The role of calcareous and shaly source rocks in the composition of petroleum expelled from the Triassic Shublik Formation, Alaska North Slope

Abstract

For nearly thirty years, the Triassic marine carbonate Shublik Formation has been suggested and confirmed as a key source rock for hydrocarbons in the North Slope of Alaska. The formation accounts for roughly one third of the oil in the supergiant Prudhoe Bay Field, and for nearly all of the oil in the second largest Kuparuk River Field. Recent studies of oil types in the vicinity of the Northstar Field suggested presence of “shaly” organofacies of the Shublik Formation based on the likely Triassic age and marine shale biomarker signatures of some analyzed oil samples. The current work fills the gap between biomarker analysis of predicted “calcareous” and “shaly” oil types and source rock geochemistry. Biomarker-based oil-source rock correlation confirms the presence of two genetically-distinct organofacies and related oil families. Both groups were deposited under a similar redox condition (anoxic to suboxic) with dominantly marine algal input but in either (1) a clay-rich or (2) a clay-poor depositional setting. Chemometric evaluation of multivariate biomarker data reveals mixtures with variable degrees of mixing between end members. Analysis of diamondoids confirms mixed oil types and establishes diamondoid signatures of source rock end-members. This allows for correlation of biomarker-poor, overmature Shublik source rock samples to oils, and extends these interpretations over large areas of the North Slope.