Page 126 - Geology and Geochemistry of Oil and Gas

98 CRUDE OILS

pattern was found in the compositional changes with increasing depth and temper-
ature. In some cases, the indications of catagenic changes (low density, high gasoline
content, high content of paraffins, and low degree of paraffin cyclicity) were obvious,
whereas in others they were barely noticeable or non-existent.
Infrared spectroscopy studies enables determination of the ratio of long to short
paraffin chains, the degree of their branching, and the number of CH 2 and CH 3
groups. In some crude oils with catagenetic alterations, the fraction of long chains in
the paraffin-naphthene crudes decreases, and of short ones, increases. This is an
indication of destruction of long-chain paraffins. In another group of crude oils,
exposed to the similar temperature and pressure, the number of long chains did not
decrease. Possibly, the resistance to destruction of paraffins in the latter case was
greater. In general, the catagenetically altered crude oils are rare.
Hypergenetic alteration affects the crude oils composition in the accumulation
more profoundly. The geochemical types of hypergenetically altered crude oils in-
clude (1) oxidized, (2) biodegraded, and (3) degassed. Hypergenetic processes include
(1) oxidization (on contact with oxygen); (2) bacterial reduction of sulfates; and (3)
loss of the light hydrocarbon fractions. The hypergenetic alterations are more pro-
nounced when several processes are operative (for instance, degassing plus chemical
and bacterial alteration).
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Hypergenetically altered oils are usually heavier (lower API gravity), higher in
resins and asphaltenes contents, and lower in the gasoline boiling point fraction. A
very informative criterion is found on infrared spectrograms of crude oils: the pres-
ence and intensity of the 1710 cm 1 absorption band that reflects the content of
oxygen-containing carbonyl groups. Modeling of hypergenetic processes due to ox-
idation, sulfate reduction, etc. showed that the crude oils that had not been subjected
to hypergenesis do not have the 1710 cm 1 absorption band in their infrared spec-
trograms.
On the other hand, the crude oils that underwent hypergenetic alterations have
1710 cm 1 absorption band ranging from 0.01 to 0.5 and even higher. The oils with
1710 cm 1 absorption band within a few hundredths do not display any significant
changes in their properties and composition. Such oils are usually found near the
oil–water contact. The crude oils with 1710 cm 1 absorption band of a few tenths do
display significant changes in their properties and composition. As a rule, those are
heavy oils with a very low content of low boiling point fractions.
The distribution of differently hypergenetically altered crude oils is presented in
Table 5.6. As shown, 86% of the very oxidized crude oils are found at depths
shallower than 400 m. Most of them in Russia occur in the Cretaceous and Jurassic
rocks of the North Caspian and North Caucasus-Mangyshlak petroleum provinces.
Alterations during migration result in changes in the composition of crude
oils. Depending on the type of migration, these alterations may be quite significant.
Some experiments (Mileshina et al., 1983) showed that the composition of crude
oil flowing through rocks changes (depending on the lithology, temperature, and
pressure).
Upon migration of crude oil through sands and sandstones, the number of par-
affin structures increases, whereas the role of naphthene cycles declines. The degree