Shelf Sandstones or Wave-dominated Deltas: A Comparison of the Nuiqsut and Nechelik Formations, North Slope Alaska

Burns, Beverly A.¹, James MacEachern², David B. Bannan³, Douglas G. Knock³, Robert J. Kunemund³, Justine Boccanera⁴, Wayne J. Campaign⁵ (1) ConocoPhillips Alaska, Stravanger, Norway (2) Simon Fraser University, Burnaby, BC (3) ConocoPhillips Alaska, Anchorage, (4) CocnocoPhillips Alaska, Anchorage, (5) Conocophillips Alaska, Anchorage, Alaska

The Oxfordian Nuiqsut and Nechelik formations comprise stacked, coarsening and cleaning-upward, fine to very fine-grained, shallow marine sandstone successions, deposited on the North Slope of Alaska. Previous workers have suggested that these sandstones were deposited in a shelf to lower shoreface settings. The Nuiqsut Formation overlies the Nechelik Formation, which in turn is overlain by the Alpine A Sandstone. The Nuiqsut and the Nechelik formations range in thickness from 200 to 300 feet (60 to 90 m). The reservoirs are thoroughly bioturbated (BI 5) and contain very few preserved physical sedimentary structures. Core plug data indicate the sandstones are characterized by moderate reservoir quality and the sands are quartz-rich. Wire line log correlation and seismic mapping demonstrate that coarsening-upward cycles prograded from the north-northwest to south-southeast across a broad low relief Jurassic shelf, but cannot be traced long distances along inferred depositional strike. The sandstones are truncated to the north by two major unconformities (Upper Jurassic and Lower Cretaceous) and pinch out stratigraphically to the south. Isopach maps and core data indicate that the best quality rock occurs in the eastern portion of the depositional sequence, which actually corresponds to the thinnest portion of the reservoir. Detailed sedimentologic and ichnological analyses of the facies indicate that the sandstones were deposited in delta-front environments, and pass upward out of markedly heterolithic prodelta sediments. Integration of core, seismic, and wire line log data suggests that the Nuiqsut and Nechelik sandstones were deposited in wave-dominated to wave-influenced deltaic settings that were minimally influenced by storms.