Petroleum Geology of Northern Part of Arctic National Wildlife Refuge, Northeastern Alaska

Kenneth J. Bird, Leslie B. Magoon, Cornelius M. Molenaar

The northern part of the Arctic National Wildlife Refuge and adjacent Native lands, an area between the Brooks Range and the Beaufort Sea encompassing about 2.4 million ac, is judged to have the geologic characteristics of a major petroleum province. Except for the underformed northwest quarter, the area is involved in an east-west-trending and northeast-trending, north-verging imbricate fold and thrust-fault system related to Brooks Range deformation.

The most likely petroleum-reservoir rocks are sandstones of Cretaceous and Tertiary age and intrabasement carbonate rocks of pre-Mississippian age. Clastic and carbonate reservoir rocks of Mississippian to Triassic age, similar to the reservoir rocks at Prudhoe Bay, were truncated by Early Cretaceous erosion related to rifting; these rocks are expected to occur in the southern part of the coastal plain.

Analyses of hydrocarbons from oil seeps and oil-stained rocks in outcrop suggest that three types of oil are present, all dissimilar to oils from the Prudhoe Bay area. The Hue Shale is postulated to be the most important source rock for oil. With a present-day geothermal gradient of about 30°C/km (1.6°F/100 ft), oil generation is expected to occur between depths of 3.7 and 6.9 km (12,000-22,500 ft), mostly within the thick Cretaceous and Tertiary (Brookian) sequence. Oil generation, accompanied by clay-mineral transformation and abnormal fluid-pressure development, probably began about 50 Ma at the southern edge of the coastal plain and progressed northward, reaching the coastline about 10 Ma.

Seismic data indicate that many more structures are present than were previously suspected. Short-wavelength folds, which may number in the hundreds, are complexly faulted and involve mostly Brookian rocks. These folds overlie one or more regionally extensive low-angle faults that structurally detach them from a relatively small number of long-wavelength, moderately faulted structures composed mostly of pre-Brookian rocks. Structural traps are believed to have formed before, during, and after oil generation and migration.

Mean value estimates of in-place oil and gas resources for a 1.5-million ac part of this area are 13.8 billion bbl of oil and 31.3 tcf of gas. The amount of economically recoverable oil in 26 seismically mapped structures is estimated at 3.2 billion bbl of oil (mean value). Evaluation showed that natural gas resources are not economically recoverable.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.