岩溶在四川盆地下二叠统储集层中的重要作用

四川盆地下二叠统致密碳酸盐岩气藏裂缝储集空间有限,岩溶发育是高产稳产的关键.喜马拉雅褶皱期,下二叠统致密灰岩孔隙水量极为有限,地表水难以渗入地腹岩层,故不具备形成岩溶的条件.对钻井获得的较大储量缝洞型气藏分布、地表充填溶洞调查、井下钻遇溶洞与断层的关系、溶洞风化壳充填物及地层水同位素的特征等分析,均反映出东吴期暴露岩溶在溶洞形成中起到重要作用.认为,下二叠统碳酸盐岩早期成岩致密化,通过溶蚀扩大节理面和层理层面发生岩溶作用,并形成以管道状为主的洞穴系统,为现今缝洞型气藏提供了天然气储渗的主要空间.图3表1参16

Important role of karstification in the Lower Permian reservoir in Sichuan Basin

The development of cavity rather than fracture is critical for gas reservoirs with high production and big reserves in the dense carbonate of Lower Permian in Sichuan Basin. During the Himalayan movement stage of forming folds and fractures, there was no condition of forming karstification because of no motion of the formation water or/and the meteoric water in the dense rock. The important role of karstification during the Dongwu Lift in the formation of cavity has been certified by the distribution of the gas reservoirs with big reserves, the investigation of the mineralized caves in the crop sections, the relationship between cavities met in drilling wells and faults, the finding of material filled in the cavities,and the oxygen isotopic compositions of the formation water in the Lower Permian. Since the Lower Permian carbonate becomes dense in the early digenesis stage, karstification mainly takes place through the solution and enlargement of fissures and bed surfaces, and the conduit cavern system is formed, which provides the major void spaces for the gas reservoirs with big reserves.