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Geological Controls on Heterogeneous Reservoir Behavior in Complex Deltaic Sediments, Brent Field, Northern North Sea

S. E. Livera

The Brent field is the largest oil field in the United Kingdom sector of the northern North Sea, and its discovery in 1971 pioneered development in the area. The field contains some 3.4 billion bbl of oil in place, trapped in two reservoirs in a simple rotated fault block on the western margin of the Viking graben failed rift system. Three-fifths of the oil is found in an 800-ft section of deltaic deposits of the Middle Jurassic Brent Group, with a single oil water contact and an 850-ft hydrocarbon column. The lower part of this reservoir is a sand-rich, wave-dominated delta-front sequence overlain by delta-plain sediments. The first oil was produced in 1976 under a downflank artificial water drive.

Three dominant controls on the reservoir geologic framework have been identified: (1) individual delta lobe progradation and abandonment, (2) irregular periods of rapid subsidence followed by relative sea level still-stand, and (3) crestal collapse. The first two primary depositional controls combined to produce a layered reservoir in the delta-plain sequences characterized by fieldwide coals marking delta-lobe abandonment.

Extensive shales resulted from the periods of rapid subsidence. These shales now form vertical permeability barriers defining subreservoirs highlighted by repeat formation tester profiles.

Late Jurassic graben-associated tectonics initiated the fault block rotation and allowed crestal unroofing and erosion. Instability in the underlying plastic marine shales created complex rotational faulting in the Brent sediments. These crestal features complicate reservoir behavior, especially in the gas cap. Continued fault movement created a number of unconformities and condensed sequences in the overlying Upper Jurassic and Lower Cretaceous reservoir sealing shales.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.