Fluid pressures in reservoir rocks are normally hydrostatic, but under certain conditions may be higher, ranging up to a maximum value ("petrostatic") governed by the weight of the overburden. The presence of clay or shale layers or rock salt deposits of great lateral extent may impede or completely prevent the escape of water from underlying porous sediments. Examples of such overpressured reservoirs are cited from the Permian basin of northwest Germany, and from Tertiary formations of New Guinea and Pakistan.

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