Helium is the second most abundant element in the universe and has the lowest melting point (−272.1 °C) and boiling point (−268.94 °C) of any known element in nature. It plays an irreplaceable role in various fields, including aviation, healthcare, cryogenic superconductivity, chip manufacturing, and civilian high-tech industries. Currently, helium used in industry mainly comes from natural gas, which is produced by the radioactive decay of uranium (U) and thorium (Th) in rocks. The content of U and Th in rocks is low, and their half-lives are extremely long, resulting in a very slow rate of helium production. As a result, it is difficult for helium to accumulate into independent reservoirs, and it is only enriched in a few natural gas reservoirs, while most natural gas reservoirs are helium-deficient ^[1]. With the development of the economy and high-tech industries, the demand for helium is increasing. Where to find and how to locate helium-rich resources are pressing concerns for explorers today. Since helium generation and enrichment laws differ significantly from those of natural gas, with distinct rules and characteristics, traditional methods used for natural gas exploration are not applicable to helium. It is necessary to establish a theory of helium enrichment to guide helium exploration efforts.

Although research on helium has been conducted for quite some time, a theory of helium enrichment has yet to be established. Researchers have investigated various aspects, such as the source, migration, and preservation conditions of helium, but there is significant disagreement among them. For instance, different views exist in the source of helium in helium-rich reservoirs of radioactive origin. Some scholars believe the helium originates from the decay of uranium and thorium in granite and pegmatite ^[2], while others suggest it comes from hydrocarbon source rocks ^[3]. Some broadly argue that helium comes from the shallow crust ^[4], whereas others propose that it is generated jointly by basement rocks and hydrocarbon source rocks ^[5]. Additionally, some argue that the main source of helium is the basement ^[6-7]. In summary, there is still no consensus on the genesis of helium in helium-rich reservoirs of radioactive origin. Regarding the migration pathways, some scholars agreed that helium migrates and accumulates through basement faults and fractures ^[5,8], and this view is widely accepted. Due to helium's small molecular diameter and high diffusivity and permeability, some believe that helium escapes more readily than other gas molecules, making it possible for helium to accumulate only in environments with good sealing conditions ^[9]. However, some researches have shown that many helium-rich reservoirs do not have excellent sealing conditions, and helium enrichment does not necessarily require an ideal cap rock. If the cap rock is too impermeable, there may be no regional pressure release channels for the upward movement of deep fluids, and the helium in these fluids may not reach the overlying reservoirs to form helium-rich gas reservoirs ^[10]. Regarding the phase of migration, many studies suggest that water-soluble helium plays a significant role in the formation of helium-rich reservoirs ^[11-12]. Additionally, research has found that almost all geothermal water or spring gases show some evidence of helium, with some regions exhibiting relatively high helium content. For example, in the Weihe Basin, helium shows are quite common in geothermal water wells, with an average helium content of about 1% in the gases released from the geothermal water. Preliminary estimates suggest that the helium resource potential of this basin reaches 33.82×10⁸ m^{3 [13]}. Despite its considerable resource potential, it is difficult to exploit and achieve large-scale production, making it unlikely to be a primary field for future helium exploration and development. Nevertheless, it supports the possibility that helium might migrate and accumulate in a water-soluble form. In the study on helium enrichment theory, some scholars have proposed the concept of weak source for helium accumulation, based on the characteristics of low U and Th contents in rocks, extremely long half-lives, and very low helium supply intensity ^[1,14].

After years of research, our team has made significant systematic progress in several aspects, including helium enrichment mechanisms ^[1,14], helium enrichment models ^[15], genesis and distribution patterns of helium in Chinese petroliferous basins ^[16], the associated coupling relationships between helium and natural gas components ^[17], the differences between helium enrichment and natural gas accumulation, and the various misconceptions faced in exploration ^[10]. On the basis of the above research, the authors' team has carried out a large amount of new research work, improved the overall understanding of helium enrichment, distribution, and main controlling factors, established a preliminary theory of helium enrichment, proposed exploration ideas for helium gas reservoirs, and guided the evaluation of helium resources and exploration of helium gas reservoirs.

Our team collected natural gas and rock samples from major petroliferous basins in China to analyze helium content, composition and isotopes, and U and Th contents in rock samples. For comparative studies, we also gathered publicly available data domestically and internationally on helium content in natural gas and U and Th contents in rocks from both domestic and international sources.

Based on the helium content analysis, a batch of relatively helium-rich gas fields or helium-rich zones was identified (Table 1). The natural gas composition in helium-rich gas fields varies significantly across different basins.

In the basins such as the Tarim, Qaidam, and Sichuan basins in central and western China, natural gas is primarily composed of hydrocarbons, but also contains a high proportion of non-hydrocarbon gases. Among hydrocarbons, the content of heavier hydrocarbons like ethane is relatively low in the natural gas from the Tarim and Sichuan basins, so it is mostly dry gas. In the Qaidam Basin, the Dongping and Jianbei gas fields contain dry gas, while the Mabei gas field has higher ethane content, classifying as wet gas reservoir. Among non-hydrocarbon gases, all fields exhibit high nitrogen content. For example, the Hetianhe gas field has an average nitrogen content of 10.4%, the Akemomu gas field 8.01%, the Dongping gas field 9.53%, and the Jianbei gas field 12.45%. And the Mabei gas field has slightly lower nitrogen content at 4.90%, while the Weiyuan gas field shows an average nitrogen content of 7.29%. Additionally, as shown in Table 1, there is a clear positive correlation between nitrogen content and helium content in helium-rich gas fields. The carbon dioxide content varies greatly between different gas fields. In the Akemomu gas field, it is 13.80%, while in the Hetianhe gas field, it is 8.76% averagely in Ma 2 and Ma 3 well areas. Ma 4 and Ma 5 well areas have much lower content, averaging only 1.25%. In the Qaidam Basin, several helium-rich gas fields show very low carbon dioxide content, and even no carbon dioxide detected in most samples. The Weiyuan gas field in the Sichuan Basin contains a certain amount of carbon dioxide, with an average content of 4.16%.

The natural gas composition in helium-rich gas fields in eastern basins differs significantly from those in central and western basins. Helium-rich reservoirs in the eastern basins are dominated by high content of non-hydrocarbon gases such as carbon dioxide and nitrogen, with hydrocarbons playing a secondary role. For example, in the Huagou gas field of the Bohai Bay Basin, Well Huagou 501 has a methane content of only 1.77%, carbon dioxide content of 34.27%, and nitrogen content of 61.86%. In the Huangqiao gas field of the Subei Basin, the average methane content is 24.98%, the average carbon dioxide content is 8.09%, and the average nitrogen content is 61.65%. The natural gas composition in the Baoyue gas field of the Sanshui Basin varies significantly between different wells. For example, in wells Shuishen 44 and Shuishen 24, carbon dioxide dominates, with contents of 83.09% and 99.48%, respectively, while the hydrocarbon content is very low. In contrast, wells Shuishen 3 and Nan 35 are dominated by hydrocarbons, with methane contents of 65.23% and 76.81%, respectively. In the helium-rich gas fields of Wuzhan and Taipingzhuang in the Songliao Basin, natural gas is still primarily composed of hydrocarbons, with an average methane content of 91.08% and average carbon dioxide content of 0.37%.

Regarding helium content, the Hetianhe gas field in the Tarim Basin has helium content ranging from 0.281% to 0.372%, with an average of 0.320%. In the Akemomu gas field, helium content ranges from 0.110% to 0.125%, with an average of 0.120%. In the Qaidam Basin, the Mabei gas field has helium content ranging from 0.074% to 0.259%, with an average of 0.212%. The helium content varies significantly across different blocks in the Dongping gas field: Block 1 has helium content ranging from 0.072% to 0.074%, with an average of 0.073%, while Block 3 has helium content ranging from 0.086% to 0.695%, with an average of 0.319%. The Jianbei gas field has helium content ranging from 0.171% to 0.258%, with an average of 0.193%.

The helium isotope ratios in helium-rich gas fields in China show significant differences across different basins and gas fields. For example, in the Hetianhe gas field of the Tarim Basin, the ³He/⁴He ratio is (11.70-14.80)×10⁻⁸, indicating a trace amount of mantle-derived helium, with a proportion of less than 2%. In the Akemomu gas field, the ³He/⁴He ratio is (65.2-106.0)×10⁻⁸, with mantle-derived helium accounting for about 10%. As for the Qaidam Basin, no helium isotopic analysis was conducted in this study, but according to previously published data, the ³He/⁴He ratio is (1.01-3.62)×10⁻⁸, equivalent to the end-member value of crust-source helium indicating that there is no mantle-derived helium mixed in it ^[24]. In the Weiyuan gas field of the Sichuan Basin and two shale gas wells, Weiyuan 201-H1 and Weiyuan 201-H3, the ³He/⁴He ratio is (1.15-1.31)×10⁻⁸, representing typical crust-source helium with no mantle-derived helium. In the Songliao Basin, the ³He/⁴He ratio is (26.1-67.8)×10⁻⁸, indicating clear evidence of mantle-derived helium, with a mixing proportion of 2.2% to 6.0%. In the Bohai Bay Basin, Sanshui Basin and Subei Basin, the ³He/⁴He ratio is (1.60-6.39)×10⁻⁶, which is two orders of magnitude higher than crust-source helium. The proportion of mantle-derived helium is relatively high, with some samples showing mantle-derived helium accounting for more than 50%, such as in wells Shuishen 44 and Shuishen 24 in the Baoyue gas field.

Helium in natural gas has both crust-source and mantle-source origins ^[25-26], both of which are of inorganic genesis. Helium in all gas fields in China is dominated by ⁴He, but the ³He content in the eastern region is higher than that in the central and western regions, and the samples have obvious mantle-source helium mixing, which is a mixed crust-mantle type. In the central region, such as the Sichuan and Ordos basins, all of the helium is crust-source type; and some of the samples of helium from the Tarim Basin in the western region show traces or small amount of mantle-source helium mixing, but it is also of crust-source genesis on the whole ^[15]. This paper focuses on crust-source helium, i.e., radiation-induced helium enrichment, and proposes the concepts of effective helium source rocks and main helium source rocks.

Corresponding to diverse types of petroliferous basins, there are various mechanisms and models of helium enrichment and accumulation.

Helium is the associated gas in natural gas, but it does not mean that finding natural gas will lead to the discovery of helium-rich reservoirs. So far, helium-rich gas fields discovered all over the world have been accidentally found in the search for natural gas, the number of such gas reservoirs is very small, and the vast majority of the world's gas reservoirs are helium-poor. Due to the limited understanding on the enrichment mechanism and distribution law of helium, no exploration idea for helium-rich gas reservoirs have been formed so far. Through research, it is found that the mechanism of helium enrichment is very different from that of natural gas, and the formation conditions and main controlling factors are not the same, therefore, the exploration methods of helium are very different from those of natural gas, and even contradictory to natural gas exploration. Natural gas exploration always selects the "sweet spot" area for drilling, but such areas are not favorable for helium enrichment due to the large degree of helium dilution resulted from high intensity of natural gas charging and large reserve abundance. It is difficult for helium content to exceed the helium enrichment criterion of 0.1% even if there is a supply of exogenous helium. Therefore, to find helium-rich gas reservoirs, it is necessary to go beyond the boundaries of natural gas exploration and search for helium-rich zones according to the characteristics and controlling factors of helium enrichment.

First of all, in the accumulation process of natural gas from hydrocarbon source rock to reservoir, only the helium generated by U and Th decay in the hydrocarbon source rock and reservoir is captured, so the amount of helium capture is limited. And coupled with the dilution effect of natural gas on helium, it will be very difficult to form helium-rich reservoirs if there is no additional helium source to supply helium to the gas reservoir. This additional source of helium comes from the ancient basement, because the ancient basement rocks are large and old with long U and Th decay time, which cumulatively generates more helium. The helium generated tends to be preserved in the groundwater through dissolution. Additionally, the overlying gas reservoir cannot be enriched with helium unless the helium-dissolved groundwater in the pores of the basement rocks migrates into the reservoir, so there must be channels to communicate the basement rocks with the gas reservoir. Besides available migration channel, the driving force of groundwater migrating upward is indispensable, which comes from tectonic uplift. If the tectonic uplift amplitude is small, or the gas reservoir is basically not uplifted, the formation water dissolved with helium in the ancient basement is difficult to move to the vicinity of the gas reservoir and supply the gas reservoir with additional helium, so a helium-rich gas reservoir can be hardly formed, which also explains why helium is often enriched in the structural high of the same stratum.

Secondly, it is important to avoid misconception of exploration for helium-rich natural gas. In view of excessive dilution of helium by natural gas, the exploration areas with high gas generating intensity and high gas charging intensity shall be evaded. High-pressure and ultra-high-pressure gas reservoir distribution areas are also not conducive to helium enrichment. These areas often have good sealing conditions, and there is no pressure relief channel, which is not conducive to the upward movement of deep fluids, so the helium in the deep fluids is difficult to reach the overlying reservoirs to form helium-rich reservoirs. Volcanic gas reservoirs should be evaded as well, for volcanic rocks come from mantle magma, but all mantle-derived helium carried with the intrusion of magma is dissipated along with other volcanic gases and is rarely left in the volcanic rock. Additionally, volcanic rocks are mostly basic rocks with very low U and Th contents, so the amount of helium generated is too small to form helium-rich gas reservoirs. Taking the Kelameili gas field in the Junggar Basin as an example, its main body consists of Carboniferous volcanic rocks, the helium isotope analyses show that all of the helium come from the crust, with low helium content of 0.2-0.3 mg/g ^[64-65]. It should be noted that not all regions with ancient basement have helium-rich gas reservoirs. Only under specific geological conditions, helium in ancient basement can migrate to gas reservoirs through carriers and form helium-rich gas reservoirs.

Therefore, the search for helium-rich resources should focus on the gas reservoirs in or near ancient basement development areas with developed fault, larger uplift amplitude under the effect of the late tectonic movement, and medium-low abundance of natural gas resource. This requires that the effective natural gas supply intensity in these areas should not be too high, otherwise the large degree of helium dilution will hinder the formation of helium-rich gas reservoirs. The areas that meet such conditions are generally located in the periphery of the gas generating center of hydrocarbon source rock, the edge of the ancient Craton basin with medium-low gas generating intensity, the collision zone between basins, the paleouplift within the basin, the discordogenic fault development area, the shattered fault zone, and the area with relatively large uplift amplitude under the effect of the Himalayan tectonic movement.

In addition, although helium is an associated gas in natural gas, the proposed coupling of helium and nitrogen enrichment has made it clear that helium enrichment is not dependent on alkane gas or CO₂, but really accompanied by N₂ in the gas reservoirs. If we can find nitrogen-rich or nitrogen-dominated gas reservoirs, it is probable that we can find helium-rich gas reservoirs. In reality, it is not necessary to search for nitrogen reservoirs in exploration and production, but if we can grasp the law of nitrogen generation and distribution in the geologic body, it will be a good reference for searching helium. In the future, we can also try to find nitrogen-dominated gas reservoirs from the perspective of nitrogen formation and accumulation. If such gas reservoirs can be found, helium-rich resources will inevitably be found.

Generally, when natural gas migrates from source rock to reservoir and accumulates, helium content is often low due to the dilution of alkane gas to helium. The formation of helium-rich gas reservoirs requires supplementation by additional helium sources, which are mainly ancient basement rocks with a certain U and Th contents. In the areas with frequent magmatic activity in tensional basins, the additional helium source can also be mantle-sourced helium. Helium-rich reservoirs are characterized by a significant associated relationship of helium and nitrogen, both of which are mainly derived from the same ancient basement rocks, with helium from U and Th radioactive decay, and nitrogen from the decomposition of nitrogenous compounds. The enrichment of helium in helium-rich gas reservoirs has gone through the process of "multi-source helium supply, main-source helium enrichment, helium-nitrogen associating, and co-enrichment". Helium generated in ancient basement is preserved and transported as water-soluble helium, which is eventually released and enriched in gas reservoirs. There are six modes of helium enrichment under different tectonic backgrounds and reservoir formation conditions. The formation of helium-rich gas reservoirs is mainly controlled by the degree of difficulty and intensity of helium supply from ancient basement rocks to the gas reservoirs, so helium-rich gas reservoirs are mostly distributed in the areas above the ancient basement, with developed discordogenic fault, strong tectonic activities, and large uplift amplitude. In view of the dilution effect of alkane gas on helium, the helium content is relatively high in the regions with low supply intensity of alkane gas. The search for the favorable helium enrichment area should focus on the zones close to the basement rocks with developed fault, large uplift amplitude under the effect of the late tectonic movement, and low gas supply intensity in the ancient basement development areas.