Gas sources and concentrations in Surat Basin shallow aquifers: a field sampling method comparison, and isotopic study

The Surat Basin hosts various industries that extract groundwater including coal seam gas (CSG), feedlots, and agriculture. With water drawdown, gas has been observed in some bores drawing groundwater from different aquifers across the Basin. While methane can occur naturally in aquifers, biogenic CSG has been extracted from the Walloon Coal Measures raising questions on the sources of gas in overlying aquifers. Current standard monitoring uses a direct fill approach to measure dissolved methane concentrations in vials; however, this approach may lose gas present above solubility levels. Water and gas sampling was performed on bores in the Springbok, Gubberamunda, Mooga, Hutton, and Precipice Sandstones, the Orallo Formation, and the Condamine Alluvium. Water bores and CSG production wells from the Walloon Coal Measures were also sampled. We compared direct fill with a closed sampling method for dissolved gas, plus a method for sampling the total free and dissolved gas in the field. Higher dissolved and total methane concentrations were measured using closed sampling methods, especially in gassy bores. The majority of our sampled aquifer gases and waters have stable isotopic signatures distinct from CSG, where methane had likely been formed insitu in shallow aquifers by primary microbial CO2 reduction or fermentation processes. In several gassy bores, the source of the methane could not be clearly identified. This study indicates that (1) current monitoring methods may be underestimating methane concentrations above identified thresholds especially in gassy bores and (2) a combination of isotopic techniques may distinguish methane sources and interaquifer disconnectivity in the majority of cases.