Facies associations and sand-body geometries in the Ness Formation of the Brent Group, Brent Field

Abstract

Summary The Brent Group in the Brent Field contains over 1000 × 10 6 bbl of recoverable oil in a simple gently dipping half-graben structure. It is 243 m (800 ft) thick and represents a sequence amplified by synsedimentary fault-block subsidence close to the axis of the Viking Graben. On a regional scale the depositional model is of a major shoreface and coastal sequence, the Rannoch and Etive Formations, underlying the partly time-equivalent shallow lagoonal to emergent delta-plain deposits of the Ness Formation. However, more localized and detailed sedimentary modelling is required to predict reservoir geometries and production behaviour, particularly in the Ness Formation. The Ness Formation is the largest reservoir-bearing unit in the Brent Field, being up to 160 m (525 ft) thick. Five main facies associations form reservoir-quality rock: varied channel sandstones, fluvial sheet sandstones, major mouth bars, lagoonal shoals (which are usually associated with, and capped by, thin beaches) and lagoonal sheet sandstones. These associations make up 50%–60% of the sand-rich sequence and are embedded in the non-reservoir facies associations: lagoonal mudstones, emergent flood-plain mudstones and allochthonous and autochthonous coals. The coals are often field wide in extent. Within the amplified Ness Formation the most important environments were the lagoonal deltas which were wave and fluvial dominated with minor tidal influence. As a result of this importance the sand bodies produced by these deltas are subjected to a more detailed description in this paper. Wave energies were sufficient to redistribute large amounts of sediment and indicate significant fetch in the brackish lagoons diluted by major freshwater input. Sand-body geometries are very variable, reflecting both localized controls on sediment distribution, particularly immediately above the compacting shoreface pile, and more regional effects including coarse-sediment supply, basinal processes and fault-block subsidence. The dominant controls on sedimentation resulted in regular drowning events on the delta plains and had a significant impact by producing a strongly layered reservoir.