鄂尔多斯盆地富县地区长7油组页岩气地球化学特征及成因

摘要

以鄂尔多斯盆地富县地区WX井长7油组页岩岩心的解析气为研究对象,分析了其气体组分、轻烃化合物、碳同位素和氢同位素等地球化学特征,并在此基础上分析了天然气的成因.结果表明,WX井长7油组不同深度页岩气的气体组分、轻烃组成、碳同位素和氢同位素特征存在较大差异.随深度增加,重烃含量、轻烃化合物的正构烷烃、异构烷烃相对含量及其与1,1-二甲基环戊烷的比值逐渐升高,干燥系数、环烷烃相对含量及其与1,1-二甲基环戊烷的比值逐渐降低,碳同位素和氢同位素逐渐变轻,碳同位素的倒转程度逐渐减弱.WX井长7油组页岩气属于油型热解气,没有无机成因气和古生界煤型气的混入,但遭受了不同程度的微生物氧化作用,微生物氧化作用由浅至深也逐渐变弱,不均一的微生物氧化作用是导致WX井页岩气气体组分、轻烃组成、碳同位素和氢同位素特征垂向差异性和碳同位素倒转的主要原因.%Taking the desorbed gas in the shale cores of the Chang 7 Oil Member in Well WX of Fuxian area,Ordos basin as the study object,the geochemical characteristics such as gas component,ligbt hydrocarbon,carbon isotope and hydrogen isotope in the shale cores are studied,based on which the genesis of the natural gas are analyzed.The results show that the characteristics of gas component,light hydrocarbon composition,carbon isotope and hydrogen isotope of the shale gas are significantly different among samples obtained from different depths of the Chang 7 Oil Member in Well WX.With the increase of the depth,heavy hydrocarbon contents,relative contents of n-alkanes and isoalkanes in light hydrocarbons and the ratios of n-alkanes and isoalkanes contents to 1,1-dimethylcyclopentanes rise gradually,drying coefficient,naphthene hydrocarbon content and the ratio of naphtbene hydrocarbon content to 1,1-dimethylcyclopentanes decrease gradually,carbon and hydrogen isotopes become lighter and the reverse degree of carbon isotope reduces.The shale gas in Chang 7 Oil Member of Well WX belongs to oil-type pyrolysis gases without the mixing with inorganic diagenetic gas and Paleozoic coal-type gas,suffering from different degrees of microbiological oxidization which becomes weaker from shallow to deep.The inhomogeneous microbiological oxidization is the main reason causing the vertical differences of gas component,light hydrocarbon composition,carbon and hydrogen isotopes and carbon isotope reversal in the shale gas of Well WX.