Impact of Formation Environment of Coal-Forming Material on Hydrogen and Carbon Isotopic Compositions of Thermogenic Coalbed Gas:Thermal Simulation of Herbaceous Marsh Peats Formed under Different Climatic Environments
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摘要: 煤层气的成因研究可以为煤层气勘探与开发提供科学依据，然而，煤层气的氢碳同位素组成受多种因素的影响，以前较多的研究是成气母质性质和成熟度对煤层气氢碳同位素的影响，对于成煤物质形成的气候环境对热解煤层气同位素的影响尚不清楚.热解模拟了高纬度寒冷干旱和低纬度热带湿润环境的草本泥炭，对热解烃类气体的氢碳同位素组成及其差异性进行了研究.研究结果表明：与低纬度热带湿润环境中形成的草本泥炭相比较，高纬度寒冷干旱环境的草本泥炭热解甲烷、乙烷和丙烷具有轻的氢同位素组成和重的碳同位素组成，并且从泥炭连续热解至Ro分别为2.5%、3.5%和5.5%时，甲烷、乙烷和丙烷δD值分别平均降低-17‰~-10‰、-32‰~-28‰和-25‰~-17‰，甲烷和乙烷δ13C值分别平均升高2.9‰~3.6‰和0.9‰~1.1‰.认为这种同位素差异起因于气候环境对形成泥炭的植物氢碳同位素组成的影响.建立了高纬度寒冷干旱和低纬度热带湿润环境中形成的成煤有机质热解烃类气体氢碳同位素组成与Ro之间的关系式，同时也建立了烃类气体的碳和氢同位素之间的关系式.这些研究成果为不同气候环境下形成的成煤有机质生成的煤层气成因研究提供了科学依据.Abstract: The study of coalbed methane (CBM) can provide scientific basis for coalbed methane exploration and development. However, the hydrogen and carbon isotopic compositions of CBM are affected by many factors. Previous studies were mainly concerned with the influence of the properties of gas-forming parent material and maturity on the hydrogen and carbon isotopic compositions of CBM. The influence of formation environment of gas-forming parent material on the isotopic compositions of thermogenic CBM is still unclear. Closed-system isothermal pyrolysis experiments were performed on herbaceous marsh peats derived from high latitude area with cold and dry climates and low latitude area with tropical moist climate. The hydrogen and carbon isotopic compositions and their differences of the hydrocarbon gases (methane, ethane and propane) generated during the pyrolysis of the samples were studied. The results show that the hydrocarbon gases generated from the herbaceous swamp peat in high latitude area had lighter hydrogen and heavier carbon isotopic compositions compared with those in low latitude area. At pyrolysis intervals from peat to vitrinite reflectance values (Ro) of 2.5%, 3.5% and 5.5%, the differences in the average δD values between the samples in high latitude and low latitude areas were from -17‰ to -10‰ for methane, -32‰ to -28‰ for ethane and -25‰ to -17‰ for propane, and in the average δ13C values between them were 2.9‰ to 3.6‰ for methane and 0.9‰ to 1.1‰ for ethane, respectively. The differences should result from the influence of climatic environment on hydrogen and carbon isotopic compositions of coal-forming original material. The relationship between Ro values and the hydrogen and carbon isotopic compositions of gases generated by coal-forming organic matter under different climatic environments as well as the hydrogen or carbon isotopic relationships of methane and ethane were established. These results may provide a basis for studying on the genesis of coalbed gas formed by coal-forming material under different climatic environments.
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Key words:
- peat /
- simulation experiment /
- gas product /
- hydrogen and carbon isotopes /
- climatic environment /
- coalbed gas origin /
- geochemistry
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图 1 甲烷(a)、乙烷(b)δD值与热解温度的关系
Fig. 1. δD of CH4 (a) and C2H6 (b) vs. pyrolysis temperature
图 4 甲烷δD值与δ13C值(a)和乙烷δD值与δ13C值(b)的关系
Fig. 4. δD vs. δ13C of CH4 (a) and δD vs. δ13C of C2H6 (b)
表 1 样品参数
Table 1. Parameters of the samples
地区 样号 样品 海拔(m) 气候 TOC (%) 雨水δD值(‰) 年均降雨量(mm) 年均蒸发量(mm) 年均气温(℃) 阿尔泰 Alt 草本泥炭 2 560 寒冷干旱 32.3 -100 350~600 1 816 -3.8~1.8 湛江 Zwx-3 草本泥炭 28 热带湿润 34.5 -53 1 393~1 798 1 700~2 200 23.4 
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表 2 不同演化阶段烃类气体平均氢同位素组成
Table 2. Average δD value of hydrocarbon gas generated from peats at different evolution stages
温度(℃) Ro(%) δDCH4 (‰) δDC2H6 (‰) δDC3H8 (‰) δDCH4 (‰) δDC2H6 (‰) δDC3H8 (‰) ΔCH4 (‰) ΔC2H6 (‰) ΔC3H8 (‰) Alt Alt Alt Zwx-3 Zwx-3 Zwx-3 Alt-Zwx-3 Alt-Zwx-3 Alt-Zwx-3 250~400 ＜2.5 -337.1 -261.7 -248.4 -326.8 -233.4 -231.2 -10 -28 -17 250~500 ＜3.5 -310.4 -225.7 -229.3 -297.9 -194.0 -204.4 -13 -32 -25 250~650 ＜5.5 -257.9 -225.7 -229.3 -241.4 -194.0 -204.4 -17 -32 -25
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表 3 不同演化阶段烃类气体平均碳同位素组成
Table 3. Average δ13C value of hydrocarbon gas generated from peats at different evolution stages
温度(℃) Ro(%) δ13CCH4 (‰) δ13CC2H6 (‰) δ13CC3H8 (‰) δ13CCH4 (‰) δ13CC2H6 (‰) δ13CC3H8 (‰) ΔCH4 (‰) ΔC2H6 (‰) Alt Alt Alt Zwx-3 Zwx-3 Zwx-3 Alt-Zwx-3 Alt-Zwx-3 250~400 ＜2.5 -41.4 -31.9 -32.6 -44.3 -32.8 -31.0 2.9 0.9 250~500 ＜3.5 -38.8 -30.1 -28.2 -42.4 -31.2 -28.0 3.6 1.1 250~650 ＜5.5 -36.2 -30.1 -28.2 -39.6 -31.2 -28.0 3.4 1.1
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