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ABSTRACT: Arctic National Wildlife Refuge--Petroleum Potential in One of the Last Alaskan Frontiers

Leslie B. Magoon

The coastal plain of the Arctic National Wildlife Range (ANWR) lies between two important petroleum provinces, the Prudhoe Bay area on the west and the Mackenzie delta on the east. The Prudhoe Bay area originally contained at least 13 billion bbl of oil and 31 tcf of gas, whereas the Mackenzie delta contains 1-2 billion bbl of oil and more than 10 tcf of gas. Presently, oil production from fields in the Prudhoe Bay area contributes 25% of the United States daily production. Estimated in-place undiscovered resources are a mean of 13.8 billion bbl of oil and 31.3 tcf of gas in part of the ANWR coastal plain.

The assessment of the ANWR petroleum potential can be viewed at four levels: basin analysis, petroleum-system study, play evaluation, and prospect evaluation. Basin analysis emphasizes regional geology, petroleum systems emphasizes the hydrocarbon plumbing systems, plays emphasize structural and stratigraphic reservoir trends, and prospects emphasize individual traps.

Rocks with petroleum potential in the coastal-plain area are mostly younger than Devonian and are divided into an older (Ellesmerian) sequence of Mississippian to Early Cretaceous age, and a younger (Brookian) sequence of Cretaceous and Cenozoic age. Basement rocks consist of Precambrian to Devonian metasedimentary and minor igneous rocks. South of the coastal plain, the Ellesmerian sequence is about 1 km (3300 ft) thick and is composed of nearly equal amounts of carbonate and clastic rocks. These rocks record marine and nonmarine deposition along a slowly subsiding continental margin in which the land lay to the north and the sea to the south. Ellesmerian reservoirs are the primary oil-producing rocks in the Prudhoe Bay area. An Early Cretaceous regional unconformity truncates the El esmerian sequence and older rocks northward across the ANWR coastal plain. This unconformity is believed to limit the occurrence of Ellesmerian units in much of the northern coastal-plain area. The Brookian sequence consists of as much as 7 km (23,000 ft) of marine and nonmarine siliciclastic deposits representing debris shed from the ancestral Brooks Range; depocenters migrated northeastward across a subsiding basement ridge and rifted margin. The Brookian sequence provided the deep burial and attendant heat necessary for hydrocarbon maturation in both Ellesmerian and Brookian source rocks on the North Slope.

Two petroleum systems, the Ellesmerian and Hue-Sagavanirktok, are important on the ANWR coastal plain. The most important source rocks in the Ellesmerian petroleum system are the Shublik Formation, Kingak Shale, and a pebble shale unit. Major migration conduits include the Sadlerochit Group and Sag River Sandstone, and important reservoirs include the pre-Mississippian and Ellesmerian-sequence carbonate and siliciclastic rocks. Traps formed during the Cretaceous and Tertiary. Early formed hydrocarbons occurred to the south and west of the coastal plain and migrated north-northwestward before Brooks Range thrusting in the Late Cretaceous to early Tertiary. The Hue-Sagavanirktok petroleum systems includes the Hue Shale as the most important source rock; major migration conduits are turb dite sandstones of the Canning Formation and faults, and important reservoirs are turbidite sandstones of the Canning Formation and deltaic sandstones of the Sagavanirktok Formation. Structural traps formed during the Tertiary in the coastal-plain area, coincident with migration of Hue-Sagavanirktok petroleum.

For the U.S. Geological Survey play assessment, the ANWR coastal plain was divided into two areas, undeformed and deformed, along the northwest flank of the Marsh Creek anticline. In-place oil and gas resources were assessed in seven plays: five to the northwest in the undeformed area and two to the southeast in the deformed area. Three-quarters of the petroleum resources is estimated to occur in the deformed area: 50% in the folded Ellesmerian/pre-Mississippian play, and 25% in the imbricate-fold belt play.

Twenty-six structural prospects, identified and mapped from seismic data, were assessed for recoverable oil by the U.S. Bureau of Land Management. Because of the high costs of development, operation, and transportation on the North Slope, a single accumulation of more than 440 million bbl of recoverable oil was judged to be the most likely minimum economic field size. Under the most likely case, 18 of the 26 prospects were modeled as being economic at least once during many Monte Carlo simulations. For these 18 prospects, conditional mean resource estimates ranged from 0.5 to more than 3.5 billion bbl of economically recoverable oil.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990