Carbon and Hydrogen Isotopic Compositions and Their Evolutions of Gases Generated by Forest Marsh Peat at Different Thermal Maturity Stages
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摘要: 煤层气的成因是石油地质学研究的热点.煤层气聚集存在着"累积聚气"和"阶段聚气"两种形式, 对于"阶段聚气" 的煤层气成因判识的地球化学研究还很薄弱.通过森林沼泽泥炭在不同温度下制备的样品进行热模拟实验, 首次获得了不同演化阶段甲烷、乙烷和二氧化碳的碳、氢同位素组成和演化规律.发现随着原始样品演化程度越高, 生成的甲烷和乙烷的碳、氢同位素组成具有变重的趋势; 同时, 甲烷和乙烷碳同位素组成明显地受原始样品演化程度的影响, 而氢同位素组成主要与成熟度密切相关.确定了成煤有机质在不同演化阶段生成的气体碳、氢同位素组成.首次获得了成煤有机质不同演化阶段热解气体碳、氢同位素组成与Ro之间的关系式.建立了甲烷与乙烷的碳、氢同位素之间的关系式, 形成了甲烷碳、氢同位素组成相关图.根据这些为研究不同成熟度区间生成的煤层气成因提供了科学数据, 为"阶段聚气"的煤层气地球化学特征认识及其成因判识提供了科学依据.并且, 将这些研究结果应用到我国沁水盆地南部煤层气研究, 认为该地区煤层气是在中侏罗世以后聚集而成, 具有"阶段聚气"的特征, 证明了热模拟研究成果对自然界煤层气成因的判识具有重要的科学意义.Abstract: Coalbed methane (CBM) accumulation models include continuous gas accumulation and staged gas accumulation. However, the study on geochemistry characteristics and evaluation index of staged accumulation CBM is inadequate. This study has obtained the carbon and hydrogen isotopic compositions and their evolution laws of methane and ethane generated at different evolution stages by thermal simulation of samples prepared using a forest marsh peat at different temperatures. The data show that the carbon and hydrogen isotopic compositions tend to increase heavy isotope with increasing evolution level of the starting sample. At the same time, it is found that the maturity level of the starting sample influences the carbon isotopic compositions of methane and ethane, while maturity of gases affects their hydrogen isotopic compositions. The relationship between Ro values and the carbon and hydrogen isotopic compositions of gases generated by coal-forming organic matter at different evolution stages as well as the carbon or hydrogen isotopic relationship between methane and ethane are established. These results provide scientific evidence for studying the genesis of CBM generated at different maturity intervals and understanding the geochemistry characteristics of staged accumulation CBM, and they were applied to study on CBM from the southern Qinshui basin, and it is found that it accumulated after Middle Jurassic and is characterized by staged gas accumulation, which indicates that the results of thermal simulation experiments are very important for judging the genesis of natural CBM.
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图 1 甲烷(a)、乙烷(b)和二氧化碳(c) 的δ13C值与热解温度的关系
Fig. 1. δ13C of CH4 (a), C2H6 (b) and CO2 (c) vs. pyrolysis temperature
图 2 甲烷(a)和乙烷(b)的δD值与热解温度的关系
Fig. 2. δD of CH4 (a) and C2H6 (b) vs. pyrolysis temperature
图 6 甲烷与乙烷δ13C值(a)和甲烷与乙烷δD值(b)相关图
Fig. 6. δ13C of CH4 vs. C2H6 (a) and δD of CH4 vs. C2H6 (b)
表 1 不同Ro区间生成的甲烷、乙烷和二氧化碳平均δ13C与δD值
Table 1. Average δ13C and δD of methane, ethane and CO2 generated from peat of different Ro intervals
样品 Ro(%) δ13CCH4 δDCH4 δ13CCO2 δ13CC2H6 δ13CC3H8 δDC2H6 250~650 ℃(C3t) 0.8~4.8 -34.7 -223 -20.1 -29.6 -26.7 -203.5 300~650 ℃(C3t) 1.1~4.8 -33.1 -208 -19.2 -28.5 -24.8 -196.8 350~650 ℃(C3t) 1.9~4.8 -30.7 -189 -19.0 -24.3 -23.9 -152.3 400~650 ℃(C3t) 2.3~4.8 -26.4 -167 -14.7 -15.6 -125.0 450~600 ℃(C3t) 3.1~4.2 -26.4 -167 -17.3 
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