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

Abstract

Natural gases produced from the Fushan Depression, Beibuwan Basin, South China Sea are generally rich in carbon dioxide. The δ 13C values of methane in these gases are within the range of −41.18‰ to −47.00‰. This, together with the linearity of the δ 13C values of methane to butane in the “natural gas plots,” indicates that the hydrocarbon gases are of organic origin. The δ 13 C CO 2 values of these gases are in the range of −5.1‰ to −7.5‰, suggesting an inorganic origin. A dominant mantle source for the CO 2 in these gases was determined on the basis of a simple binary mixing model using the δ 13 C CO 2 values and 3He/ 4He ratios of associated helium in these gases. The primary geological controls on the occurrence of CO 2 rich gas accumulations in the Fushan Depression are discussed following a comparison with other sedimentary basins in the region.