Groundwater and gas sampling informing hydrogeological conceptualisation of the Precipice and Hutton Sandstone aquifers of the southern Surat Basin

The southern Surat Basin in Queensland, Australia, may be a prospective region for geological storage of CO2 within the Precipice Sandstone with the Evergreen Formation acting as a sealing cap-rock of the reservoir. The Hutton Sandstone, a major regional aquifer, overlies the Evergreen Formation. Understanding the baseline current hydrogeological and geochemical processes and potential impacts to water resources from CO2 injection are important factors. Data from newly collected field samples of groundwater from the Precipice Sandstone and Hutton Sandstone in the southern Surat Basin are presented, focusing on the stable isotope component of a broader study. Initial results show that the Precipice Sandstone has a wider range of salinity than the Hutton Sandstone, with the highest values in the region east of the Burunga-Leichhardt fault zone. The majority of dissolved methane and carbon dioxide samples from Precipice Sandstone bores have a mixed origin isotopic signature, with 13C-DIC also indicating methanogenesis. Samples from the Moonie oil field, including the Precipice Sandstone and Evergreen Formation have a mixed thermogenic signature. The majority of Hutton Sandstone dissolved gases, however, indicate in situ primary microbial CO2 reduction producing methane. Water stable isotopes in the two aquifers span large ranges and are more depleted than modern rainfall consistent with recharge during colder climates. Initial results from DST analyses in the project also indicate flow components from north to south, and a potential pathway east to the ClarenceMoreton Basin. Analysis of groundwaters are ongoing to understand processes and flow paths.