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Microbial Mats in the Shublik and Otuk Formations, Arctic Alaska: Implications for Improving Our Understanding of Continuous Resource Plays

Abstract

Parts of the Upper Triassic Shublik and Otuk Formations in Arctic Alaska are being studied to improve our understanding of permeability controls within continuous resource plays. Our approach is to study variability in mineralogy, and in the amount and type of organic matter using petrography, scanning electron microscopy, x-ray diffraction, and geochemical analyses. The Shublik Formation is a proven source rock, and has a significant carbonate component that can enhance brittleness, both of which make it a viable continuous-resource exploration objective. It has been typed as a source for oil in Prudhoe Bay and other fields across the North Slope of Alaska. The Otuk is a distal, siliceous, partially coeval formation with variable lithofacies and organofacies across the basin. Research on these formations focuses on the unique petrographic characteristics of self-sourced petroleum systems. One of the key issues is the control of lateral and vertical variability in original mineralogy and organic content and their effects on the storage versus migration of hydrocarbons. Preliminary results from six wells and three outcrop localities suggest that microbial mats were present in several facies across the Late Triassic depositional basin, including organic-rich marls, and that these mats acted as small-scale permeability barriers. Probable mats are black, organic-rich layers that are generally horizontal, follow the underlying topography, have pyrite framboids, and contain quartz and clay particles that were entrapped when the mat was at the sediment-water interface. Mat fragments ripped up by storm or current energy are present in some facies. These fragments have torn edges, contain pyrite framboids, and locally are recumbently folded, implying cohesiveness prior to re-deposition. The mats also show evidence of bioturbation, indicating that they were present on or near the sea floor. These small-scale mats may have acted as important permeability barriers within the source intervals, as evidenced by the presence of calcite spar cement above a mat interval, but not within or below. Organic-rich, carbonate or siliceous facies that contain continuous mat layers may retain more generated hydrocarbons, and thus be a more attractive self-sourced petroleum system target than other organofacies.