## 3 Two Modes of Forming Overpressure
### Passive Pressurization (Overpressure Formed by Rapid Tectonic Uplift)
At the end of the Paleogene, this pressure coefficient had already increased to 1.57 due to uplift. As evolution progressed to the Neogene period, pressure attenuation needs to be considered. At this point, based on the length of burial time, similar to what was mentioned earlier, a qualitative formation pressure retention coefficient a = 0.8 is assigned. This reduces the pressure coefficient from 110 MPa to 88 MPa. During the Neogene period, there was a further uplift of 3 km, changing the depth from 7 km to 4 km. Similarly, due to the relatively short time since then, it can be considered as having no attenuation. Therefore, after two uplifts and based on the new depth of 4 km, the pressure coefficient \(c\) increases to 2.2. Studies by multiple scholars have indicated that the uplift during the Neogene period occurred relatively recently, likely during the Late Cenozoic era. Electron spin resonance dating has determined that the oldest age of the Dayi conglomerate in the western Sichuan Basin is between 2.6 and 3.6 Ma. Many regions along the northern edge of the Tibetan Plateau have also deposited a similar set of coarse-grained sediments, such as the Xiyu conglomerate and Jishi conglomerate, with paleomagnetic and fission track ages dating back to the Late Cenozoic (2.5 to 3.5 Ma). This reflects that the latest period of intense uplift in the Tibetan Plateau and Sichuan Basin ended around 2.5 Ma [40]. Compared to other geological timescales, this period is relatively short, and the pore fluid pressure has not had sufficient time to dissipate. Figure 3 shows the burial history of the Sichuan Basin, indicating that the era of the latest rapid uplift and denudation is relatively recent. Figure 3: Comprehensive analysis of the causes of abnormal pressure in the Dengying Formation (Z2d) of the well W117 in the Sichuan Basin [25]. If it is a recent rapid uplift, the following approximate formula can be used to calculate the uplift and denudation thickness \(\Delta h\):  
\[\Delta h=p\times 100-z_{pmax} \tag{2}\]  
where \(p\) represents the formation pressure at the depth \(z_{pmax}\), which corresponds to the maximum formation pressure coefficient, measured in MPa. Using this value for calculations is more accurate as other well sections, such as shallower depth intervals, may rapidly decompress during uplift and become distorted. This formula is only an approximation, as it may involve multiple recent episodes of uplift. This model requires that the faster the uplift speed and the closer the time of uplift to the present, the better. The closely spaced JY1, PY1, and LY1 wells (<50 km) in the Wulong area on the southeastern edge of the Sichuan Basin all began to uplift during the Cretaceous period. However, JY1 experienced rapid uplift in a relatively recent time, resulting in a 1.5-fold passive overpressure in the Wufeng-Longmaxi shale formation. In contrast, PY1 and LY1 underwent uniform and slow uplift, leading to normal or low pressure, fully validating this reasoning. Most shale oil and gas basins in the United States are characterized by normal to low pressure, which is typically associated with long-term slow uplift and denudation processes that began a considerable time ago. For example, the New Albany shale in the eastern part of the Illinois Basin has been undergoing tectonic uplift since the early Triassic period with a pressure coefficient of 0.98. The Antrim shale in the Michigan Basin has experienced prolonged uplift following the deposition of Jurassic strata, resulting in a pressure coefficient of 0.4-0.6. The Barnett shale formation in Eastland County, Fort Worth Basin, has been uplifting since the end of the Cretaceous period K1 with a pressure coefficient of 1.0. Relatively speaking, the tectonic uplift of the Eagle Ford shale formation in South Texas occurred more recently, starting with rapid uplift at 15 Ma during the Miocene, resulting in a pressure coefficient of 1.27 and weak overpressure, forming a phenomenon of rapid uplift and passive overpressure.