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PRESSURE (p) ze

5 10 103ft

w

1- \\ \ ‘
-\
x lo3 k=ie 0.5 0.6 0.7 0.8 0.9 1.0 110 6.9 d.8 6.7 0.6 0.5 k,
Fig. 14-5. The equilibrium compaction depth, ze (= 6z), decreases in the transition zone
below normal hydrostatic pore-fluid pressures.

leum geology of regressive sequences that we shall discover the true causes of
abnormal pressures and their relative importance.
The mechanical hypothesis attributes abnormal pressures to compaction
of mudstones that do not have sufficient permeability to permit pore water
to escape as fast as the other mechanical influences would require for normal
compaction and normal hydrostatic pore-fluid pressures. The increase in pres-
sure with depth in the transition zone means that the equilibrium compac-
tion depth decreases throughout the transition zone (Fig. 14-5). Compaction
of mudstone is regarded as irreversible, so the history of the equilibrium
compaction curve cannot have followed the form in Fig. 14-5, but must have
developed along different paths from a much shallower depth, each path
tending to increase the effective compaction depth with increasing depth (as
shown schematically in Fig. 14-6) because of leakage. Undercompaction there-
fore requires an early origin, the greater the undercompaction, the earlier the
origin.
The transition zone pressure gradient is caused by the overlying permeable
sand or sandstone with normal hydrostatic pressures. The potential gradient
that this creates leads to upward water flow through the mudstone in the
transition zone. The small permeability of the mudstone to water causes large
energy losses during flow, with a consequent steep gradient of pressure loss.
This pressure gradient, following Terzaghi’s principle (p. 57), leads to a
compaction gradient, and so a permeability gradient. The slope of the transi-
tion zone pressure gradient is therefore also due to the permeability of the
transition zone as a whole: the smaller the permeability, the larger the pres-
sure gradient (larger Ap/Az). Eventually, all the expellable water will be ex-
pelled and the mudstones will become normally compacted, with pore fluids
at normal hydrostatic pressures.
The thermal hypothesis postulates an “isolation depth” at which mud-