Occurrence and origin of carbon dioxide in the Fushan Depression, Beibuwan Basin, South China Sea

Natural gases produced from the Fushan Depression, Beibuwan Basin, South China Sea are generally rich in carbon dioxide. The δ13C values of methane in these gases are within the range of −41.18‰ to −47.00‰. This, together with the linearity of the δ13C values of methane to butane in the “natural gas plots,” indicates that the hydrocarbon gases are of organic origin. The δ13CCO2δ13CCO2 values of these gases are in the range of −5.1‰ to −7.5‰, suggesting an inorganic origin. A dominant mantle source for the CO2 in these gases was determined on the basis of a simple binary mixing model using the δ13CCO2δ13CCO2 values and 3He/4He ratios of associated helium in these gases. The primary geological controls on the occurrence of CO2 rich gas accumulations in the Fushan Depression are discussed following a comparison with other sedimentary basins in the region.