Kuparuk River is a giant oil field located on the Colville High, west of Prudhoe Bay field, on the North Slope of Alaska. The Kuparuk oils have values greater than 0.5 for the aromaticity (B) parameter (= toluene/n-C₇) and less than 1 for the paraffinicity (F) parameter (= n-C₇/MCH). Using two parameters in isolation, one would interpret Kuparuk as the residue from repeated episodes of phase segregation and gas-cap leakage. This process, termed “evaporative fractionation,” is experimentally proven to concentrate naphthenic and aromatic hydrocarbon with low fugacity in the parent oil phase while enriching the daughter gas-condensate phase in alkanes.

Reassessment of the geochemistry of the Kuparuk field within the context of a regional evaluation of North Slope petroleum systems suggests a very simple charge and filling model where the Kuparuk accumulation remained a single phase throughout its history. Oil and solution gas derived mainly from the Shublik Formation, together with gas and very light naphthenic-aromatic condensate derived from Kekiktuk Conglomerate coals shared a common migration path into Kuparuk field. This study illustrates that the B-F plot must be used with caution because factors other than evaporative fractionation, such as source input, can influence these ratios based on data from Southeast Asia, Alberta, and North Slope basins.

This study also suggests that the Sohio 1 Mukluk well did not successfully result from the presence of Kuparuk thief sands, thus petroleum migration continued to the Kuparuk field.