An introduction to the Brent Group: a literature review

Abstract

Abstract There is a vast volume of published literature on the Brent Group, and consequently many controversies or differing points of view regarding the age, nature and palaeogeographic evolution of the Group. Brent Group lithostratigraphy is, however, relatively uncontroversial. A five-fold subdivision of the Brent Group into the Broom, Rannoch, Etive, Ness and Tarbert Formations is used by most authors. The Oseberg Formation is used to describe Broom Formation equivalent strata in Norwegian waters. There are, however, problems of applying this simple lithostratigraphy in some parts of the basin, where the artificial subdivision may unduly constrain depositional system correlation. Many companies use their own, field-specific, stratigraphic schemes for reservoir subdivision purposes, and some authors have applied a sequence stratigraphic framework to the Brent Group in preference to the lithostratigraphic scheme. There is, however, discord over the actual stratigraphic position of sequence boundaries and the time equivalence of strata. Brent Group biostratigraphy is less well documented in the literature than lithostratigraphy, and represents the area where most new work is needed if the evolution of the depositional system is to be better understood. Analyses of the depositional environment of the Brent formations are numerous but more or less similar. The origin of the Broom Formation is the most controversial aspect, with a range of shallow to deep water interpretations; a fan delta interpretation is preferred in recent publications. The Rannoch and Etive Formations are generally interpreted as shoreface to foreshore/channel deposits, and the Ness Formation as delta top sediments. The Tarbert Formation represents an essentially transgressive unit. There are two main published models to explain the regional evolution of the Brent Group, and this is possibly the most active of the controversies in Brent Group geology. The first model considers the system as a northwards prograding delta with a southerly source and a concentric arrangement of facies belts across the basin. The second model envisages a dominantly transverse sediment supply to the basin, with subsequent localized northwards progradation within the basin.