Cracking, mixing, and geochemical correlation of crude oils, North Slope, Alaska

Abstract

Forty-one crude oil samples from the North Slope of Alaska have variable diamondoid and biomarker concentrations, indicating different extents of oil cracking. Some of the samples are mixtures of high- and low-maturity components containing high concentrations of both diamondoids and biomarkers. Compound-specific isotope analysis of diamondoids (CSIAD) shows that the Shublik Formation accounts for the higher maturity component in several mixed oil samples, whereas biomarkers, especially those providing information on the age of the source rock, show either a Cretaceous Hue-gamma ray zone (GRZ) or Triassic Shublik source for the lower maturity component. Oil samples in this study mainly correlate to six source rocks based on their biomarker characteristics and CSIAD. Chemometrics of selected source-related biomarker and isotope ratios helps to classify the oil samples into different genetic families. The source rocks include carbonate and shale organofacies of the Triassic Shublik Formation, Jurassic Kingak Shale, Lower Cretaceous Pebble shale, Lower Cretaceous Hue-GRZ, and Cenozoic Canning Formation. Oil presumed to originate from a seventh source rock interval, the Carboniferous–Permian Lisburne Group, was not clearly differentiated from well-established Shublik oil by any geochemical age-related parameter or CSIAD, which suggests that the Lisburne is not an effective source rock for any of the studied oil samples. Four oil samples collected from wells located north of the Barrow arch show unique biomarker characteristics, but age-related biomarker parameters indicate likely Triassic source rock organofacies that is not represented by any of the samples from south of the arch. The source rock for these four oil samples appears to be a clay-rich equivalent of the calcareous Shublik Formation that occurs to the north of the Barrow arch.