Isotopic Analysis of Hulium, Argon and Their Application on Accumulation Mechanism of Natural Gases

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.