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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 |
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| | 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 |
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| | isbn = 0-89181-602-X | | | isbn = 0-89181-602-X |
| }} | | }} |
− | Underpressure exists when a fluid pressure is lower than estimated from the normal hydrostatic fluid gradient for that depth at which it occurs. For this situation to exist, the fluid must be trapped within a rock unit. | + | Underpressure exists when a [[Normal hydrostatic pressure#Fluid pressure|fluid pressure]] is lower than estimated from the [[Normal hydrostatic pressure gradients|normal hydrostatic fluid gradient]] for that depth at which it occurs. For this situation to exist, the fluid must be trapped within a rock unit. |
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− | Underpressure can be caused by burial or heat decrease. | + | Underpressure can be caused by [[burial]] or [[heat]] decrease. |
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| ==Burial== | | ==Burial== |
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| if the encapsulated unit is buried deeper, its original pressure is carried to a higher pressure environment. If the rock cannot compact, the trapped pressure is abnormally low for the new depth. | | if the encapsulated unit is buried deeper, its original pressure is carried to a higher pressure environment. If the rock cannot compact, the trapped pressure is abnormally low for the new depth. |
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− | As long as a rock unit remains encapsulated by impermeable rocks, it becomes underpressured by burial as faulting or as downwarp occurs. | + | As long as a rock unit remains encapsulated by [[impermeable]] rocks, it becomes underpressured by burial when faulting or downwarping occurs. |
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| The diagrams in [[:file:formation-fluid-pressure-and-its-application_fig5-3.png|Figure 1]] illustrate this phenomenon. | | The diagrams in [[:file:formation-fluid-pressure-and-its-application_fig5-3.png|Figure 1]] illustrate this phenomenon. |
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| ==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. |
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| 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. |