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pressures in a sequence of reservoirs in Trinidad. The effective compaction
depth of all of them is less than 500 m, and the top of abnormal pressures is
a little shallower than 500 m (1640 ft).
The evidence of growth faults also supports such shallow depths. Dickinson
(1953) found that the age of the mudstones at the top of abnormal pressures
in the Louisiana Gulf Coast became younger towards the south, towards the
Gulf, in the direction of the regression. Thorsen (1963) made an interesting
study of growth faults in western Louisiana and found that the age of the
beds showing maximum thickness contrast - that is, the age of maximum
rate of growth fault movement - became younger towards the south, in the
direction of the regression. Comparison of Thorsen’s map (1963, p. 107, fig.
4) with Dickinson’s (1953, pp. 416-417, fig. 3) shows that regionally the
periods of maximum growth-fault movement occurred within a couple of
biostratigraphic subzones above the top of abnormal pressures. Dickey et al.
(1968) found that some growth faults in south-western Louisiana appeared
to control the tops of abnormal pressures, and Fowler et al. (1971) found in
the Midland field, Louisiana, that the top of abnormal pressures was strati-
graphically higher and at shallower depth in the downthrown block of a
growth fault than in the upthrown block, and the maximum rate of growth
fault movement took place within the abnormally pressured sequence.
Thorsen (1963) observed that sand percentage is most closely related to
contemporaneous structural growth near “the basinward limit of sand depo-
sition, that is, in those areas of ten per cent or less sand”. Harkins and Baugher
(1969) observed that the top of abnormally pressured mudstone normally
occurs regionally where there is less than five to ten per cent sand. It is diffi-
cult to avoid the conclusion that the two are causally related.

z
Fig. 14-9. Some transition zones contain zones of lesser abnormality.