A review of natural CO ₂ accumulations in Europe as analogues for geological sequestration

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.