CHARACTERISTICS OF CARBON ISOTOPE COMPOSITION FROM SECONDARY BIOGENIC GAS IN COALBED GASES--TAKING THE HUAINAN COAL FIELD AS AN EXAMPLE

Coalbed gases in Huainan coal field is a gas mixture resulted from thermogenic methane and secondary biogenic gas.And the δ¹³C₁ value of coalbed gases is very different among different mines and coal beds.This is mainly because the difference of mixing proportion between the remaining pyrolysis gas from coalbed and the secondary biogenic gas from post-stage in Huainan coal field.Compared with Chinese thermogenic coalbed gases and others from the world's major regions by δ¹³C₁ and δ¹³C₂ values,it is showed that the δ¹³C₁ value from Huainan coal field is much smaller.The variational property of δ¹³C₁ value is characterized by containing secondary biogenic gas.However,the distribution of δ¹³C₂ value in Huainan coal field is obviously in same position with the one that from Chinese thermogenic coalbed gases and the world's major regions,and it is characterized by property of thermogenic methane.And from the δ¹³C_CO2 value of coalbed gases,we can inferred that CO₂ in methane is mainly resulted from coal pyrolysis,and part of it is the remaining CO₂ from microbial reduction occurring in the secondary biogenic gas generating process.