Page 56 - Reservoir Geomechanics

40 Reservoir geomechanics

Mechanisms of overpressure generation

Avariety of mechanisms have been proposed to explain the occurrence of overpres-
sure in sedimentary basins. We briefly discuss these mechanisms below. The reader is
directed to the papers in Law, Ulmishek et al.(1998) and Mitchell and Grauls (1998)
and to the review of mechanisms of overpressure generation presented by Swarbrick
and Osborne (1998). It should be noted that there are several natural mechanisms lead-
ing to underpressure (lower than normal pressure). Because such situations are quite
rare in the context of the cases being considered here, underpressure is not discussed
(see Ingebritsen, Sanford et al. 2006).
Disequilibrium compaction (which is often called undercompaction)is perhaps the
most easily understood physical mechanism leading to overpressure (and the most
important, according to Swarbrick and Osborne 1998). At a given depth, ongoing sedi-
mentation increases the overburden stress which, in turn, will tend to cause compaction
and porosity loss. In a hydraulically open system, that is, in sufficiently permeable for-
mations to be hydrologically connected to earth’s surface, the compaction and porosity
loss associated with burial can be accommodated by fluid flow without excess pressure
build up. This is apparently the case with the formations at depths less than 8000 ft
in Figure 2.2.However, in a low permeability formation (such as a shale), in confined
sands isolated from other sands (such as with the formations deeper than 9000 ft in
Figure 2.2), or in regions of such rapid sedimentation and compaction fluid expulsion
cannot keep pace with the porosity loss. In this case, the increasing overburden stress
driving compaction will cause increases in pore pressure as the overburden stress is
carried by the pore fluid pressure. This state, in which externally applied stresses are
supported by pore fluid pressure, is related to the concept of effective stress (Terzaghi
1923), which is discussed at length in Chapter 3.
In the Gulf of Mexico, it is fairly obvious that sedimentation from the Mississippi
River has caused large accumulations (many km) of sediment over the past several
million years. This sedimentation has caused compaction-induced pore pressures,
which can reach extremely high values at great depth where extremely thick sequences
of impermeable shales prevent drainage (Gordon and Flemings 1998). It should be
noted that under conditions of severe undercompaction, porosity is quite high and the
lithostatic gradient can be significantly below 23 MPa/km (1 psi/ft). The transition from
hydrostatic pressure to overpressure is highly variable from place to place (depending
on local conditions) but is often at depths of between 5000 ft and 10,000 ft. That said,
there are some areas in the Gulf of Mexico where overpressure is found at very shallow
depth and is responsible for shallow-water flow zones. As will be discussed throughout
this book, compaction-induced pore pressures have a marked effect on in situ stress,
reservoir processes, physical properties, etc. Of course, the sedimentation driving com-
paction at depth need not be a steady-state process. For example, in various parts of
the North Sea, there have been appreciable accumulations of sediments in some places