These are formations whose pore pressure is greater than that corresponding to the normal gradient of 0.465 psi/ft. As shown in Figure 11 these pressures can be plotted between the hydrostatic gradient and the overburden gradient (1 psi/ft). The following examples of overpressures have been reported:
From the above list it can be seen that overpressures occur worldwide. Some results from European fields are given in Figure 11. There are numerous mechanisms which cause such pressures to develop. Some, such as potentiometric surface and formation foreshortening have already been mentioned under subnormal pressures since both effects can occur as a result of these mechanisms. The other major mechanisms are summarised below:
(a) Incomplete Sediment Compaction
Incomplete sediment compaction or undercompaction is the most common mechanism causing overpressures. In the rapid burial of low permeability clays or shales there is little time for fluids to escape. Under normal conditions the initial high porosity (+/- 50%) is decreased as the water is expelled through permeable sand structures or by slow percolation through the clay/shale itself. If however the burial is rapid and the sand is enclosed by impermeable barriers (Figure 12) , there is no time for this process to take place, and the trapped fluid will help to support the overburden.
Faults may redistribute sediments, and place permeable zones opposite impermeable zones, thus creating barriers to fluid movement. This may prevent water being expelled from a shale, which will cause high porosity and pressure within that shale under compaction.
(c) Phase Changes during Compaction
Minerals may change phase under increasing pressure, e.g. gypsum converts to anhydrite plus free water. It has been estimated that a phase change in gypsum will result in the release of water. The volume of water released is approximately 40% of the volume of the gypsum. If the water cannot escape then overpressures will be generated. Conversely, when anhydrite is hydrated at depth it will yield gypsum and result in a 40% increase in rock volume. The transformation of montmorillonite to illite also releases large amounts of water.
(d) Massive Rock Salt Deposition
Deposition of salt can occur over wide areas. Since salt is impermeable to fluids the underlying formations become overpressured. Abnormal pressures are frequently found in zones directly below a salt layer.
(e) Salt Diaperism
This is the upwards movement of a low density salt dome due to buoyancy which disturbs the normal layering of sediments and produces pressure anomalies. The salt may also act as an impermeable seal to lateral dewatering of clays.
(f) Tectonic Compression
The lateral compression of sediments may result either in uplifting weathered sediments or fracturing/faulting of stronger sediments. Thus formations normally compacted at depth can be raised to a higher level. If the original pressure is maintained the uplifted formation is now overpressured.
(g) Repressuring from Deeper Levels
This is caused by the migration of fluid from a high to a low presssure zone at shallower depth. This may be due to faulting or from a poor casing/cement job.The unexpectedly high pressure could cause a kick, since no lithology change would be apparent. High pressures can occur in shallow sands if they are charged by gas from lower formations.
(h) Generation of Hydrocarbons
Shales which are deposited with a large content of organic material will produce gas as the organic material degrades under compaction. If it is not allowed to escape the gas will cause overpressures to develop. The organic by-products will also form salts which will be precipitated in the pore space, thus helping to reduce porosity and create a seal.
Institute of Petroleum Engineering, Heriot-Watt University