莺-琼盆地天然气成藏条件及地球化学特征

研究莺一琼盆地天然气成藏条件以及地球化学特征，认为该盆地天然气成藏条件良好：高温环境促进了有机质的热演化作用，缩短了烃源岩的成熟时间；储盖组合好，孔隙型的储集层利于天然气富集，具有超压的盖层提供了优异的封闭性能；圈闭多，类型丰富；构造活动提供了天然气运移的通道；烃源岩晚期排烃的特征保证了气田不会因为长时间的扩散而损失。天然气地球化学特征揭示：莺一琼盆地天然气中烷烃碳同位素普遍较重，以煤成气为主；莺歌海盆地由于具有流体幕式充注的特点，混有生物成因气。CO2成因比较复杂，莺歌海盆地既有壳源有机成因CO2，又有壳源无机成因和壳幔混合成因CO2；琼东南盆地CO2为无机成因，包括壳源无机成因CO2和幔源无机成因CO2。图7表1参31

Natural gas accumulation and geochemical characteristics in Yinggehai-Qiongdongnan Basin, South China Sea

The accumulating conditions and geochemical characteristics of the natural gas in Yinggehai-Qiongdongnan Basin were studied, which disclosed that the natural gas accumulation was favorable in the basin. The high temperature environment accelerates the thermal evolution of the organic matters and shortens the maturation period of the source rocks. The reservoir cap combination is desirable in that the porous reservoir benefits the gases reserving and the overpressured cap provides excellent sealing ability. Multi-types of traps develop in the basin. Tectonic activities open new faults as pathways for the natural gas migration. And the source rocks expel gas in late period, which prevents the gas field from depletion due to long- term diffusion. Isotopic compositions show that the gases in Yinggehai Qiongdongnan Basin are generally coal gases with heavy alkane carbon isotopic compositions. Some biogenetic gases exist in the natural gases of Yinggehai Basin due to the episodic injection of hydrocarbon fluids. The CO2 gas in Yinggehai Basin is from organic crust origin, inorganic crust origin, and crust-mantle origin. The CO2 gas in Qiongdongnan Basin is of inorganic origin as crust origin and mantle origin.