USE OF A NEW CIRCUM-ARCTIC CHEMOMETRIC MODEL TO STUDY MIXTURES OF SHUBLIK AND YOUNGER OILS IN NORTHERN ALASKA

Chemometric (multivariate) analysis of biomarker and isotopic data were used to infer the age, lithology, and identity of the source rocks for >1000 Circum-Arctic crude oil samples collected above ~60^oN latitude. On the North Slope, two of the seven oil groups determined by this method originated from marine carbonate and marine marl facies of the Middle-Upper Triassic Shublik Formation source rock. The so-called marl oils occur mainly in Triassic reservoirs near Prudhoe Bay Field (e.g., Prudhoe Bay-1, Gwydyr Bay S-1, and Foggy Island-1 wells). The so-called carbonate oils occur mainly in Lower Cretaceous reservoirs to the northwest of Prudhoe Bay along the Barrow Arch (e.g., Fish Creek-1, Phoenix-1, and Mukluk-1 wells). Three scenarios could explain the composition of the marl oils: (1) a marl facies of the Shublik Formation occurs near Prudhoe Bay, (2) carbonate oils mixed with oils from Jurassic-Cretaceous argillaceous source rocks (e.g., Kingak Shale, Hue Shale, pebble shale unit), or (3) a combination of the above.

This paper addresses the origins of the Shublik oil groups using chemometrics, age-related biomarker ratios, and geologic setting. Chemometrics shows that the carbonate and marl oils fall on a mixing line with oils that originated from Cretaceous Hue Shale in principal component space. Age-related biomarker ratios distinguish Triassic Shublik carbonate from Cretaceous Hue Shale oils but the marl oils give intermediate ages, consistent with mixing of oils at Prudhoe Bay. Nonetheless, some marl oils occur in geologic settings that preclude mixing with younger oils and their biomarker ages are consistent with distinct carbonate and marl facies of the Shublik Formation.