Preinjection Characterisation and Evaluation of CO2 Sequestration Potential in the Haizume Formation, Niigata Basin, Japan. Geochemical Modelling of Water-Minerals-CO2 Interaction - The Research Institute of Innovative Technology for the Earth (RITE) is carrying out a small-scale CO2 injection field experiment to investigate the feasibility of geological sequestration of CO2 greenhouse gas in the south-west of Nagaoka City, Niigata Prefecture, Japan. Prior to the injection geochemical reactions caused by CO2 injections were investigated using the geochemical modelling code (EQ3/6). The injection formation is the sedimentary marine Haizume Formation (Pleistocene) in the Uonuma Group, which is covered by a mudstone seal. The formation is mainly composed of quartz, plagioclase, feldspar, pyroxene, and clays (smectite, chlorite). The sandstone shows minor consolidation and grain size is medium to coarse sand. The total dissolved solid (TDS) of the formation water is approximately 6100 mg/l and the water contains a high Ca2+ (> 20% of Na+ concentration). The geochemical model was used for an initial adjustment of the formation water chemistry to the formation conditions and a modelling of the formation water-mineral-CO2 reactions. The modelling results showed a high reactivity of the minerals in the CO2 rich environment and high mineral conversion rate within the formation. At the final state, approximately 23 mol of CO2 were taken into I kg of formation water and more than 90% of this was stored within carbonate minerals. In this simulation, some uncertainty is associated with the time scale and a more detailed investigation is planned and will address accurate evaluation.