柴达木盆地天然气中稀有气体同位素特征

为了确定柴达木盆地勘探新层系，测试并分析了该盆地5个油气田的8个天然气样品的氦、氩同位素比值，探讨了其在大地构造背景、大地热流和气源方面的示踪意义。研究结果如下：①柴达木盆地8个天然气样品³He/⁴He值分布在2.0×10^-8～5.48×10^-8的范围，总体平均为3.7×10^-8，均在10^-8量级，体现了典型的壳源成因氦同位素的特征。②据³He/⁴He计算的大地热流值较低，分布于41.2~48.2 mW/m²之间，平均44.9 mW/m²，低于东部地区大地热流平均值73 mW/m²，表明该盆地具有较稳定的构造背景。③根据天然气⁴⁰Ar/³⁶Ar的年代积累效应，结合烃源岩分布特征推测，柴达木盆地东部凹陷区伊克雅乌汝构造伊深1井天然气应源于上第三系；西部坳陷区尕斯油田尕斯8-6井和尕斯475井天然气应源于下第三系；碱山构造碱1井天然气应源于侏罗系，碱山构造碱2井的天然气应源于上第三系；北缘块断带南八仙油气田仙试1井的天然气应源于侏罗系；南八仙油气田仙试8井和马北构造马北1井的天然气可能主要源于侏罗系。

NOBLE GAS ISOTOPIC CHARACTERISTICS AND THEIR TRACING SIGNIFICANCE OF THE NATURAL GASES FROM THE QAIDAM BASIN

In order to determine the new exploratory formation, this paper analyzed the He and Ar isotopic composition of 8 natural gas samples from 5 oil fields or structures in the Qaidam Basin and discussed their tracing significance in tectonic backgrounds, geothermal flow and gas source. The research shows the results as follows: ① the 3He/4He values of natural gases in the studied area is distributed on the range from 2.0×10-8 to 5.48×10-8, in an order of 10-8, averaging 3.7×10-8, reflecting the characteristic of the typical helium isotope of crust origin. ②The geothermal flow values calculated based on 3He/4He ratios range from 41.2 to 48.2 mW/m2，averaging 44.9 mW/m2，which is lower than the average ones in the eastern China, indicating stable tectonic backgrounds of the Qaidam Basin. ③Based on an aging effect of natural gas 40Ar/36Ar and integrating the distribution of source rocks, it deduces that gases from the well Yishen 1 in the Yikeyawuru structure in the eastern depression area were derived from the Upper Tertiary; gases from the well Gasi 8-6 and Gasi 475 in the Gasi oil field in the western depression area were derived form the Lower Tertiary; gases from the well Jian 1 in the Jianshan structure were sourced form the Jurassic and that from the well Jian 2 in the Jianshan structure from the Upper Tertiary; gases from the well Xianshi 1 in the Nanbaxian oil and gas field in the northern fault block zone was derived from the Jurassic; gases from the well Xianshi 8 in the Nanbaxian oil and gas field and the well Mabei 1 in the Mabei structure were also mainly sourced from the Jurassic. The knowledge about the gas sources is of important significance to find new natural gas explored strata in the Qaidam Basin.