Abstract Gas and water geochemistry in the No.2 coal reservoir of the Yanchuannan CBM block in the southeastern Ordos basin reflects a complex interaction of tectonic, hydrodynamic, thermogenic, and biogenic processes. Stable isotope and component analyses of gas and water samples from the No. 2 coal seam of 17 CBM wells were performed to determine the correlations between stable isotopes of gases and co-produced water, and to understand the controls of structure and hydrology on isotope distribution. The results show that the co-produced water continuously ranges from brackish, isotopically lighter Na-HCO3/Na-HCO3-Cl water to hypersaline, isotopically heavier Na-Cl/Na-Ca-Cl brine. Based on the hydrochemical features, fault development, and burial depth of coal seam, four structure-controlled hydrodynamic zones are distinguished, including strong through-flow, medium through-flow, weak through-flow, and stagnant zones. The gas isotope analysis shows that thermogenic methane is the dominant source of CBM, and secondary biogenic gases only exist in the strong through-flow zone. δ2H-CH4 is ultimately inherited from formation water, and more 2H can be transferred during the formation of thermogenic gases. δ13C-CH4 and δ13C-CO2 are mainly controlled by thermal maturity and gas genesis respectively, and there is a negative correlation between them. The stable isotopes of coalbed gases and water show strong spatial heterogeneity with the changing hydrodynamic strength. Owing to the influence of water recharge, gas leakage, and secondary biogas formation, the strong through-flow zone is characterized by the most negative δ13C-CH4, δ2H-CH4, δ18O-H2O, δ2H-H2O and the most positive δ13C-CO2, Δ2H-(H2O-CH4) and Δ13C-(CO2-CH4). However, the stagnant zone has maintained stable in a long geological period, which leads to the high concentration of original connate water and mass preservation of coalbed gases, and hence its isotopic feature is completely opposite to the strong through-flow zone. This study confirms the significant controls of hydrogeological conditions on the migration of water and gas and the application of isotopic indicators in the exploration of deep CBM.

中文翻译：

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鄂尔多斯盆地东南部延川南区块煤层气与联产水结构水文控制同位素耦合与非均质性

摘要 鄂尔多斯盆地东南部延川南煤层气区块2号煤储层气水地球化学反映了构造、水动力、热成因和生物成因过程的复杂相互作用。对17口煤层气井2号煤层气水样品进行稳定同位素和成分分析，确定气体稳定同位素与联产水的相关性，了解结构和水文对同位素分布的控制. 结果表明，联产水不断变化，从微咸、同位素较轻的 Na-HCO3/Na-HCO3-Cl 水到高盐度、同位素较重的 Na-Cl/Na-Ca-Cl 盐水。根据煤层水化学特征、断层发育、埋深等特征，划分出4个构造控制的水动力带，包括强通流、中通流、弱通流和滞流区。气体同位素分析表明，热成因甲烷是煤层气的主要来源，次生生物气仅存在于强穿流区。δ2H-CH4 最终继承自地层水，在热成因气体形成过程中可以转移更多的 2H。δ13​​C-CH4和δ13C-CO2分别主要受热成熟度和气成因控制，两者呈负相关。煤层气和水的稳定同位素随着水动力强度的变化表现出强烈的空间非均质性。受补水、漏气和二次沼气形成的影响，强通流带的特征是δ13C-CH4、δ2H-CH4、δ18O-H2O最负，δ2H-H2O 和最正的 δ13C-CO2、Δ2H-(H2O-CH4) 和 Δ13C-(CO2-CH4)。但滞流带在较长的地质时期保持稳定，导致原始原生水浓度高，煤层气大量保存，因此其同位素特征与强穿流带完全相反。本研究证实了水文地质条件对水气运移的显着控制以及同位素指标在深层煤层气勘探中的应用。