Pressure compartments are found in sedimentary basins throughout the world. They are defined primarily by hydraulic potentials calculated from pressure measurements but may be indicated by differing brine and hydrocarbon chemistries; by mineralogic differences; by electrical resistivity, sonic velocity, and density of the shales; and by mud weight requirements and drilling rate changes.

Pressure compartments are characterized by an effective seal, in three dimensions, that prevents pressure equilibration to normal hydrostatic pressure. A pressure seal, as opposed to a capillary seal, restricts flow of both hydrocarbon and brine and is formed where the pore throats become effectively closed, i.e., the permeability approaches zero. A leaking pressure seal, called a "rate seal," occurs when the pressure difference caused by subsidence-sedimentation or uplift-erosion or other pressure source is greater than the seal pressure leakage. When the internal fluid pressure in the compartment exceeds the fracture pressure of the seal, the seal will fracture and fluids will escape from the compartment. The fracture and resealing may occur repeatedly.

Multiple pressure seal origins must be invoked to explain their geometric and stratigraphic occurrence. Certainly some pressure seals appear to be stratigraphically controlled with perhaps more or less diagenetic enhancement. Some seals, particularly those that cross stratigraphy, appear to be entirely diagenetic. The lateral seals, which appear to be subvertical to vertical, are possibly due to faulting and fracturing or to lateral facies changes. An extensive investigation of seal origin, recognition, and duration is being undertaken by a consortium of universities under the sponsorship of the Gas Research Institute.