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Depositional Environments and Reservoir Geology of Dibblee Sandstone, South Cuyama Field

C. D. Atkinson, G. C. Gaynor, W. J. Ebanks, Jr.

South Cuyama field is a structural trap located in the center of the Cuyama basin. The reservoir interval, the lower Miocene Dibblee sandstone, is approximately 400-500 ft (120-150 m) thick and is folded into a complexly faulted asymmetric anticline associated with a major transverse fault zone.

Active tectonically induced subsidence characterized the Cuyama basin during deposition of the Dibblee sandstone. The sandstones are massive plagioclase-rich lithic arkoses, intensely bioturbated, medium to coarse-grained, and only moderately consolidated. The sandstones were deposited as laterally extensive units, which have since been offset by numerous faults. Faunal evidence and burrow types, especially abundant Ophiomorpha, indicate that the sandstones were deposited in a marine-shelf environment. A large-scale coarsening-upward and then fining-upward grain-size trend indicates progradation and shallowing followed by transgression and deepening of the shelf.

The sandstones have porosities of approximately 20-30% and permeabilities ranging from 10 to 2,500 md. Variations in reservoir quality result from changes in grain size and sorting, degree of carbonate cementation, and amount of authigenic clay present. Major reservoir heterogeneities include numerous sealing faults, laterally extensive intercalated sandy siltstones, and discontinuous zones of carbonate cemented nodules.

The Dibblee sandstone can be considered analogous to the shallow marine Upper Jurassic Fulmar Sandstones in the central North Sea. Both units are very similar in their sedimentological characteristics and were deposited on narrow shelves in tectonically active basin settings.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.