2019 AAPG Annual Convention and Exhibition:

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Geological and Petroleum Systems Framework of the ANWR Coastal Plain (Arctic National Wildlife Refuge 1002 Area)


A 2017 law allowing exploration in the ANWR 1002 area has focused attention on the petroleum systems framework of the 1.6-million-acre Federal land. This presentation reviews the geological framework of the area, describes critical geological uncertainties, and presents ongoing work to better understand petroleum potential. This work is based on interpretation of 2,250 km of vintage 2-D seismic data in the 1002 area and a denser grid of 2-D data, sparse 3-D data, and limited well control west and north of ANWR. New field data, including structural observations, source- and reservoir-rock quality, and oil geochemistry, also are integrated into the analysis.Two main tectonic pulses influenced ANWR petroleum systems. First, Jurassic-Neocomian extension spawned grabens containing pre-Cretaceous strata near the modern coast. Contemporaneous rift shoulder uplift onshore resulted in northward truncation of pre-Cretaceous strata beneath a Lower Cretaceous unconformity (LCU) and outcrop and seismic data indicate that pre-Mississippian basement rocks subcrop the LCU north of the Sadlerochit Mountains. Critical uncertainties include the presence or absence, spatial distribution, and sedimentary fill of grabens beneath the 1002 area, as pre-Cretaceous source and reservoir rocks may be present there. Second, Paleocene-Holocene northward propagation of the Brooks Range tectonic front deformed pre-tectonic strata (including Cretaceous source rocks), built a structural wedge and wedge-top depositional system, and progressively deformed syntectonic strata in the wedge-top basins. Onshore, the result is a thick and complex structural-stratigraphic wedge that may have disrupted oil generation and migration from pre-tectonic strata and may have modified trap and seal geometry and integrity. Offshore, this wedge thickens to more than 7 km north of the rift hinge and extends to the modern toe-of-slope more than 100 km offshore. This system generated overpressure, buried Cenozoic and older source rocks to elevated thermal maturity, and inverted rift-phase grabens. This combination likely produced a heterogeneous hydrocarbon charge, which migrated an unknown distance onshore as evidenced by geochemistry of oil along the ANWR coast. The USGS is integrating multiple data sets, including newly reprocessed ANWR seismic data, to reduce petroleum systems uncertainties. Results will inform federal lease-sale preparation and provide a more robust petroleum systems framework for the region.