LAUSR

Abstract The found gas pools in the Songdong and Baodao regions in the eastern Qiongdongnan basin, South China Sea have almost failed to realize expected commercial value due to either low reserve abundance or high carbon dioxide (CO2) content. To assist in further exploration, the genetic type, source and accumulation history of natural gas have been investigated based on the analysis of the chemical compositions, carbon isotopes and light hydrocarbons in gases, isotope compositions of rare gas, basin numerical modeling and fluid inclusion analysis. The results indicate that there are three representative types of gases. The first type distributed in the BD13 and ST24-1 areas consists of biogenic and oil-derived gases from the Songdong sag. The second type distributed in the BD19-2 area is composed of coal-derived and oil-derived gases from the Baodao sag. The third type distributed in the BD15-3, BD19-4 and BD19-2 areas consists of significant amounts of volcanic mantle-derived CO2 and few organic hydrocarbon gases. The charge of oil-derived gas and formation of biogenic gas are close to the deposition stage of overlying thick mudstones. This may be a critical factor for current gas pools with low reserve abundance in the Songdong region. Intense activity of the No.2 fault and absence of overlying thick mudstones gave rise to the escape of a large quantity of oil and gas from the middle Oligocene to middle Miocene. Besides, due to volcanic activity in the Quaternary, mantle-derived CO2 was injected into the reservoirs via No.2 fault. The previous accumulative hydrocarbon gases were driven off by CO2 in various degrees, which may be mainly responsible for current situation that most gas pools have high CO2 content in the Baodao region.