Geological storage of CO is considered a solution for reducing the excess CO released into the atmosphere. Low permeability caprocks physically trap CO injected into underlying porous reservoirs. Injection leads to increasing pore pressure and reduced effective stress, increasing the likelihood of exceeding the capillary entry pressure of the caprocks and of caprock fracturing. Assessing on how the different phases of CO flow through caprock matrix and fractures is important for assessing CO storage security. Fractures are considered to represent preferential flow paths in the caprock for the escape of CO. Here we present a new experimental rig which allows 38 mm diameter fractured caprock samples recovered from depths of up to 4 km to be exposed to supercritical CO (scCO) under in situ conditions of pressure, temperature and geochemistry. In contrast to expectations, the results indicate that scCO will not flow through tight natural caprock fractures, even with a differential pressure across the fractured sample in excess of 51 MPa. However, below the critical point where CO enters its gas phase, the CO flows readily through the caprock fractures. This indicates the possibility of a critical threshold of fracture aperture size which controls CO flow along the fracture.