山西古交矿区煤层气组成特征及成因探讨

Origins and molecular composition of coalbed methane in Gujiao area,Shanxi

山西古交矿区煤层气资源丰富,是我国煤层气重点开发区块之一。区内地质条件复杂,煤层气地质研究起步晚,尤其是对煤层气组成特征及成因方面的讨论还较匮乏,制约了煤层气开发进程。煤层气组成特征及成因与煤层气母质性质、生成机理、运移和逸散规律等密切相关,对于煤层气保存条件和分布规律的认识和煤层气资源评价具有重要意义。为揭示古交矿区煤层气化学组成特征及气体成因,采集矿区内10口煤层气井口排采气样品,开展了气体化学组分和碳、氮同位素检测,分析了煤层气中甲烷及氮气的成因;结合构造演化背景、构造形态、水文地质条件和煤芯解吸气化学组成,讨论了煤层气富氮气机理及保存条件。结果表明:古交矿区内煤层气中氮气含量偏高,为0.86%~14.13%,平均6.10%,甲烷含量在83.79%~97.57%,平均91.33%,乙烷含量在0~0.46%,平均0.09%,不含2个碳原子以上的烃类,二氧化碳含量在1.47%~4.71%,平均2.48%;煤层气属于极干气体,干燥系数(C1/C1~5)为0.9949~1,甲烷δ13C值在-47.13‰^-39.26‰,平均值为-44.03‰,为有机质热解成因;氮气δ15N值在-1.16‰^-0.51‰,平均值为-0.80‰,为大气与有机质热降解混合成因,且以大气来源为主,有机成因氮气含量很少;地表水下渗与煤层发生相互作用,地表水携带的大气与煤层气发生组分交换,导致煤层气解吸、逸散,煤层含气量降低、甲烷δ13C值降低、氮气含量升高。

Gujiao area,Shanxi,is one of the main coalbed methane development area in China due to its rich resource of coalbed methane.However,the coalbed methane development has been limited by its complex geological condition and a low level of research,especially a low level of research in the chemical composition and origin of coalbed methane.The chemical composition and origin of coalbed methane are closely related to its parent material properties,generation mechanism and dissipation law,and has significance for uncovering its preserving condition,distribution and resource evaluation.In order to reveal the chemical composition and origin of coalbed methane (CBM),ten CBM samples were collected from Gujiao area,Shanxi,China,and then their molecular and isotopic compositions were analyzed to study the origins of methane and nitrogen.Tectonic evolution history,structural form,hydrogeology condition and desorbed gas composition were combined to study nitrogen enrich mechanism and gas-preserving condition.Results indicate that the CBM is composed of 83.79%-97.57% methane,0.86%-14.13% nitrogen,1.47%-4.71% carbon dioxide and minor amounts of ethane.The δ 13 C values of methane range from -47.13‰ to -39.26‰,with an average of -44.03‰,and methane was generated from the pyrolysis of organic matter.The δ 15 N values of nitrogen range from -1.16‰ to -0.51‰,with an average of -0.80‰,and nitrogen was mainly generated from the entraining of atmosphere.All low gas content and δ 13 C value of methane,and high nitrogen proportion were the results of groundwater flow.