Middle Jurassic, Upper Jurassic and Lower Cretaceous of the UK Central and Northern North Sea

Continental Shelf and Margins Programme Research Report RR/03/001

BRITISH GEOLOGICAL SURVEY

RESEARCH REPORT RR/03/001

Middle Jurassic, Upper Jurassic and Lower Cretaceous of the UK Central and Northern North Sea

The National Grid and other Ordnance Survey data are used with the permission of the Authors Controller of Her Majesty’s Stationery Office. Licence No: 100017897/2005. H Johnson, A B Leslie, C K Wilson, I J Andrews and R M Cooper

Keywords Contributors Central and Northern North Sea, Middle Jurassic, Upper Jurassic, P Egerton, E Gillespie, A F Henderson, S Jones, M F Quinn and J B Wild Lower Cretaceous

Front cover Glacial till overlying Oxfordian sedimentary rocks at Filey Brigg, North Yorkshire.

Bibliographical reference

JOHNSON, H, LESLIE, A B, WILSON, C K, ANDREWS, I J, and COOPER, R M. 2005. Middle Jurassic, Upper Jurassic and Lower Cretaceous of the UK Central and Northern North Sea. British Geological Survey Research Report, RR/03/001. 42pp.

ISBN 0 85272 509 4 Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail [email protected]. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

Maps and diagrams in this book use topography based on Ordnance Survey mapping.

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Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1EU ຜ 01793–411500 Fax 01793–411501 Foreword

The North Sea Oil Province comprises the Central and Graben rift systems. During Early Cretaceous times, much Northern North Sea and is one of the world’s major oil- of the pre-existing rift topography continued to exert a producing regions. For the past 30 years this province has strong influence on sedimentation, though tectonism and been a very important part of Britain’s resource base and a halokinesis were also locally important. Syn-rift organic- major contributor to the Nation’s wealth. The private rich marine mudstones are the source for virtually all of the sector’s success in vigorously exploring and developing the region’s hydrocarbons and are mature for hydrocarbon oil province has been supported and stimulated by the generation throughout most of the rift system. Though the regulatory activities of the Department of Trade and Central and Northern North Sea is now considered to be Industry, and by the complementary work of the British mature for exploration, plays such as the Upper Jurassic Geological Survey (BGS), which has published an syn-rift play and the deep basin-axis high pressure/high extensive range of offshore geological maps and reports. temperature gas condensate play are expected to be the This report summarises much that is known about the focus of much future activity. For example, the recent giant tectono-stratigraphic development, palaeogeography and ‘Buzzard’ discovery in Upper Jurassic sandstones of the petroleum geology of the Middle Jurassic, Upper Jurassic Central North Sea may have oil in place ranging from 800 and Lower Cretaceous rocks within the province. The to 1100 million barrels, making it one of the largest report text and figures presented here are complemented by discoveries on the United Kingdom Continental Shelf in three 1:1 000 000 map enclosures that illustrate the the last 25 years. The Lower Cretaceous succession has distribution and thickness of the Middle Jurassic, Upper also been a recent exploration target, and significant Jurassic and Lower Cretaceous lithostratigraphical discoveries have been reported within the Moray Firth successions as defined by the BGS on behalf of the United Basin. Kingdom Offshore Operators Association. The Middle Jurassic development of the province was dominated by an episode of transient thermal doming. A David A Falvey subsequent, Late Jurassic, phase of crustal extension Executive Director developed the Viking Graben, Moray Firth and Central British Geological Survey

iii Acknowledgements

This report was compiled by H Johnson, A B Leslie, C K Wilson and I J Andrews, with editorial advice from D Evans and J Thomas. Figures were prepared by J Bain, M Cherrie and A Stewart.

iv Contents

Foreword iii along the axis of the Viking Graben (after Underhill Acknowledgements iv and Partington, 1993) 12 9 Middle Jurassic palaeogeography (after Mitchener et 1 Introduction 1 al., 1992) 13 1.1 Structural development 1 10 Simplified map of the Rattray Volcanics Member of 1.2 Stratigraphic templates 8 the Forties Volcanic Province; numbers refer to UK 2 Brent and Fladen Groups (‘Middle Jurassic’) 9 quadrants (after Smith and Ritchie, 1993) 14 2.1 Introduction 9 11 Depositional model for the Rannoch, Etive and Lower 2.2 Lithostratigraphy 9 Ness formations (after Budding and Inglin, 1981) 14 2.3 Sequence stratigraphy 10 12 Depositional setting of the Brent and Fladen groups 15 2.4 Palaeogeography 10 13 Summary of diagenetic and burial history of the Brent 2.5 Depositional environments and reservoirs 12 Group (after Giles et al., 1992) 16 2.6 Source rocks 14 14 Structure of the Murchison Oilfield (after Warrender, 2.7 Traps 14 1991) 16 15 Structure of the Brent Oilfield (after Struijk and Green, 3 Humber Group (‘Upper Jurassic’) 18 1991) 17 3.1 Introduction 18 16 Structure of the Bruce Oilfield (after Beckly et al., 3.2 Lithostratigraphy 18 1993) 17 3.3 Sequence stratigraphy 19 17 Humber Group (Upper Jurassic) lithostratigraphy (after 3.4 Palaeogeography 19 Richards et al., 1993; Andrews et al., 1990; Wignall 3.5 Depositional environments and reservoirs 22 and Pickering, 1993) 19 3.6 Source rocks 23 18 Genetic stratigraphy template for the Late Jurassic of 3.7 Traps 25 the UK Central and Northern North Sea (after 4 Cromer Knoll Group (‘Lower Cretaceous’) 27 Partington et al., 1993b and Fraser et al., 2003) 20 4.1 Introduction 27 19 Upper Jurassic palaeogeography (after Rattey and 4.2 Lithostratigraphy 27 Hayward, 1993) 21 4.3 Sequence stratigraphy 28 20 Depositional setting of the Humber Group 22 4.4 Palaeogeography 30 21 Depositional model for the Fulmar Formation (after 4.5 Depositional environments and reservoirs 32 Gowland, 1996) 23 4.6 Traps 35 22 Schematic block-diagram of the depositional setting of References 38 Upper Jurassic sediments in the Brae Oilfield area (after Stoker and Brown, 1986) 24 23 Approximate thermal maturity at top Kimmeridge Clay FIGURES Formation (after Cayley, 1987; Field, 1985; Goff, 1983) 25 1 Structural framework of the Central and Northern 24 Structure of the South Brae Field (after Roberts, 1991) 26 North Sea. Approximate locations of cross-sections in 25 Structure of the Clyde Field (after Smith, 1987) 26 Figure 5 as well as Figures 14, 15, 16, 24, 25, 32, 33, 26 Cromer Knoll Group (Lower Cretaceous) 34 and 35 are shown 2 lithostratigraphy (after Johnson and Lott, 1993) 28 2 Location of Central and Northern North Sea hydrocarbon 27 Generalised Early Cretaceous sequence stratigraphy fields in the Middle Jurassic, Upper Jurassic and Lower (after Oakman and Partington, 1998; Copestake et al., Cretaceous (after Brooks et al., 2002) 3 2003) 29 3 Generalised tectonic phases of the Central and 28 Chronostratigraphic distribution of Early Cretaceous Northern North Sea 4 sequences, UK Central Graben (after Oakman and 4 Approximate extent of the ‘Mid-Cimmerian Partington, 1998) 30 Unconformity’ due to transient domal uplift and the 29 Early Cretaceous palaeogeography 31 pattern of subsequent marine onlap onto the 30 Depositional setting of the Cromer Knoll Group 33 unconformity (after Underhill and Partington, 1993; 31 ‘Fill and Spill’ model for sand emplacement, in which Underhill, 1998) 5 a channel sand feeds (a) then fills (b) a basin, leading to 5 Generalised cross-sections across the Central and bypass by the feeder channel (c) and filling of a second Northern North Sea; locations are given in Figure 1 basin (after Argent et al., 2000) 33 (after Andrews et al., 1990; Gatliff et al., 1994 and 32 Captain Sandstone depositional model for (a) the lower Johnson et al. 1993) 6 Captain Sandstone and (b) the upper Captain Sandstone 6 Brent and Fladen groups (Middle Jurassic) (after Rose, 1999) 34 lithostratigraphy (after Richards et al., 1993) 10 33 Schematic structure of the Captain Field (after Rose, 7 Generalised Middle Jurassic genetic stratigraphical 1999) 35 sequences in the Northern and Central North Sea (after 34 Structure of the Britannia Field (after Copestake et al., Partington et al., 1993b) 11 2003) 36 8 Generalised correlation of Jurassic chronostratigraphy, 35 Structure of the Scapa Field (after McGann et al., genetic sequence stratigraphy and lithostratigraphy 1991) 36

v vi 1 Introduction

Oil and gas production within the United Kingdom (UK) be the focus of future exploration activity (Munns, 2002). and its offshore-designated area continued to rise through The Mesozoic basin margin play was tested by the giant the 1990s and until recently the UK has been net self- ‘Buzzard’ discovery in May 2001, which found oil in sufficient in both oil and gas. To date, about 4000 Upper Jurassic turbidite sandstones near the western exploration and appraisal wells have been drilled on the margin of the South Halibut Basin (UK Blocks 19/5S, UK Continental Shelf (UKCS). These wells have resulted 20/1S, 19/10 and 20/6; Figures 1, 2) (Doré and Robbins, in more than 285 producing fields and another 300 plus 2003). significant discoveries (Munns, 2002). The Central and Northern North Sea area (Figure 1) is now considered to be a mature hydrocarbon province. However, there remains 1.1 STRUCTURAL DEVELOPMENT significant potential for increasing the overall reserves through additional, more focussed, exploration, Since the end of the Caledonian Orogeny, the Central and particularly in sparsely drilled basinal areas and by Northern North Sea Basin has occupied an intraplate improving recovery from existing fields. Indeed, petroleum setting. Early Jurassic strata within the North Sea Basin production on the UKCS is no longer dominated by a few accumulated during a phase of post-rift subsidence large fields, but reflects a trend towards the development of following Permo-Triassic extension (e.g. Faerseth, 1996; small satellite fields that can be brought on stream using Ziegler, 1990). In Mid Jurassic times, the Central North existing host platforms and infrastructure (Department of Sea area experienced transient thermal doming, erosion Trade and Industry, 1999). and volcanism, possibly associated with a mantle plume The exploration and development of the North Sea has (Underhill and Partington, 1993) (Figure 4). The resulting provided, and still provides, a vast amount of detailed regional unconformity is commonly termed the ‘Mid information regarding the subsurface geology. This report Cimmerian Unconformity’ although in the Central North briefly summarises much that is known about the tectono- Sea it covers a much wider timespan (Husmo et al., 2003). stratigraphic development, palaeogeography and petroleum Stratigraphical relationships indicate that the uplift was geology of the Brent and Fladen, Humber and Cromer centred upon the North Sea rift triple junction (Figure 4). Knoll groups (approximately equivalent to the Middle Although the initial formation of a simple trilete rift Jurassic, Upper Jurassic and Lower Cretaceous rocks junction may have been the result of doming and deflation, respectively) within the UK Central and Northern North subsequent extension during the Mid and Late Jurassic led Sea. It includes three 1:1 000 000 map enclosures that, to the development of a series of intra-rift fault sets respectively, illustrate the distribution and thickness of the (Davies et al., 2001). However, the detailed structural Brent and Fladen groups, the Humber Group and the evolution of the region has been a topic of many debates. Cromer Knoll Group (Enclosures 1, 2 and 3). Rattey and Hayward (1993) and Fraser (1993) proposed Representative well sections shown on these enclosures that rifting was initially most intense at the extremities of illustrate the typical lithofacies and wireline log responses the present graben system and as time elapsed it of the successions. The geographical distribution of the propagated back towards the centre of the domal uplift. lithostratigraphical units on Enclosures 1–3 is based Underhill (1991) and Glennie and Underhill, 1998) predominantly on information from released well data held suggested that the onset of major rifting probably occurred at the Department of Trade and Industry Core Store in in mid-Oxfordian to early Kimmeridgian times. However, Edinburgh. Well data up to 2002 (Release 79) were a number of pulses of Late Jurassic extension have been examined in this study and additional information was also recognised (e.g. Errat et al., 1999; Davies et al., 1999; extracted from a few regional 2D seismic profiles Davies et al. 2001). (Department of Trade and Industry, 2002). According to Davies et al. (2001), extension may have The Middle Jurassic, Upper Jurassic and Lower been initiated on north–south and north-north-east–south- Cretaceous successions were, in general terms, formed south-west trending faults (in the South Viking Graben) during the pre-rift, syn-rift and post-rift phases of basin during the Bathonian and Callovian, on north-east–south- development, respectively (Figures 2 and 3) and all three west and east–west structures (in the Moray Firth Basin) successions include important reservoir rocks. The Upper during the Oxfordian and on north-west–south-east faults Jurassic succession also includes the primary source rocks (in the Central Graben and Outer Moray Firth Basin) for the Central and Northern North Sea (i.e. units within during the Kimmeridgian and Volgian. the Kimmeridge Clay Formation). The geometry of these Seismic data reveal that the syn-rift (Upper Jurassic) successions together with lateral facies changes within successions commonly thicken dramatically towards them reflects a complex interplay between local tectonism syndepositional faults (Figure 5). This general style of and eustatic sea level fluctuations. sediment thickness variation is in contrast with the pattern A significant proportion of the remaining hydrocarbon which developed during the post-rift ‘thermal sag’ and potential within the province is believed to lie within the sediment loading phase of basin development during Early Upper Jurassic and Lower Cretaceous successions. In to Mid Jurassic times, when the basin was more ‘saucer- particular, the Upper Jurassic syn-rift, Mesozoic basin shaped’ and the thickest deposits accumulated at its centre. margin and the deep basin axis high pressure/high Rift styles vary substantially between the Northern and temperature gas condensate exploration plays are likely to the Central North Sea and there were two principal

1 4ºW 62ºN 2º 0º 2ºE

ERLEND Platform, including STRUCTURE PLATFORM Palaeozoic basins NOT RESOLVED MAGNUS Mesozoic basinal areas, BASIN including internal fault blocks

Area covered by this report 14 Limit of UK designated area

33 Approximate locations of Figures 14,15,16,24,25,32, UNST 33,34 and 35 BASIN 5a 61º EAST SHETLAND BASIN 15

TRANSITIONAL RIDGE

SHELF RONA

GRABEN

NORTH VIKING MOINE THRUST

60º WEST SHETLAND BASIN WEST FAIR ISLE 5b EAST SHETLAND BASIN PLATFORM 5c EAST 16 FAIR ISLE NORTHERN BERYL BASIN NORTH EMBAYMENT SEA 5d(i)

59º DUTCH BANK BASIN EAST ORKNEY FLADEN BASIN GROUND UTSIRA SPUR HIGH

CAITHNESS RIDGE

24 32 33 35 WITCH GROUN LING GRABEN CAPTAIN RIDGE GRABEN HALIBUT HORST D SOUTH VIKING GRABEN TH BASINSMITHBASIN BANK 34 58º 5d(ii) RENEE C INNER MORAY FIR SOUTH E HALIBUT N RIDGE BASIN T R EAST CEN A JAEREN L GRABEN HIGH FORTIES-MONTROSE TRAL S HIGH E N A O WEST CER T 5e(iii) 5e(i) 5e(ii) H NTRAL GRABEN PETERHEAD RIDGE WEST 57º CENTRAL SHELF FORTH DEVIL'S 0 50 kilometres APPROACHES HOLE BASIN HORST CENTRAL G SOUTH

RABEN 25 JOSEPHINE RIDGE

56º MID NORTH SEA HIGH

Figure 1 Structural framework of the Central and Northern North Sea. Approximate locations of cross-sections in Figure 5 as well as Figures 14, 15, 16, 24, 25, 32, 33, 34 and 35 are shown.

2 NORWAY UK 0˚ 3˚E Lower Cretaceous fields (post-rift)

Upper Jurassic fields (syn-rift)

Middle Jurassic fields (pre-rift) Murchison

Mature Upper Jurassic source rocks (after Pegrum and Spencer, 1990)

61˚N Note: arrowed fields/discoveries are mentioned in the text

Brent

VIKING GRABEN 60˚

Bruce

Beryl

59˚ Brae

Piper Scapa

IRTH MORAY F Hannah (oil accumulation)

58˚

Beatrice Britannia Captain CENTRAL GRABEN

Goldeneye Buzzard (oil accumulation) (discovery) SCOTLAND

57˚

Fulmar 0 50 kilometres Clyde

3˚W 2˚ 1˚0˚ 1˚ 2˚ 3˚E

Figure 2 Location of Central and Northern North Sea hydrocarbon fields in the Middle Jurassic, Upper Jurassic and Lower Cretaceous (after Brooks et al., 2002).

3 controlling factors. Firstly, differences in the basement composition and tectonic grain between the two regions TECTONIC PHASE AGE influenced subsequent structural development. In the Thermal subsidence and pulses Cretaceous Central North Sea, the rifts are more complex and appear of transpression to be segmented along both north-east–south-west ‘Caledonide’ and north-west–south-east ‘Trans-European Second phase of major rifting Bathonian to Volgian Fault Zone’ trends (e.g. Errat et al., 1999; Jones et al., Thermal subsidence and build Toarcian to Bajocian 1999). Secondly, in the Northern North Sea, Upper up and deflation of Central North Permian salt is largely absent, and there is no major Sea thermal dome detachment between basement and cover rocks. In the East Shetland Basin, for example, deep seismic reflection Thermal subsidence Early Jurassic to Mid - profiles suggest domino-style rotation of large crustal Jurassic fault blocks (Figure 5a) (Klemperer and White, 1989). In contrast, the Zechstein evaporites in the Central North Initial phase of major rifting ?Late Permian and Sea provide a major detachment level that essentially ?Early Triassic separates the basement from the cover succession or ‘carapace’ (e.g. Hodgson et al., 1992; Smith et al., 1993; Figure 3 Generalised tectonic phases of the Central and Helgeson, 1999). Northern North Sea. In the Northern North Sea, uplift of the footwall blocks by extensional faults led in many cases to pronounced erosion and fault-scarp degradation (Underhill et al., 1997; Cretaceous times, particularly within parts of the Moray McLeod and Underhill, 1999). Indeed, much of the oil Firth area, and may have influenced the depositional remaining in the Middle Jurassic fields of the East pattern of mass-flow clastics. For example, the Lower Shetland Basin may lie within such degradation complexes Cretaceous Scapa Sandstone Member, which forms an (Underhill, 1999). It should also be noted that many of the important oil reservoir in the Scapa Field, has been major faults probably grew through linkage of originally described as a syn-rift deposit (Harker and Chermak, isolated segments, rather than by simple radial 1992; Harker and Rieuf, 1996). However, Argent et al. propagation. Pronounced changes in fault strike, (2000) regarded the Lower Cretaceous succession of the displacement minima and transverse hanging-wall highs Moray Firth Basin as essentially post-rift in character are commonly inferred to represent palaeosegment with only local evidence for continued extensional boundaries that subsequently became breached and faulting. In contrast, Oakman and Partington (1998) incorporated into an extensive fault zone (e.g. Peacock and proposed that the transition from the Late Jurassic Sanderson, 1991; McLeod et al., 2000). A consequence of Humber Group to the Early Cretaceous Cromer Knoll fault growth by segment linkage is that transfer zones/relay Group represents a change in tectonic style, from mainly ramps at segment boundaries are transient features transtension in the Jurassic to dominantly transpression in (Jackson et al., 2002). the Early Cretaceous. The amount of Late Jurassic extension experienced by Local inversion of Central North Sea depocentres the North Sea Basin has been a controversial subject, during the Early Cretaceous is commonly considered to though beta factors of 1.1–1.2 are supported by be a response to oblique-slip faulting (e.g. Pegrum and backstripping and other structural analyses (e.g. Roberts et Ljones, 1984; Ineson, 1993). Ziegler (1990) mapped al., 1993). Within the graben axes, Late Jurassic beta many of the Cretaceous inversion axes within the North factors may rise to 1.4–2.0 (Glennie and Underhill, 1998). Sea Basin, most of which trend either west-north-west or Similarly, the orientation of Late Jurassic extensional east-north-east. Compressional events in early stress and the amount of oblique and strike-slip movement Ryazanian, Valanginian, mid-Hauterivian and mid- has been a contentious issue. The majority of the evidence Barremian times are thought to reflect north-south presented in support of significant strike-slip movement compression associated with either Tethyan sea-floor has been 2D and 3D seismic data over the Central Graben spreading or rifting in the Bay of Biscay (Oakman and (Bartholomew et al., 1993; Sears et al., 1993; Eggink et al., Partington, 1998). A compressive pulse in the mid- 1996). However, recent interpretations have tended to Aptian was possibly associated with Tethyan closure and emphasise multiple phases of differently orientated dip-slip Alpine orogenesis. This tectonic event has commonly extension (Underhill, 1999; Davies et al., 2001) rather than been referred to as the ‘Austrian Orogeny’, though the oblique-slip. associated unconformity is weakly developed within the The degree to which North Sea extensional tectonism Central and Northern North Sea. The transpressional continued into Early Cretaceous times has also been the pulses are believed to have triggered the halokinesis of subject of debate. In a recent study of the Northern North Zechstein salts within the Central Graben, which exerted Sea, Gabrielsen et al. (2001) commented that the syn- to an additional strong control on patterns of subsidence post-rift transition is unlikely to occur simultaneously and sedimentation (Oakman and Partington, 1998; throughout the entire basin, due to differences in Gatliff et al., 1994). structural configurations and thermal inhomogeneities In contrast to much of the North Sea Basin, the Magnus associated with variable stretching along and transverse Basin in the Northern North Sea experienced strong, down to the basin axis. Over most of the graben system, major to the north-west rifting in Early Cretaceous times, rifting is thought to have ended in the late Volgian associated with opening of the North Atlantic (Rattey and (Rattey and Hayward, 1993). The rifting might have Hayward, 1993). A thick syn-rift wedge of coarse clastics created a bathymetric relief of up to 2 km, which of mainly Valanginian age is developed on the downthrow persisted well into Cretaceous times. However, there side of the so-called ‘End-of-the-World-Fault’ (Nelson and appears to be evidence that tectonic uplift and erosion of Lamy, 1987) at the south-eastern boundary of the Magnus North Sea fault blocks persisted intermittently into Early Basin.

4 Limit to Intra-Aalenian Unconformity caused by thermal doming Basinal areas

Direction of marine flooding Data-poor areas (Structural highs) North Sea Triple Junction N

E Kim Early Kimmeridgian L Ox Late Oxfordian L Aal M Ox Mid-Oxfordian

E Ox Early Oxfordian E Baj L Cal Late Callovian

E/M Cal Early/mid-Callovian L Baj Bath/E Cal Bathonian/early Callovian Mid Bath Mid-Bathonian L Baj Late Bajocian

E Baj Early Bajocian 60˚N L Aal Late Aalenian L Baj E Kim

E Kim

E Ox L Ox

L Cal M Ox L Ox E-M Cal L Ox

Bath/ E Ox E cal

E Kim E M Ox L Ox Mid Bath E Kim

E Kim

L Ox

E Kim E Kim L Aal 55˚

E Kim

50˚

0 200 kilometres

5˚W 0˚ 5˚ 10˚E

Figure 4 Approximate extent of the ‘Mid-Cimmerian Unconformity’ due to transient domal uplift and the pattern of subsequent marine onlap onto the unconformity (after Underhill and Partington, 1993; Underhill, 1998).

5 5a W E EAST SHETLAND EAST SHETLAND BASIN PLATFORM 1 EOCENE AND YOUNGER Basement PALEOCENE 2 LOWER CRETACEOUS F L CRETACEOUS L CRETACEOUS a UPPER CRETACEOUS u lt

z o U JURASSIC 3 n Intra-Triassic reflector e MIDDLE JURASSIC UPPER UPPER JURASSIC JURASSIC 4 AND OLDER MIDDLE JURASSIC AND OLDER Two-way travel time (seconds) travel Two-way 0 10 kilometres

5b W E CENTRAL VIKING GRABEN Sea bed 0

1 EOCENE AND YOUNGER

2 PALEOCENE

UPPER CRETACEOUS 3 DEVONIAN AND OLDER LOWER CRETACEOUS

4 UPPER JURASSIC Two-way travel time (seconds) travel Two-way

? Fault zone LOWER TO MIDDLE JURASSIC 5 TRIASSIC AND OLDER

6 0 5 kilometres

5c W E BERYL EMBAYMENT 0 Sea bed not shown

1 EOCENE AND YOUNGER

2 PALEOCENE

UPPER CRETACEOUS LOWER

Two-way travel time (seconds) travel Two-way CRETACEOUS 3 JURASSIC TRIASSIC JURASSIC

L is Crystalline tr UPPER PERMIAN ic PERMO-TRIASSIC 4 basement fau (approximate position) lt p AND OLDER lan e

0 5 kilometres

Figure 5 Generalised cross-sections across the Central and Northern North Sea; locations are given in Figure 1 (after Andrews et al., 1990; Gatliff et al., 1994 and Johnson et al., 1993).

6 5d(i) N S EAST SHETLAND DUTCH BANK EAST SHETLAND BASIN WITCH GROUND GRABEN HALIBUT HORST PLATFORM BASIN Sea bed 0 THIN U CRETACEOUS EOCENE AND YOUNGER THIN U CRETACEOUS 1 PALEOCENE U CRETACEOUS L CRETACEOUS U JURASSIC 2 DEVONIAN U PERMIAN TRIASSIC AND OLDER 3 PERMO-TRIASSIC CARBONIFEROUS M JURASSIC AND OLDER

Two-way travel time (seconds) travel Two-way 4

5d(ii) NNW SSE

HALIBUT HORST BUCHAN GRABEN PETERHEAD RIDGE Sea bed 0 EOCENE AND YOUNGER

1 U CRETACEOUS L CRETACEOUS PALEOCENE 2

DEVONIAN 3 AND OLDER U JURASSIC TRIASSIC CARBONIFEROUS PERMO-TRIASSIC AND OLDER U PERMIAN

Two-way travel time (seconds) travel Two-way 4 0 20 kilometres

5e(i) N S PETERHEAD RIDGE FORTH APPROACHES BASIN DEVIL'S HOLE HORST 0

2 HIGHLAND BOUNDARY FIRTH OF FORTH FAULT FAULT 4 Depth below sea level (km) sea level Depth below

5e(ii) W E DEVIL'S WEST HOLE WEST CENTRAL SHELF CENTRAL Palaeogene HORST GRABEN 0 and Neogene Upper Cretaceous 2 Lower Cretaceous

4 Jurassic

Triassic 6 Upper Permian Depth below sea level (km) sea level Depth below Pre-Upper 8 Permian

5e(iii) W E WEST CENTRAL GRABENFORTIES-MONTROSE HIGH EAST CENTRAL GRABEN JAEREN HIGH 0

2

4

6 Depth below sea level (km) sea level Depth below

8 0 10 kilometres

Figure 5 Continued.

7 1.2 STRATIGRAPHIC TEMPLATES which often result in prominent high gamma-ray spikes on wireline logs. The flooding surfaces are also important in Within the North Sea Basin, a sound understanding of many oil fields where they commonly form permeability stratigraphy is vital for exploration success, particularly barriers. during the current mature phase of exploration, when the An important stratigraphic template for the North Sea remaining lightly tested plays are subtle and at the limit of Jurassic was proposed by Partington et al. (1993a, b), who seismic resolution (Boldy and Fraser, 1999). Early named each maximum flooding event by reference to the stratigraphic schemes were dominated by lithostratigraphy standard ammonite biozonation scheme. However, recent and a number of contrasting formal national nomenclature papers suggest that it may be preferable to define these events schemes for the North Sea Basin have been proposed (e.g. by their diagnostic dinocyst extinction event (Underhill, 1998; Deegan and Scull, 1977; Vollset and Doré, 1984; Isaksen Veldkamp et al., 1996; Jeremiah and Nicholson, 1999). and Tonstad, 1989; Jensen et al., 1986; Knox and Cordey Within the Jurassic succession, it has commonly proved easier 1992–94). Over the last decade, sequence stratigraphy to recognise these key surfaces, rather than sequence techniques have been applied increasingly to improve boundaries marked by unconformities and their correlative understanding of the temporal and spatial distribution of conformities, as favoured by Vail and his co-workers (e.g. van reservoir units. Wagoner et al., 1990). Consequently, the Jurassic sequence The merits of sequence stratigraphy compared with stratigraphic scheme of Partington et al. (1993a, b) subdivided lithostratigraphy within the North Sea Basin have been the succession into (third-order) genetic sequences in the debated, and several schools of thought exist. Partington et al. sense of Galloway (1989). These genetic sequences are (1993b) advocated the complete abandonment of grouped together into (second-order) tectono-stratigraphic lithostratigraphy in favour of sequence stratigraphy. In cycles that were probably controlled by large-scale tectonic contrast, Price et al. (1993) proposed a lithostratigraphical processes. nomenclature that is based in large part on chronostratigraphy, Largely on the basis of data provided by British Petroleum, with formations and members restricted to narrowly defined Oakman and Partington (1998) outlined a prototype sequence age ranges. However, Veldkamp et al. (1996) suggested that, stratigraphy for the Lower Cretaceous of the North Sea Basin. in addition to sequence stratigraphy, lithostratigraphical This scheme utilises three integral approaches: the nomenclature with little or no chronostratigraphical unconformity (sequence boundary) seismo-stratigraphy connotations (e.g. Richards et al., 1993) is needed for practical techniques of Vail et al. (1984), the maximum-flood (isochron) reasons, such as communication with non-specialists, techniques, typified in Galloway (1989) and the synchronous particularly during operational decision making, when few plate-wide stress models of Cloetingh (1988). Utilising similar biostratigraphical data are available. techniques, Jeremiah (2000) described a sequence Various sequence stratigraphic templates have been stratigraphical framework for the Lower Cretaceous of the proposed and all are calibrated by high-resolution Moray Firth Basin that is broadly compatible with the scheme biostratigraphy. However, some difficulties have been of Oakman and Partington (1998). Jeremiah (2000) encountered regarding inconsistent definition of species commented that the concept of ‘Vailian’ stratigraphy is due to the vintage of analysis and the degradation of difficult to apply in the Lower Cretaceous of the Moray Firth palynomorphs caused by the great depth of burial and high Basin, because the facies are dominated by basinal mudstone thermal maturity (Jeremiah and Nicholson, 1999). units and consequently shoreface progradation/retrogradational A key element of sequence stratigraphic schemes for the stacking patterns cannot be followed. He considered the North Sea Basin has been the recognition of widespread sequences within his scheme to be tectonic sequences with condensed sections of marine mudstone that are interpreted little calibration to eustatic sea-level changes. as essentially isochronous maximum flooding surfaces. Major revisions of genetic sequence stratigraphic The maximum flooding surfaces contain a rich and diverse templates have been carried out recently for the Upper microflora and fauna, and are commonly associated with Jurassic (Fraser et al., 2003) and Lower Cretaceous significant authigenic mineral growth, including glauconite (Copestake et al., 2003) in The Millennium Atlas (Evans and phosphate, and concentrations of radioactive minerals and Graham, 2003).

8 2 Brent and Fladen Groups (‘Middle Jurassic’)

2.1 INTRODUCTION subordinate siltstone, mudstone and coal seams that mainly formed in a transgressive shallow marine setting. The The Middle Jurassic of the Central and Northern North Sea Tarbert sandstone units are markedly time-transgressive contains one of the most significant hydrocarbon reservoirs and reflect a pulsed, southerly directed marine in the North Sea, the Brent Group. Middle Jurassic transgression that eventually drowned the Brent Delta. The hydrocarbon fields in the Beryl Embayment and Inner Tarbert Formation may be separated from the underlying Moray Firth Basin are also significant and, although the play Brent Group formations by an unconformity that reflects is mature, exploration is still active (Husmo et al., 2003). the early stages of rifting (Underhill et al., 1997).

2.2.2 Fladen Group 2.2 LITHOSTRATIGRAPHY The Fladen Group comprises the Pentland, Brora Coal, In the scheme of Richards et al. (1993) the Middle Jurassic Beatrice and Hugin formations. rocks (Figure 6) are assigned essentially to the Brent and The Pentland Formation is Toarcian–mid-Oxfordian in Fladen groups, though, in addition, the lower part of the age, but is commonly only assigned to the Bathonian. The Humber Group is commonly of Middle Jurassic age. The formation is distributed over the Beryl Embayment, South Brent and Fladen groups are themselves divided into their Viking Graben, Central Graben, Outer Moray Firth Basin component formations. Along the coast of the Inner Moray and Unst Basin and consists of up to 1200 m of Firth Basin at Brora are exposed the Brora Argillaceous interbedded sandstone, siltstone, mudstone and coal seams, and Brora Arenaceous formations, which are laterally with locally significant volcanics (tuff, lava and intrusive equivalent to the Fladen Group strata within the offshore rocks) within the Rattray and Ron Volcanics members. The basin. lavas comprise undersaturated porphyritic, alkali olivine The distribution and thickness of the Brent and Fladen basalt (Dixon et al., 1981; Fall et al., 1982). The Pentland groups, together with the well log character of selected Formation is interpreted as paralic to delta plain deposits formations, are summarised on Enclosure 1. and the products of volcanic centres. The Stroma Member of the Pentland Formation is a 2.2.1 Brent Group problematic sequence of mid-Oxfordian sediments that is assigned to the Fladen Group on the basis of lithological The Brent Group makes up the most important hydrocarbon characteristics, but is interpreted to mark the start of the reservoir sequence within the North Sea Basin. The group is Late Jurassic transgression. geographically restricted to the East Shetland Basin and The Stroma Member is recognised in the Outer Moray comprises, in ascending order, the Broom, Rannoch, Etive, Firth Basin and comprises up to 40 m of interbedded Ness, and Tarbert formations. These formations comprise a sandstone, carbonaceous mudstone and coal seams of mid- broadly regressive-transgressive wedge of diachronous Oxfordian age (Richards et al., 1993). It generally rests coastal and shallow marine sediment and reflect the unconformably on much older Pentland Formation outbuilding and retreat of a major wave-dominated delta sediments and volcanics (Rattray Volcanics Member) or largely fed from the south (Budding and Inglin, 1981; Permo-Triassic sediments. The member was deposited in Johnson and Stewart, 1985). paralic, deltaic and lagoonal environments and represents The Broom Formation is of Aalenian age and comprises the drowning of a peneplaned surface during the mid- up to 50 m of marine sandstone and conglomeratic Oxfordian. sandstone with mudstone clasts. The formation is The Brora Coal Formation is of Bajocian to early interpreted as a fan delta deposit (Graue et al., 1987). Callovian age and is geographically restricted to the Inner The Rannoch Formation is of late Aalenian to early Moray Firth Basin. The formation consists of up to 180 m Bajocian age and consists of up to 100 m of upward- of interbedded mudstone, sandy mudstone, sandstone and coarsening mudstone to fine-grained micaceous sandstone. coal seams that are interpreted as alluvial flood plain The formation is interpreted as a marine offshore to middle deposits. shoreface deposit that formed under the influence of storms. The Beatrice Formation is of early to mid-Callovian The Etive Formation is probably of late Aalenian to age and is geographically restricted to the Inner Moray early Bajocian age and comprises up to 40 m of massive, Firth Basin. The formation comprises up to 60 m of clean, cross-bedded sandstone which formed in a barrier- sandstone interbedded with subordinate mudstone that bar/delta-front setting that was transected by channels. formed in marine barrier bar and offshore bar The Ness Formation is probably of Bajocian age and environments. The precise nature of the westward passage comprises up to 180 m of interbedded sandstone, siltstone, from Beatrice Formation facies to the Brora Argillaceous mudstone and coal seams that formed in a delta-top setting. Formation facies (see below) remains uncertain. The Ness Formation is commonly subdivided into three The Hugin Formation is largely of late Bajocian to component parts: a lower interbedded unit, the mid-Ness Bathonian age, but locally may range up to Shale and an upper interbedded unit. Callovian/Oxfordian age. The formation is distributed over the The Tarbert Formation is probably of late Bajocian to Beryl Embayment, South Viking Graben and the Unst Basin Bathonian age. It consists of up to 75 m of sandstone with and comprises up to 300 m of sandstone, siltstone and

9 mudstone with minor coal seams and conglomerate. It is interpreted as having formed in storm influenced coastal barrier to shoreface/offshore settings.

2.2.3 Onshore formations Not exposed The Brora Argillaceous Formation is of early to late Brora section Non deposition/erosion Callovian age and consists of up to 89 m of MORAY FIRTH COAST BALINTORE FORMATION BRORA ARENACEOUS FM BRORA COAL FORMATION

bituminous mudstone, sandy siltstone and muddy and BRORA ARGILLACEOUS FM

(part)

FLADEN GROUP FLADEN silty glauconitic sandstone, that are interpreted to GP HUMBER have formed in a shallow marine environment (Andrews et al. 1990). ? The Brora Arenaceous Formation is of late COAL ? Callovian to early Oxfordian age and comprises more BRORA FORMATION BEATRICE FM Member HEATHER FM than 56 m of bioturbated, muddy sandstone and Inner Moray Firth HUMBER GP (part) Alness Spiculite

yellow, fine-grained sandstone with lenticular Non deposition/erosion conglomerate. These rocks probably represent the GP FLADEN

deposits of migrating bars on a shallow marine shelf GROUP FLADEN

Mbr

Rattray Volcanics Member Volcanics Rattray (Andrews et al., 1990). Stroma ?

HEATHER FORMATION FORMATION PENTLAND 2.3 SEQUENCE STRATIGRAPHY ? Outer Moray Firth CENTRAL NORTH SEA Non deposition/erosion

? Sequence stratigraphy studies within the Central and Member Volcanics Ron

Northern North Sea have divided the Middle Jurassic FM

?

FULMAR ???? into nine genetic stratigraphic units (termed J22, J24, FORMATION PENTLAND

etc) based on the recognition of ten maximum ? Rattray Volcanics Member Volcanics Rattray

flooding surfaces or marine condensed horizons ?

HEATHER FM (Figure 7) (Mitchener et al., 1992; Partington et al., GROUP FLADEN FORMATION Central Graben

1993a, b). Each genetic stratigraphic unit records an FULMAR

individual progradational pulse. A correlation GP(part) HUMBER

Member diagram along the axis of the Viking Graben Volcanics Rattray

(Figure 8) illustrates the relationship between Jurassic ? chronostratigraphy, genetic sequence stratigraphy and (part) ? FM BRAE FORMATION (part) lithostratigraphy and the truncation and onlap patterns Ling Sdst Mbr HEATHER associated with thermal doming and the ‘Mid- ?? FM HUGIN PENTLAND FORMATION

Cimmerian Unconformity’ (Figure 4). South Viking Graben FORMATION HEATHER

2.4 PALAEOGEOGRAPHY ? HUGIN ? Member

Two palaeogeographic reconstructions have been FORMATION PENTLAND FORMATION HEATHER FORMATION Bruce Sandstone

proposed for the Middle Jurassic of the Central and ??

Beryl Embayment (part) Northern North Sea. The more traditional model is that GROUP FLADEN

sediment was shed off an eroding volcanic dome in the GROUP HUMBER

(part)

BRENT GROUP BRENT Central North Sea and that delta systems prograded GROUP HUMBER radially from here along the axes of the proto-Viking (part) GROUP Graben, Moray Firth Basin and the Central Graben. DUNLIN

However, thickness and grain size trends, heavy mineral NORTHERN NORTH SEA analyses, age relations between the sediments and the ETIVE FM (part) RANNOCH FM volcanics, and facies analysis in the Beryl Embayment DRAKE FORMATION HEATHER FORMATION suggest that an additional source for the Middle Jurassic EMERALD FORMATION BROOM FORMATION East Shetland Basin TARBERT FORMATION of the Northern North Sea lay on the adjacent platforms NESS FORMATION and basin margins (e.g. Morton, 1992). From late Toarcian to early Aalenian times, large ? areas of the Central North Sea were affected by ? (part) ? ? transient regional uplift associated with the Central SPEKE HUGIN HEATHER PENTLAND FORMATION

North Sea Dome (Underhill and Partington, FORMATION FORMATION FORMATION

Unst Basin (part) GROUP

FLADEN GROUP FLADEN 1993; 1994). The rise of the dome centre probably HUMBER

continued until Bathonian to early Callovian times, (part) GROUP DUNLIN though deflation of the dome margins may have Brent and Fladen groups (Middle Jurassic) lithostratigraphy (after Richards et al., 1993). commenced in the Aalenian to Bajocian. The regional STAGE AALENIAN BAJOCIAN

CALLOVIAN BATHONIAN

uplift has resulted in a widespread stratigraphic break, OXFORDIAN

JURASSIC

MIDDLE JURASSIC MIDDLE SERIES known as the ‘Mid-Cimmerian Unconformity’, which UPPER is apparent from the North Viking Graben to the Figure 6 Southern North Sea and from the Moray Firth Basin to the Ringkøbing-Fyn High; the subcrop to this

10 S N Tectono- Genetic Absolute Beatrice Beryl Brent Magnus stratigraphic stratigraphic Stage Series age unit unit (Ma)

alva ahna Bajocian J44 Bathonian Callovian J40 Shelf and Turbidite J42 sands basinal mudrocks

J36

J34 J30 160 Shoreline- shoreface J33 Middle Jurassic J32

Coastal J26 plain

J24 170

J20 Non deposition / erosion

J22 Aalenian Fan delta

Lower 180 Toarcian Jurassic

Figure 7 Generalised Middle Jurassic genetic stratigraphical sequences in the Northern and Central North Sea (after Partington et al., 1993b). unconformity is broadly circular to elliptical (Figure 4) Broom Formation fan deltas were shedding material off the (Underhill and Partington, 1993, 1994). Estimates of the Shetland Platform. At the end of the Aalenian, Rannoch amount of topographic relief on the dome have varied Formation shelf deposits transgressed rapidly over most of considerably and range up to 2.5 km (Ziegler and Van this northern area with a mud-prone shelf (Rannoch Hoorn, 1989). However, a relatively low-lying, though mudstone) developing in the extreme north-east. highly variable relief is considered to be a more likely scenario (Underhill, 1998). The pattern of marine onlap onto 2.4.2 Early Bajocian the ‘Mid-Cimmerian Unconformity’ highlights the progressive nature of marine transgression down the During Bajocian times, the sea regressed northwards, incipient Viking and Central grabens and across the Moray taking with it the Rannoch-Etive Formation shoreline and Firth Basin (Figures 4, 8). Erosion of the exhumed Triassic associated shoreface facies belts. By the end of the early to Lower Jurassic sedimentary succession led to a major Bajocian, a coastal plain (lower Ness Formation) covered progradation of fluvial-deltaic clastic successions, including most of the East Shetland Basin. Further south, condensed those of the Brent Group in the East Shetland Basin. The sequences were formed in the Beryl Embayment. Middle Jurassic palaeogeographic/palaeofacies development of the Central and Northern North Sea is summarised in 2.4.3 Late Bajocian Figure 9, which is based on Mitchener et al. (1992). A major late Bajocian flooding event, marking the base of the 2.4.1 Aalenian mid-Ness Shale, represents the first pulse of a forthcoming transgression. Sedimentation then continued much as in the By the end of Early Jurassic times, deposition was restricted early Bajocian with fluvial sedimentation (Ness Formation) to the East Shetland Basin (Figures 1, 8). During the over much of the East Shetland Basin and deposition of the Aalenian, large areas of the Central North Sea experienced Pentland Formation resuming in the Beryl Embayment. Non- domal uplift. Coastal plain sedimentation (lower Pentland marine sedimentation of the Pentland Formation may have Formation) began in the South Viking Graben and Beryl started in the Outer Moray Firth Basin and Central Graben by Embayment and further north, in the East Shetland Basin, this time. It was possibly from mid-Bajocian times that the

11 S N UK QUADRANT 22 UK QUADRANT 16 UK QUADRANT 9 UK QUADRANT 211

KIMMERIDGE CLAY FORMATION JURASSIC LATE

FULMAR HEATHER FORMATION SST FM

PENTLAND HUGIN FM Transgression FORMATION JURASSIC MID - TARBERT FORMATION

Rattray Volcanics NESS FORMATION

Member R e

AL BAJ BATH CAL OX KIM g re ss ion JURASSIC TOA DUNLIN GROUP EARLY MID-CIMMERIAN UNCONFORMITY

NANSEN FORMATION n o i s s

HETT PL ERICKSSON AND RAUDE FORMATIONS e EARLY JURASSIC MID - JURASSIC LATE JURASSIC r

g

s

n

a

r

TRIAS T TRIASSIC HERON GROUP 0 100 kilometres PRE- PRE- TRIAS TRIASSIC

Marine condensed Non-marine/ Shallow marine Shoreline attached Non deposition/ Marine shales horizon/ maximum paralic sediments sandstones sequence boundary erosion flooding surface

Figure 8 Generalised correlation of Jurassic chronostratigraphy, genetic sequence stratigraphy and lithostratigraphy along the axis of the Viking Graben (after Underhill and Partington, 1993).

Hugin Formation transgressed south from the Beryl volcanic centres in the Central North Sea (Figures 9, 10) Embayment to the northernmost South Viking Graben and that are assigned to the Forties Volcanic Province. Three of also into the Unst Basin. these centres (whose volcanic deposits are assigned to the Rattray Volcanics Member) occur at the North Sea rift triple junction (Figure 10). A smaller volcanic centre, 2.4.4 Bathonian whose deposits form the Ron Volcanics Member, exists to A transgression brought marine conditions into the East the south on the western margin of the West Central Shetland Basin and Beryl Embayment and sandstone units Graben. The volcanic centres comprise piles of extrusive, of the Hugin Formation continued to transgress southwards. mildly undersaturated, porphyritic alkali basalt Coastal plain deposits (Pentland Formation) extended south (ankaramite) with associated hawaiite and mugearite that into the southern Viking Graben and Central North Sea. In reach up to 1500 m in thickness. The lavas were extruded parts of the Outer Moray Firth Basin and Central Graben, onto land (they are lateritised) and are associated with volcanic centres were active. Paralic sedimentation (Brora volcaniclastic sediments on the flanks of the vents. Coal Formation) began in the Inner Moray Firth Basin. By Distally, these strata are interbedded with fluvio-deltaic the end of Bathonian times, marine mudstone (Heather and paralic sediments. Formation, Humber Group) had transgressed into the East Dating of the volcanism has been a contentious issue. Shetland Basin and Beryl Embayment. Radiometric age dates led Smith and Ritchie (1993) to invoke a complex history of igneous intrusion and local uplift spanning Aalenian to Callovian times. However, 2.4.5 Mid-Callovian Underhill (1998) considered it premature to rely on By earliest Callovian times, marine mudstone deposition radiometric ages without supporting evidence from (Heather Formation) covered much of the Northern North volcanic outfall and suggested that the stratigraphical Sea. The belt of shallow marine sands was restricted to the evidence implies a Bathonian to Callovian age (from southern Viking Graben (Hugin Formation), the Inner Moray approximately 160 to 150 Ma) for the volcanism. The Firth Basin (Beatrice Formation) and the northern Central timing of Mid Jurassic volcanism relative to Late Jurassic Graben (lowermost Fulmar Formation, Humber Group). Non- rifting is consistent with a model of active rifting marine sedimentation (Pentland Formation) continued across associated with a mantle plume (Houseman and England, much of the Central Graben and Outer Moray Firth Basin. 1986), rather than a passive rifting model which requires Later, marine mudstone deposition (Heather Formation) coeval stretching and volcanism (e.g. McKenzie, 1978). spread further south into the southern Viking Graben and Inner Moray Firth Basin. Some turbiditic sands of latest Callovian age were deposited in the southern Beryl 2.5 DEPOSITIONAL ENVIRONMENTS AND Embayment and northern Viking Graben — forerunners of RESERVOIRS the processes that were going to dominate in the Late Jurassic. Middle Jurassic reservoirs commonly take the form of delta systems that prograded radially up the proto-graben 2.4.6 Volcanic centres arms from the North Sea rift triple junction, though some Smith and Ritchie (1993) used well data, seismic reflection deltaic sediments also prograded east from the Shetland profiles and potential field data to identify four Jurassic Platform to form part of the Brent Delta (Figure 11).

12 1 62ºN 2 62ºN 3 62ºN ? ? ? AALENIAN EARLY BAJOCIAN LATE BAJOCIAN

60º 60º 60º ?

?

58º 58º 58º

?

56º 56º 56º 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE

Mud-prone shelf 4 62ºN 5 62ºN ? ? BATHONIAN MID-CALLOVIAN Lower shoreface

Shoreline-shoreface

Coastal plain 60º 60º

Volcanic activity

Low-lying land

? ? 58º 58º NB. Bars of colour indicate mixed facies ? ? Direction of sediment transport

? ?

56º 56º 0 200 kilometres 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE

Figure 9 Mid Jurassic palaeogeography (after Mitchener et al., 1992).

Environmental interpretations for the Brent and Fladen Carbonate cements can be detrimental to permeability when groups are shown in Figure 12. forming concretions or cemented horizons. Fibrous illite, The Brent Group comprises one of the principal reservoir the product of vermicular kaolinite diagenesis below units within the North Sea. The initial porosity of Brent 3200 m, can have a negative effect on reservoir properties, Group sandstones might have been 35% to 50%, but this especially permeability. Secondary porosity, generated was significantly reduced during burial by a combination of during diagenesis by the dissolution of feldspar and compaction and cementation (Figure 13) (Morton et al., carbonate, is a further complication, but is typically 1992). Net reduction in porosity is normally 2.5 to 3.2% for associated with the nearby precipitation of authigenic each 330 m of burial, but anomalies are common. For quartz and kaolinite/illite such that in general the net example, over-pressure in the Middle Jurassic of the Viking porosity of a reservoir is not increased, rather redistributed. Graben is probably a major factor in inhibiting porosity The sequence of diagenetic events is often comparable reduction due to burial diagenesis. In addition, there is between reservoirs irrespective of the original lithofacies debate as to whether the early emplacement of and geographical location. In general, early diagenesis hydrocarbons into the pores suspended diagenesis. Several reflects changes in porewater chemistry at shallow depths studies do not support this theory and suggest that illite whilst later the reactions are largely isochemical and continues to grow preferentially below an oil-water contact. controlled by deeper burial and increasing temperature. The major pore-filling diagenetic phases within the Brent In the Outer Moray Firth Basin, paralic sandstone units Group comprise carbonate, kaolinite (vermicular and of the mid-Oxfordian Stroma Member (Pentland blocky forms), illite and authigenic quartz; these all have an Formation, Fladen Group) locally form secondary important effect on reservoir evaluation and performance. hydrocarbon reservoirs (e.g. parts of the Piper Field).

13 0˚ 1˚E Middle Jurassic succession are a potential source of dry 15 16 gas and might be thermally mature within the deeper parts of the graben areas. Hydrocarbons sourced from the Middle Jurassic are not thought to contribute significantly to the oil and gas accumulations in the Northern North Sea. However, oil found in the Beatrice Field in the Inner Moray Firth Basin has a complex origin. In addition to a Devonian component,

Ivanhoe NORWAY centre it appears that the oil-prone, algal-rich mudstones and coals UK of the Brora Coal Formation have also made a contribution (see Cornford, 1998 and references therein).

58˚N Glenn 2.7 TRAPS centre Fisher Bank Oil and gas fields associated with the pre-rift, Middle centre Jurassic tilted fault blocks are some of the most productive in the North Sea. However, ‘creaming curves’ suggest that the exploration play is very mature, with most remaining potential in hanging-wall closures Volcanic rocks 0 20 kilometres (Johnson and Fisher, 1998; Brooks et al., 2002). Middle 21 22 Jurassic oil and gas fields occur in three geographical areas (Figure 2): (a) the East Shetland Basin (or ‘Brent Province’) which relies on the near omnipresence of Figure 10 Simplified map of the Rattray Volcanics reservoir quality sandstones in the Brent Group; (b) in the Member of the Forties Volcanic Province; numbers refer ‘Beryl Embayment Province’ where the Hugin and to UK quadrants (after Smith and Ritchie, 1993). Pentland formations form the reservoirs and (c) in the Inner Moray Firth Basin (e.g. Beatrice Field) where Fladen Group and underlying Lower Jurassic sediments 2.6 SOURCE ROCKS act as reservoirs.

The vast majority of oil contained within Middle Jurassic 2.7.1 East Shetland Basin sediments of the North Sea was sourced from the prolific Upper Jurassic Kimmeridge Clay Formation. The coals Oilfields in the East Shetland Basin comprise a variety of and mudstones that locally form a high proportion of the tilted fault blocks with two- or three-way dip closures. The

DISTRIBUTARY MOUTH

NNE

BEACH RIDGE PLAIN

INLET CHANNEL

LAIN LAGOON

LTA DELTA P

TIDAL DE ORMATION RANNOCH FORM E T IV E

FO DUNLIN GR R M A T LOWER NESS F 0 ATION IO OUP N

1 kilometre

BROOM FORMATION

Figure 11 Depositional model for the Rannoch, Etive and Lower Ness formations (after Budding and Inglin, 1981).

14 FLADEN GROUP stratigraphical truncation of the reservoir below an unconformity forms the updip seal (e.g. Brent, Ninian and FORMATION ENVIRONMENT Statfjord fields). Shallow marine, deepening-upwards, transgressive In general, the larger the throw of the principal bounding Upper sequence of basal lags and washovers passing up to shoreface and/or offshore facies with storm influences fault, the greater the amount of footwall uplift and the Hugin Formation larger the amount of erosion over the crest of the trap. Transgression followed by shallowing-upwards, Lower regressive sequence of mass-flow, up through Fault blocks within the East Shetland Basin typically show shoreface, tidal delta, barrier inlet and lagoonal facies up to a few hundred metres of erosion from their crests. Paralic Paralic environment with sub-tidal, inter-tidal However, in some cases, the trapping mechanism did not succession and salt marsh sub-environments form simply by tilting followed by marine transgression. Pentland Lower coal marker Swamp Formation Rather, the fault scarp, being composed of poorly Shallowing-upwards sequence, with tidal Interbedded unit influences and lagoonal or sub- to inter-tidal mud consolidated sand and shale, collapsed into the deeper flat environments water of the adjacent half-graben (Figure 15) (Underhill et al., 1997; McLeod and Underhill, 1999). This process took place by rotation of slide blocks or the complete reworking BRENT GROUP of sediments. FORMATION ENVIRONMENT PHASE In some instances, such as in the Don Field, oil is Tarbert Formation Transgressive sheet sand and shoreface facies trapped within Middle Jurassic fault hanging wall blocks. Fluvial-dominated delta top and coastal Abandonment phase The principal faults of the Don Field are sealed by clay plain environments. In the north a barrier and southerly retreat of smearing across a sand-sand contact, without which the oil developed delta (possibly pulsed) would have continued its migration into higher structures. Ness Formation Open lagoon Marginal marine, back barrier environments. Main facies are back barrier, stagnant swamp and lacustrine. Some marine 2.7.2 Beryl Embayment influence. Barrier-bar complex with effects of wave/ Progradation phase storm activity dominant. Development of The Beryl and Bruce oilfields are complex, fault-bounded Etive Formation of delta with northward nearshore bar and trough systems with progradation of associated rip channels. shoreline structures. The Beryl Field (UK Block 9/13) is compartmentalised by sealing faults into two main parts: a Rannoch Formation Lower to middle shoreface sequence, probably storm dominated westward-tilted fault block and a north-north-east-oriented Reactivation of marginal horst. The component parts of the Bruce Field (UK Blocks Broom Formation Locally sourced fan-delta deposition fault systems (and rift marginal uplift) 9/8 and 9/9) are associated with a major, relatively shallowly dipping fault: a west-dipping, rotated fault block, a graben and a high flanking the Viking Graben (Figure 16). Figure 12 Depositional setting of the Brent and Fladen groups. 2.7.3 Inner Moray Firth Basin The Beatrice Field (UK Block 11/30) is a relatively simple traps vary in structural complexity, dip angle (typically north-east-trending, tilted fault block that formed as a about 5 degrees, rarely more than 10 degrees) and the result of Upper Jurassic rifting and remained unbreached amount of erosion along their crests. They thus range from despite Palaeogene and Neogene transpression along the pure fault traps where the updip seal is entirely faulted, nearby Great Glen Fault (Thomson and Underhill, 1993; such as the Murchison Field (Figure 14) to those where Davies et al., 2001).

15 Jurassic Cretaceous Palaeogene and Neogene

LowerMiddle UpperLower Upper Pal Eocene OligMiocene Pl

Brent burial curve

Early carbonate cement 40ºC 1 compaction

60ºC Depth in kilometres

80ºC 2

Aut 100ºC higenic qua lllit F 3 e e growth Ferroan carbonatesld (Ank, Dol, Sid, sp rtz a r d is so lu tio n etc) 120ºC 4

Figure 13 Summary of diagenetic and burial history of the Brent Group; line thickness indicates relative importance of the diagenetic process (after Giles et al., 1992).

NW SE

Heather Formation 2900 Cretaceous

3000 p Kimmeridge Clay Formation Brent Grou Oil/water contact

3100

3200 Dunlin Group

Depth below sea level (metres) level sea below Depth 3300

0 1 kilometre Oil/water contact 3400

Figure 14 Structure of the Murchison Oilfield (after Warrender, 1991).

16 W E

2500 Cretaceous

Gas/oil contact up Reworked Humber Gro sediment Brent Group Oil/water contact

Dunlin Group

3000

Depth below sea level (metres) Statfjord Formation

Cormorant Formation

3300 0 1 kilometre

Figure 15 Structure of the Brent Oilfield (after Struijk and Green, 1991).

WNW ESE West Flank Central Panel East High Viking Graben

2 Palaeogene and Neogene

Cretaceous

Jurassic 3 Jurassic Top Permo-Triassic reflector

Deep Two-way time (seconds) Two-way reflector Permo-Triassic and older 0 2 kilometres 4

Figure 16 Structure of the Bruce Oilfield (after Beckly et al., 1993).

17 3 Humber Group (‘Upper Jurassic’)

3.1 INTRODUCTION thickly developed along the western, fault-bounded margin of the South Viking Graben. The Brae Formation consists The Late Jurassic represents a crucial period during which of over 760 m of sandstone, conglomerate, mud-matrix- the most important North Sea source rock, the Kimmeridge supported breccia and interbedded mudstone. The Clay, was deposited. During this time sandstones with formation was deposited by a variety of gravity-flow reservoir potential were also laid down in both shallow and processes in overlapping, partly channelised, submarine fan deep water settings and tectonic activity gave rise to systems (Turner et al., 1987; Garland, 1993; Cherry, 1993). structures that formed abundant traps both within the The Piper Formation is late Oxfordian to Upper Jurassic and also in older rocks. Kimmeridgian in age (Richards et al., 1993) and is widely distributed across the Outer Moray Firth Basin. The Piper Formation consists of up to 300 m of fine- to coarse- 3.2 LITHOSTRATIGRAPHY grained sandstone with interbedded marine mudstone and accumulated from several progradational and In the UK Central and Northern North Sea, the Humber retrogradational phases of a wave-dominated delta (Harker Group incorporates most of the Upper Jurassic (Figure 17) et al., 1993). Two major depositional cycles are (Richards et al., 1993), though some Upper Jurassic recognised, and these equate to the Pibroch and Chanter sediments in the Outer Moray Firth Basin are assigned to members. In many wells, the Piper lithofacies are arranged the Fladen Group, as discussed in the previous chapter. in large-scale upward-coarsening subcycles up to 100 m Two regionally extensive units of marine mudstone are thick, with overall upward-decreasing gamma-ray profiles. recognised within the Humber Group and are known as the The Fulmar Formation is Callovian to late Ryazanian in Heather Formation and the Kimmeridge Clay Formation. age and is widespread in the UK Central North Sea, but Localised bodies of mass-flow coarse clastics enclosed displays complex patterns of distribution and thickness that within these mudstone formations are typically given were influenced by penecontemporaneous rifting, halokinesis member status (e.g. Magnus Sandstone Member); however, and salt dissolution. The Fulmar Formation comprises up to the Brae clastics in the South Viking Graben comprise a 365 m of fine- to medium-grained, generally massive, widespread and thick unit of basinal marine mass-flow bioturbated sandstone. The Fulmar Formation commonly deposits that interdigitates with both the Heather and includes large-scale upward-coarsening and upward-fining Kimmeridge Clay formations, and are assigned formation successions and was deposited in shallow marine, low to status. Within the Central North Sea, the Heather and moderately high-energy, storm-influenced environments. Kimmeridge Clay formations pass laterally into two major The Emerald Formation is of late Bathonian to early units of shallow marine sandstone known as the Piper and Oxfordian age, but is predominantly Callovian. The Fulmar formations. The distribution and thickness of the formation is geographically restricted to the transitional Humber Group, together with the well log character of shelf to the East Shetland Basin and comprises up to 30 m selected units are summarised on Enclosure 2. of sandstone and subordinate siltstone, commonly with basal conglomerate. It is interpreted as a transgressive 3.2.1 Humber Group sheet sand that formed in a nearshore to offshore setting. Heather Formation The is Bathonian to latest Oxfordian 3.2.2 Onshore formations in age and is therefore partly of Mid Jurassic age. The formation is distributed widely across the Central and Upper Jurassic rocks are exposed on the Moray Firth coast Northern North Sea and consists of up to 700 m of marine and equate to strata within the Humber Group. mudstone with sporadic thin stringers or concretions of The upper part of the Brora Arenaceous Formation is limestone and localised bodies of mass-flow sandstone early Oxfordian in age. It comprises bioturbated, muddy (e.g. Bruce, Freshney and Ling sandstone members), sandstone and yellow, fine-grained sandstone with lenticular shallow marine spiculitic sandstone (e.g. Alness Spiculite conglomerate. These strata probably represent migrating Member) and paralic mudstone (e.g. Gorse Member). bars on a shallow marine shelf (Andrews et al., 1990). Although it is commonly perceived as representing shelf The Balintore Formation is mid-Oxfordian in age, and facies, the Heather Formation also includes mass-flow conformably overlies the Brora Arenaceous Formation. It sandstones of slope or basin association. Bottom waters comprises up to 10 m of muddy calcareous sandstone in during deposition of the formation were generally aerobic. which calcitised spicules are common, glauconitic The widespread Kimmeridge Clay Formation is sandstone and limestone. These rocks are interpreted to Kimmeridgian to late Ryazanian in age. It comprises up to have been deposited within inshore marine environments 1400 m of moderately to highly organic-rich, marine (Lam and Porter, 1977). mudstone with local bodies of mass-flow sandstone (e.g. The Kintradwell Boulder Beds are early Kimmeridgian in Burns, Claymore, Magnus and Ribble sandstone members). age and comprise up to 60 m of thin-bedded sandstone, The formation was deposited mainly in a basinal setting siltstone, shale and conglomerate deposited in a slightly where bottom waters were anoxic (Miller, 1990). restricted marine environment (Wignall and Pickering, 1993). The Brae Formation is mainly Kimmeridgian to mid- The Allt na Cuille Formation is mid-Kimmeridgian in age Volgian in age, but possibly ranges into the Oxfordian. It is and up to 122 m in thickness, comprising laminated and

18 massive sandstone and siltstone deposited in a submarine canyon environment (Wignall and Pickering, 1993). The Helmsdale Boulder Beds are mid-Kimmeridgian to mid-Volgian in age and comprise up to 530 m of conglomerate, sedimentary breccia and sandstone. These Not exposed Not exposed rocks were probably deposited as submarine gravity HELMSDALE BOULDER BEDS Section at Helmsdale flows and screes. NA CUILLE FM# and (ALLT BALINTORE FORMATION BALINTORE

BRORA ARENACEOUS FM ARENACEOUS BRORA FORMATION COAL BRORA BRORA ARGILLACEOUS FM ARGILLACEOUS BRORA KINTRADWELL BOULDER BEDS) KINTRADWELL FLADEN GROUP FLADEN 3.3 SEQUENCE STRATIGRAPHY CLAY FM FM Member

Four tectono-stratigraphic sequences and thirteen COAL BRORA FORMATION KIMMERIDGE BEATRICE

Alness Spiculite genetic stratigraphic sequences have been recognised GROUP Member

within the Upper Jurassic by Rattey and Hayward KNOLL CROMER Burns Sandstone Burns Inner Moray Firth Inner Moray

(1993) and Partington et al. (1993a, b) (Figure 18). FORMATION HEATHER Harker and Rieuf (1996) applied sequence stratigraphic techniques to subdivide the Humber Group of the ?

Moray Firth Basin into eight genetic units. In a Sst Mbr Claymore Mbr Stroma contrasting (Vail-type) approach based on the PIPER FM Chanter Mbr

GROUP

FLADEN ? PENTLAND GROUP CLAY FM CLAY Mbr recognition of unconformities, Donovan et al. (1993) FORMATION Dirk Sst Member KIMMERIDGE

developed a depositional sequence stratigraphic Pitbroch

FM CROMER KNOLL CROMER Burns Sandstone Mbr Sandstone Burns FLADEN GROUP FLADEN Outer Moray Firth Outer Moray CENTRAL NORTH SEACENTRAL NORTH COAST FIRTH MORAY template to understand better the spatial and temporal Gorse Mbr HEATHER distribution of Upper Jurassic shallow marine sandstone reservoirs (e.g. Fulmar Formation). These widespread sandstone units display marked physical similarities, ?

but vary widely in age (e.g. Donovan et al., 1993; Price HEATHER FORMATION PENTLAND et al., 1993). However, Jeremiah and Nicholson (1999) FORMATION Member GROUP Sst Mbr FULMAR Freshney noted that aspects of the various stratigraphic templates FORMATION Ribble Sandstone Ribble CROMER KNOLL CROMER Central Graben FORMATION

might suffer from incorrect age allocations for a KIMMERIDGE CLAY number of their sequence boundaries. Recently, Fraser NS et al. (2003) have developed the Late Jurassic scheme of Partington et al. (1993b) and applied a subdivision FM into seven genetic sequences (A to E) based on region- CLAY PENTLAND FORMATION KIMMERIDGE wide maximum flooding surfaces (Figure 18). ? Ling Sandstone Member GROUP Birch Sst Member HUGIN BRAE CROMER KNOLL CROMER FORMATION FORMATION HEATHER FORMATION HEATHER 3.4 PALAEOGEOGRAPHY South Viking Graben

The Late Jurassic palaeogeographic/palaeofacies development of the Central and Northern North Sea is FM summarised in Figure 19. Within the Central and PENTLAND Member Northern North Sea, repeated rises of relative sea level GROUP FORMATION HUGIN FM BRAE FORMATION CROMER KNOLL CROMER FLADEN GROUP

during the Late Jurassic reflect the interplay of active KIMMERIDGE CLAY Beryl Embayment Bruce Sandstone rifting and eustatic sea level changes. Broadly, the series FORMATION HEATHER of marine transgressions progressively drowned basin margins and resulted in the expansion of basinal facies and an associated outward shift of paralic and shallow

marine facies belts (Rattey and Hayward, 1993). SEA NORTH NORTHERN Member Member Magnus Sandstone Magnus BRENT GROUP 3.4.1 Early to mid-Oxfordian Ptarmigan Sandstone FORMATION

In the Central and Northern North Sea, early to mid- KIMMERIDGE CLAY TARBERT FORMATION TARBERT HEATHER FORMATION HEATHER EMERALD FORMATION East Shetland Basin Oxfordian subsidence was confined to the developing KNOLL GROUP CROMER rifts (Figure 19). The marine transgression initiated in the Mid Jurassic continued and shelf and basinal mudstones (Heather Formation) accumulated in much ?

of the Inner Moray Firth Basin, Viking Graben and East CLAY HUGIN HEATHER

FORMATION

FORMATION FORMATION

KIMMERIDGE

Shetland Basin, and for the first time extended into the GROUP GROUP

Unst Basin GROUP HUMBER East Central Graben (Rattey and Hayward, 1993). The FLADEN CROMER KNOLL CROMER basinal facies usually comprises condensed mudstone Humber Group (Upper Jurassic) lithostratigraphy (after Richards et al. 1993; Andrews 1990; Wignall and Pickering, 1993). sequences, although some gravity-flow sandstones were deposited in the South Viking Graben and Beryl Non deposition/erosion GIAN STAGE

VOLGIAN CALLOVIAN RYAZANIAN BATHONIAN

OXFORDIAN VALANGINIAN KIMMERID-

Embayment (e.g. Bruce and Ling Sandstones). ACEOUS

PERIOD JURASSIC UPPER JURASSIC MIDDLE Widespread shelf sands accumulated marginal to the CRET- Heather mudstone in the Central Graben (Fulmar Figure 17 Formation) and in the Inner Moray Firth Basin (Alness Spiculite Member). In the Outer Moray Firth Basin and

19 CHRONO- MAXIMUM STRATIGRAPHY TSU GSS(P) GSS(F)FLOODING SURFACE CENTRAL GRABEN MORAY FIRTH VIKING GRABEN K10 Stenomphalus L J78 MFS RYAZANIAN J76 Kochii E MFS

J74 E

Preplicomphalus L MFS

J70 J73

Anguiformis VOLGIAN TEMFS

J72 E D Okusensis MFS

J71

Filtoni MFS

L J66 C2 Huddlestoni MFS

M J64

J60 Autissiodorensis C1 MFS KIMMERIDGIAN J63

Eudoxus TEMFS

E J62 B2 Baylei MFS J56

B1 Rosenkrantzi MFS L

J50 J54

OXFORDIAN Glosense MFS

M J52 A Densiplicatum MFS E J40 J46

Fan aprons and Coastal Plain Shelf sands Shelf muds Basinal muds Ravinement (erosion) basin floor fans

TSU Tectono-stratigraphic unit GSS(P) Genetic Stratigraphic Sequence (Partington et al. 1993a) GSS(F) Genetic Stratigraphic Sequence (Fraser et al. 2003) MFS Maximum flooding surface TEMFS Tectonically enhanced maximum flooding surface

Figure 18 Genetic stratigraphy template for the Late Jurassic of the UK Central and Northern North Sea (after Partington et al., 1993b and Fraser et al., 2003).

South Halibut Basin, the paralic facies of the Stroma (Figure 19). Consequently, deposition of mudstones of shelf Member (Pentland Formation) commonly overstepped pre- and basinal facies became more widespread (Rattey and Jurassic strata and mark the start of the Late Jurassic Hayward, 1993). Deposition of the Brae Formation gravity- transgression (Richards et al., 1993). flow clastics may have commenced in the late Oxfordian, or even earlier, in association with the rifting. In the Outer 3.4.2 Mid-Oxfordian to early Kimmeridgian Moray Firth Basin and marginal areas of the Central Graben, however, deposition kept pace with subsidence with the There was an increase in the amount of rifting during mid- accumulation of expanded, upward-coarsening cycles of Oxfordian to early Kimmeridgian times and over much of wave-dominated delta sands (Piper Formation: Scott the graben system subsidence outpaced sediment supply Member) and shallow marine sands (Fulmar Formation).

20 1 62ºN ? 2 62ºN EARLY TO MID-OXFORDIAN ?? ? MID-OXFORDIAN ? TO EARLY ? KIMMERIDGIAN

? ?

0 100 kilometres 60º 60º

58º 58º

56º 56º 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE

3 62ºN ? 4 62ºN EARLY ? ? MID-VOLGIAN ? KIMMERIDGIAN TO LATE ? TO MID-VOLGIAN RYAZANIAN

0 100 kilometres 60º 60º

58º 58º

56º 56º 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE

Syndepositional fault Low-lying land Near shore and shelf Basin with crossmark on downthrow side

Coastal plain Slope Submarine high

Figure 19 Upper Jurassic palaeogeography (after Rattey and Hayward, 1993).

21 3.4.3 Early Kimmeridgian to mid-Volgian HUMBER GROUP During the early Kimmeridgian to mid-Volgian, rifting continued to exert a major control on sedimentation, and Kimmeridge Clay Moderately deep water, restricted marine Formation environment with local basin floor fans prominent wedges of syntectonic clastics accumulated in fault hanging wall basins (Figure 19). Locally, these syn- Brae Formation Apron fringe and basin floor fans rift deposits include sand derived from the uplifted and eroded fault footwall blocks. The South Viking Graben Wave-dominated delta with several progradational Piper Formation formed a deep, narrow, half graben, bounded on the west and retrogradational phases by an uplifted and subaerially eroded Fladen Ground Spur. This half graben was infilled by thick gravity-flow deposits Fulmar Formation Offshore shelf with storm influence (Brae Formation) which pass basinward into relatively Emerald Formation Nearshore to offshore transgressive shelf condensed pelagic mudstones. Transfer faults or relay Low-energy, open marine shelf with local slope ramps associated with rift development strongly influenced Heather Formation channels and basin floor fans the position of feeder channels for the Brae clastics (e.g. Cherry, 1993). In the East Shetland Basin and Outer Moray Firth Basin, gravity-flow deposits of the Magnus and Figure 20 Depositional setting of the Humber Group. Claymore sandstone members, respectively, were deposited during this tectonically active phase (Enclosure 2). In the Central North Sea, rifting was Wakefield et al., 1993). On the western shelf to the Central associated with halokinesis and the amount of footwall Graben, reservoir quality within the Fulmar Formation is uplift was commonly insufficient to cause emergence and commonly determined by mainly primary depositional erosion. Consequently, associated basin floor mass-flow controls, of which the intensity and frequency of physical deposits (e.g. Ribble Sandstone Member) appear to be less reworking and the palaeobathymetric setting are the most widespread. important (Johnson and Fisher, 1998). For example, rapidly deposited, bioturbated sands have permeabilities an order of 3.4.4 Mid-Volgian to late Ryazanian magnitude higher than bioturbated sands (Veldkamp et al., 1996). In the Central Graben, a number of factors, such as Active faulting significantly decreased at the beginning of primary mineralogy (e.g. high feldspar content), burial mid-Volgian times and a general deepening led to depth, proximity to fault planes and the timing of progressive drowning of fault footwalls (Figure 19). In the hydrocarbon entrapment have influenced diagenesis. East Shetland Basin and Outer Moray Firth Basin, the start Because of the large number of controlling factors, reservoir of thermally controlled subsidence was marked by quality in the graben remains poorly understood (Veldkamp cessation of Magnus Sandstone and Claymore Sandstone et al., 1996). deposition, respectively. By late Ryazanian times, all but The Fulmar Formation forms the principal reservoir in the the highest footwalls of faults had been drowned. Although high pressure/high temperature (HP/HT) Elgin and Franklin some deep-water sands (the Burns, Birch and Dirk fields within the Central Graben. In these deeply buried sandstone members) were deposited, this was generally a reservoirs (more than 5 km subsea) there is a significant phase when subsidence exceeded sediment supply and amount of secondary porosity developed and, together with basins were starved of coarse clastic material. the extreme overpressure (in excess of 500 bars) and residual stable grain mineralogy, this has resulted in the preservation of high quality reservoirs (Lasocki et al., 1999). 3.5 DEPOSITIONAL ENVIRONMENTS AND The average porosity of the reservoirs is about 16%, but RESERVOIRS reservoir properties of up to 30% porosity and permeability of over 2 darcies are reported. Early compaction effects The interpreted genetic environments of deposition for were lessened by the early growth of authigenic minerals each formation within the Humber Group of the UK such as microcrystalline quartz and dolomite. However, the Central and Northern North Sea are tabulated in Figure 20. most significant diagenetic event in the burial history of the In the Moray Firth Basin, Viking Graben and East reservoirs was the development of secondary porosity (up to Shetland Basin, both shallow marine/deltaic and basinal half the observed porosity) resulting from the dissolution of facies contain important Humber Group reservoirs. In feldspar, sponge spicules and shell debris. The precise contrast, shallow marine strata currently provide the principal timing of this secondary porosity development is not well reservoirs in the Humber Group of the Central Graben. constrained. It began before the development of high However, Brooks et al. (2002) noted that recent wells in the overpressures and may have continued after the Central Graben have extended the play fairway for thick establishment of the ‘closed system’. The development of Upper Jurassic syn-rift basin-floor sandstones into that area. increasingly high overpressure within the reservoir has In the Central Graben area, reservoir sandstone units of helped to minimise further compactional effects. the Callovian to late Ryazanian Fulmar Formation were Thicker and better quality reservoir sandstone units of the deposited in shallow marine, low- to moderately high- late Oxfordian to mid-Kimmeridgian Piper Formation energy, storm, influenced, nearshore to offshore settings succeed them. The Piper sands accumulated during two (Figure 21) (Gowland, 1996; Johnson et al., 1986; Donovan major regressive cycles of a wave-dominated delta system et al., 1993; Clark et al., 1993). The Fulmar sands are that prograded from the Fladen Ground Spur and the Halibut commonly bioturbated (e.g. Martin and Pollard, 1996), and Horst (Harker et al., 1993). These major cycles correspond are locally cross-bedded. Large-scale, upward-coarsening to the Pibroch and Chanter members. Both of these members cycles are displayed in some sections and reflect rapid fault- comprise several vertically stacked, large scale upward- and related halokinetically-controlled subsidence during coarsening sub-cycles. The Piper Formation generally forms cycles of shelf progradation (Rattey and Hayward, 1993; an excellent reservoir, though reservoir quality can be

22 NW

C E N T R A L

G R A B E N

Active salt structure

Salt withdrawal facilitating major subsistence SE

Coastal plain fed by fluvial system; Nearshore clastic Upper Permian thin coal development in areas of Low-lying land sedimentation Salt clastic starvation Marine sands transported Siliceous sponge build- Triassic and Progradational shoreface from shelf through intra basinal ups in open marine Middle Jurassic relay ramp (FULMAR FORMATION) environment

Figure 21 Depositional model for the Fulmar Formation (after Gowland, 1996). variable. In the Piper Field, the Piper sands are grain Rattey and Hayward (1993) recognised two main types of supported and retain porosity values of 20–28% and average Upper Jurassic deep-marine fans in the North Sea Basin: permeability values of 4 darcies (Maher, 1981). However, in apron-fringe fans and basin-floor fans. The apron-fringe fans the Tartan Field (Coward et al., 1991), the formation are characterised by their small radii (less than 10 km) and exhibits markedly different petrophysical properties in each thick conglomerate facies, and form much of the Brae block, with lower porosity in the ‘downthrown block’ due to Formation in the South Viking Graben (Turner et al., 1987). cementation and an intense compaction fabric. McCants and This formation largely comprises thick units of unorganised, Burley (1996) note that in the Lowlander Prospect in UK sand-matrix conglomerate and mud-matrix breccia with Licence Block 14/20b, reservoir quality is poor with an interbedded units of sandstone and mudstone. Oil is average porosity of 9.4% and permeability of 55 commonly held within the matrix of the conglomerates. millidarcies, because mechanical compaction and diagenetic Proximal conglomerate and breccia occur immediately cementation affect the Piper sandstones. However, total adjacent to the western faulted margin of the South Viking porosities are enhanced by feldspar grain dissolution after Graben (Figure 22). Contemporaneous rifting exerted a quartz overgrowth development. strong influence on sedimentation and large-scale upward- Drowning of the Piper delta occurred in the mid- fining cycles separated by regionally extensive marine Kimmeridgian (Eudoxus Standard Zone) in conjunction condensed horizons reflect rapid changes in relative sea level with renewed rifting (Partington et al., 1993b). As a result, (Partington et al., 1993b; Garland, 1993). In the Central Brae basinal conditions were established over much of the Moray Field, reservoir porosity averages 11.5%, with average Firth Basin (Boote and Gustav, 1987). Basinal mudstone permeability of 100 millidarcies (Turner and Allen, 1991). development was locally interrupted by deposition of the Basin-floor fans, such as the Miller Fan system (also mid-Kimmeridgian to mid-Volgian Claymore Sandstone included in the Brae Formation) in the South Viking Member, which now forms an important hydrocarbon Graben and the Magnus Fan in the East Shetland Basin, are reservoir in the Outer Moray Firth Basin (e.g. Claymore characterised by their relatively large radii (10–15 km), Field). The Claymore sands were largely derived from greater basinwards extent, higher proportion of sandstone reworking of the Piper Formation on the crests of uplifted facies and relatively minor conglomerate component. fault blocks (O’Driscoll et al., 1990; Boldy and Brealy, Within the Miller Field, primary intergranular porosity 1990; Hallsworth et al., 1996) and were emplaced by a predominates and average porosity is 16% (Rooksby, range of gravity-flow processes (Turner et al., 1984). 1991). Secondary porosity, after feldspar dissolution, Within the Claymore sandstones of the Claymore Field, provides a minor contribution to overall porosity. porosity averages 20% and permeability lies in the range 10–1300 millidarcies (Harker et al., 1991). There is very little detrital or authigenic clay. A minor amount of 3.6 SOURCE ROCKS degradation of feldspar and mica to kaolinite and mixed- layer clays took place, in addition to relatively early quartz The Kimmeridge Clay Formation is the principal and feldspar overgrowths (Maher and Harker, 1987). hydrocarbon source rock of the Central and Northern North

23 NNE

FLADEN SOUTH VIKING GRABEN GROUND Apron-fringe fans SPUR Sea level

Miller Fan System

INNER FAN OUTER FAN TO BASIN PLAIN

DEVONIAN AND OLDER HEATHER FORMATION AND OLDER

0 10 kilometres

Conglomerate Brae Formation Sandstone

Mudstone Kimmeridge Clay Formation

Figure 22 Schematic block-diagram of the depositional setting of Upper Jurassic sediments in the Brae Oilfield area (after Stoker and Brown, 1986).

Sea and the majority of oil and gas fields in this region inertinite (highly oxidised terrigenous plant material). The were charged from this prolific source (Cornford, 1998). A distribution of the organic matter was influenced by the variably high gamma signature, due to high uranium proximity to palaeocoastlines, the grain size of the content, is characteristic of this formation and has led to sediment and the energy levels at sites of deposition the term ‘hot shale’ being applied to uranium-rich units of (Fisher and Miles, 1983). In general, inertinite is found in source rock within the formation. The uranium was greater proportion closer to palaeocoastlines, with vitrinite probably adsorbed onto the enclosed organic matter during to the seaward side and liptinite in the deeper axial parts of deposition on a stagnant sea floor (Bjorlykke et al., 1975). the grabens. There has been much discussion on the overall Broadly, oil generation began in the Late Cretaceous to environmental parameters necessary for source bed Palaeogene, though generation commenced earlier in the development in the Kimmeridge Clay Formation (e.g. deeper parts of the Viking and Central grabens and Outer Gallois, 1976; Tyson et al., 1979; Irwin, 1979). However, Moray Firth Basin, and later in the less deeply buried East there is general agreement that the organic-rich sediments Shetland Basin (Goff, 1983). The maturity of the Upper within the formation were deposited under anoxic Jurassic mainly reflects the amount and timing of Neogene conditions, where a lack of bottom feeding organisms and to Recent sedimentation (Figure 23) (Cornford, 1998). The aerobic bacteria resulted in preservation of oil-prone locus of maximum sedimentation migrated south with material. Within the graben areas, deposition of the source time, and consequently, source rocks in the Central Graben rock probably occurred below wave base and beneath have experienced oil-generating temperatures for shorter about 200 m or more of water (Cornford, 1998). times than equivalents in the north (Cornford, 1998). The organic component of the Kimmeridge Clay is Generally, hydrocarbons have migrated up dip from derived from both land and marine environments and deeply buried source rocks and into adjacent traps. The average values of total organic carbon are generally composition of the kerogen, together with the thermal between 5% and 10% (Barnard and Cooper, 1981). Typical maturity of the source, are factors in determining the kerogen types are amorphous liptinite of marine planktonic composition of derived hydrocarbons now entrapped in origin (bacterially degraded algal debris) and amorphous adjacent fields (Fisher and Miles, 1983), though expulsion vitrinite of terrigenous origin (degraded humic matter). efficiencies are perhaps the main control on the gross Also present is particulate vitrinite (woody debris) and gas/oil ratio of the total expelled product (Cornford, 1998).

24 3.7 TRAPS 62ºN Immature vitirinite reflectance Syndepositional rifting exerted a major control on the development of Upper Jurassic traps in the Central and Oil window R 0.5-1.3% Northern North Sea (Figure 3). In the Central Graben, the O associated effects of halokinesis of Upper Permian salt Gas window R >1.3% were an additional strong influence on the pattern of Late O Jurassic subsidence, deposition and subsequent trap Approximate limit of Kimmeridge Clay development (Smith et al., 1993). The syn-rift Upper Formation

Jurassic exploration play is largely, but not entirely, 60º confined to the syn-rift graben. It owes its success to the 0 100 kilometres widespread occurrence of high-quality sandstone reservoirs, which are juxtaposed against mature source rocks in much of the region. The producing syn-rift reservoirs include both shallow-marine and deep-marine sandstones. The syn-rift producing fields display a wide variety of trapping mechanisms, including tilted fault blocks, four-way dip closures, hanging-wall closures and combined structural-stratigraphic closures (Johnson and 58º Fisher, 1998). A number of Upper Jurassic exploration plays are expected to form the focus of much future exploration activity (Munns, 2002). These plays include the stratigraphic pinch-out of basin floor sandstones such as the recent giant ‘Buzzard’ discovery in UK Licence Block 20/6 and the deep basin HP/HT play.

3.7.1 Viking Graben 56º Late Jurassic rifting along the major fault zone that forms the 4ºW 2º 0º 2ºE western margin of the South Viking Graben was key to the development of the Brae Formation reservoir, which represents the proximal portion of a tectonically influenced, Figure 23 Approximate thermal maturity at top apron-fringe submarine fan. The submarine fan sediments Kimmeridge Clay Formation (after Cayley, 1987; Field, were derived from a shallow marine shelf on the crest of the 1985; Goff, 1983). uplifted footwall block (and the fault scarp itself) and were channelled into the rapidly subsiding South Viking Graben. In the South Brae Field (UK Block 16/7), the trap was related to syndepositional faults that detach in the formed by a combination of downfaulting, folding, underlying salt (Smith, 1987). In a contrasting model for differential compaction and stratigraphic pinch-out structural development of the Clyde Field, Gibbs (1984) (Figure 24). The maximum height of the oil column within postulated post-Fulmar Formation, Late Jurassic the field is 510 m, though structural closure on the top of the gravitational slides with detachments in Zechstein salt. Brae Formation amounts to only 60 m. The main trapping elements are the major fault zone that marks the edge of the 3.7.3 Moray Firth Basin Fladen Ground Spur, where the Brae reservoir abuts impermeable Devonian sandstone, and lateral pinch out to Hanging wall-related, slope-apron/basin-floor fan the east of the reservoir facies (Roberts, 1991). The overlying accumulations (Claymore, Galley, Perth and Saltire fields) Kimmeridge Clay Formation provides a seal. occur in the Outer Moray Firth Basin. These fields display both stratigraphic and structural trapping elements with the 3.7.2 Central Graben reservoirs in part overlain conformably by Kimmeridge Clay Formation and partly by truncation followed by onlap In the Central Graben, Upper Jurassic reservoirs within of Lower Cretaceous shale (Harker et al., 1991; Casey et producing fields dominantly comprise shallow marine al., 1993). Similar reservoir/trap systems also extend into sandstones of the Fulmar Formation. However, some deep- the Lower Cretaceous, such as in the Scapa and Claymore marine sandstone reservoirs are present (e.g. in the recently fields (McGann et al., 1991; Harker and Chermak, 1992). discovered Jacqui Field) and may be best developed in the In the Outer Moray Firth Basin, hanging wall-related relatively unexplored basinal parts of the graben areas. traps commonly contain older, shallow-marine sandstone The Clyde Field, on the western margin of the Central reservoirs such as the Piper Formation, which is trapped in Graben (UK Block 30/17), exemplifies a trap within the fault-bounded dip closures with erosional truncation to the shallow-marine Fulmar Formation. It comprises a rotated south. The Saltire Field, situated in a down-faulted terrace fault block underlain by a wedge of Upper Permian on the northern margin of the Witch Ground Graben, is a (Zechstein Group) salt (Figure 25). The lensoid geometry good example of downthrown closure in which fault seal is of the Triassic Smith Bank Formation and the Fulmar provided by juxtaposition of Upper Jurassic shallow- and Formation reflects progressive, syndepositional halokinesis deep-water sandstone reservoirs against tight Zechstein and dissolution. The wedge-shaped geometry of the carbonate and evaporite and Triassic continental mudstone overlying Kimmeridge Clay Formation is thought to be (Fraser et al., 2003).

25 W E SOUTH BRAE FIELD

2000

Palaeogene and Neogene

3000

Shetland and Chalk groups

Humber Group, undivided ? Cromer Knoll Group 4000 Oil Water ? ? ?Devonian ?

Depth below sea level (metres) sea level Depth below 5000 ? ? Heather Formation 0 2 kilometres Kimmeridge Clay Fladen Group Formation and older

BRAE FORMATION LITHOFACIES

Thick conglomerate, sandstone Thick sandstone units/ Thin sandstone units/ and mudstone units subordinate mudstone abundant mudstone

Figure 24 Structure of the South Brae Field (after Roberts, 1991).

SW NE CLYDE FIELD

Fulmar Formation Palaeogene and Neogene 3.0

Cromer Knoll Group Chalk Group Kimmeridge Clay Formation

Heron Group (Triassic)

4.0 Rotliegend Group and Devonian Two-way travel time (seconds) travel Two-way Zechstein Group Devonian 0 1 kilometre and older

Figure 25 Structure of the Clyde Field (after Smith, 1987).

26 4 Cromer Knoll Group (‘Lower Cretaceous’)

4.1 INTRODUCTION argillaceous chalky limestone with localised, sometimes thick, mass-flow sandstone and conglomerate. The thicker, more Renewed exploration interest and recent hydrocarbon widespread sandstone bodies, together with smaller bodies discoveries in a tract lying just to the south of the Halibut that produce significant volumes of hydrocarbons, are given Horst, has generated significant new data on the Lower member status within the Valhall Formation (e.g. Scapa Cretaceous in the Central and Northern North Sea Sandstone Member and Sloop Sandstone Member). (Copestake et al., 2003). From these data, detailed sequence Fine-grained strata of the Valhall Formation are divided stratigraphic models have been developed, allowing the into seven, regionally extensive, informal units (V1–V7) deliberate targeting of subtle Lower Cretaceous prospects on the basis of subtle lithological variation and wireline log (Copestake et al. 2003; Garrett et al., 2000). signatures (Figure 26). The Valhall Formation was deposited in a predominantly aerobic marine environment, although anoxic bottom-water conditions were widely 4.2 LITHOSTRATIGRAPHY established in the mid-Barremian and early Aptian when laminated, organic-rich mudstones of the Munk Marl The Lower Cretaceous within the UK Northern and Central (intra-unit V3) and Fischschiefer (unit V5) were deposited. North Sea is practically synonymous with the Cromer A thin, green, non-calcareous layer within the Munk Marl Knoll Group. Deegan and Scull (1977) proposed the first has been interpreted as a volcanic ash (Jensen and formal lithostratigraphic nomenclature for the Lower Buchardt, 1987). The Fischschiefer (V5) correlates with Cretaceous of the UK and Norwegian sectors of the Central Global Anoxic Event 1a (Arthur et al., 1990). and Northern North Sea. Subsequently, however, many The Carrack Formation is of late Aptian to early/mid- informal terms were introduced, some of which have been Albian age and is widely distributed across the UK Central applied in a number of different senses. A revision of the North Sea and South Viking Graben. The formation is formal and informal nomenclature was proposed by Johnson commonly up to about 100 m thick in basinal areas, though and Lott (1993), who rejected application of the widely used it exceeds 150 m in local depocentres. It consists of medium term Sola Formation within the UK sector, due to possible to dark grey to black, essentially non-calcareous mudstone confusion resulting from a differing, formal application of with thin bentonites and local sandstone. In general, a low the term within the Danish Sector. In its place, Johnson and interval velocity relative to both the underlying Valhall Lott (1993) erected the term Carrack Formation (Figure 26). Formation and the overlying Rødby Formation characterises Good summaries of the various Lower Cretaceous the formation. Bodies of mass-flow sandstone, such as the stratigraphic schemes are provided by Oakman and Skiff Sandstone Member, are enclosed within the Carrack Partington (1998) and Copestake et al. (2003) (Figure 27). Formation in the South Viking Graben. Individual sandstone Over most of the UK Northern North Sea, the Cromer units within the Skiff Sandstone Member commonly display Knoll Group remains undivided. Within the UK Central upward-fining wireline log profiles. The Carrack Formation North Sea and South Viking Graben, however, five mudstones contain a fauna dominated by agglutinating lithostratigraphic formations have been formally defined foraminifera and are thought to mark a phase of basin and proposed as a standard nomenclature (Figure 26) restriction, with bottom-water oxygen depletion. (Johnson and Lott, 1993). Three of these formations The Rødby Formation is of mid- to late Albian age and comprise regionally extensive units of marine is widely distributed across the Central North Sea and mudstone/chalky mudstone with associated units of South Viking Graben. The formation is commonly up to argillaceous chalky limestone and localised basin-floor 100 m thick, but exceeds 150 m in local depocentres. It sandstone. In ascending order, they are known as the consists of mainly pale to dark grey, but commonly red- Valhall, Carrack and Rødby formations. In addition two brown, calcareous mudstone and chalky mudstone with geographically restricted, sandstone-dominated formations sporadic thin limestones. The formation commonly is within the Cromer Knoll Group of the Moray Firth Basin divisible into three informal units (R1–R3) on the basis of are distinguished: the Britannia Sandstone Formation and wireline log responses and subtle lithological variation. The the Wick Sandstone Formation. These formations consist faunal content of much of the Rødby Formation suggests it largely of a range of gravity-flow coarse clastic sediments accumulated in a well-oxygenated marine environment. and pass laterally into the more argillaceous strata of the The Britannia Sandstone Formation is mid-Barremian Valhall and Carrack formations. to late Aptian in age and is confined to the south-east of the The distribution, thickness and well log character of the Outer Moray Firth Basin. The formation locally reaches Cromer Knoll Group are summarised on Enclosure 3. over 250 m in thickness and comprises mass-flow sandstone with interbedded mudstone. The sandstones are pale grey or 4.2.1 Cromer Knoll Group tan coloured and mainly fine- to medium-, but locally coarse-grained. Although there are no formal members The Valhall Formation is of late Ryazanian to early/late within the Britannia Sandstone Formation, an informal Aptian age and is widely distributed across the UK Central subdivision into ‘Lower Britannia Sandstone’ and ‘Upper North Sea and the South Viking Graben. It reaches over Britannia Sandstone’ is recognised from the character of 800 m in thickness within local depocentres and consists of the interbedded mudstone. In the lower unit, the calcareous interbedded calcareous mudstone, chalky mudstone and mudstones display lithologies typical of the Valhall

27 NORTHERN NORTH SEA CENTRAL NORTH SEA

Magnus Trough/ East Shetland Basin/ South Viking Graben Central Graben Outer Moray Firth Inner Moray Firth

GROUPS Beryl Embayment UPPER CENO- SHETLAND/ SVARTE/HIDRA E HIDRA FORMATION HIDRA FORMATION HIDRA FORMATION HIDRA FORMATION CRET MANIAN CHALK GPS FORMATIONS R3 R3 R3 R3 L RØDBY R2 R2 RØDBY RØDBY R2 R2 RØDBY M FORMATION FORMATION FORMATION FORMATION R1 R1 R1 R1 ALBIAN E CARRACK FORMATION CARRACK FORMATION CARRACK FORMATION CARRACK FORMATION

Skiff Sandstone (Upper) (Upper) L Member V7 V7 BRITANNIA BRITANNIA V7 V7 Captain SANDSTONE SANDSTONE Sandstone Sloop Sst V6 V6 FORMATION FORMATION V6 V6 Member Member APTIAN E (Lower) (Lower) 'Fischschiefer'V5 V5 'Fischschiefer' V5 V5 Yawl Sst WICK SANDSTONE FORMATION L CROMER V4 V4 Member V4 V4 KNOLL M VALHALL Munk Marl Munk Marl Munk Marl GROUP FORMATION E BARREMIAN UNDIVIDED V3 V3 V3 V3 VALHALL VALHALL VALHALL

LOWER CRETACEOUS LOWER FORMATION FORMATION FORMATION L KNOLL GROUP CROMER Coracle Sandstone Scapa Member Sandstone Member E HAUTERIVIAN V2 V2 V2 V2 Punt Sandstone L Member GINIAN VALAN- E Devil's Hole V1 Sandstone V1 V1 Member L

KIMMERIDGE KIMMERIDGE KIMMERIDGE CLAY KIMMERIDGE CLAY KIMMERIDGE CLAY E CLAY CLAY RYAZANIAN FORMATION FORMATION FORMATION FORMATION FORMATION GROUP FULMAR JUR- HUMBER

VOLGIAN FORMATION ASSIC

Figure 26 Cromer Knoll Group (Lower Cretaceous) lithostratigraphy (after Johnson and Lott, 1993).

Formation, whereas in the upper unit they comprise dark 59°25’N, the Cromer Knoll Group is not formally subdivided grey, fissile mudstone typical of the Carrack Formation. (Enclosure 3). An informal subdivision (units ‘a’ to ‘e’, in The Wick Sandstone Formation is late Ryazanian to ascending order) is commonly possible on the basis of subtle early/mid-Albian in age and is distributed across the northern, lithological variation and wireline log signatures. Unit ‘a’ central and eastern Inner Moray Firth Basin and into the consists of relatively calcareous beds at the base of the South Halibut Basin. The formation comprises sandstone with succession and probably correlates with Valhall unit 1 (V1). interbedded siltstone and mudstone and locally reaches over Broad lateral equivalents of units V2 and V3 are also apparent 1400 m in thickness on the downthrow side of major (? in the Northern North Sea and correspond to units of relatively mainly Late Jurassic) faults. The sandstones are very fine- to less calcareous and more calcareous mudstone, respectively coarse-grained and pebbly, poorly sorted, locally argillaceous, (‘b’ and ‘c’). In northern sections, a prominent high gamma- and the grains dominantly consist of clear to translucent ray spike provides a useful marker within the Aptian, but the quartz. The interbedded mudstone units are typical of those relationship of northerly Aptian and Albian strata (i.e. units ‘d’ within the Valhall and Carrack formations. On wireline logs, and ‘e’) to the Carrack and Rødby formations awaits the Wick Sandstone Formation displays both ‘blocky’ and clarification through detailed biostratigraphical studies. ‘serrated’ signatures, reflecting massive sandstone units and The Cromer Knoll Group reaches over 1400 m on the thinly interbedded sandstones and mudstones respectively. downthrow side of the major growth fault forming the The virtual absence of core material from the Lower southern boundary of the Magnus Basin. In this depocentre, Cretaceous succession of the Inner Moray Firth Basin has led thick, unnamed late Valanginian to early Hauterivian mass- to debate about its genetic interpretation. Ziegler (1990) and flow sandstones and interbedded mudstones are present Oakman and Partington (1998) postulated that shelf sandstone within the lower Cromer Knoll Group. might be present locally, but Jeremiah (2000) considered that all the preserved Lower Cretaceous sediments within the Moray Firth Basin were deposited within a deep marine 4.3 SEQUENCE STRATIGRAPHY setting. In the south and east of the Inner Moray Firth Basin, the Wick Sandstone Formation can be divided into the Punt, Largely on the basis of data made available by British Coracle and Captain Sandstone members. Petroleum, Oakman and Partington (1998) outlined a provisional sequence stratigraphy for the Cretaceous of the 4.2.2 Undivided Cromer Knoll Group Central and Northern North Sea (Figure 27). In constructing the stratigraphic template, Oakman and Within the Beryl Embayment, North Viking Graben, East Partington (1998) assumed that tectonic events occurred Shetland Basin and Magnus Basin north of approximately synchronously across the North Sea Basin and that major

28 NORTH SEA RELATIVE SEA LEVEL CURVE INTRA-PLATE PROVISIONAL STRESSES NORTH SEA Landwards STAGE AGE OROGENIC SEQUENCES (my) Tension EVENTS (Oakman and Size of maximum flood and incision et al., 2003) Pattington, 1998) SEQUENCE symbols indicates

(after Copestake Compression STRATIGRAPHY relative magnitude FLOODS INCISIONS

K60 L Late

EC-9 K55 Confidence range of dominant M maximum floods Extensional regime and incisions

E EC-8 K50 'AUSTRIAN'

108

L EC-7 Fischschiefer K45 Mid

E EC-6 K40

113 L Munk Marl M b

E EC-5 K30 116.5

L a Early

E EC-4 K20 121

L EC-3

K10

E EC-2

128 L EC-1

Part of Part of Compressional regime dominant E J70 J70 'LATE CIMMERIAN' RYAZANIAN VALANGINIAN HAUTERIVIAN BARREMIAN APTIAN ALBIAN

Figure 27 Generalised Early Cretaceous sequence stratigraphy (after Oakman and Partington, 1998; Copestake et al., 2003). anoxic events such as the Munk Marl and the Fischschiefer Albian, top early Albian, top mid-Albian and top Albian. approximate to isochrons and also mark the peaks of In contrast to Oakman and Partington (1998), Oakman marine floods. They recognised nine ‘Vail-type’ sequences (1999) interpreted the Fischschiefer to mark a transgressive bounded by incision events within the Lower Cretaceous systems tract and placed the true maximum flooding succession. The chronostratigraphic distribution of these horizon within the overlying unit (Ewaldi Marl or Valhall sequences within part of the UK Central Graben is Formation unit V6; Figure 26). illustrated in Figure 28. Subsequently, Oakman (1999) Using a similar approach, Jeremiah (2000) described the referred to 12 Early Cretaceous sequences defined by the results of a detailed biostratigraphical, sequence stratigraphical larger incision surfaces with boundaries placed at intra-late and seismo-stratigraphical study of the Lower Cretaceous of Ryazanian, top Ryazanian, top early Valanginian, the Moray Firth. He recognised thirteen unconformity- uppermost late Valanginian, top early Hauterivian, intra bounded sequences within the Cromer Knoll Group. The early Barremian, topmost Barremian/basal Aptian, intra unconformity surfaces bounding many of these sequences and early Aptian, top early Aptian, topmost Aptian/basal the maximum flooding surfaces within them differ in detail

29 0 50 kilometres

FORTIES-MONTR JAE EAST CEN B REN HIGH

TRAL GRA

OSE HIGH A WEST CENTRAL SHELF WEST CENTRAL GRABEN BEN

JOSEPHINE R Mudstone High total organic carbon maximum flood mudstone A U K IDGE Proven basin floor sand R ID and/or basin slope sand GE Proven shelf sand (Greensand) C N FULMAR Chalky limestone

TERRACE Non-deposition/erosion

Sequence boundary

mfs Maximum flood surface

ABC WEST CENTRAL WEST CENTRAL FORTIES-MONTROSE WEST CENTRAL AUK FULMAR RELATIVE COASTAL AGE SEQ. SEQ. SHELF GRABEN HIGH GRABEN RIDGE TERRACE ONLAP

EC-9 EC-9 mfs

EC-8 EC-8 mfs mfs EC-7 EC-7

EC-6 EC-6 mfs APTIAN ALBIAN

EC-5b EC-5b mfs BARR- EMIAN EC-5a EC-5a mfs

EC-4 EC-4 mfs HAUTERIVIAN

EC-3 EC-3 mfs EC-2 EC-2

VALAN- GINIAN mfs EC-1 EC-1 mfs Part of Part of

RYAZ- ANIAN J70 J70

Figure 28 Chronostratigraphic distribution of Early Cretaceous sequences, UK Central Graben (after Oakman and Partington, 1998). from those recognised by Oakman and Partington (1998) and south of its present position, at the southern fringe of the some are correlated with the lithostratigraphical boundaries Boreal Ocean and within the Laurasian continent (Zeigler, recognised by Johnson and Lott (1993). Jeremiah (2000) 1990). The Early Cretaceous was a time of overall eustatic regarded these sequences as tectonically controlled with little sea level rise, with marked pulses interpreted in the calibration to eustatic sea level changes. Barremian and in the mid- to late Aptian (e.g. Rawson and Copestake et al. (2003) present a sequence stratigraphic Riley, 1982; Oakman and Partington, 1998). scheme including eight sequences (K10–K55) based on log The extent of emergence of highs in the North Sea area response, lithology and biostratigraphy that they have applied during the Early Cretaceous is difficult to assess (Hancock across the UK, Norwegian and Danish sectors (Figure 27). and Rawson, 1992). Though many Jurassic tilt blocks remained uplifted in Early Cretaceous times, they may not have been subjected to significant subaerial erosion. Indeed, 4.4 PALAEOGEOGRAPHY although seismic data commonly suggest strong marine onlap of the Cromer Knoll Group onto pre-existing structural highs, The generalised Early Cretaceous palaeogeographical/ in many places condensed sections are found considerable palaeofacies development of the Central and Northern distances beyond the apparent Lower Cretaceous seismic North Sea is summarised in Figure 29. During the Early pinch-out (Rattey and Hayward, 1993). However, the Fladen Cretaceous, the North Sea Basin lay about ten degrees Ground Spur and Halibut Horst may have remained emergent

30 1 62ºN 2 62ºN LATE RYAZANIAN TO HAUTERIVIAN TO VALANGINIAN BARREMIAN

0 200 kilometres

60º 60º

58º 58º

56º 56º 4ºW 2º 0º 2ºE 4ºW 2º 0º 2ºE

62ºN 3 Low-lying land/ non-deposition APTIAN TO ALBIAN Shallow marine shelf

Deep marine

Major Late Jurassic / Early Cretaceous fault, crossmark on downthrow side

60º

58º

56º 4ºW 2º 0º 2ºE

Figure 29 Early Cretaceous palaeogeography. throughout much of Early Cretaceous times, when they were widespread over platform areas, whereas a deeper-water periodically important sources of sand (Boote and Gustav, ‘outer sublittoral-upper bathyal’ biofacies is developed in 1987; Crittenden et al., 1997, 1998). the Central Graben, Moray Firth Basin, Viking Graben, East North Sea depositional settings during the Early Shetland Basin and Magnus Basin. In addition a ‘restricted Cretaceous have been broadly characterised using basin’ biofacies is widespread at several levels from the assemblages of foraminifera, and two main biofacies have Hauterivian to the mid-Albian, and a ‘carbonate biofacies’ been identified (King et al., 1989). A ‘shelf’ biofacies is occurs in clastic-starved settings developed over intrabasinal

31 highs at several levels in the Valanginian, Hauterivian and Ineson, 1993; Kühnau and Michelson, 1994), though a Barremian. The ‘restricted basin’ biofacies is developed in change to a drier climate may also have reduced the input to both shelf and deeper water ‘flysch-type’ environments. The the basin of fine siliciclastics (Jensen and Buchart, 1987; ‘carbonate facies’ is presumed to have developed in mid- to Ruffell and Batten 1990). In early/mid-Barremian times a outer-shelf environments (King et al., 1989). phase of bottom-water anoxia produced the thin, but widespread Munk Marl. Coarse clastic sedimentation 4.4.1 Late Ryazanian to Valanginian appears to have been confined mainly to the Inner Moray Firth Basin, South Halibut Basin (Wick Sandstone Initially, the late Ryazanian palaeogeography probably Formation, Coracle Sandstone Member) and Central changed little from that established in the Late Jurassic Graben (Lower Britannia Sandstone). (Figure 29). Basin modelling suggests that significant rift basin bathymetry, established in the Late Jurassic, persisted 4.4.3 Aptian to Albian into Early Cretaceous times (Rattey and Hayward, 1993). Some sub-basins are bounded on one side by a major fault Tectonism at the start of the Aptian rejuvenated many of the (e.g. the Witch Ground Graben, immediately north of the pre-existing structural highs and these acted as source areas Renée Ridge), others, especially in the Central Graben, are for coarse clastic sediment, even though global sea level synclines resulting from halokinesis of Zechstein salt, with continued to rise (Figure 29). In particular, the footwall block no distinct half-graben geometry. Within the Witch Ground of the Wick Fault, the Halibut Horst and the Fladen Ground Graben, the Valhall unit V1 displays only a small lateral Spur were important source areas for clastic sediment. The variation in the thickness and this, together with its wide thin, but widespread dark mudstones of the Fischschiefer distribution, suggests relatively little syndepositional (unit V5) mark an early Aptian phase of bottom-water differential subsidence there during late Ryazanian to mid- anoxia. The Aptian section overlying the Fischschiefer Valanginian times (O’Driscoll et al., 1990). However, includes a distinctive late Aptian unit of ‘chestnut-brown’ adjacent to relict and new highs, a new generation of deep- calcareous mudstone (unit V7) with abundant red-stained marine sandstone was developed locally. Mass-flow sand planktonic foraminifera, which probably marks a phase of units within the Inner Moray Firth Basin were probably widespread sediment starvation and sea-floor oxygenation derived from shallow-shelf greensand facies developed over (Lott et al., 1985; Guy, 1992; King et al., 1989). the footwall blocks of the Wick Fault and Halibut Horst. Further tectonism during late Aptian times contributed to Differential subsidence increased significantly in mid- a major change in sedimentation that marks the onset of a Valanginian times, and Valhall Formation unit V2 commonly phase of bottom-water oxygen-depletion within a basin shows marked lateral thickness variation. Jurassic basinal with restricted access to open marine currents (King et al., areas may have been modified by transpressive downwarping 1989). Thick, but aerially restricted bodies of gravity-flow or were partially inverted (Oakman and Partington, 1998). sandstones were deposited in the Central North Sea and Within the generally underfilled basinal areas, thick South Viking Graben during this time (e.g. Upper Britannia successions of slope and basin-floor mudstone and chalky Sandstone). These sandstones, together with associated thin mudstone are widespread. Over some palaeohighs in the bentonites, reflect a phase of regional tectonic instability Witch Ground Graben, unit V1 is absent, and this has been accompanied by localised uplift, erosion and volcanic interpreted as evidence for a significant early Valanginian activity. Speculatively, sandstone equivalent to the unconformity (O’Driscoll et al., 1990). Thick, mass-flow Britannia Sandstone Formation might also be developed conglomerate and sandstone units (Scapa Sandstone and form potential traps for hydrocarbons within the deeper Member) of mid-Valanginian to mid-/late Hauterivian age, parts of the Central Graben (Enclosure 3), where relatively accumulated in a half-graben-like depocentre on the little deep drilling has taken place. downthrown side of the Halibut Shelf (Riley et al., 1992; Within the Inner Moray Firth Basin, gravity-flow Harker and Chermak, 1992). Similarly, mass-flow sandstone sandstones continued to be deposited until final drowning (Wick Sandstone Formation, Punt Sandstone Member) of the source areas by the mid-Albian transgression. Red or accumulated within the Inner Moray Firth Basin. pink stained mid-Albian chalky mudstones contain rich Within the Magnus Basin, thick units of late and varied planktonic foraminiferal faunas and mark the Valanginian to early Hauterivian sandstone and associated widespread transgression. A subsequent phase of more mudstone were deposited in a developing half-graben restricted water circulation (Rødby Formation unit R2) was associated with North Atlantic rifting. followed in latest Albian times by a return to the red stained transgressive facies. 4.4.2 Hauterivian to Barremian Tectonism reduced during Hauterivian times, though a 4.5 DEPOSITIONAL ENVIRONMENTS AND minor phase of tectonic inversion and submarine erosion RESERVOIRS occurred within the Central Graben in the Danish sector (Vejbaek, 1986; Ineson, 1993; Kühnau and Michelson, The interpreted genetic environments of deposition for 1994). The dominant control upon sedimentation during each formation within the Cromer Knoll Group of the UK this time appears to have been eustatic sea level rise (Figure Central and Northern North Sea are tabulated in Figure 30. 29) (Oakman and Partington, 1998). In mid-Hauterivian to Argent et al. (2000) proposed a depositional model to mid-/late Barremian times, the relative proportion of account for the distribution and architecture of the Punt pelagic carbonate to siliciclastic mud sedimentation Sandstone Member of the Wick Sandstone Formation in the increased, resulting in the widespread deposition of Inner Moray Firth Basin, and also suggested that this model argillaceous chalky limestone (unit V3 and in the North may be applicable to other Lower Cretaceous massive deep- Viking Graben Cromer Knoll ‘unit c’, Enclosure 3, well water sands in the Moray Firth Basin. They examined core 3/25a-4). This lithological change is thought to reflect the from well 12/26–2, which is the only core to be taken to date onset of a transgressive phase (Crittenden et al., 1991; within the Lower Cretaceous of the Inner Moray Firth Basin,

32 and found that the Punt Sandstone comprises massive and generally structureless sandstone, with the exception of dish a structures that indicate pervasive dewatering. However, the sands are partially consolidated and these sedimentary structures are poorly preserved. The sands within this core are very fine to coarse-grained and poorly sorted and have an erosive basal contact with the underlying mudstone unit. They are quartzose, with clear translucent grains, minor amounts of glauconite, carbonaceous debris and mica. Thin carbonate- cemented zones in these sands form high-sonic spikes on wireline logs. The underlying mudstone is pale to dark grey, b micromicaceous, variably calcareous, fissile and bioturbated. Implicit to their depositional model is the presence of pre- existing basin topography, which provided an underlying control to sand deposition within the available accommodation space. They suggested that several mechanisms could have acted together to develop the subtle, uneven basin floor topography in the Lower Cretaceous Inner Moray Firth Basin: c • underfilled Jurassic fault-controlled lows • differential compaction of a filled Jurassic graben • local, active Lower Cretaceous extensional faulting

Argent et al. (2000) envisaged that sand progressively filled depressions in the basin floor, beginning with those most proximal to the sediment input points in the Inner Moray Firth Basin to the west. Once this basin was filled, a feeding channel propagated by incision across the filled basin and 0 5 kilometres interbasin high, allowing sediment to be transported eastwards into a neighbouring basin. This model is Figure 31 ‘Fill and Spill’ model for sand emplacement, comparable to the ‘fill and spill’ process for massive sand in which a channel sand feeds (a) then fills (b) a basin, deposition illustrated by Weimer et al. (1998) in the Gulf of leading to bypass by the feeder channel (c) and filling of a Mexico, where the basin floor is characterised by numerous second basin (after Argent et al., 2000). ovoid ‘mini-basins’ caused by salt withdrawal. The linked architectural elements principally consist of ‘ponded’ sheets and back-filled, linear, incised channels (Figure 31). developed in a north-north-west-trending sand fairway interpreted as a backfilled submarine canyon that cuts across 4.5.1 Wick Sandstone Formation the Captain Ridge (Figure 32). The upper Captain Sandstone is developed as a continuous sheet that systematically thins Mass-flow sandstones of the Captain Sandstone Member and pinches out to the south-south-east. On wireline logs, the form the reservoir for heavy oil in the Captain Field (UK sandstones display a blocky character, and form Block 13/22a) that overlies the western tip of the Captain amalgamated sets that can be over 30 m thick. Core evidence Ridge (the western extension of the Halibut Horst, indicates that the sandstones are predominantly massive Figure 32). The reservoir can be informally divided into the suspension fall-out sediments. Rare bedding, parallel upper and lower Captain sandstones, separated by a unit of laminations and dewatering structures, including vertical tuffaceous mudstone known as the mid-Captain Shale (Rose, pipes and sporadic dish structures are described from the 1999). The Captain sandstones were sourced from shallow cores. Evidence for traction current deposition is uncommon, marine greensand that was originally deposited to the north but sporadic mudstone clasts are concentrated into imbricated of the Wick Fault. The lower Captain Sandstone is best horizons and there are a few occurrences of graded beds with parallel laminated horizons overlain by cross-bedded units (Rose, 1999).

CROMER KNOLL GROUP 4.5.2 Britannia Sandstone Formation Rødby Formation Deep water, open marine environment The Britannia Sandstone Formation forms the reservoir for gas condensate within the Britannia Field, which is the Carrack Formation Deep water, restricted marine largest producing hydrocarbon accumulation in Lower Cretaceous sediments of the North Sea. The formation Britannia Sandstone Formation Basin floor fans comprises mainly fine- to medium-, but locally coarse- grained gravity-flow sandstone and associated mudstone. Wick Sandstone Formation Apron fringe and basin floor fans Both the bioclastic debris and the abundant carbonaceous material within the sandstone units suggest derivation from Deep water, open marine with sporadic Valhall Formation a high-energy, shallow marine shelf, which may have been anoxia located on the Fladen Ground Spur. The Lower Britannia Sandstone is associated with an open marine hemipelagic Figure 30 Depositional setting of the Cromer Knoll mudstone similar to that of the Valhall Formation, whereas Group. the Upper Britannia Sandstone is associated with

33 Slumping on margin of Captain Ridge Less continuous sands a in channel overspill area

N

Degraded fault scarp

Thick amalgamated sandstone beds deposited from high- density turbidite flows CAPTAIN RIDGE

ASIN

SOUTH HALIBUT B

Unconfined turbidite flows in West Halibut Basin

0 1 kilometre

b

N

High-density turbidite flows deflected by Rejuvenated Captain Ridge fault scarp

CAPTAIN RIDGE

ASIN

SOUTH HALIBUT B

0 1 kilometre

Lower Wick Upper Captain Lower Captain Valhall/Carrack Sandstone Jurassic Devonian Sandstone Sandstone formations Formation

Hemi-pelagic deposition Direction of turbidite on crest of ridge flows

Figure 32 Captain Sandstone depositional model for (a) the lower Captain Sandstone and (b) the upper Captain Sandstone (after Rose, 1999).

34 SW NE Smith Bank High Captain Ridge South Halibut Basin

0 1 kilometre

Upper Cretaceous Wick Sandstone Permo-Triassic Conglomerate facies Chalk Group Formation

Captain Sandstone Jurassic Devonian Sandstone facies

Figure 33 Schematic structure of the Captain Field (after Rose, 1999). disaerobic mudstones characteristic of the Carrack of gravity-flow deposits and are interbedded with Formation. In the western part of the field (UK Blocks calcareous and chalky mudstone typical of the Valhall 15/29a and 15/30), the lower Britannia Sandstone forms Formation. The sandstone units are fine- to medium- the main reservoir. Clean, high-density turbidite sandstone grained and mainly massive to poorly laminated. Core units are well developed in the lower Britannia Sandstone. evidence indicates that both non-graded and upward-fining These turbidites are massive or show dish structures, sandstone is present. Clasts of micrite and mudstone, commonly with near vertical water escape pipes and together with comminuted carbonaceous debris and broken sandstone dykes towards the top of the beds. The upper shelly material are common. The widespread occurrence of Britannia Sandstone is dominated by units of argillaceous, structures associated with sediment traction suggests that laminated sandstone with dewatering structures, interpreted bottom currents reworked the sands. The associated as the deposits of muddy slurry flows (Guy, 1992; Jones et conglomerate facies are matrix-supported and poorly al., 1999), together with thick-bedded and vertically sorted. Matrix composition varies from sandy mudstone to stacked units of massive, relatively mud-free sandstone coarse sandstone or conglomerate. Slump structures and with dewatering structures. These facies are interbedded deformed clasts are common in the conglomerate facies. with upward-fining and upward-coarsening sandstone and Biostratigraphic studies indicate that the locus of sand conglomeratic sandy mudstone. deposition across the Scapa Field shifted through time (Riley et al., 1992), possibly in response to continued 4.5.3 Scapa Sandstone Member tectonism. The Scapa Sandstone Member forms an important oil reservoir in the Scapa Field and in nearby fields. The 4.6 TRAPS Scapa Sandstone Member in the Scapa Field was derived from Jurassic, Permian and Carboniferous sources eroded A number of important hydrocarbon-producing fields and from the Halibut Shelf, possibly during a phase of recent discoveries occur in the Lower Cretaceous deep- localised tectonism (Harker and Chermak, 1992; Oakman water sandstones of the Moray Firth and South Halibut and Partington, 1998). Conglomeratic facies fringe the basins (Figure 3) (Garrett et al., 2000). Law et al. (2000) fault scarp to the north of the Halibut Shelf and pass summarised many of the trapping mechanisms associated laterally into reservoir sandstone that accumulated in more with this exploration ‘play fairway’ in the Inner Moray distal areas. The reservoir sandstone units comprise a range Firth and South Halibut basins. They recognised four-way

35 S N 15/30-7 15/30-1 15/30-2

28000

30000

Two Way Time (ms) Way Two 32000

0 1 kilometre 34000

Upper Cretaceous Britannia Sandstone Cromer Knoll Group Jurassic and older Disconformity and younger Formation (undivided)

Figure 34 Structure of the Britannia Field (after Copestake et al., 2003).

SW NE Halibut Claymore Scapa Field Shelf tilt-block 14/19-9 14/19-18 14/19-15

2200

2500 Mainly Devonian but with some Carboniferous oil/water contact ? -2686m ?

Kimmeridge Clay Formation Depth below sea level (metres) sea level Depth below

3000

Claymore Sandstone Member 0 500 metres

Main reservoir sandstone Cromer Knoll Group (undivided) Chalk Group Scapa Sandstone Member Conglomerate facies Upper Jurassic

Figure 35 Structure of the Scapa Field (after McGann et al., 1991). dip closure traps (e.g. the ‘Hannay’ oil accumulation) and the west, counter to the regional dip from west to east, and combination traps involving structural closure and second either structural closure or sand pinch-out to the stratigraphic pinch-out of the reservoir (e.g. the north or east, countering the rotation of dip approaching ‘Goldeneye’ oil accumulation). Law et al. (2000) the Halibut Horst (e.g. ‘Goldeneye’, Captain). Structural commented that almost all the successful traps they closure is provided either by inversion of pre-existing reviewed have two key elements. First, structural closure to structures or by drape over them.

36 The abundance of fields marginal to the Halibut Horst may feet of gas in place and condensate reserves of 150 million be a function of the exploration activity in the area. Outside barrels. The field includes the discoveries that were this area, many Lower Cretaceous depocentres within the UK formerly known as Kilda and Lapworth. The hydrocarbons Central and Northern North Sea have not yet been tested by are contained in the Britannia Sandstone Formation and the drilling and may still yield discoveries. The Agat Field in trapping mechanism incorporates a combination of Norwegian Waters, comprising Aptian to Albian mass-flow stratigraphic pinch out of the reservoir over the Fladen sands on the margin of the North Viking Graben, indicates Ground Spur to the north and structural closure (Figure 34) that conditions for Lower Cretaceous reservoir charging do (Jones et al., 1999). Mudstone units of the Cromer Knoll exist in the Northern North Sea (Copestake et al., 2003), but Group provide the seal. until further exploration takes place it remains uncertain whether significant additional reserves will be found. 4.6.3 Scapa Field

4.6.1 Captain Field The Scapa Field in UK Block 14/19 of the Outer Moray Firth Basin is a combined structural and stratigraphic trap The Captain Field lies in UK Block 13/22a on the eastern (Figure 35) (McGann et al., 1991; Harker and Chermak, margin of the Inner Moray Firth Basin. The trap overlies 1992). Oil is contained in the Scapa Sandstone Member, the Captain Ridge at the western end of the Halibut Horst, which forms a wedge-shaped body that abuts the faulted and is estimated by the field operators to contain up to 1.5 northern margin of the Halibut Shelf and thins northwards billion barrels of oil in place within the Captain onto the Claymore tilt-block. Deposition of these coarse Sandstone Member. The trap combines elements of dip clastic sediments may have been accompanied by active closure and stratigraphic pinch out of the reservoir oblique slip tectonism along a pre-existing Jurassic half sandstones (Figure 33). The dip closure to the north, west graben. The tectonism, together with subsequent and south is caused by drape over the west-plunging differential compaction, has resulted in the present day Captain Ridge. Closure to the east is controlled by structure of the field; a fault bounded south-east-plunging stratigraphic pinch out of the reservoir. The oil is of high syncline with relatively little internal faulting. The field is viscosity and is biodegraded (Rose, 1999). There are two terminated to the south-west by a combination of major main reservoirs, known informally as the upper and lower ‘down-to-the-north’ faults and a facies change from Captain sandstones (Figure 32), separated by the mid- reservoir sandstone to tightly cemented conglomerate. Captain Shale. The ‘sandstones’ are largely Closure to the north-east is controlled by onlap onto the dip unconsolidated and lie at relatively shallow depths of slope of the Claymore tilt-block and by lateral passage from about 900 m below sea bed. reservoir sandstones to mudstones and chalky mudstones. The south-eastern limit of the field is determined by the dip of the 4.6.2 Britannia Field plunging syncline. The structure is not filled to spill point. To the north-west of the Scapa Field, trapping probably results The Britannia Field lies in UK Blocks 15/30 and 16/26 in from a combination of dip and fault closure and stratigraphic the Outer Moray Firth Basin and contains 4.3 trillion cubic pinch-out of the reservoir sandstone (McGann et al., 1991).

37 References

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The Elgin and Franklin fields: UK Blocks the Geological Society of London, Vol. 159, 175–187. 22/30c, 22/30b and 29/5b. 1007–1020 in Petroleum Geology of JENSEN, T F, HOLM, L, FRANDSEN, N, and MICHELSEN, O. 1986. Northwest Europe: Proceedings of the 5th Conference. Jurassic–Lower Cretaceous lithostratigraphic nomenclature for FLEET, A J, and BOLDY, S A R (editors). (London: Geological the Danish Central Trough. Danmarks Geologiske Society.) ISBN 0903317559 Undersøgelse, Serie A, No. 12. LAW, A, RAYMOND, A, WHITE, G, ATKINSON, A, CLIFTON, M, JENSEN, T F, and BUCHARDT, B. 1987. Sedimentology and ATHERTON, T, DAWES, I, ROBERTSON, E, MELVIN, A, and geochemistry of the organic carbon-rich Lower Cretaceous Sola BRAYLEY, S. 2000. The Kopervik fairway, Moray Firth, UK. Formation (Barremian-Albian), Danish North Sea. 431–440 in Petroleum Geoscience, Vol. 6, 265–274. Petroleum Geology of Northwest Europe: Proceedings of the 3rd LOTT, G K, BALL, K C, and WILKINSON, I P. 1985. Conference. 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40 MAHER, C E, and HARKER, S D. 1987. Claymore Oil Field. Cretaceous of the Central North Sea: Regional Setting and 835–845 in Petroleum Geology of Northwest Europe: depositional architecture, 18–19 May 1999, Programme and Proceedings of the 3rd Conference. BROOKS, J, and GLENNIE, K W Abstracts,1–7. Kings College Conference and Visitor Centre, (editors). (London: Graham and Trotman.) University of Aberdeen. MARTIN, M A, and POLLARD, J E. 1996. The role of trace fossil OAKMAN, C D, and PARTINGTON, M A. 1998. Cretaceous. ichnofabric analysis in the development of depositional models for 294–349 in Petroleum geology of the North Sea, basic concepts the Upper Jurassic Fulmar Formation of the Kittiwake Field and recent advances, 4th edition. GLENNIE, K W (editor). Quadrant 21 UKCS. 163–183 in Geology of the Humber Group: (London: Blackwell Science Limited.) ISBN 0632038454 Central Graben and Moray Firth, UKCS. HURST, A, JOHNSON,HD, O’DRISCOLL, D, HINDLE, A D, and LONG, D C. 1990. BURLEY, S D, CANHAM, A C, and MACKERTICH, D S (editors). The structural controls on Upper Jurassic and Lower Cretaceous Geological Society of London, Special Publication, No. 114. reservoir sandstones in the Witch Ground Graben, UK North Sea. ISBN 0903317826 299–323 in Tectonic events responsible for Britain’s oil and gas MCCANTS, C Y, and BURLEY, S D. 1996. Reservoir architecture reserves. HARDMAN, R F P, and BROOKS, J (editors). Geological and diagenesis in downthrown fault block plays: the Lowlander Society of London, Special Publication, No. 55. Prospect of Block 14/20b, Witch Ground Graben, Outer Moray ISBN 0903317559 Firth, UK North Sea. 251–285 in Geology of the Humber Group: PARTINGTON, M A, COPESTAKE, P, MITCHENER, B C, and Central Graben and Moray Firth, UKCS. HURST, A, JOHNSON,HD, UNDERHILL, J R. 1993a. Biostratigraphic calibration of genetic BURLEY, S D, CANHAM, A C, and MACKERTICH, D S (editors). sequences in the Jurassic of the North Sea and adjacent areas. Geological Society of London, Special Publication, No. 114. 371–386 in Petroleum Geology of Northwest Europe: ISBN 0903317826 Proceedings of the 4th Conference. PARKER, J R (editor). MCGANN, G J, GREEN, S C H, HARKER, S D, and ROMANI, R S. (London: Geological Society.) ISBN 0903317850 1991. The Scapa Field, Block 14/19, UK North Sea. 369–376 PARTINGTON, M A, MITCHENER, B C, MILTON, N J, and FRASER,AJ. in United Kingdom Oil and Gas Fields: 25 Years 1993b. Genetic sequence stratigraphy for the North Sea Late Commemorative Volume. ABBOTTS, I. (editor). Memoir of the Jurassic and Early Cretaceous: distribution and prediction of Geological Society of London, No. 14. ISBN X780644025 Kimmeridgian–Late Ryazanian reservoirs in the North Sea and MCKENZIE, D P. 1978. Some remarks on the development of adjacent areas. 347–370 in Petroleum Geology of Northwest sedimentary basins. Earth Planetary Science Letters, Vol. 40, Europe: Proceedings of the 4th Conference. PARKER, J R (editor). 25–32. (London: Geological Society.) ISBN 0903317850 MCLEOD, A E, DAWERS, N H, and UNDERHILL, J R. 2000. PEACOCK, D C P, and SANDERSON, D J. 1991. Displacements, The propagation and linkage of normal faults; insights from the segment linkage and relay ramps in normal fault zones. Strathspey-Brent-Statfjord fault array, northern North Sea. Journal of Structural Geology, Vol. 13, 721–733. 263–284 in Processes and controls in the stratigraphic PEGRUM, R M, and LJONES, T E. 1984. 15/9 Gamma-Gas Field development of extensional basins.GUPTA, S, and COWIE, P A offshore Norway, new trap type for the North Sea Basin with (editors). (London: Blackwell Science Limited.) ISSN 0950-091X regional structural implications. American Association of MCLEOD, A E, and UNDERHILL, J R. 1999. Processes and Petroleum Geologists Bulletin, Vol. 68, 874–902. products of footwall degradation, northern Brent Field, Northern PRICE, J, DYER, R, GOODALL, I, MCKIE, T, WATSON, P, and North Sea. 91–106 in Petroleum Geology of Northwest Europe: WILLIAMS, G. 1993. Effective stratigraphical subdivision of the th Proceedings of the 5 Conference.FLEET, A J, and BOLDY, S A R Humber Group and the late Jurassic evolution of the UK Central (editors). (London: Geological Society.) ISBN 0903317559 Graben. 443–458 in Petroleum Geology of Northwest Europe: MILLER, R G. 1990. A paleoceanographic approach to the Proceedings of the 4th Conference. PARKER, J R (editor). Kimmeridge Clay Formation. 13–26 in Deposition of Organic (London: Geological Society.) ISBN 0903317850 Facies. HUC, A Y (editor). Studies in Geology, No. 30, (Tulsa, RATTEY, R P, and HAYWARD, A B. 1993. Sequence stratigraphy Oklahoma: American Association of Petroleum Geologists). of a failed rift system: the Middle Jurassic to Early Cretaceous ISBN 0891810382 basin evolution of the Central and Northern North Sea. 215–249 MITCHENER, B C, LAWRENCE, D A, PARTINGTON, M A, BOWMAN,MBJ, in Petroleum Geology of Northwest Europe: Proceedings of the and GLUYAS, J. 1992. Brent Group: sequence stratigraphy and 4th Conference. PARKER, J R (editor). (London: Geological regional implications. 45–80 in Geology of the Brent Group. Society.) ISBN 0903317850 MORTON, A C, HASZELDINE, R S, GILES, M R, and BROWN, S (editors). RAWSON, P F, and RILEY, L A. 1982. Latest Jurassic–Early Geological Society of London, Special Publication, No. 61. Cretaceous events and the ‘Late Cimmerian Unconformity’ in the ISBN 0903317680 North Sea area. American Association of Petroleum Geologists MORTON, A C. 1992. Provenance of Brent Group sandstones. Bulletin, Vol. 66, 2628–2648. 227–244 in Geology of the Brent Group. MORTON, A C, RICHARDS, R P, LOTT, G K, JOHNSON, H, KNOX, R W O’B, and HASZELDINE, R S, GILES, M R, and BROWN, S (editors). RIDING, J B. 1993. Jurassic of the Central and Northern North Geological Society of London, Special Publication, No. 61. Sea. In: Lithostratigraphic nomenclature of the UK North Sea. ISBN 0903317680 KNOX, R W O’B, and CORDEY, W G (editors). (Nottingham: MORTON, A C, HASZELDINE, R S, GILES, M R, and BROWN, S British Geological Survey.) ISBN 0852722206 (editors). 1992. Geology of the Brent Group. Geological Society RILEY, L A, HARKER, S D, and GREEN, S C H. 1992. of London, Special Publication, No. 61. ISBN 0903317680 Lower Cretaceous palynology and sandstone distribution in the MUNNS, J. 2002. Remaining potential and opportunities on the Scapa Field, UK North Sea. Journal of Petroleum Geology, United Kingdom Continental Shelf (UKCS) and the need for New Vol. 15, 97–110. Entrants. Petroleum Exploration Society of Great Britain, ROBERTS, A M, YIELDING, G, KUZNIR, N J, WALKER, I, and Newsletter, March 2002, Abstract, 9–12. DORN-LOPEZ, D. 1993. Mesozoic extension in the North Sea: NELSON, P H H, and LAMY, J M. 1987. The Møre/West Shetland constraints from flexural backstripping, forward modelling and area: a review. 775–784 in Petroleum geology of Northwest fault populations. 1123–1136 in Petroleum Geology of Northwest Europe: Proceedings of the 3rd Conference. BROOKS, J, and Europe: Proceedings of the 4th Conference. PARKER, J R (editor). GLENNIE, K W (editors). (London: Graham and Trotman.) (London: Geological Society.) ISBN 0903317850 OAKMAN, C D. 1999. An overview of the tectono-sedimentological, ROBERTS, M J. 1991. The South Brae Field, Block 16/7a, UK sequence stratigraphic and play fairway models for the Early North Sea. 55–62 in United Kingdom Oil and Gas Fields: 25 Cretaceous of the Central North Sea: where have we been, where are Years Commemorative Volume. ABBOTTS, I. (editor). Memoir of we now and which way are we going? Extended abstract in: Lower the Geological Society of London, No. 14. ISBN X780644025

41 ROOKSBY, S K. 1991. The Miller Field, Blocks 16/7b, 16/8b, Petroleum Geology of Northwest Europe: Proceedings of the 5th UK North Sea. 159–164 in United Kingdom Oil and Gas Fields: Conference. FLEET, A J, and Boldy, S A R (editors). (London: 25 Years Commemorative Volume. ABBOTTS, I. (editor). Memoir Geological Society.) ISBN 0903317559 of the Geological Society of London, No. 14. ISBN X780644025 UNDERHILL, J R, and PARTINGTON, M A. 1993. Jurassic thermal ROSE, P T S. 1999. Reservoir characterization in the Captain doming and deflation in the North Sea: implications of the Field: integration of horizontal and vertical well data. 1101–1113 sequence stratigraphic evidence. 337–346 in Petroleum Geology of in Petroleum Geology of Northwest Europe: Proceedings of the Northwest Europe: Proceedings of the 4th Conference. PARKER,JR th 5 Conference. FLEET, A J, and BOLDY, S A R (editors). (London: (editor). (London: Geological Society.) ISBN 0903317850 Geological Society.) ISBN 0903317559 UNDERHILL, J R, and PARTINGTON, M A. 1994. Use of maximum RUFFELL, A H, and BATTEN, D J. 1990. The Barremian-Aptian flooding surfaces in determining a regional control on the Intra- arid phase in western Europe. Palaeogeography, Aalenian Mid-Cimmerian sequence boundary: implications of North Palaeoclimatology, Palaeoecology, Vol. 80, 197–212. Sea basin development and Exxon’s Sea-Level Chart. 449–484 in SEARS, R A, HARBURY, A R, PROTOY, A J G, and STEWART, D J. Recent Advances in Siliciclastic Sequence Stratigraphy. 1993. Structural styles from the Central Graben in the UK and POSAMENTIER, H W, and WIEMER, P J (editors). American Association Norway. 1231–1243 in Petroleum Geology of Northwest Europe: of Petroleum Geologists Memoir, No. 58. ARKER Proceedings of the 4th Conference. P , J R (editor). UNDERHILL, J R, SAWYER, M J, HODGSON, P, SHALLCROSS, M D, and (London: Geological Society.) ISBN 0903317850 GAWTHORPE, R L. 1997. Implications of fault scarp degradation SMITH, K, and RITCHIE, J D. 1993. Volcanic centres in the for Brent Group prospectivity, Ninian Field, Northern North Sea. Central North Sea. 519–531 in Petroleum Geology of Northwest American Association of Petroleum Geologists Bulletin, No. 81, Europe: Proceedings of the 4th Conference.PARKER, J R (editor). 295–311. (London: Geological Society.) ISBN 0903317850 VAIL, P R, HARDENBOL, J, and TODD, R G. 1984. Jurassic SMITH, R I. 1987. The structural development of the Clyde unconformities, chronostratigraphy, and sea-level changes from field. 523–531 in Petroleum Geology of Northwest Europe: seismic stratigraphy and biostratigraphy. 129–144 in Proceedings of the 3rd Conference. BROOKS, J, and GLENNIE, K W International Unconformities and Hydrocarbon Accumulation. (editors). (London: Graham and Trotman.) SCHLEE, J S (editor). American Association of Petroleum SMITH, R I, HODGSON, N, and FULTON, M. 1993. Salt controls on Geologists Memoir, No. 36. Triassic reservoir distribution, UKCS Central North Sea. VAN WAGONER, J C, MITCHUM, R M, CAMPION, K M, and 547–558 in Petroleum Geology of Northwest Europe: RAHMANIAN, V D. 1990. Siliciclastic sequence stratigraphy in well Proceedings of the 4th Conference. PARKER, J R (editor). logs, cores and outcrops: concepts for high-resolution correlation of (London: Geological Society.) ISBN 0903317850 time and facies. Methods in Exploration Series, No. 7 (Tulsa, STOKER, S J, and BROWN, S S. 1986. Coarse clastic sediments of Oklahoma: American Association of Petroleum Geologists.) the Brae Field and adjacent areas, North Sea: a core workshop. VEJBÆK, O. 1986. Seismic stratigraphy and tectonic evolution British Geological Survey Marine Report, 86/8. of the Lower Cretaceous in the Danish Central Trough. STRUIJK, A P, and GREEN, R T. 1991. The Brent Field, Block 211/29, Danmarks Geologiske Undersøgelse Serie A, Vol. 11, 46pp. UK North Sea. 63–72 in United Kingdom Oil and Gas Fields: 25 VELDKAMP, J J, GAILLARD, M G, JONKERS, H A, and LEVELL, B K. Years Commemorative Volume. ABBOTTS, I. (editor). Memoir of the 1996. A Kimmeridgian time-slice through the Humber Group of Geological Society of London, No. 14. ISBN X780644025 the central North Sea: a test of sequence stratigraphic models. 1–28 THOMSON, K, and UNDERHILL, J R. 1993. Controls on the in Geology of the Humber Group: Central Graben and Moray development and evolution of structural styles in the Inner Moray Firth, UKCS. HURST, A, JOHNSON, H D, BURLEY, S D, CANHAM, A C, Firth Basin. 1167–1178 in Petroleum Geology of Northwest and MACKERTICH, D S (editors). Geological Society of London, Europe: Proceedings of the 4th Conference. PARKER, J R (editor). Special Publication, No. 114. ISBN 0903317826 (London: Geological Society.) ISBN 0903317850 VOLLSET, J, and DORÉ, A G. 1984. A revised Triassic and TURNER, C C, and ALLEN, P J. 1991. The Central Brae Field, Jurassic lithostratigraphic nomenclature for the Norwegian North Block 16/7a, UK North Sea. 49–54 in United Kingdom Oil and Sea. Norwegian Petroleum Directorate Bulletin, No. 3. Gas Fields: 25 years Commemorative Volume. ABBOTTS, I. WAKEFIELD, L L, DROSTE, M R, GILES, M R, and JANSSEN, R. (editor). Memoir of the Geological Society of London, No. 14. 1993. Late Jurassic plays along the western margin of the ISBN X780644025 Central Graben. 459–468 in Petroleum Geology of Northwest TURNER, C C, COHEN, J M, CONNELL, E R, and COOPER, D M. 1987. Europe: Proceedings of the 4th Conference. PARKER, J R A depositional model for the South Brae oilfield. 853–864 in (editor). (London: Geological Society.) ISBN 0903317850 Petroleum Geology of Northwest Europe: Proceedings of the 3rd WARRENDER, J. 1991. The Murchison Field, Block 211/19a, UK Conference. BROOKS, J, and GLENNIE, K W (editors). (London: North Sea. 165–173 in United Kingdom Oil and Gas Fields: 25 Graham and Trotman.) years Commemorative Volume. ABBOTTS, I. (editor). Memoir of TURNER, C C, RICHARDS, P C, SWALLOW, J L, and GRIMSHAW, S P. the Geological Society of London, No. 14. ISBN X780644025 1984. Upper Jurassic stratigraphy and sedimentary facies in the WIGNALL, P B, and PICKERING, K T. 1993. Palaeoecology and Central Outer Moray Firth Basin, North Sea. Marine and sedimentology across a Jurassic fault scarp, NE Scotland. Petroleum Geology, Vol. 1, 105–117. Journal of the Geological Society of London, Vol. 150, 323–340. TYSON, R V, WILSON, R C L, and DOWNIE, C. 1979. A stratified WEIMER, P, VARNAL, P, BUDHIJANTO, F M, ACOSTA, Z M, and water column environmental model for the Kimmeridge Clay. MARTINEZ, R E. 1998. Sequence Stratigraphy of Pliocene and Nature, Vol. 277, 377–380. Pleistocene Turbidite Systems, Northern Green Canyon and Ewing UNDERHILL, J R. 1991. Controls on Late Jurassic seismic Bank (Offshore Louisiana), Northern Gulf of Mexico. American sequences, Inner Moray Firth, UK North Sea: a critical test of a Association of Petroleum Geologists Bulletin, No. 82, 918–960. key segment of Exxon’s original global cycle chart. ZIEGLER, P A. 1990. Geological atlas of western and central Basin Research, Vol. 3, 79–98. Europe. (Amsterdam: Elsevier for Shell Internationale Petroleum UNDERHILL, J R. 1998. Jurassic. 245–293 in Petroleum geology Maatschappij BV.) ISBN 0444420843 th of the North Sea, basic concepts and recent advances, 4 edition. ZIEGLER, P A, and VAN HOORN, B. 1989. Evaluation of North GLENNIE, K W (editor). (London: Blackwell Science Limited.) Sea rift system. 471–500 in Extensional tectonics and ISBN 0632038454 stratigraphy of the North Atlantic margins. TANKARD, A J, and UNDERHILL, J R. 1999. Regional syntheses, tectono-stratigraphic BALKWILL, H R (editors). American Association of Petroleum analyses and structural studies: introduction and review. 3–6 in Geologists Bulletin, No. 46.

42

210 / 15b-4 0 GR 150 140 T 40 LITHOLOGICAL ORNAMENT 62º N SHETLAND SVARTE (Well sections) 210 211 GROUP FORMATION

209 Mudstone/siltstone Limestone Enclosure 3 600 , 4200 1000 Lower Cretaceous’ (Cromer Knoll Group) 600 1200 400 1000 Sandstone 200 1200 of the UK Central and Northern North Sea Chalky mudstone 800 MARGARETA SPUR 400

1:1 000 000 200

800 Conglomerate 1400 BGS Report RR/03/01: Middle Jurassic, Upper Jurassic Coal

and Lower Cretaceous of the UK Central and Northern North Sea 200 4300

Chalky limestone Tuffs and lavas MAGNUS MAGNUS RIDGE BASIN KEY e Punt Sandstone Member Skiff Sandstone Member Britannia Sandstone Formation Land areas (Wick Sandstone Formation) (Carrack Formation) N O R T H 4400 Well where Cromer Knoll Group is absent S H E T L A N D Captain Sandstone Member Rødby, Carrack Undivided Cromer

P L A T F O R M 200 (Wick Sandstone Formation) & Valhall Formations Knoll Group Well where Cromer Knoll Group is present 208 Subsurface faults affecting TERN-EIDER HORST Coracle Sandstone Member Cromer Knoll Group 200 Salt diapir Unnamed clastic units (Wick Sandstone Formation) Isopach of Cromer Knoll 4500 200 Group in metres 400 Devil’s Hole Sandstone Scapa Sandstone Member Boundary of more than one Wick Sandstone Formation CORMORANT RIDGE Member (Valhall Formation) (Valhall Formation) stratigraphic unit present EAST Note: For lithostratigraphical relationships, see Report Figure 26. 61º SHETLAND Some colours differ from those used in Figure 26 to aid clarity of the enclosure 207 1 2 3 3 / 25a-4 UNST BASIN 0 GR 200 140 T 40 4600

200

BASIN 800

SHETLAND SVARTE GROUP FM 4000

600

400 200 T 4700 e Shetland R A Islands N d S IT IO 4100 N 200 A L

S d H 4800

E 600

L 400

F

c 4200

4900 CROMER KNOLL GROUP UNDIVIDED

600 400 800

4300 60º 4 5 5000 6 7 8 9 E A S T 203 S H E T L A N D 10

CROMER KNOLL GROUP UNDIVIDED P L A T F O R M 400

b 5100 4400 BERYL 200 EMBAYMENT

5200

4500

5300

a

4600 202 HUMBER KIMMERIDGE GROUP CLAY FM 200

DUTCH 5400 59º BANK 201 11 Orkney BASIN CRAWFORD SPUR Islands 13 14 / 20-8 12 14 15 16 EAST 400 400 0GR 200 190 T 40 ORKNEY 200

CHALK 200 HIDRA FM BASIN GROUP 5500

R3 FLADEN GROUND ? JURASSIC 200 SPUR 16 / 26-16 2600 R2 400 0GR 150 140 T 40 CAITHNESS RIDGE 200

RØDBY 1800 FMORMATION 600 CHALK HIDRA FM GROUP 3900 Wick Fault 1600 400 1000 400 WITCH 1200 600 800 600 1400 800 200 SOUTH 200 1200 1000 GROUND R3 R1 Wick GRABEN VIKING 1000 600 400 800 GRABEN R2 HALIBUT 800 RØDBY FORMATION 600 600 800 R1 400 800 PLATFORM 2700 600 400200 400 SMITH BANK HIGHFault CARRACK ith Bank 200 Sm FM 1000 1200 1000 400 4000 CARRACK FORMATION Helmsdale Fault

1000 800 800 600 200 V7 HALIBUT HORST 400 200 400 V6 400 Fault 200 MORAY FIRTH BASIN 400 2800 en 600 V5 Gl 800 lt V4 au nk F 58º 600 B a 800 Great ies 17 os 200 600

200 Upper Britannia Sandstone CROMER KNOLL GROUP 18 B 400 400 4100 V3 400 19 200 600 21 600 22 400 600 FISHER 20 600 400 Munk Marl 400 400 400 BANK Banff Fault 600

200 CROMER KNOLL GROUP 800 200 BASIN 200 1000 200 600 BRITANNIA SANDSTONE FORMATION V3 CENTRAL RIDGE 800 2900 600 400

au 200 ad F lt Lower Britannia Sst rhe Pete 4200

200 VALHALL FORMATION 600 JAEREN 23 200 HIGH 400 200

FORTIES-MONTROSE HIGH VALHALL Scapa Sandstone Member 3000 PETERHEAD RIDGE 600 FORMATION

200 V2 Inverness EAST 4300 400 600 CENTRAL V1 GRABEN HUMBER KIMMERIDGE W E S T GROUP CLAY FM C E N T R A L

S H E L F 200 12 / 30-1 400 200 29 / 24-1 0 150 190 T 40 GR 800 0GR 100 190 T 40

CHALK HIDRA 400 GROUP FM 900 CHALK GROUP 1600 CARRACK Aberdeen FORMATION RØDBY FM WICK Captain 600 200 57º CARRACK FM SST Sst 200 800 FM Mbr 400 1000 400 26 30 27 28 29 600

FORTH APPROACHES BASIN DEVIL’S 1000 600 HOLE HORST 400 200 1700 WEST 400 400 200 1100 800 VALHALL FORMATION 25 CENTRAL CROMER KNOLL GROUP Devil's Hole GRABEN Sst Mbr 400 800 400 JOSEPHINE 800 RIDGE HUMBER FULMAR 600 GROUP SST FM

VALHALL 1200 200 FORMATION 400

SOUTH CENTRAL Dundee GRABEN Limit of UK offshore designated area 600 1300 400 200 CROMER KNOLL GROUP

WICK Coracle SST Sst FM Mbr 1400

VALHALL 200 FM 31

WICK Punt SST Sst 1500 FM Mbr

56º N VALHALL FM 4º W HUMBER KIMMERIDGE 3º 3º E GROUP CLAY FM 1600 2º 2º 1º 0º 1º Copyright of BGS maps The copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council [NERC]. No part of these materials may be reproduced or transmitted in any Scale 1:1 000 000 form (analogue or digital), or by any means, or stored in a retrieval system of any nature, without the prior written permission of the BGS Intellectual Property Rights Manager. [British Geological Survey, Kingsley 25 050100 kilometres This map is not suitable for use in navigation Dunham Centre, Keyworth, Nottingham, Notts, NG12 5GG. Telephone 0115 936 3100].

British Geological Survey © NERC 2005 LITHOLOGICAL ORNAMENT 62º N 210 211 211/29-3 (Well sections) 0 GR 150 200 T 100 m 209 ft CROMER Enclosure 1 Siltstone KNOLL GP 2577 ’ Middle Jurassic’ (Fladen/Brent group) 8455

of the UK Central and Northern North Sea Mudstone/siltstone MARGARETA SPUR GROUP HUMBER 2602.5 1:1 000 000 8539

BGS Report RR/03/01: Middle Jurassic, Upper Jurassic Sandstone MAGNUS BASIN

and Lower Cretaceous of the UK Central and Northern North Sea TARBERT FORMATION 2633.5 8640 Chalk MAGNUS RIDGE

Limestone ST R O N O R T H H KEY R Dolomite S H E T L A N D E ID E P L A T F O R M - 208 N Stroma Member R Beatrice Formation Coal 150 E 150 (Pentland Formation) T NESS FORMATION Altered volcanics, lavas 300 600 Brent Group Onshore formations (undifferentiated) and intrusions

CORMORANT RIDGE

150

150 Brora Coal Formation Land areas Metamorphic rocks 61º 207 1 2 3 EAST UNST SHETLAND Callovian strata of Heather or Well where Fladen/Brent group is absent BASIN BASIN 2772 Beatrice Formation facies ETIVE 9095 FM 2783 150 9130

300 Fladen/Brent group Well where Fladen/Brent group is present RANNOCH Hugin Formation Subsurface Late Jurassic faults affecting Fladen/Brent group Shetland FORMATION T 2818 Islands R BROOM 9245 A N FM 2829.5 150 S 9283 Pentland Formation Isopach of Fladen/Brent group in metres IT IO N A

Rattray Volcanics Member L DARWIN

Boundary of more than one stratigraphic unit present FORMATION

(Pentland Formation) S

H E

Ron Volcanics Member L (Pentland Formation) F

Note: For lithostratigraphical relationships, see Report Figure 6. 21/3b-3 Some colours differ from those used in Figure 6 to aid clarity of the enclosure. 0GR 100 140 T 40 m ft CHALK GP 2900.5 CROMER 9516 KNOLL GP 2940 9646 60º 4 5 6 7 8 9 E A S T 10 203 S H E T L A N D P L A T F O R M

300 600 BERYL EMBAYMENT 9/13-12

0 GR 200 140 T 40 m ft

150

600 HEATHER FORMATION 3362 11030 300

202 HUGIN FORMATION DUTCH BANK 3501 59º BASIN 11486 201 11 Orkney Islands 13 12 14 15 16 CRAWFORD SPUR EAST ORKNEY PENTLAND FORMATION BASIN 3584.5 PENTLAND FORMATION

11760 Rattray Volcanics Member FLADEN GROUND

DARWIN SPUR FORMATION

CAITHNESS RIDGE k Fault ic 300 W 150 WITCH SOUTH 150 GROUND Wick VIKING 150 GRABEN GRABEN

300 600 SMITH BANK HIGHith Bank Fault 11/25-1 Sm HALIBUT PLATFORM 0 GR 120 140 T 40

m 150 ft HEATHER FORMATION 2921 300 9584 Helmsdale Fault HALIBUT HORST

Fault 600 150 Carr MORAY FIRTH BASIN 300 Member en 150 Gl BEATRICE Louise lt FORMATION Fau Mbr 2975 58º ank Great B 9760 17 sies 18 Bo 19 20 21 600 22 FISHER BANK Banff Fault BASIN

CENTRAL RIDGE 900 600 300 150 4409.5 ZECHSTEIN 14467 GROUP 4433.5 ROTLIEGEND 14545 Peterhead GROUP BRORA COAL FORMATION Fault JAEREN 23 HIGH

3141 29/12-1 FORTIES-MONTROSE HIGH 10305 Inverness 0 GR 200 140 T 40 m W E S T EAST ft ORRIN CENTRAL

FORMATION 3189 C E N T R A L LADY'S WALK 10462 GRABEN FORMATION S H E L F HEATHER FORMATION 2846.5 150 9339

150

Aberdeen 300 FORTH APPROACHES BASIN 57º

26 30 27 28 29 DEVIL’S

HOLE PENTLAND FORMATION HORST WEST 150 25 CENTRAL GRABEN

300 300

3078.5 S 10100 OU TH C JOSEPHINE EN 300 TR RIDGE A TRIASSIC L G RAB EN Dundee Limit of UK offshore designated area

31

56º N 4º W 3º 3º E 2º 1º 2º Copyright of BGS maps 0º 1º The copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council [NERC]. No part of these materials may be reproduced or transmitted in any form (analogue or digital), or by any means, or stored in a retrieval system of any nature, without the prior written permission of the BGS Intellectual Property Rights Manager. [British Geological Survey, Kingsley Scale 1:1 000 000 Dunham Centre, Keyworth, Nottingham, Notts, NG12 5GG. Telephone 0115 936 3100]. 25 050100 kilometres This map is not suitable for use in navigation British Geological Survey © NERC 2005 62º N LITHOLOGICAL ORNAMENT 211 211 / 12-2 210 m (Well sections) 0 GR 220 190 T 40 ft 209 CROMER KNOLL Enclosure 2 Mudstone/siltstone GROUP 3209 10529 ’ Upper Jurassic’ (Humber Group) 3224 of the UK Central and Northern North Sea 10577 Sandstone MARGARETA SPUR 1:1 000 000

BGS Report RR/03/01: Middle Jurassic, Upper Jurassic Conglomerate MAGNUS BASIN and Lower Cretaceous of the UK Central and Northern North Sea

750

Chalky limestone MAGNUS RIDGE

250

500 Magnus Sandstone Member

3391 Limestone 250 T 11125 KEY RS N O R T H HO KIMMERIDGE CLAY FORMATION R Chalky mudstone S H E T L A N D E 3449

Alness Spiculite Member Magnus Sandstone Member ID 250 11316 E P L A T F O R M 250 (Heather Formation) (Kimmeridge Clay Formation) 208 - RN Coal Member Ptarmigan E Sandstone 3501 Birch Sandstone Member T Piper Formation and equivalents 11486 (Kimmeridge Clay Formation) 500 3513

500 250 11526 Tuffs and lavas 250 Brae Formation and equivalents Ptarmigan Sandstone Member (Kimmeridge Clay Formation) CORMORANT RIDGE EAST 61º SHETLAND Bruce Sandstone Member Ribble Sandstone Member 207 1 2 500 3 BASIN (Heather Formation) (Kimmeridge Clay Formation) UNST 500 BASIN 750 250

250 HEATHER FORMATION 500 Burns Sandstone Member 500 Unnamed clastic units (Kimmeridge Clay Formation) Total depth 3678 m Claymore Sandstone Member Onshore formations (undifferentiated) (Kimmeridge Clay Formation) 250 250 750 250 Shetland 250 14 / 19-4 Emerald Formation Land areas Islands T R 0 GR 150 190 T 40 A 500 m N ft S CROMER IT 250 KNOLL Fulmar Formation and equivalents Well where Humber Group is absent IO GROUP 2453 N 8047

A 250 L Well where Humber Group is present 2484 S

Heather Formation H 8150 E

Subsurface faults affecting Humber Group L 250 F 250 Kimmeridge Clay Formation

250 Isopach of Humber Group in metres Member

Ling Sandstone Member Boundary of more than one stratigraphic 1000 Claymore Sandstone 2566.5 unit present KIMMERIDGE CLAY FORMATION 8420 (Heather Formation) 2583 8474 750 2616 500

FM Pibroch 8583 Chanter Note: For lithostratigraphical relationships, see Report Figure 17. Member PIPER Member PENTLAND 2617 Some colours differ from those used in Figure 17 to aid clarity of the enclosure. FORMATION 250 8586 2627 60º 750 4 8619 5 SKAGERRAK Stroma 6 FORMATION Member 7 8 9 E A S T 10 203 250 S H E T L A N D 500 2/10a-8 P L A T F O R M 16 / 7a-8 m m 0GR 120 140 T 40 SHETLAND 0GR 200 140 T 40 ft ft GROUP 3621.5 KIMMERIDGE 11882 CLAY BERYL 1000 3629.5 FORMATION 250 2684.5 EMBAYMENT 500 500 11908 HEATHER 8808 1000 750 CLAY

FORMATION 2706.5 GROUP

FORMATION 3679.5 8880 KIMMERIDGE EMERALD CROMER KNOLL 12071 FORMATION 2734 250 8970 PRECAMBRIAN

250 250

500 250

15 / 21a-15 750 m 1000 0GR 140 140 T 40 ft Orkney Islands VALHALL FORMATION 500 3158 250 10361 202 DUTCH BANK 59º BASIN 201 11

12 13 15 16 EAST 14 250 CRAWFORD SPUR ORKNEY BASIN

1000 FLADEN

GROUND BRAE FORMATION KIMMERIDGE CLAY FORMATION SPUR 500 1500

750 500 CAITHNESS RIDGE Fault 3383.5 ick W 750 250 11100 250 WITCH 1000 1000 SOUTH 750 GROUND Wick 1500 500 VIKING 2000 GRABEN 750 1000 750 250 GRABEN Chanter Member 250 3449.5 750 500 11318 SMITH BANK HIGH 500 250 2500 ith Bank Fault 500 Sm 1500 HALIBUT 250 250 750 1000 750 1000 2000 PLATFORM 1500 500 250 750 PIPER FORMATION 250 Pibroch Member 2000 250 1500 2000 1500 3529.5 1000 HALIBUT HORST 1000 750 11580 Fault MORAY750 FIRTH BASIN750 250 250 HEATHER 1500 750 Helmsdale1000 Fault 500 FORMATION en 500 1500 Gl 750 750 ult 500 Fa 1000 1000 58º ank 500 Gorse Great B Total depth 4271 m

750

750 Mbr 3584.5 ies 750 250 os 17 1000 18 B Stroma 11760 19 500 22 250 20 250 21 250 FISHER 500 Mbr 3607 500 250 750 250 500 500 1000 750 11834 500 BANK Rattray 750 750 PENTLAND Volc Mbr 750 FORMATION 1500 Banff Fault 500 750 BASIN 30 / 16-7

500 500 750 0 GR 200 140 T 40 m CENTRAL RIDGE 500 500 500 500 ft 1000 250 250 CHALK 250 750 GROUP 250 3033 250 250 20 / 2-5 Peterhead 9951 Fault 250 0 GR 200 140 T 40 m 500 ft 500 250 JAEREN CROMER HIGH KNOLL GROUP 2927 9603 FORTIES-MONTROSE HIGH 23

250 Inverness PETERHEAD RIDGE EAST W E S T 500 CENTRAL C E N T R A L GRABEN

S H E L F 500 KIMMERIDGE CLAY FORMATION 250 500

500 750

500 3262 750 250 Burns Sandstone Member 1000 10702 Ribble Member

750 Sandstone 3305 Aberdeen 750 3285.5 10843 FORTH APPROACHES BASIN 750 10779 3333

KIMMERIDGE CLAY FORMATION 57º 10935 500 250 250 500 750 500 26 500 30 500 27 28 29 DEVIL’S 250 HOLE

250 HORST WEST 1000 500 3489.5 CENTRAL

25 500 11448 GRABEN

1000 JOSEPHINE FULMAR FORMATION

HEATHER RIDGE FORMATION SOUTH 750 CENTRAL 750 500 250 GRABEN

Gorse Member 3651.5 250 3541 500 11980 SMITH 11618 PENTLAND FM 3687 BANK FM 12096 250 Mbr 3733 Dundee Limit of UK offshore designated area Stroma 250 TRIASSIC 12248

500

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56º N 4º W 3º 3º E 2º 1º 2º Copyright of BGS maps 0º 1º The copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council [NERC]. No part of these materials may be reproduced or transmitted in any form (analogue or digital), or by any means, or stored in a retrieval system of any nature, without the prior written permission of the BGS Intellectual Property Rights Manager. [British Geological Survey, Kingsley Scale 1:1 000 000 Dunham Centre, Keyworth, Nottingham, Notts, NG12 5GG. Telephone 0115 936 3100]. 25 050100 kilometres This map is not suitable for use in navigation British Geological Survey © NERC 2005