Gary J. Hampson¹, Peter J. Sixsmith¹, J. Anthony Cooke¹, Howard D. Johnson¹, Winfried Leopoldt², Sandy W. Smith², Steve R. Taylor²

(1) Imperial College, London, United Kingdom
(2) Shell UK Exploration & Production, Aberdeen, United Kingdom

ABSTRACT: Reservoir Development in a Transgressive, Syn-Rift Sandstone Complex: the Tarbert Formation, UK North Sea

The Tarbert Formation is a shallow marine sandstone of variable thickness and quality that hosts significant hydrocarbon reserves in Middle Jurassic Brent Group reservoirs of the North Sea. Regional and reservoir-scale analysis using an integrated core, well-log, biostratigraphic and seismic dataset reveals that reservoir thickness and quality trends in the Formation are controlled by transgressive facies architecture during rift initiation.

The Tarbert Formation comprises two regionally extensive, unconformity-bounded sequences stacked in a retrogradational pattern. Each sequence-bounding unconformity is overlain by a discontinuous, coarse-grained, extrabasinal lag. Variations in thickness and facies architecture within each sequence are controlled by the initiation and early growth of syn-rift fault systems, which strongly influenced the local generation of accommodation and the position of sediment delivery routes. For example, highly bioturbated, tidally influenced shoreface sandstones exhibit local progradational stacking patterns along the axes of evolving half-grabens that acted as sediment conduits. These shoreface sandstones are only weakly wave reworked, reflecting structural isolation of the half-grabens from the open seaway located in the main rift axis. Wave-dominated shoreface sandstones are rare and occur only in early syn-rift depocentres adjacent to the rift axis. Evolving half-grabens that do not lie on structurally controlled sediment fairways are starved of sediment and contain only oolitic ironstones and offshore mudstones. Finally, Tarbert Formation thickness has been locally modified by post-depositional, submarine erosion along the crests of the rift fault systems as they grew and linked during rift climax.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado