Reservoir forming characteristics and favorable area evaluation of deep coalbed methane in Daning-Jixian Block
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摘要: 鄂尔多斯盆地东缘大宁–吉县区块下二叠统太原组埋深大于2 000 m的8号煤是国内首个千亿方级别的深层煤层气田，但是深层煤层气成藏特征尚不明确。综合应用地质、测试、生产资料，开展深层煤层气成藏特征及有利区评价2方面研究。结果表明：研究区深层煤储层全区发育、厚度大、热演化程度高、两期成藏及古热流体侵入，使其具备大量生烃的条件；深层煤储层裂隙、微孔广泛发育，储层具备吸附气和游离气共同赋存的条件；顶底板以灰岩及泥岩为主，封盖能力强，具备游离气保存条件；深层煤层气具有“广覆式生烃、高含气、高饱和、高压束缚游离气与吸附气共存”的赋存特征。建立了深层煤层气“地质–工程”双甜点识别指标体系12项，划分了3类工程–地质甜点区，其中，地质–工程Ⅰ类甜点区位于研究区的西北部，地质–工程Ⅱ类甜点区位于研究的中部，地质Ⅱ类–工程Ⅰ类甜点区位于研究东北部和南部；在地质–工程Ⅰ类甜点区内实施的JS-01井自喷生产，最高日产气9.4~9.7万m3，展现了良好的上产潜力。研究成果有效指导了深层煤层气先导试验区的优选及国内首个千亿方级别的深层煤层气田探明。Abstract: The No.8 coal with a buried depth of more than 2 000 m of Lower Permian Taiyuan Formation in Daning-Jixian block, eastern margin of Ordos Basin is the first deep CBM field with an output of 100 billion m3 in China, but the reservoir forming characteristics of deep CBM are still unclear. In this paper, geological information, test and production data are synthetically applied to study the characteristics of deep CBM reservoir formation and evaluation of favorable areas. The results show that the deep coal reservoir in the study area is extensively distributed, with large thickness and high degree of thermal evolution. Two-stage reservoir formation and paleothermal fluid intrusion provide premise for substantial hydrocarbon generation. In addition, fractures and micropores are widely developed in deep coal reservoir, where adsorbed gas and free gas coexistence. The roof and floor are mainly limestone and mudstone, with strong capping ability and free gas preservation conditions. The deep CBM possesses the characteristics of “extensive hydrocarbon generation, high gas content, high saturation, high pressure bound free gas and adsorption gas coexistence”. Originally, twelve geological-engineering double sweet spot identification index systems for deep CBM are established, and three types of geological-engineering sweet spots are identified. Among them, geological engineering class I sweet spot is located in the northwest of the study area, geological engineering class sweet spot is located in the middle of the study area, and geological class Ⅱ engineering class Ⅰ sweet spot is located in the northeast and south of the study area; The flowing well JS-01 implemented in the geological engineering class I sweet spot has a maximum daily gas production of 94 000-97 000 m3, showing a good production potential. The research results effectively guide the optimization of the pilot trial area of deep CBM and the exploration of the first deep CBM field of 100 billion cubic meters in China.
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图 3 研究区8号煤埋藏史及吸附史
Fig. 3 Burial history and adsorption history of No. 8 coal in the study area
图 5 Dj20压汞孔隙分布频率
Fig. 5 Distribution frequency of mercury injection pores in well Dj20
图 8 DJ-P20井8号煤碳同位素变化曲线
Fig. 8 Carbon isotope variation curve of No.8 coal in well DJ-P20
图 10 大宁−吉县区块“地质–工程”甜点区分布
Fig. 10 Distribution of geological-engineering sweep spots in Daning-Jixian Block
表 1 吉县中浅层与深层煤储层特征对比
Table 1 Comparison of middle-shallow and deep coal reservoirs in Jixian
参数 中浅层(埋深小于1 500 m) 深层(埋深大于2 000 m) 地层组 C–P C–P 开采深度/m 300~1 200 2 000~2 600 煤层厚度/m 3~5 5~12 煤体结构 构造煤为主 原生结构煤为主 镜质体反射率/% 1.2~2.0 2.3~2.8 含气量/(m3·t−1) 8~15 20~35 渗透率/(10−3 μm2) 0.01~0.8 0.01~0.05 孔隙率/% 5~15 3~8 有机质体积分数/% 80.7 89.7 无机矿物成分 黏土矿物为主 碳酸盐岩类、氧化硅类为主 压裂后单井日产气量/104 m3 0.03 0.5~10 
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表 2 研究区地层水矿化度
Table 2 Salinity of formation water in the study area
井号 离子质量浓度/(mg·L−1) 总矿化度/
(mg·L−1)水型 Ca2+ Mg2+ Na+ K+ HCO− 3 SO4 2− Cl− DJ1 68855 2663 40967 911 300 39 207829 321262 CaCl2 DJ2 27990 1454 23875 1 953 300 43 100876 156190 CaCl2 DJ3 1358 309 13764 261 100 33 24434 40158 CaCl2 DJ4 9971 1505 8449 1244 900 36 61818 83023 CaCl2 DJ5 25246 790 22193 1189 100 54 93387 142859 CaCl2 DJ6 16182 1458 19034 1465 950 36 69326 107501 CaCl2
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表 3 大宁−吉县区块深层与浅层煤含气量、含气饱和度对比
Table 3 Comparison of deep and shallow coal gas content and gas saturation in Daning-Jixian Block
浅层 深层 井号 含气量/(m3·t−1) 含气饱和度/% 深度/m 井号 含气量/(m3·t−1) 含气饱和度/% 深度/m J1 9.14 51.78 1 144.8 DJ1 21.31 87.90 2 118 J2 9.42 58.48 1 139.3 DJ 2 23.45 99.98 2 276 J3 11.70 49.41 1 145.0 DJ 3 28.50 100.00 2 173 J4 14.21 78.80 1 176.0 DJ 4 29.59 100.00 2 143 J5 14.76 82.50 1 055.0 DJ 5 26.24 98.47 2 166
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表 4 大宁−吉县区块煤岩柱样核磁共振测试结果
Table 4 Nuclear magnetic resonance test results of coal and rock cores
样品编号 深度/m 样品类型 总孔隙率/% 水占孔隙率/% 含水饱和度/% 含气饱和度/% 23 2278.15~2278.40 柱样 5.73 3.90 68.14 31.86 25 2278.66~2278.95 柱样 5.50 2.20 40.02 59.98 27 2279.04~2279.29 柱样 5.72 3.92 68.50 31.50 29 2279.44~2279.68 柱样 4.14 3.78 61.26 38.74
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表 5 深层煤层气“地质−工程”甜点区划分指标
Table 5 Classification index of deep CBM “geological-engineering” sweet spots
地质甜点区 工程甜点区 指标 Ⅰ类 Ⅱ类 指标 Ⅰ类 Ⅱ类 构造 构造平缓带，
地层倾角小于3°构造陡坡带，
地层倾角3°~8°顶板岩性 泥岩+薄灰岩 厚灰岩 煤层厚度/m >6 >6 底板岩性 脆性指数小于30的
泥岩、灰岩脆性指数大于30的泥岩、
砂质泥岩、砂岩埋深/m >2000 1500~2000 顶板隔层应力差/MPa >6 <6 含气量/(m3·t−1) >16 <16 底板隔层应力差/MPa >6 <6 气测峰值/% >80 <80 可压性(煤体强度、
裂隙发育程度)割理裂隙发育 割理裂隙发育差 录井显示 槽面有气泡
液面
上涨2 cm以上槽面有气泡
液面
上涨小于2 cm可改造性(煤储层
矿物质种类及含量)填充物溶蚀率高 不含填充物或填充物溶蚀率低
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