## 3 Date and Methods
### Methods  
#### 3.2.1 Anomalous Pressure Prediction  
To accurately predict the pressure of the Qiongzhusi Formation in the study area, this research employed the equivalent depth method, Eaton method, and Bowers method. The logging data utilized in this study were obtained from the Southwest Petroleum Field Branch of China National Petroleum Corporation. Furthermore, to enhance the accuracy of  
Figure 1: Geological background of the research area: (**a**) Geographical location of Sichuan Basin; (**b**) Geographical location of Zizhong area; (**c**) Contour map of mudstone thickness in Qiongzhusi Formation in Zizhong area; (**d**) Stratigraphic chart of Qiongzhusi Formation (modified after [18,19]). the predicted pressure values for the Qiongzhusi Formation, the aforementioned methods were calibrated using measured pressure data. The equivalent depth method posits that the acoustic time-difference curve exhibits a distinct compaction trend line within normally compacted strata [10; 28; 29; 30]. This method assumes that, in normally compacted strata, the acoustic time-difference curve follows this clear compaction trend, while in abnormally compacted areas, the curve often deviates from the established trend. If the acoustic time difference between a point in the abnormally compacted zone and a point in the normally compacted zone is equal, and both points possess identical pore structure characteristics and compaction degrees, they are considered equivalent. Consequently, it can be inferred that the effective stress at the point in the abnormally compacted zone is equal to that at the point in the normally compacted zone. Therefore, the formation pressure in the abnormally compacted zone can be derived from the measurements at the point in the normally compacted zone. The Eaton method was developed by [PERSON] [14; 31; 32] while analyzing the intrinsic relationship between formation pore pressure and logging parameters, such as acoustic time difference, in regions like the Gulf of Mexico. This method parallels the equivalent depth method in that it constructs a normal compaction trend line based on the theory of normal compaction and introduces a compaction correction factor (C), modeled as follows:  
\[P_{\text{P}}=P_{0}-\left[(P_{0}-P_{\text{n}})(V_{\text{n}}\div V_{\text{s}})^{ \text{C}}\right] \tag{1}\]  
where \(P_{\text{P}}\) is the abnormal pore pressure, MPa; \(P_{0}\) is the overlying formation pressure, MPa; \(P_{\text{n}}\) is the normal formation pore pressure, MPa; \(V_{\text{n}}\), \(V_{\text{s}}\) are the acoustic time difference between the normal and abnormal formations, \(\upmu\text{s}/\text{m}\); and \(C\) is the compaction correction factor. Bowers method  
[PERSON] [33; 34] found that there is a curvilinear relationship between sonic velocity and effective stress for normal and unbalanced compaction-induced pressure anomalies in strata, and that the relationship between effective stress and sonic velocity is:  
\[V=1500+A\sigma^{B} \tag{2}\]  
where \(V\) is the velocity, \(\text{m}/\text{s}\); \(\sigma\) is the effective stress, MPa; and \(A\) and \(B\) are virgin-curve parameters. The empirical relation for the unloading curve is:  
\[V=1500+A\left[\sigma_{\text{max}}\left(\sigma/\sigma_{\text{max}}^{1/U}\right) \right]^{B} \tag{3}\]  
where \(U\) is the coefficient, dimensionless; \(\sigma_{\text{max}}\) is the effective pressure at the starting point of the unloading curve, MPa. \[\sigma_{\text{max}}=\left[(V_{\text{max}}-1500)/A\right]^{1/B} \tag{4}\]  
where \(V_{\text{max}}\) is the velocity at the onset of unloading, \(\text{m}/\text{s}\). Evaluation Methods for Overpressure Contribution  
In order to quantify the contribution of different overpressure genesis to reservoir overpressure, the evaluation method of overpressure contribution established by [PERSON] et al. [35] was used in this study. According to the study by [PERSON] et al. [36], the pressurization resulting from disequilibrium compaction can be determined by calculating the equivalent depth of density.