Abstract: Deep coalbed methane (CBM) is a new field of hydrocarbon exploration with huge resource potential. In 2021, a great breakthrough was achieved in the exploration and development of CBM at depths greater than 2 000 m in the Daning-Jixian block on the eastern margin of Ordos Basin. The initial gas production from Well Jishen 6-7P-01 reached 10×10⁴m³/d, marking the beginning of large-scale exploration and development of deep CBM in the basin. The lack of a systematic study on targeted pore characteristics has restricted the efficient development of deep CBM in the study area. Based on the test data of cores, scanning electron microscopy, full-diameter CT scans, reservoir properties, low-pressure CO₂ adsorption, low-pressure N₂ adsorption, and high-pressure mercury injection, the paper systematically analyzes the reservoir characteristics and pore structure of the No.8 coal seam in Taiyuan Formation of Daning-Jixian block. The results indicate that:(1) The No.8 deep coal reservoir formed in waterlogged forest and swamp with lagoon facies. Bright and semi-bright coal are mainly found in the study area, with a high organic matter thermal maturity (average R_o of 2.81%). Cleats and fractures are well developed but often filled with secondary minerals, resulting in a low effective fracture ratio. (2) The of the No.8 deep coal seam has poor reservoir properties, of which the matrix porosity ranges from 3.60% to 6.11%, averaging at 3.65%, and the matrix permeability ranges from 0.001 mD to 0.060 mD, averaging at 0.016 mD; it is classified as an ultra-low porosity and ultra-low permeability reservoir. Micropores are the dominant pore type, followed by macropores, while mesopores are poorly developed. Micropores with the specific surface area ratio exceeding 99% serve as the main storage space for adsorbed methane. (3) Compared with the No.8 mid-shallow coal reservoir and the Longmaxi Formation shale reservoir in Sichuan Basin, the No.8 deep coal reservoir has lower permeability. This is primarily due to less effective cleats and fractures, and more micropores with poor connectivity. Based on this understanding, a shift in reservoir stimulation strategies in combination with super-large-scale volume fracturing techniques have significantly increased gas production from the No.8 deep coal reservoir and effectively guided the exploration and development of deep CBM in the study area.

Key words: Daning-Jixian block, No.8 deep coal seam, pore characteristics, full-scale pore size distribution, ultra-low porosity and ultra-low permeability, super-large-scale volume fracturing

摘要： 深部煤层气是煤层气勘探新领域，资源潜力巨大，2021年，鄂尔多斯盆地东缘大宁—吉县区块（大吉区块）2 000 m以深的煤层气勘探开发取得重大突破，吉深6-7平01井初期产气量达10×10⁴m³/d，揭开了盆地深部煤层气规模勘探开发的序幕。缺少针对性孔隙特征的系统研究，制约了研究区深部煤层气的高效开发。基于岩心和扫描电镜、全直径CT扫描、储集物性测试、CO₂低压吸附、N₂低压吸附、高压压汞等测试资料，系统分析了大吉区块太原组深部8号煤层的储层特征和孔隙结构，结果表明：①深部8号煤储层形成于潟湖相覆水森林沼泽，煤岩类型以光亮煤和半亮煤为主，有机质热演化程度高（R_o平均为2.81%），割理和裂隙较为发育，但多被次生矿物充填，有效裂缝占比低。②深部8号煤储层储集物性较差，基质孔隙度在3.60%~6.11%，平均为3.65%，基质渗透率在0.001~0.060 mD，平均为0.016 mD，属于特低孔、特低渗储层；孔隙类型以微孔为主，宏孔次之，介孔欠发育；微孔的比表面积占比达99%以上，是吸附态甲烷的主要赋存场所。③与盆地中—浅部8号煤储层和四川盆地龙马溪组页岩储层相比，研究区深部8号煤储层的渗透性更差，这主要是由于有效割理和裂隙较少、孔隙以微孔为主且连通性差。基于研究认识，转变储层改造思路，采用超大规模体积压裂工艺，显著提升了深部8号煤储层的产气量，有效指导了研究区深部煤层气的勘探开发。

关键词: 大吉区块, 深部8号煤层, 孔隙特征, 全尺度孔径分布, 特低孔特低渗, 超大规模体积压裂