Enrichment Mechanism of Helium in the Jiaoshiba Shale Gas Field, Sichuan Basin, SW China

Abstract

In Sichuan Basin, uranium (U) and thorium (Th) are more abundant in organic-rich shale than in granitic rocks, suggesting that organic-rich shale is a significant source rock for helium (He). However, the He generation potential of organic-rich shale and He enrichment mechanisms in shale gas fields remain limited in understanding. Based on the He, U and Th concentration tests, fluid inclusion analysis in the Jiaoshiba shale gas field, it was found that the He concentrations in shale gas is 340–730 ppm. The helium-rich shale gas (≥ 500 ppm) was primarily found in synclinal units. The biggest helium abundance area (0.27 m³/m²) is located at the core of the Jiaoshiba anticline. The He is not solely derived from Wufeng–Longmaxi shale itself, for the current helium abundance (0.15–0.27 m³/m²) exceeds its historical He generation intensity (0.063–0.104 m³/m²) calculated by the U and Th concentration of the shale. Low-angle deep faults are more favourable for He capture than high-angle deep faults. The generation intensity of hydrocarbon is 21,851–29,224 times greater than He, which indicates that the dilution of hydrocarbon to He in organic-rich shale is more significantly than in common source rocks (3000 times). The He enrichment model in the study area includes three stages: continuous burial, fold-dominated, and fault-dominated stages. In the anticline zone, He is continuously accumulated as carrier gas migration. In the syncline zone, He is mainly accumulated as the migration of carrier gas along faults and the water flows the aquifer, at the fault-dominated stage. He-rich gas is preferentially accumulated in shallow gas reservoirs associated with deep faults, and the shale gas layers in synclinal zones adjacent to low-angle deep faults.