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be very sensitive, but there have been hints that abnormally high pore pres-
sures may inhibit maturation, and that high effective stress may facilitate
maturation by deforming the kerogen and so accelerating its transformation.
There is much work in progress on quantitative modelling of these interac-
tions so that we may be able to estimate the quantities of hydrocarbons gen-
erated under any conditions (Lopatin, 1971, 1980; Connan, 1974, Tissot
and Welte, 1978, p. 500ff; Waples, 1980).
For the reasons stated on p. 69, we shall not go into details of the chem-
ical transformations of kerogens and other organic matter buried with sedi-
ment: for these, the reader is referred to Tissot and Welte (1978) and Hunt
(1979). We merely note that empirical data obtained by analysis of extracts
from supposed source rocks indicates that low-temperature cracking of large
molecules into smaller molecules takes place, and similar results have been
obtained by heating laboratory samples in the presence of clay minerals. In
addition, the common clay mineral smectite (montmorillonite) can acquire
organic compounds by cation exchange, and these, when heated without
oxygen, yield hydrocarbons similar to petroleum (Weiss, 1963).

Coal and petroleum

It is natural that one should enquire whether the two great fossil fuels,
coal and petroleum, have any significant geological relationship; and this en-
quiry has been going on for more than a century. Coal results from the dia-
genesis of vegetable organic matter that accumulated in an environment largely
devoid of sediment. Conditions on the actual surface of accumulation may
have been reducing or oxidizing; but close below this surface, reducing con-
ditions prevailed. Coal consists largely of carbonized plant tissues, wood and
bark, with spores (particularly the more durable spore coatings), leaf cuticles,
waxes and resins. Coals form a series, with peat at one end and graphite at
the other, and they are ranked according to their degree of alteration from
lignites to anthracites. The type of coal groups coals of similar composition.
Cannel coal, for example, is a coal rich in volatiles that burns easily and com-
monly contains significant proportions of spore coatings. Heating of a “bitu-
minous” coal results in distillation of a gas that consists largely of hydrogen
and methane, with numerous other components in small proportions (some
being hydrocarbons). The coal-tar residue contains hydrocarbon oils (benzene,
toluene, etc.) with other components.
Not surprisingly, early work on the association between coal and petroleum
was carried out in Pennsylvania, USA. - a coal mining area in which com-
mercial oil production began in the United States. The early work culminated
in White’s “Carbon Ratio Theory”, in which he related the occurrences of oil
and gas to the percentage of “fixed carbon” (see Glossary) in associated coals.
He ranked petroleum from low-ranking heavy oils to high-ranking light oils,
and noted that oil occurred where the fixed carbon is less than 65%, mostly