Compositions of non-hydrocarbon and noble gases in natural gas samples from Tarim Basin, China

Abstract

Compositions of non-hydrocarbon and noble gases in natural gas samples collected from the Kuqa Depression, Tabei Uplift Belt, North Depression, Tazhong Uplift, and Southeast Depression in the Tarim Basin, China were analyzed to investigate the origin of the natural gases. The results show that the largest component of the natural gases is hydrocarbon gas with a concentration of 33.46% to 99.26%. The non-hydrocarbon gases are mainly composed of CO₂ and N₂ with concentrations of 0.01% to 17.95% and 0.1% to 52.45%, respectively. The helium content varies from 0.005% to 0.30%. The ³He/⁴He ratio varies from 0.01 Ra to 0.55 Ra (where Ra denotes atmospheric ³He/⁴He ratio = 1.4 × 10^–6), and the ⁴He/²⁰Ne ratio varies from 8.3 to 7400. Most of the ³He/⁴He values are below 0.1 Ra. The low ³He/⁴He ratios indicate crustal radiation to be the main source of the helium. There are no positive correlations between the ³He/⁴He ratio and helium abundance, non-hydrocarbon abundance, heat flow, and the age of reservoir formations. The ratios of ³He/⁴He are related to the time or stage of the natural gas accumulation. The ratios of ³He/⁴He in the reservoirs that were filled in the later stage (Himalayan period) are lower than those in the reservoirs that were filled in the earlier stage (Yanshan period).