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 CH 4 , δ 13 C-CH 4 , δ 2 H-CH 4 , δ 13 C-DIC, δ 37 Cl, δ 2 H-H 2 O, δ 18 O-H 2 O, Na, K, Ca, Mg, HCO 3 , Cl, Br, SO 4 , NO 3 and DO), in combination with a novel application of isometric log ratios, this study describes hydrochemical and thermodynamic controls on dissolved CH 4 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-CH 4 data in the gas reservoir (−58‰ to −49‰) and shallow coal measures underlying the alluvium (−80‰ to −65‰) are distinct. CO 2 reduction is the dominant methanogenic pathway in all aquifers, and it is controlled by SO 4 concentrations and competition for reactants such as H 2 . At isolated, brackish sites in the shallow coal measures and alluvium, highly depleted δ 2 H-CH 4 (<310‰) indicate acetoclastic methanogenesis where SO 4 concentrations inhibit CO 2 reduction. Evidence of CH 4 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 CH 4 at different depth profiles within and between aquifers. Further research, including culturing studies of microbial consortia, will improve our understanding of the occurrence of CH 4 within and between aquifers in these basins.