Experimental Study on Permeability Behavior of Helium Within Its Source Rocks in the Sichuan Basin, Southwest China

Abstract

Understanding helium migration in source rocks is critical to resolving constraints imposed by helium's weak sourcing on modern enrichment models. This work investigates the permeability behaviour of helium in granite and shale across the Sichuan Basin to elucidate their contrasting controls on helium accumulation. Basing on the permeability behaviour of helium and nitrogen in shale and granite in the southwest of Sichuan Basin, it is found: (1) Helium permeability decreases by 81.06%–98.03% under confining pressure escalation (2–60 MPa), exhibiting negative correlations with rock strength/water saturation and a positive correlation with porosity. (2) Single-phase helium systems maintain higher permeability than two-phase gas-water pore systems, where dissolution/exsolution cycling governs flow. At > 60% water saturation, gas properties no longer exert a dominant control over the sample's permeability. When water saturation > 60%, gas-phase dominance ceases—granites show solubility-driven permeability evolution, while shales remain compaction-controlled. (3) Granite permeability plunges about 95.7% when heated from 25°C to 85°C, attributed to thermal grain expansion reducing pore-throat volumes. (4) Subsurface helium transport progresses through three stages. In shallow burial stages, it is dominated by groundwater-driven dissolved helium migration. In the middle/deep burial stages, flow decelerates due to the Jamin effect and aqueous phase trapping, with permeability increasingly controlled by dissolution/exsolution cycling in both lithologies. (5) In the Weiyuan gas field, helium accumulation derives from water-dissolved migration during granite burial. Shales indirectly control enrichment through (i) post-maturation migration retardation via the Jamin effect/aqueous phase trapping and (ii) compartmentalisation of underlying helium-saturated aquifers. Bedding orientation governs shale permeability anisotropy, contrasting with granite's isotropic pore geometry.