The organic and isotope geochemistry of the Permian Greta coal, Australia

Changing environments accompanying invasion by the Permian oceans of regions of land plant deposition are reflected in the organic and isotope geochemistry of the Australian Greta coal. In traversing from the lower (non-marine) to the upper (marine) margins of the Greta seam (about 7 m thick), the vitrinite reflectance values decrease from 0.66% to 0.54% and the amount of extractable organic matter increases markedly, suggesting that suppression of vitrinite reflectance is occurring in the marine-influenced coal. Widely differing inputs of organic material are suggested by n-alkane distributions.
Aliphatic hydrocarbon fractions from the bottom of the seam contain high molecular weight n-alkanes with an odd carbon number predominance, typical of a land-plant source, together with abundant hopane and sterane biological markers. These latter compounds become dominant towards the top of the seam with few n-alkanes remaining, reflecting a greater bacterial contribution due to the marine incursion. However, the distribution of individual hopanes, steranes, methylphenanthrenes and the great bulk of other aromatic compounds vary less markedly through the seam. The marine transgression is manifested isotopically by the {18}O/{16}O ratio of clays associated with dirt hands in the coal and by the concentration and {34}S content of sulphur species.