川西拗陷天然气丁烷地球化学特征

摘要

丁烷与甲烷、乙烷、丙烷同为天然气的烷烃气组分,但是对其研究及应用明显不如甲烷、乙烷及丙烷.本文在川西拗陷天然气地球化学分析测试的基础上,重点对天然气中丁烷的地球化学特征进行分析,探讨丁烷与天然气成因、天然气运移的关系.测试结果显示,研究区天然气以烷烃气为主,其中甲烷含量最高,其体积分数＞85％,平均值＞93％;重烃含量相对较低.研究区天然气中丁烷含量较低,其体积分数＜1％;其中异丁烷与正丁烷含量相当,体积分数均不超过0.5％.iC4/nC4值能示踪未受到明显次生变化天然气的成因类型,煤型气iC4/nC4 ＞0.8,油型气iC4/nC4 ＜0.8;受到次生作用影响的天然气,iC4/nC4值可能失去判别天然气成因类型的功能.目前,对iC4/nC4值能否示踪天然气运移及其示踪机理存在较多争议,在研究区iG/nC4值就未能有效示踪天然气的垂向运移过程,因此在选择该指标示踪天然气运移时应慎重.%Butane together with methane,ethane and propane is the component of alkane of natural gas.The paper studies the natural gas in West Sichuan depression,analyzes geochemical characteristics of butane in natural gas,and explore the relationship among butane,natural gas origin and gas migration.It shows that the natural gas in the studied area is mainly alkane and the methane content is high with volume fraction over 85％,and with the average values over 93％.By contrast,heavy hydrocarbon content is relatively low.In the research area the content of butane is low with the volume fraction less than 1％ and the content of isobutene and normal butane are fairly the same with the volume fractions less than 0.5 ％.The iC4/nC4 value and gas genetic type analysis can be used as an index to distinguish the oil-type gas from coal-type gas,especially for natural gas which has not been affected by secondary changes obviously.When iC4/nC4＞0.8,the gas is coal-type gas and when iC4/nC4＜0.8,the gas is oil-type gas.However,if gas is affected by the secondary changes,the iC4/nC4 value can not be used to distinguish the genetic type of natural gas like the studied area.Presently,there are many controversies whether the iC4/nC4 value can be used to trace gas migration or not.Therefore,caution should be exercised when applying the index to trace migration of gas.