大气甲烷碳同位素测试方法及其在雅克拉凝析气田上方大气中的应用

研制了一套与气体稳定同位素质谱仪联机在线分析大气甲烷碳同位素的制样技术,该方法具有精度较高(±0.4×10^-3)、用样量少(200mL)、耗时短(45min)和操作相对较为简便的优点.利用该系统对新疆塔里木盆地雅克拉凝析气田上方大气甲烷的碳同位素组成做了初步测试.结果表明,其大气甲烷碳同位素组成平均值为-45.0×10^-3,相对全球大气甲烷碳同位素平均值明显偏重1.2×10^-3～2.0×10^-3,显示了其甲烷主要来源为深部凝析气藏中的甲烷等气体的渗漏和扩散.并且由于白天日照强,温度高,气田渗漏和扩散到地表的甲烷部分被氧化,因此白天大气甲烷的浓度略低于夜晚,且¹³C_CH4较夜晚偏轻.

On-line Method for Measurement of the Carbon Isotope Ratio of Atmospheric Methane and Its Application to Atmosphere of Yakela Condensed Gas Field

An on-line method for measurement of the 13C/12C ratio of methane by a gas chromatography/high-temperature conversion/ isotope ratio mass spectrometry (GC/C/MS) technique was developed. This method is less laborious, more rapid (45 min), of high precision (+/- 0.4 x 10(-3)) and by using a small amount of sample (about 200 mL of atmosphere). Its application to isotopic characterization, and hence methane source identification, was demonstrated by examination of atmosphere sample collected in Yakela condensed gas field, China. The average 13C/12C ratio of atmospheric methane in Yakela field was -45.0 x 10(-3) heavier by 1.2 x 10(-3) -2.0 x 10(-3) than the global average. This is caused by seepage and diffusing of methane from Yakela condensed gas reservoir. The concentrations of atmospheric methane in daytimes are found to be lower than those in nighttimes, and the corresponding 13C/12C ratios in daytimes are lighter compared to those in nighttimes, a phenomena probably caused by the fact that a small part of methane from Yakela condensate reservoir is consumed in soil's surface under sunlight.