Geochemical Study of Natural Gas Pools Combining He, Ne and Ar Isotopic Tracers and Major Elements

Abstract

Samples from 20 CO2,N2-enriched natural gas wells have been collected using the copper tube technique (Weiss, 1969). In the aim to constrain the origin and migration of gas, He, Ne and Ar concentrations and He and Ar isotopic compositions have been determined using a VG 5400 mass spectrometer. Major element analyses show that CH 4 is the main component, but that CO2 an N2 can be relatively high for some of the wells, reaching maximum values of 69% and 23% respectively. Maturity indicators clearly correlate with depth of pool indicating that gas accumulation is 'near the kitchen', so that gas migration has been very short. 3He/'~Ie isotope ratios are generally very low, i.e. of 0.02-0.05 x Ra, a range typical of continental helium, indicating no admixture of mantle helium. Therefore, a mantle origin for COz and N2 can be discarded. Argon isotope ratios are low, generally between 296 and 500 (with one exception reaching 800) which indicates that argon is dominated by the atmospheric component. He and Ar concentrations exhibit systematic variations. The atmospheric isotopes, 36Ar and 2~ or total 4~ show no clear correlation with depth of pool, while the radiogenic isotopes, 4He or Universit6 Paris Sud, 91405 Orsay Cedex, France