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Source, Migration and Exploration Potential of Gases in the Deep-Water Area, The Qiongdongnan Basin, South China Sea


The deep-water area in the Qiongdongnan Basin has received great attention because of its huge thickness (>10 km) of Tertiary-Quaternary sediments, and great gas potential, especially the recently discovered SS22-1gas pool located in the Central Submarine Canyon System in a water depth over1300m. The geochemical data with combination of kinetic modeling results and geological data are applied to investigate the origin, accumulation of these gases and exploration potential in the deep-water area. The results reveal that the gases are composed predominantly of hydrocarbon gas (98%-99%), with high dryness coefficients (C1/C1–5), relatively heavy δ13C1 values (−38.8‰ to −39.4‰) and δ13C2 values (−26‰ to −25‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The compositions of C5–7 light hydrocarbon in these gases are dominated by isoparaffins (35%-65%), implying higher plant source. Their carbon isotopic finger-printings and the high abundance of bicadinanes and oleanane in the associated condensates show close affinity with the coal-bearing sequence of the Oligocene Yacheng Formation in the Basin. Source rock sample data for the Yacheng Formation in the deepwater area contain variable amounts of TOC (0.4%–21%) with type II2–III kerogens, indicating excellent gas source potential. The kinetic modeling results show that the δ13C1 values of the gas generated from the source rocks during 3 Ma to Present-day has a better match with those of the reservoir gases. This means that the gas pool is young, and likely formed after 3 Ma. Geologic data indicate that the deep-water area developed massive sandstone reservoirs during the Oligocene and Miocene. In particular, abundant seismic bright spots could be seen in some of these reservoirs. Fluid-flow modeling results show that the gas chimney/faults act as the main pathways for upward gas migration from the deep hydrocarbon kitchen into the shallow reservoirs, and that the deep overpressure is the key driving force for vertical and lateral migration of the gases. This gas migration pattern implicates that the South Uplift and the No.2 fault zone are on the pathway of migrating gases, and are also favorable places for gas accumulation besides the Central Canyon System. This research provides guidance for the future gas exploration in this area. Key words: gas source; migration modeling; exploration potential; the deep-water area; South China Sea