Squinting Through Leaded Glass: A Public Domain View of the Alpine Play in the National Petroleum Reserve in Alaska (NPRA)

David W. Houseknecht: USGS, Reston, VA

Abstract

The 1994 discovery of the Alpine oil field (>400 MMBO recoverable), the resultant 1999 Federal lease sale in northeast NPRA, and the 2001 announcement of discoveries of commercial quantities of hydrocarbons apparently in Alpine-type traps within NPRA have reinvigorated exploration interest in a formerly moribund part of the Alaska North Slope. Limited data available in the public domain suggest that key ingredients of Alpine-type accumulations include a high gravity oil charge apparently sourced from a condensed section in the Lower Jurassic and stratigraphic traps in the Upper Jurassic comprising a transgressive assemblage of lenticular, fine-grained, well winnowed shoreface sands effectively sealed by condensed mudstone.

Regional mapping of Jurassic depositional sequences using public domain, 2-D seismic and well data indicates excellent potential for extending the Alpine play westward in NPRA. Although the Upper Jurassic play extends across the entire NPRA, the greatest potential may exist in eastern NPRA where a Lower Jurassic condensed section is present basinward (south) of a well defined shelf margin. Upper Jurassic depositional sequences offlap that shelf margin and include thick clinoforms that downlap onto the Lower Jurassic condensed section. Those clinoforms either toplap progradational shoreface sands or are truncated by sequence bounding unconformities capped by transgressive systems tracts that locally contain Alpine-type stratigraphic traps. From the Colville delta, this prospective depocenter trends southwestward for more than 100 miles. However, play depths increase southwestward, thereby increasing the chances of reduced reservoir quality and increased proportions of thermogenic gas.

Elsewhere in NPRA the apparent absence of both the Lower Jurassic basinal condensed section and clear migration pathways within Upper Jurassic strata may indicate increased charge risk and lower resource potential.