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{{Merge|Normal hydrostatic pressure}}
{{publication
{{publication
| image = exploring-for-oil-and-gas-traps.png
| image = exploring-for-oil-and-gas-traps.png
| part = Critical elements of the petroleum system
| part = Critical elements of the petroleum system
| chapter = Formation fluid pressure and its application
| chapter = Formation fluid pressure and its application
| frompg = 5-1
| frompg = 5-10
| topg = 5-64
| topg = 5-10
| author = Edward A. Beaumont, Forrest Fiedler
| author = Edward A. Beaumont, Forrest Fiedler
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
| link = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
==Heat decrease==
==Heat decrease==
The major factor causing underpressure is the cooling of pore fluids as they are uplifted and the overburden erodes. For example, drain a bottle filled with hot water and immediately seal the bottle back up by screwing on the cap. The bottle will be underpressured as it cools to room temperature.
The major factor causing underpressure is the cooling of [[Petroleum reservoir fluid properties|pore fluids]] as they are uplifted and the [[overburden]] erodes. For example, drain a bottle filled with hot water and immediately seal the bottle back up by screwing on the cap. The bottle will be underpressured as it cools to room temperature.
This same phenomenon occurs when an encapsulated rock unit is uplifted into a region of lower temperature. However, predicting pressure in uplifted rock units is difficult. Because uplift brings a rock unit from a region of high pressure to a region of low pressure, the uplifted unit may be at a higher-than-expected pressure, a lower-than-expected pressure, or normal pressure, depending on the state of equilibration.
This same phenomenon occurs when an encapsulated rock unit is uplifted into a region of lower temperature. However, predicting pressure in uplifted rock units is difficult. Because uplift brings a rock unit from a region of high pressure to a region of low pressure, the uplifted unit may be at a higher-than-expected pressure, a lower-than-expected pressure, or normal pressure, depending on the state of equilibration.