Study on formation mechanism of CO2-enriched CBM reservoirs in low-rank coal seams from southern Zhunggar Basin

The low-rank coalbed methane（CBM）reservoirs in part of the southern Zhunggar Basin has higher CO2 enrichment degree,which not only reflects the multiple intervention in the process of formation and transformation of CBM system,but also affects the quality of CBM resources.To find out mechanism on the formation of CO2-enriched coalbed methane reservoirs,geochemical research was carried out based on gas samples from production wells.The results indicate that the carbon isotope （δ13C CH4=-7.83%～-4.06%; δ13D CH4=-29.00%～-30.41%） of methane is generally lighter.The Miquan and Baiyanghe areas are dominated by biogenic gas and mixed causative gas,respectively,while the deep seams in Sigonghe and Manasi areas mainly developed thermally derived gas.There are significant vertical variations in CO2concentration,with obvious multiple geological factors intervention phenomena.Isotope analysis suggested that δ13C CO2 is generally heavier （δ13C CO2=-1.54%～2.51%）.The sources of CO2 are mainly from the pyrolysis at the beginning of coalification and microbial hydrocarbon generation.Hydrodynamic conditions played an important role in CO2 enrichment:the CO2 generated in the early hydrocarbon generation stage of coalification accumulates at stagnant condition.The associated CO2 of microbial action mixed and preserved under weak runoff conditions.The spontaneous combustion of coal seams leads to the development of burnt rocks and the enhanced underwater seepage,which resulted the participation of non-hydrocarbon gases in the formation of CBM.It is in line with the affected of CBM wind oxidation.Comprehensive analysis shows that the differential groundwater dissolution and bio-hydrocarbon reformation affected the distribution characteristics of CO2 in coal seams.CO2 intervention formed the closed-stagnant-in-situ,semi-closed perturbed,and open connected coalbed methane reservoirs,which carries key information such as the origin and the occurrence of coalbed methane resources.