## 2 Formation Mechanisms of Effective Reservoir in Ultra-Deep
### Sedimentation Lays the Foundation for Mineral Fabric and Primary Porosity of Reservoirs
#### 2.2.4 Protective Role of Hydrocarbon Charging on Porosity  
The role of hydrocarbon charging in preserving porosity has been extensively debated [58; 59; 60]. Extensive exploration practices and research indicate that early hydrocarbon charging can modify the wettability of reservoir grains, leading to the formation of continuous oil films on grain surfaces. These films inhibit the precipitation of minerals such as quartz, illite, and kaolinite [61; 62; 63]. For instance, in the Silurian Kalpintag Formation of the Tarim Basin, significant hydrocarbon charging occurred from the late Caledonian to the late Hercynian periods. Subsequent tectonic uplift and erosion exposed the strata, resulting in crude oil degradation into reservoir bitumen. These early hydrocarbon charges adhered to quartz grain surfaces, altering their wettability and effectively inhibiting later quartz overgrowths. In reservoirs containing bitumen, quartz overgrowths are almost absent, with the combined area percentage of bitumen and residual porosity reaching up to 8.5% (Figure 8a). Conversely, in reservoirs without early hydrocarbon charging, authigenic  
Figure 6: Relationship between geothermal gradients and reservoir properties in the four major basins in the central and western regions of China [11].  
minerals like quartz overgrowths are more common, resulting in an average area porosity percentage of only 5.6% (Figure 8b).  
However, the protective effect of hydrocarbon charging on reservoirs largely depends on the wettability of mineral particle surfaces and the timing of oil and gas charging. In many quartz-rich reservoirs, the mixed wettability of quartz surface means some particle surfaces remain hydrophilic [46], making it challenging to form continuous oil films. Consequently, material exchange and mineral precipitation can still occur on mineral surfaces with discontinuous oil films, such as the growth of quartz overgrowth rims (Figure 8c). Therefore, in most cases, hydrocarbon charging can only slow down cementation to some extent, but cannot entirely prevent it.  
The timing of hydrocarbon charging significantly influences its effectiveness in inhibiting cementation within reservoirs. In the Kuqa Depression, studies [4] have identified multiple hydrocarbon charging episodes. For instance, research indicates that low-maturity oil entered the reservoirs approximately 5 to 3 million years ago, followed by high-maturity hydrocarbons between 3 and 1.2 million years ago, and a subsequent gas charge from about 1.2 million years ago to the present.  
Given that cementation processes typically occur over extended geological periods, these relatively recent hydrocarbon charging events may have had a limited impact on inhibiting cementation within the Kuqa Depression's reservoirs.  
Figure 7: Burial history and pore evolution of key strata in the four major basins in the central and western regions are illustrated [56; 57]. **(a)** Triassic strata in the Xinhe area of the southern slope of the Kuqa Depression; **(b)** Cretaceous Qingshuihe Formation in the southern margin of the Junggar Basin; **(c)** Xujiahe Formation in the Puguang area of the Sichuan Basin; **(d)** Permian Shihezi Formation in the Eastern Ordos Basin. These diagrams depict the sedimentary burial processes and their impact on reservoir pore development across the respective basins.