Detailed chemical analyses of crude oil from the North Slope of Alaska began with United States Bureau of Mines efforts about 35 years ago. The discovery of major commercial accumulations within the past 15 years has resulted in routine application of modern analytical techniques, with the resulting classification of North Slope oils into two chemically distinct, and presumably separately sourced, families. This report will review published analytical results obtained for North Slope oils to date, in light of data for nine specific North Slope oils analyzed by Union Oil.

The nine oils analyzed are from the National Petroleum Reserve in Alaska and Prudhoe Bay field, and include a condensate and at least four biodegraded oils. Gravity and sulfur content variations are 65-54.1° API and 0.01-1.85%, respectively. Carbon isotope ratios of total (untopped) oils vary between -29.4 and -25.3 ^pmil, and are a discriminating parameter for grouping these oils into two chemical families. Other distinguishing chemical attributes include vanadium, nickel, and sulfur concentrations, V/(V + Ni) ratios, carbon number distribution of the major 5(^agr), 14(^agr), 17(^agr), 20R-steranes, and i-C₁₉/i-C₂₀ isoprenoid ratios. Using these distinctions, the oils are successfully grouped into two types. Type A oils, typified by the Prudhoe crude, are r latively high in vanadium, nickel, and sulfur content, isotopically light, and high in tricycle terpane content. Type B oils, typified by the Umiat and Simpson crudes, are low in sulfur and metals and contain relatively high concentrations of 5(^agr), 14(^agr), 17(^agr), 20R-ethylcholestane. Although little definitive published work on potential source rocks of maturities less than peak generation is available, the oil typing demonstrated here and elsewhere strongly suggest at least two distinct source sequences. Based on the biological marker geochemistry of the oil types, the nature of these sequences may be predictable.

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