## 3 Two Modes of Forming Overpressure
### Active Overpressure (Overpressure Formed by Rapid Subsidence)  
Statistics show that overpressure is prone to occur in strata at the mesodiagenetic and telodiagenetic stages when there is rapid deposition and subsidence in the basin during recent geological periods, such as the Neogene, especially the Quaternary. The following three conditions need to be met: (1) The strata where overpressure occurs require a higher degree of diagenesis, ranging from the mesodiagenetic to the telodiagenetic stage. (2) The rapid subsidence should have occurred relatively recently, during the Neogene or Quaternary; more distant rapid subsidence, such as in the Paleogene or earlier strata, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Pore Size** & \multicolumn{6}{c}{**Thickness (m)**} \\ \hline
**(nm)** & **50** & **100** & **200** & **500** & **1000** & **2000** \\ \hline  & \multicolumn{6}{c}{Pressure relief to 1.6 times:} \\
1 & 0.297 & 1.189 & 4.756 & 29.723 & - & - \\
2 & 0.074 & 0.297 & 1.06 & 7.43 & 29.723 & - \\
5 & 0.012 & 0.048 & 0.190 & 1.189 & 4.756 & 19.023 \\
10 & 0.003 & 0.012 & 0.048 & 0.297 & 1.189 & 4.756 \\
20 & 0.001 & 0.003 & 0.012 & 0.074 & 0.297 & 1.189 \\
50 & 0 & 0 & 0.002 & 0.012 & 0.048 & 0.190 \\ \multicolumn{6}{c}{Pressure relief to 1.2 times:} \\
1 & 0.376 & 1.506 & 6.022 & 37.640 & - & - \\
2 & 0.094 & 0.376 & 1.506 & 9.410 & 37.640 & - \\
5 & 0.015 & 0.060 & 0.241 & 1.506 & 6.022 & 24.090 \\
10 & 0.004 & 0.015 & 0.06 & 0.376 & 1.506 & 6.022 \\
20 & 0.001 & 0.004 & 0.015 & 0.094 & 0.376 & 1.506 \\
50 & 0 & 0 & 0.002 & 0.015 & 0.06 & 0.241 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Timetable for overpressure relief of mudstone with different pore sizes and thicknesses. cannot form overpressure. (3) The current depth of deposition and burial exceeds the maximum burial depth in its geological history. At this time, compaction, cementation, and other diagenetic processes are still continuing, pores are decreasing, pore water is pressurized, and there is not enough time for pressure relief. The strata are still in the process of deposition and subsidence, leading to the formation of overpressure. This process is called active pressurization, or rapid subsidence overpressure, i.e., deposition and subsidence occur faster than the permeation rate of formation water. The widespread existence of overpressure in the Shahejie Formation of the Nanpu Sag, with a maximum overpressure of up to 1.6 times the normal pressure [35], is shown in Figure 1. Weak overpressure is observed in the second and third members of the Dongying Formation, while overpressure occurs in the first member of the Shahejie Formation. The second and third members of the Shahejie Formation exhibit abnormally high pressure. The pressure distribution within the formation does not completely align with the formation trend and can cross different geological times and layers. The following is an explanation of the reasons for overpressure in the Nanpu Sag using the high permeability of formation water as an example. Recently, two sets of strata have been rapidly deposited. One is the Late Miocene Guantao Formation with a maximum thickness of 1 km, and the other is the Pliocene Minghuazhen Formation with a thickness of 2 km. This has resulted in a time lag between the permeation of formation water and the speed of tectonic movement. In the late rifting stage of the Nanpu Sag,  
Figure 1: Reservoir pressure profile and sedimentary burial history characteristics of Nanpu Sag. **(a)** Profile of formation pressure in the eastern part of Nanpu sag (NNW-SSE direction); **(b)** Comprehensive stratigraphic column; **(c)** Sedimentation and burial history. towards the end of the Dongying Formation deposition, faulting activities ceased, resulting in a relatively long period of tectonic stability. During this time, the third member of the Shahejie Formation (Sha-3) was buried at a depth of 3 km, and the formation pressure was hydrostatic, allowing for the completion of the permeation process of formation water.