VITRINITE REFLECTANCE SUPPRESSION IN COAL DUE TO A MARINE TRANSGRESSION: CASE STUDY OF THE ORGANIC GEOCHEMISTRY OF THE GRETA SEAM, SYDNEY BASIN

Abstract

Vitrinite reflectance suppression in marine sequences is a major problem facing some petroleum exploration companies. It leads to considerable difficulties in determining thermal maturity in exploration provinces like the North West Shelf of Australia. The Permian Greta seam, northern Sydney Basin, is a classical example of a marine-influenced coal which displays a vitrinite reflectance suppression of 0.25 per cent R0. It is an ideal candidate for detailed organic geochemical investigation of this problem. The amount and composition of extractable hydrocarbons in the coal appear to be only partially related to vitrinite reflectance, so it is unlikely that suppression is directly caused by the adsorption of hydrocarbons into the vitrinite matrix. Vitrinite reflectance is inversely proportional to the H/C atomic ratio, suggesting that the observed suppression is caused by the more perhydrous nature of vitrinite in marine-influenced coals. At the molecular level the marine influence can clearly be distinguished in the top metre of the seam by depleted amounts of n-alkanes with a lower carbon preference index and a slightly bimodal distribution, a lower pristane/phytane ratio and considerably more hopanes and diasteranes. In addition to the peat-derived humic compounds prevalent throughout the seam, these geochemical parameters indicate bacterial re-working of the newly deposited peat and a direct contribution of marinederived lipids at the top of the seam. Fluctuations in n-alkane and isoprenoid distributions and abundances elsewhere in the seam indicate that the depositional environment changed periodically.