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ABSTRACT: Hydrous Vitrinite as a Liquid Hydrocarbon Source in Permian and Mesozoic Eastern Australian Coal

FIELDING, CHRISTOPHER R., and GLIKSON MIRYAM, Department of Geology and Mineralogy, University of Queensland, Queensland, Australia

The traditional view of hydrocarbon generation in nonmarine source rocks is that "humic" organic matter produces gas, whereas "sapropelic" residues may give rise to liquid hydrocarbons. It has been acknowledged in the past ten years, however, that coals may act as source facies for oil accumulations in certain Mesozoic and Cainozoic sedimentary basins. Studies where a coal source has been identified or inferred have to date invoked the hydrogen-rich macerals resinite, cutinite and exinite as the oil source.

Several workers have pointed out the enormous discrepancy between the amount of oil generated in some cases (e.g., Bass Strait, Australia) and the minute contribution possible from liptinite macerals. Geochemical studies by Bertrand and Durand and Parratte have suggested a link between coal-sourced oil and vitrinites, the dominant "humic" maceral in coals.

Exinite has been shown in a previous study to polymerise in the vicinity of the source. In the Callide coal, with vitrinite (A) reflectance of 0.56% Ro, oil has already undergone some primary migration by moving into and through coal cleats. Given exinite and vitrinite B contents of 2.6% and 30%, respectively, the Callide coals must be considered as a potential oil source facies. In the higher rank German Creek coal, considerable amounts of bitumen residue were noted as cell cavity infills in fusinite, interpreted as representing the earlier migration of oil through microporosity in the coal matrix. Smith and Cook pointed out on volumetric grounds that liptinites alone were inadequate in explaining volumes of oil generated in coal-bearing source intervals in the Gippsland basin of sou heast Australia, and suggested that hydrous vitrinite could be a liquid hydrocarbon source. Some previous studies of Australian and other coals have established the presence of both low- and high-reflecting forms of vitrinite.

The present study, however, is the first to document the role of low-reflecting, per-hydrous vitrinite (B) in generating liquid hydrocarbons:

This discovery may have profound implications in characterizing the source of some known oil accumulations (such as those in the Gippsland and Eromanga basins of Australia) and in the evaluation of potential exploration targets. Some organic-rich formations which have been conventionally regarded as gas-prone may need to be reevaluated petrographically.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)