Central Arctic foothills, Alaska: a unique challenge in frontier exploration

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 3000 km (1965 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. 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. 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, perhapsmore » 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.« less