<p>The “sweet area” within the tight reservoir of the Shan-2 member of the Shanxi Formation in the Ordos Basin is identified in this study by exploring the relationship between reservoir densification and hydrocarbon accumulation. The diagenesis process of different sandstone facies are revealed by combining cast thin section, X-ray diffraction and scanning electron microscope. Based on fluid inclusion petrography, microthermometry and laser raman probe testing, combined with single well burial history and thermal history simulation, the hydrocarbon charging period were determined. The results show that there are mainly two periods of hydrocarbon charging in Shan-2 member of Shanxi Formation. The first period occurred in the late Triassic to early and middle Jurassic between 220-170 Ma, and the second period occurred in the late Jurassic and early Cretaceous between 160-100 Ma. The reservoir densification process and hydrocarbon charging sequence of different sandstone facies are obviously different. The pure quartz sandstone, quartz-rich and plastic-poor debris sandstone have good physical properties before hydrocarbon charging in the second period, the hydrocarbon is accumulation meanwhile the reservoir is dense in middle diagenetic stage B. The tuffaceous-rich quartz sandstone and plastic-rich debris sandstone have been basically compacted in early diagenetic stage, while carbonate cemented sandstone has been dense in middle diagenetic stage A, which are not conducive to the charging of hydrocarbon. Therefore pure quartz sandstone and quartz-rich and plastic-poor debris sandstone become the most favorable sandstone facies for natural gas exploration.</p>