Differences in geological conditions of deep and shallow coalbed methane and their formation mechanisms

Deep coalbed methane (CBM) boasts abundant resources and great potential for exploitation. However, there is a lack of in-depth studies on the internal connections between deep and shallow CBM. This study investigated the Upper Paleozoic coalbed in the Ordos Basin. From the perspective of coalbed formation and evolution, it obtained three burial depth evolution modes of coalbed through summaries: both the maximum and present burial depths exceeding 2000 m, the maximum burial depth over 2000 m while the present burial depth less than 2000 m, and both the maximum and present burial depths less than 2000 m. This study systematically analyzed the differences in the temperature and pressure conditions, gas content, in-situ stress, reservoir permeability, metamorphic grade, and water content of deep and shallow coal seams, as well as their formation mechanisms. The results indicate that, due to the influence of burial depth and evolutionary process, deep and shallow coal reservoirs show maximum temperature and pressure differences of up to 100°C and about 40 MPa, respectively. As a result, from deep to shallow, the CMB occurrence state transitions from the predominance of adsorbed gas to the coexistence of adsorbed state and free gas, the in-situ stress field shifts from the predominance of horizontal stress to that of vertical stress, and coal reservoirs’ porosity, permeability, and water content gradually decreased. Accordingly, this study ascertained the typical characteristics of deep CBM. Specifically, under high-temperature and high-pressure conditions, deep CBM occurs as coexisting adsorbed and free gas in coal reservoirs below a certain depth. Under the action of predominant vertical stress, the coal reservoirs of deep CBM feature extremely compressed pore and fissure spaces, an extremely low water content, and an extremely high degree of mineralization, with microfissures acting as major seepage channels. Based on the differences in the critical depth for gas-bearing property and the transformation depth of in-situ stress field between deep and shallow coal reservoirs, this study proposed that there exist transitional zones between shallow and deep coal seams, exhibiting the characteristics of atypical deep CBM or the coexistence of geological conditions for deep and shallow CBM. During exploration and exploitation, it is necessary to figure out a tailored scheme based on specific analysis to achieve efficient and coordinated development of shallow and deep CBM.