Thermodynamic and hydrochemical controls on CH4 in a coal seam gas and overlying alluvial aquifer: new insights into CH4 origins

Using a comprehensive data set (dissolved CH4, δ(13)C-CH4, δ(2)H-CH4, δ(13)C-DIC, δ(37)Cl, δ(2)H-H2O, δ(18)O-H2O, Na, K, Ca, Mg, HCO3, Cl, Br, SO4, NO3 and DO), in combination with a novel application of isometric log ratios, this study describes hydrochemical and thermodynamic controls on dissolved CH4 from a coal seam gas reservoir and an alluvial aquifer in the Condamine catchment, eastern Surat/north-western Clarence-Moreton basins, Australia. δ(13)C-CH4 data in the gas reservoir (-58‰ to -49‰) and shallow coal measures underlying the alluvium (-80‰ to -65‰) are distinct. CO2 reduction is the dominant methanogenic pathway in all aquifers, and it is controlled by SO4 concentrations and competition for reactants such as H2. At isolated, brackish sites in the shallow coal measures and alluvium, highly depleted δ(2)H-CH4 (<310‰) indicate acetoclastic methanogenesis where SO4 concentrations inhibit CO2 reduction. Evidence of CH4 migration from the deep gas reservoir (200-500 m) to the shallow coal measures (<200 m) or the alluvium was not observed. The study demonstrates the importance of understanding CH4 at different depth profiles within and between aquifers. Further research, including culturing studies of microbial consortia, will improve our understanding of the occurrence of CH4 within and between aquifers in these basins.