Geology and stratigraphy of fluvio-deltaic deposits in the Ivishak Formation: Applications for development of Prudhoe Bay Field, Alaska

Significant remaining reserves in Prudhoe Bay field are confined within deltaic rocks at the base of the Triassic Ivishak sandstone. The initial stratigraphic characterization of the Prudhoe Bay reservoir was lithostratigraphically based, and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarsegrained, fluvial sandstones. A reassessment of this interval based on cores and genetic-stratigraphic correlations depicts en echelon, offlapping, fluvially dominated deltaic wedges.

Reservoir-quality rocks occur in distributary mouth bar, distributary channel, and fluvial facies associations. 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, tidal, and biogenic structures. Slumping and growth faulting locally replaced coarsening-upward deltaic successions with sharpbased, overthickened mouth bar and distributary channel deposits.

Mudstones deposited following delta-lobe abandonment form laterally extensive f low 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, stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes. Previously, poor well performances were attributed to sandstone pinch-outs. In some cases, production can be enhanced with recompletions rather than infill drilling. Nonconventional wells planned and completed with the benefit of detailed facies-association correlations currently are recovering millions of barrels of previously bypassed oil.