Objective  With the continuous advancement in the exploration and exploitation of coalbed methane (CBM), the focus of research and practices is gradually shifting from shallow to deep parts. Deep CBM, an important unconventional natural gas resource, has become a strategic replacement to ensure energy security and prompt industrial development in China. However, deep coal reservoirs exhibit strong heterogeneities, posing a core geological challenge to their efficient exploration and development.

Methods  This study investigated the deep No. 8 coal seam in the northern part of the eastern margin of the Ordos Basin. By integrating multiple technical methods, including geostatistics, spatial interpolation, substantial core tests, and numerical simulation, this study systematically examined the multi-scale heterogeneities of reservoirs in the No. 8 coal seam in the lateral and vertical directions. Furthermore, the controlling effects of reservoir heterogeneities on CBM productivity were quantitatively assessed.

Results and Conclusions  The results indicate that deep coal reservoirs in the northern part of the eastern margin of the Ordos Basin exhibit a representative heterogeneous mode characterized by a homogeneous geological background but differential key parameters. The burial depth, vitrinite reflectance (R_max), and vitrinite content of the reservoirs show coefficients of variation (CVs) of less than 0.15, suggesting the high stability of the regional tectonic setting, thermal evolution history, and coal-forming environments. In contrast, the Langmuir volume, gas content, and adsorbed gas saturation of the reservoirs exhibit CVs ranging from 0.27 to 0.33, implying moderate heterogeneities. Furthermore, the spatial differentiations of these parameters are governed by the coupling effects of local factors. Based on these findings, three CBM enrichment patterns were identified in the study area. Specifically, the southeastern part exhibits gas generation-dominated CBM enrichment, which is driven by thermal anomalies of concealed rock masses and features high R_max and gas content. The southwestern part manifests a CBM enrichment pattern characterized by great burial depth and high saturation. This enrichment pattern can be attributed to both weak adsorption effects under high temperatures due to deep burial and favorable preservation conditions, which jointly contribute to the formation of a free gas enrichment area with high adsorbed gas saturation. The northern part displays low hydrocarbon generation and weak preservation conditions, identified as an area with poor gas-bearing properties. Based on two representative wells, this study revealed the binary control mechanisms behind the vertical heterogeneity of gas-bearing properties in the No. 8 coal seam. Under the geological setting of poor preservation conditions, the intralayer sealing effect predominates, with gas-bearing properties tending to be higher in the middle part and lower in the upper and lower parts vertically. In this case, sweet spot intervals are situated in the middle part of the coal seam. In areas with favorable preservation conditions, the vertical distribution of gas-bearing properties is primarily governed by the static differentiation of original coal-forming materials. Numerical simulations corroborate that the heterogeneous distributions of vertical gas-bearing properties exert significant impacts on CBM productivity. The 10-year cumulative gas production of the scheme with precise perforation in a sweet spot interval was 4.1 times higher than that of the scheme with perforation arranged in an interval with poor gas-bearing properties. Integrating research on the vertical heterogeneity of coal reservoirs into the precise positioning of sweet spot intervals and the optimization of perforation schemes is key to the fine-scale and efficient exploitation of deep CBM. The results of this study are of great significance for gaining deep insights into the enrichment patterns of deep CBM and for guiding the exploration and exploitation practices of deep CBM under similar geological conditions.