ABNORMAL PRESSURES
Pore pressures which are found to lie above or below the “normal” pore pressure gradient line are called abnormal pore pressures (Figure 5 and 6). These formation pressures may be either Subnormal (i. e. less than 0.465 psi/ft) or Overpressured (i. e. greater than 0.465 psi/ft). The mechanisms which generate these abnormal pore pressures can be quite complex and vary from region to region. However, the most common mechanism for generating overpressures is called Undercompaction and can be best described by the undercompaction model.
|
|
|
I————— 1 Depth |
|
‘Abnormal’ Pressure Gradient > 0.465 psi/ft |
|
Overpressured (Abnormally Pressured) Formation |
|
Pressure, psi |
Figure 5 Overpressured Formation
|
|
|
Pressure, psi |
|
Figure 6 Underpressured (Subnormal pressured) formation |
The compaction process can be described by a simplified model (Figure7) consisting of a vessel containing a fluid (representing the pore fluid) and a spring (representing the rock matrix). The overburden stress can be simulated by a piston being forced down on the vessel. The overburden (S) is supported by the stress in the spring (a) and the fluid pressure (p). Thus:
|
|
S = a + p
If the overburden is increased (e. g. due to more sediments being laid down) the extra load must be borne by the matrix and the pore fluid. If the fluid is prevented from leaving the pore space (drainage path closed) the fluid pressure must increase above the hydrostatic value. Such a formation can be described as overpressured (i. e. part of the overburden stress is being supported by the fluid in the pore space and not the matrix). Since the water is effectively incompressible the overburden is almost totally supported by the pore fluid and the grain to grain contact stress is not increased. In a formation where the fluids are free to move (drainage path open), the increased load must be taken by the matrix, while the fluid pressure remains constant. Under such circumstances the pore pressure can be described as Normal, and is proportional to depth and fluid density.
|
|
|
PORE FLUID |
|
ROCK GRAINS |
|
DRAINAGE PATH CLOSED |
|
|
|
OVERRURDEN |
|
PORE FLUID |
|
ROCK GRAINS |
. Pore Fluid
x. Pressures . Increase
DRAINAGE PATH OPEN
|
|
|
OVERBURDEN |
|
ROCK GRAINS |
|
Figure 7 Overpressure Generation Mechanism |
|
Pore Fluid Pressure Gradient Remains Constant |
In order for abnormal pressures to exist the pressure in the pores of a rock must be sealed in place i. e. the pore are not interconnecting. The seal prevents equalisation of the pressures which occur within the geological sequence. The seal is formed by a permeability barrier resulting from physical or chemical action. A physical seal may be formed by gravity faulting during deposition or the deposition of a fine grained material. The chemical seal may be due to calcium carbonate being deposited, thus restricting permeability. Another example might be chemical diagenesis during compaction of organic material. Both physical and chemical action may occur simultaneously to form a seal (e. g. gypsum-evaporite action).