Petroleum source rock assessment in non-marine sequences: pyrolysis and petrographic analysis of Australian coals and carbonaceous shales

Abstract

A series of Australian coals and terrestrial sediments ranging in age from Permian through tertiary have been analyzed to assess their petroleum source character and the suitability of various techniques for assessment of source rock potential in non-marine sequences. The procedures used include organic petrography, Rock-Eval pyrolysis, elemental analysis and quantitative pyrolysis-gas chromatography. The latter procedure was used to assess the potential yield of paraffins which is critical to the assessment of most non-marine source rocks. Principal components analysis was used to assist in the analysis of the data. Although the petroleum potential of the samples follows the broad trends in petrographic composition established for Australian coals, i.e. relative proportions of vitrinite, inertinite and liptinite, there is much variation which cannot be explained petrographically at the maceral group level. Further, pyrolytic hydrocarbon yield is not related to overall elemental composition below an atomic H/C ratio of 1.0. The yield of phenols in flash pyrolysis is related to depositional setting as well as degree of maturation. The yield and carbon number distribution of normal hydrocarbons in flash pyrolysis varied widely depending on the age, nature and amount of liptinite macerals particularly in samples with Hydrogen Indices below 300. Liptinite-poor (< 10%) samples may yield significant amounts of hydrocarbons, but typically they have a low wax content. Sporinite-rich and liptodetrinite-rich samples give lower yields of normal hydrocarbons. These are predominantly of lower molecular weight. Suberinite and to a lesser extent cutinite are associated with high yields of waxy normal hydrocarbons, but some samples with high yields did not contain large amounts of these maceral. The correlation of T max to vitrinite reflectance varies with petrographic composition. The results have implications for the way source rock analyses are conducted and interpreted in non-marine sequences.