Evolution of continental formation pressure in the middle part of the Western Sichuan Depression and its significance on hydrocarbon accumulation

Based on the distribution features of present formation pressure, analysis on logging data, and genetic mechanisms of abnormal pressure, the overpressure evolution was reconstructed using numerical basin simulation and other quantitative analysis methods. The phase differences of pressure increasing mechanisms, enhancement differences among different mechanisms, and the control of geopressure evolution on gas migration and accumulation were discussed. The causes of overpressure in the Xujiahe Formation include under-compaction (from early Late Triassic to the end of the Jurassic), hydrocarbon generation (from end of the Early Jurassic to the Neogene) and tectonic compression (since the Cretaceous). It is suggested that tectonic compression and hydrocarbon generation are the principal factors of the present overpressure in the Xujiahe Formation. Overpressure transmission is the main cause of overpressure in the Jurassic strata which occurred during the intense tectonic activity periods since the Cretaceous. The overpressure is the main force for hydrocarbon migration and overpressure-related microfractures are the major pathways for gas migration. Fault transport and overpressure driving caused the formation of Jurassic secondary gas reservoirs with distant sources. The areas with high pressure coefficient in the Jurassic and high-overpressure zones in the Xujiahe Formation are optimum targets for exploration. Key words: abnormal pressure, under-compaction, hydrocarbon generated overpressure, tectonic compression, overpressure transmission, hydrocarbon accumulation, Western Sichuan Depression