The genesis of a reservoir is a result of the combined action of deposition,diagenesis,tectonic reworking,and interaction of rock and fluid and the evolutionary environment.We discuss the genetic and evolution mechanism of a low-permeability reservoir bed of the Xujiahe Formation in the western Sichuan Depression on the basis of the study of diagenesis,diagenetic reservoir facies and the diagenetic evolution sequence.The research indicated that this reservoir bed can be divided into five types of diagenetic reservoir facies,namely strong dissolution,chlorite-lined intergranular pores,compaction and pressure solution,carbonate cementation and secondary quartz increase.There are,however,just two diagenetic reservoir facies which provide low-permeability reservoir beds,namely strong dissolution and chlorite-lined intergranular pores.We also analyzed their diagenetic evolution sequences and the origin of the low-permeability reservoir bed.Besides,it was also indicated that the composition and structure of sandstones,types of sedimentary microfacies,diagenesis history as well as the tectonic reworking in later periods are the main factors controlling the formation of the low-permeability reservoir bed.The above- mentioned factors establish the foundation for the forecasting the distribution of high quality reservoir beds. The genesis of a reservoir is a result of the combined action of deposition, diagenesis, tectonic reworking, and interaction of rock and fluid and the evolutionary environment. We discuss the genetic and evolution mechanism of a low-permeability reservoir bed of the Xujiahe Formation in the western Sichuan Depression on the basis of the study of diagenesis, diagenetic reservoir facies and the diagenetic evolution sequence. the research indicates that this reservoir bed can be divided into five types of diagenetic reservoir facies, saying strong dissolution, chlorite-lined intergranular pores, compaction and pressure solution, carbonate cementation and secondary quartz increase. there are however just two diagenetic reservoir facies which provide low-permeability reservoir beds, suggesting strong digestion and chlorite-lined intergranular pores. We also analyze their diagenetic evolution sequences and the origin of the low-permeability reservoir bed.Besides, it was also indicated that the composition and structure of sandstones, types of sedimentary microfacies, diagenesis history as well as the tectonic reworking in later periods are the main factors controlling of the formation of the low-permeability reservoir bed. above above mentioned factors establish the foundation for the forecasting the distribution of high quality reservoir beds.