Page 115 - Geology and Geochemistry of Oil and Gas

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Chapter 5

CRUDE OILS

5.1. COMPOSITION OF CRUDE OILS 10

Crude oil is a natural multicomponent mixture. Its major part is composed of
hydrocarbons (alkanes, naphthenes, and aromatics). Their content in oils ranges
between 30% and 100%. Most important among the non-hydrocarbon components
are resins and asphaltenes. The other non-hydrocarbon compounds are metal–po-
rphyrin complexes and trace elements; their content is usually low. Some compounds
in oils lost structural features of the parent organic matter, whereas some other
molecules preserved these features. They are called ‘‘relic hydrocarbons’’ or ‘‘chem-
ical fossils’’.
Alkane hydrocarbons (C 5 –C 40 ) include normal and branched molecules (is-
oprenoids). Carbon number distribution in the normal alkanes reflects the compo-
sition of the original organic matter. For example, lipids of the continental biomass
are dominated by normal C 25 –C 33 alkanes that consequently are inherited by the oil.
The pristane/phytane ratio is used as a genetic criterion for the isoprenoids. The
pristane is associated with the continental deposits, whereas the phytane is associated
with the marine deposits.
Cyclic paraffins (naphthenes) include monocyclic (5–6 carbon atoms) as well as
polycyclic molecules. The latter molecules may contain 1–6 rings. This feature was
probably inherited from the parent organic matter (naphthene index). But most
polycyclic naphthenes (such as steranes) were not present in the parent organic
matter and have been formed during catagenesis (Petrov, 1984).
Arenes (aromatic hydrocarbons) are usually not as important as the other classes
of hydrocarbons in crude oils. Aromatic compounds may include exclusively aro-
matic rings, or may contain complex structures with naphthene rings. Some arenes
are directly related to the parent organic matter.
Polishchuk and Yashchenko (2003) showed that there are cyclic changes in the
chemical properties of oils (contents of paraffin, asphaltenes, resins, and sulfur) with
geologic age of rocks. This cyclicity is controlled by the cyclicity of ocean trans-
gressions and processes of formation and development of paleo-oceans in the ge-
ological history of the Earth.
Recently developed equipment and techniques drastically increased the informa-
tion on oil composition. The researchers are now able to determine not only the
group hydrocarbon composition, but also the composition of individual hydrocar-
bons and their structure. The new techniques include gas and liquid chromato-
graphy, spectral and isotope methods, and nuclear magnetic and paramagnetic
resonances. Among the new highly sensitive equipment are chromatographs,

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After T.A. Botneva (in: Eremenko and Chilingar, 1996).