|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|
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).
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 in Figure 1 illustrate this situation.
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.
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 rises. This 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.
- Normal hydrostatic pressure
- Geostatic and lithostatic pressure
- Normal hydrostatic pressure gradients
- Abnormal hydrostatic pressure
- Causes of underpressure