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tary facies was the most important control on petroleum occurrence, and
that structure was important because it controlled facies. He concluded that
the optimum sand/shale ratio in La Brea-Parifias is about 0.6: at this ratio
the reservoirs appear to be just large enough to pool all the oil generated. At
smaller ratios, he suggested, the reservoirs are not large enough to pool the
oil generated.
The interpretation of the role of the sand/shale ratio is more easily under-
stood in the context of the discussion of Walther’s law in Chapter 1 (p. 8)
and the assertion made on p. 181 that the facies of a mudstone contiguous
with a sandstone is unlikely to be a true source rock. Thus the true source
rock, laterally more distant from the reservoir facies, can only appear in the
sequence above or below a potential reservoir when the proportion of “shale”
is large enough to include it (Fig. 11-4).
Hunt (1953, p. 1862) found a correlation between sand/shale ratios and
the gravity of crudes in Wyoming, U.S.A., the lighter crude oils being asso-
ciated with smaller sand/shale ratios, and concluded from this and other con-
siderations that the major differences between Wyoming crudes are due to
source material and the environment of deposition.
Recently, Saikia and Dutta (1980) reported that high-wax crude oils in
Tertiary sandstone reservoirs in fields of the Assam basin at depths between
two and three kilometres have consistently high specific gravities. There are
wide variations in wax content both vertically and laterally, and the variations
from field to field could be related to depositional environment. The wax
content varies from 8 to 16% by weight, and the API gravities from 22” to
38” (s.g., 0.92-0.83). The sandstone bodies are not persistent, and they con-
cluded that there had been little or no migration, and that the effect of tem-
perature on the quality of the crudes has been “insignificant”.
All these examples serve to illustrate the geological arguments that have
been applied to areas around the world concerning the source and migration
paths of crude oil found in traps. However, there are some important petro-

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Fig. 11-3. Sand/shale ratiosappear to influence the content of reservoirs in La Brea-Parifias,
Peru. (After Youngquist, 1958, p. 711, fig. 10.)