During CO2 geological storage, low porosity and permeability cap-rock can act as a structural trap, preventing CO2 vertical migration to overlying fresh water aquifers or the surface. Clay and organic matter rich shales, fine-grained sandstones and mudstones often act as cap-rocks and may contain substantial sub-micron porosity. CO2-brine-rock interactions can open or close pore throats through dissolution, precipitation or migration of clay fines or grains. This could affect CO2 migration if the porosity is accessible, with unchanging or decreasing accessible porosity favourable for trapping and integrity. Two cap-rock core samples, a clay and organic-rich mudstone and a more organic-lean feldspar-rich fine grained sandstone, from a well drilled for a CO2 storage feasibility study in Australia were experimentally reacted with impure CO2 ( + SO2, O-2) and low salinity brine at reservoir conditions. Mercury injection capillary pressure indicated that the majority of pores in both cores had pore throat radii similar to 5-150 nm with porosities of 5.5-8.4%. After reaction with impure CO2-brine the measured pore throats decreased in the clay-rich mudstone core. Dissolution and precipitation of carbonate and silicate minerals were observed during impure CO2 reaction of both cores via changes in water chemistry. Scanning electron microscopy identified macroporosity in clays, mica and amorphous silica cements. After impure CO2-brine reaction, precipitation of barite, Fe-oxides, clays and gypsum was observed. Ion leaching from Fe-rich chlorite was also apparent, with clay structural collapse, and fines migration. Small-angle neutron scattering measured the fraction of total and non-accessible pores (similar to 10-150 nm radii pores) before and after reaction. The fraction of pores that was accessible in both virgin cap-rocks had a decreasing trend to smaller pore size. The clay-rich cap-rock had a higher fraction of accessible pores (similar to 0.9) at the smallest SANS measured pore size, than the feldspar rich fine-grained sandstone (similar to 0.75). Both core samples showed a decrease in SANS accessible pores after impure CO2-water reaction at CO2 storage conditions. The clay-rich cap-rock showed a more pronounced decrease. After impure CO2-brine reaction the fraction of accessible pores at the smallest pore size was 0.85 in the clay-rich cap-rock and similar to 0.75 in the feldspar-rich fine-grained sandstone. Reactions during impure CO2-brine-rock reaction have the potential to close cap-rock pores, which is favourable for CO2 storage integrity.