Sedimentology and stratigraphy of the Nanushuk formation and related foreland basin deposits, central Brooks Range foothills, Alaska

Abstract

I interpret sedimentary facies and depositional environments from the Albian-Cenomanian Nanushuk Formation of Alaska's North Slope from sedimentary structures observed in core samples and in outcrop in the National Petroleum Reserve - Alaska (NPRA) and surrounding areas, and support these interpretations with supplemental analyses of geophysical well logs and ������Ar/�_��_Ar dating of volcanogenic deposits. The results have implications for the reservoir characterization of a shallow, frozen oil field at Umiat, Alaska, and for interpretations of the Colville foreland basin. In the central Brooks Range foothills the Nanushuk Formation comprises shallow marine and non-marine fades associations that can be grouped into marine-distributive and river-distributive systems. In wells at Umiat, Alaska, shoreface and wave-influenced deltaic sandstones (marine-distributive systems) occur at the base of the Nanushuk Formation, followed by a marine transgression and subsequent progradation of a delta complex (river-distributive system). The shift from marine-distributive to river-distributive systems is related to shelf building processes during the Albian-Cenomanian. Marine-distributive systems occurred at or near the shelf edge in a high-energy coastal environment, whereas river-distributive systems rapidly prograded over the shallow Nanushuk shelf following transgression. Marine-distributive conditions resumed when rapidly progradational river-distributive systems reached the shelf edge. This pattern occurs in the Umiat, Wolf Creek, and Grandstand No. 1 subsurface wells. The reservoir quality of the Nanushuk Formation varies strongly with facies. Well-sorted sandstones at the top of upward-coarsening successions in both marine- and river-distributive units have the highest permeabilities within the Nanushuk Formation. Marine-distributive units have low permeability anisotropy in contrast to river-distributive units due to better sorting in wave-influenced environments and the higher frequency of impermeable lamination surfaces in deltaic sandstones. Despite coarser grain size and similar depositional environments as lower Nanushuk Formation sandstones, transgressive units at the top of the Nanushuk Formation have low permeability, probably due to the presence of calcite cement. These results suggest hat the highest quality Nanushuk Formation reservoirs most likely occur in time transgressive, regionally extensive marine-distributive sandstones of the lower Nanushuk Formation.