## 3 Two Modes of Forming Overpressure
### Passive Pressurization (Overpressure Formed by Rapid Tectonic Uplift)  
For strata in the mesodiagenetic and telodiagenetic stages, overpressure can also form after recent rapid tectonic uplift, known as passive pressurization (or passive overpressure). This is because strata with higher diagenesis have poor permeability, and pore fluid pressure will be maintained for a period of time after uplift. The cementation process surrounds the fluid and slows down the pressure release process. Although the uplift process may cause some pressure loss due to pore rebound and cooling, it is an order of magnitude smaller compared to the new pressure coefficient \(c\) converted at a shallower depth. Passive pressurization also needs to follow the aforementioned conditions (1) and (2). The mainstream view of most scholars currently is that uplift should result in a low-pressure trend. Their main argument is that after uplift, temperature decreases, pressure drops, and rebounding increases pores, leading to an increase in the available space for fluid volume within the pores. According to the formula of state for liquid water, formation pressure would then decrease [38]. However, these viewpoints do not adequately consider the permeation process of formation water. Both the Wufeng-Longmaxi shale formation and the Triassic Xujiahe formation in the Sichuan Basin commonly exhibit high overpressure, as shown in Figure 2. Essentially, this is the result of recent (Pliocene) rapid tectonic uplift exceeding the rate of water permeation process completion. Similar to the aforementioned active overpressure, the closer the tectonic uplift occurred to the present day, the better, especially during the Neogene and Quaternary periods. According to [PERSON] et al. (2009) [39], their research on the restoration of surface denudation in the Sichuan Basin reveals that the basin experienced two intense uplifts during the Cenozoic Era. Specifically, the Paleogene uplift averaged approximately 4000 m, while the Neogene uplift was about 3000 m. The target layer, the Xujiahe Formation of the Upper Triassic, currently has a burial depth of approximately 4 km. Assuming an initial state at the end of the Cretaceous, where the formation was under normal pressure with a burial depth of 11 km and an initial formation pressure of 110 MPa. This formation pressure is assumed to be the result of long-term permeation of pore water, forming a normal hydrostatic pressure due to decompression, independent of the effective stress on the framework. After the Paleogene uplift of 4 km, the depth decreased to 7 km. If this process had occurred over a prolonged period, the pressure would have  
Figure 2: Distribution of residual pressure in current formations in the western Sichuan region [21]. theoretically dropped to a hydrostatic pressure of 70 MPa due to decompression. However, water permeation has a certain lag and would not have completely reduced to hydrostatic pressure. Additionally, the reduction in overlying strata weight only serves to \"decompress\" the formation matrix. The effective stress (supporting stress) experienced by the formation matrix (framework) should be significantly higher than 110 MPa. Assuming a matrix density of 2.6 g/cm\({}^{3}\), the effective stress on the matrix would be approximately 286 MPa (calculated as 11 km \(\times\) 26 MPa/km = 286 MPa). Moreover, the overpressured stratum has already lithified. Temporarily disregarding factors such as uplift rebound of stratum thickness, pore enlargement, and pressure reduction due to cooling (their magnitudes will be estimated in later chapters), the pore pressure remains essentially unchanged. A reduction in the weight of the overlying stratum can only decrease the skeletal support stress, while the pressure within the pores can still be maintained at 110 MPa. Only through a prolonged period of pressure relief can it slowly decrease to the normal hydrostatic pressure of 70 MPa, corresponding to a depth of 7 km.