Abnormal hydrostatic pressure

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Formation fluid pressure and its application
Author Edward A. Beaumont, Forrest Fiedler
Link Web page
Store AAPG Store


Abnormal hydrostatic pressure is a departure from normal fluid pressure that is caused by geologic factors. The term “geopressure” was introduced originally by Shell Oil Company to refer to overpressured intervals in the U.S. Gulf Coast. “Geopressure” is gradually being replaced by the more descriptive terms “overpressure” and “underpressure.”


Abnormal fluid pressures may be caused by any of the following:

Abnormal pressures develop when fluid is unable to move into or out of the local pore system fast enough to accommodate to the new environment. Such a pore system must be isolated from the surrounding system by impermeable barriers for abnormal pressure to exist.

The table below shows the generally accepted major causes of abnormal fluid pressure.

Overpressure Underpressure
Uplift Burial
Heat increase Heat decrease
Compaction, Generation of hydrocarbons Dilation of pores

Multiple simultaneous causes

More than one mechanism may operate simultaneously or sequentially to create abnormal pressure. For example, burial of a sealed compartment carries a trapped fluid pressure into a deeper environment. The pressure in the compartment compared with the surrounding environment would be abnormally low. The higher temperature at depth would slowly raise the pressure in the compartment to normal.

It may not be possible to predict the existing condition of the pressure system in examples like this because the combined effects of all the variables are often not well known in advance.

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