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Abstract: Geology and Hydrocarbon Potential of the Nation River Basin and Yukon Thrust, East-Central Alaska and Canada


The Kandik area of east-central Alaska has long attracted attention as a potential petroleum province due to the presence of numerous surface bitumen seeps, excellent hydrocarbon source rocks, and an extensive Mesozoic to Paleozoic stratigraphic section. The area includes the Nation River basin, a half-graben of Tertiary age, and the Yukon Thrust, a major east-directed overthrust extending from the US about 8 miles into Canada. Geological, geochemical, and geophysical work was conducted across the US-Canada border to evaluate the hydrocarbon potential in Alaska and adjacent Yukon Territory. Prospective plays include Tertiary and pre-Tertiary traps in the Nation River basin in the US, and potential sub-thrust traps beneath the Yukon Thrust in the US and Canada. The prospective areas are remote from roads and infrastructure and are currently undrilled.

This study includes helicopter-supported field mapping over about 400 mi2, stratigraphic analysis, structural cross-sections, source rock geochemistry, one seismic line in the Nation River basin, regional gravity surveys and modeling, outcrop rock density, surface soil geochemistry, age dating, and fission track analysis. The data was collected in 1982-1992 by Arco Alaska, Inc., to evaluate the hydrocarbon potential of Doyon, Limited, lands and adjacent areas. This effort was extended from Alaska into the Yukon Territory because the US-Canada border bisects the Yukon Thrust, a dominant structural feature with at least 20 miles of eastward movement, emplaced Precambrian Tindir Group over Cretaceous and older rocks. Working in similar detail on both sides of the international border allows the Yukon Thrust system to be evaluated as a kinematic whole.

Two general areas were identified with hydrocarbon potential: the Nation River basin in Alaska and the Yukon Thrust in both Alaska and Yukon Territory. The Nation River basin play involves Tertiary sands as well as possible pre-Tertiary thrust rollovers beneath the Tertiary sediments. Bitumen veins up to 4 inches thick occur along the southeast margin of the Nation River basin, usually closely associated with Triassic Glenn Shale Formation. Geochemical analysis shows the bitumen is not biodegraded and was generated from the Glenn Shale, the equivalent rock unit in this area to the Shublik Formation, a prolific source rock on the North Slope of Alaska. The Glenn Shale at Michigan Creek, about 8 miles southwest of the Nation River basin, is 600 ft thick and contains an average TOC of 3.5%, and an average HI of 500. At the south end of the Nation River basin at Hard Luck Creek, 480 ft of Glenn Shale is exposed and averages about 2% TOC. HI values increase from 100-200 at the base to 300-420 at the top of the exposed section with an overlying covered interval. A seismic line about 8 miles long in the Nation River basin shows a half graben shape and hints of possible pre-Tertiary thrust structures. Bouguer and residual gravity maps show that the basin can be divided into a deeper basin to the south and a smaller, shallower basin to the north. The second derivative gravity map highlights the basin margins as well as several potential structures within the basin that may correspond to pre-Tertiary thrust features.

The Yukon Thrust spans the US-Canada border and is composed of upper plate Precambrian Tindir Formation overlying Cretaceous Monster Formation and older rocks in the lower plate. The presence of lower plate Paleozoic rocks is confirmed by gravity modeling which incorporates density measurements from 1100 outcrop samples. Bitumen occurs in the upper plate primarily along faults that allow vertical migration to the surface. Bitumen is also found in scattered localities in the footwall rocks in Canada. These solid hydrocarbons evidently originate from Paleozoic source rocks, such as the Calico Bluff, McCann Hill Chert, and Road River Formations, which in outcrop show good hydrocarbon source potential. The Glenn Shale Formation, mapped as Shublik in Canada, is missing over a wide area at the front of the Yukon Thrust and is also likely absent beneath at least some of the Yukon Thrust. The Glenn Shale was removed by a late Triassic or younger unconformity that also thinned and removed the Permian Tahkandit Limestone at the front of the Yukon Thrust. Reservoir rocks beneath the Yukon Thrust are likely Paleozoic carbonates, including the Ogilvie Formation which contains bitumen in a reef facies outcrop and is a gas reservoir in the Eagle Plains basin in Canada. Several mapped surface anticlines in the upper plate of the Yukon Thrust may be indications of sub-thrust traps.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah