Abstract To better understand the law of gas generation, migration and accumulation of the No. 3 coal seam in the Zhengzhuang (ZZ) area, Southern Qinshui Basin, North China, isotopic geochemical experiments, including determination of the molecular and isotopic compositions of coalbed gas and the hydrogen and oxygen isotope decompositions of the coalbed methane (CBM) co-produced water, as well as the geological and hydrogeological surveys, were conducted. The results reveal that CBM from the No. 3 coal seam is dominated by CH4 (46.5–99.54 vol%) and the carbon isotope ratio of the produced CH4 (δ13C1) ranges from −28.7‰ to −57.2‰. Secondary thermogenic CH4 caused by a tectonic thermal event in the Yanshan Epoch is the main gas source, with a small amount of secondary biogenic gas generated from methyl-type fermentation. The carbon isotope is fractionated due to the terrain change of the No. 3 coal seam, a discrepancy in internal structures of basin and the flushing of the groundwater. Geological and hydrogeological information shows that fluid pressure traps for gas preservation will easily form in the northern and eastern deep stagnation areas, or local syncline in the central part. Based on the experimental data and CBM well data, the gas content and gas saturation have a logarithmic correlation with δ13C1. These investigations may serve the exploration of favorable zones for gas accumulation and exploitation of CBM with the geochemistry and carbon isotope composition of CH4 in the study area.

中文翻译：

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沁水盆地南部郑庄地区CH4碳同位素特征及其对煤储层气性的意义

摘要 为更好地认识华北沁水盆地南部郑庄（ZZ）地区3号煤层天然气的生成、运移和聚集规律，开展了同位素地球化学实验，包括煤层气分子和同位素组成的测定。并对煤层气联产水进行氢氧同位素分解及地质、水文地质调查。结果表明，3号煤层煤层气以CH4为主（46.5～99.54vol%），产出CH4（δ13C1）碳同位素比为-28.7‰～-57.2‰。燕山期构造热事件引起的次生热成因 CH4 是主要气源，少量次生生物气由甲基型发酵产生。由于3号煤层地形变化、盆地内部构造差异和地下水冲刷，碳同位素分馏。地质和水文地质资料表明，北部和东部深部滞留区或中部局部向斜容易形成储气流体压力圈闭。根据实验数据和煤层气井数据，含气量和含气饱和度与δ13C1呈对数相关。这些调查可用于探索研究区CH4地球化学和碳同位素组成的煤层气成藏和开采有利区带。地质和水文地质资料表明，北部和东部深部滞留区或中部局部向斜容易形成储气流体压力圈闭。根据实验数据和煤层气井数据，含气量和含气饱和度与δ13C1呈对数相关。这些调查可用于探索研究区CH4地球化学和碳同位素组成的煤层气成藏和开采有利区带。地质和水文地质资料表明，北部和东部深部滞留区或中部局部向斜容易形成储气流体压力圈闭。根据实验数据和煤层气井数据，含气量和含气饱和度与δ13C1呈对数相关。这些调查可用于探索研究区CH4地球化学和碳同位素组成的煤层气成藏和开采有利区带。