25th International Meeting on Organic Geochemistry IMOG 2011

P-097
Geochemistry of low-boiling С5-С8 hydrocarbons from Middle
Ob’ oils (West Siberia)
Elena Fursenko
Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russian Federation
(corresponding author:fursenkoea@ipgg.nsc.ru)
The object of this study is C5-8 hydrocarbons (HC) in
oils from the Mesozoic deposits (K1 - 47 samples, J3 -
20 samples, J2 - 12 samples) in the Middle Ob‘ region
of West Siberia. Additionally, it was used another data
on physical and chemical composition (density; sulfur
and paraffins contents; structural and group
composition; V and Ni content; δ 13 С of saturated and
aromatic fractions), geochemical parameters by the
composition of n-alkanes C10 +, acyclic isoprenoids,
high polycyclic hydrocarbon biomarkers (steranes,
terpanes).
The comparative analysis of the C5-8 HCs distribution
in the studied oils points to the aquagene genesis of
the organic matter they were, mainly, formed of /3/. By
a bigger half, the group composition of C5-8 HCs in
these oils is represented by alkanes, light arenes
concentrations are low, the values of cyclopentanes/
cyclohexanes ratios are high (> 0,7), as well as those
of ethylbenzene/ xylenes (> 0,3), and n-heptane/
methylcyclohexane (> 0,9); the values of m-xylene/ oxylene
ratios are low (< 2,0). Genetic parameters in
the context of high-molecular saturated hydrocarbons
composition (low values of n-C27/ n-C17 - < 0,4,
pristane/ phitane - < 1,5, C29 steranes/ C27 steranes
2,0), and light isotopic composition
(δ 13 C < -30,0 ‰) altogether attest to the genotype of
the studied oils being aquagene /1, 3, 5, 6/.
These oils are most likely to have derived from
organic matter (OM) of the Bazhenovo Fm /2, 4/.The
differences observed in the studied oils, can be
explained by differences in biogeochemical facies of
the Bazhenov Sea (the oxidation-reducing conditions
of diagenesis, hydrodynamics and salt regime of the
sea basin, and possibly, the biota). On the other
hand, the vertical migration of oil components from
source-rock stratum J1-2 with mixed composition of
OM (aquagenous - terragenous, terragenous-
aquagenous) can not be excluded /2, 4/.
Several samples from the examined samples from
J2 deposits differ significantly both in terms of C5-8
hydrocarbons composition (cyclopentanes/
cyclohexanes < 0,4); ethylbenzene/ xylenes < 0,1;
m-xylene/ o-xylene > 2,0) and by the high-molecular
hydrocarbon biomarkers composition (pristane/
phytane > 3,0; C29 steranes/ C27 steranes > 1,0,
elevated concentrations of tricyclanes C19-20, etc.).
Such characteristics of hydrocarbon composition are
indicative of their genesis being predominantly
terragenous /1, 3, 5, 6/. The source of these oils could
have been the OM accumulated in shallow-water
marine and lacustrine facies in the Lower-Middle
Jurassic time /2, 4/.
In general, the studied oils are mature. Similar
maturity of hydrocarbon fluids caused the insignificant
scattering of katagenesis coefficients in the
composition of light HСs. Furthermore, the ratio of
even и odd n-alkanes (CPI) in most samples is ≈1. The
data on the steranes‘ and terpanes composition also
prove the studied oils to have been formed in the ―oilwindow‖
zone /1, 3, 5, 6/.
Some single samples from the studied oils showed
low value of some ratios (С5-8 alkanes/ С5-8 cyclanes,
n-heptane/ methylcyclohexane, n-alkanes/ isoalkanes),
and high coefficient Kiso = (Pr+Ph)/ (n-C17+n-C18).
N-alkanes С10+ are not identifiable in these oils. They
are confined to the Lower Cretaceous shallow-depth
deposits with low formation temperature (≤ 70-75 0 С).
The distributions in the steranes and terpanes
compositions, likewise in oils diagnosed as aquagenous.
Therefore, these oils can be considered biodegraded to
a moderate degree /3/.
References
[1] Hunt, J.M., (1982). Petroleum geochemistry and
geology. (in Russian) Moscow: Mir, p. 703
[2] Kontorovich, A.E., et al. (1975). Geology of oil and
gas of West Siberia: Moscow, Nauka, 679p
[3] Peters, K.E. The biomarker guide/ 2 nd ed./
K.E. Peters, C.C. Walters, J.M. Moldovan/ USA, New
York: Cambridge University Press, 2007. – P. 1155
[4] Petroleum-bearing basins and areas in Siberia.
Edition 2. West Siberian basin/ Edited by
А.E. Kontorovich. Novosibirsk: UIGGM SB RAS,
1994. p. 200
[5] Petrov A.A. (1984) Hydrocarbons of crude oil.
Moscow: Nauka, p.243
[6] Tissot, B. P. and Welte, D.H., (1981). Petroleum
Formation and Occurrence. (in Russian) Moskow: Mir,
p. 502
240