A review of natural CO2 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 CO2-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 CO2-rich fluids to migrate, limited serf-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.