Origins and molecular composition of coalbed methane in Gujiao area,Shanxi

Gujiao area,Shanxi,is one of the main coalbed methane development area in China due to its rich resource of coalbed methane. However,the coalbed methane development has been limited by its complex geological condition and a low level of research,especially a low level of research in the chemical composition and origin of coalbed meth-ane. The chemical composition and origin of coalbed methane are closely related to its parent material properties,gen-eration mechanism and dissipation law,and has significance for uncovering its preserving condition,distribution and re-source evaluation. In order to reveal the chemical composition and origin of coalbed methane ( CBM),ten CBM sam-ples were collected from Gujiao area,Shanxi,China,and then their molecular and isotopic compositions were analyzed to study the origins of methane and nitrogen. Tectonic evolution history,structural form,hydrogeology condition and de-sorbed gas composition were combined to study nitrogen enrich mechanism and gas-preserving condition. Results indi-cate that the CBM is composed of 83. 79% -97. 57% methane,0. 86% -14. 13% nitrogen,1. 47% -4. 71% carbon dioxide and minor amounts of ethane. The δ13 C values of methane range from -47. 13‰ to -39. 26‰,with an average of -44. 03‰,and methane was generated from the pyrolysis of organic matter. The δ15 N values of nitrogen range from -1. 16‰ to -0. 51‰,with an average of -0. 80‰,and nitrogen was mainly generated from the entraining of atmos-phere. All low gas content and δ13 C value of methane,and high nitrogen proportion were the results of groundwater flow.