25th International Meeting on Organic Geochemistry IMOG 2011

P-318
Kinetic study of primary cracking gases from a marine kerogen
by stepwise open system and its implications
Yunpeng Wang 1 , Hongjun Wang 2 , Changyi Zhao 2 , Jinzhong Liu 1 , Jialan Lu 1
1 SKLOG, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China,
2 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China (corresponding
author:wangyp@gig.ac.cn)
Marine hydrocarbon gases can be generated from
kerogen itselt as well as its product oil or liquid
hydrcargons. The former is primany cracking gases
generated from the cracking of kerogen, and the latter
is secondary gases generated from the caracking of
oil. The widely used simulation apparatus in
laboratory is pure closed or open system. In closed
system, gas yields in higher temperatures are often
the mixture of primary and secondary gases, while in
open system, the gas yileds measured are always an
instanuous one which is proved very low and hard to
be measured. In this study, we developed a stepwise
open system, which is composed of a pyrolysis cavity
and computer-cotrolled multi-traps in vacuum tanks.
Kerogen is heated to the pre-set temperatue and all
the gases generated in the temperature range from
beginning to this temperature point are collected in a
tank. Gas chemical composition and carbon isotopes
will be measured by a on-line GC and GC-IRMS
nennected to the tanks using a multi-way valve.
Because the heating caivty is connected to a vacuum
tank, the yields will expeled to the tank without
retaining. Due to the gases collected in each tank is a
cumulative gases from the room temperature to the
pre-set temperature point, the result is a cumulative
primary gases rather than a instanuous one. This
apparatus provides an ideal tool to study the primary
cracking gases.
The sample we used is a typical marine kerogen
extracted from a Protezoic shale from Zhangjiakou,
north of China, which have been studied in closed
system and the results have been reported before. It
can be compared with the reults of this study.
The generation process of gases look similar with
those in closed system, but the gas yields are quite
low. The maximum yiled of C1 is about one third of
that in closed system, and the maximum yiled of C2,
C3 and C4+C5 in this study are more lower than those
in closed system.
The kinetc parameters of gas generation were
calculated which are 50-72 kcal/mol of activation
energy with a frequency factor of 6.5х10 15 s -1 for C1,
44-61 kcal/mol of activation energy with a frequency
factor of 2.63х10 13 s -1 for C2, 38-49 kcal/mol of
activation energy with a frequency factor of 2.56х10 11
s -1 for C3, and 38-51 kcal/mol of activation energy
with a frequency factor of 6.5х10 15 s -1 for C4+C5,
respectively. In general, the values of activation
ennergy are relatively lower than those in closed
system, especially for the heavy gases of C2, C3 and
C4+5.
The kinetic parameters were used to extropolate our
resluts to geological condittions of 5 o C/My. It is clear
that C4+5 and C3 gases start their generation around
105 o C (0.4%Ro) and reach 90% conversion at around
150 o C (1.1%Ro), while C2 gases begin its generation
at 110 o C(0.5%Ro) and reach 90% conversion at
around 160 o C (1.5%Ro%). Although C1 gas begins its
generation at around 120 o C, it reaches its fast
generation process only after 150 o C, and reaches its
90% conversion at around 220 o C (2.5%Ro). The
complete conversion of C1 gas will reach 245 o C
(3.4%Ro). These results suggest that primary heavy
gases of C2-C5 start at very early maturities and
lower temperatures, while primary C1 gas is mainly
generated in later stages greater than 1.5%Ro, which
is similar to the secondary C1 gas in closed system.
In the diagram of ln(C1/C2)-ln(C2/C3), primary gases
are mainly plotted in the area of lower C2/C3 and
C1/C2. Comparing with the gases in closed system,
primary gases show less fractionation of defferent
molecular compostions. Less C2/C3 and C1/C2
suggest that the generation of primary gases is mainly
from the cracking of kerogen rather than the cracking
of oil. In the diagram of C2/C3-δ 13 C2-C3, the primary
gases are also show less fractionation of carbon
isotopes, while gases from closed system show both
greater compositional and isotopic fractionation. It
also suggests that gases from the closed system are
a mixture of primary and secondary ones.
Acknowledgments
China Giant S&T Programs (2011ZX05008-002-13),
China 973 program (2007CB209501) and NSFC
project (40872091) are thanked for the support.
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