Abstract: Densification process of the Upper Triassic Xujiahe sandstones were described based on petrologic,diagenesis and basin modeling analyses.The diagenetic events that affect reservoir quality mainly include mechanical and chemical compactions,precipitation of carbonate cements,and cementation by quartz.Compaction is the dominant process of porosity loss.Sandstones with high content of quartz,feldspar and carbonate rock fragment exhibit higher mechanical stability.Pervasive mesogenetic carbonate cementation occurs in the calcarenaceous sandstones in T₃x⁴ and the fine-grained sandstones in T₃x² and reservoir quality is significantly deteriorated by extensive carbonate cementation.Open and paleo-high environment is favorable for relatively good reservoir quality.Quartz cements are mainly sourced by the pressure dissolution of detrital quartz and are more abundant in sandstones rich in detrital quartz and illite and poor in carbonate cements and chlorite.Thick and continuous chlorite pore-linings help to preserve porosity by inhibiting pressure dissolution and quartz cementation and thus can be doubly beneficial to the Xujiahe sandstones.