Abstract: Typical tight sandstone reservoir are well developed in the deep Triassic Xujiahe Formation in western Sichuan depression. It experienced multi-stages of tectonic evolution, hydrocarbon generation and expulsion and reservoir formation, and also experienced complicated processes of reservoir densification, thus, its genetic types and formation mechanisms of gas reservoir are difficult to be recognized. The genetic types and its formation mechanisms have been discussed, through dissecting the typical gas reservoir, combining the tectonic evolution, history of hydrocarbon generation and expulsion and the process of sandstone reservoir densification. The comprehensive study determined the genetic type of tight sandstone reservoir and analyzed its formation mechanism. The results show that the tight sandstone reservoir in deep Triassic Xujiahe Formation in the western Sichuan depression has complex geological characteristics. It characterizes as both high structural and low structural enriched with gas, coexistence of high porosity and low porosity reservoir, coexistence of high pressure and low pressure reservoir, complicated gas-water distribution etc. The present gas reservoir is the recombination and superimposition of two types of reservoirs:pre-existing tight sand gas reservoir and subsequent ones. There are two limits to identify the formation of tight gases, the lower limit of buoyancy-controlled accumulation and the lower limit of hydrocarbon accumulation during the evolution of deep Xujiahe Formation. These two lower limits can divide the hydrocarbon-bearing basin into three kinds of fluid dynamic fields and control the formation and distribution of tight sandstone reservoir. Based on the history of hydrocarbon generation and expulsion, reservoir densification history, reservoir formation and the evolution of structures, the genetic model of the formation and distribution of superimposed continuous hydrocarbon reservoirs has been established, which can be divided into four stages. ① From the Early Triassic to the end of Triassic, a stage of gas accumulation preparation; ② From the end of the Late Triassic to the Middle Jurassic, an early stage of formation of conventional gas reservoir; ③ From the end of the Middle Jurassic to the end of Jurassic, a stage of formation of subsequent tight sand gas reservoir and pre-existing tight sand gas reservoir; ④ From the end of the Late Jurassic to the Middle Cretaceous, a stage of formation of superimposed continuous gas reservoir.

Key words: deep petroleum-bearing basin strata, tight sandstone reservoir, superimposed continuous gas reservoir, hydrocarbon accumulation mechanism, Xujiahe Formation in western Sichuan depression