--> --> Abstract: Secondary Biogenic Coalbed Gas, by Minxin Tao, Xiaobin Li, Zuodong Wang, Zhongping Li, Baoguang Shi, and Jing Li; #90082 (2008)

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Secondary Biogenic Coalbed Gas

Minxin Tao1, Xiaobin Li1, Zuodong Wang2, Zhongping Li2, Baoguang Shi2, and Jing Li1
1College of Resources Sciences and Technology, Beijing Normal University, Beijing, China
2Key Laboratory of Gas Geochemistry, Chinese Academy of Sciences, Lanzhou, China

Secondary biogenic coalbed gas (SBCG) is an important type of coalbed gas. But pure SBCG rarely exists, it usually occurs with a certain amount of thermogenic gas. Variation in the ratio of the two is an important factor for such type of coalbed gas in terms of geochemical composition and change. With the increase of SBCG proportion, its changing tendencies are as follows: C1/C2+3 ratio rises, CO2 content decreases, δ13C1 value becomes lighter, the isotopic value of residual CO2 and heavy hydrocarbons get heavier, and the value of △δCCO2-CH4 rises correspondingly. So its various average values of geochemical indicators are of more representative and scientific significance. According to a typical coalbed gas with its main components of SBCB,its geochemical characteristics and tracer indicators are as follows: the ratio of C1/C2+3 is generally more than 1000, the CO2 content is less than 5%, CDMI index < 2%, δ13C1<-55‰, δDCH4 value ranges about from -229‰ to -179‰, from -27.5‰ to -21.5‰ for δ13C2, -18.8‰ to +5‰ for δ13CCO2. The coal seams contain SBCG generally experienced tectonic uplift and dissipation of thermogenic gas, then SBCG was formed in the shallow environment suitable for methanogen, meanwhile.
This work was supported by the National Basic Research Program of China (No. 2002CB211701) and the Key Projects of the Chinese Ministry of Education (No.307007).

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