Origins and carbon isotopic composition of coalbed methane and carbon dioxide from Taiyuan Formation,Northern Qinshui Basin,China

A total of eleven coal seam gas (CSG) samples (production gases) were collected from the Taiyuan Forma- tion of Sijiazhuang District,Northern Qinshui Basin,China,and then their chemical and isotopic compositions were an- alyzed to study the origins of coalbed methane (CH4 ) and carbon dioxide (CO2 ). Results indicate that the CSG of Si- jiazhuang District is dominated by CH4(average:98. 6% ,air-free basis) with minor amounts of CO2(average:0. 25% ) and N2(average:1. 07% ). The δ13 C values of CH4 range from -33. 2‰ to -40. 8‰,with an average of -37． 1‰(n = 11),and CH4 was mainly generated from thermal cracking during coalification,with small proportion from microbial CO2 reduction. The desorption-diffusion-migration effect plays a dominant role in carbon isotope fractionation of CH4 . The CH4 with lighter carbon isotope in shallow reservoirs mainly desorbs from deep ones,then diffuses and moves to the upper part ( shallower reservoirs) and finally escapes into the atmosphere. The δ13 C values of CO2 are between -15． 9‰ and +0． 05‰,with an average of -8. 6‰,and the CO2 may be generated from thermal degration or thermal cracking or both at the early stage of coalification or after the latest coal seam uplift. The CO2 with heavier carbon iso- tope may be associated with groundwater or microbial CO2 reduction. The carbon isotopes of coalbed CO2 become heav- ier as the coal seam becomes shallower,because of the stronger CO2 reduction effect in shallower coal seam.