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Abstract

The Permo-Triassic Ivishak Formation is the main reservoir interval of the Prudhoe Bay field, North Slope, Alaska. Studies of cores from the field area reveal that porosity development within the Ivishak Formation has a complex relationship dependent on both depositional (lithofacies) and post-depositional (diagenetic) history.

Four dominant lithofacies are identified: (1) interbedded very fine sandstones and mudstones; (2) parallel laminated carbonaceous fine sandstones; (3) multistory upward-fining medium sandstones; and (4) conglomerates. These lithofacies occur everywhere as upward-coarsening to conglomerate sequences. In the main field area the coarsening sequence is overlain by a gross upward-fining sequence of gravelly to medium-grained multistory sandstones, which thins dramatically to the north. Consideration of lithofacies and thickness variation leads to an interpretive model concerning evolution of the basin with respect to tectonics and sedimentation. Thus initial progradation of an active alluvial fan-delta system from the northeast was replaced by progressive transgression from the south of more distal upon proximal facies.

Petrographic characteristics of the rocks reveal that porosity development is related to the diagenetic history of each lithofacies. Porosity within the medium-grained sandstones is predominantly secondary because of dissolution of grains and early grain-replacement calcite. Porosity within the conglomeratic intervals appears to be much more of a primary (textural) origin.

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