Gas in Alaska's Western North Slope: A USGS Assessment of Older Strata

By the USGS Western Assessment Team: David W. Houseknecht, Tracey J. Mercier, Christopher J. Schenk, Thomas E. Moore, William A. Rouse, Julie A. Dumoulin, William H. Craddock, Richard O. Lease, Palma J. Botterell, Margaret M. Sanders, Rebecca A. Smith, Christopher D. Connors, Christopher P. Garrity, Katherine J. Whidden, Jared T. Gooley, John W. Counts, Joshua H. Long and Christina A. DeVera. USGS Fact Sheet 2021–3003, February 2021.

Using a geology-based method, the U.S. Geological Survey estimated a mean of 1,407 billion (1.4 trillion) cubic feet of gas in conventional accumulations in Upper Devonian to Lower Cretaceous rocks of Alaska's western North Slope.

The three assessment units, tectonic elements and exploration wells. Base: National Park Service. Map: USGS.

The area

The western North Slope lies north of the Brooks Range, west of the National Petroleum Reserve–Alaska (NPR–A) and east of the Chukchi Sea, including adjacent State waters. Earlier assessments covered its younger Nanushuk and Torok formations (2017) and the whole North Slope's continuous resources (2012), so this one covers only rocks older than the Torok and younger than the acoustic basement.

Data are sparse: three exploration wells drilled between 1978 and 1982, and about 500 miles (800 kilometers) of old (1970–1971) 2D seismic lines, backed by geologic maps and data from the NPR–A and Chukchi Sea.

A crossroads of tectonic domains

The area sits where several tectonic domains, from Late Devonian to Paleogene, meet:

1. Hanna trough: a failed rift of the Late Devonian, a series of north–south grabens.
2. Sag basin: after rifting stopped in the Mississippian, the trough became a broad sag basin between the Chukchi and Arctic platforms.
3. Colville foredeep: Jurassic–Early Cretaceous Brooks Range tectonism loaded the crust, forming a foredeep that merges with the southern Hanna trough here.
4. Lisburne Hills: deformed in the Early Cretaceous as the Wrangel-Herald arch thrust belt rode over the Chukchi platform.
5. Paleogene Brooks Range tectonism thrust older rocks in the south and folded the Torok and Nanushuk in the north.
6. Hope Basin: extensional faulting buried parts of these under thick Eocene and younger sediment.

Generalized rock column of the assessment area. Chart: USGS.

Why gas, not oil

• Deeper water: parts of the Hanna trough hold deeper-water, more distal rocks than elsewhere on the North Slope.
• Source rocks: as across the region, the Triassic Otuk-Shublik Formation and the Jurassic–Lower Cretaceous Kingak Shale, plus the Hue Shale. In the Kugrua, Tunalik and Klondike wells, even the upper Sadlerochit Group is source rock — the same interval that is reservoir at Prudhoe Bay.
• Too hot: the rift, sag and foredeep buried everything deeper and made it more mature than elsewhere, too hot to preserve oil.
• Only gas found: the Tunalik well, less than 20 miles east, went through more than 9,000 feet of rock and found only overpressured gas. The gas cannot be traced to a source, so the area is treated as part of the Arctic Alaska Composite Total Petroleum System.

Three assessment units

Inputs for the two assessed units (minimum / median / maximum, mean):

The results

Only accumulations of at least 30 billion cubic feet of gas (about 5 million barrels of oil equivalent) were counted. Oil was not assessed: the chance of even one field that size was negligible.

Gas in billion cubic feet (BCFG). F95 means a 95 percent chance of at least that amount. NGL: natural gas liquids, ranging 0 to 39 million barrels in total.

More assessments: the USGS Energy Resources Program.

Sources

Based on Assessment of Undiscovered Gas Resources in Upper Devonian to Lower Cretaceous Strata of the Western North Slope, Alaska, 2021, USGS Fact Sheet 2021–3003, U.S. Geological Survey; a work of the United States government in the public domain. The map, the rock column and both tables are taken from the fact sheet's PDF.