Methane in aquifers and alluvium overlying a coal seam gas region: Gas concentrations and isotopic differentiation

• Published in: The Science of the total environment Vol. 861; p. 160639
• Main Authors: Pearce, J.K., Golding, S.D., Baublys, K., Hofmann, H., Gargiulo, G., Herbert, S.J., Hayes, P.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 25-02-2023
• Subjects: Aquifers; Carbon Dioxide; Coal; Coal bed methane; Condamine alluvium; Environmental Monitoring; Gases; Great Artesian Basin; Groundwater; Methane - analysis; Oil and Gas Fields; Stable isotopes; Surat Basin; Water Pollutants, Chemical - analysis; Surat Basin; Coal bed methane; Stable isotopes; Great Artesian Basin; Condamine alluvium; Aquifers
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2022.160639

The detection and attribution of methane in aquifers overlying oil and gas reservoirs has recently gained increasing attention internationally. The Surat Basin, in the Great Artesian Basin (GAB), Australia, hosts a coal seam gas (CSG) reservoir, with feedlots, town water supply, mines and agriculture that extract groundwater from aquifers that underly and overly the gas reservoir. This study aimed to use a multi-isotopic approach to differentiate biogenic methane generated in situ in GAB aquifers and the Condamine Alluvium, from the biogenic CSG produced from the underlying Walloon Coal Measures reservoir, to understand if gas had migrated or not. Dissolved methane (0.001 to 160 mg/l) and total methane concentrations (up to 91,818 ppmv) were measured using closed sampling methods and were higher than from open direct fill sampling (<0.001 to 25.4 mg/l), especially in gassy bores that contain dissolved methane above 10 to 13 mg/l. The CSG production waters and a gassy overlying aquifer bore had the most depleted water isotopes, and also the most enriched δ13C-DIC indicating strong methanogenesis. The majority of aquifers have isotopic signatures (δ13C-DIC, CH4 and CO2) indicating in situ methane production by primary CO2 reduction or fermentation, distinct from secondary microbial CO2 reduction in the CSG reservoir. Fractionation factors support methane production mainly via CO2 reduction, with fermentation in a subset of aquifer samples. The gas wetness parameters (636 to 20,000) are consistent with mainly microbial gases, with low dissolved ethane (max 0.04 mg/l). The majority of aquifer and alluvium samples in this study are consistent with in situ methane production, not migration, however in several gassy bores the methane source could not be clearly identified. This study is broadly applicable to understanding methane sources in aquifers overlying CSG reservoirs.
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•Field study differentiating in situ produced aquifer methane from potential migration.•Aquifer and alluvium methane production via primary CO2 reduction and fermentation•Multi-isotope method, majority of GAB aquifer and alluvium methane produced in situ.•Majority of aquifer and alluvium methane different from CSG reservoir methane•Several gassy bores could not be distinguished from CSG, methane source unknown.