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

Abstract: In recent years, a lot of lithologic/structural-lithologic gas reservoirs were found in the Xujiahe Formation, Upper Triassic, Sichuan Basin, and most of the reservoirs contain water. We have carried out the study of 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 with 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 with degasification with reduction of pressure & temperature for strata uplift and with reduction of pressure & temperature for lateral migration of soluble gas along layer are the main forming mechanisms. The strong ground uplift, fluid decompression and reduction of temperature, local concentration of water salinity, and a wealth of water-soluble gas resources are the favorable conditions for the accumulation. We conclude that the accumulation of water-soluble gas with 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. 摘要: 近年来四川盆地上三叠统须家河组发现了大量岩性气藏及构造-岩性复合气藏 且普遍含水。通过开展流体性质与特征研究 发现了水溶气脱溶成藏的天然气组分变化依据与碳同位素特征变化依据远离气源-心区的水溶气脱溶气藏甲烷含量高、干燥系数较大、异构烷烃与正构烷烃比值大、甲烷与乙烷碳同位素较重。水溶气脱溶成藏主要存在地层抬升减压降温脱溶成藏与水溶气顺层侧向运移减压脱溶成藏两种模式 川-地区具有水溶气脱溶成藏所需的有利条件 即丰富的水溶气资源、地层强烈抬升、流体减压降温及地层水矿化度局部浓缩等。水溶气的脱溶成藏是大川-地区大面积岩性气藏发育及天然气规模聚集的重要成藏机理之一 也是合川—安岳地区须一段烃源岩发育很差而须二段却能形成大规模天然气聚集的重要原因。图6参36 关键词四川盆地 须家河组 水溶气 脱溶作用 成藏机理-图分类号TE122.1 文献标识码A Key words: Sichuan Basin, Xujiahe Formation, water-soluble gas, degasification, accumulation mechanis