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overpressure Overpressured zones occur in subsurface horizons in which the fluid pressure is greater than the normal hydrostatic pressure (see hydrostatic stress) for the depth in question. Overpressures develop in rapidly deposited sediments which have been sealed by impermeable horizons, which prevent the pore fluids from escaping. Drilling into overpressured horizons can cause hazardous blowouts and well-caving.