25th International Meeting on Organic Geochemistry IMOG 2011

P-118
The cracking kinetics of two oil samples from Sichuan Basin,
China
Liangliang Wu, Ansong Geng, Yuhong Liao, Yunxin Fang
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China (corresponding
author:asgeng@gzb.ac.cn)
It has been widely accepted that Oil cracking gas is
one of the main sources of natural gas in the Sichuan
Basin, China. Actually, thermal decomposition of oil
lead to the generation of gaseous hydrocarbons and
pyrobitumen at high maturity. The thermal
decomposition can be kinetically described by
chemical kinetic equation (Ungerer et al.,1987; Wang
et al. 2004). Sealed gold tube pyrolysis is an effective
research means for kinetic study of oil cracking in
closed system (Hill et al.,2004). Two oil samples with
different maturity from the Sichuan basin, China were
selected to perform the thermal simulation experiment
and the cracking kinetic parameters were calculated.
W-oil sample was slight biodegraded, while C-oil is a
transparent condensate oil with little heavy fraction.
The total yields of hydrocarbon gases generated
from the C-oil and the W-oil are similar (Fig. 1). For Coil
at the rate of 2℃/h, when the yield of C2-5 reached
its maximum value (250ml/g.oil) at 470℃, the yield of
C1 is 290ml/g.oil, whereas the maximum yield of C1
is 730mg/g.oil at 600℃.The results indicated that C2-
5 gradually increases with the rising thermal stress at
the early stage of oil-cracking, liquid hydrocarbons
crack accompanying the generation of C2-5 gases. At
the late stage of oil-cracking, decomposition of C2-5
becomes dominated process which may significantly
contribute to the increase of methane and
pyrobitumen. C1 is the stable end gaseous product at
high maturation stage.
Fig. 1 The yields of the two oil samples for C1, C2,
C3 and C2-5 respectively.
Figure 2 illustrates the activation energy
distribution and frequency factor for kinetic models
derived from the selected oil samples. A single
frequency factor (A) of 1.0E+14s -1 was assumed for
all different parallel reactions to make the data
comparable. The average value of activation energy
of C1 is 65.40 Kcal/mol for C-oil and 65.84 Kcal/mol
for W-oil. The average values of activation energy of
C2-5 for C-oil and for W-oil are 59.37 Kcal/mol and
59.56 Kcal/mol, respectively. Obviously, the average
activation energy of C1 is higher than that of C2-5.
The distribution of activation energies of W-oil is wider
than that of C-oil, which could be attributed to the
difference in maturity between the two oils. Oil at
lower thermal stress usually has more heavy
constituents than that at higher thermal stress. It is
obvious that the maturity of W-oil is lower than that of
C-oil which is mainly composed of saturated
hydrocarbon with smaller molecular weight and of
lower density. The total H/C atomic ratio of the oils
rich in heavy constituents usually is lower than that of
condensate oil. Hydrogen content should be factor
controlling the ultimate volume of C1 generated.
Condensate oil is rich in saturated hydrocarbon and
total H/C ratio of bulk oil should be higher. Therefore,
C-oil has narrower distribution of activation energies
and higher yield in C1 than W-oil.
Fig. 2 The activation energy distribution(Ea)and the
frequency factor (A) calculated for the two oil samples.
References
Ungerer, P., Pelet, R., 1987. Nature 327,52-54
Wang, Y. P., Geng, A. S., Liu, D. Y., Xiong, Y. Q.,
Shen, J. G., 2004. Acta Sedmentologica Sinica 22
Suppl., 106-110
Hill, R. H., Tang, Y. C., Kaplan, I. R., 2003. Organic
Geochemistry 34, 1651-1672
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