Geochemical tracers associated with methane in aquifers overlying a coal seam gas reservoir

Abstract

Understanding inter-aquifer connectivity or leakage of greenhouse gases and groundwater to aquifers overlying gas reservoirs is important for environmental protection and social licence to operate. Australia's Great Artesian Basin (GAB) is the largest artesian groundwater system in the world with groundwater extracted for agriculture, livestock, mines, energy, private or town water supply. Microbial coal seam gas (CSG) and production water are also extracted from the GAB. Here a range of groundwater tracers is used to investigate the potential for gas and groundwater connectivity between the CSG reservoir and aquifers.

The GAB aquifer and alluvium contained a range of methane concentrations (0.001 to 2100 mg/L) that exhibit an increase with depth and δ¹³C-CH₄. Aquifer and alluvium groundwater ⁸⁷Sr/⁸⁶Sr were in the range 0.7042 to 0.7082. CSG production waters however had non-radiogenic, distinctive ⁸⁷Sr/⁸⁶Sr signatures <0.7036, indicating a lack of significant groundwater leakage. One gassy aquifer bore with 160 mg/L methane conversely has ⁸⁷Sr/⁸⁶Sr, δ¹³C-CH₄, δ²H-CH₄ and δ¹³C-DIC values overlapping the CSG waters. In several aquifers δ³⁴S-SO₄ and δ¹⁸O-SO₄ are sourced from windblown surface salts of inland Australian playa lakes in recharge waters. Bacterial sulphate reduction is additionally occurring in a regional aquifer. Cosmogenic isotopes and tritium show recent recharge and mixing with older groundwaters in several shallow aquifers.

Groundwater and gas signatures indicate that leakage of groundwater and methane from the CSG reservoir was not occurring in the majority of areas investigated here. Methane was consistent with in situ generation in shallow GAB aquifers by primary microbial CO₂ reduction or acetate fermentation. Connectivity of one alluvial bore and the underlying GAB aquifer could not be completely ruled out. Separately, one gassy Springbok GAB aquifer bore is either connected to the underlying CSG gas reservoir, or has in situ secondary microbial CO₂ reduction producing methane from interbedded coal within the aquifer. This study is relevant to other basins in Australia and internationally where gas is observed in aquifers that overly conventional, unconventional or coal seam gas reservoirs.