The discovery and origin of helium-rich gas on the northern margin of the Qaidam Basin

In this paper, high-concentration helium was found for the first time in the coal drill hole and the mud-shale adsorbed gases in Quanjishan and Tuanyushan area on the northern margin of the Qaidam Basin. Six methane samples from these two areas were analyzed for carbon and helium isotopes. The data obtained show that two samples' carbon isotope values of methane are-38.4‰and-39.9‰, suggesting organic origin. In addition, the ³He/⁴He isotope values of four samples are between 0.03×10^-6 and 1.3×10^-6, showing that they were mainly crust-derived matter and subordinately mantle-derived matter. Through the analysis of regional geology and geophysical data, the authors have reached the conclusion that crust-derived helium was sourced from the granite with rich uranium and thorium on the basin basement. The deep faults of the northern Qaidam Basin might have provided better conditions for migration of the helium. Helium was produced in combination with other hydrocarbon or non-hydrocarbon gases in vertical migration process, and probably formed the unique helium-rich natural gas enrichment environment under the good reservoir and sealing condition in Jurassic, Neogene and Paleogene.