## 4 Formation of Normal or Low Pressure Patterns
### Large Uplift and Small Subsidence Low Pressure Pattern  
Large uplift and small subsidence refer to a long-term significant uplift followed by a rapid but small-scale subsidence that does not reach the maximum burial depth in history. Taking the Sulige gas field as an example, it is an abnormally low-pressure gas field. Since the Late Cretaceous (about 95 Ma), it has been in a state of uplift and denudation for an extended period, with a relatively slow denudation rate, allowing sufficient time for the internal overpressure to release. Another possible reason is that the Mesozoic strata in the northern Ordos region have relatively coarse lithology, with no development of source rocks and poor capping layers [45]. The sealing ability of the overlying strata is poor, and the mudstone layers have thinner individual and cumulative thicknesses, leading to rapid water permeation and pressure relief, resulting in low pressure. A more important reason may be that the ancient uplifted terrain was higher than it is now. According to the research by [PERSON] et al. (2009), the Weihe, Yinchuan, and Hetao fault depressions formed successively at the end of the Paleocene to Eocene, and their timing and tectonic patterns are no different from those of the Bohai Bay and other basins in the east. The Shanxi Graben System developed later [46]. [PERSON] et al. (2012) concluded that the Sulige gas field experienced a small-scale sedimentary subsidence during the Neogene [47]. This means that since the Cenozoic era, there have been fluctuations rather than continuous uplift, resulting in a possible uplift to a higher elevation than present after the Late Cretaceous, followed by a long period of static conditions. After releasing pressure, hydrostatic pressure (normal pressure) was formed, as shown in Figure 5. The shallowest top surface of the P\({}_{2}\)s in the Shihezi Formation is 3120 m; during the Neogene, there was a small-scale subsidence, and the P\({}_{2}\)s lowered to 3210 m, not having enough time to adjust by \"replenishing water\" to the dense He 8 reservoir section and not yet returning to normal hydrostatic pressure, forming the present abnormally low-pressure gas reservoir. In the aforementioned active overpressure model, when a new sedimentary load is added to the original equilibrium state, there is a tendency for pore space to shrink, allowing fluids within the pores to share the load of the overlying strata. However, this does not occur during small-scale subsidence after uplift. Because the latest subsidence did not reach the maximum depth in history (the P\({}_{2}\)s lowered to 4150 m at the end of the Early Cretaceous, 97 Ma), the organic matter maturity and porosity would remain at their maximum states,  
Figure 4: Burial history of well Ba 5 in the Erlian Basin, the red dotted lines in the figure show temperatures history [42]. and the diagenesis process would stagnate. The fluids within the pores would not share the additional overlying load with the skeleton until the latest subsidence exceeds the original maximum depth, at which point compaction and increased maturity and other diagenesis processes would resume. Therefore, the numerical deviation of abnormal low pressure from normal pressure can be used to qualitatively calculate the relative height of the highest uplift during geological history compared to the present. The formula is as follows:  
\[\Delta h=100(P_{j}-P)=100(z_{j}/100-P)=z_{j}(1-c) \tag{3}\]  
where \(\Delta h\) represents the uplift height in meters (m), \(P\) represents the formation pressure at the measuring point in megapascals (MPa), \(Pj\) represents the hydrostatic pressure at the corresponding depth \(z_{j}\) of the measuring point in megapascals (MPa), \(z_{j}\) represents the depth of the measuring point in meters (m), and \(c\) represents the pressure coefficient. The pressure coefficient of the Sulige gas reservoir is relatively low (with an average of 0.85), and the main layer has a deep burial depth (with an average of 4000 m).