Abstract: Analyses based on thin section,SEM,mercury intrusion and logging data were combined with outcrop and core observations to study the characteristics and diagenesis of tight sandstone reservoirs in the fourth member of the Xujiahe Formation in northern Sichuan Basin.The results show a calcareous sandstone-dominated west part,a lithic sandstone-dominated north part and a feldspar/lithic sandstone-dominated east part,in the area of interest.The rock type zoning was found to be closely linked to the distribution of the three provenances at the periphery of northern basin.Physical properties can be summarized as ultra-low porosity and permeability.Types of pores were mainly secondary solution pores with small diameter and fine throat and micro fractures.Diagenesis were complicated and varied from compaction and pressure solution,to cementation,dissolution,replacement and alteration as well as fracturing.And all the processes had once played important roles in the formation of ultra-low permeability tight sandstone reservoirs.Compaction caused loss of primary pores by 77.52% and cementation destroyed reservoir properties through porosity reduction by 30%.Dissolution generated secondary pores and increased porosity by 3.28%.Later tectonic activities also helped shaping the reservoirs with secondary solution pores-fractures being the main accumulation space.

Key words: reservoir characteristics, diagenetic evolution, tight sandstone, Xujiahe Formation, northern Sichuan Basin