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Chevron U.S.A. has under lease nearly 2 million acres (800,000 ha.) of Arctic Slope Regional Corporation lands in the foothills of the Brooks Range between the Chukchi Sea and Canning River. In the central foothills, between the National Petroleum Reserve-Alaska and the Alaska pipeline, Chevron has conducted extensive field programs and air photo mapping, recorded 3,000 km (1,865 mi) of seismic data, and drilled three exploratory wells.
The Brooks Range foothills are underlain by complex thrust plates and associated foreland folds which contain deformed rocks ranging in age from Devonian to middle Cretaceous. Main thrusting occurred in latest Jurassic to Albian time, corresponding to an arc-continent collision possibly associated with the widening of the Canada basin. First orogenic pulses are recorded by Upper Jurassic turbidites and olistostrome units which reveal a southern clastic source, a major reversal in source direction from older sedimentary units. Lower Cretaceous foreland turbidites show progressive northward migration of underthrusted imbricating plates.
In the thrust belt, the primary reservoir objective is Lisburne limestone and dolomite, Mississippian to Permian in age. Seismic data identify a variety of structural styles of Lisburne plates ranging from complex stacks of imbricates to a single leading-edge plate underthrust by Lower Cretaceous foreland clastic units. Pore space in dolomitic Lisburne is filled with solid bitumen nearly everywhere on the surface in the central foothills, suggesting that extensive amounts of oil have migrated through
the rocks, and in at least one area, accumulated in older traps. Rich, oil-prone source beds and oil shales occur in rocks ranging from Devonian to Early Cretaceous in age. These rocks are typically mature or post-mature on surface thrust plates in the heart of the thrust belt, but become significantly less mature to the north along the leading edge thrusts.
In the fold belt, the primary objective is Lower Cretaceous sandstone. Surface and seismic mapping reveal numerous open folds, whose location is controlled by more deeply seated thrust fault geometry. Cretaceous shale units are typically gas-prone and organic-lean, but the Umiat field demonstrates that oil has migrated into shallow structures, perhaps from a significant distance. Also, residual oil shows are common in many wells throughout the fold belt. The relationship of the source of the Umiat oil to the fold and thrust belt and its implications for exploration potential are yet to be fully understood.
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