Page 91 - Petroleum Geology

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another part of the reservoir rock). We are therefore forced to conclude that
oil migrates into a reservoir as a “stream” in which physical continuity of the
oil is maintained. Such streams have not been identified with confidence.
There is much conflicting evidence in this fundamental topic of petroleum
geology, and there is no general agreement amongst geologists on the nature
of oil migration.
Many geological and petroleum geological inferences are based on associa-
tions. The association of lithologies in a sequence, and the association of the
contained fossils, is the basis for correlating rock units and for the interpre-
tation of the environment in which the sediment accumulated. Consistent
association of a particular fauna with a lithology leads to the conclusion that
the fauna is “facies-bound”. The association of petroleum with anticlines
rather than synclines indicates accumulation under the influence of gravity,
with an upward component of movement of the less dense petroleum
through the denser formation* water.
Associations are notoriously difficult to use in geological (and other) in-
ference because a causal relationship does not necessarily exist. Associations
such as those between the consumption of alcohol and teachers’ salaries, or
between Denmark’s birthrate and her stork population, are amusing. Perhaps
we would find some geological associations amusing if we knew all the facts.
Consider the following common associations with depth:
- Temperature increases.
- Fluid pressure increases.
- Water salinity tends to increase.
- Water density, as measured at the surface, tends to increase.
- Oil density tends to decrease in successive reservoirs in one field.
- Mudstone bulk density tends to increase.
These six associations with depth, seven items in all, lead to 21 consequent
associations that can be shown in a correlation half-matrix (Fig. 4-1). Some
of these are clearly not causal relationships. For example, the association of
increasing water density with increasing temperature is demonstrably false
(indirect) because water expands when heated more than the pressure com-
presses it, and its density decreases (Fig. 4-2). Is the increase in water salinity
with depth due to increasing temperature? . . . or pressure?. . . or both? Or
is it perhaps related to the increase in mudstone bulk density, that is, with
the expulsion of pore water on compaction? Is the commonly observed
decrease of oil density in successive reservoirs with depth to be attributed to
temperature, or the salinity of the water, or to none of these?
The increase in temperature is caused by none of the listed factors, nor is
the increase in mudstone density. It could be that the only causal relation-

* “Formation” does not have a strict connotation in petroleum geology outside its purely
geological context.