Page 101 - Petroleum Geology

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One conclusion is clear: we cannot reasonably postulate that petroleum
generation is not going on today, for that would make this period of time
unique in the last 600 m.y. or so of world history. Until the present-day
source rocks can be identified with certainty and studied, the processes will
necessarily remain rather speculative. Nevertheless, geological arguments can
narrow the areas of doubt and unify to some extent the conflicting evidence
(as we shall see in Chapters 9-11). Above all, it must be remembered that
the problems of petroleum in the rocks are fundamentally geological prob-
lems. No chemical or physical explanation can be satisfactory unless the
geological explanation is also satisfactory. If a geologist feels in need of en-
couragement in this point of view, he should read some of the geological pro-
nouncements of the great 19th-Century physicist, Sir William Thomson
(Lord Kelvin)!

Migration of petroleum

Migration of petroleum into accumulations is thought of largely in terms
of permeability paths. It must also be thought of in terms of fluid potential
gradients or energy gradients - that is, deviations from hydrostatic equili-
brium in the fluids in the pore spaces of the rocks. Permeability, as we have
seen, is but one component necessary for flow; it is a measure of the trans-
missibility of fluids through a rock. An energy gradient in the fluid is neces-
sary for it to flow, and migration paths are paths of continuously decreasing
energy, such that the energy of each particle of the fluid migrating decreases
continuously during its migration.
Migration paths may be long or short. Hypotheses vary from simple mi-
gration from the source rock into the adjacent carrier bed, and through the
carrier bed to the trap, on the one hand, to multiple-stage migration from
a distant source, some with intermediate trapping from which the petroleum
is spilled by subsequent geological events, on the other. Much of the uncer-
tainty is because of the uncertainty about the true source rocks and their
positions.
Migration is divided into two stages: primary migration from the source
rock to the permeable carrier bed; secondary migration from there to the
trap, through one or more carrier beds (Illing, 1933, p. 232).
Opinions differ widely on the state of petroleum during migration, whether
in solution, in colloidal solution, or as a separate phase in water. Petroleum
occurs in solution in formation waters; it occurs as emulsions in the produc-
tion processes; and it occurs as a separate phase in the trap. Each possibility
has its merits and its problems.
Migration in aqueous solution has the merit of requiring least work for its
transport through the rocks. The problems relate to the quantitative suffi-
ciency of this process in view of the relatively low solubility of petroleum in
water, and the need for some physical or chemical change to be imposed on