氦、氩同位素分析在天然气成藏规律研究中的应用——以川西盆地中部天然气He、Ar同位素组成分析为例

The purpose of the present paper is to show He, Ar isotopic application to studying the accumulation mechanism of natural gases in the central areas of ( Xiaoquan, Xincheng and Hexinchang ) Western Sichuan Basin. The gas reservoirs occur at depths ranging from 560m to 4566m, in which CH4 dominates, occupying up to 92% to 99% by volum of the gas samples, and being believed to be thermogenetic in source. The samples all have ^40Ar/^36Ar ratios greater than the atmospheric value(295.5). The calculated radiogenic ^40Ar content ranges in 20% to 60% by the total volum of each sample. And the ^3He/^5He ratios(R/Ra=0.01-0.1), suggesting that all the He in these samples is entirely crust radiogenic. The radiogenic ^4He/^40Ar ratios are 1.3 times to 29 times greater than the crustal production ratio(4.92), being attributed to the preferential transport of ^4He relative to ^40Ar in fluxes from the basement into the upper strata. The observed correlation between the CH4/^36Ar and ^4He/^40Ar ratios clearly indicates that the thermogenetic CH4, and crust radiogenic ^4He and ^40Ar, as well as atmopheric ^36Ar are intimately associated. These characteristics require a common carrier intermediate to the sampled reservoirs, where these differentially originated gases are well mixed and stored together in prior to entrapment into gas reservoirs.