Geological characteristics and enrichment-accumulation patterns of coalbed methane within the Jurassic strata in the Miquan area along the southern margin of the Junggar Basin

ObjectiveThe southern margin of the Junggar Basin (also referred to as the southern Junggar) possesses abundant medium- to low-rank coalbed methane (CBM) resources, with significant progress achieved in CBM exploration and production. However, there remains a lack of systematic understanding of the genetic mechanisms of CBM within the Middle-Lower Jurassic strata, along with its accumulation and evolution laws, in the Miquan area, southern Junggar. This absence restricts the commercial development of CBM in these strata.MethodFocusing on CBM within the Middle-Lower Jurassic Xishanyao and Badaowan formations in the Miquan area, this study further identified the CBM origins and characterized the CBM accumulation patterns in detail. Using the Petromod basin modeling software, this study simulated the geological evolution process of the Jurassic coal seams and elucidated CBM enrichment and accumulation laws. Accordingly, it established and improved the CBM accumulation patterns of the Jurassic strata in the Miquan area. Results and Conclusions In the Miquan area, the CBM within the Xishanyao Formation is dominated by secondary biogenic gas, while that in the Badaowan Formation consists of thermogenic gas formed after modification by microbial degradation (similar to the Fukang block). Laterally, the Badaowan syncline exhibits a significantly higher gas content (6.62 m3/t) than the Northern monocline (5.40 m3/t). Vertically, there is a negative correlation between gas content and methane concentration. Specifically, with increasing coal seam burial depth, the gas content trended upward, while the methane concentration showed a decreasing trend. Further investigation reveals that the CBM accumulation evolution in the Miquan area can be divided into five key stages: the generation and dissipation of primary biogenic gas during the Early−Middle Jurassic (199‒163 Ma); the generation of secondary biogenic gas in the Xishanyao Formation during the Middle−Late Jurassic (163‒135 Ma); the generation of thermogenic gas in the Badaowan Formation during the Cretaceous (135‒78 Ma); the continuous recharge of thermogenic gas in the Badaowan Formation during the Late Cretaceous−Oligocene (78‒23 Ma), and the recharge of secondary biogenic gas and the modification of thermogenic gas since the Neogene (23 Ma to present). The control and enrichment laws of CBM within the Jurassic strata in the Miquan area were also revealed, characterized by syncline-controlled enrichment, structure-controlled accumulation, and hydrodynamic sealing. Accordingly, three accumulation pattern types were established: Badaowan syncline-fault type, the Qidaowan anticline-fault-lithologic trap type, and the hydrodynamic sealing type of steeply inclined coal seams in the Northern monocline. The results of this study can provide a reference for understanding the genetic mechanisms and developing exploration schemes of other medium- to low-rank CBM.