## 5 Discussion
### Inhomogeneity of Helium Distribution
In the mantle, it is mainly produced by the degassing of mantle magmas, and in the crust it is produced by the radioactive decay of uranium and thorium. Previous studies showed that the proportion of mantle-derived helium in the Ordos Basin is very small and can be ignored; so, helium in the Ordos Basin is mainly crust-derived [15; 21]. There are two main sources of crust-derived helium, one consists of uranium-rich and thorium-rich metamorphic rocks and granitidis in the basin basement undergoing decay, and the other consists of uranium-rich and thorium-rich sedimentary rocks within the basin, usually organic-rich mud shales, undergoing decay. In this study, the mudstones appeared to supply helium to the sandstone and could also act as a cap layer, reducing the upward helium transport, which is important for helium preservation. In addition, these high-helium-concentration sandstone sections showed poor hydrocarbon generation capacity and could not generate large amounts of hydrocarbon gases. Since they are located far from the main hydrocarbon-rich rocks of the reservoir, they showed a small dilution effect of hydrocarbon gas on helium and a relatively high helium concentration. The three wells for helium concentration detection in this study are all located in the same work area, have the same geological background, and are close to each other. According to the traditional oil and gas accumulation theory, the three wells should provide similar detection results [21]. Although a high hydrocarbon gas concentration was detected in all three wells, helium was detected in only one well, which is inconsistent with traditional knowledge, indicating that helium has a more complex accumulation mechanism. This phenomenon may be related to the different molecular kinetic features of methane and helium. Previous studies found that the molecular mean free path of helium is greater than that of methane. Therefore, in rock formations, the adsorption of methane reduces the channel size, increases surface roughness, and thus suppresses the molecular slip phenomenon. This results in a lower apparent permeability of methane compared to helium. Therefore, helium has a greater migration capacity relative to methane [40], which leads to a more complex accumulation mechanism for helium. Through the above analysis, we found that in the examined natural gas field, the horizontal distribution of helium was not uniform, showing a high degree of heterogeneity. At the same time, in the vertical direction, we found that the natural gas dessert layer might not coincide with the helium dessert layer. The helium concentration in the helium dessert layer was often 1 to 2 times higher than in the natural gas dessert layer, but these layers are not the traditional natural gas target layers, so they are easy to be missed in the traditional exploration process. Using the helium detector to conduct real-time helium detection during drilling processes can promptly allow for detecting abnormally high levels of helium, compensating for the deficiencies of the traditional exploration process, and is of great significance for helium exploration.