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for all the known oil and gas accumulations of the area. In a long but inter-
esting argument, he asserted that pore-water movement may generally be up-
ward relative to the stratigraphic levels, but in a subsiding basin it will be
downwards in an absolute sense. Accepting the temperature of onset of oil
generation as 90°C and that of peak generation as 120"C, he found that a
28°C drop of temperature since the early Pliocene could have exsolved the
known quantities of oil and gas in the area. Bonham pointed out that this
was not the only possible process. The difficulty with this hypothesis from
our point of view is that the depths of onset of oil generation and primary
migration in Bonham's area are believed to be below 3000 m, while there is
much oil in the US. Gulf Coast above 2000 m.
Treating deltaic sequences as a class apart, as Jones (1981) did, does not
really help very much. But it must be noted that the regressive sequences
with important zones of abnormally pressured mudstones (shales) are com-
mon and important in petroleum geology, and the apparent general lack of
maturity in the mudstones associated with the reservoirs in such areas will be
taken up in the next chapter.
The chemical evidence of accumulations in the context of solubility is
confusing to the non-specialist : some authors have claimed that the chemistry
of accumulations supports the solution process (Baker, 1959, 1960, 1962),
others that it does not.
Price (1976) found that the solubility of hydrocarbons in water increased
markedly with increase of temperature above lOO"C, and postulated very
deep sources with migration in faults upwards in aqueous solution, the petro-
leum exsolving at shallower, cooler levels.
We conclude that solution is a process of primary migration, but that in
those areas where a shallow source is indicated by geological or other argu-
ment it is not the main process quantitatively (although its indirect importance
in other processes may be great).
Colloidal or micellar solution are similar processes that were proposed by
Baker (1959) and Meinschein (1959), and revived by Cordell (1973), because
of the possibility of solubilizing petroleum in pore water at low temperatures.
The difficulties are much the same as for molecular solutions, with two more.
Colloids and micelles are larger than the individual molecules because they
are disordered and ordered groups of molecules, respectively. There will there-
fore be an even greater tendency to restrict movement mechanically. The
larger micelles solubilize hydrocarbons better than the smaller, but by less
than an order of magnitude. The soap concentrations required to form micelles
is about two orders of magnitude greater than that found in formation waters.
The size problem is not amenable to numerical assessment as yet because
we do not know the sizes of pore throats in mudstones, and the pore-size
estimate of about 5 nm (see Cordell, 1973, pp. 1619, 1623-1625) may be
misleading in pores that have a smaller vertical dimension than the lateral.
The conclusion reached by Tissot and Welte (1978, p. 276) and Hunt