Geopressure

Geopressure refers to subsurface pore-fluid pressures that exceed the hydrostatic pressure expected at the same depth. It is a component of formation pore pressure and can be normal (hydrostatic) or abnormal (overpressure). Geopressured zones are particularly important in sedimentary basins and petroleum systems because excess pressure can influence rock strength and fluid flow.

Causes of geopressure include rapid sedimentation with limited pore-fluid drainage, undercompaction of sediments, diagenetic changes that reduce permeability, tectonic compression, and fluid generation or expansion from hydrocarbon maturation. Trapped fluids and mineral reactions can also contribute to pressure buildup. Overpressure is often most pronounced in thick, poorly permeable mudstones or shales overlain by rapidly deposited, permeable formations.

Geopressure is common in mature sedimentary basins such as deltaic and passive-margin sequences. Examples include parts of the Gulf Coast of the United States and several offshore basins in the North Sea and Asia. The regime can range from normal hydrostatic pressures to significantly elevated pressures at depth, depending on burial history and fluid dynamics.

Measurement and prediction of pore pressure rely on direct and indirect methods. Direct methods include formation tests and drill stem tests during drilling. Indirect approaches use well logs and modeling, such as sonic velocity, density, and resistivity data, with empirical tools like Eaton’s method or Robertson-type techniques to estimate pore pressure from downhole measurements.

Mitigation focuses on accurate pressure prediction, mud-weight management, real-time pressure monitoring, and, when necessary, managed pressure drilling, proper casing and cementing, and contingency planning for potential blowouts or kicks.