Keywords: 
• Xujiahe group /
• Dissolution characteristics of particle boundary /
• The formation mechanism /
• Geological significance

Abstract: [Objective] The dissolution of quartz particles in tight sandstone has attracted more and more attention. There is a clear correlation between the dissolution boundary characteristics of quartz particles and the dissolution mechanism, but the coupling relationship has not been systematically summarized to determine the types and characteristics of quartz dissolution. In order to clarify the relationship between the dissolution characteristics of quartz particle boundary and the dissolution mechanism. [Method] Based on the methods of thin section identification, cathodoluminescence, and scanning electron microscopy, combined with the test and analysis data, the fine observation and statistical analysis of the quartz boundary dissolution characteristics of the second member of Xujiahe Formation in western Sichuan are carried out. By establishing different types of quartz boundary dissolution, the dissolution process of quartz is determined, and the dissolution mechanism is analyzed. It can provide new ideas and references for the dissolution analysis of sandstone skeleton particles and the genesis of pores in tight sandstone. [Result] The boundary dissolution characteristics of quartz particles are divided into two categories: 1 smooth boundary-dissolved pore-increasing type, which shows that the boundary of quartz particles is smooth and clear, and pores are formed after dissolution; the fuzzy boundary-dissolution metasomatism type is characterized by the fuzzy and rough boundary of quartz particles, which is the result of dissolution and metasomatism of carbonate minerals and clay minerals. According to the different metasomatism minerals, it is further divided into boundary fuzzy-dissolution-carbonate metasomatism type and boundary fuzzy-dissolution-clay mineral metasomatism type. The smooth boundary-dissolution pore-increasing type is the result of organic acid dissolution under acidic conditions. The fuzzy boundary-dissolution-carbonate metasomatism type is the result of metasomatism between carbonate ions and quartz particles due to the difference in ion concentration under alkaline conditions. The boundary fuzzy-dissolution-clay mineral metasomatism type is that under alkaline conditions, clay minerals release alkali metal ions, produce a “salt effect”, accelerate the dissolution rate of quartz particles, and metasomatized quartz particles through clay film. The evolution of quartz particle dissolution boundary tends to change from smooth boundary-dissolution pore-increasing type to fuzzy boundary-dissolution metasomatic type, which not only reveals the characteristics of smooth boundary-dissolution pore-increasing type but also reflects that the evolution of quartz particle boundary is in the direction of favorable reservoir. Among the quartz particles corroded in the second member of the Xujiahe Formation in the study area, the corrosion loss part accounts for 4.33 % ~ 8.67 % of the quartz particle area, with an average of 6.37 %. The proportion of dissolved quartz particles to all quartz particles reached more than 55.33 %, with a maximum of about 72 % and an average of 63.02 %. The content of quartz in the thin section is about 45 % -96 %, with an average of 75.3 %. Therefore, the statistical results of quartz dissolution surface porosity are between 2.05 % and 4.09 %, with an average of 3.19 %. [Conclusion] The dissolution of quartz particles is carried out in a favorable direction for the reservoir and can provide a certain amount of secondary pores, increase the oil and gas reservoir space, and effectively improve the pore structure of tight sandstone.