25th International Meeting on Organic Geochemistry IMOG 2011

P-115
The carbon isotopic composition of Upper Jurassic oil shales
Russian plate organic matter and its formation conditions
Dmitry Bushnev, Nadezhda Burdelnaya, Irina Smoleva
Institute of Geology of Komi SC of RAS, Syktyvkar, Russian Federation (corresponding
author:boushnev@geo.komisc.ru)
The rock samples with a wide range of Сorg content
were selected to study the carbon isotopic
composition of Middle Volgian kerogen of Russian
plate. The collection represents rocks from outcrops
Vazh-yu, Koigorodok, Ib, Sinegorye, Ayyuva, Kashpir
and Gorodishche. Totally 31 kerogen samples,
isolated from oil shales and clays, were studied. The
� 13 C values of kerogen were measured using mass
spectrometer Delta V Advantage (Thermo), linked to
element analyzer Flash EA. The � 13 C values of
kerogen vary from -22.2 to -28.3‰. The organic
matter of the studied deposition was characterized by
our previous works (Bushnev and Burdelnaya, 2008),
the model of chemical composition for ―average‖
kerogen was suggested (Burdelnaya and Bushnev,
2010).
The analysis of the data we received testifies the
kerogen carbon enrichment by 13 C isotope with the
growth of the Сorg content in rocks. The same trend
was observed for Kimmeridge Clay Formation (KCF)
where � 13 C depends by Corg content of decarbonated
rock. Previously published data (Van Kaam-Peters et
al., 1998), related to isotope composition of organic
carbon of KCF rocks, were interpreted as the
evidence of preservation of isotopically heavy
carbohydrate of organic matter due to their early
diagenetic sulfurisation.
The part of dots on the plot of � 13 C vs. Сorg ―jumps‖
toward relatively lighter kerogen carbon. These dots
correspond to the samples from outcrops of Ayyuva
and Vazh-yu. We believe that such heterogeneity of
the sample can testify to the presence of certain
source of isotopically light OM affecting the total
isotope composition of kerogen of the given samples.
The other part of the collection is related to outcrops
Kashpir, Gorodishche, Koygorodok, Sinegorye, Ib,
where accurate correlation between Сorg content
decarbonated residue of the rocks and � 13 Ckerogen was
observed. The comparison of the published data (Van
Kaam-Peters et al., 1998) on � 13 CTOC for KCF and our
data testifies to their coincidence both in the sense of
variation trend, and absolute values (Figure). It is
possible to consider that this coincidence is not
casual, and testifies to the generality of processes of
organic matter accumulation on the vast territory of
Upper Jurassic sedimentation basins.
� 13 C of kerogen
-20
-21
-22
-23
-24
-25
-26
-27
R² = 0,55
our data
KCF by Van Kaam-Peters et al., 1998
-28
0 20 40 60 80
Corg of decarbonated residue of rock, %
Figure. � 13 C of kerogen vs. Corg content of
decarbonated rock. Our data is related to Kashpir,
Gorodishche, Koygorodok, Sinegorye and Ib samples.
The isotope composition of kerogen carbon and
such index as 2,5-DMT/(2-ET+2,3-DMT+2,4-DMT)
ratio, assessed by the composition of pyrolysis
products of kerogen, are perfectly correlated for our
collection of Middle Volga oil shales, as well as for
KCF, that additionally testifies to the wide
development of early diagenetic sulfurisation of
carbohydrates.
References
Bushnev D.A., Burdel‘naya N.S. // Geochemistry
<strong>International</strong>, 2008. Vol. 46, No. 10, pp. 971 – 984.
Burdel‘naya N.S and Bushev D.A. // Geochemistry
<strong>International</strong>, 2010. Vol. 48. No. 5. Pp. 492 – 504.
Van Kaam-Peters H.M.E., Schouten S., Köster J.,
Sinninghe Damsté J.S. // Geochim. Cosmochim. Acta,
1998. Vol. 62. Pp. 3259–3284.
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