The value of δ

13

C

1

of coal bed gas in Shanxi and Taiyuan Formations in Qinshui Basin is between-2.963%～-3.539%

which is clearly lower than the δ

13

C

1

value(-2.701%) of simulation test and that calculated by empirical formula.The gas is thus considered to be secondary coal bed gas.The contents of methane in coal bed gas in the study area is more than 98%

and there is no exchange of methane and CO

2

isotope

the preservation condition is good.So it is suggested that desorption diffusion migration shuld be the real origin of the secondary coal bed gas in the study area.