Lithofacies, Diagenesis, and Porosity, Ivishak Sandstone, Prudhoe Bay Area, Alaska: ABSTRACT

Abstract

End_Page 605------------------------------The Permo-Triassic Ivishak Sandstone is the main reservoir interval of the Prudhoe Bay field, North Slope, Alaska. Studies of cored sequences from the field and offshore (Reindeer Island) reveal that porosity development within the Ivishak Sandstone has a complex relationship dependent on both depositional (lithofacies) and postdepositional (diagenetic) history. These factors are related to the tectonic history of the basin. Five dominant lithofacies are identified: (1) interbedded very fine sandstones and mudstones, (2) parallel laminated carbonaceous fine sandstones, (3) multistory upward-fining medium sandstones, (4) conglomerates, and (5) multistory upward-fining coarse to granular sandstones. These lithofacies occur everywhere as upward-coarsening to conglomerate sequences. In the onshore (Prudhoe Bay field) area the coarsening sequence is overlain by a gross upward-fining sequence of gravelly to medium-grained multistory sandstones. This thins dramatically to the north, and is absent at Reindeer Island. Consideration of lithofacies and thickness variation leads to an interpretative model concerning evolution of the basin with respect to tectonics and sedimentation. Thus initial progradation of an ac ive 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 intimately related to lithofacies. Porosity within the medium-grained sandstones is predominantly due to dissolution of early nodular calcite. Porosity within the conglomeratic intervals appears to be much more of a primary (textural) origin. Hypothetical porosity profiles can thus be constructed based on predicted lithofacies distribution across the area in any direction. End_of_Article - Last_Page 606------------