Abstract: Abstract:Tight sandstones as an important carrier of unconventional oil and gas is great significance to study its densifying cause and formation of the reservoir.Through using drilling core, thin section observation, cathode luminescence and fluid inclusion, this article has researched diagenesis and the reservoirs formation mechanism of tight sandstones in the Xujiahe Formation, the eastern margin of Longnvsi, Sichuan. The results show that the Xujiahe Formation of the eastern margin of Longnvsi suffered mechanical compaction, pressure solution, cementation, replacement, dissolution and fracture. Mechanical compaction and cementation are the main factors that lead to densification of sandstones. Dissolution and fracture are the main factors that lead to formation of the reservoir. At present,the Xujiahe Formation is in the middle diagenetic stage AB. Main experienced fluid activities are early calcite cementation and chlorite film of surface water precipitation in syndiagenetic stage. Pressolution controls the formation of secondary quartz increase and pore fluid dissolution of clay minerals,feldspars,easily dissolved cuttings etc. in the early diagenetic stage B. A large number of oil and gas injecting promote secondary increase of quartz and dissolute a small amount of calcites in the middle diagenetic stage A. The formation of the Yanshan and Himalayan movement promote the formation of the reservoir in the middle diagenetic stage B. The formation mechanism of the reservoir is mainly covered by the preservation of the residual primary hole and the formation of the secondary pores. Formation of the reservoir is jointly controlled by favorable sedimentary environment, dissolution, abnormal high pressure and tectonic rupture. Favorable sedimentary environment, dissolution and abnormal high pressure are mainly to protect the primary hole and to form secondary pores. A large number of micro cracks are produced by tectonic disruption, not only improving seepage ability of the reservoir, also providing the conditions for forming the dissolution pores.

Key words: tight sandstone, diagenesis;reservoir, Xujiahe Formation, eastern margin of Longnvsi