## 2 Formation Mechanisms of Effective Reservoir in Ultra-Deep  
### Sedimentation Lays the Foundation for Mineral Fabric and Primary Porosity of Reservoirs  
The four major basins in central and western China have undergone distinct sedimentation phases: marine sedimentation in the Early Paleozoic, transitional marine-terrestrial sedimentation in the Late Paleozoic, and terrestrial sedimentation since the Mesozoic and Cenozoic. Reservoir properties vary significantly across these basins (Table 1). Overall, the Tarim and Junggar Basins exhibit relatively better reservoir quality, followed by the Ordos Basin, while the Sichuan Basin has the poorest reservoir properties. Reservoirs' characteristics across different basins and sedimentary environments are largely controlled by original sedimentation processes. On a macro scale, sedimentary environments determine the distribution of sedimentary facies, thereby influencing the formation of reservoir sand bodies in oil and gas accumulations [35]. On a micro scale, they affect key factors such as  
Figure 1: The favorable exploration formations in the central and western regions of China. provenance, grain size, sorting, roundness, and matrix content, which form the material foundation for reservoirs at later stages [36; 37]. In marine sedimentary environments, the Devonian Donghetang Formation in the Tarim Basin developed high-energy foreshore facies, depositing pure medium- to fine-grained quartz sandstone with quartz content exceeding 90% (Table 1). Despite long-term deep burial, the grains remain primarily in point-to-line contact, maintaining an average porosity of 18.3% (Figure 2a, Table 1). In contrast, the Silurian Kalpintag Formation in the Shun-9 well area of the Tarim Basin, also comprising coastal sedimentary facies, is characterized by offshore deposits with lithic content generally exceeding 50% (Table 1). As a result, these reservoirs have experienced more intense compaction at similar burial depths. Some exhibit almost no visible pores under a microscope, with plastic lithic fragments compacted into grain pores, leading to relatively poor reservoir quality (Figure 2b, Table 1). Similarly, the Silurian Xiaotheba Formation in the Sichuan Basin primarily developed distal sand bars in a delta-front environment. Coarse clastics are largely absent, and the reservoir consists mainly of siltstone to argillaceous siltstone, resulting in relatively poor reservoir properties (Table 1). Figure 2: Typical core and thin-section photos of different sedimentary facies zones in the four major basins of central and western regions of China. (**a**) Tarim Basin, D3d, foreshore bar fine sandstone, Shun-1 well, 5343 m; (**b**) Tarim Basin, S1k, offshore sand bar fine sandstone, Shun-9 well, 5900 m; (**c**) Junggar Basin, T1b, delta plain glutene body, Ma-18 well, 3874 m; (**d**) Junggar Basin, T2k, delta front fine sandstone, Zheng-10 well, 6701 m; (**e**) Sichuan Basin, T3x, delta front fine sandstone, Fenggu-110 well, 4624 m; (**f**) Ordos Basin, P1s, Delta Diversion Channel, Xinfu-3 Well, 2663 m.  
Since the Permian-Triassic period [13; 24; 31], the four major basins in central and western China have progressively transitioned into continental sedimentary environments. In the Junggar basin, thick fan deltas and braided-river delta sandbodies developed during the Triassic. Near the basin margin, in the delta plain close to the provenance area, thick glutenite bodies were deposited. Despite their coarse grain size, sands, gravels, and muds were rapidly mixed and deposited, with tuffaceous material and argillaceous matrix commonly filling the pores (Figure 2c), resulting in poor reservoir properties. Towards the sag area, the sorting and roundness of delta-front sandbodies improve significantly (Figure 2d,f), enhancing reservoir quality and making them prime exploration targets.