Abstract: This paper studies the largest gas field-Weiyuan gas field in Sichuan ? basin . There are four gas-bearing layers in the gas pool, i.e. Triassic, Permian, Cambrian and Sinian. The main industry reservoir in the dolomite of the Siniansy stsyem. The oldest layer is about 700~800 x 10⁶years. It is the largest and oldest gas field which has been found in China so far. It is significant for the study of early organic matter evolution, mineral and energy resources to study out the genesis ofgaspool, especially, to decide whether the source of Sinian gas pool may be relative to Precambrian layers.
The characteristics of the gas constituent and the i so topic composition of C， H， S， Ar，He from this gas field are the following: 1. Compared with the other gas pools, Sinian gas pool gas high content of N₂(9%), C0₂+H₂S (5%), He (0.18) and Ar (0.05%)； 2.The value of ⁴⁰Ar/³⁶Ar in Sinian gas pool is very high, from 4400 to 9255, the average is 7000, showing high age-accumulating effect. The value of ³He/⁴He is low. only n x 10^-8. This means that the He in the Wciyuan gas pool is mainly from the crustal matter; 3. He /Ar is from 5.39 to 6.14, the lowest value which has been found by us; 4. The values of N2/Ar and N2/He have better linear relativity; S.Triassic, Permian, Cambrian and Sinian gas pools can be classified with the isotopic composition of C, H, S， Ar, which shows the gas in these gas pools is from different gas sources; 6. The components of absorbed hydrocarbon from Sinian source rock are diiTcrent from other source rocks, and the value of C₂/C₃from the source rock can be well compared with that from gases.
To sum up, we think that the Sinian gas pool is of autochthonous origin. It was formed by organic matter at the early stage of earth f s evolution. The CH₄ and rare gas are not characteristics of mantle mater; but we cannot rule out a possibility that there may be some mantle matter being pux in this gas pool.