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Overpressure, in geology, is a term used to describe the pressure regime in a stratigraphic unit that exhibits higher-than-hydrostatic pressure in its pore structure. This phenomenon is the primary cause of "oil gushers". Overpressure is also used, in military terminology, to refer to the pressure caused by an explosion over and above normal atmospheric pressure, especially when measuring the effects of nuclear weapons or thermobaric bombs.

Overpressure in stratigraphic layers is fundamentally caused by the inability of connate pore fluids to escape as the surrounding mineral matrix compacts under the lithostatic pressure caused by overlying layers. Fluid escape may be impeded by sealing of the compacting rock by surrounding impermeable layers (such as evaporites, chalk and cemented sandstones). Alternatively, the rate of burial of the stratigraphic layer may be so great that the efflux of fluid is not sufficiently rapid to maintain hydrostatic pressure.

A common type of situation where overpressure may occur is in a buried river channel filled with coarse sand that is sealed on all sides by impermeable shales.

It is extremely important to be able to diagnose overpressured units when drilling through them, as the drilling mud weight (density) must be adjusted to compensate. If it is not, there is a risk that the pressure difference down-well will cause a dramatic decompression of the overpressured layer and result in a blowout at the well-head with possibly disastrous consequences.

Because overpressured sediments tend to exhibit better porosity than would be predicted from their depth, they often make attractive hydrocarbon reservoirs and are therefore of important economic interest.

• Underpressure
• Lithification
• petroleum geology