Sequence Stratigraphic Framework for Oil- and Gas- Prospective Brookian Turbidites, National Petroleum Reserve - Alaska (NPRA)

David W. Houseknecht: USGS, Reston, VA; and Christopher J. Schenk: USGS, Denver, CO

Abstract

Integrated field and subsurface studies of the Albian Torok and Nanushuk formations (coeval clinoform and topset couplet) have produced a sequence stratigraphic framework for identifying fairways most likely to contain stratigraphic traps in Brookian turbidites in NPRA. Stratigraphic traps in the Torok involve lowstand, sand-rich turbidite facies sealed by condensed mudstones deposited during transgression. The lowstand deposits commonly downlap onto, and interfinger basinward with, oil-prone GRZ source rocks.

Optimum reservoirs occur in amalgamated turbidites deposited in channels incised into mudstones of the mid-lower slope or in channel-levee systems on submarine fans; the former may grade downslope into the latter within a single stratigraphic trap. Reservoir potential also may occur in backstepping fan lobes deposited during earliest transgression.

At least 15 Albian shelf margins displaying lowstand sequence boundaries have been mapped in NPRA using public domain, 2-D seismic data. Shelf margins are generally oriented north-south in northern NPRA and swing eastward to parallel the Brooks Range in southern NPRA. Sediment dispersal patterns and accommodation may influence the size and distribution of potential stratigraphic traps in turbidite facies along these shelf margins. For example, deposition in distal parts of the sediment dispersal system combined with low accommodation may favor the occurrence of point-sourced, isolated lenses of turbidite facies in northern NPRA. In contrast, deposition in proximal parts of the sediment dispersal system combined with high accommodation may favor the occurrence of line-sourced, laterally coalescing aprons of turbidite facies in southern NPRA.