a b s t r a c tVariations in the pore system of sandstones from the so-called Utrillas Formation (Lower Cretaceous, Iberian Peninsula) after CO 2 injection have been investigated in a laboratory on a micro scale. In this study, we present results regarding variations in the pore spaces of sandstones caused by the injection of CO 2 and its permanence in supercritical conditions in contact with a rock sample for two months. The modifications produced in the porosity and pore size distribution have been evaluated on two geological samples, using a 3D modelling of the results obtained by Hg intrusion porosimetry. Reconstructions of the pore structure of the rock before and after CO 2 injection from mercury intrusion-extrusion curves, generating virtual models of pores that reproduce the experimental porosity. By analysing the results, a drastic modification in the mesoporosity of the rock is confirmed, which may have a paramount influence not only on the total storage capacity but also on the percolation of fluid through the rock.