## 5 Discussion
### Implications for the Helium Accumulation Mechanism
Since for the HJQ-H501 well the hydrogen isotope test was not conducted, it was impossible to determine the hydrogen source in this well based on isotope data. Usually, the helium isotope ratio can be used to determine whether there is a mixture of mantle-derived fluids. A large number of previous test results showed that the helium isotope ratio in the Ordos Basin suggests a typical crustal source for helium [14; 41]. No mantle-derived fluid was found in the basin; so, a deep source of hydrogen can be excluded. In addition, the basement of the Hangjinqi area is mainly composed of Archean and Paleo-Mesoproterozoic granitic gneiss, quartzite, and quartz sandstone. These rocks contain less Fe\({}^{2+}\) minerals (olivine and pyroxene), and the contribution of serpentinization to hydrogen production is also extremely limited. Therefore, hydrogen in the HJQ-H501 well is mainly produced by the irradiation of water. Through logging curve analysis and by examining the results of previous studies, it can be seen that the distribution of groundwater in this area is complex [47], and the granite rocks widely distributed in the basement of this area are rich in radioactive elements such as uranium and thorium. The energy generated by the decay of uranium and thorium and the widely distributed water provide sufficient conditions for the irradiation of water to produce hydrogen. Figure 10: Correlation of hydrogen and helium concentrations. (**a**) The first member of the Lower Shihezi Formation in the HJQ-H501 well, (**b**) the high-helium-concentration area at the top of the first member of the Lower Shihezi Formation in the HJQ-H501 well, (**c**) the Huoshiling Formation and basement in the SK-2 well, (**d**) the Huoshiling Formation in the SK-2 well. During the late Mesozoic and early Cenozoic, the Ordos Basin experienced a series of tectonic uplift events, particularly during the early Cenozoic period. Continual subduction and compression of the Pacific plate and Indian Ocean plate led to a sustained uplift along the basin margins [48; 49]. This resulted in a decrease in the original formation temperature and pressure, causing gases dissolved in groundwater to exsolve, allowing hydrogen and helium to migrate upward together. This process is akin to that observed in the Weiyuan gas field in the Sichuan Basin, which underwent a significant tectonic uplift of nearly 4000 m during the Himalayan orogen [50]. Consequently, the pressure and temperature of the gas field significantly decreased, leading to the migration of helium, previously trapped in fractures and pores of the granite basement, from high-pressure zones to reservoirs, where it accumulated at normal pressure [51]. However, because of its high reactivity, hydrogen may react with unsaturated long-chain hydrocarbons during its upward migration to form saturated short-chain hydrocarbons [52; 53; 54], thus decreasing the hydrogen concentration. Nonetheless, regardless of whether hydrogen is transformed into hydrocarbon gases, helium remains inert during this process. Therefore, both hydrogen and hydrocarbon gases can serve as carrier gas for transporting helium upward. Based on the above discussion, this paper proposes a hydrogen-helium migration and accumulation model (Figure 11). In the diagram, Well-1 represents the situation of a sedimentary basin, where there is no obvious mantle-derived material. Helium is mainly derived from the crust and is produced by the radioactive decay of the basement granite of the basin, and hydrogen is mainly derived from the irradiation of water. Under the influence of the basin tectonic movements, hydrogen produced by water irradiation extracts the helium dissolved in pore water, and the complex hydrodynamic conditions and fracture distribution in the basin provide a migration path for hydrogen and helium, so that they continue to migrate upward along the faults and accumulate in structural high points and hydrocarbon gas reservoirs, which become enriched in helium; this process was observed in well HJQ-H501.