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) delta 13 C and delta D values for methane of -60 + or -10 per mil PDB and -217 + or -17 per mil SMOW, respectively, (3) carbon dioxide contents of less than 5%, and (4) delta 13'C(CO 2 -CH 4 ) values of 55 + or -10 per mil PDB. These data suggest that microbial reduction of CO 2 rather than traditional thermogenic reactions is mainly responsible for gas composition. Invasions by CO 2 of deep-seated external origins are readily recognized by their isotopic compositions (-7 + or -2 per mil PDB).

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