25th International Meeting on Organic Geochemistry IMOG 2011

P-072
High molecular alkanes С40+ in West Siberian oils
Marina Mozhayskaya 1 , Galina Pevneva 1 , Julia Golovko 2 , Anatoly Golovko 1
1 Institute of Petroleum Chemistry SB RAS, Tomsk, Russian Federation, 2 University of Houston, Houston,
United States of America (corresponding author:mozhayskaya@ipc.tsc.ru)
Organic geochemistry is concerned with the
study of biomarkers containing carbon atoms in a
molecule ranging from С1 to С40. They are widely
spread in geosphere and are analyzed by gas-liquid
chromatography and chromatography-mass
spectrometry. The development of instrumental
methods for the analyses enables us to study
composition and distribution of high molecular
alkanes C40+ in oils.
The oils occurring in Cretaceous, Jurassic and
Paleozoic deposits of West Siberian oil-and-gas
bearing province were the objects of the investigation.
High-molecular alkanes were extracted using a
modified Burger method.
In the oils from Cretaceous deposits a content of
waxes varies from 2.8 to 11.3 % mass and that of nalkanes
in the same factions varies from 10 to 40 %.
In major oils a homologous series of n-alkanes in wax
fractions is composed by the compounds with a
number of carbon atoms up to С44-50 and maximum
falls on n-alkanes С23-С26.
In the oils from Jurassic deposits the content of
waxes increases up to 2.9-16.8 % as compared with
Cretaceous deposits. The oil from Gorstovoye oil field
contains an abnormally high amount of waxes – 47.4
%. A content of n-alkanes can reach 40 % mass in all
wax fractions occurring in Jurassic oils as well as in
analogues fractions of Cretaceous oils. Compounds
with a number of carbon atoms in a molecule up to
С65 compose a homologous series of n-alkanes in a
wax fraction, whereas in the waxes of oils from
Cretaceous deposits it does not exceed 51 carbon
atoms in a molecule.
MMD of n-alkanes in major oils from Jurassic
deposits is unimodal. The concentration of С23-С26
compounds is maximal. There are samples among
Jurassic deposits in which maximum of n-alkanes
distribution is shifted to a high-molecular region (С39-
С41).
As opposed to Cretaceous and Jurassic oils a
higher content of waxes was determined in Paleozoic
oils (14.6-26.5 % mass). n-Alkanes are the main
components of this fraction as in the waxes of oils
occurring in the deposits of other geological ages.
Compounds containing up to 58 carbon atoms in a
molecule compose a homologous series of n-alkanes
in waxes of major Paleozoic oils.
Besides n-alkanes the oils occurring in the
deposits of all geological ages contain homologous
series of 2- and 3-methylsubstituted alkanes and
alkylcyclopentanes. Their content is smaller as
compared with that of n-alkanes and varies ranging
from trace amounts to 0.82 % mass for methylalkanes
and up to 0.34 % mass for alkylcyclopentanes.
Along with other biomarkers the data on
distribution of n-alkanes, methyl-substituted and
alkylcyclopentanes occurring in wax fractions can be
used as additional information about the conditions of
the initial OM sedimentation. The authors of
publications [1, 2,] proposed to use a ratio of even
and odd (parameter CPI) of n-alkanes, methylalkanes
(CPI 2,3 ) and alkylcyclopentanes (CPI a ) to estimate a
type of the initial OM and sedimentation conditions.
CPI 2,3 and CPI a values for the major oils under study
vary from 1.2 to 1.3, that corresponds to a marine
environment of sedimentation. The oil from
Verkhnesalatskoye oil field is an exception to the rule:
its CPI a is equal to 0.7 and it probably indicates
evolution of oil in salt lacustrine environment [2].
Thus, the analysis of wax fraction demonstrated
that a homologous series of n-alkanes and
concentration of waxes in oil increases with the
increases in the age of the enclosing sediments. The
study of these hydrocarbons enables us to obtain new
information about compositional features and
distribution of high molecular n-alkanes, which can be
used to determine sedimentation conditions.
References
1. Philp R.P., Mansuy L. // Energy & Fuels – 1997 -
№4 - p. 749-760.
2. Hong Z., Guanghui H., Cuishan Z., Peirong Z.,
Yongxin Y. // Org. Geochem. – 2003 - v. 34, № 7, p.
1037-1046
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