Reconstruction of Oil Formation and Accumulation in North Slope, Alaska, Using Quantitative Gas Chromatography-Mass Spectrometry: SOURCE ROCK EVALUATION INCLUDING ISOTOPES AND BIOMARKERS

Fifteen sediments, 8 oils, and a condensate from the North Slope of Alaska have been investigated with a range of geochemical techniques. in addition to organic carbon determination and Rock-Eval pyrolysis, the sedimentary rock extracts, the oils, and the condensate have been extensively examined with computerized gas chromatography-mass spectrometry (GC-MS), including the application of high-resolution (~2,500) mass spectrometry. In this way, information on the distributions of aliphatic and aromatic biological marker by hydrocarbons and of phenanthrene, methylphenanthrene, and dimethylphenanthrene isomers was aquired. Additional information on bulk composition was derived from the GC-MS analysis of the aromatic hydrocarbon fractions of the oils and the condensate with l w electron energy (12 eV) mass spectrometry. These results were integrated with carbon, hydrogen, and sulfur isotope data. The results could be best explained by a model, which envisages that most of the oil presently accumulated in the North Slope of Alaska derived from the Jurassic Kingak Shale. In the Coastal Plain, this shale presently lies within the zone of oil generation, but in the Northern Foothills of the Brooks Range it is overmature and hence below it. In the model, most of the oil from this shale has migrated laterally and northward into the Sadlerochit Sandstone (Permian) and Lisburne Limestone (Mississippian) reservoirs around the Barrow arch. Minor contributions from the units overlying and underlying the Kingak Shale are also predicted (Neocomian pebble shale unit and Permo-Triassic Shublik Formation). Only when the deeper reservoirs were full did oil, following the same migration pathway migrate further upward into younger and shallower reservoirs. Thus, the increase in oil maturity with decreasing reservoir depth is explained. This maturity trend was apparent from the oils' sulfur contents, carbon and sulfur isotope compositions, bulk compositions (percent hydrocarbons, percent asphaltenes), and the distribution of their methyl-phenanthrene isomers (methylphenanthrene index). The most mature oils are also the shallowest oils and may therefore have migrated the longest distance. Any increase in the relative amounts of lighter and less polar components could either be the result of increased maturation, increased migration, or both. Not only have the distributions of biological marker compounds been considered, but also their absolute concentrations (µg/g Corg, ppm in hydrocarbon fractions) by the addition of a known amount of an internal standard. This has shown that the concentrations of these components in the C15+ hydrocarbon fractions of sedimentary rock extracts decrease sharply with the onset of petroleum generation and that in immature rocks these are an order of magnitude greater than the concentrations of the same components in the hydrocarbon fractions of the oils. That the End_Page 319------------------------ most unstable components (fastest decrease in concentration) considered (monoaromatic steroid hydrocarbons) suggests a correlation of the oils with immature sediments, while more stable components of the oils (e.g., the hopanes) match better with those of shales well within the zone of petroleum generation, implies that expulsion of components from shales occurs over a wide range of maturities. This in turn suggests oil accumulations could be averaged mixtures of organic fluids representing a range of maturities. The decreases in concentrations of the biological markers with the onset of petroleum generation mean their distributions could exaggerate the contributions of immature sources to a given oil pool. Such a case is thought to exist in the North Slope of Alaska. Although the shallower oils are more mature on a number of measurements, the biological markers suggest the reverse. If the shallower oils were more mature, they should have lower concentrations of biological markers. The addition of small amounts of immature oil, from either the pebble shale unit or Torok Formation in the region where the oils have accumulated, may have had a major influence on the biological marker patterns without contributing significantly to the vast bulk of the oils. Magoon and Claypool (1981) proposed a division of the North Slope oils into two families: Simpson-Umiat and Barrow-Prudhoe. This was mainly based on carbon and sulfur isotopic compositions and sulfur content. It now appears that both families came from similar sources but that the Simpson-Umiat oils are products of a later generation than the Barrow-Prudhoe oils. The Simpson-Umiat oils are therefore more mature and have migrated further into shallower reservoirs, where the addition of small amounts of immature oils from different sources is also more likely. The condensate derived from highly mature sources. It was found in the Northern Foothills, and the most favored source is the deep Kingak Shale (>4,000 m) in this region.