Abstract: Stratigraphic Controls on Reservoir Performance and Development in Deltaic Deposits of the Ivishak Sandstone, Prudhoe Bay Field, Alaska

Robert S. Tye, Janok P. Bhattacharya, James A. Lorsong, Scott T. Sindelar

Significant remaining reserves in Prudhoe Bay field are confined within deltaic rocks at the base of the Triassic Ivishak Sandstone. Traditional lithostratigraphic correlations treated this basal reservoir interval as tabular zones between marine shale and overlying coarse-grained, fluvial sandstones. A reassessment of this interval based on cores and genetic stratigraphic correlations, depicts en echelon, offlapping, fluvially dominated deltaic wedges.

A paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low sinuosity rivers. This alluvial plain graded into a delta plain cut by distributary channels that fed distributary-mouth bars on a broad delta front. River dominance is inferred from the abundance of unidirectional current structures, normally graded beds, soft-sediment deformation, and general absence of wave-formed and biogenic structures. Slumping and growth faulting locally replaced upward-coarsening deltic successions with sharp-based, over-thickened mouth-bar and distributary-channel deposits.

Reservoir-quality rocks occur in delta front, distributary-mouth bar, and distributary channel facies. Mudstones deposited following delta-lobe abandonment form laterally extensive flow barriers between lobes. Compartmentalization is most pronounced distally, where deltaic sandstones are overlain by and pass laterally into marine shale. Proximally, fluvial and deltaic sandstones are juxtaposed across erosional contacts, improving reservoir continuity.

This stratigraphic interpretation is corroborated by production and surveillance data, plus an interference test. Locally, poor waterflood performance reflects completions in different lobes rather than previously interpreted lateral discontinuities. In some cases, production can be enhanced with recompletions rather than infill drilling.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994