Abstract
Using a comprehensive data set (dissolved CH4, δ13C-CH4, δ2H-CH4, δ13C-DIC, δ37Cl, δ2H-H2O, δ18O-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. δ13C-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 δ2H-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.
Highlights
Using a comprehensive data set, 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. δ13C-CH4 data in the gas reservoir (−58‰ to −49‰) and shallow coal measures underlying the alluvium (−80‰ to −65‰) are distinct
A single coal sample from the shallow coal measures underlying the alluvium at Cecil Plains showed small amounts of pyrite; SO4 concentrations at this site ranged from 8–12 mg/L (0.16–0.25 meq/L), Figure 1
The lack of evidence of CH4 leakage from the gas reservoir to the shallow coal measures, and the abrupt shift from enriched δ13C-CH4 (−58‰ to −49‰) of the gas reservoir to the depleted δ13C-CH4 of the shallow coal measures indicate that the migration of CH4 from the gas reservoir to the alluvium at these sites is not a plausible scenario based on these data
Summary
Using a comprehensive data set (dissolved CH4, δ13C-CH4, δ2H-CH4, δ13C-DIC, δ37Cl, δ2H-H2O, δ18O-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. δ13C-CH4 data in the gas reservoir (−58‰ to −49‰) and shallow coal measures underlying the alluvium (−80‰ to −65‰) are distinct. While information on the complex behaviour of CH4 isotopes is readily available, it creates some uncertainty about the value of CH4 isotopes as indicators of broader processes, such as fugitive emissions or aquifer connectivity (see Figure S1 for examples of δ13C-CH4 values under a range of different pathways and conditions). Some previous research in shale-gas-bearing basins has shown that CH4 can migrate with brines from underlying gas-bearing aquifers but, in the absence of hydrocarbon reservoirs, it can be generated in situ[18,19,20]. These two isotopes allow more informative assessments of potential CH4 origins than δ13C-CH4 alone. The ratio of isotope values between source carbon/hydrogen and that of CH4 provides insight into the production and consumption pathways[3,5,26]
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