The natural gas is exploited from over 25 distinct gas-bearing layers across different regions of the Sichuan Basin, with production layers varying by location (Figure 3; Table 1). In the North low steep fold and thrust belt, natural gas is mainly exploited from the upper Triassic Xujiahe Formation (T₃ x), lower Permian (P₁), upper Permian Changxing formation (P₂ ch), and the upper Carboniferous Huanglong Formation (C₂ h). Helium concentration in the Xujiahe Formation (T₃ x) ranges from 70 ppm to 400 ppm, with an average of 140 ppm, while it is in the range of 10–290 ppm, with an average of 150 ppm in Permian strata. Helium concentration in the Huanglong Formation (C₂ h) is 370 ppm.

In the West depression, natural gas is mainly exploited from Cretaceous, the Middle and upper Jurassic Suining Formation (J₂ sn) and Penglaizhen Formation (J₃ p), and the upper Triassic Xujiahe Formation (T₃ x). The helium concentration in Cretaceous is 79 ppm. For the Jurassic strata, helium levels range from 39 to 410 ppm, with an average of 190 ppm. In the Xujiahe Formation (T₃ x), helium concentrations range from 37 to 200 ppm, averaging 190 ppm.

In the Southwest low-flat structural belt, natural gas is mainly extracted from Triassic and Permian formations. Helium concentrations in the Triassic range from 180 to 640 ppm, with an average of 410 ppm. In the Permian, the helium concentration is around 170 ppm.

In the Central flat and gentle fold belt, natural gas is produced from almost all sedimentary strata (Table 1). Helium concentrations in the Jurassic range from 230 to 2013 ppm (average 799 ppm). The Triassic shows helium concentrations of 172–469 ppm (average 315 ppm). Helium concentration in Permian ranges from 30–2,120 ppm, (average 1,152 ppm), while the Ordovician contains helium concentrations of 1,670–1,750 ppm (average 1,710 ppm). The Cambrian and Sinian formations show helium concentration in the ranges of 100–2,620 ppm (average 935 ppm) and 90–18,770 ppm (average 1,290 ppm).

In the East high and steep structural zone, natural gas is mainly sourced from the upper Triassic Xujiahe Formation (T₃ x), Permian and the upper Carboniferous Huanglong Formation (C₂ h). Helium concentration in the Xujiahe Formation (T₃ x) ranges of 10–470 ppm (average 167 ppm), compared to 56–1,500 ppm (average of 389 ppm) for the Permian. Helium concentration in the Huanglong Formation (C₂ h) in the range of 120–947 ppm (average 445 ppm).

In the South broom-shaped structural belt, natural gas is mainly extracted from the lower Triassic Feixianguan (T₁ f) and Jialingjiang Formation (T₁ j), and the lower Permian Maokou Formation (P₁ m). Helium concentration in Triassic ranges from 270 to 600 ppm (average 435 ppm), while in the Permian helium concentration is in the range of 470–650 ppm (average 586 ppm).

Overall, helium-rich gas fields (≥500 ppm) are predominantly located in the Central flat and gentle fold belt (Wang et al., 2020). The helium concentration tends to increase from the periphery toward the center of the Sichuan Basin (Weiyuan), indicating that the central part of the basin is more favorable for helium enrichment.

In the Sichuan Basin, shale gas is primarily extracted from the Wufeng-Longmaxi Formation (O₃ w-S₁ l). In the southern part of the basin, helium concentrations in shale gas varies from 173 to 414 ppm (average below 300 ppm), as observed in the Weirong, Changning, and Zhaotong shale gas fields (Table 2). Other shale gas fields, such as those in Fuling, Weiyuan, and Fushun-Yongchuan, have an average helium concentration of approximately 400 ppm. In the eastern Sichuan Basin, helium concentrations are somewhat higher, with the Pengshui shale gas field exhibiting a concentration of about 993 ppm. Additionally, helium concentrations in the lower Cambrian Qiongzhusi shale (Є₁ q) are greater than those in the Wufeng-Longmaxi (O₃ w-S₁ l) shale, reaching up to 1,400 ppm. Overall, there is no distinct pattern in the distribution of helium across shale gas fields in the basin.

As shown in Figure 3, helium concentrations are highest in the Sinian Dengying Formation (Z₂ d) and lowest in the Cretaceous deposits. Vertically, there is a slight decrease in helium concentration upward, suggesting that helium primarily originates from deeper layers. High helium concentration layers (≥1,000 ppm) are predominantly found in the deeper strata, such as the Sinian, Cambrian, Permian, and a shallow layer (J₂) in the Jinqiu gas field. These helium-rich layers are interbedded with helium-poor layers (<500 ppm), indicating that helium enrichment is influenced by factors beyond vertical diffusion, including other geological processes.

It has been proved that the gas in Dengying Formation (Z₂ d) originated from the Qiongzhusi shale (Є₁ q) (Zhu, 2006). While helium concentrations in the Cambrian are lower than in the Dengying Formation, they are higher compared to those in the Ordovician and middle-upper Cambrian gas reservoirs. This suggests that the Cambrian Qiongzhusi Formation may influenced helium enrichment in Sinian gas reservoirs. Conversely, gas from the Huanglong (C₂ h) and Maokou formation (P₁ m) originated from the Wufeng-Longmaxi shale (O₃ w-S₁ l) (Zhu, 2006). However, helium concentrations in the Wufeng-Longmaxi shale (O₃ w-S₁ l) are considerably lower than in the double layers, indicating that the influence of the Wufeng-Longmaxi shale on vertical helium diffusion is limited.

Helium in natural gas is derived from three types of sources, as indicated by the ³He/⁴He ratio (R): (1) atmospheric helium, with R=1.4 ×10⁻⁶ (Ra), is mainly released through volcanic eruptions, magma degassing, and rock weathering (Wang et al., 2020; Chen et al., 2021); (2) mantle-derived helium, originated volatile compounds that entered the sedimentary crust along fault systems in tectonically active regions. This type of helium usually has a ³He/⁴He ratio greater than Ra (Wang et al., 2020). (3) crust-derived helium is radiogenic ⁴He produced by the decay of U, Th, and other elements in crustal rocks and minerals (Kennedy et al., 2002), and often shows a ³He/⁴He ratio of 0.02 Ra. If the R/Ra ratio exceeded 0.1, it indicates that mantle-derived helium constitutes more than 12% of the total helium (Zhao et al., 2023a).

In the Sichuan Basin (Figure 4A), the R/Ra ratio in gas fields varies from 0.002 to 0.05 (average 0.016). For shale gas, the R/Ra ratio ranges from 0.007 to 0.04 (average 0.015). All these values are below 0.1, indicating that the helium in the Sichuan Basin is predominantly crust-derived. However, mantle-derived helium has been reported in several hot springs in the western Sichuan Basin (Du et al., 2006), with R/Ra values ranging from 0.79 to 2.59 (Figure 1). Additionally, R/Ra ratios of 0.23 were observed in the Longmenshan Fold Belt (Du et al., 2006). The highest R/Ra value observed in gas fields is 0.05, as observed in the Xinchang and Xiaoquan gas fields, suggesting a minor contribution of mantle-derived helium in the western depression.

Nitrogen (N₂) in natural gas originates from several sources, including primordial nitrogen from the mantle, volcanic magma, radioactivity, the atmosphere, sedimentary organic matter, and clay minerals (Liu et al., 2006; Zhao et al., 2023a; Mansour et al., 2020; Mansour and Wagreich, 2022). Zhao et al. (2023a) indicated that N₂ in the central Sichuan Basin derived from sedimentary organic matter in argillaceous source rocks, with N₂ content being proportional to the maturity of the organic matter. Conversely, Wang et al. (2023b) suggested that the nitrogen is homologous with helium and that it originated from granite or the metamorphic rock basement of the basin. Hence, consequently, N₂ concentration is often used as an indicator of helium origin in natural gas.

In the Sichuan Basin, N₂ concentration varies widely, ranging from 0.01% to 37.37% (Figure 4B). For shale gas, N₂ concentrations range from 0.01% to 4.05% (average 0.65%). In contrast, N₂ concentrations in gas fields variy from 0.02% to 37.37% (average 2.39%). This indicates that N₂ is significantly more abundant in natural gas than in shale gas, suggesting that N₂ in the Sichuan Basin did not primarily originate from sedimentary organic matter in hydrocarbon source rocks.

In the Jinqiu gas field, natural gas predominantly comes from the Xujiahe Formation (T₃ x) (Liu et al., 2024). Despite the lower maturity of this gas compared to the shale gas in the Wufeng-Longmaxi Formation (O₃ w-S₁ l), N₂ concentrations in the Jinqiu gas field range from 0.2% to 5.06% (average 1.27%), which are higher than the average 0.65% observed in the Wufeng-Longmaxi shale. This provides further evidence that N₂ in the Jinqiu gas field is likely derived from granite or the metamorphic rock basement of the Sichuan Basin (Wang et al., 2023b), rather than from sedimentary organic matter.

In the Weiyuan gas field, N₂ concentrations range from 3.09% to 37.37%, (average 8.16%), which is notably higher than in other gas fields. This indicates that the N₂ in the Weiyuan gas field likely derived from the granite basement of the Sichuan Basin. Conversely, N₂ concentrations are lowest in the basin periphery, with values in the Pengshui, Zhaotong, and Changning shale gas fields being less than 0.63% (average 0.34%). This suggests that minimal N₂ in these areas is derived from the granitic basement of the Sichuan Basin.

Overall, it can be inferred that the source of helium differs between within and outside the Sichuan Basin. Within the basin, helium primarily originated from the granite basement of the Sichuan Basin, whereas in the basin periphery, helium sources differ, with minimal contribution from the basin basement.

It is accepted that both CO₂ and low helium concentration (<200 ppm) are mantle-derived (Dai et al., 2017; Wang et al., 2023a). A R/Ra ratio greater than 2.0 can serve as an indicator for identifying CO₂ gas fields. As shown in Figure 4A, no samples in the Sichuan Basin have R/R_a ratios >0.1, and no CO₂ gas fields have been discovered in this region, indicating that helium in the Sichuan Basin is primarily crust-derived.

Globally, gases from mid-ocean ridge basalts exhibited a similar CO₂/³He molar ratio, averaging 2 × 10⁹. In eastern China, as the molar ratio of CO₂ to ³He decreases, helium concentration increases by three orders of magnitude in gas reservoirs (Wang et al., 2023b). Wang et al. (2022) concluded that the loss of CO₂ is the primary mechanism driving helium enrichment in gas reservoirs. CO₂ is deposited as carbonate veins when the partial pressure and temperature decrease as fluids migrate from deep to shallow layers. In the East High and Steep Structure Zone (Figure 1), the molar ratio of CO₂ to ³He decreases from 2 × 10¹¹ to 2 × 10⁸, and helium concentrations increase proportionally with the loss of CO₂ (Figure 4C). This phenomenon is likely related to several deep faults in the East High and Steep Structure Zone (He et al., 2018), which facilitated the migration of deep-layer fluids to shallower layers.

Based on extensive CH₄/³He data from various global locations (Dai et al., 2008), it has been concluded that methane with CH₄/³He ratios ≤10⁶, ≥ 10¹¹, and 10⁶-10¹⁰ are indicative may be of either organic or inorganic origin (Dai et al., 2017). As shown in Figure 4D, gas in the Sichuan Basin is close to the crustal end (CH₄/³He = 3 × 10¹³, R/Ra = 0.01) (Ni et al., 2014), and is far from the abiotic values observed in geothermal fluids from the 21°N East Pacific Rise (EPR) (Dai et al., 2017) and gas seeps from the Zambales Ophiolite in the Philippines (ZO) (Abrajano et al., 1988). The highest CH₄/³He ratios are found in the North Low Steep Fold and Thrust Belt, ranging from 5.5 × 10¹¹ to 5.8 × 10¹³, while the lowest CH₄/³He ratios are observed in the Weiyuan gas field, ranging from 5.6 × 10¹⁰ to 2.8 × 10¹¹. The CH₄/³He ratio in the Anyue-Moxi gas field falls between these values. This suggests that some methane in the Weiyuan gas field has an inorganic origin (Dai, 2003). The proportion of inorganic methane increases from the North Low Steep Fold and Thrust Belt to the Weiyuan gas field in the Central Flat and Gentle Fold Belt, indicating that helium may accumulate with methane from the North Low Steep Fold and Thrust Belt to the Weiyuan gas field.

Generally, ²⁰Ne in natural gas originates from the atmosphere and enters the crust through groundwater recharge (Byrne et al., 2021). ²⁰Ne in crustal fluids rarely derived from crustal or mantle origins. However, ⁴He in the crust is primarily radiogenic (Ballentine and Burnard, 2002). In Weiyuan shale gas field, ²⁰Ne concentrations range from 3.64 × 10⁻⁹ to 1.14 × 10⁻⁷, and there is a strong linear relationship with ⁴He concentration (R ² = 0.9428). A similar relationship is observed in the Jinqiu gas field(R ² = 0.85), suggesting that ²⁰Ne and ⁴He may have similar migration processes, likely involving dissolution in groundwater and transport with water (Zhang et al., 2024a).

In contrast, this linear relationship is not evident in the Changning shale gas field (R ² = 0.123), indicating that helium may not primarily migrate with groundwater flow in this field (Figure 5A). The ⁴He^/20Neratios in the Sichuan Basin, ranging from 3,951 to 67,228, are significantly higher than the atmospheric ratio of 0.32 (Liu et al., 2024). This suggests that atmospheric helium can be disregarded. Moreover, all data points fall within the range of crust-derived helium, with less than 0.5% mantle-derived helium (Liu et al., 2024) indicating that mantle-derived helium constitutes less than 0.5% of the total helium in the Sichuan Basin (Figure 5B).

The relationship between helium-rich gas distribution and shale is typical in the central Sichuan Basin (Figure 11). In the Gaoshi-Moxi gas reservoir and Weiyuan gas reservoir, the reservoirs are interbedded with mudstone or shale layers. In the Gaoshi-Moxi gas reservoir, helium concentration decreases progressively upwards. In Member 2 of the Dengying Formation, helium concentrations range from 0.018% to 0.102%, while in Member 4, they range from 0.012% to 0.029%. Both are higher than the concentrations found in the Cambrian gas reservoir, which range from 0.005% to 0.008%. A similar pattern is observed in the Weiyuan gas reservoir, where helium concentrations are lowest in the Permian gas reservoirs.

Within a single gas formation, helium concentration tends to increase as burial depth decreases. For example, in the Dengying Formation, helium concentrations are only 0.012%–0.102% in the Gaoshi-Moxi gas reservoir but increase to 0.12%–0.25% in the Weiyuan gas reservoir. This pattern is consistent with the accumulation of hydrocarbons (Su et al., 2020) and is closely related to the solubility of helium in water. As shown in Figure 11, the burial depth of the Gaoshi-Moxi gas reservoir is about 2,500 m greater than that of the Weiyuan gas reservoir. Helium solubility in water increases significantly with increasing partial pressure, approximately ninefold from 10 to 100 MPa (Abrosimov and Lebedeva, 2013).

As fluids migrate from the Gaoshi-Moxi gas reservoir to the Weiyuan gas reservoir, the formation pressure decreases by more than 25 MPa, which lead to a notable drop in helium solubility in water. Consequently, some of the helium that was dissolved in the water is released and accumulated in the Weiyuan gas reservoirs. Regional shale layers facilitated the long-distance migration of helium alongside carrier gases. This migration help in partial offset the relatively low helium generation rates (liters produced annually per cubic kilometer of rock) (Li et al., 2017; Li et al., 2022), and contributing to systematic helium enrichment.