Gas Leaking From 20,000 Feet Down: The USGS Assessment of the North Chukchi Basin

Seismic images of the North Chukchi Basin show gas chimneys rising from the deep basin — signs that gas is leaking upward from more than 20,000 feet (6.1 km) down. The U.S. Geological Survey, assessing the basin's conventional resources, estimates a mean of 119,901 billion cubic feet of gas, 1,759 million barrels of oil and 842 million barrels of natural gas liquids, all undiscovered and technically recoverable.

Where it is

The basin lies beneath the outer continental shelf, slope and rise of the Chukchi Sea and East Siberian Sea, 150 to 215 miles (240–345 km) northwest of Arctic Alaska. It covers more than 135,000 square miles (355,000 km²) — about 690 mi (1,100 km) west to east and 285 mi (460 km) south to north — and straddles the outer Exclusive Economic Zones of the United States and Russia and the international waters beyond them.

The North Chukchi Basin assessment unit, the U.S.–Russia maritime boundary, the outer limits of both Exclusive Economic Zones, and nearby exploration wells. USGS.

Almost no one has drilled it. The nearest exploration wells are 20 to 100 miles to the south and southeast on the U.S. Chukchi shelf. The assessment rests mainly on about 20,000 miles (32,000 km) of two-dimensional seismic data from commercial, academic and public sources — densest in U.S. waters, sparser elsewhere — plus regional gravity and magnetic maps.

How the basin formed

Rifting pulled the continental crust of the Chukchi and East Siberian shelves on the south away from the Chukchi Borderland, the Chukchi Plain (oceanic crust) and the Mendeleev Rise on the north, likely between the Middle or Late Jurassic and the Early Cretaceous. The basin then collapsed and subsided from the Early Cretaceous through the Cenozoic, filling with sediment carried in from the south and southeast — probably from the Chukotka Orogen in Siberia and the western Brooks Range in Alaska. Along its axis the fill reaches about 75,000 feet (23 km) thick, much of it Cretaceous and Cenozoic strata of the Brookian sequence, spanning perhaps the Aptian to the Quaternary.

Depth to the base of the sediments, in two-way seismic travel time; the maximum of 11 seconds corresponds to about 75,000 feet. USGS.

Why mostly gas

The likely source rocks are the Hauterivian pebble shale unit, the Barremian-to-Turonian Hue Shale and upper Paleocene to middle Eocene strata. Modeling of "pseudowells" built where seismic lines cross shows the Cretaceous source rocks buried so deep and so hot that gas is likely the only stable hydrocarbon — they are in the gas window. Only the Paleogene source interval reached the oil window without passing into the gas window across much of the basin.

Four lines of evidence point to overpressured, basin-centered gas leaking upward through every available pathway: the gas chimneys; a zone of unusually slow seismic velocity marking the top of overpressure; and modeled strata that are overmature and overpressured throughout the deep basin. The top of the gas window and of overpressure generally lies between 15,000 and 20,000 feet. So oil is likely limited to depths shallower than about 20,000 feet, while gas may occur at any depth and is likely the only hydrocarbon below that.

The numbers

Inputs were based largely on amplitude anomalies in the seismic images, pointing to stratigraphic traps (shelf-margin deltas, slope channels, basin-floor fans) and structural traps (tilted fault blocks, and fold-and-thrust structures in the east). Only accumulations of at least 50 million barrels of oil or 300 billion cubic feet of gas were counted.

F95 means a 95-percent chance of at least that amount; the other columns are defined the same way. Of the gas, a mean 118,494 billion cubic feet is expected in gas fields and 1,407 as gas associated with oil.

Sources

Based on "Assessment of Undiscovered Oil and Gas Resources of the North Chukchi Basin," U.S. Geological Survey Fact Sheet 2024–3015, by the North Chukchi Basin Assessment Team (David W. Houseknecht, Craig P. Markey, Tracey J. Mercier, Christopher J. Schenk, Christopher D. Connors, Jared T. Gooley, Palma J. Botterell, Rebecca A. Smith, William A. Rouse and Christopher P. Garrity); a work of the United States government in the public domain.