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 delta C-13 values of methane in these gases are within the range of -41.18 parts per thousand to -47.00 parts per thousand. This, together with the linearity of the delta C-13 values of methane to butane in the natural gas plots, indicates that the hydrocarbon gases are of organic origin. The delta C-13(CO2) values of these gases are in the range of -5.1(parts per thousand) to -7.5 parts per thousand, 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 delta C-13(CO2) values and He-3/He-4 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. (C) 2007 Elsevier Ltd. All rights reserved.