## 5 Discussion  
### The Influence of MSR Mixing Degree on Pore Structure  
The reservoir quality of sandstone can be influenced by grain size, pore types, sedimentary structures, and facies types [39]. The carbonate rock reservoirs are heavily impacted by lithofacies classification [40]. However, under mixed sedimentation, the degree of mixing has a significant effect on the pore structure of the MSRs, thus affecting the reservoir quality. When large particle-sized terrigenous sand (20-200 \(\upmu\)m) and small particle-sized dolomite (1-20 \(\upmu\)m for mud and microcrystals) are mixed together during sedimentation, the stacking pattern of particles with different sizes changes with the sand content, and its impact on physical properties also varies (Figure 8). When sandstone undergoes mixed deposition with dolomite, that is, when a small amount of dolomite is mixed into terrigenous sands, the original intergranular pores among the sands (with close packing generally about 25%) cannot be completely \"filled\" by dolomite at a low mixing degree. As a result, many residual intergranular pores remain. The permeability of sandstone-bearing dolomite decreases as the dolomite content increases (Figure 7). A double-pore-throat mode can appear when the crystalline gap increases (a double-peak structure will appear on the pore throat radius distribution diagram). When the degree of mixing becomes high, the pore structure of MSRs changes significantly: the intergranular pores basically disappear and are replaced by intercrystalline pores (dissolved intercrystalline pores) and intercrystalline gap assemblies. With fewer sandy particles and more fine-crystalline dolomites per unit volume, the number of intercrystalline pores (dissolved intercrystalline pores) increases. Theoretically, both porosity and permeability are significantly reduced (Figure 7). Figure 7: The cross-plot of porosity and permeability of sandstone, MSRs, and dolostone reservoirs. (**a**) The cross-plot of porosity and permeability of sandstone, dolomite sandstone, and sandstone-bearing dolomite. (**b**) The cross-plot of porosity and permeability of dolostone, sandy dolostone, and dolostone-bearing sand. Note that these samples were collected from wall cores of drilling wells. When mixed sedimentation occurs in dolostones with a crystalline structure, that is, when a small amount of sand is mixed into fine-crystalline dolomites, the addition of large sand particles can reduce the porosity of the rock (Figure 7). Because the sand particles are \"suspended\", the pore structure of dolostone-bearing sand is not significantly affected by sand mixing, and the pore structure type remains primarily intercrystalline (dissolved) pores and crystalline gap assemblies. In the sedimentary environment of a tidal shoal with a large number of dolomitic allocations, granular dolostone-bearing sand occur. In this type of MSR, the stacking pattern of a small amount of sand and a large amount of dolomite is similar to that of fine-crystalline dolomite mixed with sand. However, the difference lies in the pore types: granular dolostone-bearing sand exhibit large-scale intergranular pores, dissolved intragranular pores, and moldic pores. Small sand particles can fill these pores, resulting in a slight reduction in porosity and permeability (Figure 7). Because the tidal shoals are deposited in the subtidal zone, which is generally less influenced by the delta and contains fewer sands, the granular dolostone-bearing sand is mainly deposited in mixed tidal shoals with limited sand input. The pore-throat pattern of granular dolostone-bearing sand is basically the same as that of fine-crystalline dolostone-bearing sand deposited in normal tidal shoals. Therefore, the pore and throat patterns of MSRs reservoirs can change with the mixing degree index, which is very different from those of pure sandstone and dolostone. When the content of dolomite in sandstone exceeds 25%, the pore-throat characteristics change significantly.