APPLICATION OF REGIONAL STRUCTURAL INTERPRETATIONS TO OIL AND GAS ASSESSMENTS OF FEDERAL LANDS IN THE FRONTAL BROOKIAN DEFORMED BELT OF ALASKA'S NORTH SLOPE

On Alaska's North Slope, the frontal part of the Cretaceous to Tertiary Brooks Range orogenic belt underlies the foothills and part of the coastal plain in the Arctic National Wildlife Refuge (ANWR) and the National Petroleum Reserve-Alaska (NPRA). Several gas accumulations and one oil accumulation have been discovered in this deformed belt, although all of the North Slope's producing oil fields lie in the undeformed foreland beneath the northern part of the coastal plain. As part of the 1998 USGS oil and gas assessment of the 1002 area in ANWR and the current USGS assessment of NPRA, we evaluate the fold-and-thrust-controlled petroleum plays. To do this, we develop a systematic understanding of the evolution of the thrust system through synthesis of regional seismic reflection data, field observations and thermal data; this provides constraints on deformational geometry and timing that are integrated with the stratigraphic framework, physical properties, source-rock geochemistry and thermal modeling in the oil and gas assessment process.

In the 1002 area of ANWR, the principal deformed-area petroleum plays include several Paleocene to Miocene passive-roof duplexes (frontal triangle zones) affecting Lower Cretaceous to Tertiary (Brookian) foreland basin strata, folded Mississippian through Jurassic (Ellesmerian and Beaufortian) strata, and two broad antiforms that developed beneath the northeastern part of the coastal plain above basement-involved thrusts. In southern NPRA, Brookian rocks are deformed in a broad Paleocene passive-roof duplex, and the frontal part of the thrust system is defined by a series of detachment folds in Brookian strata. Beneath the foothills of southernmost NPRA, allochthonous Mississippian carbonates and Devonian clastic rocks are involved in antiformal thrust stacks that have been drilled and imaged on seismic profiles; modest anticlinal closures are also present in parautochthonous Ellesmerian carbonate and clastic strata. The thermal history of the deformed belt indicates that these are largely gas-prone plays. In general, the timing of Tertiary thrust faulting may not have been ideal for trapping hydrocarbons, and reservoir quality and seal integrity may limit prospectivity in the deformed belt, compared to the relatively undeformed region to the north.