Is an expression which has been used commonly to refer to high pressure found in some formations; super-normal pressure or surpressure. Technically, it should be said that overpressure is that amount of pore pressure which is in excess of normal pore pressure in overpressured formations.
Any pore pressure greater than normal pore pressure can result from a number of conditions, some of which are listed below:
(1) Abnormally high pore pressure related to geostatic load. As geostatic load increases. porous clay rock compresses with the resulting expulsion of associated water. Fractures and porous and permeable reservoir beds serve as conduits to carry off the expelled water. If the water in the pore spaces of the reservoir rock cannot be expelled, the water will be trapped. Under these conditions, as overburden is increased, the clay rock is prevented from further compaction and the compressive stress is transmitted to the interstitial water. Pore pressure in isolated reservoir beds will increase along with the pore pressures in the overlying clay rock (shale).
(2) Abnormally high pore pressure related to the density contrast between reservoir fluid (if oil or gas) and interstitial water. Some reservoirs (which when filled with water, exhibit normal pore pressures) exhibit abnormal pressure at the crest of the structure when containing a column of oil or gas. For example. in a gas-filled reservoir, the normal pore pressure at the level of the gas-water contact may be transmitted through the gas column to the crest of the structure with only a small reduction (since the pressure exerted by the weight of a substantial column of gas is low compared to that of the same height of interstitial water). This results in higher than normal pore pressure at the crest of the structure.
(3) Abnormally high pore pressure related to causes other than those found in (1) or (2). For example, high pressure may result from the charging of one bed in communication with another at higher pressure.