Bituminous coal seam gases from the Permian Sydney and Bowen basins, Australia, are characterized by (1) methane/ethane ratios greater than or equal to 1000, (2) δ13C and δD values for methane of –60 ±10‰ PDB and –217 ±17‰ SMOW, respectively, (3) carbon dioxide contents of less than 5%, and (4) δ13C(CO2–CH4) values of 55 ±10‰ PDB. These data suggest that microbial reduction of CO2 rather than traditional thermogenic reactions is mainly responsible for gas composition. Invasions by CO2 of deep-seated external origins are readily recognized by their isotopic compositions (–7 ±2‰ PDB).

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