Lithofacies, Organic Geochemistry, Petrology, and Sequence Stratigraphy of the Ellesmerian Sequence

Abstract

Abstract The Tenneco Phoenix #1 well (OCS-Y-0338) cored a nearly complete section through the Pebble Shale (Cretaceous), Sag River and Shublik Formations (Triassic), and the Eileen and Ivishak Formations (Permo-Triassic). Cores of the Shublik Formation and the underlying Eileen and Ivishak Formations. (7, 800 to 8, 200 ft) were subjected to detailed geochemical and sedimentological analyses. These cores allowed comprehensive study of source rock and reservoir lithofacies from a known petroleum system within a sequence stratigraphic context. The basal (reservoir) section of the studied interval (Ivishak Sandstone) records nonmarine (fluvial) deposition during highstand. The overlying Eileen and lower Shublik Formations are marine lithofacies that developed during a major transgression (source rock and seal lithofacies). A distinctive glauconitic, pyritic and phosphatic unit (condensed interval) occurs near the top of this transgressive interval. Overlying upper Shublik strata record deposition during highstand conditions. The distribution of oil-prone facies within the Shublik Formation is related to stratigraphic position and is predictable within a sequence stratigraphic framework. The lower (transgressive) part of the Shublik Formation is dominated by oil-prone facies, whereas upper (regressive) Shublik strata are predominantly gas-prone to nonsource. Ivishak sandstones in the Phoenix #1 well are equivalent to the main reservoir interval in the Prudhoe Bay field. These sandstones have average porosity ranging from 18 to 22 percent. Cross-stratified medium-grained Ivishak sandstones have average horizontal permeability that exceeds 2 Darcies. Other cored Ivishak lithofacies have average permeability values that range from 138 to 353 md. Compaction and the precipitation of quartz and siderite cements reduced the effectiveness of pore systems in Ivishak sandstones. Overlying Eileen sandstones are cemented pervasively with calcite. Marine shales (Shublik and Pebble Formations) provide potential seals for the North Slope Alaskan petroleum system.