<sup>1</sup> Authigenic clay mineral cementation

According to the casting slice and SEM analysis, chlorite and kaolinite are authigenic clay minerals in the Chang 8 reservoir of Zhenjing area, accompanied by some illite/smectite formation and illite. Chlorite, which was developed in the early diagenetic periods, was found in slice and foliated structures, manifested as looped lining cementation and pore-lining of cementation (Figure 4c,d). The supporting framework that was formed by the looped lining cementation has the ability to protect pores from cement filling effectively and it can also inhibit mechanical compaction to some extent. Moreover, it can be used as a separation layer between the silica-containing fluid and quartz particles to inhibit the nucleation of SiO2 on quartz granules, as well as the secondary expansion of authigeneic quartz, and promote the storage of primary pores. This mechanism has a constructive effect on the evolution of pores (Figure 5a).

**Figure 5.** Analysis of clay mineral cementation in the Chang 8 reservoir [28]: (**a**) Relationship between chlorite content and porosity and particle size; (**b**) relationship between carbonate cement content and porosity.

Kaolinite cement appeared as book-like and worm-like under SEM imaging (Figure 4e). Its formation is closely related to the alteration of feldspar. Feldspar is one of the most commonly observed clastic particles in the region and it may produce kaolinite after dissolving in acid water. In other words, the occurrence of kaolinite is accompanied by the dissolution of feldspar.

The authigenic illite has a relatively low content in the Chang 8 reservoir of the Zhenjing area and it is frequently adhered onto particle surfaces as thin films, schistose, hair-like, and fibrous structures (Figure 4f). Montmorillonite illite may cause illite/smectite formation, which appeared as honeycomb shapes under SEM imaging (Figure 4g) [28].
