The deep coalbed methane (CBM) resources (buried deeper than 1500 m) in North China exceed 30 trillion cubic meters, with significant breakthroughs in exploration, making it a new frontier for natural gas production growth. However, it faces key issues, including complex preservation conditions, unclear controls on enrichment and high productivity, and large production variations. In order to clarify the control factors of preservation conditions of deep CBM, the coal quality, lithology combination, and structural preservation characteristics of typical deep CBM exploration wells in North China are systematically analyzed with multidisciplinary analysis methods of geology, geochemistry, and geophysics. This study reveals that the differential preservation of deep CBM is controlled by the dynamic coupling mechanism of adsorption self-sealing, physical property sealing, and structural preservation system: (1) Favorable coal quality is the basis of strong adsorption. Coal with low ash content and a high evolution degree is characterized by a large Langmuir volume, a great critical depth, and strong self-sealing ability. (2) Tightly lithology combination is a barrier to deep free gas dissipation. Limestone and mudstone exhibit low porosity, a small throat radius, and high breakthrough pressure, resulting in good physical sealing conditions and a high total retained gas content. (3) Continuous and stable structural preservation is the key to free gas enrichment. Late-stage uplift with low amplitude and weak tectonic activity is conducive to free gas accumulation, resulting in a high-pressure coefficient. In summary, areas characterized by deep burial, low-ash coal, limestone/mudstone roof rocks, and structural stability are favorable for deep CBM preservation. The study of differential preservation of deep CBM deepens the understanding of enrichment and high productivity, and provides a theoretical basis for optimizing favorable exploration and development targets.

China boasts abundant deep coalbed methane (CBM) resources. Positive progress in the exploration in recent years has established CBM as a strategic replacement for current unconventional natural gas in the future. However, the commercial exploitation of deep CBM faces challenges of complex geological and engineering conditions. In this study,we investigate the geology of deep CBM in typical regions across China and review advances in relevant theoretical and technical study, proposing the prospects for the exploration and production of deep CBM. The results indicate that deep CBM reservoirs exhibit the geological and engineering characteristics of strong heterogeneity, enrichment in both free and adsorbed gas, and high plasticity compared to their shallow counterparts. Deep medium- to low-rank coal reservoirs provide substantial storage space dominated by primary plant tissue pores. In contrast, deep medium- to high-rank coal reservoirs contain micropores and fissures, with the former dominated by organic pores and the latter consisting primarily of cleats and exogenetic fractures. Over years of addressing technological challenges, SINOPEC has preliminarily developed a series of technologies for the selection and assessment of deep CBM target areas, sweet spot prediction,horizontal well drilling, and hydraulic fracturing with fractures effectively propped, which serve to provide effective support for breakthroughs achieved in deep CBM exploration. It is recommended to focus on the accumulation patterns,sweet spot identification, production technologies and policies, and production rules of deep CBM in future study. Additionally, it is advisable to develop efficient drilling and completion technologies for horizontal wells in thin coal seams, along with technologies for fracturing with reduced cost and enhanced efficiency for reservoir stimulation.

In recent years, significant breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) in the Daniudi area, northern Ordos Basin. This study aims to determine the potential for deep CBM exploration in this area. Based on regional geology and data derived from core and maceral observations, scanning electron microscopy (SEM) image analysis, proximate analysis, mercury injection capillary pressure (MICP) analysis, lowtemperature liquid-nitrogen adsorption measurements, and CO₂ adsorption and methane isothermal adsorption experiments, along with on-site gas content tests, we delve into the geological characteristics of deep coal seams in the Daniudi area in terms of their spatial distribution, lithotypes, coal quality, reservoir properties, and gas-bearing properties. The No.8 coal seam of the Taiyuan Formation is identified as the dominant coal reservoir in the Daniudi area. The results indicate that this coal seam is consistently distributed throughout the Daniudi area, with a thickness ranging from 2 to 14 m and vitrinite reflectance (R_o) of 1.7 %. Bright to semi-bright coals occur in the upper part of the coal seam as featuring low ash content, indicating high coal quality and providing a material basis for CBM accumulation. The No.8 coal seam shows well-developed micropores and macropores, with micropores being predominant, accounting for 67.5 %. The bright to semi-bright coals with low ash content therein display high total pore volume, specific surface area of pores,micropore proportion, and porosity. The roof of the coal seam contains thickly laminated limestones and dark mudstones, as characterized by simple structure and low formation water content, suggesting a dry-coal gas-bearing system. With total gas content ranging from 20.3 to 47.1 m³/t and free gas proportion from 33.2 % to 66.2 %, the No.8 coal seam is rich in both adsorbed and free gases. This coal seam manifests a gas-bearing area of 2003 km², CBM resources of 5422 × 10⁸ m³, and CBM resource abundance of 2.71 × 10⁸ m³/km². These findings indicate that deep coals in the Ordos Basin are characterized by favorable geological condition for CBM generation, representing a significant exploration and development target.