfaces and phosphate-rich nodule beds, are evidence for relatively small-scale, tectonically-controlled episodes of relative sea-level fluctuations, These low and stillstand events are similar to those deduced for sequences onshore in NW Europe (Crittenden, 1987b, 1988),

A decrease in tectonic activity and the continuing sea-level rise caused sediments to onlap the basin margins, leading to a progressive increase in the lateral continuity of the younger strata. This is well seen in the southern and Central North Sea. The upper pHI1 of the Red Chalk Formation (R2 and R3 units, and their Gault Clay equivalents) of mid die to late Albian age is the oldest unit to transgress completely the London-Brabant Massif. The "transgression" (second-order), with third-order stillstands and regressions, continued through the Cenomanian with the deposition of the Hidra Formation and its equivalents, For example, in some parts of the North Sea and adjacent areas, sands become prominent

15 S. Crittenden et al.

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Fig. 7. Well 15/30-1 in the Central North Sea (in feet , and adapted from the original composite log held at the DTI).

r 16 Distribution of Aptian Sandstones, Central/Northern North. Sea: pal'! 1

in the Cenomanian both as shelfal Greensands (Carter and Hart, 1977), and as turbidite sand fans. Shelf-margin wedges of limestone deposited at this time may also have reservoir potential (e.g . in the We st of Shetland area).

The early Turonian Plenus Marl Formation (Unit A of Crittenden et al., 1991), or the Black Band Bed (part of the GO unit) of the Herring Formation (Johnson and Lott, 1993) and its equivalents have a remarkable lateral persistence. The unit can be correlated from the Bay of Biscay to the Northern North Sea and the NE Atlantic (Hart and Ball, 1986). The formation represents deposition during an Oceanic Anoxic Event associated with extensive shelfal seas, and is interpreted as a transgressive maximum (a maximum flooding event).

APTIAN SANDSTONES AND CLAYSTONES

In mo st of the wells in the studied region, the late Aptian sandstone s of the Sola Formation overlie the Val hall Formation of the Cromer Knoll Group. The top two units ofthe Valhall Formation de signated V5 and V6 (Crittenden et al., 1991) are early Aptian in age (see Figs. 1,6,7 and 10). These three units ­ V5 and V6 units, and Sola Formation - are described in the following section.

•

The Valhall Formation (pars) V5 unit

This sapropel- rich unit represents deposition in an anoxic sea-bottom environment. The high organic content probably results from preservation of prolific surface-water plankton.

This unit commonly contains an abundant, but poorly preserved (compressed and brown-stained), planktonic foraminiferal fauna (equatable to the Blefuscuiana apt iana (Bartenstein) = H.D9 fauna of Hecht, 1938 ; the Zone 3 of Banner et al, 1993 ; and our KL7 Biozone. The unit can be directly related to the coeval Tock Facies of Heligoland (Bartenstein and Kaever, 1973), and to the Fischschiefer of the Lower Saxony Basin of Germany (Kemper, 1973 ; Crittenden, 1982a, 1987a,b; Meyn and Vespermann, 1987). Thurow tpers. comm. ) refers thi s tran sgressive maximum flooding event to the Oceanic Anoxic Event IA (Arthur et al., 1987) .

The V5 unit, regionally early Aptian in age (forbesi - deshayesi ammonite biozones) , is distinctive and widespread in the North Sea region (Crittenden, 1987a,b) and the rest of NW Europe. It is thought to equate with the III major maximum flooding surface (Haq et al., 1987). The unit is a major condensed interval associated with relatively high sea level, and hence a transgressive maximum onto the shelfal areas. It is coeval with part of the Atherfield Clay sensu stricto in South England and the Isle of Wight (Crittenden, 1982a,b, 1983a; A. H. Ruffell,pers.comm.), marking the return offully-marine conditions which followed non-marine We alden deposition. Interestingly, this unit may be ofsource­rock potential if sufficiently thick in the Atlantic Basin area west of the UK and Scandinavia, particularly in the Faeroe Trough and Moere Basin.

•

V6 unit

This unit comprises red-brown and orange, calcareous claystones and limestones deposited in well-oxygenated open-marine conditions. It is dated as latest early Aptian to perhaps earliest late Aptian (top part of the de shayesi Biozone, the bowerbanki Biozone, and the lower part of the drewi Biozone). The contained microfaunas are dominated by calcareous benthonic and planktonic foraminiferal species characteristic ofwarm. clear, full y-marine waters. One or more influxes ofred/orange stained planktonic foraminifera (Praehedbergellids: equivalent to the Hedbergella D9 interval of Hecht, 1938; Zone 4 of Banner et al., 1993; our Biozone KL6) characterises this interval , and

17 S. Crittenden et al.

Anoxic FacM. Fitlchachief... equivalent V5 (AnoxYl)

•• V4 HIATUS/CONDENSED

LATE?·MID BARREMIAN

MODEL 2

MODEL 3

53

52

51

Reworked microfaunall/llorall Including Reworked Red Pr.........~ app.

la. claata'

/ ,/

-: ./

• Ewald Marts e<Pvalent VII - - - - - - ­

==5:0C==:::H=:a:::E==a:::::l== ~ Pr.........~ app . horizon~ _ - _ -

Ewald Marts e<Pvalent-VII ----:_ - Mot1fad IthoIogIes _ - ­

~~;F~=---=-=--~~=~- -A~T~ ~ - ------...:t

LATE?·MID BARREMIAN V4

Fig. 8. Cartoons illustrating alternative models for the timing of the onset of Sola Formation Sandstones deposition. See text for discussion.

I 18 Distribution of Aptian Sandstones. Central/Northern North Sea: part

are important for correlation purposes. In some wells, an influx of radiolaria is also observed. These marls were deposited in response to a latest early Aptian relative highstand of sea-level, basin flushing, and a decrease in clastic input to the basins and distal shelfal areas due to inundation of the clastic source area. At this time, peneplanation of the hinterland topography had probably created a low-lying, arid area, which provided little clastic material and very little run-off into the basin. This desert-like hinterland, together with erosion of the Old Red Sandstone and New Red Sandstone bedrock geology, may explain the red colouration of the V6 unit.

The unit is interpreted as a highstand systems tract which, in a basinal situation, is represented by relatively condensed intervals of hemipelagic calcareous claystones (i.e. a primary condensed section). However, coarse clastics would have accumulated on the landward, shallow-marine margins of the shelf (cf. Lower Greensands in South England).

The V6 and V5 units are not present in all the studied wells. Where they are absent, it is assumed that they have been removed by erosion associated with major regression and relative fall in sea-level (and the onset of Sola Formation deposition) rather than non­deposition. Indeed, in some areas, the erosion associated with this regression is severe, incision of the Valhall Formation being such that the V6, V5, V4 and part of the V3 units e, are missing. We interpret the top of the Val hall Formation as a type 1 sequence boundary (109.5 - martinioides Biozone).

The Sola Formation (sensu Crittenden et al., 1991)

The Sola Formation represents deposition in a relatively deep-marine environment, and comprises dark coloured, non- to slightly-calcareous, fine-grained clastics, containing a predominantly agglutinating and green-stained foraminiferal population. However, siderite horizons are recorded in the upper part, and may represent minor flooding surfaces. The calcareous content of the formation is considerably less than in the underlying Valhall Formation, indicating an increase in the supply of terrigenous sediments to the depocentre. Influxes of green-stained calcareous benthonic and planktonic foraminifera occur at discrete horizons (in the lower Sola S I unit and the S2 unit), and suggest that periodic influxes of oceanic waters affected the region during minor flooding events.

Within the middle part of the Sola Formation, tuffaceous intervals have been recorded (Crittenden et al ., 1991). Widespread volcanic activity occurred in the late Aptian and early Albian throughout much of NW Europe, and sediments rich in volcanic material (usually bentonites) have been described from this interval in wells from the North Sea (including BGS borehole 81/40) and from South England (Sandgate Beds) and Germany (W . Zimmerle, pers. comm.. 1992). The source of this volcanic material is not known accurately, although it has been suggested that volcanic centres could have been located e in the Baltic and southern North Sea areas, including The Netherlands.

This phase of volcanic acti vity coincides with increased tectonism at the plate margins to the west of Britain associated with the opening of the Rockall Trough and rotation of the Iberian Peninsula. It is thus intimately associated with basin-margin uplift, increased terriginous sediment supply, and initial relative fall in sea-level followed by sea-level rise and transgression as oceanic basaltic plateaux were developed.

The Sola Formation consists of a thick interval of claystones in basinal areas distal from sources ofcoarse clastics, and has been divided into three claystone sub-units based on wireline-log characteristics and lithology: S3, S2 and S 1 (see Crittenden et al., 1991 for definition and discussion). The lower units, S 1 and S2, are considered by us to be replaced by sandstones in some circumstances.

The Soia Formation sandstones

The classic area (quadrants 15129, 15/30, 16/26, 16/27 and 16/28) for these sandstones is in the southern Witch Ground Graben (Bisewski, 1991 ; Guy, 1992) at the graben

19 S. Crittenden £'1 al.

z L1THOSTRAT L1THOSTRAT a ~

CLAYSTONE w!;(

::E CLAYSTONE ::Ea: a: FAUNAL EVENTS AND SAND -I­a SEQ UENCE 1-(1) u. SEQUENCE

fu L. gyroidinaeformis KL4A

w z 0 R1 R1

5 oe(

~ Biozone o -:i ~ ~ < cr ,

L. gyroidinaeformis & i1i , S3 V. chapmani &siderite S3 ,

KL4B and KL5A Biozone « ...J 0 Green-stained Praehedbergellids wCJ) S2 Sands SKl5B Biozone

8 1 KL5B Biozone Sands

?HIATUS? 'zloe(

OR CONDENSATION I i=

I c, oe(

Red-stained PraehedbergelJids

i=' V6, Ewaldi (H. 011), very abundant and V6, Ewaldi

a: Marls commonly in clumps. Marls ~

~ KL6 Biozone a:

-c ...J Brown-stained PraehedbergelJids w ...J V5, V5,« (H. 09), flattened . I Fischschiefer KL7 Biozone Fischschiefer ...J;;;

?HIATUS? OR CONDENSATION

Fig. 9. Lithostratigraphy a nd faunal eve nts in clayston es a nd sa nds to nes in th e So la Fo rmation and upper pa rt of th e Va lhall Forma tio n in th e Ce n tral No r th Sea.

.e

intersect ion area of the Central North Sea (Britannia fiel d area) and surrounding region of the Buchan Graben .

The sandstones of the So la Formation have been referred to as the Kopervik Sa nds Formation (Hesjeda l and Hamar, 1983) and the Bosun Sands (Crittende n et al., 199 1) (Figs 1. 6, 7 and 10). Sa nds of sim ilar age in the so uthern part of the Viking Graben have been referred to as the Shi rley Sa nds (on we ll / 6// 7-13 DTI co mposite log). while Isaksen and Tonstad ( 1989) refer to them as the Ran sands tone unit s. John son and Lott ( 1993) apply a rang e of names to them . depending on location in the Ce ntral North Sea. including the Britannia Sand stone Format ion and the Skiff Sandstone Member and the Ca ptain Sand stone Member of the Wick Formation.

We co nsider all the above sandstones to be coeval wit h the lower part of the So la Formation (S I and S2 uni ts) , and to rep resent lowstand sys tems tract deposits (F ig. 5). The units co nsis t of interbedded sandstones and siltstones, with minor claystones, of mass-flow origin (including turb idites - see Gu y, 1992). and are of late Aptian to per haps ea rly Albian age . Th e onset of sand sedime ntation may be as old as latest ea rly Aptia n in some areas (see below). Palynofacies studies indicate a high content of terrestria lly­derived pollen and spores in these sedi ments.

Lithostratigraphical subdivision and correlation of the clastic sediments deposited by mass-flow processes in the study area is difficult if only wire line logs are used. It is apparent that the relative proportions of claystones. siltstones and sandstones are very var iable within the Sola Formation in each we ll. This is a characteristic of sediments of

20 Distribution of Aptian Sandstones, Central/Northern North Sea: part J

mass-flow origin. Detailed palynofacies and biofacies studies by the Authors in specific areas have been used for detailed correlations.

The accurate and precise dating of these sandstones by the usually sparse and facies­controlled foraminiferal faunas is difficult, because the recovered faunas are essentially agglutinated forms and non-age diagnostic. However, influxes ofgreen-stained calcareous benthonic and planktonic foraminifera within thin claystones ("hemipelagic drapes") are important marker horizons equatable with the major martinioides/nutfieldiensis transgressive pulse seen onshore southern England (Crittenden et al.. 1991). These hemipelagic drapes seal the sandstone units. This is further evidence for dating the regression responsible for the emplacement of the sandstones as late Aptian.

The presence of reworked lithologies, foraminiferal faunas and palynofloras within the sandstones indicate that complications may exist. Such reworked elements comprise material from the original shelfal "source" sands of early Aptian age (the upper Valhall Formation), from the whole of the Valhall Formation and from much older stratigraphical units (of Devonian, Carboniferous, Permo-Triassic and Jurassic ages), thus corroborating the concept ofshelfal incision associated with regression and a type 1sequence boundary. •

Incidentally, Vail's early charts for the North Sea area maintained that the major • regression was mid-Aptian, and was the 112 sequence boundary (see Vail and Todd, 1981, figure 11). However, the 1987, March version 3.1b Vail Chart places the 112 sequence boundary in the basal ammonite biozone of the early Aptian. This discrepancy, perhaps an illustration of scientific progress, emphasises the need for: (l) a complete understanding of the data-set used by Vail and colleagues in the construction of their so-called global curves; and (2) the need to use an agreed absolute age and chronostratigraphy in relation to the stage nomenclature. sedimentology, lithostratigraphy and biostratigraphy of onshore sections of NW Europe used as a data-set in constructing these "sea-level" curves. These points are well illustrated by Hart et al., (1989, table 7.1). In order to overcome such problems of calibration, a firm agreement must be made regarding the relationships of various biostratigraphic models (eg. ammonites, dinoflagellates, foraminifera) to lithostratigraphical and sedimentological models ofdocumented onshore sections (see discussion in Bartenstein, 1978).

The late Aptian coarse siliciclastics discussed in this paper are interpreted as mass­flow deposits of a lowstand systems tract immediately above the sequence boundary (109.5) which defines the top of the Valhall Formation. The lowstand systems tract is divisible into a number of units (the basin-tloor fan, the slope fan and the prograding complex), all of which are formed during a relative sea-level low (see Fig. 5), but which are not necessarily distinguishable in the late Aptian lowstand mass-tlow sediments of • the North Sea.

The Sola Formation 53 Unit

The topmost claystone unit of the Sola Formation, the S3 unit. is early Albian in age (although a late Aptian age cannot be discounted for the basal part). and is characterised by sideritic material. Authigenic siderite is a characteristic of condensed sections associated with flooding surfaces. In some wells, there is a relatively high percentage of coarse clastic material. There is also an up-hole increase in planktonic foraminifera. which is indicative ofmore open-ocean influences, suggesting transgressive and highstand systems tracts. The S3 unit is characterised in the upper part by the occurrence of green-stained Lingulogavelinella gyroidinaeformis, and tuffaceous material is present at discrete horizons throughout.

THE ONSET OF APTIAN SAND DEPOSITION

The in-place foraminiferal faunas recovered from the Sola Formation claystones and sandstones include green-stained, planktonic foraminifera of the family Praehedbergellidae

21 S. Crittende n et al.

Ie

e

~ LITHO- TIME ~

40 STAAT. STAAT. ~ SONIC

140

RESISTIVITY OHMIM

LITH.

100

GAMMA

AP.I .U. o

z 0 1= ~ a: 0 u,

~ a: gw a:l ~ w

1:::;; z c w < Z ~ ~ c:i (}j < z

l~~ s ~fa:l a. <!i~

~ ~

Ita:

"' ~

:::i! Jtu,

::J ,£

s -c I -' ~

SC. I990

Fig. 10. The original composite log of Well 21/la-12 illustrating the Aptian interval as interpreted by the present Authors (in metres. and adapted from the original composite log

held by the DTI).

r 22 Distribution of Aptian Sandstones. Central/Northern North Sea : part 1

(Zone 5 of Banner et al ., 1993 ; our Biozone KL5B of Fig.9). However, in some of the studied wells (eg. /5/30-/, see Fig.7), a horizon oftiny , red-stained, planktonic foraminifera, and red-stained calcareous benthonic foraminifera is present within claystones, and in some cases sandstones in the lower part of the Sola Formation. This fauna is similar, if not identical, to those ofthe early Aptian V6 unit (ewaldi Marls) ofthe Val hall Formation (Zone 4 of Banner et al., 1993; our Biozone KL6).

Three models have been proposed to explain the presence ofthis planktonic foraminiferal fauna in the basal part of the Sola Formation sandstones, and each model has important implications for dating the onset of sandstone deposition (see Fig. 8):

Model 1 This model assumes that the red-stained planktonic foraminifera within the sandstones

are in place , and it implies that sand deposition and emplacement was coeval with the V6 unit tewaldi marls) of the Valhall Formation. Thus, sand deposition commenced in the early Aptian . If deposition occurred at this time, then such "lowstand sands" were deposited (as slumps off the shelf-margin and shelf/slope triggered by tectonic activity) in an environment in which condensed calcareous clays were also being deposited, e. elsewhere interpreted as a highstand systems tract.

Model 2 The red-stained planktonic and benthonic foraminifera within the sandstones could

also be interpreted as being reworked, and associated with erosion and slumping of the Valhall Formation (V6 unit), during sand deposition. There is an abundance of reworked fauna and flora (both pre-early Cretaceous and intra-early Cretaceous), within the sandstones which causes immense confusion and difficulty in dating. Because of this reworking, the original dating of this unit in well/5/30-/ was Barremian (see DTI composite log: Fig. 7). This model indicates that sand emplacement commenced in the late Aptian after deposition of the V6 unit, which is early Aptian in age.

Model 3 The red-stained planktonic and benthonic foraminifera are interpreted to be in place

in the sands and in the V6 unit. This implies that there are more than one interval of red­stained planktonic and benthonic foraminifera. If this were the case, there would probably be more than one interval of red-stained planktonic and benthonic foraminifera observed in wells which have no sand member within the Sola Formation - that is, one interval in the lower part of the Sola Formation and one interval in the V6 unit. However, this has not been observed by the present Authors in any of the released wells examined. e

DISCUSSION

It is difficult to determine which is the most applicable model for the onset ofdeposition. However it is revealing to look at this stratigraphical interval elsewhere in NW Europe. Biostratigraphical and lithostratigraphical information from boreholes located in depositional centres in the Central North Sea has been interpreted to derive the scheme in Fig . 9.

The Early Cretaceous section of the island of HeJigoland in the German sector of the North Sea (Bartenstein and Kaever, 1973), the Lower Saxony Basin of Germany (Hecht, 1938; Bartenstein, 1978), and boreholes in the Southern North Sea Basin (Crittenden, 1987a,b) display the same biostratigraphical and lithostratigraphical sequence (Crittenden et al., 1991; Banner et al., 1993). The complete interval is illustrated (Fig. 6) by released we1l2113a-4 from the Central North Sea (see Banner et al., 1993 for other well sections). It is important to note that, in this well, the Valhall Formation beneath the sandstones is complete, as the V5 and V6 units are present. Another complete interval, from the

I

II

23S. Crittenden et at.

15/30-/ well, is shown in Fig. 7, with what we interpret to be a truncated V6 unit. In the 15/30-1 well, there is an abundance of Barremian and early Aptian faunal and floral taxa, interpreted as reworked (plus pre-Cretaceous reworking), in the lower partof the sandstones. On the original composite log held by the DTI, this interval has been dated as Barremian, initiating the confusion about the correct age range of the sandstones. The presence of the reworked, red-stained Praehedbergellid fauna in the sandstones may tempt some geologists to date the onset of sand deposition as early Aptian (intra-V6 unit), as depicted by Model 1 and Model 3 of Fig. 8.

.e

The wells examined by the Authors indicate that the degree of erosion of the top of the Valhall Formation varies from location to location. For example in well 21/2-4, the sandstone rests unconformably on a truncated Valhall Formation, while in well 21/2-1, the Sola Formation lies above the V6 unit of the Valhall Formation, with the basal part of the Sola thinly developed and perhaps associated with condensation/erosion and the generation of sand nearby. Well 2//1a-12 illustrates the sandstone member of the Sola Formation above the V6 unit (characterised by an abundance of red-stained planktonics) of the Valhall Formation (Fig. 10) . The interpretation is therefore that sand-member emplacement is post-V6 unit and is late Aptian in age . The depositional processes, such as slumping and gulley erosion, associated with mass-flow sedimentation from a shelfal source have eroded the top of the Val hall Formation, thus accounting for the reworked red-stained planktonic foraminifera (and other faunal and floral elements) within the sandstones, and for the variation of the age of the Valhall Formation subcrop beneath the sandstone member.

As a regional model applicable to the search for sandstones of reservoir potential of late Aptian age, Model 2 of Fig. 8 is preferred for the timing of the onset of emplacement of the sand member. Some degree of diachroneity may indeed be present within the region, associated with the different timing ofthe onset of tectonic activity and reactivation of fault movements (discussed further in Crittenden et al., in press). This triggered slumping of sand-rich slope and shelfmaterial into the basin, where condensed hemipelagic sediments of the V6 unit. a highstand systems tract (primary condensed section), were accumulating.

CONCLUSIONS

This study of the V5 and V6 units of the Valhall Formation and the Sola Formation in the Central and Northern North Sea indicates that deposition of the Sola Formation and the associated sandstones commenced as a result of a late Aptian regression. This regression may have been tectonically enhanced. The boundary between the Val hall and Sola Formations represents a profound change in depositional regime, ret1ected by the sediments and the floras and faunas, and is interpreted as a major sequence boundary. The sandstones in the Sola Formation represent deposition as a result of marine mass­flow processes, and therefore a lowstand systems tract. These sandstones overly the highstand systems tract deposits of the early Aptian V6 unit of the Valhall Formation.

ACKNOWLEDGEMENTS

The Authors acknowledge helpful discussion with Chris Harlow (HRH Ltd) and discussions with numerous oil company geologists working in the UK sector of the North Sea. Our particular thanks go to Bryan Moseley (Amerada), Frank Barker (Area), Tim Chapman (Mureo) , Martin David (Deminex) , Mark Partington (Shell) and Martin Jakubowski (BMNH). Steve Crittenden thanks Brit E. Sauar and Finn Livbjerg of Norsk Hydro, Oslo, for many hours of stimulating discussion on the Cretaceous of NW Europe. Finally, we thank Professor K. W. Glennie (Aberdeen University; Editorial Board) for constructive Journal reviews and for useful suggestions which have greatly improved the paper.

~

24 Distribution of Aptian Sa ndstones, Centra l/No rthern North Sea: part !

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