Geoscience Visual Presentation G06: Tracers for potential gas reservoir-aquifer leakage: 87Sr/86Sr

Geoscience Visual Presentation G06 Groundwater tracers such as strontium isotopes (87Sr/86Sr) are increasingly used by governments and regulators to identify gas reservoir-aquifer leakage or connectivity. For example, they are used to identify potential connectivity between the Surat Basin Walloon Coal Measures (WCM) coal seam gas (CSG) reservoir and the overlying Springbok Sandstone aquifer. Generally, values of the 87Sr/86Sr signature of groundwaters sampled from the Springbok Sandstone that are overlapping with the CSG reservoir have been interpreted as a potential leak. However, overlapping 87Sr/86Sr signatures may not be an indicator of connectivity/leakage, rather of similar rock formations/mineral sources. 87Sr/86Sr signatures of groundwater can be a combination of recharge water and its interaction with the host rock. This study characterised 87Sr/86Sr signatures and sources in rock cores and interburden of the Springbok Sandstone and WCM. Both formations have a wide range of mineral components, with calcite and plagioclase potential Sr sources, and clays and K-feldspars Rb sources. Core whole rock digestions and acid extractions were characterised, with whole rock signature 87Sr/86Sr = 0.703–0.710 and acid extracts 0.7032–0.7048. Our recent groundwater measurements from the WCM and Springbok Sandstone aquifer overlap in the range 0.7034–0.7043. The overlapping signatures from acid extractions of the WCM and Springbok Sandstone indicate that the similarity of the two host formations may be the reason for overlapping groundwater signatures, not necessarily any occurring leakage. Future work should expand the study sites and also investigate reservoir aquifer pairs in the Bowen Basin. To access the Visual Presentation click on 'Supplementary data' below. To read the full paper click here