Geology of the Brent Group: Introduction

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