The occurrence, origin, and enrichment of helium in the Wufeng-Longmaxi shale gas in the Sichuan Basin, China

Helium is a valuable natural resource used widely in high-tech industries because of its unique physical and chemical properties. The study of helium in shale gas is still in its infancy, and the content, genesis, and enrichment patterns of helium in shale gas are not yet clear. In this paper, the concentrations and isotopic characteristics of helium were investigated in the Wufeng-Longmaxi shale gas in the Sichuan Basin and the periphery areas. The analytical results show that the concentrations of helium in the southern Sichuan shale gas fall in the range of 0.018–0.051 vol% with an average of 0.029 vol%. The helium abundance in Weiyuan shale gas are relatively low compared to those in conventional natural gas pools from the same area (generally greater than 0.20 vol%), reflecting the significance of long distance migration to the enrichment of helium in gas pools. The relatively low ratios of ³He and ⁴He in shale gas indicate that most of the helium are crustal derived helium. Further quantitative estimate based on helium, neon, and argon isotopic ratios suggest almost 100% crustal helium source. The helium residing in shale reservoirs can be deconvoluted into the indigenous helium generated in-situ by shale and exogenous helium generated from external helium source rocks and charged through faults and/or fractures networks. According to preliminary calculations, external helium source is required to meet the threshold of an economic helium-rich field of helium concentration of 0.1 vol% except for particular areas with extraordinarily high uranium and thorium concentration. Based on detailed study on typical helium-rich shale gas reservoirs, major advantageous features for helium's enrichment in shale gas include: (1) high-quality helium source rocks, (2) effective migration paths, and (3) diminished dilution effects of shale gas. Shale gas plays with underlying ancient cratonic basement, well developed source-connecting faults, and moderate pressure coefficient are potential targets for helium exploration.