Hydrocarbon Habitats Basement Highs: Exploration Results ...geologi.no/images/Konferanser/HH/Abstract_2-2013.pdf · With both the 22nd round and APA 2012 behind us, it is now time

Hydrocarbon HabitatsBasement Highs: Exploration Results and Future Possibilities

Oslo, February 28th, 2013

NGFNumber 2, 2013

Abstracts and Proceedings of the Geological Society of Norway

www.geologi.no

© Norsk Geologisk Forening (NGF), 2013

ISBN: 978-82-92-39479-3

NGF Abstracts and ProceedingsNGF Abstracts and Proceedings was first published in 2001. The objective of this series is to generate a common publishing channel of all scientific meetings held in Norway with a geological content.

Editor:Halfdan Carstens

Front photo/illustration:Cartography: Exploro

Printing:Skipnes Kommunikasjon, Trondheim

241 PRINTED MATTER 7

31

Orders to:Norsk Geologisk Foreningc/o Norges geologiske undersøkelseN-7491 TrondheimNorwayE-mail: [email protected]

Published by:Norsk Geologisk Foreningc/o Norges geologiske undersøkelseN-7491 Trondheim, NorwayE-mail: [email protected]

The map on the front page shows onshore the basement units (Gee et al 2008); offshore the top regional basement map (Barents Sea (Marello et al. 2013), Norwegian Sea (Ebbing et al 2010), N-North Sea (Hospers and Ediriweera 1991)), with structural elements wells and oil/gas field from NPD.

NGF Abstracts and Proceedings of the Geological Society of Norway

Number 2, 2013

Hydrocarbon HabitatsBasement Highs:

Exploration Results and Future Possibilities

February 28th, 2013

ABSTRACTS

http://www.geologi.no/basementhighs2013/

2 NGF Abstracts and Proceedings, No 2, 2013

Hydrocarbon Habitats – Basement Highs: Exploration Results and Future Possibilities Tom Bugge Det norske President of the Geological Society of Norway (NGF) As president of the Geological Society of Norway (NGF) it is a pleasure to welcome you all to this seminar, and I am very glad to see that nearly all seats are taken also today. This is the fifth seminar in this Hydrocarbon Habitats series, which is a joint venture between the Geological Society, GeoPublishing and Exploro. We plan to have one more before summer and two during fall. The first one will be on 30th May with the working title: “Exploring the Northern North Sea ‐ No quick fix”. Although we are already discussing certain topics for the fall, I invite you to propose possible topics for these seminars. Today we will address hydrocarbon plays and finds in conjunction with basement highs. With both the 22nd round and APA 2012 behind us, it is now time to glance back and start reflecting over the future. We still have the astonishing Utsira High discoveries fresh in mind. Oil has been proven in both weathered and fractured crystalline basement rocks, and we know that the Norwegian Petroleum Directory has oil in basement as a play model offshore Lofoten and Vesterålen. The basement highs, however, have a lot more to offer. More finds and fields have reservoirs connected with basement highs, being sandstones from the Jurassic, or Permian carbonates. The possibilities are plenty. For this reason, NGF has taken the initiative to sum up some of the knowledge that we have acquired so far, and to look forward based on the possibilities the basement highs have given us. We will have key notes from Robert Trice about offshore Shetland and from Alvar Braathen about the Suez Rift as well as updates from the Utsira High by Statoil and Lundin. Fridtjof Riis at the NPD will discuss weathering of basement off Lofoten‐Vesterålen in a regional context. Further, there will be talks dealing with geophysics, chemistry and methodology for studying basement. It has been a success with these half days meetings where we start with lunch, have several coffee breaks and a beer or two at the Justisen Pub after the seminar. I hope you appreciate this opportunity to meet colleagues, friends and – not the least – establish new contacts and listen to the talks and take part in the discussions.

www.geojobb.noLEDIGE STILLINGER I GEOMILJØET

Skålatårnet er bygd på toppen av Skåla (1843 m o.h.) som ligger ovenfor Loen i Stryn. I forgrunnen blokkmark med øyegneis som er byggematerialet i tårnet. Skålatårnet har 22 overnattingsplasser. Øyegneisen kan du studere i detalj på hele topplatået så vel som på vei opp.

Les mer om øyegneisen på Skåla her: geofunn.no

Foto

: Hal

fdan

Car

sten

s

NGF Abstracts and Proceedings, No 2, 2013 3

Hydrocarbon Habitats – Basement Highs: Exploration Results and Future Possibilities Tom Bugge Det norske President of the Geological Society of Norway (NGF) As president of the Geological Society of Norway (NGF) it is a pleasure to welcome you all to this seminar, and I am very glad to see that nearly all seats are taken also today. This is the fifth seminar in this Hydrocarbon Habitats series, which is a joint venture between the Geological Society, GeoPublishing and Exploro. We plan to have one more before summer and two during fall. The first one will be on 30th May with the working title: “Exploring the Northern North Sea ‐ No quick fix”. Although we are already discussing certain topics for the fall, I invite you to propose possible topics for these seminars. Today we will address hydrocarbon plays and finds in conjunction with basement highs. With both the 22nd round and APA 2012 behind us, it is now time to glance back and start reflecting over the future. We still have the astonishing Utsira High discoveries fresh in mind. Oil has been proven in both weathered and fractured crystalline basement rocks, and we know that the Norwegian Petroleum Directory has oil in basement as a play model offshore Lofoten and Vesterålen. The basement highs, however, have a lot more to offer. More finds and fields have reservoirs connected with basement highs, being sandstones from the Jurassic, or Permian carbonates. The possibilities are plenty. For this reason, NGF has taken the initiative to sum up some of the knowledge that we have acquired so far, and to look forward based on the possibilities the basement highs have given us. We will have key notes from Robert Trice about offshore Shetland and from Alvar Braathen about the Suez Rift as well as updates from the Utsira High by Statoil and Lundin. Fridtjof Riis at the NPD will discuss weathering of basement off Lofoten‐Vesterålen in a regional context. Further, there will be talks dealing with geophysics, chemistry and methodology for studying basement. It has been a success with these half days meetings where we start with lunch, have several coffee breaks and a beer or two at the Justisen Pub after the seminar. I hope you appreciate this opportunity to meet colleagues, friends and – not the least – establish new contacts and listen to the talks and take part in the discussions.

www.geojobb.noLEDIGE STILLINGER I GEOMILJØET

Skålatårnet er bygd på toppen av Skåla (1843 m o.h.) som ligger ovenfor Loen i Stryn. I forgrunnen blokkmark med øyegneis som er byggematerialet i tårnet. Skålatårnet har 22 overnattingsplasser. Øyegneisen kan du studere i detalj på hele topplatået så vel som på vei opp.

Les mer om øyegneisen på Skåla her: geofunn.no

Foto

: Hal

fdan

Car

sten

s


Hydrocarbon Habitats: Basement Highs: Exploration Results and Future possibilitiesFebruary 28th, 2013, Oslo Kongressenter Folkets hus

Programme

10.00 Registration

11.00 Lunch

11.55 Welcome Tom Bugge, President Geological Society of Norway

12.00 Keynote: Strategies for fractured basement exploration: A case study of experience from the West of Shetlands Robert Trice, Hurricane

12.30 Keynote: Basement fracturing and weathering in rift-shoulder fault blocks; a reservoir analogue from the Sinai Suez Rift (Egypt) Alvar Braathen et al, UNIS

13.00 Coffee break

Exploration potential of the basement on the Utsira High

Chaired by Evy Glørstad-Clark, Det norske

13.25 Structural framework and geophysical mapping Jan Erik Lie, Lundin

13.45 Well results and reservoir potential Else Grandal, Lundin

14.05 Characterisation of Fracturing and Weathering of the Utsira High Granitic Basement Paul Gillespie, Statoil

14.25 Coffee break


Hydrocarbon Habitats: Basement Highs: Exploration Results and Future possibilitiesFebruary 28th, 2013, Oslo Kongressenter Folkets hus

Programme

10.00 Registration

11.00 Lunch

11.55 Welcome Tom Bugge, President Geological Society of Norway

12.00 Keynote: Strategies for fractured basement exploration: A case study of experience from the West of Shetlands Robert Trice, Hurricane

12.30 Keynote: Basement fracturing and weathering in rift-shoulder fault blocks; a reservoir analogue from the Sinai Suez Rift (Egypt) Alvar Braathen et al, UNIS

13.00 Coffee break

Exploration potential of the basement on the Utsira High

Chaired by Evy Glørstad-Clark, Det norske

13.25 Structural framework and geophysical mapping Jan Erik Lie, Lundin

13.45 Well results and reservoir potential Else Grandal, Lundin

14.05 Characterisation of Fracturing and Weathering of the Utsira High Granitic Basement Paul Gillespie, Statoil

14.25 Coffee break

Geophysical imaging of basement rocks

Chaired by Odleiv Olesen, NGU

14.50 Improved understanding of basement highs through broadband seismic Berit Osnes, PGS

15.10 Mapping fractured and/or deeply weathered base-ment from geophysical data Marco Brönner, NGU

Miscellaneous

15.30 An integrated geological and geophysical approach to investigating the petroleum potential of basement highs Laura Marello, Gaetano Salvaggio & Tomas Kjennerud, Exploro

15.50 Coffee break

Chaired by Halfdan Carstens, GeoPublishing

16.15 Keynote: Chemical weathering of basement rocks in Norway. What do we know and what are the implications for the petroleum industry? Ola Fredin, NGU

16.45 The offshore basement highs in the Lofoten-Vesterålen area and some observations on their regional structural context Fridtjof Riis, NPD

17.05 End of conference

17.20 Informal gathering at Justisen pub


Sinai basement reservoir analogue 1

Basement fracturing and weathering in rift‐shoulder fault blocks; a reservoir analogue from the Sinai Suez Rift (Egypt) Braathen1, A., Abdel Fattah2, M.M., Ogata1, K., Olaussen1, S., and Abdel‐Gawad2, G. 1‐ University Centre in Svalbard (UNIS), Norway 2‐ Beni‐Suef University, Egypt Summary This study describes deformation and weathering effects observed in basement rocks in the proximal footwall of rift‐bounding faults and in the rift shoulder of the Suez Rift, revealed in world‐class exposures in Sinai, Egypt. These outcrop‐based datasets reflect those reported from hydrocarbon producing fields in the Suez Rift, and may represent valid analogues to hydrocarbon reservoirs encountered in granitoid rocks in the North Sea and elsewhere. The fracture intensity of basement rocks peaks marginal to major faults, and declines away from the faults in damage zones that stretches out to the background fracturing level over 50‐200 m. Within these major damage zones, small faults seen as 1‐5 m‐wide fracture corridors show high fracture intensity, which is similar to smaller faults encountered in the rift‐shoulder. Sometimes such fracture corridors and small scale faults develop along dykes, which represent structural discontinuities prone to be reworked. Especially, major basement‐seated faults show mineralogical alteration and diagenetic mineralogy in associated fault rocks and partial sealing along fractures due to circulating fluids. This is most striking in the mafic basement lithologies. In granites, the rocks of the fault cores are altered but the damage zone reveals open fracture networks. Furthermore, the analysis documents associated thickness variations of the tropical to arid‐style weathering profile on top of the basement, depending on lithology and amount of erosional unroofing. Schistose and mafic rocks appear to have thicker weathering profiles than granitoids. The overlying sandstones show an overall fining upward profile some 30 to 35 m thick, deposited as fluvial sandstone/gravel/conglomerate in lower part which gradually passing upward to flood plain or tidal flat deposits. The basement source is reflects by angular quartz pebbles and kaolinite. Kaolinite is observed as both diagenetic pore fill and as clastic grains. Very well rounded coarse and very coarse grained quartz arenites shows origin from previous sorted sediments. In sum, the basal sandstones and the weathering profile make up a volumetrically significant unit with considerable porosity. These units may act as a reservoir, draining the underlying tight and fractured basement. Introduction For decades, exploration of rift basins has successfully targeted sedimentary plays above the basement in the footwall of major faults, since this location offers the upper summit of pre‐rift reservoirs as well as the apex of syn‐rift reservoir clastics. The underlying basement is seldom targeted. As major faults offset source rocks downwards into the rift zone, these become juxtaposed with fractured basement rocks (e.g. fault damage zone) across the fault. Further, tropical to arid style weathering below the top‐basement unconformity and fluid‐driven



corrosion of fracture systems will potentially create porous and permeable weathering profiles. Thereby, migration of hydrocarbons out of the source rocks can either migrate up into overlying sedimentary reservoir successions, or across the fault into the fractured basement. With a sealing cap rock succession above basement, a significant hydrocarbon column can be stored in fracture networks and weathering profiles in the otherwise tight basement, as shown by significant hydrocarbon columns in basement of the Suez Rift (e.g., Salah and Alsharhan 1998). This study analyzes basement reservoir analogies encountered in the onshore part of the Suez Rift, covering: (i) fracture systems in the basement around rift‐bounding and rift‐shoulder faults and in the background domain of the rift shoulder, (ii) thickness and character of the tropical to arid weathering profile along the top of several basement lithologies, (iii) diagenesis and weathering products encountered on top of basement and in fracture systems, and (iv) sedimentary deposits overlying the weathered basement. Regional setting of Sinai Suez Rift The exposed eastern rift shoulder of the Suez Rift, in Sinai, Egypt, offers unique localities for basement reservoir analogue studies (Fig. 1), as the eastern margin of the rift has been uplifted and exhumed during crustal scale extensional tectonics since the Miocene (e.g., Sharp et al. 2000; Mustafa 2004). Two major normal fault zones demark the rift zone on the east side: (i) the Coastal Fault Belt that juxtapose the offshore syn‐tectonic rift fill with the onland Paleozoic to Eocene pre‐rift sedimentary succession in the footwall and, more distal to the rift, (ii) the interlinked Thal‐Baba fault zones, which have a hanging wall of mainly the Paleozoic to Eocene pre‐rift successions, at places overlain by syn‐rift successions. The footwall consists of a basement complex of Pan‐African granitoids and subordinate schists below the pre‐rift Nubian Sandstones. Both these major faults show displacement of the basement‐cover contact in the range of 1‐3 km. Outside the main rift center, exhumed 20‐30 km wide terraces towards the uplifted rift shoulder show subsidiary faults with up to 200‐500 m throw that cut from the basement into the overlying Nubian Sandstone and younger cover rocks. Presented datasets target the fracture system and weathering profile in the basement of the footwall of the Thal and Baba faults. We describe the cores of the master faults and the fracture sets encountered in the footwall damage zones, and document the effects of lithological variations in the basement rocks. Different protolith mineralogy observed between mainly coarse‐grained microcline granite, coarse‐medium grained hornblende granodiorite, and medium‐fine grained hornblende‐biotite schist, affected the encountered diagenetic alterations. Datasets Fracture intensity distribution datasets from rift‐bounding and rift‐shoulder faults: The rift‐bounding Thal fault offers exhumed footwall basement in a 700 m high mountain range topped by klippen of Nubian Sandstones (see Mustafa 2004). Exposures of the fault core at several sites along the fault show 10‐20 m thick zones of fault rocks that have experienced diagenesis (Fig. 2). Both the fault zone width and the diagenetic effect seem lithology‐dependent and the latter is more extensive in mafic lithologies. The nearby footwall damage zone in the basement shows a dense network of fractures in an inner damage zone, with fracture intensity of the outer damage zone declining towards a background level of 1‐2 fracture/meter over 50 to 200 meters (Fig. 3). This damage zone is dissected by subsidiary faults that define 2‐5 m to 10‐20 m wide fracture corridors.



The rift shoulder offers smaller offset faults, representing zones of intense fracturing similar to the fracture corridors of the major faults. They typically show 5‐10 m wide inner damage zones next to the fault core that is followed by 10‐30 m wide outer damage zones that are transitional into the background level. Fracture systems in the pristine tight basement rocks contain some long fractures, mostly encountered in the fracture corridors next to faults. Such fractures could be highly permeable giving the main contribution to fluid flow (Odling 1997; Braathen and Gabrielsen 1998; Venvik Ganerød et al. 2006). Notably, some of these fracture corridors concentrate along granite dykes, representing anisotropic features of the basement rocks where the bulk of the deformation is accommodated. Weathering profile thickness variation datasets: The basement‐cover contact in Sinai is mostly well exposed, showing a weathering profile in the basement rocks (Fig. 4). This profile can be mapped out in a general sense by the characters of outcrops (superb versus moderate quality of exposures). A first assessment suggests this profile to be 5‐10 m deep in granite, slightly deeper in granodiorite and 10‐20 m deep in dark schists for areas with few fractures (i.e. background level), but seems to extend much deeper in zones of intense fracturing, for instance in fault damage zones (see Fig. 4). Such relief is well known from weathering profiles in tropical to arid regions. The cover section: The weathered granite is overlain by classical Nubian Sandstones, probably of Cambrian age. The up to 35 m thick logged section at five localities up to 15 km apart show an overall fining upward section (Figs. 4 and 5) with a very coarse grained cross stratified sandstone to conglomeratic, 2 to 5 m thick lower part. The logged section shown in Fig. 5b has a lower, 4 meter unit with well‐rounded and very coarse to coarse grained cross stratified sandstone, with scattered gravel and pebbles. This unit is interpreted as northwards prograding and aggrading braidplain. Incomes of interbedded fines in upper part of this unit suggest waning of the braidplain. Apart for an intersected fluvial stream channels around 10 m up from the base of the sedimentary rock section, the upper part of the section consists of very fine to fine grained sandstone with wave and current ripples, horizontal laminated, low angle cross stratification and hetereoliths (Fig 5a). In the southern outcrops, the lower braidplain part is passing upward to flood plain, over bank, lacustrine and single story fluvial channel deposits. Here the sandstones are whitish due to abundant kaolinite as pore filling clays. In the northern outcrops the fine grained upper part is interpreted as tidal flat and lacustrine deposits occasionally intersected by fluvial stream channel and overbank deposits. In contrast to the southern outcrops the coarser grained sandstones in the northern outcrops are cleaner with less kaolinite and have much better observable porosity. Micro‐textural analysis and mineralogy of fault rocks and top‐basement weathering profile: Ongoing work on micro‐textures (optical and SEM) and mineralogy (XRD) suggests that feldspars and mafic minerals are labile in the weathering profile, whereas quartz is stable. The former minerals are replaced by clays, kaolinite and subordinate zeolite. The alteration varies from mild in quartz‐dominated units to significant in mafic schists, in both cases creating secondary porosity by dissolution of mineral phases. This is reflected in offshore basement reservoirs in the Suez Rift, revealing porosities around 10‐20% and good permeability (Alsharhan and Salah 1994; Salah and Alsharhan 1998).



Discussion of scientific challenges Fundamental outcrop data based on fracture mapping and weathering characterization of granitoids and schistose granodiorite illustrate the challenges of predicting basement reservoirs in the subsurface (Fig. 6). For instance, the tight fractured basement can offer hundreds of meters of hydrocarbon columns, basically limited by the base level of the source rock(s) in the nearby, down‐faulted basin. Migration from source rocks across altered fault rocks into the fractured damage zones in the footwall basement is a possible hydrocarbon fairway type, as shown by producing oilfields in the Suez Rift. Accumulation of hydrocarbons would likely firstly build up in the top‐basement weathering profile and overlying recycled sandstones, and thereafter gradually fill underlying fracture systems in the basement. In order to predict the fairways and column heights, with bearing on production strategies, insight has to be found in combined understanding of faults, fracture systems, weathering‐diagenesis and sedimentology. The presented ongoing study addresses all four approaches in an effort to mature the concepts applied to exploitation of basement reservoirs. References Alsharhan, A.S., and Salah, M.G., 1994: Geology and hydrocarbon habitat in a rift setting:

southern Gulf of Suez, Egypt. Bulletin of Canadian Petroleum Geology 42 (3), 312‐331. Braathen, A., and Gabrielsen, R.H., 1998. Lineament architecture and distribution in

metamorphic and sedimentary rocks, with application to Norway. NGU Report 98.043, Geological Survey of Norway, pp. 78.

Moustafa, A. R., 2004, Geologic Maps of the Eastern Side of the Suez Rift (Western Sinai Peninsula), Egypt: AAPG map series.

Odling, N.E., 1997. Scaling and connectivity of joint systems in sandstone from western Norway. Journal of Structural Geology 19 (10), 1257–1271.

Salah, M.G., and Alsharhan, A.S., 1998: The Precambrian basement: A major reservoir in the rifted basin, Gulf of Suez. Journal of Petroleum Science and Engineering 19, 201–222.

Sharp, I. R., R.L. Gawthorpe, B. Armstrong, and J.R. Underhill, 2000: Propagation history and passive rotation of mesoscale normal faults: implications for syn‐rift stratigraphic development. Basin Research, v. 12, p. 285‐306.

Vevik Ganerød, G., Braathen, A., and Willemoes Wissing, B., 2008: Predictive permeability model of extensional faults in crystalline and metamorphic rocks; verification by pre‐grouting in sub‐sea tunnels in Norway. Journal of Structural Geology 30, 993‐1004.



Fig. 1. Oblique aerial view of the Suez Rift shoulder as seen on‐land the western Sinai, Egypt (from Google Earth Pro, 2012). Note major faults that separate topographic domains, seen as light‐colored pre‐rift units of mainly Cretaceous to Eocene age, dark gray Pz‐Mz pre‐rift Nubian Sandstones on basement, and dark basement in the footwall of the rift‐bounding Thal fault (center of figure). The mountain range in the center is 700 m high, showing a klippen of Nubian Sandstones resting on mainly basement granitoids. Faults are outlined in red.



Fig. 2. Exposure of the Thal rift‐bounding fault, showing a wide zone of brecciated and altered granodiorite in the footwall (dark color). Details of the fault core next to the hanging wall show a sand injectite and highly sheared and altered basement fault rocks (dark red) next to down‐faulted sandstones of the hanging wall. A scan line of fracture intensity is located on the photograph and shown in Fig. 3.


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Fig. 4. Basement‐cover contact of the rift‐shoulder, with a c. 200 m throw fault to the left (east), as seen in this 200 m high mountain side with a view from the south. The basement here is red, coarse microcline granite, intruded by a few m–thick black, mafic dikes. The upper 5 m of the granite is deeply weathered as suggested by the change in color into a white tint and degrading quality of outcrops. This weathering profile extends deeply into basement along the inner damage zone of the fault (right/east), which likely is of Miocene age. The overlying Nubian Sandstones sedimentary section starts with mature, coarse to medium grained and pebbly fluvial sandstone (Cambrian?) followed by flood‐plain deposits including probable eolian dunes. Note encircled person for scale.


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Fig. 6. Conceptual model for a weathered basement to cover‐sandstone reservoir of the rift center to rift‐shoulder domain.


Mapping fractured and/or deeply weathered basement from geophysical data Brönner, M.1, Nærheim, M.2, Hogstad, K.3, Olesen, O.1 1) Geological Survey of Norway, 7491 Trondheim 2) Norges Teknisk Naturvitenskaplige Universitet, Trondheim, now ConocoPhillips, Stavanger 3) Det norske oljeselskap, Harstad Systematic mapping and investigation of fractured and deeply weathered basement are still an underexplored and challenging topic, which is why hydrocarbon exploration in basement rocks is commonly shackled with words like by accident, exotic or unconventional. Even though quite a few fields worldwide are producing from reservoirs within the basement and some have been highly prolific over several decades. K.K. Landes et al. (1960) had already urged at an AAPG meeting in 1960 that “Commercial oil deposits in basement rocks are not geological ‘accidents’ but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement should be explored with the same professional skill and zeal as accumulations in the overlying sediments”. This perception is affirmed for the Norwegian shelf where of more than 50 wells drilled down into basement quite a few show signs of weathered and/or fractured basement rock, and in the last few years a couple of companies have begun to explore systematically for hydrocarbon reservoirs in the basement on the Norwegian and UK continental shelf using high‐resolution seismic data. A fractured basement and subsequent alteration by weathering processes, however, cause noticeable changes of the rock’s physical parameters, which are also likely to be detected using geophysical methods other than seismics. Although seismics is certainly the most powerful technique for producing a very detailed picture of the subsoil, it is also the most expensive method, which is economically challenging in many cases. It is well known that fracturing and weathering of basement rocks also effect changes in the rock resistivity, density and magnetisation. Several studies from onshore Norway have proved the benefits of local 2D resistivity profiling (Brönner et al. 2010a), but for larger areas especially the joint interpretation of high‐quality aeromagnetic data with the landscape morphology (AMAGER method, Olesen et al. 2007, Brönner et al. 2010a) proved to be a reliable technique for mapping of fractured and deeply weathered basement. Onshore Norway different epochs of glaciations eroded and removed almost entirely the overlying sedimentary successions. The method is based on the not necessarily but for Norway common observation that deeply weathered basement shows a noticeable reduced magnetisation of the parent material and is moreover easily erodible. Relative magnetic lows correlating with topographic depressions in eroded and exposed basement landscapes are hitherto likely to indicate the location and distribution of deep weathering. Offshore, on the Norwegian shelf the AMAGER method is not easily applicable due to sedimentary coverage, but in general, also marine high‐resolution gravity and magnetic data have the potential to map fractured/weathered basement and at a significantly smaller budget.

For the E&P industry the quality of the basement especially on the various basement highs along the Norwegian shelf are of particular interest for both bottom seal and/or migration paths. Gravity and magnetics in particular in conjunction with additional observations from seismics and wells can give fast and economically reliable indications of the basement quality. However, little knowledge exists or has been published about the potential and limitations of such data applications. We will present results of approaches to map fractured and deep weathering in the basement from mainland Norway and the Loppa High in the southwestern Barents Sea, where we applied marine gravity data together with an automated fracture tracking technique from seismic data to identify fractured basement.


Mapping fractured and/or deeply weathered basement from geophysical data Brönner, M.1, Nærheim, M.2, Hogstad, K.3, Olesen, O.1 1) Geological Survey of Norway, 7491 Trondheim 2) Norges Teknisk Naturvitenskaplige Universitet, Trondheim, now ConocoPhillips, Stavanger 3) Det norske oljeselskap, Harstad Systematic mapping and investigation of fractured and deeply weathered basement are still an underexplored and challenging topic, which is why hydrocarbon exploration in basement rocks is commonly shackled with words like by accident, exotic or unconventional. Even though quite a few fields worldwide are producing from reservoirs within the basement and some have been highly prolific over several decades. K.K. Landes et al. (1960) had already urged at an AAPG meeting in 1960 that “Commercial oil deposits in basement rocks are not geological ‘accidents’ but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement should be explored with the same professional skill and zeal as accumulations in the overlying sediments”. This perception is affirmed for the Norwegian shelf where of more than 50 wells drilled down into basement quite a few show signs of weathered and/or fractured basement rock, and in the last few years a couple of companies have begun to explore systematically for hydrocarbon reservoirs in the basement on the Norwegian and UK continental shelf using high‐resolution seismic data. A fractured basement and subsequent alteration by weathering processes, however, cause noticeable changes of the rock’s physical parameters, which are also likely to be detected using geophysical methods other than seismics. Although seismics is certainly the most powerful technique for producing a very detailed picture of the subsoil, it is also the most expensive method, which is economically challenging in many cases. It is well known that fracturing and weathering of basement rocks also effect changes in the rock resistivity, density and magnetisation. Several studies from onshore Norway have proved the benefits of local 2D resistivity profiling (Brönner et al. 2010a), but for larger areas especially the joint interpretation of high‐quality aeromagnetic data with the landscape morphology (AMAGER method, Olesen et al. 2007, Brönner et al. 2010a) proved to be a reliable technique for mapping of fractured and deeply weathered basement. Onshore Norway different epochs of glaciations eroded and removed almost entirely the overlying sedimentary successions. The method is based on the not necessarily but for Norway common observation that deeply weathered basement shows a noticeable reduced magnetisation of the parent material and is moreover easily erodible. Relative magnetic lows correlating with topographic depressions in eroded and exposed basement landscapes are hitherto likely to indicate the location and distribution of deep weathering. Offshore, on the Norwegian shelf the AMAGER method is not easily applicable due to sedimentary coverage, but in general, also marine high‐resolution gravity and magnetic data have the potential to map fractured/weathered basement and at a significantly smaller budget.

For the E&P industry the quality of the basement especially on the various basement highs along the Norwegian shelf are of particular interest for both bottom seal and/or migration paths. Gravity and magnetics in particular in conjunction with additional observations from seismics and wells can give fast and economically reliable indications of the basement quality. However, little knowledge exists or has been published about the potential and limitations of such data applications. We will present results of approaches to map fractured and deep weathering in the basement from mainland Norway and the Loppa High in the southwestern Barents Sea, where we applied marine gravity data together with an automated fracture tracking technique from seismic data to identify fractured basement.


Chemical weathering of basement rocks in Norway. What do we know and what are the implications for the petroleum industry? Ola Fredin1,2, Ronald Sørlie3, Odleiv Olesen1, Jan‐Erik Lie3, Jochen Knies1, Else Margrethe Grandal3,Marco Brønner1, Horst Zwingmann4, Axel Müller1, Christoph Vogt5

1: Geologcial Survey of Norway, Trondheim, Norway 2: Norwegian University of Science and Technology, Trondheim, Norway 3: Lundin Petroleum Norway, Oslo, Norway 4: CSIRO, Kensington, Australia 5: University of Bremen, Bremen, Germany Fractured and weathered basement is an important part of the hydrocarbon play on southern Utsira high in the Norwegian North Sea. Chemical weathering of basement rocks on‐ and offshore Scandinavia needs to be studied in more detail. However, it turns out there are important onshore sites with preserved weathering soils (saprolite), which are used by the petroleum industry to improve offshore exploration models. For the past century there has been a debate about the role of deep weathering as a landscape forming agent onshore Scandinavia. The discussion ranges from that Scandinavia largely was influenced by Quaternary processes and most of the older weathering imprint has been obliterated, to the notion that effects of past chemical etching still dominates the landscape. In recent years, more and more evidence is pointing at the importance of past deep weathering processes as an important factor in the geology of Scandinavia. Extensive studies have been made in southern Sweden and Denmark, where it has been shown that significant deep weathering took place in sub‐Cretaceous time with the most intense, kaolinitic, weathering occurring in late Triassic and early Jurassic times. Less is known in Norway but relatively widespread occurrences of saprolite in South to Western Norway and in the Lofoten/Vesterålen area have been reported, but the age of this weathering remains unclear. The only confirmed locality with sub‐Mesozoic weathering is the Ramså basin, Andøya where weathered basement gneisses are overlain by Jurassic and Cretaceous sedimentary rocks. These Norwegian saprolite localities have recently been investigated with respect to for example geophysical properties, grain size, petrography, geochemistry, clay mineralogy and palaeomagnetism. In addition, the same spectrum of techniques has also been applied to weathered basement found in offshore exploration wells (Edvard Grieg 16/1‐15 and Johan Sverdrup 16/3‐4) on southern Utsira High in discoveries made by Lundin Norway. Generally, many similarities can be found between the onshore‐ and offshore saprolites. We also attempted K‐Ar dating of illite diagenesis in the southern Utsira High wells, thus giving an age of when deep weathering took place. Preliminary data suggest saprolite formation in late Triassic, which thus is consistent with observations in southern Scandinavia. We also conducted Quartz provenance studies on the Draupne sst. together with weathered basement on southern Utsira High and concluded that the Draupne sst. to a large extent is derived from erosion and short transported weathered material. Again, this shows that past weathering of basement directly have influenced the petroleum system on southern Utsira high. cgg.com

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Chemical weathering of basement rocks in Norway. What do we know and what are the implications for the petroleum industry? Ola Fredin1,2, Ronald Sørlie3, Odleiv Olesen1, Jan‐Erik Lie3, Jochen Knies1, Else Margrethe Grandal3,Marco Brønner1, Horst Zwingmann4, Axel Müller1, Christoph Vogt5

1: Geologcial Survey of Norway, Trondheim, Norway 2: Norwegian University of Science and Technology, Trondheim, Norway 3: Lundin Petroleum Norway, Oslo, Norway 4: CSIRO, Kensington, Australia 5: University of Bremen, Bremen, Germany Fractured and weathered basement is an important part of the hydrocarbon play on southern Utsira high in the Norwegian North Sea. Chemical weathering of basement rocks on‐ and offshore Scandinavia needs to be studied in more detail. However, it turns out there are important onshore sites with preserved weathering soils (saprolite), which are used by the petroleum industry to improve offshore exploration models. For the past century there has been a debate about the role of deep weathering as a landscape forming agent onshore Scandinavia. The discussion ranges from that Scandinavia largely was influenced by Quaternary processes and most of the older weathering imprint has been obliterated, to the notion that effects of past chemical etching still dominates the landscape. In recent years, more and more evidence is pointing at the importance of past deep weathering processes as an important factor in the geology of Scandinavia. Extensive studies have been made in southern Sweden and Denmark, where it has been shown that significant deep weathering took place in sub‐Cretaceous time with the most intense, kaolinitic, weathering occurring in late Triassic and early Jurassic times. Less is known in Norway but relatively widespread occurrences of saprolite in South to Western Norway and in the Lofoten/Vesterålen area have been reported, but the age of this weathering remains unclear. The only confirmed locality with sub‐Mesozoic weathering is the Ramså basin, Andøya where weathered basement gneisses are overlain by Jurassic and Cretaceous sedimentary rocks. These Norwegian saprolite localities have recently been investigated with respect to for example geophysical properties, grain size, petrography, geochemistry, clay mineralogy and palaeomagnetism. In addition, the same spectrum of techniques has also been applied to weathered basement found in offshore exploration wells (Edvard Grieg 16/1‐15 and Johan Sverdrup 16/3‐4) on southern Utsira High in discoveries made by Lundin Norway. Generally, many similarities can be found between the onshore‐ and offshore saprolites. We also attempted K‐Ar dating of illite diagenesis in the southern Utsira High wells, thus giving an age of when deep weathering took place. Preliminary data suggest saprolite formation in late Triassic, which thus is consistent with observations in southern Scandinavia. We also conducted Quartz provenance studies on the Draupne sst. together with weathered basement on southern Utsira High and concluded that the Draupne sst. to a large extent is derived from erosion and short transported weathered material. Again, this shows that past weathering of basement directly have influenced the petroleum system on southern Utsira high. cgg.com

Today the industry has a new Geoscience leader.

Our global community of talented geoscientists work closely with our clients to deliver innovative solutions for the exploration and sustainable development of the Earth’s natural resources.

With a proven track record and a proud heritage of more than 80 years, we are your geoscience partner of choice.

We are CGG.

PASSION FORGEOSCIENCE


An integrated geological and geophysical approach to investigate the petroleum potential of basement highs Laura Marello, Gaetano Salvaggio, Tomas Kjennerud Exploro AS, Stiklestadveien 1, 7041, Trondheim, Norway. Abstract The basement in the Norwegian Shelf is the extension of the onshore units and consists of an aggregate of different terranes spanning from the Archeaen to the Silurian. The understanding of offshore basement has seen a strong improvement in the last years (e.g. Ebbing and Olesen 2010; Slagstad et al 2011; Marello et al. 2013). The recent models provide a regional knowledge of depths to basement and of basement composition, which can be used to further investigate the petroleum potential of the Norwegian Continental Self. In the present work we study the applicability of different methods to identify the conditions that play and important role in having hydrocarbons in basement or associated with basement highs. In a first stage we made a regional overview of the Norwegian Shelf basement structure in relation with the location of oil/gas discovered hydrocarbons. We estimate statistically the range of basement depths and basement thickness where the largest numbers of oil/gas fields have been found (Fig. 1). In a second part of the study we use a wealth of methods to understand basement structures and composition in areas characterized by shallower basement. Seismic data may provide a good understanding of basement highs morphology. High quality 3D seismic data allows for mapping depth to basement with a high degree of confidence, and to outline the main faults and lineaments. In addition seismic attribute analysis provides a first estimation of internal basement heterogeneities. The results obtained from seismic data are then integrated with gravity and magnetic studies. Different mathematical transformations have been applied to the potential field and furnish a qualitative interpretation of the main density and magnetic sources (the basement). Especially where we have good control of the basement geometry the changes in gravity and magnetic anomalies are interpreted in terms of density and magnetic variations in the bedrock. These petrophysical heterogeneities in the basement can further be related with weathering processes, fractures in basement or with a lithological change. Furthermore the results of seismic and potential field interpretation are also integrated with well data (where available) and onshore geology. This combined approach provides a good understanding of the basement, allowing to define distinct geological basement units (e.g. different Caledonian allochthons, Precambrian basement, weathered basement and fractured basement) dominated by different properties and with well‐defined geometries, which are the key elements in order to understand the petroleum potential of basement. References Ebbing, J. & Olesen, O. 2010: New compilation of top basement and basement thickness for the Norwegian continental shelf reveals segmentation of the passive margin system. Petroleum Geology Conference series; v. 7; p. 885‐897.

Hospers, J., Ediriweera, K.W., 1988. Mapping of the top of the crystalline crust in the Viking Graben Area, North Sea. In: Kristoffersen, Y. (Ed.), Progress of the Studies of the Lithosphere in Norway: Norges Geologiske Undersøkelse, Special Publication, 3, pp. 21–28. Marello, L., Ebbing, J., Gernigon, L., 2013. Basement inhomogeneities and crustal setting in the Barents Sea from a combined 3D gravity and magnetic model. Geophys. J. Int. (in press). Slagstad, T., Davidsen, B., Daly, J.S., 2011. Age and composition of crystalline basement rocks on the Norwegian continental margin; offshore extension and continuity of the Caledonian–Appalachian orogenic belt. Journal of the Geological Society 168, 1–19. Fig. 1. Oil/gas fields (from NPD) plotted on a regional top basement compilation (Barents Sea (Marello et al. 2013), Norwegian Sea (Ebbing et al 2010), N‐North Sea (Hospers and Ediriweera 1991)). Most of the oil/gas fields are located in areas with top basement depths in the range of ‐4.8 to ‐8.2 km (brown areas).



An integrated geological and geophysical approach to investigate the petroleum potential of basement highs Laura Marello, Gaetano Salvaggio, Tomas Kjennerud Exploro AS, Stiklestadveien 1, 7041, Trondheim, Norway. Abstract The basement in the Norwegian Shelf is the extension of the onshore units and consists of an aggregate of different terranes spanning from the Archeaen to the Silurian. The understanding of offshore basement has seen a strong improvement in the last years (e.g. Ebbing and Olesen 2010; Slagstad et al 2011; Marello et al. 2013). The recent models provide a regional knowledge of depths to basement and of basement composition, which can be used to further investigate the petroleum potential of the Norwegian Continental Self. In the present work we study the applicability of different methods to identify the conditions that play and important role in having hydrocarbons in basement or associated with basement highs. In a first stage we made a regional overview of the Norwegian Shelf basement structure in relation with the location of oil/gas discovered hydrocarbons. We estimate statistically the range of basement depths and basement thickness where the largest numbers of oil/gas fields have been found (Fig. 1). In a second part of the study we use a wealth of methods to understand basement structures and composition in areas characterized by shallower basement. Seismic data may provide a good understanding of basement highs morphology. High quality 3D seismic data allows for mapping depth to basement with a high degree of confidence, and to outline the main faults and lineaments. In addition seismic attribute analysis provides a first estimation of internal basement heterogeneities. The results obtained from seismic data are then integrated with gravity and magnetic studies. Different mathematical transformations have been applied to the potential field and furnish a qualitative interpretation of the main density and magnetic sources (the basement). Especially where we have good control of the basement geometry the changes in gravity and magnetic anomalies are interpreted in terms of density and magnetic variations in the bedrock. These petrophysical heterogeneities in the basement can further be related with weathering processes, fractures in basement or with a lithological change. Furthermore the results of seismic and potential field interpretation are also integrated with well data (where available) and onshore geology. This combined approach provides a good understanding of the basement, allowing to define distinct geological basement units (e.g. different Caledonian allochthons, Precambrian basement, weathered basement and fractured basement) dominated by different properties and with well‐defined geometries, which are the key elements in order to understand the petroleum potential of basement. References Ebbing, J. & Olesen, O. 2010: New compilation of top basement and basement thickness for the Norwegian continental shelf reveals segmentation of the passive margin system. Petroleum Geology Conference series; v. 7; p. 885‐897.




Improved understanding of basement highs through broadband seismic Berit Osnes PGS Introduction The North Sea still holds opportunities for large discoveries through the use of geological insight and creativity coupled with new technology. Basement highs play a key role in hydrocarbon exploration and can make for an attractive target, as shown through the recent success story of the Utsira High, which may open up exploration of other basement highs. Understanding the role of basement highs in exploration can be complicated and difficult to model. Improved seismic imaging and resolution can help our understanding of the role of basement highs in terms of hydrocarbon migration, trapping mechanisms, fractured reservoirs and sources of sediment. In this paper we will review how GeoStreamer broadband seismic and advanced imaging technology has improved the understanding of the Utsira and Mandal basement highs as potential reservoirs alongside the lateral sealing potential for associated traps. Case histories The first towed streamer broadband survey on the southern part of the Utsira High was acquired by PGS on behalf of Lundin and partners in 2009 and the results have been published in many papers. Between 2009 and 2011 PGS also acquired GeoStreamer® multiclient data across the northern part of the Utsira High and over the Mandal High in the Central Graben. The seismic section in Figure 1 shows a half‐graben north of the giant Johan Sverdrup discovery on Utsira High. If migration through basement is a working model, then pre‐Upper Jurassic sediments can represent interesting opportunities to be pursued. Whilst most of the traps in the Norwegian‐Danish Basin are salt induced, there are other opportunities where the Mandal High may act as both a source of sediment and lateral seal. Examples will be discussed in the presentation. GeoStreamer delivers both deep penetration through low frequencies and enhanced resolution from high frequencies . This is achieved by combining data from co‐located pressure and particle velocity sensors in the streamers. The streamers are towed deeper than in conventional acquisition, thereby improving the low frequency signal/noise. This is very important for the seismic imaging of a fractured basement. Both Utsira and Mandal Highs have been depth‐imaged with PGS Beam migration, which has improved the understanding of the fracture patterns within the basement highs. The PGS beam migration is well suited for application to broadband data as wavelet picking is designed to preserve the original amplitudes. The difference between Kirchhoff (left) and Beam (right) depth migration on the Mandal

High is shown in Figure 2 and the benefits of using the two styles will be presented. The technology itself and the implications of the imaged fault patterns for the Mandal High as a possible reservoir will be presented and discussed.

Figure 1: Half‐graben on the Utsira High, north of the Johan Sverdrup discovery. Figure 2: Fracture pattern on the Mandal High, depth‐imaged by Kirchhoff algorithm (left) and PGS Beam algorithm (right)


Improved understanding of basement highs through broadband seismic Berit Osnes PGS Introduction The North Sea still holds opportunities for large discoveries through the use of geological insight and creativity coupled with new technology. Basement highs play a key role in hydrocarbon exploration and can make for an attractive target, as shown through the recent success story of the Utsira High, which may open up exploration of other basement highs. Understanding the role of basement highs in exploration can be complicated and difficult to model. Improved seismic imaging and resolution can help our understanding of the role of basement highs in terms of hydrocarbon migration, trapping mechanisms, fractured reservoirs and sources of sediment. In this paper we will review how GeoStreamer broadband seismic and advanced imaging technology has improved the understanding of the Utsira and Mandal basement highs as potential reservoirs alongside the lateral sealing potential for associated traps. Case histories The first towed streamer broadband survey on the southern part of the Utsira High was acquired by PGS on behalf of Lundin and partners in 2009 and the results have been published in many papers. Between 2009 and 2011 PGS also acquired GeoStreamer® multiclient data across the northern part of the Utsira High and over the Mandal High in the Central Graben. The seismic section in Figure 1 shows a half‐graben north of the giant Johan Sverdrup discovery on Utsira High. If migration through basement is a working model, then pre‐Upper Jurassic sediments can represent interesting opportunities to be pursued. Whilst most of the traps in the Norwegian‐Danish Basin are salt induced, there are other opportunities where the Mandal High may act as both a source of sediment and lateral seal. Examples will be discussed in the presentation. GeoStreamer delivers both deep penetration through low frequencies and enhanced resolution from high frequencies . This is achieved by combining data from co‐located pressure and particle velocity sensors in the streamers. The streamers are towed deeper than in conventional acquisition, thereby improving the low frequency signal/noise. This is very important for the seismic imaging of a fractured basement. Both Utsira and Mandal Highs have been depth‐imaged with PGS Beam migration, which has improved the understanding of the fracture patterns within the basement highs. The PGS beam migration is well suited for application to broadband data as wavelet picking is designed to preserve the original amplitudes. The difference between Kirchhoff (left) and Beam (right) depth migration on the Mandal

High is shown in Figure 2 and the benefits of using the two styles will be presented. The technology itself and the implications of the imaged fault patterns for the Mandal High as a possible reservoir will be presented and discussed.

Figure 1: Half‐graben on the Utsira High, north of the Johan Sverdrup discovery. Figure 2: Fracture pattern on the Mandal High, depth‐imaged by Kirchhoff algorithm (left) and PGS Beam algorithm (right)


The offshore basement highs in the Lofoten‐Vesterålen area and some observations on their regional structural context Fridtjof Riis, Christian Magnus and Jon Arne Øverland Norwegian Petroleum Directorate Abstract Fractured and/or weathered crystalline basement rocks may constitute reservoirs for petroleum. In the Norwegian shelf, approximately 60 exploration wells are drilled into pre‐Devonian rocks (Figure 1). Weathered zones and fracture permeability have been observed in many of these wells. The reservoir quality of the basement rocks of the Norwegian shelf is thought to depend on their position relative to the Caledonian orogenic belt (Figure 1), on the duration and paleo‐climate of their exposure before burial and on the style and degree of tectonization and fracturing. For the study of basement petroleum geology it is also necessary to consider the overlying source rocks. In Figure 1, the basement has been divided into provinces based on the considerations above. In the Baltic Shield provinces in the south and north‐east, Precambrian rocks are overlain by Cambrian source rocks. The climate prior to burial was cold and no extensive weathering is indicated. In the southernmost part of the North Sea and in the Barents Sea, basement rocks belong to the external part of the orogen. Following Carboniferous (possibly also Devonian) rifting, they were covered by Carboniferous sediments in a warm and humid climate. Weathering is observed in a few cores. Carboniferous source rocks are mainly gas prone. In the northern North Sea, Norwegian Sea and offshore Lofoten‐Vesterålen, the Stord Basin, Froan and Helgeland Basins (purple lines in Figure 1) were filled with Triassic sediments following Permian‐Triassic rifting. The paleo‐climate at that time was dry and warm. The basement highs in the Møre and Lofoten‐Vesterålen area and in the Norwegian onshore were gradually transgressed in the Jurassic (locally in the Cretaceous), when the climate was humid. In many locations, there are only thin stratigraphic sections between the Upper Jurassic source rocks and the basement. In the Lofoten‐Vesterålen high, the mid Jurassic transgression of the basement is observed in Andøya and in shallow stratigraphic wells drilled by the IKU (now SINTEF Petroleumsforskning). In Andøya, the basement below the Jurassic outcrops is strongly weathered. Offshore seismic interpretation reveals large rotated basement blocks with a thin cover of Jurassic sediments and a thick Lower Cretaceous shaly section. Due to its position close to the active margin, the faults were reactivated in several phases in the late Cretaceous, Paleogene and Neogene. The onshore morphology of Lofoten and of the strandflat east of Vestfjorden seem to be controlled by the exhumed cores of Mesozoic fault blocks. In these large blocks, weathering surfaces which were initially semi‐horizontal are now rotated. Gravity data suggest that narrow basement cored ridges extend from the Lofoten‐Vesterålen high towards the south (Utgard High) and to the north along the Senja Fracture zone (Figure 1). In the evaluation of the petroleum potential of the Lofoten‐Vesterålen area (NPD website), a play model in fractured and weathered basement in the major rotated fault blocks was identified. The reservoir properties of these basement rocks are poorly constrained, and the parameters used for volume estimates were based on literature studies, mainly from the Utsira High.

Figure 1. Overview of different basement provinces in the Norwegian shelf and their relation to the Caledonian orogen. Wells drilled into basement are shown as black dots. The black line along the coast in the north shows the basement subcrop to the sea floor. Basins and highs are indicated by the Base Cretaceous surface. The subdivision of the Caledonian orogeny is sketchy and simplified.


The offshore basement highs in the Lofoten‐Vesterålen area and some observations on their regional structural context Fridtjof Riis, Christian Magnus and Jon Arne Øverland Norwegian Petroleum Directorate Abstract Fractured and/or weathered crystalline basement rocks may constitute reservoirs for petroleum. In the Norwegian shelf, approximately 60 exploration wells are drilled into pre‐Devonian rocks (Figure 1). Weathered zones and fracture permeability have been observed in many of these wells. The reservoir quality of the basement rocks of the Norwegian shelf is thought to depend on their position relative to the Caledonian orogenic belt (Figure 1), on the duration and paleo‐climate of their exposure before burial and on the style and degree of tectonization and fracturing. For the study of basement petroleum geology it is also necessary to consider the overlying source rocks. In Figure 1, the basement has been divided into provinces based on the considerations above. In the Baltic Shield provinces in the south and north‐east, Precambrian rocks are overlain by Cambrian source rocks. The climate prior to burial was cold and no extensive weathering is indicated. In the southernmost part of the North Sea and in the Barents Sea, basement rocks belong to the external part of the orogen. Following Carboniferous (possibly also Devonian) rifting, they were covered by Carboniferous sediments in a warm and humid climate. Weathering is observed in a few cores. Carboniferous source rocks are mainly gas prone. In the northern North Sea, Norwegian Sea and offshore Lofoten‐Vesterålen, the Stord Basin, Froan and Helgeland Basins (purple lines in Figure 1) were filled with Triassic sediments following Permian‐Triassic rifting. The paleo‐climate at that time was dry and warm. The basement highs in the Møre and Lofoten‐Vesterålen area and in the Norwegian onshore were gradually transgressed in the Jurassic (locally in the Cretaceous), when the climate was humid. In many locations, there are only thin stratigraphic sections between the Upper Jurassic source rocks and the basement. In the Lofoten‐Vesterålen high, the mid Jurassic transgression of the basement is observed in Andøya and in shallow stratigraphic wells drilled by the IKU (now SINTEF Petroleumsforskning). In Andøya, the basement below the Jurassic outcrops is strongly weathered. Offshore seismic interpretation reveals large rotated basement blocks with a thin cover of Jurassic sediments and a thick Lower Cretaceous shaly section. Due to its position close to the active margin, the faults were reactivated in several phases in the late Cretaceous, Paleogene and Neogene. The onshore morphology of Lofoten and of the strandflat east of Vestfjorden seem to be controlled by the exhumed cores of Mesozoic fault blocks. In these large blocks, weathering surfaces which were initially semi‐horizontal are now rotated. Gravity data suggest that narrow basement cored ridges extend from the Lofoten‐Vesterålen high towards the south (Utgard High) and to the north along the Senja Fracture zone (Figure 1). In the evaluation of the petroleum potential of the Lofoten‐Vesterålen area (NPD website), a play model in fractured and weathered basement in the major rotated fault blocks was identified. The reservoir properties of these basement rocks are poorly constrained, and the parameters used for volume estimates were based on literature studies, mainly from the Utsira High.

Figure 1. Overview of different basement provinces in the Norwegian shelf and their relation to the Caledonian orogen. Wells drilled into basement are shown as black dots. The black line along the coast in the north shows the basement subcrop to the sea floor. Basins and highs are indicated by the Base Cretaceous surface. The subdivision of the Caledonian orogeny is sketchy and simplified.


Strategies for fractured basement exploration: A case study from the West of Shetland

Dr Robert Trice

CEO Hurricane Exploration

Extended Abstract

Commercial production of hydrocarbon from fractured crystalline basement is well documented, with petroleum basins across the globe hosting fractured basement fields. The UK is an anomaly within this global phenomenon, as despite numerous serendipitous discoveries of basement oil in the North Sea and the West of Shetlands (WoS), currently there is no commercial field on the United Kingdom Continental Shelf (UKCS) that is reliant on oil production from fractured basement. Recognising that this situation presented an exploration niche, Hurricane Exploration plc (Hurricane) was formed in 2005 to focus on UKCS basement exploration and undertook a strategy for acquiring and evaluating fractured basement prospective exploration acreage in the West of Shetlands.

The current results of deploying this basement-focused strategy are two discoveries, Lancaster and Whirlwind. Lancaster has a contingent resource range (1C-3C) of 62 - 456 MMboe and is associated with 38° API oil. Whirlwind, which has very similar reservoir properties to Lancaster, is associated with either volatile light oil or a gas condensate. Contingent resource volumes for Whirlwind are of a similar magnitude to Lancaster, ranging from 98-373 MMstb of oil and 236-1017 Bscf of gas for the volatile oil case and 91-301 MMstb oil and 437-1308 Bscf of gas for the gas condensate case.

In addition to the two discoveries Hurricane has two further basement prospects Lincoln and Typhoon, which combine as a resource base of 444 MMboe of un-risked mean prospective resource.

The strategy deployed to acquire these basement resources is summarised with a focus on the geoscience aspects of acquiring and evaluating basement prospectivity. A key element of this strategy is that Hurricane has maintained 100% control of its basement discoveries to date allowing a consistent approach to evaluating the basement play in the WoS. The geological aspects of the applied strategy which will be focused on during the presentation are:

a) Work flow

b) Identifying the fracture network

c) Visualizing and modelling the fracture network

These aspects will be discussed in context to generating a basement prospect, fractured basement data acquisition and generating basement resource estimates.

Generating a basement prospect

In contrast to clastic and carbonate fields, basement fields and their respective reservoirs are poorly documented. That stated there are global examples of productive basement fields, outcrop analogue data and technical papers devoted to basement evaluation. Through evaluating such data sources it is clear that basement prospectivity is associated with traps such as basement highs, basement ridges, buried hills and fault blocks. Combinations of these trapping elements are common. In addition basement fields are typically located proximal to a prolific kitchen and may benefit from an overlying hydrocarbon bearing clastic succession, which sits unconformably on the basement. A key element in evaluating a prospective region for basement prospectivity is to establish what geological properties are most likely to be present and thereby what is the likely characteristics of a given basement prospect or lead. Such a process is no different to any method of prospect generation, however in the case of basement there is a relatively small volume of offset and analogue data to work with and consequently it is advisable to start the exploration process with the absolute basics. The approach taken by the author follows a prescriptive work flow (Figure 1), which can be applied to numerous phases of the exploration/appraisal process.

Figure 1 - Consistent generic workflow applied in fractured basement reservoir evaluation

The workflow consists of the underlying steps all of which benefit from cross reference to one another:

Internal data – Data package comprising previous data acquired on a Lead or Prospect, e.g. seismic or well data (not always available).

In the case of Hurricane’s basement prospects data was available for review from previous operators who had penetrated the basement and in some cases undertaken testing and/or coring both of which reduced uncertainty on basement oil presence.


Strategies for fractured basement exploration: A case study from the West of Shetland

Dr Robert Trice

CEO Hurricane Exploration

Extended Abstract

Commercial production of hydrocarbon from fractured crystalline basement is well documented, with petroleum basins across the globe hosting fractured basement fields. The UK is an anomaly within this global phenomenon, as despite numerous serendipitous discoveries of basement oil in the North Sea and the West of Shetlands (WoS), currently there is no commercial field on the United Kingdom Continental Shelf (UKCS) that is reliant on oil production from fractured basement. Recognising that this situation presented an exploration niche, Hurricane Exploration plc (Hurricane) was formed in 2005 to focus on UKCS basement exploration and undertook a strategy for acquiring and evaluating fractured basement prospective exploration acreage in the West of Shetlands.

The current results of deploying this basement-focused strategy are two discoveries, Lancaster and Whirlwind. Lancaster has a contingent resource range (1C-3C) of 62 - 456 MMboe and is associated with 38° API oil. Whirlwind, which has very similar reservoir properties to Lancaster, is associated with either volatile light oil or a gas condensate. Contingent resource volumes for Whirlwind are of a similar magnitude to Lancaster, ranging from 98-373 MMstb of oil and 236-1017 Bscf of gas for the volatile oil case and 91-301 MMstb oil and 437-1308 Bscf of gas for the gas condensate case.

In addition to the two discoveries Hurricane has two further basement prospects Lincoln and Typhoon, which combine as a resource base of 444 MMboe of un-risked mean prospective resource.

The strategy deployed to acquire these basement resources is summarised with a focus on the geoscience aspects of acquiring and evaluating basement prospectivity. A key element of this strategy is that Hurricane has maintained 100% control of its basement discoveries to date allowing a consistent approach to evaluating the basement play in the WoS. The geological aspects of the applied strategy which will be focused on during the presentation are:

a) Work flow

b) Identifying the fracture network

c) Visualizing and modelling the fracture network

These aspects will be discussed in context to generating a basement prospect, fractured basement data acquisition and generating basement resource estimates.

Generating a basement prospect

In contrast to clastic and carbonate fields, basement fields and their respective reservoirs are poorly documented. That stated there are global examples of productive basement fields, outcrop analogue data and technical papers devoted to basement evaluation. Through evaluating such data sources it is clear that basement prospectivity is associated with traps such as basement highs, basement ridges, buried hills and fault blocks. Combinations of these trapping elements are common. In addition basement fields are typically located proximal to a prolific kitchen and may benefit from an overlying hydrocarbon bearing clastic succession, which sits unconformably on the basement. A key element in evaluating a prospective region for basement prospectivity is to establish what geological properties are most likely to be present and thereby what is the likely characteristics of a given basement prospect or lead. Such a process is no different to any method of prospect generation, however in the case of basement there is a relatively small volume of offset and analogue data to work with and consequently it is advisable to start the exploration process with the absolute basics. The approach taken by the author follows a prescriptive work flow (Figure 1), which can be applied to numerous phases of the exploration/appraisal process.

Figure 1 - Consistent generic workflow applied in fractured basement reservoir evaluation

The workflow consists of the underlying steps all of which benefit from cross reference to one another:

Internal data – Data package comprising previous data acquired on a Lead or Prospect, e.g. seismic or well data (not always available).

In the case of Hurricane’s basement prospects data was available for review from previous operators who had penetrated the basement and in some cases undertaken testing and/or coring both of which reduced uncertainty on basement oil presence.




Figure 3 Pre-drill conceptual model of the basement reservoir at Lancaster Prospect.

The conceptual model summarises key statistical information gathered from outcrop analogues such as, lithology, fracture frequency, fracture connectivity, fault zone widths, degree and distribution of weathering, stress direction and key aspects of the anticipated reservoir quality.

The final product of the prospect generation work flow is the prospect summary (Figure 4), which indicate that oil can accumulate outside of structural closure (especially in flank locations), that fault zones will be subvertical and that the anticipated oil would be light. Estimates of resource potential therefore need to be subdivided (for risking purposes) between the GRV within structural closure and an anticipated GRV below structural closure that could also be associated with in place hydrocarbon. Offset data and global field analogues were helpful in establishing potential oil down to depths.

Figure 4 Pre-drill conceptual model and volumetric assessment of the fractured basement reservoir prospect Lancaster (note - post well result indicate the contingent resource range for recoverable oil is, C1 60 MMstb, 2C 200MMstb, 3C 437 MMstb)

From the analysis of analogue and offset undertaken during the “Generating a Basement Prospect” it was evident that numerous basement wells and discoveries had been drilled without optimal consideration of the fracture network. This situation was either due to limited seismic data, limited geological knowledge or where basement penetrations were not the primary exploration target. Consequently many basement fields are associated with wells that are interpreted as having low permeability/tight reservoir. Hurricane’s strategy was focused to optimise the penetration of quality reservoir and a GRV volume containing hydrocarbon. To this end well, defined seismic scale faults were mapped and evaluated for their potential connectivity to a broader fault/fracture network (Slightam 2012). Such seismic scale faults where the drilling target to be evaluated by deviated wells drilled perpendicular to the fault strike. Once drilled a detailed data acquisition program was undertaken to ensure that the seismic scale faults could be characterised by well based measurements.


Figure 3 Pre-drill conceptual model of the basement reservoir at Lancaster Prospect.

The conceptual model summarises key statistical information gathered from outcrop analogues such as, lithology, fracture frequency, fracture connectivity, fault zone widths, degree and distribution of weathering, stress direction and key aspects of the anticipated reservoir quality.

The final product of the prospect generation work flow is the prospect summary (Figure 4), which indicate that oil can accumulate outside of structural closure (especially in flank locations), that fault zones will be subvertical and that the anticipated oil would be light. Estimates of resource potential therefore need to be subdivided (for risking purposes) between the GRV within structural closure and an anticipated GRV below structural closure that could also be associated with in place hydrocarbon. Offset data and global field analogues were helpful in establishing potential oil down to depths.

Figure 4 Pre-drill conceptual model and volumetric assessment of the fractured basement reservoir prospect Lancaster (note - post well result indicate the contingent resource range for recoverable oil is, C1 60 MMstb, 2C 200MMstb, 3C 437 MMstb)

From the analysis of analogue and offset undertaken during the “Generating a Basement Prospect” it was evident that numerous basement wells and discoveries had been drilled without optimal consideration of the fracture network. This situation was either due to limited seismic data, limited geological knowledge or where basement penetrations were not the primary exploration target. Consequently many basement fields are associated with wells that are interpreted as having low permeability/tight reservoir. Hurricane’s strategy was focused to optimise the penetration of quality reservoir and a GRV volume containing hydrocarbon. To this end well, defined seismic scale faults were mapped and evaluated for their potential connectivity to a broader fault/fracture network (Slightam 2012). Such seismic scale faults where the drilling target to be evaluated by deviated wells drilled perpendicular to the fault strike. Once drilled a detailed data acquisition program was undertaken to ensure that the seismic scale faults could be characterised by well based measurements.


Whilst it is important to have a clear geological target that best tests the properties of the prospect, it is also essential to plan the mechanics of drilling basement targets as there are a number of potential drilling issues to accommodate including, a) management of fluid losses, b) optimising bottom hole assembly (bits motors, stabilisers MWD etc), and, c) drilling methods (managing ROP over vibration etc). Whilst analogue data and published papers provide a significant amount of data on which to formulate a drilling strategy, there is no substitute, in basement operations, for working as an integrated team with the well planning and operational team. Key learning from drilling the Lewisian Basement includes:

Drilling fluid losses:

Work with drilling contractor to define benchmark for acceptable hourly loss rate(s) and to define mitigation measures for manageable loss criteria. Define solution for “total” loss scenario.

Drilling:

Whilst underbalance and managed pressure drilling systems offer an optimum method of reducing formation damage these techniques require specialised equipment, trained personal and HSE approvals which are not readily available on the open rig market. Consequently basement drilling needs to be achieved through use of conventional pressure management systems. The combination of tricone rock bits, drilling brine and use of MWD data are more than capable of drilling basement reservoirs and Hurricane has achieved rates of penetration equivalent of 3-5 m/hour (depending on fracture properties). High resolution mudlogging has also proven invaluable in typing/detecting hydrocarbon.

Fractured basement data acquisition

Effective data acquisition in Type 1 Fractured Reservoirs such as basement requires the gathering of a broad range of static and dynamic data. This requires detailed wireline and testing (DST) data with wireline testing and production logging (flow meter) providing the key measurements that integrate depth based static properties such as fracture attitude to permeability and flow. Well testing of discrete intervals such as an individual fault zone is also of significant benefit in helping to understand the behaviour and properties of the fracture network. Key wireline/LWD measurements that have proved helpful in evaluating the Lewisian Basement are summarised below:

Lateralog – fracture detection, fracture aperture estimate

Electrical imaging – fracture identification, fracture orientation, fracture aperture estimate

Acoustic imaging – fracture identification, fracture orientation, fracture aperture estimate

Wireline pressure – fracture permeability & formation fluid

NMR – bulk porosity – fracture detection

Density-neutron & pef – lithology and fracture identification, aperture estimate

Spectral gamma – lithology and permeable fracture detection

Sidewall cores – lithology and hydrocarbon presence, fracture properties

PLT – multiple rate passes – fracture permeability, deliverability, formation fluid

From the integration of the above data with production logging data it was possible to determine that many permeable/productive fractures are of sufficient aperture to be detected by conventional wireline and LWD measurements. Such aperture magnitudes are interpreted as having been associated with permeability enhancement through hydrothermal and epithermal process, an interpretation supported by the mineralogical assemblages noted from sidewall core analysis.

A key learning from the data analysed in the wells so far drilled is that, contrary to the initial pre-drill conceptual model, fracture frequency is not associated with seismic scale fault zones and fracture frequency is significantly higher than predicted with permeable fractures being equivalently distributed within and without seismic scale fault zones. However it is apparent that seismic scale fault zones are associated with preferential permeability (consistent with the pre-drill conceptual model) indicating that such rock volumes are better connected to the bulk fracture network.

Of the wireline measurements acquired, sonic waveform data has proven to be the least responsive to permeable fractures, an observation clearly supported by comparison to PLT and wireline tester data. This is attributed to a function of relative fracture azimuth to the borehole (tool), fracture tortuosity and very high fracture frequency.

During DST data acquisition it is recommended to acquire pressure data on the seabed to accommodate tidal effects that occur within the reservoir. Cyclic pressure variation of @1 Psi have been noted in the Lewisian basement during a 24 hour well test. In addition, variable rate choke with PLT provides important information on fluid movement and fracture productivity.

Generating basement resource estimates

Fracture reservoirs are a challenge to evaluate from the perspective of establishing hydrocarbon resource volumes. Currently there is no acknowledged industry standard for evaluating fractured reservoirs. Type 1 fractured reservoirs bring their own specific challenges and Hurricane has worked closely with third party specialists to develop a robust methodology for establishing fractured basement resource volumes. The details of this approach remain proprietary, however the basic approach is a mechanism for evaluating and risking hydrocarbon within structural closure and hydrocarbon outside of structural closure. The four basic steps applied to volumetric analysis are summarised using the Lancaster discovery as an example as Figure 5.


Whilst it is important to have a clear geological target that best tests the properties of the prospect, it is also essential to plan the mechanics of drilling basement targets as there are a number of potential drilling issues to accommodate including, a) management of fluid losses, b) optimising bottom hole assembly (bits motors, stabilisers MWD etc), and, c) drilling methods (managing ROP over vibration etc). Whilst analogue data and published papers provide a significant amount of data on which to formulate a drilling strategy, there is no substitute, in basement operations, for working as an integrated team with the well planning and operational team. Key learning from drilling the Lewisian Basement includes:

Drilling fluid losses:

Work with drilling contractor to define benchmark for acceptable hourly loss rate(s) and to define mitigation measures for manageable loss criteria. Define solution for “total” loss scenario.

Drilling:

Whilst underbalance and managed pressure drilling systems offer an optimum method of reducing formation damage these techniques require specialised equipment, trained personal and HSE approvals which are not readily available on the open rig market. Consequently basement drilling needs to be achieved through use of conventional pressure management systems. The combination of tricone rock bits, drilling brine and use of MWD data are more than capable of drilling basement reservoirs and Hurricane has achieved rates of penetration equivalent of 3-5 m/hour (depending on fracture properties). High resolution mudlogging has also proven invaluable in typing/detecting hydrocarbon.

Fractured basement data acquisition

Effective data acquisition in Type 1 Fractured Reservoirs such as basement requires the gathering of a broad range of static and dynamic data. This requires detailed wireline and testing (DST) data with wireline testing and production logging (flow meter) providing the key measurements that integrate depth based static properties such as fracture attitude to permeability and flow. Well testing of discrete intervals such as an individual fault zone is also of significant benefit in helping to understand the behaviour and properties of the fracture network. Key wireline/LWD measurements that have proved helpful in evaluating the Lewisian Basement are summarised below:

Lateralog – fracture detection, fracture aperture estimate

Electrical imaging – fracture identification, fracture orientation, fracture aperture estimate

Acoustic imaging – fracture identification, fracture orientation, fracture aperture estimate

Wireline pressure – fracture permeability & formation fluid

NMR – bulk porosity – fracture detection

Density-neutron & pef – lithology and fracture identification, aperture estimate

Spectral gamma – lithology and permeable fracture detection

Sidewall cores – lithology and hydrocarbon presence, fracture properties

PLT – multiple rate passes – fracture permeability, deliverability, formation fluid

From the integration of the above data with production logging data it was possible to determine that many permeable/productive fractures are of sufficient aperture to be detected by conventional wireline and LWD measurements. Such aperture magnitudes are interpreted as having been associated with permeability enhancement through hydrothermal and epithermal process, an interpretation supported by the mineralogical assemblages noted from sidewall core analysis.

A key learning from the data analysed in the wells so far drilled is that, contrary to the initial pre-drill conceptual model, fracture frequency is not associated with seismic scale fault zones and fracture frequency is significantly higher than predicted with permeable fractures being equivalently distributed within and without seismic scale fault zones. However it is apparent that seismic scale fault zones are associated with preferential permeability (consistent with the pre-drill conceptual model) indicating that such rock volumes are better connected to the bulk fracture network.

Of the wireline measurements acquired, sonic waveform data has proven to be the least responsive to permeable fractures, an observation clearly supported by comparison to PLT and wireline tester data. This is attributed to a function of relative fracture azimuth to the borehole (tool), fracture tortuosity and very high fracture frequency.

During DST data acquisition it is recommended to acquire pressure data on the seabed to accommodate tidal effects that occur within the reservoir. Cyclic pressure variation of @1 Psi have been noted in the Lewisian basement during a 24 hour well test. In addition, variable rate choke with PLT provides important information on fluid movement and fracture productivity.

Generating basement resource estimates

Fracture reservoirs are a challenge to evaluate from the perspective of establishing hydrocarbon resource volumes. Currently there is no acknowledged industry standard for evaluating fractured reservoirs. Type 1 fractured reservoirs bring their own specific challenges and Hurricane has worked closely with third party specialists to develop a robust methodology for establishing fractured basement resource volumes. The details of this approach remain proprietary, however the basic approach is a mechanism for evaluating and risking hydrocarbon within structural closure and hydrocarbon outside of structural closure. The four basic steps applied to volumetric analysis are summarised using the Lancaster discovery as an example as Figure 5.


Figure 5. The basic four steps used in evaluating the resource potential of the Lancaster Prospect. (a) Depicts the seismic scale faults mapped across the Lancaster structure. Faults planes are interpreted to be vertical. (b) A subset of the fault volume extracted for modelling purposes. (c) Represents the modelled GRV. The upper surface is defined by the top basement reflector and the lower surface is defined by an oil down to, (d) Portrays the GRV spit into the two reservoir facies Pseudomatrix and Fault zones. (e) Reservoir properties are probabilistically populated within the model and in this example porosity is presented within Fault Zones and Pseudomatrix.

A key component of this approach is the splitting of the reservoir GRV into two facies, Fault Zones (FZ Figure 6) and Pseudomatrix (PM Figure 6). Fault zone GRV is the rock volume interpreted as being associated with fault zones associated with seismic scale faults. Detailed analysis of wireline data indicates that such zones are 30 -70m in width and up-to 90m in width based on outcrop data. Pseudomatrix is the remaining GRV and comprises sub seismic scale faults.

Figure 6. Basic reservoir description for volumetric analysis including reservoir parameters.

Figure 7. Summary of reservoir calibration as an integrated workflow. Key elements include the calibration of fracture porosity/permeability with PLT data and the testing of geometric models (DFN’s) with pressure data.


Figure 5. The basic four steps used in evaluating the resource potential of the Lancaster Prospect. (a) Depicts the seismic scale faults mapped across the Lancaster structure. Faults planes are interpreted to be vertical. (b) A subset of the fault volume extracted for modelling purposes. (c) Represents the modelled GRV. The upper surface is defined by the top basement reflector and the lower surface is defined by an oil down to, (d) Portrays the GRV spit into the two reservoir facies Pseudomatrix and Fault zones. (e) Reservoir properties are probabilistically populated within the model and in this example porosity is presented within Fault Zones and Pseudomatrix.

A key component of this approach is the splitting of the reservoir GRV into two facies, Fault Zones (FZ Figure 6) and Pseudomatrix (PM Figure 6). Fault zone GRV is the rock volume interpreted as being associated with fault zones associated with seismic scale faults. Detailed analysis of wireline data indicates that such zones are 30 -70m in width and up-to 90m in width based on outcrop data. Pseudomatrix is the remaining GRV and comprises sub seismic scale faults.

Figure 6. Basic reservoir description for volumetric analysis including reservoir parameters.

Figure 7. Summary of reservoir calibration as an integrated workflow. Key elements include the calibration of fracture porosity/permeability with PLT data and the testing of geometric models (DFN’s) with pressure data.


Once the 3D reservoir model is constructed various sensitivities can be run including varying porosity, water saturation and oil down-to. The results are then cross-checked with further detailed analysis/calibration, (Figure 7) which is basically a workflow consisting of detailed analysis of geophysical, wireline and test data cross referenced to analogue fields and discrete fracture network models of the fracture network.

Summary

An important conclusion from this work is that whilst there are many geological and production similarities between analogue basement reservoirs it is evident that the fracture network is unique to each field and each reservoir. Given that it is the fracture characteristics that define the reserves of Type 1 Fractured reservoirs the most important objective of any fractured basement evaluation strategy is the acquisition and interpretation of data that helps define the productive fracture network.

Fractured basement reservoirs remain a potentially significant unproven play type for the UKCS and NCS (Norwegian Continental Shelf). Whilst fractured basement is generally ignored by the industry it is clear from basement discoveries such as Lancaster & Whirlwind and Lundin’s /Statoil’s Tellus, that there is a growing volume of data which supports the potential for basement reservoir. The Norwegian and UK continental shelves are blessed with a prolific source rock and large basement structures and it is clearly a matter of time before fractured basement reservoirs are contributing to commercial production in the UK and Norway.

Reference

Slightam, C. 2012. “Characterizing seismic-scale faults pre and post-drilling; Lewisian Basement, West of Shetlands, UK”. In; Spence G.H., Redfern J., Aguilera R., Bevan T.G., Cosgrove J.W., Couples G.D. & Daniel J.M. (eds.) 2012. Advances in the study of Fractured Reservoirs. Geological Society, Special Publications, 374, .In press.

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