Page 116 - Geology and Geochemistry of Oil and Gas

88 CRUDE OILS

chromato-mass spectrometers, and infrared, ultraviolet, quasi-linear, and isotope
spectrometers.
A heightened interest in the molecular- and atomic-level information on the oil
composition was caused by two factors: technological and geochemical.
Petroleum hydrocarbons currently serve as a source of wide spectrum of synthetic
substances used for the manufacturing of various goods in food industry and other
industries. This required detailed studies of the composition of individual hydro-
carbons. The current technology provides an opportunity to obtain information
on the detailed composition and structure of hydrocarbons found in the high-boiling
oil fractions. Such information covers carbon atom distribution in the paraffin
chains and in the naphthene and aromatic rings. Lately, this information also be-
came insufficient.
The emergence of such analytical techniques as the gas–liquid chromatography
and chromato-mass spectrometry enabled scientists to
(1) obtain new information on the composition and structure of petroleum hydro-
carbons,
(2) study in detail their homological series, and
(3) determine the distribution patterns of normal and branched alkanes, methyl-
alkanes, and isoprenoid alkanes in oils.
In studying naphthenes, new techniques led to the elucidation of the proportions
of mono-, bi-, tri-, and tetracyclic naphthenes, steranes and tri-terpanes (hopanes).
Detailed studies of aromatic hydrocarbons in crude oils (using various techniques
including spectral) resulted in the establishment of the presence and proportions of
not only mono-, bi-, and tricyclic, but also polycyclic (4–6 cycles) hydrocarbons that
were almost impossible to identify earlier. The latter include hydrocarbons such as
perylene, 1,12-benzoperylene, 3,4-benzopyrene and their homologes.
Nuclear-magnetic and paramagnetic resonance techniques developed in the 1950s
enabled to study the properties of nuclei in different states. This is important in
studying the free radicals (kinetically independent), atoms and atom groups, and
chain reactions (polymerization, pyrolysis) in biochemical processes, in which the
free radicals actively participate.
A new approach in studying the crude oil hydrocarbons involves the stereo-
chemistry of saturated aliphatic and alicyclic hydrocarbons.
Stereochemical studies of the normal and branched alkanes and mono-, bi-, tri-,
and tetracyclic hydrocarbons (including hopanes) are becoming more important in
geochemical studies. It was shown that the transformations (aging) of biomolecules
in the Earth’s crust is closely related to the changes in their stereochemistry (Petrov,
1984).
There is an increase in the trace element studies. Contents of the trace elements in
crude oils vary significantly. Most of the iron series elements are found in crude oils
in amounts below the clarke amounts (sedimentary rock clarke). Some elements
(zinc, nickel, copper, arsenic, and silver) are found in near-clarke amounts, and four
elements (vanadium, molybdenum, bromine, and mercury) are present in the
amounts an order of magnitude above the clarke. This offers an opportunity of their
recovery directly from crude oils. The recovery of trace elements from crude oils is