天然气运移的气体同位素地球化学示踪

本文通过鄂尔多斯等含油气盆地内岩石酸解烃、罐顶气和同源多产层天然气碳同位素组成的变化，从实例剖析出发，探讨了天然气运移时气体同位素组成的变化及其对天然气运移的示踪。天然气在通过沉积地层中孔隙系统和微裂隙运移时，天然气中的甲烷碳同位素会发生一定的分馏，而乙烷以上重烃碳同位素几乎不发生分馏；在天然气层所在深度，罐顶气甲烷碳同位素组成与天然气一致，在天然气层附近，罐顶气甲烷碳同位素则明显偏离了热演化趋势线；烃源岩酸解烃与其同源的天然气重烃碳同位素组成具有较好的一致性和可比性。由此，可利用气体组分碳同位素的上述变化特征，追索天然气的运移作用。

Isotopic Geochemical Traces of Gaseous Hydrocarbons for Natural Gas Migration

A lot of theoretical and practical studies have been done on applying the carbon and hydrogen isotopic compositions of gaseous hydrocarbons to indentify the type of natural gas and trace the type and maturation of potential gas source rocks. However, there are few studies on the application of the isotopic character of natural gas to gas migration. The main argument point on the above case is whether the isotopic fractionation occurs druing gas migration. In this paper, according to the characteristics of the carbon isotopic composition from acidlysis gas, canned gas and natural gas of Ordos and other oil - gas bearing basins, the changes of carbon isotope compositions of gaseous hydrocarbons during gas migration are discussed and the following conclusions are obtained.1.While natural gas migrates a long distance through micro fractures and the pore system in strata, a certain isotopic fractionation for methane occurred but little isotopic fractionation for heavier hydrocarbons.2. A relatively good concordant and comparative relation exsits between the carbon isotope compositions of heavier hydrocarbons from acidlysis gas of source rocks and those from natural gas orginated from the same source rocks. The correlation of direct gas and its source rocks was carried on from this coincidence.3. The carbon isotope ratios of methane from canned gas are equal to those of natural gas at gas production layers and show much heavier than the normal increase tendency with depth near gas production layers.The exsitence of gas layers may be indicated according to the vertical isotope characters of methane from canned gas.