The tight siltstone reservoirs of the lower Triassic M Formation in the Western Canadian Basin are rich in oil and gas resources，but it has a series of problems such as fine grain size，low porosity and permeability，complex sedimentary and diagenesis processes，large differences in rock fabric between layers，and strong vertical and horizontal heterogeneity，resulting in low accuracy in geological sweet spot prediction and difficulties in oil and gas exploration and development. To better understand the reservoir characteristics and achieve efficient development，research on the restoration of the reservoir densification process should be deepened. Based on the core description and thin section analysis，X-ray diffraction，scanning electron microscopy，and high-pressure mercury injection were comprehensively used to determine the reservoir pore development characteristics and diagenetic types of the three main layers of the lower Triassic M Formation in the study area. On this basis，the evolution process of diagenesis-reservoir formation was further clarified，and the causes of reservoir densification were divided. The results show that the pores in the tight siltstone reservoirs of M Formation are dominated by residual intergranular pores and dissolution pores，and the pore throat radius is concentrated between 10 and 150 nm，which belongs to tight siltstones with ultra-low porosity and ultra-low permeability. The diagenesis types are dominated by compaction，cementation，and metasomatism. The present diagenesis falls between the middle stage A and middle stage B. The dissolution is relatively developed in the LE layer. Due to the influence of sedimentary environments，the processes of porosity variation in the three main layers differ greatly. LC layer is dominated by compaction-based porosity reduction，supplemented by carbonate cementation-based porosity reduction. In the LE layer，carbonate cementation is the main factor of porosity reduction，and dissolution has improved the reservoir properties. LG layer has a higher shale content，and early compaction is the primary mechanism for porosity reduction. Based on the diagenesis and porosity variation，the causes of densification in the LC，LE，and LG layers are classified as primary mixed origin，secondary cementation origin，and primary fine-grained origin，respectively.