GAS AND OIL PLAYS RELATED TO FRONTAL THRUSTS AND FOLDS OF THE BROOKS RANGE FOOTHILLS AND ARCTIC COASTAL PLAIN, CENTRAL NORTH SLOPE, ALASKA

The Brooks Range foothills and adjacent coastal plain of Alaska's central North Slope record a transition in structural style from thin-skinned in the west to strongly basement-involved in the east. Additionally, there is a structural transition from south to north across the Colville Basin from a complexly deformed thrust belt in the south to a fold belt in the north as deformation dies out at the front of the orogenic belt. These regional structural variations formed the basis for nine structural plays used in the recent USGS oil and gas assessment of the State lands on the central North Slope.

The frontal fold belt, defined by Tertiary detachment folding in Lower Cretaceous to Paleogene Brookian foreland basin clastic rocks, is the structural setting of four Brookian structural plays. The dominant structures are anticlines with thousands of feet of structural relief. Broad, low-amplitude detachment folds characterize the deformation front; the timing and position of these northernmost folds might have been ideal for trapping oil, whereas more southerly folds are likely more prospective for gas.

A Beaufortian structural play is based on widespread thrust imbrication of Lower Cretaceous Kemik Sandstone and of Lower Cretaceous shelf-margin sandstones in the Kingak Shale. This imbrication is localized where the primary thrust detachment steps up from the Jurassic to Lower Cretaceous Kingak Shale into the Lower Cretaceous Brookian rocks. This is primarily a gas play, as are others described below.

Prominent basement-involved faults and folds north of the northeastern Brooks Range front underlie a basement-involved structural play. Potential reservoirs in the Mississippian through Triassic Ellesmerian passive-margin sequence are folded with the basement. The undeveloped Kavik and Kemik gas fields exemplify this play.

In the southern foothills, two thrust belt plays are defined based on: (1) an early Tertiary triangle zone dominated by Lower Cretaceous clastic rocks; and (2) a regionally extensive, thrust-imbricated antiformal stack dominated by Mississippian Lisburne Group carbonates. This stack was assembled during the main (Neocomian) phase of Brookian deformation, and was redeformed and transported northward during the early Tertiary deformation that produced the final configuration of the foothills belt.