## 5 Discussion
### Pore Origin and Diagenesis Sequence  
In the penectemporaneous stage, due to the evaporation pump and reflux infiltration, a large amount of mud powder crystal dolomite is formed, which contains primary  
Figure 12: Microscopic characteristics of dolomite in the fifth member of the Majiagou Formation in the Ordos Basin. (**A**) T66, 3740 m, microite dolomite, horizontal laminae, wavy laminae; (**B**) T39, 3565.71 m, micrite dolomite, bird’s eye structure, gypsum is directional distribution; (**C**) J2, 3599.84 m, crystal powder dolomite, the bright edge structure of cloudy core can be seen in dolomite, automorphic-hyautomporuic; (**D**) J2, 3587.9 m, the bright edge structure of cloudy core can be seen in dolomite; (**E**) J2, 3608.17 m, fine crystalline dolomite, saddle dolomite (PPL); and (**F**) J2, 3608.17 m, fine crystalline dolomite, saddle dolomite (CPL).  
intergranular pores and a small amount of intercrystalline micropores. Although the original porosity can reach 60-70%, due to its micrite dolomite state, it is difficult to form a good reservoir in the later diagenesis process. However, its brittleness is strong, it is easy to form fractures under tectonic action, and it contains highly soluble anhydrite, which provides conditions for dissolution. Dissolution can act on each diagenetic stage of carbonate rocks, increase the porosity of rocks, and improve the reservoir performance by dissolving and transforming the original pores. It is a constructive diagenesis. The dissolution of carbonates in the penecontemporaneous environment and the supergene environment is often fabric-selective. For example, it can be identified by some mold pores or intragranular pores. The dissolution of carbonates in the supergene karst environment is related to tectonic uplift or unconformity. We can attribute the non-selective dissolutions developed near the regional unconformity surface to the dissolution of carbonates in the supergene karst environment. However, the dissolution of carbonate in the medium-deep burial environment is due to the continuous discharge of acidic pressure-release water from the overlying strata during the compaction process and the karst produced by the migration. In this study area, the dissolution of anhydrite, dolomite, and calcite in the penecontemporaneous and near-surface shallow burial environment and the dissolution of dolomite and calcite in the medium-deep burial environment are the main observations.  
The diagenetic evolution sequence refers to the sequence of various diagenesis processes. According to the research, some diagenesis can occur in different diagenetic stages, such as dolomitization and karstification, which can occur not only in the same diagenetic stage but also in other diagenetic stages. In this case, the diagenetic changes in rocks caused by different diagenesis in the non-dienetic stage have different characteristics. Diagenetic stages can be divided according to various diagenesis processes and the resulting diagenetic mineral assemblages and structural changes [49; 50; 51]. Figure 13 shows the influence of the main diagenesis on pore evolution in the fifth member of the Majiagou Formation in the study area systematically and vividly. The diagenetic evolution sequence from the penetemporaneous period to the middle-deep burial period is penecontemporaneous dolomite, anhydrite dissolution \(\rightarrow\) seepage silt filling, freshwater dolomite, calcite, and gypsum filling, pressure solution compaction, calcite partial dissolution \(\rightarrow\) gypsum filling, karst cave, buried hydrothermal dolomite, dolomite partial dissolution \(\rightarrow\) calcite complete dissolution, pore dissolution expansion, and quartz pyrite filling. Recrystallization and compaction pressure solution affect the reservoir to varying degrees from the near-surface shallow burial period to the middle-deep burial period.  
Figure 13: Diagenetic sequence diagram of dolomite in the fifth member of the Ordovician Majiagou Formation in the Ordos Basin.