## 5 Discussion
### Implications for the Helium Accumulation Mechanism  
At present, no independent helium gas reservoir has been found. Helium is mostly coupled with hydrocarbon gases, and sometimes with nitrogen and carbon dioxide. Helium in natural gas reservoirs is mainly produced by the radioactive decay of basement granite and then dissolved in pore water [4; 5; 6]. When free gas passes through pore water, helium dissolved in water will enter the gas phase due to Henry's law and follow the gas, further migrating and accumulate. Free gas plays an important role in the migration and accumulation of helium [7; 10; 11; 12; 13]. The formation and accumulation of hydrocarbon gases often occur in the overlying strata far from the basement; so, it is impossible to directly extract helium from basement granite. In addition to hydrocarbon gases, nitrogen and carbon dioxide play an important role in the extraction of helium. However, previous studies showed that the concentration of nitrogen and carbon dioxide in the natural gas of the Ordos Basin is very low, and the test results of this paper further verified this phenomenon [41]. Therefore, the extraction of helium by nitrogen and carbon dioxide in this area is very limited. Through further analysis of the data, we found an obvious relationship between hydrogen and helium. Figure 10 shows the correlation diagram of hydrogen and helium concentrations. Figure 10a,b represent the first member of the Lower Shihezi Formation in the HJQ-H501 well and the high-helium-concentration area at the top of this member, respectively. Figure 10c,d represent the Huoshiling Formation to the basement and the Huoshiling Formation of well SK-2, respectively. The data of well SK-2 are quoted from [PERSON] et al., 2023 [42]. The SK-2 well is located in the Songliao Basin and was drilled through the Cretaceous strata into the underlying basement. In previous works, interval gas sampling was conducted during the drilling process, and these test data could be used to analyze the vertical gas migration. It can be seen in Figure 10a,b that the correlation between hydrogen and helium concentrations in the first member of the Lower Shihezi Formation was poor, but there was a good correlation in the high-helium-concentration area at the top of the formation. The SK-2 well showed the same pattern. The correlation between hydrogen and helium concentrations in the Huoshiling Formation was obviously better than that in the Huoshiling Formation, including the basement (Figure 10c,d). Generally, there was no correlation between hydrogen and helium concentrations from different sources, and these gases also appeared diluted to different degrees by the main gas components in the later stage. However, the positive correlation between hydrogen and helium concentrations in Figure 10b,d shows that hydrogen and helium were mixed before their upward migration, which was observed also in previous studies on the correlation between nitrogen, carbon dioxide, and helium concentrations [7; 10; 11; 12; 13] and proves that hydrogen plays an important role in the migration of helium. At present, it is generally believed that the inorganic genesis of hydrogen mainly occurs in three ways, which are serpentinization, deep degassing, and irradiation of water. Serpentinization refers to the process of generating hydrogen by the reaction between Fe\({}^{2+}\)-rich minerals (olivine or pyroxene) and water in basal ultramafic rocks; deep degassing refers to the process of releasing hydrogen from the mantle and the earth's core, excluding the process of degassing within the earth's crust; water irradiation refers to the process of decomposing hydrogen from water by the energy generated by the decay of radioactive elements (e.g., uranium and thorium) in the earth's crust, which is also considered to be an important source of hydrogen [43; 44; 45; 46]. Previous studies on hydrogen isotopes in the SK-2 well suggested that the hydrogen in the basement and the Huoshiling Formation mainly derives from the mantle [42].