25th International Meeting on Organic Geochemistry IMOG 2011

P-269
Distribution of alkylbenzenes in oils from deposits of different
geological ages
Galina Pevneva, Anatoly Golovko
Institute of Petroleum Chemistry, Siberian Branch of the Russian Academy of Sciences, Tomsk, Russian
Federation (corresponding author:pevneva@ipc.tsc.ru)
The investigation of composition and distribution
of aromatic hydrocarbons in oils and organic matter of
rocks along with saturated hydrocarbons enables one
to obtain information about the source and
accumulation conditions of the initial organic matter
and its transformation in geological period.
We investigated the composition of alkyl
derivatives monocyclic aromatic hydrocarbons in
order to reveal regularities in their compositions and
distribution in Phanerozoic oils of different oil-and-gas
bearing basins. We have studied compositions of
alkylbenzenes in oils from Cainozoic (Sakhalin,
Pannonsky basin (Serbia), China, Vietnam, Saxony),
Cretaceous (West Siberia, Mongolia), Jurassic (West
Siberia, Baltic Republics) and Paleozoic (West
Siberia, Timano-Pechersky and Volgo-Ural oil-andgas
bearing basin) deposits occurring at the depths
from 200 to 4500 m.
Using chromatography-mass-spectrometry we
have identified the following homologous series of
alkylbenzenes containing a chain of normal structure:
n-alkylbenzenes (n-AB) (m/z 91); 1-methyl,2alkylbenzenes
(1,2-MAB); 1-methyl,3-alkylben-zenes
(1,3-MAB); 1-methyl,4-alkylbenzenes (1,4-MAB) (m/z
105); ethyl- or dimethylalkylbenzenes (m/z 119); as
well as 1-alkyl-2,3,6–thimethylbenzenes containing an
isoprenoid chain of irregular structure (m/z 133). They
were identified by characteristic ions.
Homologous series of alkylbenzenes are
composed by the compounds containing from 10 to
30 carbon atoms in a molecule. In major oils their
molecular-mass distribution has a unimodal character
with a maximum falling on low-molecular
alkylbenzenes of С14-С17 compositions. It should be
also noted that n-AB21 predominates over the content
of adjacent members in the homologous series and it
is the most pronounced in oils from Jurassic and
Paleozoic deposits. According to the opinion of
authors in such an abnormal content of n-AB21 is
probably connected with its relict character. Cortisalin
is considered as a probable precursor of n-AB21. It is
a natural polyene pigment having a bacterial origin,
though possible formation of n-AB21 from natural fat
acids and alcohols should not be excluded.
A homologous series of 1-alkyl-2,3,6–trimethyl
benzenes (i-AB) with an isoprenoid chain of irregular
structure was identified in Paleozoic oils in Timano-
Pecherskaya oil-and-gas bearing province, Ulyanovsk
Region, in Middle Jurassic oils in Nizhne-
Tabaganskoye oil field and in Paleozoic oils in
Gerasimovskoye and Severo-Kalinovskoye oil fields
of West Siberian oil-and-gas bearing basin, as well as
in some oils from Cainozoic deposits located on
Sakhalin island. Homologous of these compounds
contain from 13 to 23 carbon atoms in a molecule.
According to literature data i-AB are isorenieratene
derivatives. This carotinoid is included into the
composition of lipids of green sulfur bacteria which
inhabit waters contaminated by hydrogen sulfide. The
presence of such AB in the oils can testify to the
conditions of hydrogen sulfur contamination at the
stage of sedimentogenesis. Paleozoic oils occurring
in Usinskoye, Dzhyerskoye and Yaregskoye oil fields
of Timano-Pecherskaya oil-and-gas bearing province
contain mainly i-AB.
Among alkylbenzenes in the oils occurring in
deposits of all ages 1,2-MAB predominate over n-AB,
1,3-MAB, 1,4-MAB and ethylAB. The ratio of total
content of n-AB homologous to total content of MAB
varies from 0.3 for Cainozoic to 0.5 for Paleozoic oils.
In oils of Middle, Lower and Paleozoic deposits the
concentration of 1,4-MAB increases as compared with
the content of 1,3-MAB. Accumulation of 1,4-MAB,
which are the most thermodynamically stable isomers
among methyl-substituted alkyl benzenes, depends
on the increased transformation from Cainozoic to
Paleozoic oils.
Thus based on the results of the study on composition
and distribution of alkylbenzenes in the oils occurring
in deposits of different ages it was shown that
alkylbenzenes along with alkanes are carriers of
genetic information; differences in the distribution of
alkylbenzenes depend on the effect of catagenetic
factors which promote secondary isomerization
reactions.
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