煤型气中丁烷的地球化学特征及意义——以塔里木盆地库车坳陷为例

Significance and Geochemical Characteristics of Butane in Coal Gas:An Example from the Kuqa Depression,Tarim Basin

为研究母质类型、热演化程度和运移方向等对煤型气中丁烷地球化学特征的影响,统计分析了塔里木盆地库车坳陷天然气中丁烷组分和碳同位素的变化特征。结果表明,母质类型、烃源岩热演化程度及成藏过程、天然气成熟度共同控制了库车坳陷煤型气i C4/n C4值;库车坳陷煤型气正、异丁烷的碳同位素比值与δ13C1均具有明显的正相关性,即随着天然气成熟度增加,正、异丁烷均变重,且异丁烷相对更富集重碳同位素;在砂岩疏导层中,煤型气i C4/n C4值沿天然气渗滤运移方向有减小的趋势,大北气田天然气总体上来讲由南向北运移。天然气中的丁烷组分和碳同位素特征研究具有重要的理论意义,与甲烷、乙烷等常规研究组分类似,丁烷组分和同位素特征也可成为天然气成熟度的替代指标。

The component and carbon-isotope composition of butane were analyzed to study the influences of source-rocktype,thermal evolution and migration direction on the geochemistical characteristics of butane in coal-type gases in the Kuqa Depression of the Tarim Basin. The results showed that the values of i C4/ n C4 of coal-type gas samples from the Kuqa depression were influenced by the type and thermal evolution of source rocks,the accumulation process and gas maturity.A significant positive correlation was found between δ^13C values of methane and δ^13C values of n C4,or i C4. Moreover,theδ^13C of i C4 became heavier than those of n C4 with the increase of gas maturity. The ratios of i C4/ n C4 in coal type gas decreased along the migration direction of gases in clastic rocks. So,we concluded that,similar to those of methane and ethane,the component and carbon-isotope compositions of butane could also indicate the maturity,the migration and the accumulation of nature gases.