overpressures

Overpressures, in geology and geotechnical engineering, are pore fluid pressures that exceed the surrounding hydrostatic pressure at a given depth. In many sedimentary basins the normal pressure gradient increases with depth, but overpressures occur when the pore pressure rises faster than the rock can drain. They are also called geopressure or abnormal pressure.

Most common mechanisms are undercompaction from rapid burial with limited drainage, and hydrocarbon generation that releases fluids and elevates pore pressure. Other processes include diagenetic changes that reduce permeability, clay mineral swelling, and tectonic loading that traps fluids beneath seals. Low permeability cap rocks impede pressure relief and allow pressure to build.

Overpressures are inferred from drilling data, formation tests, and seismic and well-log indicators. A higher mud weight than expected to maintain a stable wellbore suggests overpressure. Seismic velocity reductions, resistivity anomalies, and compressibility data can indicate elevated pore pressure. Predictive methods and pore pressure prediction charts (e.g., Eaton, Roegner) help estimate normal versus abnormal pressures.

Consequences include increased drilling risk, potential blowouts, reduced fracture gradients, and challenges for reservoir development and cap rock integrity. They influence well planning, casing programs, cementing, and drilling fluids strategies.

Outside geology, the term overpressure is used in other contexts to describe pressures exceeding ambient in systems such as mining, explosives, and process engineering, but in geoscience it refers specifically to elevated pore fluid pressures within rocks.