Helium signatures of gases from the Sichuan Basin, China

Natural gases from the Sichuan Basin, China were analyzed to investigate helium abundances and isotopic composition in order to assess crustal vs. mantle inputs into the Sinian to Jurassic gas reservoirs that occur at depths up to 4900 m. In general, gases from the Sinian and pre-Sinian strata in south Sichuan have relatively high abundances of helium compared to those from the east, central and west Sichuan, implying an increase of crustal helium with time. All gas samples demonstrate very low helium isotopic ratios (3He/4He designated R) with an average of 0.016 Ra (n = 78, where Ra is the isotope ratio found in the atmosphere), indicative of a crustal origin. This is further supported by the relatively stable upper mantle found in west China. In contrast, the thin and active lithosphere in the east part of China is favorable for the injection of mantle-derived volatiles into gas reservoirs. This explains why gases from the east part of China generally have R/Ra > 1 while those in the west have R/Ra ⩽ 0.1.