Geological Definition of the Subsurface Pressure System and Its Significance in Hydrocarbon Exploration

ABSTRACT Lithology and principal stress are the primary factors influencing the development of the subsurface pore pressure profile. The distribution of lithology creates conditions that lead to compartmentalization in both geological time and space. Identifying the paleoenvironment and understanding sediment distribution within a basin, along with the dynamics of sedimentation and tectonics, are essential for understanding subsurface pressure systems. While most basins are mature regarding shallow prospectivity, they remain immature for deeper plays. The scarcity of deep calibration data makes pressure prediction a challenging task. The importance of pore pressure prediction cannot be overstated, as errors can be costly while drilling. Understanding the mechanisms of overpressure generation and the regional geology is closely connected to pressure distribution. Each depositional environment has its own pore pressure profile. Therefore, an integrated geological approach is recommended to develop a reliable model for exploration. To understand the geological factors affecting overpressure distribution, analysis of pressure data is necessary to assess the influence of individual lithostratigraphic units and the distribution of low-permeability mudrocks and other seals within those units. The lateral spread of overpressures may be structurally controlled. It can also be partly affected by the interaction of permeable proximal deltaic sediments, low net-to-gross ratios, and disconnected shelf-margin and slope facies. The onset of overpressure is more broadly influenced by regional shale distributions, which are most developed during the maximum flooding stage. The analysis also involves a complex relationship between distal sealing facies distributions and their proximal permeable correlatives. Broadly, where sandier deltaic/shelfal facies are connected to the surface, they allow rapid pressure dissipation and are consequently hydrostatically pressured. In more distal pro-delta/seaboard facies, lower permeability facies do not enable effective pressure dissipation, and overpressures begin to develop. Higher burial rates and distal facies development coincide; thus, increased overpressure generation typically occurs in facies developed at the shelf margin or beyond. Examples from the North Sea and the Gulf of Mexico are presented to illustrate the importance of the geological definition of subsurface pressure systems in petroleum exploration, particularly in the context of petroleum entrapment. Key words: Pore pressure prediction, Geology, Pressure cell, Loading, Unloading, Protected trap, Fault valve, Top seal, Blown trap, Sweet spot, Headroom, and Vertical effective stress.