郑庄-胡底煤层气地球化学特征及成因探讨

Analysis of geochemistry characteristics and its origin of CBM in Zhengzhuang and Hudi blocks

结合沁水盆地南部地质构造史和热演化过程,从甲烷碳同位素与煤岩镜质组反射率、甲烷碳与氢同位素、甲烷与水的氢同位素值定量关系3方面探讨了郑庄和胡底区块煤层气成因,结果表明:煤层气组分中,甲烷体积分数占绝对优势,为96.83%~98.55%,为极干燥气体;甲烷δ13C1和δD分别为-33.1‰^-30.8‰和-179.32‰^-160.53‰;煤层气主要为经次生改造的热解成因煤层气。煤层气δ13C1实测值与推测值-27.89‰相比明显偏轻,其原因是燕山期异常古地温阶段,横向地下水循环和竖向热液循环过程中流体优先溶解13CH4产生的甲烷碳同位素分馏作用的结果。15号煤抽采煤层气甲烷氢同位素值与伴生水氢同位素值定量关系表明,由于CO2还原次生生物作用造成3号煤和15号煤抽采煤层气甲烷δD的差异。

Based on the history of geological structure and geothermal evolution in southern Qinshui Basin,this study investigated the origins of coalbed methane(CBM) in Zhengzhuang and Hudi blocks from the aspects of the relationship between carbon isotope and hydrogen isotope of methane,the empirical relationship between carbon isotope of methane and vitrinite reflectance of coal,the quantitative relationship between δD( CH4)and δD(H2O). The results reveal that the content of methane is predominant,from 96. 83% to 98. 55%,and is extremely dry gas. The contents ofδ13C1and δD(CH4)is-33.1‰ to-30.8‰ and-179.32‰ to-160.53‰,respectively. The relationship between carbon isotope and hydrogen isotope of methane indicate that the CBM is the thermogenic gas reconstructed by secondary role. The contents of δ13C1is much lower than the calculated value(- 27. 89%) by empirical formula. The main reason can be defined as the effect of the carbon isotope fractionation of methane due to the priority dissolved13CH4 inunderground fluid during the process of lateral groundwater circulation and vertical hot fluid circulation in the Yanshan period of abnormal paleogeothermal. The quantitative relationship between δD( CH4)of No.15 coal and δD(H2O) of the associated formation water indicate that the differences between the δD( CH4)value of the methane of No.3 and No. 15 are due to the effect that thermogenic gas mixed with secondary microbial gas generated by carbon dioxide.