25th International Meeting on Organic Geochemistry IMOG 2011

P-262
Minsterry of the organic matter component during biogenic gas
formation: based on the Quaternary of Qaidam Basin
Yanhua Shuai 1 , Shuichang Zhang 1 , Pingan Peng 2 , Hong Lu 2
1 Research Institute of Petroleum & Development of CNPC, Beijing, China, 2 Guangzhou Institute of
Geochemistry,CAS, Guangzhou, China (corresponding author:yhshuai@petrochina.com.cn)
At present, the formation mechanism of earlygeneration
biogenic gas is still mysterious because of
many processes unknown to us. According
conventional thinking, many previous studies on
biogenic gas generation suggest that reactive organic
matter (ROC) be the main and effective substance for
sustaining microbial thriving during biogenic gas
forming. And the ROC was inborn and well reserved
at the beginning of deposit and burial history because
of special geological conditions such as cool
circumstance and high burial rate. However, the
results of organic composition in the Quaternary
deposit in Sanhu depression of Qaidam basin, NW
China, tell us a different story.
Sanhu depression is representative of earlygeneration
biogenic gases area with a proved reserve
of 300 bil steres. Isotopic data of natural gases have
δ 13 CCH4 value ranging from -62‰ to -68.6‰ (VPDB)
and δDCH4 from -218‰ to -245.8‰. Most values of
δ 13 CC2H6 range from -42.4‰ to –44.5‰. Gases
contain 0~0.5% ethane, propane. These data suggest
that gases should be bacterial gases generated by
bacterial reduction of CO2. Almost all the previous
studies think a quick average deposition rate of 800
m/Ma and the frigid climate with an annual average
temperature of 2 O C during the whole Quaternary are
the keys in preserving reactive organic matter which
would be converted into biogenic gases by
methanogen after buried to some depth.
However, the total reactive organic carbon
ultrasonic extracted by acid solution at ambient
temperature (Ingalls et al., 2004) is relatively low,
ranging from 0.4mg/g.sediment to 1.1mg/g.sediment.
ROC is in good relationship with depths, and
decreases with the increasing depth from 100m to
2000 m. Similarly, simple type of reactive organic
matter, as protein and cellulose, were detected in very
low concentrations. The content of protein range from
4.2ug/g to 1570.2ug/g based on per gram of
sediment, and from 10ug/g to 1000ug/g based on per
gram organic carbon. The content of Cellulose is
much higher, ranging from 0.26mg/g.sediment to
7.7mg/g.sediment, and from 0.57mg/g.TOC to
21.2mg/g.TOC, respectively. All those data
demonstrate that reactive organic matter in the
Quaternary is too low to sustain methanogen to
produce such a large scale of biogenic gas.
As a supplement, quantity of subsequent thermal
―reactive organic matter‖ (ROC) was produced by
weak thermal degradation of organic matter. ―ROC‖
content extracted at 80 o C experiment condition was
dramatically higher than that at ambient temperature
of 20 o C. The increment for some samples was even
up to 200%. It demonstrates that secondary ―ROC‖ by
thermo degradation of organic materials, other than
inborn ROC, would be the major nutrient for the deep
biospheres in most geological basins.
As therogenic gases, rapid and sustained longterm
burial background should promote ―ROC‖
generation, and promote biogenic gas formation.
Reference:
Ingalls AE., et al. GCA 2004, 68(21):4363~4379
*Funded by NSFC (40306015,40873031)
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