Morris, Jenny E.¹, Gary Hampson¹, Gavin Ward², David Richards³
(1)Imperial College of Science, Technology and Medicine, London, United Kingdom 
(2) BP Exploration, Dorset, United Kingdom 
(3) BP, Dorset, United Kingdom

ABSTRACT: Sequence Stratigraphy and Depositional Model of a Bioturbated Shelf Sandstone Reservoir: the Bridport Sands, Wessex Basin, UK

Intensely bioturbated shallow marine sandstones form productive hydrocarbon reservoirs in many basins, but poor preservation of primary sedimentary structures and a lack of detailed 3D data on their stratigraphic context, distribution and geometry has thus far handicapped attempts to construct predictive depositional models of their internal facies architecture. These issues are being evaluated in one such reservoir interval, the Jurassic Bridport Sands of the Wessex Basin, UK, using an integrated outcrop and subsurface dataset (cores, wireline logs, 3D seismic, reservoir production data). 
The Bridport Sands belong to a mixed siliciclastic-carbonate shoreface-shelf system that underwent a complex progradational history. In the Wytch Farm oilfield, the Bridport Sands comprise storm-dominated, lower-shoreface sandstones in the upper part of a 200 m thick, relatively steep (2°), prograding muddy slope. Forced-regressive progradation of the slope was punctuated by flooding episodes, identified using ichnofabric analysis, and slope deposits are capped by a sequence boundary marked by a firmground Glossifungites surface and an abrupt basinward facies shift. In proximal locations, sequence boundaries are marked by sediment bypass and the development of conglomerates containing bored, cemented sandstone clasts which truncate underlying lower-shoreface sandstones. During transgression, bioclastic limestones were reworked across the shelf and siliciclastic sediment accumulated in lagoons and tidal embayments near the basin margins. The Bridport Sands are highly diachronous across the Wessex Basin, suggesting that this lithostratigraphic unit was deposited in multiple sequences that are offset laterally. Regional reservoir distribution is further complicated by the occurrence of local depocentres in the hangingwall of active rift faults.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.