Helium migration patterns in the ultra-deep reservoirs of the Anyue gas field

Abstract

The Sinian–Cambrian reservoirs in the Anyue gas field contain over one trillion cubic meters of natural gas. These reservoirs are close to basement helium sources and have well-developed ancient formation water as a potential carrier. However, despite these favorable conditions, helium concentrations remain abnormally low. Traditional studies have not quantitatively constrained helium migration pathways. This hampers the evaluation of helium enrichment potential in ultra-deep strata. To address this, we analyzed noble gas isotopes (He, Ne, Ar, Kr, Xe) and elemental ratios in 23 reservoir samples, aiming to trace helium sources and identify migration mechanisms. The results suggest that: (1) Helium in the Anyue gas field is mainly crustal-derived. The Sinian reservoir exhibits higher R / R a ratios (0.008–0.062) and a greater crustal helium proportion than the Cambrian reservoir (0.003–0.019). (2)The 20 Ne/ 36 Ar, 84 Kr/ 36 Ar, 132 Xe/ 36 Ar ratios consistently indicate open-system behavior in the Sinian and Cambrian strata. The Sinian shows a stronger He–N₂ correlation ( R 2 = 0.82) and contains more atmospheric noble gases, indicating more intense basement groundwater circulation. (3) Helium in the Sinian reservoir is primarily derived from exsolution of water-soluble helium from basement formation water. Correspondingly, helium in the Cambrian reservoir mainly originates from the Qiongzhusi source rocks, with minor exogenous contribution. (4) Poor fault connectivity and overpressure in the Cambrian (pressure coefficient 1.51–1.70) have contributed to inefficient helium migration pathways. This study establishes a quantitative model for helium differential enrichment controlled by the activity of formation water. It provides a testable theoretical framework for integrated helium concentration in basement groundwater circulation.