Page 348 - Petroleum Geology

318

There are, of course, other possible causes of abnormally high pore pres-
sures in mudstones and in permeable strata associated with mudstones.

OTHER POSSIBLE CAUSES OF ABNORMAL PRESSURES

Reservoir geometry

We have already seen (p. 161) that a column of oil or gas of great vertical
extent can create high pressures near the top of the reservoir due to the weight
density’s of oil and gas being generally less than that of water. These are nor-
mal; but if the reservoir itself is abnormally pressured, reservoir geometry will
add to the pressures. We shall not discuss this further*.

Clay -m ineral d iagenesis

The diagenesis of smectite (montmorillonite) to illite involves the release
of inter-layer water. Powers (1967) suggested that this release of water could
aid primary migration, and the hypothesis that this process could lead to ab-
normal pressures grew from this suggestion. If the density of inter-layer water
is greater than that of free water, there is expansion on liberation that would
result in a net decrease of bulk density of the mudstone, and abnormal pres-
sures, unless the water can escape.
There is ample evidence that this diagenesis is real, but little that it contrib-
utes significantly to abnormal pressures. Powers regarded 1800 m (6000 ft)
as the ceiling of this diagenesis. Burst (1969) concluded that it did not operate
between depths of 800 and 2500 m in the U.S. Gulf Coast. Comparable depths
were found by Perry and Hower (1970, p. 171), and rather greater depths
were found by Weaver and Beck (1971, p. 18).
Magara (1978, pp. 100-109) has shown that the observed degree of under-
compaction cannot be accounted for by smectite diagenesis because it can-
not account for the bulk density decrease observed within the transition
zone.
This diagenesis cannot contribute, it seems, to those abnormal pressures
that lie above its ceiling, so it cannot be a general cause. If it can be establish-
ed that inter-layer water has significantly greater density than free water, it is
possible that release of this water to the pore spaces will contribute to abnor-
mal pressures, and primary migration, at depths below two or three kilometers.
As with thermal processes, the rate at which the diagenesis takes place is im-

* It is interesting, however, to read the discussion of Dickinson’s 1951 paper to the World
Petroleum Congress because some of those who spoke had Middle East experience of high
pressures that were due to reservoir geometry.