³He/⁴He ratios of methane-rich natural gases in Japan

Abstract

Measurements of ³He/⁴He and ²⁰Ne/⁴He ratios in 12 CH₄-rich natural gas samples were made with a magnetic deflection mass spectrometer equipped with a metallic high-vacuum purification line. In order to check the resolving power, sensitivity and stability of the mass spectrometer, atmospheric air in Tokyo was measured repeatedly. The mean ³He/⁴He ratio in Tokyo air was (1.43 ± 0.03) × 10^-6. CH₄-rich gases with significantly high ³He/⁴He ratios were first observed in this study. The ³He/⁴He ratios for 5 natural gas samples collected from hot springs and mineral springs in inland basins were as high as (1.7–7.3) × 10^-6, probably due to the large contribution of the upper mantle-derived He. In comparison, the ³He/⁴He ratios for 2 samples from water wells distributed in coastal areas facing the Pacific Ocean and 5 samples from gas fields in the southern Kanto district were as low as ∼10^-7. Most of the He in these gases is inferred to be radiogenic and of crustal origin.