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Significance of microbial activity in Australian coal bed methane reservoirs — a review

CSIRO (Commonwealth Scientific and Industrial Research Organisation) Petroleum, North Ryde, NSW, 2113, Australia
CSIRO Molecular and Health Technologies, North Ryde, NSW, 2113, Australia

Coal bed methane (CBM) is rapidly becoming a significant contributor to energy needs along the eastern seaboard of Australia. The prospective coal seams for methane production in Australia range from Jurassic to Permian in age with ranks varying from sub-bituminous to low volatile bituminous coal. These coals contain mixed gas compositions comprising mainly methane and carbon dioxide with subsidiary amounts of ethane and higher hydrocarbons. Geochemical data for gases and coal indicate extensive microbial activity, especially in coal seams shallower than about 600 m. Microbial activity possibly occurred subsequent to uplift of the eastern Australian basins during the Late Cretaceous and Tertiary. Such microbial activity has contributed to considerable volumes of methane presently stored in the shallow coals of these basins.

The two main pathways of biogenic methane generation in coal are the carbon dioxide (CO₂-reduction) and acetate dependant (aceticlastic-reaction) methanogenesis by archaea. Carbon and deuterium isotope data indicate that CO₂ reduction is the main pathway of secondary biogenic methane generation in the eastern Australian coal seams. ‘Sweet spots’for CBM production are likely to be mainly confined to permeable coal seams where microbial activity has enhanced the methane saturation levels of the coals. In the Sydney Basin, for example, the CBM production rates are up to an order of magnitude higher in areas where coal contains considerable volumes of secondary biogenic gas compared to areas containing only thermogenic gas. In the high producing wells of the Sydney Basin, the isotope fractionation factor for CO₂ and CH₄ is >1.06 which indicates extensive methane generation from CO₂ dependant methanogenesis.