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 del taic rocks at the base of the Triassic Ivishak sandstone. The initial strat igraphic characterization of the Prudhoe Bay reservoir was lithostratigraph ically based, and it depicted this basal reservoir interval as tabular zone s between marine shale and overlying coarse-grained, fluvial sandstones. A reassessment of this interval based on cores and genetic-stratigraphic corr elations depicts en echelon, offlapping, fluvially dominated deltaic wedges . Reservoir-quality rocks occur in distributary mouth bar, distributary chann el, and fluvial facies associations. A paleogeographic reconstruction of on e delta lobe includes an alluvial plain crossed by channels of possibly bra ided or low-sinuosity rivers. This alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad d elta front. River dominance is inferred from the abundance of unidirectiona l current structures, normally graded beds, soft-sediment deformation, and general absence of wave-formed, tidal, and biogenic structures. Slumping an d growth faulting locally replaced coarsening-upward deltaic successions wi th sharp-based, overthickened mouth bar and distributary channel deposits. Mudstones deposited following delta-lobe abandonment form laterally extensi ve flow barriers between lobes, Compartmentalization is most pronounced dis tally, where deltaic sandstones are overlain by and pass laterally into mar ine shale, Proximally, fluvial and deltaic sandstones are juxtaposed across erosional contacts, improving reservoir continuity. This stratigraphic interpretation is corroborated by production and surveil lance data plus an interference test. Locally stratigraphy and poor waterfl ood performance reflect completions in diachronous sandstones that originat ed in separate deltaic lobes, Previously, poor well performances were attri buted to sandstone pinch-outs, In some cases, production can be enhanced wi th 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.