Sources and concentrations of methane, ethane, and CO2 in deep aquifers of the Surat Basin, Great Artesian Basin

Understanding the occurrence and sources of methane in aquifers is an international issue, especially in regions that host oil and gas reservoirs. Multi-isotopic studies are important to understand sources and concentrations of dissolved gases in aquifers of basins that host oil and gas extraction; or future CO2, natural gas, hydrogen storage, or compressed air energy storage. The largest artesian basin in the world, the Great Artesian Basin, extends across more than one fifth of Australia providing a fresh water source from various aquifers. The Precipice and Hutton sandstones, aquifers of the Surat Basin in the eastern Great Artesian Basin, are important water resources for town water supply, agriculture, mines, feedlots, and private landholders in Queensland. Overlying and underlying formations host gas extraction. The southern part of the basin in Queensland additionally hosts an oil field, and potential CO2 storage sites, therefore understanding the aquifer gas concentrations, processes and sources is important for fluid management. The dissolved concentrations of methane and ethane in the Hutton Sandstone were up to 96 mg/L and 1 mg/L respectively, and in the Precipice Sandstone methane ranged up to 2100 mg/L and ethane 6.5 mg/L in the southern Surat Basin. Dissolved gas concentrations were measured by an open and closed sampling method, with the concentrations lower from open sampling, and the gas wetness parameter systematically slightly lower. The Hutton Sandstone aquifer dissolved gas isotopic signatures are mainly consistent with primary microbial CO2 reduction generating methane in situ, with fractionation factors indicating acetate fermentation in a few cases. The Hutton Sandstone δ13C-DIC, δ13C-CO2, and δ13C-CH4 have an enrichment trend with increasing depth that indicate a microbial substrate depletion effect. Moonie Oil Field Precipice Sandstone and Evergreen Formation samples plot in the thermogenic region, with enriched δ13C-CO2 indicating microbial biodegradation is also occurring. The Precipice Sandstone aquifer samples have a mixed microbial CO2 reduction and early mature thermogenic gas signature in both isotope cross plots and gas wetness diagrams, with a subset of sampled δ13C-C2H6 and isotope factors consistent with early mature thermogenic gas. A potential historic source of thermogenic gas is the underlying gas reservoir of the Bowen Basin. Large volumes of water are abstracted from these aquifers, hence dissolved gases may contribute to greenhouse gas emissions.