ABSTRACT: Petroleum Geology of the Coastal Plain of the Arctic National Wildlife Refuge, Northeastern Alaska

C. M. Molenaar, K. J. Bird, L. B. Magoon

The coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska has the potential for major petroleum accumulations. This area has many anticlinal structures, good oil-prone source rocks, and oil seeps and other surface indications of oil. The thickness and extent of reservoirs, however, are problematic, which places a wide range on estimated petroleum resources. In this remote area, resources must be very large to be economic.

Sedimentary rocks in the area range in age from Precambrian through Cenozoic and aggregate more than 20,000 ft in thickness. Post-Devonian strata generally are considered prospective for petroleum. In addition, underlying Precambrian to Devonian carbonate rocks, which are locally present in the Brooks Range to the south and in a few boreholes west of ANWR, are potential reservoirs in areas where they could be charged by overlying source rocks.

The Mississippian through lowermost Cretaceous section consists of shelf carbonate rocks and shallow-marine and nonmarine sandstone and shale that were deposited along a slowly subsiding, south-facing continental margin bordering a northern (present-day orientation) land area. Known as the Ellesmerian sequence, these rocks are about 3500 ft thick along the mountain front. The major reservoir rocks that are oil productive at Prudhoe Bay 75 mi to the west occur in this sequence. Early Cretaceous erosion related to Canada basin rifting, however, has removed much of this sequence in parts of the ANWR coastal plain. The overlying Brookian sequence, derived from an orogenic southern provenance, consists of at least 13,000 ft of Lower Cretaceous through Tertiary, northeasterly and northerly rograding basin, slope, and deltaic deposits. Excellent oil-prone source rocks occur at the base of this sequence, and overlying turbidites are potential reservoirs.

All but the undeformed northwest quarter of the ANWR coastal plain is part of a foreland fold and fault belt related to an Eocene and later phase of Brooks Range deformation. Seismic data indicate that Brookian rocks are deformed into many short-wavelength, complexly faulted folds that are structurally detached from underlying long-wavelength, moderately faulted structures in pre-Brookian rocks. Structural traps are believed to have formed before, during, and after oil generation and migration.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990