25th International Meeting on Organic Geochemistry IMOG 2011

P-322
Сarbonaceous rocks of the Lower and Middle Cambrian Inican
Formation in the southeastern Siberian platform
Igor Korovnikov, Tatyana Parfenova, Victorya Eder
Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russian Federation
(corresponding author:parfenovatm@ipgg.nsc.ru)
In the north and east of the Siberian Platform, the
organic-rich Cambrian Kuonamka complex
(Kuonamka, Sinskaya, Shumnaya and Sekten
Fomations), a potential oil-generating sequence, has
been established [3 and oth.]. The goal of
investigation is to specify the age and composition of
carbonaceous rocks and the conditions of their
formation in the Lena-Amga interfluve (southeast of
the Siberian Platform). A collection of samples from
well Khotochu-7 from the interval of 388-309 m has
been examined. It is known the Lena-Amga interfluve
borders on the Sinskaya Formation in the west and on
the Inikan Formation in the east. Paleontologic
observations show that carbonaceous rocks of well
Khotochu-7 contain trilobite imprints, characterizing
the older zones as compared to the reported for the
Sinskaya Formation. The rocks of the upper part of
the section, containing the fauna of the Tomagnostus
fissus zone and generally characterizing Inikan
carbonaceous deposits, are composed of light
limestones depleted in OM. The results of lithologic
and geochemical examination of rocks and OM
allowed separation of sample collection into two
groups [5]. The first group includes predominantly
low-carbonaceous and carbonaceous limestones
silicites of the lower part of the section (354-388 m)
with TOC from 0.2 to 7.5 % (in average 3.48 %). The
carbonate material mainly consists of calcite. Clays
generally account for less than 5 %. The βα/(αα+ββ)
ratio of cholestane, methylcholestane and
ethylcholestane is 0.09-0.19. The average content of
С23 tricyclane is higher than 30 % of total tricyclanes.
The concentration of С29 norhopane is higher than
that of С30 hopane. The average ratio
(steranes+pregnanes)/terpanes is 0.1 (!). Only in 4
samples from 13, homohopane content decreases in
the series С31>C32>C33>C34>C35, and С35/С34 is 0.8-
09. Usually, С35/С34 is higher than 1. The second
group includes low-carbonaceous rocks, generally of
mixed siliceous-dolomitic composition, less
commonly, silicites and limestones of the upper part
of the section (309-346 м). The average content of
TOC in them is 1.6 %, dolomite– 35-40 %, and clays
– 5-15 %. The βα/(αα+ββ) ratio of cholestane,
methylcholestane and ethylcholestane is 0.22-0.38.
The average content of С23 tricyclane is lower than
30 %. The С29/С30 ratio is less than 1. The average
ratio of (steranes+pregnanes)/terpanes is 0.4. The
С35/С34 ratio is less than 1.
Saturated HCs of two groups show common features:
generally, maximum concentrations of n-С16-n-С19
alkanes; absence of 12- and 13-monomethylalkanes;
high (more than 30 %) concentrations of tricyclanes;
tricyclane index 2*∑(С19-20)/∑(С23-26) is less than 1,
gammacerane content accounts for less than 1 %;
ethylcholestane is predominant (more than 40-50 %).
These features are characteristic of OM from the
Kuonamka Formation [1; 2; 4 and oth.].
Conclusion. Paleonthological, lithologic and
geochemical studies have shown that the section of
OM-enriched rocks from Lower and Middle Cambrian
deposits of the Lena-Amga interfluve is transitional
between the known sections of the Sinskaya and
Inikan Formations and has two-member structure.
Carbonaceous sediments accumulated in the marine
basin with normal water salinity. The rocks of the first
group formed under sharply reducing conditions and
the rocks of the second group – in normal reducing
ones. OM is likely to be derived mainly from algae
and bacteria. The difference in terpane and sterane
ratios indicates that the share of bacterial matter is
four times as much as in carbonaceous silicites and
carbonate rocks as compared to that of their overlying
low-carbonaceous rocks. It is likely that either the
contribution of prokaryotes into the composition of
microbiocenoses was higher, or the primary OM in
sediments of the lower part of the section was
essentially transformed by bacteria.
References
[1] Kashirtsev V.A. Organic geochemistry of
naphthides in the east of the Siberian Platform. Yakutsk:
Yakutsk Affiliate of SB RAS Publishing House, 2003, 159 p.
[2] Kontorovich A.E., et al. // DAN. 2005. V. 402. № 5.
[3] Kontorovich A.E., Savitsky V.Е. On
paleogeography of the Siberian Platform in the Early and
Middle Cambrian // Problems of lithology and
paleogeography of Siberia. SNIIGGiMS. Novosibirsk. 1970.
[4] Parfenova T.M., et al. // Rus. Geol. and Geophys.
2004. V. 45. № 7.
[5] Parfenova T.M., et al. // Geologiya nefti i gaza.
2009. № 1. P. 87-91.
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