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SHALE RESISTIVITY - R,h SHALE TRAVEL TIME -A t,h
ohm- metres ELSlft
1 2 3 4 5 80 100 120 140160180
I 1 1 0 , I.,.,
L

1-

-- 5

2-

rn ft
x103
Fig. 14-3. Plot of logarithm of shale resistivity and logarithm of shale transit time in mud-
stones against depth in a well in Borneo. (Data courtesy of the Royal Dutch/Shell Group.)

made on their pore fluids because their permeability is too small to allow
equilibration in the borehole. Such mudstones came to be called “undercom-
pacted” because they showed the properties that were characteristic of mud-
stones at much shallower depths.
Harkins and Baugher (1969) found that abnormal pressures were associated
with those parts of the sequence in which the sand/shale ratio was less than
5-1 0%.
In some areas, pressures were found to return to normal hydrostatic at
greater depths, or a zone of smaller abnormality was found. Examples of the
former are to be found in eastern Venezuela (Funkhouser et al., 1948, p.
1891) and north Sumatra (Mulhadiono and Marinoadi, 1977) and some other
examples are given by Frederick (1967). A particularly interesting example
of reduction in abnormality followed at greater depth by increasing abnor-
mality is described by Fowler (1970) in a study of pressures measured in
reservoirs, their petroleum accumulations, and the water salinities in a field
in Texas, USA.
During the late 1960s, the mechanical hypothesis (which had been generally
accepted) began to be questioned. The dehydration of smectite to illite, which
Powers (1967) had suggested could be important for primary migration, was
thought to be a possible cause of abnormal pressures. This diagenesis was ob-
served at depths below about 1800 m (5900 ft) (Burst, 1969; Perry and