25th International Meeting on Organic Geochemistry IMOG 2011

P-124
Compositional features of organic matter in Jurassic and Lower
Cretaceous deposits located in the east of West Siberia
Natalya A. Krasnoyarova, Olga V. Serebrennikova
Institute of Petroleum Chemistry Siberian Branch of the Russian Academy of Sciences, Tomsk, Russian
Federation (corresponding author:natalex@ipc.tsc.ru)
Compositions of bitumens occurring in Lower
Cretaceous and Jurassic deposits and in crude oil
from a conjunction zone of ancient East-Siberian and
later West-Siberian consolidated crustal blocks of
Earth‘s crust have been analyzed.
The analyzed collection of 140 samples
included cores from Jurassic - Lower Cretaceous
section of four parametrical oil wells: Vostok 1, Vostok
3, Vezdekhodnaya 4 and Yuzhno-Pyzhinskaya 1, as
well as oil of Yuzhno-Pyzhinskaya 1 area.
The concentration of bitumens recovered from
Yuzhno-Pyzhinskaja-1 well is not high in siltstones
occurring in Lower Cretaceous deposit (0.05 %) and it
increases up to 0.20 % in mudstones occurring in
Middle Jurassic deposit. Considerable fluctuations of
bitumens content from 1.1 up to 0.08 % were
observed in Lower Jurassic section.
OM of Cretaceous siltstones is characterized
by the predominance of С23-С27 n-alkanes and the
presence of perylene, vanadyl (VO-P) and nickel (Ni-
P) porphyrin complexes. This organic material was
deposited under reducing conditions (ratio of pristane
(Pr) to phytane (Ph) equals 1.1). A high value of СPI –
an oddness coefficient (1.9) and estimated reflective
capacity of vitrinite (Rc) indicate low thermal
transformation of OM in this section area.
The СPI value decreases in OM of Upper
Jurassic deposit and estimated reflective capacity of
vitrinite is 0.6. OM is lower matured according to the
ratio of isoprenoid to normal alkanes: Pr/n-С17 and
Ph/n-С18, the value of isomers С31 to hopane
homologues S/(S+R) – a homohopane index (0.50)
also indicates low thermal transformation. The
maximum of n-alkanes distribution is shifted in a lowmolecular
area and the ratio of pristane to phytane is
1.9.
The concentration of perylene sharply
decreases and the contents of Ni-P and VO-P
increase in Upper Jurassic mudstones as compared
with Cretaceous ones. A specific set of alkylbenzenes
was registered in organic substance. Along with nalkylbenzenes
isomers of alkylbenzene С17 with С11
alkyl substituent branched on �-atom of the alkyl
chain (i-C11) occur in OM.
The higher Pr/Ph values (> 2) were observed
in OM nearly across the whole section of Lower
Jurassic deposit. The OM of Lower Jurassic deposit is
mature and capable to generate hydrocarbon fluids
according to the ratio of isoprenoid alkanes to normal
ones. The values of the homohopane index (0.54-
0.70) and Rc (0.75-0.80) testify the same.
Comparison of oil composition and that of
dispersed OM of even-aged subjacent rocks indicates
their genetic commonality. The oil and bitumen of
rocks from the depth of 2987.4 meters well correlate
with each other not only by alkanes structure, but also
by the content of separate groups of aromatic
hydrocarbons and heteroatomic compounds; by the
hopanes structure the oil is similar to bitumen
occurring in rocks from depth of 2990.1 meters.
The vanadyl complexes of porphyrins are
absent in bitumens across the whole investigated
section of Vostok 1 oil well. Thermal transformation of
OM is low: Rc – 0.51-0.62 and CPI-1 – 1.4-3.4. The
relative content of isoprenoid and normal alkanes and
homohopane index (0.40) also testify low maturity of
OM. The bitumen molecular composition indicates
dominating contribution of land and coastal water
plants to structure of the initial OM, shallowness of
sedimentation pool and frequent replacement of
oxidation-reduction conditions. The presence of
alkylbenzenes with a substituent branched on �-atom
of the alkyl chain was registered in organic substance
of Upper Jurassic deposits.
In the section of Vostok 3 oil well the OM of
Lower Cretaceous and Upper and Middle Jurassic
deposits is characterized by low maturity. The
sediments of Lower Jurassic deposit can be
considered as the source sediments by OM
composition data. They were formed in the marine
environment and phytoplankton mainly served as OM
source, Rc was equal to 0.72-0.73. The presence of
significant amount of alkylbenzols with substituent
branched on �-atom of the alkyl chain was observed
in Upper Jurassic deposits.
In the presence of VO-P and Ni-P in overlying
rocks, and for underlying ones – only Ni-P, the
predominance of dibenzothiophenes over
dibenzofurans was determined in one of the Lower
Jurassic interlayers of Vostok 3 and Vezdehodnaja-4
oil wells as opposed to over- and underlying
interlayers. It can serve as a marker to correlate the
sections of these oil wells.
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