Basalts are igneous rocks that contain CaO and MgO, both of which react with CO2 to give carbonates. Basalt containing CaO are commonly near 10 wt.%, while those of MgO commonly in the range 5 to 12 wt.%. This provides theoretical CO2 sequestration levels, unfortunately, the best estimates are that even after 2 years at most 95% of the injected CO2 is mineralized, leaving significant CO2 still mobile. This slow reaction is predominantly due to the requirement of diffusion of the CO2 through the porous rock and makes assumptions of the accessibility of the CO2 to all the reservoir rock. As a consequence, and the inhomogeneity of the basalt, it is essentially impossible to determine the actual sequestration achieved in the long term.