**6.3 Diagenetic purification process of quartz sandstone in the Member 1 of Suotoushan Formation**

The microscopic observation shows that the contact relationship between quartz particles in the Member 1 of Suotoushan Formation has developed from simple grain support to concave and convex intercalation, and the intergranular pores are filled by secondary expansion edge of quartz. It is generally believed that pressure dissolution first occurs at the contact position of quartz particles. Simple pressure can locally improve the solubility of minerals or rocks, and pressure on a certain point on the surface of dry solid material can improve the solubility of the material at that point, which is the basic theoretical basis of pressure dissolution [15]. In the process of pressure dissolution, elements such as silicon, aluminum, sodium, potassium and calcium enter pore water with mineral dissolution, leading to material redistribution [15]. SiO2 can be precipitated around quartz sand particles again, forming quartz enlarged edge, while other materials may be taken away with pore water, further purification of quartz sandstone. Pressure solution phenomenon not only exists between particles, but also generally exists between layers and cracks. Sutures and pressure solution pits are relatively developed (**Figure 3h**). It can be seen that the quartz sandstone of the Member 1 of Suotoushan Formation has undergone the diagenetic process of pressure dissolution and SiO2 reprecipitation, which undoubtedly further purified the quartz sandstone of the Member 1 of Suotoushan Formation. It can be considered that the high purity quartz sandstone of the Member 1 of Suotoushan Formation not only has the deposition of high purity quartz sand but also has the diagenetic purification process. The specific diagenetic purification process and mechanism need to be further studied.
