Composition of Outer Shelf Sandstones in the Torok Formation, Brooks Range Foothills, Alaska

By

R.A. Kirkham (University of Alaska, Fairbanks)

Aptian to Albian age sandstones of the Torok Formation were deposited by sediment gravity flows in outer shelf to basinal settings in the Colville basin. These sands have largely been ignored in past exploration efforts, with previous focus dominantly on shallow-water facies in the Nanushuk Group or younger strata. This study focuses on distal shelf sandstones of the Torok Formation from Arc Mountain, Slope Mountain, and the Grandstand test well. Sandstones from these locations are fine grained and include abundant detrital grains of quartz, chert, various lithics, and feldspar. Compositionally these sandstones plot as litharenite to sublitharenite and are derived from a quartzose to transitional recycled orogenic provenance. Visible porosity in the majority of sandstones is absent. Clay and mica make up most of the matrix. Initial analysis shows that chlorite, mica, kaolinite, and possibly some mixed-layer clays are present, but their abundance has not been determined. Composition and distribution of cementation is not clear from thin section analysis. Details of cementation are being addressed using XRD and SEM analysis. The high clay and mica content of these sandstones indicate limited reservoir potential in distal shelf settings. These results, combined with recent outcrop studies that document thick amalgamated packages of cleaner and coarser grained sandstones in base of slope settings, indicate that exploration efforts within the Torok Formation should focus on depositional settings off the shelf.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.