Abstract: The Shaximiao Formation in the transition zone between the central and western Sichuan basin has great potential for natural gas exploration, which has become a hot spot of exploration in  Sichuan basin in recent years. At present, there are many aspects restricting the exploration and development of tight sandstone natural gas in the Shaximiao Formation. The formation mechanism and synthesized prediction of “relatively high-quality reservoirs” are one of the key issues that need to be resolved urgently. In this paper, a series of analysis methods were performed, including systematic physical property analysis, blue-dye thin section identification, scanning electron microscopy analysis, and reservoir geochemical analysis, to determine the genetic mechanism of relatively high-quality reservoirs in the Shaximiao Formation. The results indicate that the tight Shaximiao sandstones are dominated by lithic feldspar sandstone and feldspar lithic sandstone. The average volume fraction of quartz，feldspar，and rock debris in the Second Member of the Shaximiao Formation (SMSF) are 35.27%,  35.37%, and 29.36% respectively. The average volume fraction of quartz，feldspar，and rock debris in the First Member of the Shaximiao Formation (FMSM) are 37.79%, 24.01%, and 38.20% respectively. The volume fraction of feldspar is relatively high compared to the FMSM，and the volume fraction of rock debris is relatively high compared to the SMSF. In terms of type and content of lithic fragments, sandstones in the SMSF are dominated by metamorphic fragments, accounting for 55% of the whole lithic fragments. By contrast, those in the FMSF are dominated by magmatic fragments, making up to 43% of the whole lithic fragments. In addition, the reservoir pore system is predominately composed of primary pores followed by feldspar dissolution pores, in which the FMSF is characterized by a higher content of those pores than the SMSF. In the SMSF, samples with porosity above 12%  account for 38.00%, but with permeability above 1×10-3μm2 account for 25.25%. Those values in the FMSF are 3.09% and 41.22%, respectively. It is generally reflected that some relatively high-quality reservoirs tend to be developed under a tight background. The fact, that the SMSF reservoir is characterized by better porosity and the FMSF has better permeability, demonstrates that the pore structure of the FMSF reservoir is relatively better. Besides, The tight sandstone reservoir in Shaximiao Formation is controlled by litho-mineral association, hydrocarbon source system and diagenetic system. The litho-mineral association provided a material basis for the diagenetic evolution. The hydrocarbon source system controlled the dissolution intensity, and provided ions for main authigenic minerals precipitation in sandstone. The opening degree of the diagenetic system had an effect on the ions’ in-and-out, and affected the dissolution and precipitation of the diagenetic products. The difference of litho-mineral association, variation of sedimentary environment, hydrocarbon source system and opening degree of diagenetic system in SMSF and FMSF are the main reasons for the differential diagenesis.

Key words: transition zone between the central and western Sichuan basin, Shaximiao Formation, tight sandstone, relatively high-quality reservoirs, genetic mechanism, Sichuan basin