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.”
- Uplift
- Burial
- Rock compaction or dilation
- Abnormal heat flow
Overpressure
Underpressure
Uplift
Burial
Heat increase
Heat decrease
Compaction 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 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 .
- Causes of Overpressure :
When a fluid pressure is higher than estimated from the normal hydrostatic fluid gradient for a given depth , it is called overpressure .For this situation to occur , the fluid must first be trapped within a rock unit (pressure compartment ) ;
Overpressure can be caused by uplift , increased heat , compaction , generation of hydrocarbons , or a combination of these factors .
Uplift :
A unit can be uplifted into a regime of lower normal pressure .The encapsulated fluid then is at a pressure higher than that found at the new depth in surrounding formations where the fluid is under normal constraints .
The diagrams below illustrate this situation .
Heat increase:
Perhaps the most common way that pressure is increased is for the encapsulated fluid to be heated .The trapped fluid , unable to expand into adjacent pore systems , rises in pressure. Fluids outside the area of trapping are free to adjust to the heating , so they remain at about normal pressure .
Compaction :
As an encapsulated rock mass is buried , it tends to compact .Under normal conditions , as the porosity is reduced , the interstitial fluid is expelled. When the fluid cannot escape , the pressure within the encapsulated rock mass risks .The higher fluid pressure takes on some of the overburden load , limiting the amount of compaction .In such cases , the fluid is overpressured and the rock matrix is undercompacted .
2. Causes of Underpressure :
Under pressure 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 can be caused by burial or heat decrease .
Burial :
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 .As long as a rock unit remains encapsulated by impermeable rocks , it becomes underpressured by burial as faulting or as downwarp occurs .
The diagram below illustrate this phenomenon .
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 .This same phenomenon occurs when an encapsulated rock unit is uplifted into a region of lower temperature .However , predicting pressure in uplifted rock unit 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 .