Methods for Differentiating Methane and Carbon Dioxide Sources in Aquifers Overlying Gas Reservoirs or Associated with CO₂ Storage Sites

Understanding the sources of gases, including methane and carbon dioxide, and their concentrations and isotopic signatures in aquifer groundwater is important in regions where gas, coal or oil are extracted. It is also important where CO₂ or other gases are stored or planned to be stored geologically. Methane and CO₂ can exist naturally in an aquifer, for example when generated by microbial processes. Aquifer groundwater is often extracted by a variety of overlapping users for agriculture, stock, town and domestic water supply as well as by industries including mines and energy. Water bores drilled into these aquifers can be gassy or may become gassy over time, and gas shows can occur in shallow groundwater, soils, or rivers. Where these are overlying or adjacent to mines, gas and oil extraction or storage reservoirs it poses questions as to the source of gases and may raise queries about leakage. We show several examples where we have sampled gas and waters from Great Artesian Basin aquifers that overly coal seam gas (CSG) reservoirs. In addition, we regionally sampled deep bores in a proposed CO₂ storage reservoir, and the surrounding and overlying aquifers. We used a combination of stable isotopes of gases and waters, major and minor ions, strontium isotopes, ³⁶Cl, ¹⁴C, and other tracers to understand gas sources and inter aquifer connectivity. In the majority of cases the data support a lack of “leakage” with gases from natural in situ processes. The methods and tools described here can help distinguish sources of gases, and avoid environmental and social licence issues while protecting water resources. They are also applicable to understand sites and potential, real or claimed leakage from shale gas and oil reservoirs, or from other gas storage sites such as for H₂, compressed air, or methane.