# Causes of Multi-Mechanism Abnormal Formation Pressure in Offshore Oil and Gas Wells
## 2 Mechanisms of Abnormal Formation Pressure
### Data Sorting and Analysis  
Based on the collected logging data, this study extracted and organized P wave velocity and density data at equivalent depths in the block, presenting them in a velocity-density cross-plot. The cross-plot shows the relationship between P wave velocity and density over the entire well depth, with each data point corresponding to specific values at certain depths. Detailed analysis of the cross-plot can extract important information about formation properties, especially the mechanisms behind abnormal formation pressures. In the plot, normal formations typically exhibit a linear trend representing the relationship between porosity and mineral composition. However, any anomalies deviating from this trend may indicate abnormal formations. The velocity-density cross-plot's variations over the entire well depth are shown in Figure 1.  
From the P wave velocity and density cross-plot, three main mechanisms for abnormal pressure can be identified: undercompaction, fluid expansion, and mudstone diapirism.  
Figure 1: Velocity–density cross-plot variations over well depth.  
1. Undercompaction: Rapid sedimentation may not allow for proper compaction, resulting in higher porosity and lower density. P wave velocity and resistivity can reflect porosity to some extent; thus, lower velocity and higher resistivity may indicate undercompaction. This corresponds to the lower value area along the normal trend line in the velocity-density cross-plot.
2. Tectonic Compression: Additional compaction beyond that due to the sedimentary one lead to overcompaction, where pores are compressed and fractures are closed, resulting in increased P wave velocity (reduced resistivity, decreased porosity), though density increases only slightly. Mudstone diapirism, differs in the direction of compression compared to expected tectonic offset and transport. This corresponds to the higher value area along the normal trend line in the velocity-density cross-plot, similar to an inverse unloading line.
3. Fluid Expansion: Including hydrothermal pressurization and hydrocarbon generation (oil and gas). On the basis of normal sedimentary compaction, the rock volume is expanded by fluids, reducing P wave velocity (increasing resistivity), though density changes slightly. The temperature relative to the normal geothermal gradient is higher. Pressure changes caused by hydrothermal pressurization can be quantified by the thermal expansion coefficient of water. Structural compression can be seen as overloading, while fluid expansion as unloading. This corresponds to the lower velocity value area along the normal trend line in the velocity-density cross-plot, positioned on the unloading line.  
In the studied area's velocity-density cross-plot, actual data for high-pressure anomalies show distinct characteristics. Specifically, undercompaction occupies a larger proportion throughout the area, concentrated in shallower and mid-level formations. Further analysis indicates fluid expansion phenomena are mainly in mid-level and deeper formations, while mudstone diapirism is primarily found to occur in deep formations.