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

Abstract

The Brent Group in the Brent Field contains over 1000x 106bbl 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 nonreservoir 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.

Role of forced regression in controlling Brent Group reservoir architecture and prospectivity in the northern North Sea

“…14e). These shoreface to delta plain facies form lateral equivalents of the well-documented Mid-Ness Shale highstand deposits, which are widely developed in the Brent province further to the south (Livera 1989;Hampson et al 2004). In the study area, lagoonal facies typical of the Ness Formation characterize the highs (Facies 7), while lower shoreface deposits of Facies 3 dominate in structural lows and in areas located down depositional dip to the north.…”

“…Richards & Brown 1986;Brown & Richards 1989;Livera 1989;Livera & Caline 1990;Hampson et al 2004) and it is not the purpose of this paper to review each facies in great detail. Nevertheless, the sequence stratigraphic significance of certain facies is sometimes overlooked, and an overview of facies interpretations is a helpful foundation for describing Brent architecture and sequence stratigraphy.…”

The use of field analogues in the correlation and static reservoir methodology used in the Tern Field, Northern North Sea, UK

“…The Lower Ness varied from 1 to 17 m in thickness, and was capped by the Mid Ness Shale. It consists principally of metre-scale interbedded sands and shales (Jennette & Riley, 1996;Livera, 1989). The succession is aggradational, but downcuts in some places into the underlying Etive Formation.…”

“…The succession is aggradational, but downcuts in some places into the underlying Etive Formation. The Lower Ness is overlain by a thick shale, the Mid Ness Shale, which represents a major drowning event (Livera, 1989).…”

“…The Lower Ness is commonly viewed as comprising back barrier and lagoonal facies, which are time equivalent to the Etive Formation (Jennette & Riley, 1996). Sub-environments include washover fans, tidal channels, lagoonal mudstones, tidal flats, coal swamps, flood-tidal deltas, crevasse splays and small bay head deltas (Budding & Inglin, 1981;Jennette & Riley, 1996;Livera, 1989).…”