Overpressure 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

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

Figure 1 Encapsulated fluid in an uplifted unit.

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.

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 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.

See also

• Normal hydrostatic pressure
• Geostatic and lithostatic pressure
• Normal hydrostatic pressure gradients
• Abnormal hydrostatic pressure
• Causes of underpressure

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