Some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins

Large volumes of carbon dioxide (i.e. up to 90 v/v%) have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic. Evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the CO2 including mantle degassing, metamorphic reactions or magmatic processes. The formation of large accumulations of large volumes of CO2 in sedimentary basins is discussed with specific reference to published data from the Pannonian Basin, Hungary; the Cooper–Eromanga Basin, Australia; and the South Viking Graben, North Sea. These basins have occurrences of large volumes (i.e. over 15 v/v%) of isotopically heavy carbon dioxide (thought to be of inorganic origin), and, although they differ somewhat in terms of sedimentary fill, all have major structural features close to the gas accumulations. The available geochemical data are currently neither sufficient to confirm the specific origin(s) of the CO2 (e.g. mantle-derived, carbonate metamorphism) nor unscramble CO2 mixed from two or more sources: ongoing studies, especially those involving noble gas analyses, should provide fuller answers.