25th International Meeting on Organic Geochemistry IMOG 2011

P-312
Molecular stable carbon isotopic composition of light
hydrocarbons as a method to decipher hydrocarbon generation,
accumulation and secondary alteration in superimposed
petroliferous basin: a case study from Tarim basin, NW China
Yongge Sun 1 , Qilin Xiao 2 , Aizhu Jiang 2 , Chupeng Yang 2
1 Department of Earth Science, Zhejiang University, Hangzhou 310027, China, 2 State key laboratory of
Organic geochemistry, Guangzhou Institute of geochemistry, CAS, Guangzhou 510640, China
(corresponding author:ygsun@zju.edu.cn)
Light hydrocarbons (LHs, C5-C10), as one of the most
abundant constituents in crude oil, have received
special consideration in petroleum geochemistry for
long history. Theoretically, LHs from kerogen are
directly degraded from macromolecular network and
will migrate out of the source rock via generation and
expulsion once hydrocarbon concentration reaches
up to the saturated threshold of rock, whereas
gasoline hydrocarbons from oil-cracking are formed
via C-C bond cleavage of high molecular weight
compounds, cyclization and aromatization, and
involved an isotopic kinetic fractionation, although
they will be destroyed under very high temperature.
Different pathways of LHs generation led us to
construct new parameters to characterize processes
involved in oil and gas generation, accumulation and
destruction, using molecular and isotopic
measurements.
Compared to the measurements on normal alkanes
and biomarkers in crude oils, individual LHs have
analytical constraints because many compounds in
this range have high vapor pressures and low boiling
points, and tend to be the gas phase at room
temperature. Although care can be taken to minimize
the evaporation losses during oil sample collection,
transportation, storage, and preparation, the success
application of molecular and individual δ 13 C values of
LHs requires a full evaluation induced by evaporation
under natural conditions, especially on the magnitude
and direction determination during the progressively
vaporized losses. Here, a vaporization experiment on
crude oil was conduced before applications. The
results showed that most of molecular parameters
associated with C7 hydrocarbons are influenced by
evaporation process except interpretive schemes
such as P2 versus N2/P3 and the invariant ratio
developed by Mango. It was also found that stable
carbon isotopic fractionation effects of LHs due to
vaporization were strongly compound classdependant,
and within the analytical error, δ 13 C
values of cycloalkanes remained similar to that of
fresh sample, and can be used with confidence for
petroleum geochemistry.
Taking the theoretical consideration and laboratoryverified
parameters, molecular stable carbon isotopic
Composition of LHs was used to decipher
hydrocarbon generation, accumulation and secondary
alteration in Tarim basin, NW China, a typical
superimposed petroliferous basin with multiple source
kitchens, multiple phase filling histories, and severely
secondary in-reservoir alterations. Crude oils from
Paleozoic petroleum system in Central Tarim Basin
were studied. Combined with the δ 13 C values of nalkanes
and biomarker assemblages, it was found
that, (1) oil reservoirs from the Central Uplift of
Tazhong experienced multiple periods of oil
generation, migration, accumulation and in-reservoir
cracking as indicated by maturity range of crude oil;
(2) oil and gas from reservoirs in the Slope-break Belt
of No.1 and the Structural Belt of Tazhong 10 were
mainly derived from deep buried in-reservoir oil
cracking as revealed by distributions of
monoaromatics and δ 13 C compositions of
methylcyclohexane (Fig.1).
Fig.1 The distribution of monoaromatics V.S. δ 13 C of
methylcyclohexane in crude oils from The Central
Uplift of Tazhong (▲), the Structural Belt of Tazhong
10 (□) and the Slope-break belt of No.1 (×) in the
central Tarim basin.
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