Geological characteristics and development potential of deep coalbed methane resources in Shenmu-Jiaxian Block, Ordos Basin, China
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摘要: 我国深部(以下均指埋深大于2 000 m)煤层气资源丰富，勘探开发潜力巨大。与中浅部相比，深部煤层气在富集成藏规律与开发方式方面均具有显著的差异性，急需针对重点区块开展解剖性分析研究。神木–佳县区块位于鄂尔多斯盆地东北部，目前尚处于深部煤层气勘探的起步阶段，深部煤层气富集特征及开发潜力等尚不明确。基于研究区近期实施的地震资料、300余口井的测井资料和4口取心井的分析资料等，系统分析煤储层基础地质特征，总结煤层气富集主控因素与富集规律，类比剖析研究区深部煤层气勘探开发潜力。研究区深8号煤的镜质体反射率介于0.7%~1.8%，大部分区域煤层演化已处于热解生气高峰，区域上煤层净煤厚度高达7~8 m，煤层发育稳定，构造相对简单、水动力封闭性较强、顶底板封盖条件较好，为煤层气大面积连续成藏提供了优越条件，其中佳县南区的煤层气资源条件最好。与鄂尔多斯盆地东部其他区块深部煤层相比，研究区煤层中游离气占比明显更高(15.21%~46.47%)，煤层中吸附气主要受吸附压力封存控制，而游离气受毛管力封闭与浮力重力分异双重控制，两种封存机制共同决定了深部煤层含气量的垂向分带与平面分区，以佳县南部为例总结了吸附气吸附压力主控、游离气毛管压力封闭与重力分异耦合控制的典型深部煤层气富集模式。与临兴区块和大宁–吉县区块类比分析结果表明，神木–佳县区块具有较好的煤层气资源基础、储层改造条件和高产潜力，预示了此区块深部煤层气规模化开发的美好前景。此外，大宁–吉县区块深部属典型的高煤阶煤，而神木–佳县深部为中煤阶煤，因此，该研究认识进一步丰富完善了深部煤层气富集理论，对于全国深部煤层气勘探开发实践有重要指示意义。Abstract: China possesses abundant deep coalbed methane (CBM) resources with significant exploration and development potential, referring to depths greater than 2000 m. Deep CBM differs significantly from shallower formations in terms of accumulation and development patterns, necessitating detailed research for key blocks. The Shenmu-Jiaxian Block, situated in the northeastern part of the Ordos Basin, is currently in the early stages of deep CBM exploration, and its enrichment characteristics and development potential remain uncertain. In this study, we conducted a comprehensive analysis of the fundamental geological features of coal reservoirs in the study area using recent seismic data, well logs from over 300 wells, and analytical data from four core wells. We identified the main controlling factors and accumulation mechanism of CBM, and analogically assessed the development potential of deep CBM in the region. The targeting No. 8 deep coal, has the vitrinite reflectance of 0.7%~1.8%, indicating the coalification in the most parts of the study area is currently in the peak stage of thermogenic gas generation. The region exhibits considerable coal thickness, stable development, relatively simple tectonics, well hydrodynamic sealing, and favorable roof and floor sealing conditions, providing favorable conditions for widespread and continuous CBM accumulation. Among them, the southern area of Jiaxian demonstrates the most favorable CBM resource conditions. Compared to other deep blocks in the eastern Ordos Basin, the study area exhibits a notably higher proportion of free gas in coal seams. It is suggested that the adsorption gas in coals is mainly controlled by adsorption pressure, whereas the free gas is subject to dual controls of capillary pressure and buoyancy-gravity differentiation. These two trapping mechanisms jointly determine the vertical zonation and regional distribution of gas content in the deep coal seams. Taking the southern area of Jiaxian as an example, we summarized a typical enrichment pattern of deep CBM. Comparative analysis with the Linxing and Daning-Jixian Blocks indicates that the Shenmu-Jiaxian Block possesses a solid foundation of CBM resources, favorable reservoir alteration, and high production potential, hinting at promising prospects for large-scale development of deep CBM in this region. In addition, the Daning-Jixian block is characterized by high-rank coals, whereas the Shenmu-Jiaxia block consists of medium-rank coals. Therefore, this study contributes to the enrichment and refinement of the theory of deep coalbed methane accumulation, which holds significant instructive implications for the exploration and development of deep coalbed methane in China.
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Keywords:
- deep coalbed methane /
- eastern Ordos basin /
- Shenmu-Jiaxian /
- No.8 coal /
- gas accumulation mechanism
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