Revisiting implications of coalbed methane development in the San Juan Basin

Significant breakthroughs have been achieved in the exploration and exploitation of deep coalbed methane (CBM) in China, establishing CBM as an emerging focus of academic research. The San Juan Basin stands out as the most successful basin in CBM development to date. Therefore, the geological understanding and development experience of this basin can provide critical implications. In this study, we systematically organize geological insights into CBM in the San Juan Basin and analyze gas well production data up to 2019. The results indicate that the gas produced from the fairway zone of the basin consists of approximately 25%~50% in-situ thermogenic gas, 12%~60% migrated thermogenic gas, and 15%~30% secondary biogenic gas. The basin contains three pressure systems: overpressured, transitional, and underpressured. The overpressured zones are characterized by vitrinite reflectance (R_o) values of coals generally exceeding 0. 8% and an average CO₂ content of up to 6. 5%, with dry gas predominating. In contrast, the underpressured zones show R_o values of coals below 0. 7% and CO₂ contents generally below 2. 0%, with wet gas being dominant. The primary factors governing the high productivity of the fairway zone include a favorable sedimentary setting (thick coal seams + shale roof), moderate coalification (generation of thermogenic gas and suitable for the generation of biogenic gas), and significant hydrological regulation and well-developed structural framework for preservation. The productivity of CBM wells in the basin is jointly controlled by pressure systems, reservoir physical properties, and hydrogeological regulation, with gas and water contributions varying across different pay zones. The San Juan Basin has cumulative CBM production exceeding 650 × 10~9 m³ of CBM. Within the fairway zone, approximately one-third of CBM wells have single-well cumulative production of greater than 113 × 10~6 m³. Wells in this zone exhibit gas production cycles lasting more than 300 months, with most yielding over 18. 2 × 10~6 m³ of gas in the first 24 months of production. To date, two high-production models of CBM development have been identified worldwide. The first model is exemplified by the San Juan, Powder River, and Surat basins, where large-scale stable gas production is primarily supported by favorable natural conditions, including shallow burial depths, large coal seam thickness, high permeability, and recharge by secondary biogenic gas. The second model is represented by the Ordos Basin, characterized by great burial depths, high coal ranks, low permeability, favorable preservation conditions, and the predominance of in-situ thermogenic gas. In this case, the effective CBM recovery relies heavily on stimulation technologies. For the future development of the CBM industry, it is necessary to continuously advance the integrated geological and engineering understandings to achieve large-scale commercial CBM recovery across different coal ranks and basin types.