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Source, Accumulation and Exploration Potential of Gases in the Central Submarine Canyon in the Qiongdongnan Basin, South China Sea


Great attention has been paid to the central canyon in the Qiongdongnan Basin because of its close link to deepwater systems and the recently discovered SS22 gas field in a water depth over 1300m. Using integrated geochemical, kinetic modeling, geological and 3D seismic data, the current study documents the sources, accumulation and exploration potential of gases in the Submarine Canyon. The results reveal that the discovered gases are composed predominantly of hydrocarbon gas (98–99%), with the ratios of C1/C1-5 ranging from 0.92 to 0.94, relatively high d13C1 (−38.8 to-−39.4‰) and dDCH4 values (−144 to −147‰). 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. Source rock sample data indicate excellent gas source potential for the Yacheng Formation in the deepwater area of the basin. The kinetic modeling results show that the d13C1 values of the gas generated from the Yacheng source rock during −3 Ma to Present-day has a better match with those of the reservoir gases. Geologic data indicate that the Central Canyon is a large unusual submarine canyon in the Qiongdongnan Basin, South China Sea, has a total length of about 570 km with an area of 6800 km2 and is oriented sub-parallel to the continental slope, and it developed massive sandstone reservoirs during the Miocene and Pliocene. The gas chimney/faults below the central canyon act as the main pathways for upward gas migration from the deep hydrocarbon kitchen into the shallow reservoirs. The interpreted reservoirs in the zone have developed abundant seismic bright spots which may reflect the presence of gas. The gas accumulation pattern implicates the Central Canyon have great gas exploration potential. This research provides guidance for the future exploration efforts in this area. Key words: gas origin, gas accumulation model, exploration potential, the central canyon, South China Sea.