Page 100 - Petroleum Geology

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portant factor. One should view it rather as the difference between the
supply of organic matter and the ability of the environment to destroy it.
Large amounts of organic matter may lead to reducing conditions in an en-
vironment that would otherwise be oxidizing.
There is no obvious correlation between the composition of crude oils of
different ages and the record of life on Earth, except possibly the occurrence
of waxy oils in rocks of Mesozoic age and younger, after the establishment
of vegetation on the continents. There is as much variation between crude oils
in rocks of the same age as between those in rocks of different ages. From
this it is inferred that the precursors of petroleum are the fundamental bio-
logical molecules or compounds, such as protein, fats, waxes, humus and
the like.
The doubts and uncertainties that surround the processes of petroleum
generation (and migration), despite the great advances in petroleum geo-
chemistry, arise from one main factor: many scientific disciplines are clearly
required for the elucidation of the processes, but we have only a limited
capacity to master the knowledge we need. Judgements therefore tend to be
based on inadequate knowledge of geology, biology, chemistry and physics.
Much of the evidence is apparently contradictory. Research into Holocene
sediments of the Orinoco delta at Pedernales indicated that petroleum of a
sort is being generated at depths shallower than 60 m (200 ft), some of it
accumulating in a sand at a depth of about 35 m (120 ft) that is probably
less than 10,000 yrs old from 14C dating (Kidwell and Hunt, 1958). But
there seems to be a consensus amongst petroleum geochemists that the
ceiling of major oil generation is at depths around 1500 m (5000 ft). Not all
oil fields can be reconciled with such a deep ceiling, and Hunt (1979, p. 355,
table 8-2) gave the richest depth interval for the 236 largest fields known in
1956 as only little deeper than 1000 m.
Research into the petroleum content of mudstones in the Los Angeles
basin, California, tends to support a conclusion of late generation, because
petroleum hydrocarbons were not apparently generated at shallow depth,
and the composition of the hydrocarbons in the fine-grained rocks does not
approach that of the accumulated petroleum until depths of about 4000 m
(12,000-13,000 ft; Philippi, 1965).
We therefore return to the problems of geological inference. Does the ab-
sence of petroleum at shallow depths in an area indicate that it was only gen-
erated deeper? . . . or it is that there are no source rocks at shallow depth?
Should a petroleum source rock contain petroleum of the same composition
as that expelled? . . . or do some components migrate more readily than
others? . . . or does petroleum in the reservoir alter during burial? Does the
presence of hydrocarbons at shallow depth in the Orinoco delta mean that it
will become a significant accumulation, given the right conditions, in a few
million years time? . . . or is this part of the immense quantities of petroleum
that are considered to be lost through lack of a trap?