## 4 Results and Discussion
### Quantitative Evaluation of Overpressure in Different Genesis Types
#### 4.3.1 Disequilibrium Compaction and Pressure Transmission  
The disequilibrium compaction arises from rapid sedimentation and compaction of the formation, which prevents timely fluid expulsion, resulting in suppressed pore volume compression and alterations in density and acoustic time difference curves. However, fluid expansion and pressure increases have a more pronounced effect on acoustic waves and a lesser effect on density. Therefore, the equivalent depth method for constructing density curves can be employed to quantitatively calculate the pressure increases resulting from disequilibrium [35; 48; 49]. Table 2 presents the pressure increases resulting from disequilibrium compaction in the Qiongzhusi Formation. The findings indicate that the pressure increase attributed to balanced compaction ranges from 3.86 to 15.76 MPa (average value = 9.44 MPa). Notably, Well Z201 exhibits the smallest pressure increase due to the disequilibrium compaction (Figure 7a).  
Pressure transmission is an important mechanism contributing to pressure increase. Previous studies have indicated that the pressure increase resulting from overpressure transmission can be calculated using the sonic velocity vs. vertical effective stress diagram. An increase in overpressure transmission can lead to a reduction in effective stress within the formation, while a small decrease in acoustic velocity may cause overpressure data points to deviate from the loading curve. The difference in vertical effective stress before and after this change represents the increment of overpressure transmission [16; 35; 36]. Based on the acoustic velocity versus vertical effective stress diagram (Figure 6), the overpressure transmission and pressure increase in the Qiongzhusi Formation were calculated. The  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Wells**} & \multicolumn{3}{c}{**Pressure Increase Value/MPa**} & \multicolumn{3}{c}{**Pressure Decrease Value/MPa**} & \multirow{2}{*}{\begin{tabular}{c} **Residual** \\ **Pressure/MPa** \\ \end{tabular} } \\ \cline{2-2} \cline{4-6}  & \begin{tabular}{c} **Disequilibrium** \\ **Compaction** \\ \end{tabular} & \begin{tabular}{c} **OM Generales** \\ **Hydrocarbons** \\ \end{tabular} & \begin{tabular}{c} **Pressure** \\ **Transmission** \\ \end{tabular} & \begin{tabular}{c} **Uplift Erosion** \\ **Unloading Effect** \\ \end{tabular} & \
\begin{tabular}{c} **Geothermal** \\ **Decline** \\ \end{tabular} &
\begin{tabular}{c} **Pressure/MPa** \\ \end{tabular} \\ \hline ZJ2 & 4.35 & 77.6 & 18.46 & 14.07 & 32.28 & 54.06 \\ Z201 & 3.86 & 76.66 & 17.13 & 25.70 & 35.66 & 36.29 \\ GS17 & 15.76 & 90.35 & 45.32 & 24.88 & 67.2 & 39.35 \\ DS1 & 13.56 & 83.51  
results demonstrated that the overpressure transmission and pressure increase ranged from 17.13 to 54.36 MPa (mean of 37.98 MPa). Well Z201 exhibited the smallest value at 17.13 MPa, while Well DS1 displayed the highest value at 54.36 MPa (Figure 7b).  
Figure 7: The contribution of different causes of overpressure to the pressure of the Qiongzhusi Formation: (**a**) disequilibrium compaction; (**b**) pressure transmission; (**c**) uplift erosion unloading effect; (**d**) geothermal decline.