# Causes of Multi-Mechanism Abnormal Formation Pressure in Offshore Oil and Gas Wells
## 2 Mechanisms of Abnormal Formation Pressure  
Abnormal formation pressure or abnormal formation pore pressure primarily includes abnormally low pressure and abnormally high pressure, both of which are widely present in major oil and gas basins around the world. Among these, abnormally high pressure is more common, while abnormally low pressure is relatively rare. The mechanisms behind abnormally high pressure are highly complex and can be triggered by a single factor or by the combined effect of multiple factors. Geological deposition, chemical reactions, and physical processes can all contribute to an abnormal increase in underground fluid pressure. Common causes of abnormally high pressure include undercompaction, hydrocarbon generation from source rock cracking, tectonic compression, fault activity, and diapirism. Undercompaction occurs when sediment layers fail to consolidate adequately due to rapid sedimentation, leading to an increase in pore pressure [6]. Hydrocarbon generation involves the thermal cracking of organic matter in source rocks, which can produce additional fluid and increase pore pressure [12]. Tectonic compression arises from the interaction of tectonic plates and can significantly alter stress conditions in the subsurface [8]. Fault activity can create pathways for fluid migration, resulting in localized pressure increases [1]. Diapirism, characterized by the upward movement of less dense materials, can also impact fluid dynamics and pressure distributions [9]. These factors often interact, leading to even more complex pressure systems. For instance, [PERSON] et al. [6] demonstrated that undercompaction and tectonic forces can coexist, complicating the pressure profiles in sedimentary basins. To better understand these interactions, [PERSON] et al. [13] classified the causes of abnormally high pressure into four categories based on changes in vertical effective stress during sedimentary loading. These categories include:  
1. Primary Sedimentary Loading Mechanism--Undercompaction During diagenesis, as sediments accumulate, pressure from the overlying rock layers increases, continuously squeezing water out of the sediments. As a result, vertical effective stress increases and porosity decreases with the enhanced compaction. The normal compaction process involves an increase in vertical effective stress, i.e., continuous loading. Undercompaction occurs when water in the sediments cannot be expelled smoothly, resulting in porosity that does not decrease with increased compaction. Vertical effective stress either slows its rate of increase or remains unchanged, while the overlying rock pressure continues to increase normally. According to effective stress theory, this leads to the occurrence of abnormally high pressure [14; 15; 16]. 2. Reloading Mechanism--Tectonic Activity Under certain conditions, tectonic activity can close open fractures, making it difficult for fluids to be expelled, reducing porosity while the overlying rock pressure remains unchanged. As vertical effective stress decreases, abnormally high pressure naturally occurs according to rock effective stress theory. Common tectonic compression involves the distribution of stress between rock volume and fluids, including rock porosity, rock and fluid compressibility properties, and rock stress in sedimentary basins. Stress can only be converted into pore fluid pressure when fluid flow is restricted (tectonic compression can locally open fluid channels, leading to abnormally low pressure). Similar effects are seen with diapirism, fault activity, and salt domes, provided that fluid channels can be closed. Since fault activity can sometimes connect fluid pathways, geological conditions must be carefully considered when dealing with such abnormal high pressures [17; 18; 19; 20]. 1. Unloading Mechanism--Pore Fluid Expansion  
During or after compaction, pore fluid volume increases or the overlying rock pressure decreases due to one or more reasons, reducing vertical effective stress. 1.