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Fig. 14-6. Postulated development of the z-ze relationship during burial, assuming irre-
versibility of mudstone compaction.

stones become virtually impermeable to water. Further burial increases the
temperature of the pore water and the water's tendency to expand, resisted
by the overburden, results in elevated pressures. These have a theoretical
capacity to exceed the overburden pressure, but when that pressure is reach-
ed, fractures develop along which excess water flows, dissipating some of the
excess pressure but still leaving it close to the overburden pressure.
Figure 14-7 shows the relationship between pressure, temperature, and
specific volume of pure water. On this has been plotted the normal hydrostatic
line for a geothermal gradient of 36"C/km; and the isopycnic lines have
been terminated at the approximate overburden line. If, once the isolation
depth and temperature are reached, the pore volume remains constant and
no more pore water escapes, then further burial will lead to pressure/temp-
erature and pressure/depth gradients parallel to the isopycnic lines. These are
similar to some observed gradients.
The thermal hypothesis requires relatively late generation of abnormal
pressures, after the isolation depth has been reached, and limits the volumes
of pore water expelled to those generated by thermal expansion.

Discussion

The question is not whether the thermal or the mechanical hypothesis is
correct, but which is the dominant process, and what are the conditions that
favour one or the other in particular circumstances. The critical parameter is
the permeability of the mudstones because this affects the thermal hypothesis
much more than the mechanical. So it could well be that the importance of