Abstract:

Systemic analysis on the potential causes of the overpressure in the northeastern Sichuan basin shows that the overpressures in tight gas reservoirs in Xujiahe Formation has been resulted mainly from the hydrocarbon accumulation and tectonic compression during the medium Yanshian to the Himalaya period, rather than compaction disequilibrium. By the SRK gas state equation, the pressure value of 25.21 MPa is calculated in Yuanba gas reservoirs, which indicates that there is an average pressure amplitude of 60.86% and resulted from hydrocarbon accumulation. Meanwhile, the pressure value of 45.81 MPa is calculated in Tongnanba gas reservoirs, which has average pressure amplitude of 72.04%. The additional excess pressures have been generated mainly by tectonic compression, which account for 39.14% and 27.96% of the excess pressures in Yuanba gas reservoirs and Tongnanba gas reservoirs respectively. The difference of the tectonic compression intensity during the Medium Yanshanian and Himalaya period controlled the distribution of present pressure, the distribution of stress release area as well as the pressure-temperature reducing extent for tectonic uplift. The Tongnanba area is located in stress release area nowadays, and the pressure reducing process after hydrocarbon accumulation and tectonic compression is the major cause of the currently lower pressure than in the Yuanba area.

Key words: Xujiahe Formation, compaction disequilibrium, hydrocarbon accumulation, tectonic compression, tight sandstone, natural gas, northeastern Sichuan basin