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Remote area has petroleum potential The North Chukchi basin straddles the Russian and US outer continental shelves, and international waters of the Arctic Ocean Alan Bailey Petroleum News
Despite its extremely remote location, the North Chukchi basin, a large sedimentary basin under the northern Chukchi Sea, is of considerable interest, both from an international political perspective and from its potential as a source of oil and gas. During the American Association of Petroleum Geologists Pacific Section annual meeting on May 22, U.S. Geological Survey geologist Dave Houseknecht overviewed what is known about the geology of the basin. The 950-kilometer-long basin straddles an area that includes the U.S. Chukchi shelf, the outer Russian Siberian shelf and international waters of the Arctic Ocean, Houseknecht said.
Data about the basin is sparse, consisting of gravity and magnetic data, and some recently acquired seismic data. Data are available from five exploration wells drilled in the U.S. Chukchi Sea shelf to the south of the basin. The nearest well to the west is located on the Russian Siberian shelf at a distance of some 4,000 kilometers, Houseknecht said.
Brookian fill The data indicate that the basin is filled with strata of the Brookian sequence, the youngest and shallowest petroleum bearing rock sequence of the Arctic Alaska region. The use of seismic data coupled with the known characteristics of Brookian rocks elsewhere indicates that the basin attains a depth of 20 kilometers or more, Houseknecht said.
By using seismic data to trace the strata in the North Chukchi basin back to rocks penetrated by the Chukchi Sea wells it is possible to infer the general nature of the stratigraphy of the basin. It appears that the oldest rocks in the basin are of lower Cretaceous age, possibly a little older, broadly equivalent to the Torok and Nanushuk formations of the Alaska North Slope. There are multiple geologic unconformities within the rock sequence, with a major unconformity in the middle Brookian appearing to be associated with a substantial thickness of Brookian strata not found on the Chukchi shelf, Houseknecht said.
It appears that on the Russian side of the basin, the Brookian sediments rest directly on oceanic crust rather than on older sedimentary strata as is observed in Arctic Alaska. But, on its eastern side the basin floor appears to transition towards continental crust. It, thus, appears that the basin opened radially, beginning in the Jurassic, at the same time that the Canada basin, to the east, was opening to form the region of the Arctic Ocean to the north of present-day Alaska - the Russian side of the North Chukchi basin presumably fully opened as an oceanic basin, while the Alaska side did not open as completely.
Sediments derived from the Chukotka mountain belt in eastern Russia and the emerging Brooks Range of Arctic Alaska poured northward into the basin to form the Brookian sequence. Of particular note in this sequence in the basin are massive clinothems, systems of sedimentary deposits that build out from land into a marine basin. A giant clinothem that straddles the Eocene and Pliocene epochs is one of the largest of its kind anywhere in the world, Houseknecht said.
Petroleum potential Based on what is known about regional petroleum source rocks and the stratigraphy of the basin inferred from the seismic data, there appear to be three source rock intervals in the basin. But the two older intervals have probably been heated to metamorphic grade, Houseknecht said. So, there is probably only one known viable petroleum source, a rock interval referred to as the Paleocene-Eocene Thermal Maximum, or PETM. This is an Arctic regional source rock which has been penetrated by a research well drilled into the Lomonosov Ridge, a submarine feature in the central Arctic Ocean.
It appears that the oil window in the North Chukchi basin, the depth range at which subsurface temperatures would generate oil from petroleum source rocks, straddles an unconformity in the mid-Eocene and the giant clinothem above that unconformity. This clinothem has significant potential for holding source rocks, especially in the region underlying international waters, Houseknecht said.
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