Page 248 - Applied Petroleum Geomechanics

242 Applied Petroleum Geomechanics

Figure 7.6 Schematic porosity (A) and corresponding pore pressure (B) in a sedi-
mentary basin. The dashed porosity profile represents normally compacted formation.
In the undercompaction section, porosity (f) is larger than that in the normal
compaction trend (f n ), and the porosity reversal occurs, corresponding to
overpressure.

overburden and ability to expel fluids is maintained (Mouchet and Mitchell,
1989). The normal compaction generates hydrostatic pore pressure in the
formation, as shown in the shallow section of Fig. 7.6. When the sediments
subside rapidly and the formation has extremely low permeability, fluids
can only be partially expelled, and the remained fluid in the pores must
support all or part of the weight of overburden sediments. Consequently,
the pores are less compacted, which results in a higher porosity than the
normally compacted formation. This generates abnormally high pore
pressure, causing porosity to decrease less rapidly than it should be with
depth, and formations are undercompacted, i.e., in the state of under-
compaction or compaction disequilibrium. It mainly occurs in mudstones
(shales) because of their low permeability. The compaction disequilibrium
is often recognized by higher-than-expected porosities at a given depth and
the porosities deviated from the normal porosity trend (e.g., the deep
section of Fig. 7.6.).
Fig. 7.6 illustrates how to identify undercompaction and overpressure
from porosity profile. In a normally compacted formation, porosity should
decrease gradually as depth increases. When this porosityedepth relation is
reversed, the undercompaction occurs and overpressure generates. The
starting point of the porosity reversal is the top of undercompaction or top