Surface to subsurface correlation of the Shublik Formation: implications for Triassic paleoceanography and source rock accumulation

Abstract

The Middle to Late Triassic Shublik Formation in northern Alaska is the dominant source rock for Prudhoe Bay, the largest conventional hydrocarbon accumulation in North America, and is being studied as an unconventional reservoir because of recent shale oil exploitation in North America. This heterogeneous shale unit in the subsurface of northern Alaska provides insight on the evolution of upwelling systems during relative sea level change over geologic time. The accumulation, dilution, and more importantly preservation of organic matter are linked to relative changes in sea level, shelf bathymetry and paleoredox conditions. Multiple well correlations and isochore maps indicate thickness trends were influenced by topographical features and relative sea level during the Middle-Upper Triassic in northern Alaska. Sedimentary features, primary production, Corg preservation and oxygen concentrations were controlled by the upwelling-influenced position of the oxygen minimum zone during fluctuations in relative sea level. The Shublik Formation is a world class resource play with Corg concentrations up to 10 wt. % within the predominantly carbonate mudstone to wackestone transgressive systems tracts. Previous sequence stratigraphic interpretations of Middle-Upper Triassic deposits in northern Alaska indicate three, 3rd order depositional sequences. Evaluation of the closely spaced samples and general trends in lithology has led to a revised sequence stratigraphic architecture that includes an additional fourth depositional sequence in comparison to previous interpretations.