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been observed that holes drilled with too heavy a mud drill more slowly than
those drilled with a lighter mud. Part of the drilling break is evidently due to
the reduction and/or reversal of the potential gradient in the fluids across the
bottom of the hole while drilling.
(b) The significance of the rate of increase of pore pressure with depth is
that if it exceeds the overburden gradient, an explanation is required. Two
explanations have been offered: upward flow of pore water through the rela-
tively impermeable mudstone, and thermal expansion of the pore water in
almost totally impermeable mudstone during burial down a geothermal gra-
dient.We shall return to this question later.
(c) The increase in geothermal gradient, while not well documented in the
literature (but see Lewis and Rose, 1970; Fertl, 1976, p. 144), is a real effect
because it has been general practice in several areas (including the U.S. Gulf
Coast) to monitor the temperature of the mud returns. Anomalous increases
warn of abnormal pressures. The physical explanation has been offered that
the thermal conductivity of the abnormally-pressured mudstone is decreased
by the abnormality, so that the geothermal gradient is increased to maintain
constant heat flow.
(d) The log responses are all consistent with an increase in mudstone poro-
sity with depth, but there are other effects as well. Any increase in tempera-
ture, other things being equal, reduces the resistivity of the mudstone. Tem-
perature alone cannot account for the great relative decreases observed. There
is also some evidence (see Magara, 1978, p. 222) that abnormally pressured
pore water in mudstone is less saline than that in adjacent sands.
Perhaps of greater significance is the observation that formation density,
resistivity, and sonic transit time plots do not always indicate the same depth
of top of abnormal pressures (in the author’s experience, commonly in that
order of increasing depth). Drilling experience tends to support the sonic log
as being the most accurate from a practical point of view: it indicates accu-
rately the depth at which an appreciable abnormality has developed, but the
others may well indicate the real divergence from normal trends.
It seems quite certain that these anomalies indicate that the porosity of
mudstone in the transition zone increases with depth while the pore pressures
increase at a greater rate (usually) than the overburden pressures. We cannot
understand abnormal pressures without understanding this major anomaly.
The alternative hypotheses concerning the cause of the features of the
transition zone involve the permeability of the mudstone and the reversibility
of its compaction. The mechanical hypothesis rests on small but real perme-
ability to water or petroleum, and the irreversibility of compaction: the ther-
mal hypothesis rests on near-zero permeability and reversibility of compac-
tion due to expansion of the pore water during burial down the geothermal
gradient. These differing requirements lead to totally different conclusions
concerning the timing of generation of abnormal pressures and the quantities
of pore fluids expelled, so it is essential for our understanding of the petro-