The noble gas concentrations and isotope ratios of seven natural gas samples from the central area of the Western Sichuan Basin were measured. The samples all have 40Ar/36Ar ratios greater than the atmospheric values, and the 3He/4He ratios (R/Ra) are entirely consistent with the crustal radiogenic He values. The vertical variation of the calculated CH4/36Ar ratios with depth clearly indicates that the CH4and 36Ar are intimately associated, indicating a common reservoir intermediate to the sampled reservoirs, where they are well mixed and stored together prior to entrapment into gas reservoirs. Meanwhile, the calculated CH4/36Ar ratios range between 8×106 and 64×106 very much greater than the CH4/36Ar values for pure water and 5 mol/L NaCI brine at low temperature and hydrostatic conditions, reflecting the presence of 'excess' thermogenic CH4 over that supplied by a CH4-saturated groundwater at low temperature, and the excess CH4 saturation and dissolution to be at depth greater than the sampled reservoirs. This conclusion is consistent with the δ13C(CH4) and δ13C(C2H6) values. In addition, the 4He/36Ar ratio is correlated with depth, showing that the crustal radiogenic 4He are well mixed with the atomosphere-derived 36Ar before introducing into gas reservoirs. The 4He/36Ar ratio vertical variation with depth can be attributed to the preferential transport of 4He relative to 36Ar in fluxes from lower aquifers through water-filled pores into the upper one. The increasing 4He/40Ar* ratios with decreasing depth, from 1.3 times to 29 times greater than the crustal production ratio, are also assumed to be the results of preferential transport 4He relative to 40Ar* from the production site into groundwater.
