Abstract Natural geological accumulations of carbon dioxide occur widely throughout Europe, often close to population centres. Some of these CO 2 deposits leak, whereas others are sealed. Understanding these deposits is critical for selecting and designing underground storage sites for anthropogenic CO 2 . To provide confidence that the potential risks of geological CO 2 storage are understood, geologists are required to predict how CO 2 may behave once stored underground. Natural CO 2 accumulations provide a unique opportunity to study long-term geochemical and geomechanical processes that may occur following geological storage of anthropogenic CO 2 . In addition, natural CO 2 springs and gas vents can provide information on the mechanisms of gas migration and the potential effects of CO 2 leakage to the surface. This paper provides a description of some natural, European CO 2 occurrences. CO 2 accumulations occur in many basins across Europe. In addition, volcanic areas and seismically active areas allow CO 2 -rich fluids to migrate to the near surface. Many of these occur in areas that have been populated for hundreds and thousands of years. Stratigraphic traps have allowed CO 2 to accumulate below evaporite, limestone and mudstone caprocks. Comparisons between reservoir sandstone and equivalent nearby sandstones that contain no CO 2 indicate that reservoir sandstones may experience increased secondary porosity development through feldspar dissolution. Where fracture reactivation allows CO 2 -rich fluids to migrate, limited self-sealing may take place through calcite precipitation. Gas migration experiments indicate that, due to geochemical interactions, fine-grained seals would be able to trap smaller volumes of CO 2 compared to, for example CH 4 . In natural systems most leakage from depth occurs along fractures and is typically extremely localized on a metre-scale.

Abstract: A natural CO 2 reservoir system with a sandstone lithology in NW Hungary has been studied due to its similarities to a large saline reservoir formation that is widespread in the the Pannonian Basin (Central Europe) and is suggested to be one of the best candidates for industrial CO 2 storage. A range of analytical techniques has been used on core samples from CO 2 -containing sandstone layers that represent a wide range of pressures (90–155 bar), temperatures (79–95°C) and pore fluid compositions (total dissolved solids between 18 000 and 50 700 mg l −1 ) to identify the mineralogy and textural characteristics of the natural reservoir. The only clear CO 2 -related feature in the studied lithology was the occurrence of dawsonite (NaAlCO 3 (OH) 2 ) in a close textural relationship with albite. This is in clear agreement with our geochemical modelling results, which also underline the presence of albite as a precondition for the crystallization of dawsonite at the given P – T – X conditions. Our results suggest that, at least in the Pannonian Basin, dawsonite may be an important mineral for the safe sequestration of industrial CO 2 in the subsurface.