Forty oil samples from across the North Slope of Alaska have been analyzed by the U.S. Bureau of Mines and the U.S. Geological Survey. Results of these analyses suggest two separate genetic oil types. The first, the Simpson-Umiat oil type, occurs in reservoir rocks of Cretaceous and Quaternary age and includes oil from seeps in the Skull Cliff, Cape Simpson, Manning Point, and Ungoon Point areas, and oils from Wolf Creek test 3, and the Cape Simpson and Umiat oil fields. 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. Also, these oils have ^dgr¹³C values ranging from -29.1 to -27.8 parts per thousand (ppt) and ^dgr³⁴S values from -10.3 to -4.9 ppt. The s cond type, the Barrow-Prudhoe oil type, occurs in reservoir rocks of Carboniferous to Cretaceous age and includes oils 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 slight even-numbered n-alkane predominance and pristane to phytane ratios less than 1.5. Also these oils have ^dgr¹³C values of -30.3 to -29.8 ppt and ^dgr³⁴S values from -30.0 to +2.1 ppt. The two types are believed to originate 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 a siliciclastic source rock. Occurrences of the two oil types when outlined on a map, indicate at least two areas for additional exploration: for the Barrow-Prudhoe type, in stratigraphic traps along and adjacent to the Barrow arch, and for the Simpson-Umiat type, in Cretaceous rocks along the trend between the Simpson and Umiat oil fields and in Cretaceous and Tertiary rocks from Prudhoe Bay field to the William O. Douglas Arctic Wildlife Range.