The North Slope of Alaska is a proved petroleum province containing numerous seeps, many small undeveloped oil fields, and the largest oil field on the North American continent, Prudhoe Bay. Genetic relations among oils in the NPRA (National Petroleum Reserve in Alaska), the Prudhoe Bay area, and the Arctic Wildlife Range have important implications for future exploration.

Forty-two oil samples from across the North Slope analyzed by the U.S. Bureau of Mines and the U.S. Geological Survey suggest two separate oil types, even though some oils are biodegraded. The first, the Barrow-Prudhoe oil type, is present in reservoir rocks of Carboniferous to Tertiary age and includes oils from South Barrow gas field, Prudhoe Bay oil field, and the Fish Creek 1 test well. Physical properties of Barrow-Prudhoe oils are variable, but in general the oils are medium-gravity, high-sulfur, with a slight even-numbered n-alkane predominance and pristane-to-phytane ratio of less than 1.5. The second type, the Simpson-Umiat oil type, is present in reservoir rocks of Cretaceous to Quaternary age and includes oils from seeps in the Skull Cliff, Cape Simpson, Manning Point, and ngoon Point areas, the Wolf Creek 3 test well, and the

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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 two types probably originate from different sources, the Barrow-Prudhoe type from a carbonate or other iron-deficient source rock, and the Simpson-Umiat type from a siliciclastic source rock. Distribution of the two oil types indicates at least two exploration fairways. The fairway for the Barrow-Prudhoe type is along the Barrow arch, and the fairway for the Simpson-Umiat type coincides with the area of best reservoir development of the Nanushuk Group.

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