Carbon dioxide (CO 2) storage in geological media has attracted global interest as a mitigation strategy for reducing anthropogenic CO 2 emissions. Aquifers are particularly attractive CO 2 sinks, in part due to their extensive storage capacities, favourable petrophysical characteristics, and their proximity to CO 2 source nodes. To date, studies of CO 2-water-rock interactions have almost exclusively focussed on CO 2 storage in deep saline aquifers. However, in locations lacking suitable saline reservoirs, such as in southeast Queensland, Australia, where aquifers are generally of low salinity, alternative storage options are being considered. One such option under active assessment is the lower interval of the Jurassic-aged Surat Basin, comprising the Precipice Sandstone, a potential low-salinity primary reservoir, the overlying Evergreen Formation, an interformational seal, and the uppermost Hutton Sandstone, an internally-baffled unit with mixed reservoir-seal characteristics. A detailed mineralogical study of the targeted reservoir system, utilising and comparing several investigatory techniques, including optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and hyperspectral logging (HyLogger), revealed differing lithologies for the three units and provided insight into their CO 2 storage characteristics. The Precipice Sandstone is geochemically poorly-reactive, with a clean, quartzose mineralogy dominated by quartz and kaolins. The Evergreen Formation is geochemically reactive, containing abundant feldspar, smectite, chlorite, carbonate and kaolinite. Nonetheless, the fine-grained nature and generally poor reservoir quality indicate an effective interformational seal. The Hutton Sandstone is a heterogeneous unit with mixed reservoir-seal characteristics and reactive intervals containing smectite, chlorite, feldspars and lithics, in addition to common CaFe -Mn-Mg carbonates. Variable clay matrix content and porosity, and locally-significant cementation, indicate a heterogeneous unit with the capacity to buffer emplaced CO 2 , providing an additional vertical component of capture and storage to the Surat Basin reservoir system. The CO 2-water-rock interactions in low-salinity host formations remain largely unexplored for conditions relevant to CO 2 injection and storage. In particular, there are uncertainties surrounding the impact of these interactions on mineralogy and rock quality characteristics of both reservoir and seal lithologies in the near-wellbore environment. Batch-reaction experiments conducted under simulated reservoir conditions (T = 60 °C, P = 120 bar) for 16-18 days, coupled with highresolution mineralogical and microstructural studies of rock samples, established that injection of CO 2 into low-salinity formations will result in geochemical reactions that are highly dependent on lithology. Changes in aqueous fluid chemistry are attributed to the dissolution and desorption of i material into the fluid phase. Where present, carbonates dissolved rapidly in the early ...
