鄂尔多斯盆地东缘煤层气中氮气的地球化学研究

Geochemical study on the molecular nitrogen in the coal bed methane from eastern Ordos basin

氮气(N_(2))作为煤层气的重要成分,其地球化学特征对煤层气勘探与开发具有一定的指导意义。以鄂尔多斯盆地东缘地区煤层气为研究对象,对煤层气中N_(2)的地球化学特征、影响因素及成因进行分析和探讨。鄂尔多斯盆地东缘煤层气中N_(2)的体积分数较低,平均为3.33%,含气量平均值为0.119 m^(3)/t,δ^(15)N平均为+4.470‰。煤化程度对煤层气中N_(2)的地球化学特征有显著影响。鄂尔多斯盆地东缘由北向南(保德地区—柳林-三交地区—韩城地区),煤化程度逐渐升高,相应煤层气中N_(2)的含气量呈现逐渐升高的趋势,这主要是煤化过程中N_(2)不断生成并积累所致;N_(2)的体积分数呈现先降低后增高的趋势,这是因为柳林-三交地区在早白垩世经受岩浆的烘烤作用,导致该区煤层气中N_(2)体积分数偏低而CH_(4)体积分数偏高;氮气的同位素组成具有先变轻后变重的趋势,岩浆活动引发的烘烤作用及岩浆源N_(2)的混入导致柳林-三交地区煤层气中氮气同位素组成偏轻。鄂尔多斯盆地东缘煤层气中N_(2)属于热氨化-热裂解混合成因,其中柳林-三交地区由于受岩浆的烘烤作用,煤层气中N_(2)兼具岩浆源N_(2)的特征。

N_(2)is a major component in coal bed methane(CBM).The geochemical characteristics of N_(2)is significant for the CBM exploration and exploitation.In the present study,the geochemical characteristics,influencing factors and formation of the N_(2)in CBM are analyzed and discussed in the eastern of Ordos basin.The N_(2)in CBM of eastern Ordos basin has an average proportion of 3.33%,an average content of 0.119 m^(3)/t and an averageδ^(15)N of+4.470‰.The coalification has significant effects on the geochemical characteristics of the N_(2)in CBM.From north to south of the eastern of Ordos basin(Baode district-Liulin-Sanjiao district-Hancheng district),the coalification degree gradually increases,and the N_(2)content increases simultaneously,mainly due to the N_(2)formation and accumulation in the process of coalification.The N_(2)proportion first decreases and then increases along with coalification,due to the heat of the early cretaceous igneous intrusion in the Liulin-Sanjiao district,resulting in the lower proportion of N_(2)and higher proportion of CH_(4).Theδ^(15)N first decreases and then increases along with coalification,mainly due to the magmatic heat and the mixture of isotopically light N_(2)derived from magma.The N_(2)in CMB form the eastern Ordos basin has a multiple origin including thermal ammoniation and thermal cracking.The N_(2)form in the Liulin-Sanjiao District also has the magma-origin.