Page 347 - Petroleum Geology

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Perhaps the most interesting phenomenon in abnormally pressured mud-
stones is the occurrence of zones of lesser abnormality (Fig. 14-9; see Fowler,
1970, p. 413). Such zones are necessarily below the “isolation depth” postu-
lated by the thermal hypothesis, but the zone above the anomaly cannot be
a zone of constant water density. If the thermal process is important, it is
perhaps a zone of fractures. But it is logically unsatisfying to have different
causes for the two sides of such an anomaly. The mechanical hypothesis
would regard the anomaly as a drain with downward flow above it and up-
ward flow below it, and lateral migration of the expelled fluids within the
relatively permeable bed (see p. 61).
Finally, if the thermal process is dominant, there should be a tendency for
the thickness of the transition zone to increase with increasing depth to the
top of abnormal pressures. No world-wide study of this has been published,
but experience of several important areas with abnormal pressures lends no
support to such a relationship - rather the reverse. Transition zones may
tend to be thinner with increasing depth to the top of abnormal pressures.
Dickinson (1951, 1953, p. 420, fig. 6) plotted measured reservoir pressures
against depth for six Louisiana Gulf Coast wells, and these suggest thinner
transition zones with increasing depth to top of abnormal pressures. And in
the Midland field, mentioned above, the thickness of the transition zones
varies from block to block in one field. This is consistent with the mechanical
hypothesis, which attributes the thickness of the transition zone to its perm-
eability.
We therefore conclude, on present evidence, that at depths down to about
3 km at least, the mechanical process is dominant in most areas, and there-
fore that abnormal pressures are usually generated at shallow depth and, in
thick mudstones, retained for significant periods of time.
We have concentrated on these two hypotheses because of their importance
to petroleum geology. They are not just “academic”, because several aspects
of petroleum generation and migration depend on a proper understanding of
the processes of development of abnormal pressures. If the thermal process is
dominant in an area, the volume of fluids migrating from the mudstone is
limited approximately to the volume created by thermal expansion, and the
mudstone is, by the requirements of the hypothesis, an impermeable barrier
separating the sequence below from the sequence above. If, therefore, a pet-
roleum source rock is postulated in or below an abnormally pressured mud-
stone for an accumulation above it, consequential hypotheses have to be devel-
oped to account for primary migration. This seems to be the origin of the re-
newed interest in faults as conduits for petroleum migration, and their wide
acceptance in that role. This topic has been discussed at length on pp. 245-
251, but it is worth noting here that Dickinson’s observation of abnormal
pressures in sandstones isolated by faults (Dickinson, 1953, fig. 10, pp. 415
and 418 and p. 422 and fig. 11) indicates that these faults, at least, do not
act as conduits. Many areas of the world are affected, and the problem must
be resolved.