Accumulation of water-soluble gas by degasification: One of important mechanisms of large gas accumulations in the Xujiahe Formation, Sichuan Basin

In recent years, a lot of lithologic/structural-lithologic gas reservoirs were found in the Upper Triassic Xujiahe Formation, Sichuan Basin, and most of the reservoirs contain water. We studied the properties and characteristics of the fluid, and found the changes of natural gas components and carbon isotope which confirm the accumulation of water-soluble gas by degasification, namely, such accumulation far away from gas source features high methane, high drying factor, high ratio of isoalkane to normal paraffin hydrocarbons, and heavy carbon isotope in methane and ethane. Accumulations of water-soluble gas degasification by reduction of pressure & temperature for strata uplift and by reduction of pressure & temperature for lateral migration of soluble gas along layer are the main forming mechanisms. The strong strata uplift, fluid decompression and reduction of temperature, local concentration of water salinity, and rich water-soluble gas resources are the favorable conditions for the accumulation. We conclude that the accumulation of water-soluble gas by degasification is not only one of the important mechanisms of development of large lithologic gas reservoirs and large gas accumulations in Central Sichuan, but also a key reason for poor source rocks in the Xu1 Member but large gas accumulation in the Xu2 Member in the Hechuan-Anyue area.