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The compositions and the carbon isotope characteristics of methanes of the coal-bed gases from 14 coal mines and 3 deep to medium boreholes are studied in this paper.It is found that the coal-bed gases from the coal mines are characterized by the drying or lightening phenomena, which are absent in coal-bed gases from the boreholes.The drying and lightening features were not observed in the coalbed gases derived from both the primary zones and the lignite pyrolysis simulation experiments.Therefore, it is considered that the drying and lightening of coal-bed gases in the coal mines result from the secondary destructive processes.The origins of dried or lightened coal-bed gases may be due to (ⅰ) desorption-diffusion, (ⅱ) the exchange equilibrium of carbon isotopes between CH_4 and CO_2, and (ⅲ) mixing of primary coal-bed gases with syncoalification or postcoalification of biogenetic gases, among which the desorption-diffusion origin is of primary importance.
The compositions and the carbon isotope characteristics of methanes of the coal-bed gases from 14 coal mines and 3 deep to medium boreholes are studied in this paper. It is found that the coal-bed gases from the coal mines are characterized by the drying or lightening phenomena, which are absent in coal-bed gases from the boreholes. the drying and lightening features were not observed in the coalbed gases derived from both the primary zones and the lignite pyrolysis simulation experiments.Therefore, it is considered that the drying and lightening of coal-bed gases in the coal mines result from the secondary destructive processes. The origins of dried or lightened coal-bed gases may be due to (i) desorption-diffusion, (ii) the exchange equilibrium of carbon isotopes between CH 4 and CO 2 , and (iii) mixing of primary coal-bed gases with syncoalification or postcoalification of biogenetic gases, among which the desorption-diffusion origin is of primary importance.