Page 203 - Petroleum Geology

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perature, with lower energy, over a longer time. Chemical reactions in the
laboratory are subject to cosmic radiation: those in the presumed source
rock are subject to gamma radiation from some clay minerals, and we cannot
say for certain that this difference is unimportant.
It is impossible at present to distinguish in logic between the generation of
petroleum and its primary migration. We believe we can recognize petroleum
source rocks from the nature of their organic contents: we can identify petro-
leum accumulations. No migration path has ever been recognized physically
with confidence and reported, so the connection between source and accu-
mulation is inferred from analyses of the oil and analyses of the organic con-
tent of the supposed source rock, and from geological considerations. It is
for this reason that we cannot claim to understand the origin and migration
of petroleum. At best, we can construct plausible hypotheses.
There is another, less creditable, cause of difficulty:. the widespread mis-
understanding of the physics of fluid movement through porous rocks. Per-
haps the commonest error is the assertion that water moves from high pres-
sure to low pressure. Geologists holding this view cannot defend their posi-
tion because there are artesian basins in which the water is demonstrably
flowing from low pressures near the intake area to higher pressures in the
aquifer at depth. This misconception has bred others, particularly the widely
held view that petroleum migration is always upwards (stratigraphically or
absolutely). Downward migration over parts of the migration path is not a
new concept, nor one that depends on mathematical arguments: King (1899,
p. 80 and fig. 9, and p. 99 fig. 14) clearly understood the movement of ground
water without the use of mathematical formulation. Others have postulated
downward movement in ground-water and petroleum contexts, notably Ver-
sluys (1919), Hedberg (1926) and Hubbert (1940).
The growth of petroleum geochemistry over the last decade or so has been
spectacular, and the conclusions reached have been widely accepted. It is not
easy for geologists to assess geochemical hypotheses because of their general
lack of familiarity with chemical arguments. By the same token, it is not
easy for chemists to follow geological arguments. There are books, of course,
and all can read; but understanding also requires doing, because it is in prac-
tising our science that we acquire a feel for it.
For these reasons, we shall take the topic of origin and migration of petro-
leum in three parts. First, the hydrodynamic aspects will be considered with
the geological because this is the physical context of petroleum migration,
and they put some constraints on the geochemical hypotheses. We shall then
consider the geochemical aspects, finishing with a discussion of the whole
topic. The only matter that we shall accept uncritically is that petroleum has
its origin in organic matter that accumulated with fine-grained sediment in a
low-energy environment deficient in oxygen.