Abstract:

The overpressured gas reservoirs are common in the Upper Triassic strata,Western Sichuan basin.On the basis of the distribution of measured reservoir pressures in the Western Sichuan basin,the effects of uncompaction,hydrocarbon generation, and tectonic compression on the formation of abnormal pressure are analyzed.The analysis indicates that the Upper Triassic sedimentation and lithologic association are conductive to the formation of compaction.The evidence from compaction curves also indicates that the undercompaction since the end of Jurassic has caused a highly abnormal pressure and its effect still exists in the Upper Triassic strata.Owing to the low permeability in the Xujiahe Formation,the rapid buildup of reservoir pressure is most likely interpreted to be caused by hydrocarbon generation,especially the generation of larger amount of gas.The actual pressure increment depends on such factors as the duration of pressure increase caused by undercompaction and the closing degree of fluid system.In the areas with rapid tectonic stress relief,the tectonic compression is unlikely to induce fluid pressure buildup.In the areas with slow tectonic stress relief,the fluid within the closed system bears most of overburden load;the abnormal pressure caused by tectonic compression accounts for 80～100% of measured pressure data.The pressure increase by tectonic compression occurred mainly during the period of Himalaya orogeny.

Key words: abnormal fluid pressure, undercompaction, hydrocarbon generation, tectonic compression, genesis of pressure, Upper Triassic, West Sichuan area