Geochemistry, origin, and deep-water exploration potential of natural gases in the Pearl River Mouth and Qiongdongnan basins, South China Sea

More than 10 gas pools have been discovered since 1983 in the shallow-water region of the Pearl River Mouth (PRM) Basin and the Qiongdongnan (QDN) Basin, offshore South China Sea. Gases produced from the QDN basin are characterized by high contents of benzene and toluene and relatively heavy 13C2 values (25 to 27). The associated condensates have a high abundance of bicadinanes and oleanane, providing a good correlation with the coal-bearing sequence of the Oligocene Yacheng Formation in the basin. In contrast, the gases from the PRM basin contain lower amounts of benzene and toluene and lighter 13C2 values (24 to 34). Widely variable concentrations of bicadinane and oleanane were identifiied from the associated condensates, which can be mostly correlated with the lower Oligocene Enping Formation source rocks formed in a swamp to shallow lake environment. Oil-cracked gases sourced from the Eocene oil-prone source rock may also provide some contribution to the PRM basin gases. The available geochemical data indicate that both the Yacheng and Enping formations contain mainly type III and II2 kerogens with dominant gas potential. Regional geological information indicates that the deep-water regions of the two basins share the same hydrocarbon source sags with the shallow-water areas, and they developed massive sandstone reservoirs during the Oligocene and Miocene. Fluid-flow modeling results show that the deep-water regions were on the pathway of lateral migrating gases, and the interpreted reservoirs in these zones have developed abundant seismic bright spots, which may reflect the presence of gas. The deep-water regions of the offshore South China Sea are believed to have great gas exploration potential.