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

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 δ