Two oil types on the North Slope of Alaska: implications for future exploration

Abstract

Forty oil samples from across the North Slope of Alaska have been analyzed by the US Bureau of Mines and the Us Geological Survey. Results of these analyses suggest two separate genetic oil types. The first type, the Simpson-Umiat oil type, occurs in reservoir rocks of Cretaceous and Quaternary age and includes oils from seeps in the Skull Cliff, Cape Simpson, Manning Point, and Ungoon Point areas; the Wolf Creek test well 3, and the Umiat oil field. These are higher gravity, low-sulfur oils with no, or slight, odd-numbered n-alkane predominance and pristane to phytane ratios greater than 1. 5. The second type, the Barrow-Prudhoe oil type, occurs in reservoir rocks of Carboniferous to Cretataceous age and includes ails from South Barrow gas field, Prudhoe Bay oil field, and the Fish Creek test well 1. Physical properties of Barrow-Prudhoe oils are variable, but in general the oils are medium-gravity, high-sulfur oils with a slight even-numbered n-alkane predominance and pristane to phytane ratio of less thon 1. 5. The two types are believed to originated from different source rocks; the Barrow-Prudhoe type may have originated from a carbonate or other iron-deficient source rock, and the Simpson-Umiat type from asiliciclastic source rock. Occurrences of fine two oil types, when outlined on a map, indicate atleast two exploration fairways. The fairway for the Barrow-Prudhoe type is along the Barrow arch, and the fairway, for the Simpson-Umiat type is the area of the best reservoir development for the Nanushuk Group.