Distributions and accumulation mechanisms of helium in petroliferous basins

Helium is an irreplaceable strategic mineral resource, and commercial helium-rich gas fields(He>0.1%) worldwide are typically discovered serendipitously during hydrocarbon exploration efforts. According to an analysis of 75 helium-rich gas fields and 1048 natural gas samples worldwide, helium in natural gas generally exhibits “scarce”, “accompanying”, and“complex” properties, and helium-rich gas fields often occur at depths <4500 m. Helium concentrations in He-CH_(4) and He-CO_(2) gas fields are notably lower than those in He-N_(2)gas fields(He>1%). However, geological reserves in the former two types of gas fields are mainly in the range of 10^(7)–10^(11)m^(3), whereas in the latter, they are only in the range of 10^(5)–10^(7)m^(3). There are nevertheless notable disparities in the genesis and migration patterns between helium and gaseous hydrocarbons. Helium necessitates carriers(such as formation water, hydrocarbon fluids, N_(2), mantle-derived fluids, etc.) during both accumulation and long-distance migration processes, where migration conduits are not confined to sedimentary strata, and may extend to the basin's basement, lower crust, and even lithospheric mantle. However, the accumulation conditions of both helium and gaseous hydrocarbons are generally considered equivalent. The presence of gaseous hydrocarbons facilitates both the rapid exsolution of helium within helium-containing fluids and subsequent efficient aggregation in gaseous hydrocarbons, while both reduce helium diffusion and diminish escape flux. In terms of caprock, gypsum, salt, and thick shale as sealing layers contribute to the long-term preservation of helium over geological timescales. Large helium-rich gas fields, predominantly crust-derived gas fields, are primarily concentrated in uplifted zones of ancient cratonic basins and their peripheries. Based on a diagram of the He concentration versus He/N_(2) ratio, crust-derived helium fields can be categorized as basement, combined basement-sedimentary rock, and sedimentary rock helium supply types. Comprehensively given China's helium grade, helium resource endowment,natural gas industrialization process, and current helium purification processes, the foremost deployment zones for the commercial production of helium should be the helium-rich gas fields located in the Ordos, Tarim, Sichuan, and Qaidam Basins in western and central China. In addition, certain(extra) large helium-containing gas fields serve as important replacement zones.