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Geology of the Brent Group: Introduction

A. C. MORTON 1, R. S. HASZELDINE 2, M. R. GILES 3 & S. BROWN 4 1 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK 2 Department of Geology and Applied Geology, The University of Glasgow, Glasgow G12 8QQ, UK 3 KoninkIi]ke/Shell Exploratorie en Produktie Laboratorium, Volmerlaan 6, 2280 AB Rijswi]k, Netherlands 4 The Petroleum Science and Technology Institute, Research Park, Riccarton, Edinburgh EH14 4AS, UK

The Middle Jurassic Brent Group sediments Province, and to discuss future exploration po- and their correlatives on the Norwegian shelf tential. Although the Brent Group was orig- are, in economic terms, the most important inally interpreted to have been deposited during hydrocarbon reservoir in NW Europe. In 1971 a period of active rifting and basin subsidence, the Brent Field was discovered by Shell/Esso Yielding et al. show that the major phase of and tested in 1972 with 1.8 billion barrels of rifting occurred earlier, with the Brent deposited recoverable oil; nine major Brent sandstone during a thermal subsidence phase. fields were discovered by the end of 1973 Several papers deal with the sedimentological (Brennand et al. 1990). In 1980 the northern framework of the Brent Group, beginning with North Sea (overwhelmingly comprising fields the review by Riehards. Although the original with Brent Group reservoirs) was ranked as the interpretation of the sequence as 'deltaic' 13th largest petroleum province in the world, (Bowen 1975) is still broadly acceptable, the containing 1.6% of produced and recoverable book illustrates the controversy over the precise oil equivalent reserves (Ivanhoe 1980). By 1988, setting of Brent Group deposition. Cannon discovered Brent hydrocarbons comprised some et al. and Heiland-Hansen et al. consider the 49% of the UK's recoverable reserves, totalling sequence as a prograding wave-dominated 22.5 billion barrels of oil equivalent. Brent delta, supported by the detailed work of Scott recoverable hydrocarbons currently known in on the nearshore and coastal Rannoch-Etive the Norwegian sector add approximately 8 successions. Alternatively, Richards invokes a billion barrels of oil equivalent (Brennand significant estuarine component and Alexander et al. 1990). Now that the UK Brent Province makes an analogy with the coeval prograding has reached maturity in exploration terms, this coastal plain sequences of Yorkshire. Problems book provides a timely review of the geology of interpretation exist largely because of the and petroleum geology of one of the worlds difficulty in establishing clear time lines through major petroleum reservoirs. The book provides the sequence, making palaeogeographic recon- a wide-ranging coverage of Brent Group ge- structions speculative. Three papers (Cannon ology, including exploration history, structural et al., Helland-Hansen et al., Mitchener et al.) evolution, sequence stratigraphy, sedimento- have dealt with this problem by using a combi- logy, diagenesis, palynology, hydrocarbon nation of sedimentology, sequence stratigraphy generation and migration, and petrophysics. and palynology: the earlier paper by Eynon Accounts of the geology of individual Brent (1981) may be considered a forerunner of this Group fields are not included, as these are approach. These have provided a series of available in the books of Spencer et al. (1986) palaeogeographical 'snapshots', rather than one and Abbotts (1991). The book shows that de- individual palaeogeography. The differences be- spite the long passage of time since the original tween the models depend to a large extent upon discovery was made, over 20 years ago, and de- the interpretation of palynological events, so spite the subsequent drilling of several hundred that the discussions by Williams and Whitaker exploration and development wells, major con- et al. are especially relevant. troversies still exist, particularly over the de- Three papers cover the topic of provenance positional environment and diagenetic models. of Brent Group sediments, underplayed in pre- The book commences with Bowen drawing vious sedimentological interpretations of the upon his records and personal experience to sequence. Morton and Stattegger & Morton outline the depositional history of the Brent concentrate on the recently-developed garnet

From MORTON, A. C., HASZELDINE, R. S., GILES, M. R. & BROWN, S. (eds), 1992, Geology of the Brent Group. Geological Society Special Publication No. 61, pp. 1-2. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION geochemical technique, and Mearns uses the One paper deals with the important aspect of equally novel samarium-neodymium model petroleum generation and migration. Larter & age approach. Both methods demonstrate the Horstad discuss the secondary migration of importance of laterally-fed sediment to the hydrocarbons through very restricted pathways basin, and show that input from Norway and and carrier beds into the Gullfaks structure the Shetland Platform was considerably more of the Norwegian sector. Finally, two papers important than axially-transported material. bridge the gap between geologist and petroleum After the depositional facies, the most import- engineer: Kantorowiez et al. draw on experience ant individual process affecting the quality of of producing Brent fields to predict reservoir Brent reservoirs is diagenesis. Past and current properties in the Pelican Field area, and Moss research activity is reflected by the inclusion of describes and interprets the petrophysical seven papers on this topic, which again demon- properties of Brent sandstones in a geological strate a lack of concensus of opinion. Giles et al, context. and Bj0rlykke et al. provide the first regional syntheses of Brent sandstone diagenesis, both References of which make use of extensive data sets ac- quired from the British and Norwegian sectors AaaoaTrs, I. L. (ed.) 1991. United Kingdom Oil and respectively. Both these papers identify good Gas Fields, 25 Years Commemorative Volume. correlations between cement abundances and Geological Society, London, Memoir 14. present-day subsidence depth, implying that BOWEN, J. M. 1975. The Brent oilfield. In: WOODLAND,A. W. (ed.) Petroleum and the Con- burial is the main control on diagenetic cemen- tinental Shelf of North-West Europe, Vol 1: tation and inferring that pore fluid movement is Geology. Applied Science Publishers, London, of minor importance. By contrast, the other 353-362. papers all invoke a degree of pore fluid move- EYNON, G. 1981. Basin development and sedimen- ment during cementation. Hogg et al. utilize tation in the Middle Jurassic of the northern new cathode luminescence instrumentation North Sea. In: ILLING, L. V. ~,~ HOaSON, G. D. to identify growth zones within quartz over- (eds) Petroleum Geology of the Continental Shelf growths, the major porosity-reducing cement. of North-West Europe. Heyden & Son, London, Haszeidine et al. combine petrographic, iso- 196-204. BRENNAND, T. P., VAN HOORN, B. & JAMES, K. H. topic and geological information to consider the 1990. Historical review of North Sea exploration. importance of open and restricted porefluid In: GLENNIE, K. W. (ed.) Introduction to the movement in Brent diagenesis. Hamilton et al. Petroleum Geology of the North Sea. Blackwell, critically examine the evidence for dating Oxford, 1-33. cementation by illite, the major permeability- IVANHOE, L. F. 1980. World's giant petroleum prov- reducing cement. The remainder concentrate inces. Oil and Gas Journal, 30 June, 146-147. on particular aspects or geographical areas. SPENCER, A. M. et al. (eds) 1986. Geology of the Both Glasmann and Harris examine feldspar Norwegian oil and gas fields. Graham & Trotman, diagenesis and its influence on present-day rock London. mineralogy.