Background In recent years, breakthroughs have been constantly achieved in the production of coalbed methane (CBM) at burial depths exceeding 2 000 m in China, with the gas-bearing properties of coal reservoirs identified as a primary geological parameter controlling the CBM production. The western section of the southern margin of the Junggar Basin (also referred to as the study area) hosts abundant CBM resources. In this section, the burial depths of extensive coal seams exceed 2 000 m. However, limited explorations of deep coal reservoirs have restricted a systematic understanding of the gas-bearing properties of coal reservoirs in the study area.

Methods Coal samples were collected from the Middle Jurassic Xishanyao Formation at burial depths ranging from 2 400 m to 2 600 m in well QX-1—a typical deep CBM test well in the study area. These deep coal samples were tested for lithotypes and coal quality, gas content, and geochemical characteristics of CBM. Based on the test results and previous data on shallow CBM, this study presented a summary of the differences in gas-bearing properties between deep and shallow coal reservoirs. Furthermore, it revealed the geological factors controlling the differences in the gas content, occurrence state, and composition between deep and shallow coal reservoirs.

Results and Conclusions The deep coal samples showed measured gas content (on an air-dried basis) ranging from 10.28 m³/t to 18.99 m³/t, with an average of 14.95 m³/t. The adsorbed gas saturation generally exceeded 100%, indicating that CBM occurs in both adsorbed and free states, with an average free gas proportion of 36.56%. Regarding the gas composition, CH₄ accounted for a volume fraction close to 90% in most samples, while N₂ and CO₂ exhibited average volume fractions of 5.72% and 1.78%, respectively. Compared to those of the shallow coal reservoirs, the gas-bearing properties of the deep ones exhibited a slight increase in total gas content, a pronounced increase in the free gas proportion, but a notable decrease in the volume fractions of CO₂ and N₂. The deep coal reservoirs exhibit elevated organic matter maturity, coupled with changes in temperature and pressure, which changed the adsorbed and free gas distribution patterns. These lead to differences in the gas content and occurrence state. Moreover, the deep strata feature an enhanced sealing performance and a configuration consisting of old source rocks and young reservoirs, representing the primary causes of differences in gas-bearing properties between deep and shallow coal reservoirs. Analysis of the gas production characteristics and performance of shallow and adjacent deep CBM wells reveals that the high gas content and free gas proportion and low volume fractions of non-hydrocarbon components in deep coal reservoirs in well QX-1 collectively suggest the considerable potential for gas production. The key to high and stable production while avoiding rapid depletion of the deep coal reservoirs lies in unraveling the release patterns of CBM in different occurrence states and, accordingly, formulating a dynamic pressure control regime. The results of this study provide a critical theoretical basis in terms of basic geology for deep CBM production in the study area.