Over pressured formations are much more common than subpressures and are encountered world wide in sediments varying in age from Pleistocene to Cambrian. In this case by forcing the formation to maintain it's fluid content and in doing so, cause it to become abnormally pressured, the permeability barrier acts as a pressure seal. In such a sequence the ratio of shale to sand must be fairly high in order to increase the possibility of a sand unit being completely isolated and encapsulated by the surrounding shales. The creation of an over pressured formation is related to many physical, geochemical and mechanical processes and their order. Generally for a formation to be producing reservoir it must be porous.
Potential reservoirs are either geologically or hydrologically pressured.
Other minor factors which are thought to contribute to the formation of an over pressured zone are, osmotic phenomenom, massive salt bed deposition, and permafrost encroachment. For a hydro pressured system, these conditions can occur only at relatively shallow depth, where the degree of compaction is not such as will cause the formation to nonporous. The natural structure of a reservoir may lead to pressuring of the capped area. Water pressures which may be normal at the base of the zone are transmitted to over lying oil and gas pockets thereby creating an overpressured gas formation at the top of the layer. Most of normally pressured formations, however, are located within shale sequences who's initial sediments were deposited at a fast rate causing a premature development of a permeability barrier which restricts fluid expulsion before compaction is complete.