Source and accumulation of helium in the Dongsheng gas Field, Ordos Basin, China: Insights from noble gas isotopes

Abstract

Helium, vital for advanced technologies, is generated in the crust via uranium (U) and thorium (Th) decay. The crystalline basement is a proven source of helium. In contrast, the role of sedimentary rocks, while often considered significant, has lacked definitive proof. This study provides the isotopic evidence to deconvolve these two crustal sources and verify the significant role of sediment. Our analysis of the Dongsheng Gas Field, Ordos Basin, China, reveals that air-derived noble gases reflect open-system water–gas equilibration, a process occurring within the reservoir. Whereas crust-derived noble gases likely pre-aggregated and migrated together with hydrocarbons, thereby preserving a clear signature of their original sources. Within this preserved signature, a depletion of ²¹Ne* (where * denotes a crustal origin) is observed. This depletion produces an unusual negative correlation between ²¹Ne*/³⁶Ar and ⁴⁰Ar*/³⁶Ar, in contrast to the expected positive trend. This pattern is interpreted as the mixing of a fluid from coal-bearing strata (⁴⁰Ar*/³⁶Ar = 130 ± 10) with a ²¹Ne*-depleted fluid from the basement (⁴⁰Ar*/³⁶Ar = 2250 ± 150). Modeling suggests that only ∼ 0.26 % of basement-generated ²¹Ne* is retained. We propose a helium accumulation model in which initial preservation was inefficient owing to the absence of a seal, whereas the main accumulation was driven by hydrocarbon charging, which acted as a carrier fluid stripping helium from coal-bearing strata. These results demonstrate that U- and Th-rich sedimentary rocks can serve as a potential helium source in natural gas, which refines exploration strategies for helium in cratonic basins.