# Causes of Multi-Mechanism Abnormal Formation Pressure in Offshore Oil and Gas Wells
## 1 Introduction  
The Rio del Rey Basin in Cameroon is located on the northeastern edge of the Niger Delta Basin and is rich in oil and gas resources, making it the primary oil-producing region in Cameroon. The deep turbidite sand bodies within this block represent the next targets for exploration and production. These deep turbidite sandstone reservoirs are buried at depths ranging from 2500 to 3500 m, with a maximum formation pressure coefficient of 2.10, the highest bottom-hole temperature reaching 161 \({}^{\circ}\)C, and a maximum geothermal gradient of 4.6 \({}^{\circ}\)C/100 m, classifying them as typical high-temperature and high-pressure reservoirs. These reservoirs are relatively young in terms of diagenesis and are influenced by fault blocks and mud diapirism. The causes of overpressure are complex, potentially influenced by the undercompaction of strata, high-temperature fluid expansion, mud diapirism, and fault block development, leading to a multifaceted pressure system. Conventional prediction methods based on sediment compaction theory primarily apply to the undercompaction mechanism of sandstone and mudstone strata, while the Bowers unloading model is predominantly used for fluid expansion strata [1]. However,for the multi-mechanism superposition observed in this area, single-pressure mechanism prediction methods can yield significant errors and fail to provide precise predictions. Since the 1990s, the importance of accurately determining the pressure mechanism has been increasingly recognized. Early studies primarily relied on qualitative inferences related to sedimentary evolution, stratigraphic mineral composition, and hydrocarbon migration characteristics; however, these methods often lack substantial data support and can vary greatly depending on the individual researcher's understanding of the geological context. In later studies, the incorporation of well logging data and rock mechanics has led to more intuitive and convincing judgments [2]. For instance, [PERSON] [3] summarized well logging data from a Norwegian field, noting that low acoustic velocity and resistivity, without distinct abnormal high-pressure sections in porosity and density curves, are likely indicative of fluid expansion. [PERSON] [4] introduced the acoustic-density crossover method to discern the causes of undercompaction, pore fluid expansion, and tectonic compression by analyzing the positional relationships between measured data points and the ideal loading curve. Nevertheless, this method has limitations in identifying subtle causes such as diapirism. [PERSON] et al. [5] proposed a method to distinguish between undercompaction and tectonic compression mechanisms, considering specific regional structures and sedimentation. Although the processes of undercompaction and tectonic compression share similarities, they fundamentally differ; undercompaction typically involves a one-dimensional perspective where vertical stress from sediment loading leads to abnormal pressure due to incomplete dewatering, while tectonic compression operates in three dimensions, with horizontal and vertical stresses interacting due to tectonic forces such as faulting or folding. [PERSON] et al. [5] pointed out that in areas where the folds are not tight, high-stress tectonic compression generally does not occur; however, near mountain fronts and active large reverse faults, tectonic compression can be significant. The acoustic emission test of underground coring can thus be used to determine the magnitude of in situ stress. Building on this line of reasoning, [PERSON] et al. [6] added the factor of overpressure transmission and proposed an identification pattern diagram based on these four mechanisms, which can distinguish between unbalanced compaction and tectonic compression; however, they noted difficulties in estimating fluid expansion and overpressure transmission. In 2017, [PERSON] et al.