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王芳芳,吴财芳,杜明洋,等. 雨汪区块煤层气气体同位素变化特征及成因判识[J]. 煤炭科学技术,2024,52(7):148−157. doi: 10.12438/cst.2023-1253
引用本文: 王芳芳,吴财芳,杜明洋,等. 雨汪区块煤层气气体同位素变化特征及成因判识[J]. 煤炭科学技术,2024,52(7):148−157. doi: 10.12438/cst.2023-1253
WANG Fangfang,WU Caifang,DU Mingyang,et al. Gas isotope variation characteristics and genesis identification of coalbed methane in Yuwang Block[J]. Coal Science and Technology,2024,52(7):148−157. doi: 10.12438/cst.2023-1253
Citation: WANG Fangfang,WU Caifang,DU Mingyang,et al. Gas isotope variation characteristics and genesis identification of coalbed methane in Yuwang Block[J]. Coal Science and Technology,2024,52(7):148−157. doi: 10.12438/cst.2023-1253

雨汪区块煤层气气体同位素变化特征及成因判识

Gas isotope variation characteristics and genesis identification of coalbed methane in Yuwang Block

  • 摘要: 研究煤层气地球化学特征及其成因,对煤层气成藏及资源勘查开发具有重要意义。为探究滇东雨汪区块煤层气成因及其地球化学特征,采集该区6口典型煤层气井的产出气气样,开展了组分测定、碳同位素和稀有气体同位素测试,系统分析了煤层CH4、N2等气体的成因,并结合水文地质环境,探讨了雨汪区块煤层气排采过程中气体的同位素变化及其控制因素。结果表明:雨汪区块煤层气井产出气中CH4体积分数平均为73.95%~92.90%,乙烷、CO2、N2体积分数分别为0.20%~0.40%、0.05%~1.05%、6.15%~25.32%。此外,He仅在产气第1天检测到,而碳原子数大于2的烃类未检测到;煤层气干燥系数(C1/C1~5)为0.999 5~0.999 7,属于高成熟干气;CH4δ13C值为−36.7‰~−27.6‰,为有机质热成因气;CO2δ13C值介于−19.1‰~−6.6‰,主体分布在−18.0‰~−14.3‰,为有机成因;氮气体积分数平均为12.63%,含量相对较高,其φ(N2)/φ(Ar)远大于84,以有机成因为主;氩气体积分数平均为0.06%,远大于0.017%,以大气成因为主;氦气体积分数平均为0.02%,φ(3He)/φ(4He)为0.29×10−8~19.6×10−8,主体范围为2.00×10−8~9.38×10−8φ(3He)/φ(4He)与大气氦的比值介于0.01~0.09,属于典型的壳源氦气,且φ(4He)、φ(20Ne)的变化趋势与φ(4He)/φ(20Ne)具有一致性;随着排采时间增加,气体纯度增高,产出气的烷烃同位素值呈现逐渐变轻的趋势。研究认为,雨汪区块煤层气“变干、变轻”是气体解吸、扩散和水溶解作用综合影响的结果。

     

    Abstract: The research on the geochemical characteristics and genesis of coalbed methane (CBM) is of great significance to the formation and resource exploration-development. In order to investigate the genesis and geochemical characteristics of CBM in the Yuwang Block in eastern Yunnan, the gas samples from six typical CBM wells in the area were collected, and the composition determination, carbon isotope and rare gas isotope tests were carried out. The genesis of CH4, N2 and other gases in coal seam were systematically analyzed. Combined with the hydrogeological environment, the isotope variation of gases and their controlling factors during the CBM drainage process in Yuwang Block were investigated. The results shown that, the volume fraction of CH4 in the gas produced from CBM wells in the Yuwang Block averaged 73.95%-92.90%, and those of ethane, CO2 and N2 were 0.20% - 0.40%, 0.05% - 1.05% and 6.15% - 25.32%, respectively. In addition, helium was only detected on the first day of gas production, while hydrocarbons with a carbon atom number greater than 2 were not detected. The drying coefficient (C1/C1 ~ 5) of coalbed methane was 0.999 5 - 0.999 7, which belonged to high mature dry gas. The δ13C value of CH4 was −36.7‰ - −27.6‰, which was an organic matter thermogenic gas. The δ13C value of CO2 was −19.1‰- −6.6‰, with the main distributed from −18.0‰ to −14.3‰, which was of organic origin. The volume fraction of nitrogen averaged 12.63%, with relatively high content, and its φ(N2)/φ(Ar) value was much larger than 84, dominated by organic genesis. The average content of argon was 0.06%, much larger than 0.017%, dominated by atmospheric genesis. The volume fraction of helium averaged 0.02%, φ(3He)/φ(4He) was 0.29×10−8 - 19.6×10−8, with the main distributed from 2.00×10−8 to 9.38×10−8, the ratio of φ(3He)/φ(4He) to atmospheric helium was 0.01 ~ 0.09, which was typical shell source helium, and the variation trends of φ(4He) and φ(20Ne) were consistent with φ(4He)/φ(20Ne). The alkane isotope values of the produced gas shown a tendency to become progressively lighter as the increasing discharge time and gas purity. The study concluded that the “drying and lightening” of CBM in Yuwang Block was the result of the comprehensive influence of gas desorption, diffusion and water dissolution.

     

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