The Brent Group (Middle Jurassic) of the Brent Field, Northern North Sea

Abstract

The Brent Field is located within a N-S trending fault terrace, situated in the east Shetland Basin on the western margin of the North Viking Graben, UKNS. The Brent field was the first discovery (1971) in this part of the North Sea and hydrocarbons were encountered in the Middle Jurassic Brent Group and the Lower Jurassic/Triassic Statfjiord Formation. The Brent Group was cored in several of the early appraisal wells but the core on display comes from well 211/29 BC06 which was drilled as a down-dip water injector in 1980. The Brent Group comprise shallow marine, marginal marine and non-marine deposits of Middle Jurassic age (Aalenian - Bathonian). Five lithostratigraphic formations are recognised, Broom, Rannoch, Etive, Ness and Tarbert. These five formations are widely considered to record a major regressive-transgressive episode in which the Broom, Rannoch, Etive and Lower Ness Formations represent overall regression of a and the Upper Ness and Tarbert Formations record subsequent transgression of a wave dominated deltaic system. More recent studies of the regional Brent Group succession have recognised a variable number of high-frequency stratigraphic cycles within the major regressive-transgressive episode. These models interpret the Broom to represent a separate depositional episode to the overlying Rannoch-Etive-Lower Ness interval. The Upper Ness and Tarbert Formations are also interpreted to record multiple phases of transgression typically show tide- and wave-influenced features, with transgressive Tarbert shorelines trending along the increasingly active rift structures developing at that time. Selected intervals of core from the 5 lithostratigraphic formations in 211/29-BC06 are displayed in order to examine key sedimentological features, reservoir properties of the key Brent reservoirs and the stratigraphic relationships between some of the formations.