Shales are identified as suitable caprock formations for geolocigal CO₂ storage thanks to their low permeability. Here, small-sized shale samples are studied under field-representative conditions with X-ray tomography. The geochemical impact of CO₂ on calcite-rich zones is for the first time visualised, the role of pre-existing micro-fissures in the CO₂ invasion trapping in the matererial is highlighted, and the initiation of micro-cracks when in contact with anhydrous CO₂ is demonstrated.