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The southern Junggar Basin, Xinjiang, has abundant coalbed methane (CBM) resources. Currently, the Baiyang River development pilot test area (BYR block for short) in the Fukang east block has achieved large-scale CBM development, but the productivity characteristics and its controlling factor are still unclear. Based on the field production data of the BYR block and experimental tests, this paper summarizes the gas and water production characteristics and presents the analysis results of the geological and geochemical responses to the productivity of CBM wells. The productivity of CBM wells in the BYR block was generally characterized as medium-to-low yield. The productivity was jointly controlled by the burial depth, structure condition, thickness and number of co-production coal seams, and hydrogeological conditions. The gas production first increased and then decreased with the increase in the burial depth of the coal seam, and a burial depth between 750 and 1000 m was the most beneficial to increasing the gas production due to the good gas preservation conditions and suitable permeability and stress conditions. The total thickness of the co-production coal seams had a positive effect on the productivity of gas wells, but the productivity was also affected by the number of co-production coal seams and interlayer interference. In the BYR block, the co-production of the nos. 41 and 42 coal seams was the most favorable combination form for CBM drainage. The productivity of CBM wells had a good response to the Na+, K+ and HCO3− concentrations but a poor response to δD-H2O and δ18O-H2O. Based on the concentrations of the main ions and TDSs of the coal seam water, a productivity response index δ* was established, and there was a good positive correlation between the productivity and δ*. [ABSTRACT FROM AUTHOR]

This paper mainly focused on the genesis of coalbed methane (CBM), CO 2 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 CO 2 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 CO 2 concentration and extremely positive δ13C-CO 2 are all related to microbial methanogenesis. The CO 2 in the ER and JMSE areas and partial CO 2 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 CO 2 concentration with increasing burial depth in the ER and BYR areas. The CO 2 in other areas is originally derived from the deoxidation reaction of early coalification which then undergoes secondary biotransformation. The CO 2 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-CH 4 and δ13C-CO 2 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. • Multiple genetic types of gases coexist in the southern Junggar Basin. • The early coalification, structural and hydrogeological conditions control CBM accumulation. • Abnormally high CO 2 and extremely positive δ13C-CO 2 are related to microbial methanogenesis. • The CO 2 in SGR area was originally derived from the deoxidation reaction of early coalification. [ABSTRACT FROM AUTHOR]