Effects of Marine Transgression on the Geochemistry and Oil Generation Potential of Coals: An Example from the Sydney Basin, Australia

D. J. Curry, C. F. K. Diessel, L. C. Gammidge, R. Rigby

The sequence stratigraphic position of a source rock has a significant influence on its geochemistry and hydrocarbon generation potential. In this study, the effects of a marine transgression on the geochemistry of the Permian-age Greta Seam in the Sydney Basin were investigated.

Petrographic and geochemical data indicate these coals were derived from relatively uniform terrestrial precursors. HI values range from 313 to 421, and py-gc data indicate the coals are potentially oil-generative. However, there are systematic variations in many parameters going upward through the seam. Vitrinite reflectance decreases regularly from Rv = 0.69% near the base to Rv = 0.53% near the top of the seam, while fluorescence intensity increases smoothly. Other significant variations include systematic increases in the diasterane/sterane ratios (from 0.50 to 0.93) and H/C ratios, and decreases in pristane/phytane ratios and hopane concentrations. These variations correlate with increased concentrations of thiophenes and benzothiophenes and decreased concentrations of one-ring a omatics in the pyrolysates. However, aromatic isotope ratios and dibenzothiophene concentrations remain constant, and there are no systematic changes in the concentrations of paraffinic groups in the pyrolysates. These variations appear to result from the epigenetic incorporation of sulfur into the peats shortly after deposition. Although many correlation and maturity parameters are significantly altered, the oil generation potential does not appear to be affected. These data indicate the sequence stratigraphic position of coals and terrestrial sediments is very significant when evaluating maturity and assessing organic facies in paralic environments.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995