## 2 Formation Mechanisms of Effective Reservoir in Ultra-Deep
### Sedimentation Lays the Foundation for Mineral Fabric and Primary Porosity of Reservoirs
#### 2.2.1 Inhibition of Quartz Cement Growth by Grain Coatings  
Grain coatings play a crucial role in preventing the widespread nucleation and precipitation of quartz cement on grain surfaces. This mechanism is primarily attributed to the protective barrier formed by grain coatings, which inhibits direct contact between grain surfaces and diagenetic fluids, thereby suppressing cementation. These coatings mainly consist of authigenic clay minerals (predominantly chlorite) and microcrystalline quartz [38; 42; 43; 44]. Among clay minerals, including illite and mixed layers, authigenic chlorite is the most effective at inhibiting quartz cementation in sandstone. For instance, in the Jurassic Toutunhe Formation of the Junggar Basin, chlorite coatings form continuous rims around quartz grains, significantly reducing quartz overgrowths (Figure 4a). In the Triassic Yanchang Formation of the Ordos Basin, where chlorite coatings are particularly well-developed, their thickness can exceed 10 \(\upmu\)m, effectively hindering quartz cementation (Figure 4c). Scanning electron microscopy (SEM) images further confirm that in areas with extensive chlorite development, only minor microcrystalline quartz growth occurs, with little to no siliceous cementation, thus preserving primary porosity (Figure 4d). Additionally, other authigenic minerals, such as hematite and siderite, can serve similar roles. In the Cretaceous red beds of the Tarm Basin, hematite coatings are common along grain edges, leading to minimal quartz or other cementing minerals observed in thin sections (Figure 4d). More importantly, chlorite coatings contribute to the formation of oil-wet rock surfaces [45]. Under oil-wet conditions, the driving force required for hydrocarbon charging is significantly reduced, effectively enhancing reservoir potential despite otherwise poor reservoir properties [46; 47].  
Figure 3: Longitudinal distribution of lithology of Silurian Kepingtag Formation in Tarm Basin and typical microscopic photos. (**a**) Shun-9 well; (**b**) 5584.3 m; (**c**) 5585 m; (**d**) 5586.4 m. ms: mudstone; It: siltstone; vf: very-fine-grained sandstone; f: fine-grained sandstone; m: medium-grained sandstone.  
The effectiveness of grain coatings in inhibiting quartz overgrowth largely depends on the extent of coverage. The greater the surface area of a quartz grain covered by the coating, the more effectively quartz overgrowth is suppressed. Conversely, if coverage is incomplete, quartz overgrowth can still occur [39]. Microscopically, it is often observed that in the same quartz grain, areas not covered by chlorine film can still develop quartz overgrowths due to uneven coating distribution (Figure 4b). Moreover, excessive chlorite content can be detrimental. When chlorite densely fills intergranular spaces, it can significantly reduce or destroy primary intergranular porosity and reservoir permeability [48].  
Figure 4: Typical microscopic photos of the effect of grain coating on pore preservation. (**a**) Junggar Basin, Jzt1, Dong-7 well, 4514m; (**b**) Junggar Basin, Jzt1, Dong-2 well, 4429m; (**c**) Ordos Basin, T3y2, HH45 well, 2350m; (**d**) Ordos Basin, T3y2, HH45 well, 2350 m; (**e**) Sichuan Basin, T3x, YB wells, 4075.72 m; (**f**) Tarim Basin, K1 bs, Xinghuo-5 well, 5600 m.  
Overall, grain coatings play a crucial role in enhancing the porosity and permeability in deep sandstone reservoirs, which often exhibit anomalously high values. Statistical analyses indicate that higher chlorite content generally correlates with better reservoir properties. However, in cases where cementation is dominated by carbonate, sulfate, or zeolite cements and quartz cementation is relatively weak, the role of grain coatings in preserving primary porosity is diminished. This is particularly evident in sandstones with lower compositional maturity, where quartz cementation is minimal, rendering the impact of grain coatings on porosity preservation negligible.