Noble gas constraints on hydrocarbon accumulation and groundwater flow in the central area of Western Sichuan Basin

The noble gas concentrations and isotope ratios of seven natural gas sample s from the central area of the Western Sichuan Basin were measured. The sam ples all have Ar-40/Ar-36 ratios greater than the atmospheric values, and t he He-3/He-4 ratios (R/R-a) are entirely consistent with the crustal radiog enic He values. The vertical variation of the calculated CH4/Ar-36 ratios w ith depth clearly indicates that the CH4 and Ar-36 are intimately associate d, indicating a common reservoir intermediate to the sampled reservoirs, wh ere they are well mixed and stored together prior to entrapment into gas re servoirs. Meanwhile, the calculated CH4/Ar-36 ratios range between 8x10(6) and 64x10(6) very much greater than the CH4/Ar-36 values for pure water and 5 mol/L NaCl brine at low temperature and hydrostatic conditions, reflecti ng the presence of "excess" thermogenic CH4 over that supplied by a CH4-sat urated groundwater at low temperature, and the excess CH4 saturation and di ssolution to be at depth greater than the sampled reservoirs. This conclusi on is consistent with the delta C-13(CH4) and delta C-13(C2H6) values. In a ddition, the He-4/Ar-36 ratio is correlated with depth, showing that the cr ustal radiogenic He-4 are well mixed with the atomosphere-derived Ar-36 bef ore introducing into gas reservoirs. The He-4/Ar-36 ratio vertical variatio n with depth can be attributed to the preferential transport of He-4 relati ve to 36Ar in fluxes from lower aquifers through water-filled pores into th e upper one. The increasing He-4/Ar-40 ratios with decreasing depth, from 1 .3 times to 29 times greater than the crustal production ratio, are also as sumed to be the results of preferential transport He-4 relative to Ar-40 fr om the production site into groundwater.