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in others. It is most unlikely that the continguous environment in which
mud accumulates close to sand is also a favourable environment for the pre-
servation of organic matter because it is probably relatively well oxygenated.
The lateral oscillation of these areas leads to the alternating sequence, with
true source rock near the middle of the mudstones (Fig. 10-3). To cross this
barren mudstone, injection pressures of sufficient magnitude must be gen-
erated. During this crossing, the petroleum may come into contact with dif-
ferent clay minerals in the different facies of the mudstone.
If Meissner (1978) is correct in attributing the abnormal pressures in the
Bakken Shale of the Antilope field, North Dakota, to oil generation, the con-
version of kerogen resulting in larger volumes of oil, it would be evidence of
oil generation as a separate phase as well as a process that would contribute
to the attainment of a long continuous phase, and provide energy for primary
migration.
In the Gulf of Paria, illitic clays are associated with the delta platform of
the Orinoco river and waters of lower salinity; montmorillonite (smectite) is
associated with more saline environments (Van Andel and Postma, 1954, p.
78). Gibbs (1977) found that the montmorillonite proportion increased
away from the Amazon. Likewise, Porrenga (1966) found that the offshore
Niger delta has roughly parallel zones of increasing montmorillonite content,
with smaller proportions in the sandier facies, to more than 50% offshore.
Kaolinite decreased in the seaward direction. Both Porrenga and Gibbs con-
cluded that this was due to physical segregation (i.e., different baselevels), not
to chemical changes. If these relationships are typical, a regressive sequence

Fig. 10-3. Petroleum source rock is a facies: by Walther’s Law, the vertical distribution of
facies reflects the horizontal. (Idealized crosssection.)