Key controlling factors of coal-rock gas of Benxi Formation in Ordos Basin and its practical significance
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摘要:
鄂尔多斯盆地煤岩气资源丰富，多个区块勘探开发取得重大突破，然而煤岩气的含气性主控因素尚未明确，制约整体勘探开发进程。基于含气量、煤岩煤质、煤岩地化、孔隙结构等大量实测资料，系统分析了本溪组煤岩气地质条件，厘定了含气性主控因素，并开展了煤岩气资源条件分区评价。研究表明：沉积、煤阶、埋深奠定了深层煤岩含气量的整体格局。① 沉积环境制约着煤岩生烃物质基础（包括煤层厚度、煤层结构及煤岩煤质）、煤岩气原始赋存空间及保存条件的平面分布。成煤环境演化则决定了煤岩煤质的纵向差异分布，进而影响含气量的纵向非均质性。② 煤阶不仅控制了煤岩生气强度，还通过影响孔裂隙结构从而影响煤岩气的赋存空间及储集性能。随着变质程度提高，煤岩的吸附性能得到了强化。③ 现今埋深主要影响吸附气和游离气的比例，常压储层在临界深度以下，随着埋深的增加，游离气的比例有所增加，
3000 m以深总气量呈减小趋势。④ 微幅构造对发育于缓坡带的煤岩气起到了局部富气的作用。⑤ 综合考虑生、储、保有利条件，明确了盆地煤岩气富集带为榆林到宜川的南北条带，平均含气量高达23 cm3/g。⑥ 依据煤岩气的内涵及含气性主控因素，结合煤层展布，构建了煤岩气资源条件评价体系，将盆地本溪组煤岩划分为4大类13个小区，Ⅰ类区主要分布在榆林地区、大宁—吉县地区和延安地区，Ⅱ类区主要分布在乌审旗一带。研究成果以期为鄂尔多斯盆地煤岩气有序勘探开发提供借鉴与参考。Abstract:The Ordos Basin is rich in coal-rock gas resources, and significant breakthroughs have been made in exploration and development in many blocks. However, the key controlling factors for the gas content of coal have not yet been clarified, which restricts the overall exploration and development process. Based on a large amount of measured data such as gas content, coal rock and coal quality, coal geochemistry,and pore structure, the geological conditions of coal-rock gas in Benxi Formation were systematically analyzed, the main controlling factors of gas content were determined, and the coal-rock gas resource conditions were evaluated by zoning. The results show that sedimentation, coal rank and buried depth have laid the overall pattern of gas content of coal. ① The sedimentary environment restricts the hydrocarbon material base (including coal seam thickness, coal seam structure and coal quality), the original occurrence space of coal-rock gas and the plane distribution of preservation conditions. The evolution of coal-forming environment determines the longitudinal difference distribution of coal quality, which in turn creates the longitudinal heterogeneity of gas content. ② Coal rank not only controls the gas generation intensity of coal, but also affects the occurrence space and reservoir performance of coal-rock gas by affecting the pore structure, and the adsorption performance of coal is strengthened with the increase of metamorphism. ③ The burial depth mainly affects the proportion of adsorbed gas and free gas, and below the critical depth, with the increase of burial depth, the proportion of free gas increases, and the total gas volume decreases after the depth exceeds 3000m. ④ The micro-amplitude structure plays a role in local gas enrichment in the development and gentle slope zone. ⑤ Taking into account the favorable conditions for hydrocarbon generation, storage, and preservation, coal-rock gas enrichment zone is clearly defined as the north-south strip from Yulin to Yichuan, with an average gas content of up to 23 cm3/g. ⑥ According to the connotation and key control factors of coal-rock gas, combined with the distribution of coal seams, the evaluation system of coal-rock gas resource conditions was constructed, and the coal seam of Benxi Formation in the basin were divided into 4 categories and 13 communities. The class I area was mainly distributed in Yulin area, Daning-Jixian area, and Yanan area, and the class II area was mainly distributed in Wushenqi area. The research results are expected to provide reference and guidance for the orderly exploration and development of coal-rock gas in the Ordos Basin.
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Keywords:
- coal-rock gas /
- key control factors /
- resource conditions /
- Benxi Formation /
- Ordos Basin
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图 1 鄂尔多斯盆地上古生界煤系地层柱状图和8号煤埋深、厚度、Ro分布
Figure 1. Stratigraphic column of Upper Paleozoic coal-bearing strata in Ordos Basin and distribution of burial depth, thickness, and Ro of No. 8 coal
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图 2 本溪组8号煤含气性特征
Figure 2. Gas content characteristics of No. 8 coal of Benxi Formation
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图 3 不同沉积相带中的煤层结构和灰分含量
Figure 3. Coal seam structure and ash content of different sedimentary facies zones
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图 4 绥德1H井成煤环境制约下的8号煤的煤质、含气性纵向特征
Figure 4. Vertical characteristics of coal quality and gas content of No. 8 coal of Suide1H well as constrained by coal-forming environments
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图 5 8号煤含气量与灰分的关系
Figure 5. Relationship between gas content and ash content of No. 8 coal
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图 6 镜质组含量与游离气量及其比例与的关系
Figure 6. Relationship between vitrinite content and free gas content and its ratio
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图 7 本溪组8号煤、岩组合特征
Figure 7. Assemblage characteristics of No. 8 coal and surrounding rock of Benxi Formation
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图 8 本溪组8号煤围岩封闭性对含气量的影响
Figure 8. Impact of surrounding strata sealing on gas content of No. 8 coal of Benxi Formation
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图 9 不同煤阶煤岩的生烃模式、孔隙演化及储气性能
Figure 9. Hydrocarbon generation modes, pore evolution, and gas storage performance of coal at different coal ranks
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图 10 本溪组8号煤的Ro和孔隙结构参数的关系
Figure 10. Relationship between Ro and pore structure parameters of No. 8 coal of Benxi Formation
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图 12 本溪组8号煤含气量随埋深的变化规律
Figure 12. Variation pattern of gas content with burial depth of No. 8 coal of Benxi Formation
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图 14 鄂尔多斯盆地煤层气与煤岩气地质条件对比
Figure 14. Comparison of geological conditions between coalbed methane and coal-rock gas in Ordos Basin
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图 15 鄂尔多斯盆地本溪组8号煤煤岩气富集区分布
Figure 15. Distribution of coal-rock gas enrichment zones of No. 8 coal of Benxi Formation in Ordos Basin
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图 16 鄂尔多斯盆地本溪组煤岩气资源条件评价结果
Figure 16. Evaluation results of coal-rock gas resource conditions of Benxi Formation in Ordos Basin
表 1 泥炭沼泽环境判识指标
Table 1 Indicators for recognizing peat swamp environment
古环境指标 煤相指标 ST/% 低硫 中硫 高硫 GI/% 强凝胶化 中凝胶化 弱凝胶化 <1 1～3 >3 >10 5～10 ＜5 Sr/Ba/% 淡水 半咸水 咸水 TPI/% 保存不完整 保存完整 <0.6 0.6～1.0 >1.0 <1 >1 V/(V+Ni)/% 缺氧 弱氧 氧化 GWI/% 值越大代表地下水影响越大 >0.6 0.45～0.6 <0.45 TI/% 值越大代表水体活动越强 
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表 2 鄂尔多斯盆地本溪组煤岩气资源条件评价指标体系
Table 2 Evaluation index system for coal-rock gas resource conditions in Benxi Formation of Ordos Basin
关键参数 好 中 差 厚度/m >10 4～10 <4 灰分/% <20 >20 Ro/% >1.3 1.0～1.3 0.65～1.0 埋深/m 1800 ～3000 3000 ～3750 > 3750 ；1500 ～1800 煤、岩组合类型 煤−灰岩、煤−泥岩、煤−灰岩+泥岩 煤−砂岩+泥岩 煤−砂岩
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