RARE GAS ISOTOPIC COMPOSITION OF NATURAL GASES
Xu Yongchang Wang Xianbin Wu Renming Shen Ping Wang Youxiao He Yongping
A hundred natural gas samples of different geological periods collected from Various localities were determined for their nitrogen, helium and 'argon contents, as well as the isotopie composition of argon. The range of Ar40/Ar36 ratio was found to be 305-9255. The helium-argon chronological formula for calculating natural gas age was corrected. Based upon' the chronological accumulation effect of radiogenic argon-40, an argon-40-argon-36 empirical formula for calculating natural gas age has been proposed.It can be seen from the ratio of nitrogen to atmospheric argon that most of nitrogen in these natural gases could not be derived from atmospheric nitrogen, but elsewhere.The 'range of radiogenic He/Ar40 turn's out to be 5.19 to 23.18, with the range between 7 to 14 for most samples. The He/Ar40 ratios of Sinian samples are in the range of 5.39 to 6.14, indicating their deep derivation from the earth,It should be noted that the Ar40/Ar36 ratio shows, a general trend of approaching the atmospheric value from the Sinian to the Tertiary Period, or from old to young in geological age, mirroring, to a great extent, the evolutionary tendency of the rare gas isotopie composition of natural gases with the continuous degassing of the earth.