Lithofacies, Diagenesis and Porosity of the Ivishak Formation, Prudhoe Bay Area, Alaska
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.
Figures & Tables
Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.