Underpressure causes
| 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 |
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 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 when faulting or downwarping occurs.
The diagrams in Figure 1 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 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.
See also
- Normal hydrostatic pressure
- Geostatic and lithostatic pressure
- Normal hydrostatic pressure gradients
- Abnormal hydrostatic pressure
- Causes of overpressure
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