1. . 
The pressure within the Earth, or formation pressure. The common oilfield usage, however, is to indicate anomalous subsurface pore pressure that is higher or lower than the normal, predicted hydrostatic pressure for a given depth, or the pressure exerted per unit area by a column of fresh water from sea level to a given depth. Abnormally low pore pressure might occur in areas where fluids have been drained, such as a depleted hydrocarbon reservoir. Abnormally high pore pressure might occur in areas where burial of water-filled sediments by an impermeable sediment such as clay was so rapid that fluids could not escape and the pore pressure increased with deeper burial.