National Petroleum Reserve-Alaska: A Review of Recent Exploration

Abstract

The National Petroleum Reserve-Alaska (NPRA) is a vast frontier with only about 50 exploratory wells drilled within its borders during the past half-century. Located west and southwest of the supergiant Prudhoe Bay-Kuparuk River complex, the NPRA is composed of four major structural- stratigraphic provinces: Barrow arch, Colville foreland basin, foothills belt, and northern Brooks Range overthrust. Stratigraphy is divided into three megasequences, including the Ellesmerian (Mississippian-Early Jurassic), Beaufortian (Middle Jurassic-Early Cretaceous), and Brookian (Early Cretaceous-Tertiary) sequences. Known oil and gas accumulations within the NPRA are associated with Ellesmerian, Beaufortian, and Brookian source rocks and Beaufortian and Brookian reservoir sandstones, and occur in local structures along the Barrow arch and in the foothills province. Due to their relatively small size and distance from existing infrastructure, these fields are noncommercial at present. Recent discovery of commercial reserves at Alpine field in the Colville River Delta area, immediately adjacent to the northeastern border of the NPRA, marks a new stage in the exploration of this region. Alpine contains an estimated 1.0 billion bbl oil in place and 340-370 MMbbl (million barrels) recoverable reserves in previously undiscovered Upper Jurassic shallow-marine sandstones (Alpine sandstone). The Alpine reservoir is the uppermost of three separate oil-bearing sandstones in the upper Kingak Shale, a major source rock interval. Although data from most wells in the new field remain confidential, existing information suggests the accumulation is stratigraphically entrapped. If this information is confirmed, this would vitalize previously overlooked play concepts in the NPRA. The building of new facilities at Alpine will also lower the minimum threshold field size for commercial production in the northeastern NPRA.