Exploration Challenges and Opportunities in the Alaska Foothills, North Slope, Alaska

By

J.M. Van Fleet, G.F. Hebertson, J.L. Burton, and D.L. Buscarello (Anadarko Petroleum)

Following clues from surface geology and oil seeps in the 1940s, early Brooks Range foothills explorers drilled the Umiat discovery, a nonproduced oil field containing approximately 100 MMBO. Subsequent drilling in the region has resulted in eight gas discoveries. With the recent emphasis on producing stranded North Slope gas through a newly proposed pipeline, exploring for both oil and gas in northern Alaska has become attractive.

The Brooks Range foreland basin targets consist of Carboniferous carbonates and Mesozoic clastics. Clastic reservoir quality is challenged due to burial depths of 8,000 to 15,000 feet (2438–4572 m), but may be enhanced by secondary porosity and fractures. The classic northern Alaska source rocks range in age from Triassic to Cretaceous. Basin modeling suggests they entered the oil window due to burial by overlying, prograding Cretaceous sediments prior to trap formation at 60 ma. Continued burial and the critical timing of hydrocarbon generation and trap formation explain the preponderance of gas discoveries.

Seismic, apatite fission track and field data suggest several episodes of foothills tectonism between about 120–25 ma. Present day south central foothills structures resulted from the 60 ma event and form the basis for all known traps. Combination structural-stratigraphic traps, however, may exist. Seal rock at the discovered accumulations consists of siltstones and shales with seal capacities able to support hydrocarbon columns ranging up to 6500 feet (1981 m). Exploring in the foothills is both sensitive and difficult. In addition to challenging technical issues, harsh, remote operating conditions and native and environmental concerns must be addressed to insure success for operators. Gas from the foothills will be important in sustaining a long-term northern Alaskan supply as existing gas is depleted.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.