CO2-enriched CBM accumulation mechanism for low-rank coal in the southern Junggar Basin, China

Abstract

This paper mainly focused on the genesis of coalbed methane (CBM), CO2 origin and the CBM accumulation mechanism in the southern Junggar Basin using a combination of geochemical and geological analysis methods. From this work, it is found that the degree of early coalification and the current structural and hydrogeological conditions jointly determine the regional differences in CBM genesis, accumulation and gas-bearing capacity. Microbial (biogenic) gases are widely developed in the whole study area, while their generation types and contributions vary greatly in different areas and structural belts. The CBM in the eastern Urumqi (ER) and Jimusaer (JMSE) areas is primary microbial (biogenic gas) that have been generated through CO2 reduction pathway and mixed with a small amount of early mature thermogenic gas. The Baiyang River (BYR) area has a mixed gas source of early mature thermogenic gas, primary microbial gas and secondary microbial gas. The CBM in other areas is mixed gas of thermogenic and secondary microbial origins. Abnormally high CO2 concentration and extremely positive δ13C-CO2 are all related to microbial methanogenesis. The CO2 in the ER and JMSE areas and partial CO2 in the BYR area are produced by the microorganism-mediated degradation of sedimentary organic matter. The selective consumption by microbial reduction and differential dissolution of groundwater result in an increase in CO2 concentration with increasing burial depth in the ER and BYR areas. The CO2 in other areas is originally derived from the deoxidation reaction of early coalification which then undergoes secondary biotransformation. The CO2 intervention commonly runs through the whole process of CBM accumulation in low-rank coal in the study area. Due to isotope fractionation caused by groundwater flow and differential modification of microorganisms, the vertical variations of δ13C-CH4 and δ13C-CO2 are positively correlated with burial depth, and the genetic types and gas-bearing characteristics of CBM have obvious vertical zoning under the condition of high-dip structures.