Effects of Hydrocarbon Generation on the Occurrence of Organic Nanopores during Thermal Maturity of Organic Matters
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摘要: 纳米有机孔隙的发育情况在一定程度上关系到非常规天然气藏的经济效益，但有机质生烃对其形成演化的影响尚需深入研究，本文利用封闭热‒压模拟实验系统对鄂尔多斯盆地二叠系低熟腐殖煤样进行了生烃热模拟实验.基于TOC、岩石热解、气体吸附和扫描电镜观测等结果，刻画了低熟‒过熟不同阶段，煤样内不同类型纳米孔隙形成演化特征，探讨了有机质生烃对有机孔隙发育的影响及其程度.结果显示：低熟‒成熟干酪根初次裂解阶段，有机质颗粒内纳米有机孔隙不发育；高熟阶段中期‒过熟阶段早期，烃类二次裂解导致不同类型有机孔隙大量发育；过熟阶段中‒晚期，芳环缩聚反应促进了有机孔隙发育，尤其是孔径小于2 nm的有机孔隙强烈发育.可见，高演化阶段烃类二次裂解和芳环缩聚反应是有机质颗粒内有机孔隙形成的主要途径.该研究对深入理解沉积盆地深层页岩气和煤层气富集机理具有指导意义.Abstract: The development of organic nanopores is to some extent related to the economic benefits of unconventional natural gas reservoirs, but the influence of organic hydrocarbon generation on its formation and evolution needs in-depth study. In this paper, hydrous pyrolysis experiments were conducted on a Permian humic coal with low thermal maturity from the Ordos basin. The experimental results of TOC, Rock-Eval, gas adsorption and scanning electron microscope observation for the fresh and cocked coal samples are utilized to delineate the formation and evolution process of various nanopores within the fresh and cocked coal samples, and hence achieve a better understanding of the effects of hydrocarbon generation on the occurrence of organic nanopores from the low mature to overmature stages. The experimental results show that organic nanopores within the organic particles of pyrolyzed coal samples cannot be detected at the low mature to mature stages during which petroleum hydrocarbons were generated mainly by kerogen primary cracking. At the middle of high mature to the early of overmature stages, hydrocarbon generation from the secondary cracking of early formed crude oils resulted in the significant occurrence of various organic nanopores; At the middle to late of overmature stage, the polycondensation of aromatic rings within coal samples promoted the intensive formation of organic nanopores, in particular those with pore size < 2 nm. Accordingly, hydrocarbon secondary cracking and the polycondensation of aromatic rings within the pyrolyzed coals at high thermal maturity levels play a major role in the formation of organic nanopores. This study should be helpful to improving our understanding of the enrichment mechanisms for the deeply buried shale gas and coalbed methane.
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图 1 煤样抽提前与抽提后TOC与热解参数分布直方图
Fig. 1. The distribution histograms of TOC and Rock-Eval parameters of the coal samples before and after organic solvent extraction
图 2 原始煤样和热解样品不同相对压力下的CO2（a~c）和N2（b~f）吸附量
Fig. 2. The gas adsorption capacity of CO2 (a‒c) and N2 (b‒f) of the original and pyrolyzed coal samples at the various P/P0
图 3 原始煤样和热解煤样气体吸附量与Ro（a）和抽提后TOC（b）交汇图
Fig. 3. The crossplots showing the correlations between gas adsorption capacity of the original and pyrolyzed coal samples and Ro (a), TOC (b) with organic solvent extraction
图 4 原始煤样与热解煤样经索氏抽提后的孔径分布
Fig. 4. Pore size distribution of the original and pyrolyzed coal samples with organic solvent extraction
图 5 原始煤样与热解煤样平均孔径与Ro的相关关系
Fig. 5. The crossplots showing the correlations between average pore size of original and pyrolyzed coal samples and Ro
图 6 有机质熟化过程中原始煤样和热解煤样比表面积和孔体积分布
Fig. 6. Pore volumes and specific surface areas of original and pyrolyzed coal samples during thermal maturity of organic matters
图 7 原始煤样和热解煤样纳米孔隙扫描电镜观测
Fig. 7. Scanning electron microscope observation of nanopores within original and pyrolyzed coal samples
图 8 原始煤样和热解煤样BJH孔体积(a)和BET比表面积(b)与TOC交汇图
Fig. 8. The crossplots showing the correlations between TOC and BJH pore volume (a) and BET specific surface area (b) of original and pyrolyzed coal samples
表 1 原始煤样和热解煤样抽提前后TOC与岩石热解参数
Table 1. The TOC and Rock-Eval parameters of the original and pyrolyzed coal samples before and after organic solvent extraction
样品名称 实验温度(℃) 加热时间(h) 成熟度Ro (%) TOC(%) S1(mg/g) S2(mg/g) Tmax (℃) HI(mg/g TOC) 演化阶段 抽提前 抽提后 抽提前 抽提后 抽提前 抽提后 抽提前 抽提后 抽提前 抽提后 W0 0.60 39.30 38.00 0.11 3.40 15.81 17.25 448 449 40.23 45.39 低熟 W1 350 16.8 1.06 54.10 50.30 2.72 3.03 23.76 14.57 470 489 43.92 28.97 成熟 W2 350 48.0 1.38 57.60 53.30 11.64 3.26 35.88 14.06 507 531 62.29 26.38 高熟 W3 400 25.2 1.76 51.10 50.60 18.00 1.50 23.35 5.63 556 564 45.69 11.13 高熟 W4 400 84.0 1.88 55.80 53.30 8.60 1.73 7.85 3.50 568 567 14.07 6.57 高熟 W5 450 12.0 2.09 48.90 45.70 8.16 2.14 5.52 6.17 578 578 11.29 13.50 过熟 W6 450 24.0 2.29 59.40 53.60 9.99 1.80 3.94 2.66 580 575 6.63 4.96 过熟 W7 450 72.0 2.47 60.70 56.00 17.32 0.78 7.86 0.57 586 577 12.95 1.02 过熟 W8 500 12.0 2.70 57.50 54.20 10.25 1.16 2.85 0.87 367 396 4.96 1.61 过熟 W9 500 72.0 3.00 61.60 56.30 8.90 1.32 1.23 0.27 339 439 2.00 0.48 过熟 W10 550 24.0 3.67 59.90 55.80 2.70 1.39 0.40 0.18 414 420 0.67 0.32 过熟 W11 600 24.0 4.01 62.60 58.20 0.54 5.11 0.52 0.58 332 324 0.83 1.00 过熟 
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表 2 原始煤样和热解煤样抽提后气体吸附测试结果
Table 2. The gas adsorption analysis experimental results of original and pyrolyzed coal samples with organic solvent extraction
样品名称 Ro(%) 气体吸附量(cm3/g) 平均孔径(nm) 孔体积(cm3/g) 比表面积(m2/g) CO2 N2 总孔 微孔 介孔 宏孔 总孔 微孔 介孔 宏孔 W0 0.60 16.41 7.13 12.24 0.013 0.000 6 0.004 3 0.007 7 3.94 1.95 1.74 0.25 W1 1.06 19.76 8.62 20.36 0.022 0.000 6 0.007 0 0.014 9 4.61 1.72 2.41 0.47 W2 1.38 21.88 17.40 14.82 0.031 0.000 6 0.013 5 0.017 0 8.18 1.54 6.04 0.60 W3 1.76 19.60 35.47 11.54 0.087 0.004 3 0.034 4 0.048 2 30.24 9.73 18.86 1.65 W4 1.88 21.70 28.58 8.08 0.081 0.005 5 0.039 3 0.036 5 39.58 12.94 25.41 1.22 W5 2.09 18.80 21.41 17.58 0.077 0.002 0 0.032 3 0.042 7 18.56 5.45 11.61 1.50 W6 2.29 22.14 27.87 20.21 0.088 0.001 5 0.030 2 0.056 0 17.20 3.94 11.37 1.89 W7 2.47 23.89 31.51 11.46 0.094 0.004 8 0.041 8 0.048 0 38.89 10.76 26.48 1.65 W8 2.70 25.70 24.77 5.62 0.151 0.017 3 0.081 2 0.052 7 115.88 38.69 75.40 1.79 W9 3.00 28.61 56.42 2.57 0.140 0.044 8 0.041 7 0.053 2 166.95 135.08 30.16 1.71 W10 3.67 32.20 135.22 2.65 0.171 0.046 4 0.038 3 0.086 8 175.30 146.91 25.84 2.55 W11 4.01 36.74 110.98 2.14 0.162 0.071 3 0.030 8 0.059 7 256.11 230.12 24.29 1.70 注：孔径 < 2 nm为微孔，孔径在2~50 nm为介孔，孔径在50~300 nm为宏孔.
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