David W. Houseknecht¹, Christopher J. Schenk²
(1) U.S. Geological Survey, Reston, VA
(2) U.S. Geological Survey, Denver, CO

Abstract: Sequence stratigraphic framework for oil-prospective Cretaceous turbidites, National Petroleum Reserve – Alaska (NPRA)

Alaska North Slope exploration has focused increasingly on stratigraphic objectives, including turbidites within the Cretaceous-Tertiary Brookian Sequence. A regional 2-D seismic grid, sparse well control, and field work in the Brooks Range foothills provide constraints for an ongoing effort to establish a sequence stratigraphic framework for potential oil accumulations in Brookian turbidites, mostly in the Torok Formation.

The Torok Formation and overlying Nanushuk Group (both mostly Albian) display the overall geometry of a clinoform-topset couplet indicating northeastward migration of a shelf margin. Torok clinoforms reflect the evolution of spatially and temporally complex, slope and base-of-slope depositional systems that ranged from mud-dominated to mixed san/mud-dominated. Common associations include lowstand systems tracts, including sandstone-rich submarine fan facies, buried by highstand systems tracts, including muddy slope facies downlapping onto the lowstand deposits. Mass wasting, due to both lowstand grade adjustment and highstand oversteepening, appears to have played an important role in shaping the depositional surface and influencing sediment dispersal. Lowstand mass wasting commonly lead to development of turbidite channels incised into slope muds. Resulting amalgamated turbidite channel facies represent favorable objectives, as indicated by subsurface shows and oil-stained outcrops. An exhumed example of a lowstand incision displays a vertical succession of debris flow and slide block deposits overlain by oil-stained, amalgamated sandstones overlain by downlapping slope mudstones. The external dimensions ( 1 mi X 12 mi X 300 ft) and reservoir characteristics (9 – 20% porosity) of this incised channel deposit suggest it is a valid analogue for economically viable oil accumulations in NPRA.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana