Geochemical indicators of the origins and evolution of methane in groundwater: Gippsland Basin, Australia.

Recent expansion of shale and coal seam gas production worldwide has increased the need for geochemical studies in aquifers near gas deposits, to determine processes impacting groundwater quality and better understand the origins and behavior of dissolved hydrocarbons. We determined dissolved methane concentrations (n = 36) and δ13C and δ2H values (n = 31) in methane and groundwater from the 46,000-km2 Gippsland Basin in southeast Australia. The basin contains important water supply aquifers and is a potential target for future unconventional gas development. Dissolved methane concentrations ranged from 0.0035 to 30 mg/L (median = 8.3 mg/L) and were significantly higher in the deep Lower Tertiary Aquifer (median = 19 mg/L) than the shallower Upper Tertiary Aquifer (median = 3.45 mg/L). Groundwater δ13CDIC values ranged from -26.4 to -0.4 ‰ and were generally higher in groundwater with high methane concentrations (mean δ13CDIC = -9.5 ‰ for samples with >3 mg/L CH4 vs. -16.2 ‰ in all others), which is consistent with bacterial methanogenesis. Methane had δ13CCH4 values of -97.5 to -31.8 ‰ and δ2HCH4 values of -391 to -204 ‰ that were also consistent with bacterial methane, excluding one site with δ13CCH4 values of -31.8 to -37.9 ‰, where methane may have been thermogenic. Methane from different regions and aquifers had distinctive stable isotope values, indicating differences in the substrate and/or methanogenesis mechanism. Methane in the Upper Tertiary Aquifer in Central Gippsland had lower δ13CCH4 (-83.7 to -97.5 ‰) and δ2HCH4 (-236 to -391 ‰) values than in the deeper Lower Tertiary Aquifer (δ13CCH4 = -45.8 to -66.2 ‰ and δ2HCH4 = -204 to -311 ‰). The particularly low δ13CCH4 values in the former group may indicate methanogenesis at least partly through carbonate reduction. In deeper groundwater, isotopic values were more consistent with acetate fermentation. Not all methane at a given depth and location is interpreted as being necessarily produced in situ. We propose that high dissolved sulphate concentrations in combination with high methane concentrations can indicate gas resulting from contamination and/or rapid migration as opposed to in situ bacterial production or long-term migration. Isotopes of methane and dissolved inorganic carbon (DIC) serve as further lines of evidence to distinguish methane sources. The study demonstrates the value of isotopic characterisation of groundwater including dissolved gases in basins containing hydrocarbons.