The E₃s², E₂s³, E₂s⁴, and Upper Permian gas samples from the Dongpu Sag display the helium contents of 0.0091–0.0204%, 0.0031–0.0162%, 0.0130–0.0202%, and 0.0188–0.0217%, respectively, with the average contents of 0.0116% (N = 5, N refers to the number of samples), 0.0085% (N = 5), 0.0160% (N = 7), and 0.0202% (N = 2), respectively (Table 1; Figure 3A). Helium contents in natural gas from different strata are lower than 0.05%, which is consistent with the results of other areas in the Bohai Bay Basin according to previous studies (Figure 3B).

The ³He/⁴He ratios for natural gas from different strata in the Dongpu Sag range from 0.148 × 10^–7 to 11.986 × 10^–7 (Table 1). The R/Ra values of the E₃s², E₂s³, E₂s⁴, and Upper Permian natural gas are 0.014–0.856, 0.011–0.575, 0.191–0.212, and 0.078–0.079, respectively, with the average values of 0.274 (N = 5), 0.169 (N = 5), 0.200 (N = 7), and 0.079 (N = 2), respectively (Table 1). The R/Ra ratios of natural gas from different strata are lower than 1 (Table 1; Figures 4, 5). The ⁴⁰Ar/³⁶Ar ratios range from 487.2 to 3,270.4 (Table 1), which are significantly higher than that in the air (295.5, Allègre et al., 1987).

The CH₄/³He ratios for the E₃s², E₂s³, E₂s⁴, and Upper Permian natural gas from the Dongpu Sag are (3.7–516.7) × 10⁹, (6.8–1826.3) × 10⁹, (17.3–26.8) × 10⁹, and (38.7–43.7) × 10⁹, respectively (Table 1), with the average values of 130.1 × 10⁹, 618.9 × 10⁹, 21.6 × 10⁹, and 41.2 × 10⁹, respectively. The corresponding CO₂/³He ratios are (0.05–8.89) × 10⁹, (0.10–35.04) × 10⁹, (0.33–0.51) × 10⁹, and (2.24–2.97) × 10⁹, respectively (Table 1), with the average values of 2.34 × 10⁹, 12.56 × 10⁹, 0.41 × 10⁹, and 2.61 × 10⁹, respectively.

Natural gas commonly includes five categories according to different helium contents, i.e., extremely depleted (Helium% < 0.005%), helium depleted (0.005% ≤ Helium% < 0.050%), helium general (0.050% ≤ Helium% < 0.150%), helium rich (0.150% ≤ Helium% < 0.500%), and helium extremely rich (Helium% ≥ 0.500%) gases (Dai et al., 2017). The average helium content of natural gas from the Panhandle-Hugoton gas field in the U.S. is 0.586% (Brown, 2019), suggesting the characteristics of helium extremely rich gas. Natural gas from the Weiyuan gas field in the Sichuan Basin in China display the average helium content of 0.251% for 215 gas samples (Dai et al., 2017), and helium contents of natural gas from the Hetianhe gas field in the Tarim Basin range from 0.30% to 0.37% (Tao et al., 2019), both suggesting helium rich gas.

The helium contents of natural gas from the Bohai Bay Basin range from 0.0005% to 0.26% with an average of 0.0197% (Zhang et al., 2008; Dai et al., 2017), which are mainly helium depleted and extremely depleted gases. Among the 72 gas samples from the basin, only 3 and 2 samples reach the standard of helium general and rich gases, respectively (Figure 3B). Natural gas from the Dongpu Sag has the helium contents ranging from 0.0031% to 0.0217% averaging 0.0133%. Natural gas from different strata is helium depleted and extremely depleted rather than helium rich despite various contents of heliumAlthough natural gas from different strata vary in helium contents, the gas is helium depleted and extremely depleted rather than helium rich (Table 1; Figure 3). Helium contents of natural gas from the Dongpu Sag are significantly lower than those from the Sichuan Basin, and this may be derived from the differences of helium source and accumulation mechanism.

Based on the helium amount in the natural gas reserves, helium gas fields can be divided into very small, small, medium, large, and extra large gas fields, with the corresponding helium reserves of <5 × 10⁶ m³, (5–25) × 10⁶ m³, (25–50) × 10⁶ m³, (50–100) × 10⁶ m³, and ≥100 × 10⁶ m³, respectively (Dai et al., 2017). The helium amounts in the Panhandle-Hugoton gas field in the U.S. were about 18,000 × 10⁶ m³ at the time of discovery (Brown, 2019), and the total proved helium reserves in the Hetianhe gas field in the Tarim Basin in China are 195.91 × 10⁶ m³, both being extra large helium gas field. The proved gas reserves in the Dongpu Sag are 138.2 × 10⁹ m³ with the average helium content of 0.0133%. The calculation based on these two parameters indicates that the proved helium reserves in the sag are 18.38 × 10⁶ m³, which suggest a small helium gas field. This indicates that natural gas in the Dongpu Sag displays a low helium abundance with a considerable total amount.

Extraction from helium-bearing natural gas is the only approach to industrially produce helium, and the lower abundance limit for industrially valuable helium reservoirs was commonly believed to be 0.1% (Tao et al., 2019; Chen et al., 2021). Since the boiling point of helium is significantly lower than that of methane, during the process of compressing natural gas to produce liquefied natural gas (LNG) by some countries such as Qatar, helium is relatively enriched in the residual gas which is the by-product of producing LNG (Tao et al., 2019). The required helium contents for helium production by this approach can be as low as 0.04% (Anderson, 2018). 8 of 72 gas samples from the Bohai Bay Basin display the helium contents no less than 0.04% (Zhang et al., 2008; Dai et al., 2017), meeting the required helium abundance to produce helium by LNG. Most gas samples from the Dongpu Sag and other areas in the Bohai Bay Basin display helium contents lower than 0.04%. However, the required helium abundance to produce helium from natural gas may further decrease as the technologies of helium separation and enrichment continuously improve, which makes the effective use of more helium resource possible in the future.

Helium in natural gas includes atmospheric, crustal, and mantle-derived helium, in which the typical mantle-derived helium displays the ³He/⁴He of 1.1 × 10^–5 (Xu, 1996; Lupton, 1983) with the R/Ra ratio of 7.9, whereas the R/Ra ratio of typical crustal helium is about 0.01 (Jenden et al., 1993). Helium in natural gas reservoirs in China is mainly composed of crustal and mantle-derived helium (Xu et al., 1995a; Chen et al., 2021). The proportion of mantle-derived helium can be calculated on the basis of a two-endmember (crustal and mantle-derived) mixing model, and the calculation equation is as follows (Jenden et al., 1993). H e m a n t l e %=100 × ( R sample − R c r u s t ) / ( R mantle − R c r u s t ) = 100 × [ ( R / R a ) s a m p l e − ( R / R a ) c r u s t ] / [ ( R / R a ) m a n t l e − ( R / R a ) c r u s t ] where R refers to the ³He/⁴He ratio.

The helium content in natural gas from the Dongpu Sag ranges from 31 × 10^–6 to 217 × 10^–6 (Table 1), which is significantly higher than that in the atmosphere (5.24 × 10^–6, Porcelli et al., 2002). The ⁴⁰Ar/³⁶Ar ratio ranges from 487.2 to 3,270.4 (Table 1; Figure 6) and is also significantly higher than the atmospheric value (295.5, Allègre et al., 1987). Therefore, natural gas in the Dongpu Sag is unlikely mixed by atmosphere. Calculation based on the R/Ra ratios of natural gas from different strata in the sag (Table 1) and the above-mentioned equation indicates that, the proportion of mantle-derived helium in the gas ranges from 0.01% to 10.72% with an average of 2.39%. The proportions of mantle-derived helium in several samples are less than 1%, suggesting representative characteristics of crustal helium (Table 1).

Helium isotopic compositions have been commonly used to trace mantle-derived volatiles (Poreda et al., 1988; Xu et al., 1997; Oxburgh et al., 1986). R/Ra < 0.1 generally suggests typical crustal source, and 0.1 ≤ R/Ra<1 suggests mixing by a small amount of mantle-derived helium, whereas R/Ra ≥ 1 indicates remarkable addition of mantle-derived helium (Xu et al., 1998; Chen et al., 2021; Li et al., 2017). Natural gas from different strata in the Sichuan Basin displays R/Ra ratios of 0.002–0.05 with an average of 0.015 (Wang et al., 2020), suggesting typical crustal helium (Ni et al., 2014). The R/Ra ratios of natural gas from the Bohai Bay Basin range from 0.059 to 3.74 (Figure 5) with an average of 1.013 (Zhang et al., 2008; Dai et al., 2017), indicating general mixing of mantle-derived helium by various degrees (Figure 6). The R/Ra ratios of natural gas from the Dongpu Sag range from 0.011 to 0.856 (Table 1), displaying the characteristics of mixing between crustal and mantle-derived helium. The R/Ra distribution is consistent with that for gas samples from other areas in the Bohai Bay Basin (Figure 4). In the correlation diagram between ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios, 9 of 29 gas samples from the Dongpu Sag are of crustal origin, whereas the other samples are dominated by crustal helium and mixed by a small amount of mantle-derived helium (Figure 6). These are in accordance with the calculation results of the proportion of mantle-derived helium (Table 1).

The alkane gas in the Dongpu Sag is demonstrated to be derived from the organic matters in the E_2-3s and C₃t-P₁s source rocks (Ni et al., 2015; Lyu and Jiang, 2017; Wang et al., 2018), however, the helium is believed to be derived from both the mantle and crustal decay of uranium (U) and thorium (Th) in the rocks. Since the rocks from different strata in the sag contain various contents of U and Th, it needs further study on which strata are the main source of crustal helium.

The Sinian and pre-Sinian gas in southern Sichuan Basin has higher helium abundances than natural gas from other areas of the basin, with the average helium content (0.24%) reaching the standard of commercial exploitation (Ni et al., 2014). The high content of helium in the ancient strata suggests the accumulation of crustal helium with time (Ni et al., 2014), suggesting accumulation effect of helium derived from radioactive decay of U and Th in the rocks. Several gas wells in petroliferous basins in eastern China (e.g., Songliao and North Jiangsu) display the helium contents between 0.05 and 0.1%, and the proportions of mantle-derived helium range from 33.5% to 65.4% (Xu et al., 1997). These indicate that mantle-derived helium in sedimentary strata can form industrial accumulation (Xu et al., 1997). The helium contents and R/Ra ratios of natural gas from the Bohai Bay Basin display little positive correlation (Figure 5). Several gas samples have the helium contents higher than 0.05% with R/Ra ratios around 0.5, whereas the samples with higher R/Ra ratios does not display higher helium contents (Figure 5). This is probably associated with the various original contents of crustal helium in natural gas. Helium contents and R/Ra ratios of natural gas from the Dongpu Sag are positively correlated to some degree. Gas samples with R/Ra < 0.03 have helium contents lower than 0.01%, whereas those with R/Ra > 0.1 have helium contents generally higher than 0.01% (Figure 5). These indicate that the mixing of mantle-derived helium enhanced the helium abundance in natural gas to a certain extent. Moreover, the Upper Permian gas in the Dongpu Sag does not have remarkably higher helium contents than the E_2-3s gas (Figure 5), which suggests that the accumulation effect of crustal helium with time is unobvious. The reservoirs in the Sichuan Basin are generally older than those in the Dongpu Sag, and thus the decay time of U and Th in the Sichuan Basin is supposed to be longer than that in the Dongpu Sag. Therefore, natural gas from the Sichuan Basin generally displays higher helium contents than that in the Dongpu Sag.

Since mantle-derived helium in sedimentary strata can form industrial accumulation (Xu et al., 1997), and crustal helium is favorable to be enriched in ancient strata (e.g., Sinian and pre-Sinian strata in southern Sichuan Basin) due to longer decay time of U and Th, helium is likely to accumulate more easily in ancient strata or along the deep faults which connect the mantle.

Helium in natural gas is generally accompanied with alkane gas (e.g., CH₄) and CO₂, and the association of geochemical characteristics and origin between helium and CH₄/CO₂ has attracted wide attention (Poreda et al., 1986; Dai et al., 2017; Liu et al., 2016; Liu et al., 2021). The CH₄ contents of natural gas from the Dongpu Sag are higher than 80% except one sample of 59.78% (Table 1). The CO₂ contents range from 1.0 to 6.18% (Table 1), and both CH₄ and CO₂ contents have unremarkable correlation with R/Ra ratios (Figures 7A,B). Natural gas from Chinese petroliferous basins display similar characteristics, and only a few gas samples from the North Jiangsu, Songliao, and Bohai Bay basins have experienced mixing by mantle-derived gas with high CO₂ contents and R/Ra ratios (Figures 7A,B). Dai et al. (2017) have demonstrated that, natural gas from the cratonic basins in central-western China (e.g., Sichuan and Ordos) has the CO₂ contents generally lower than 5%, and the CO₂ is considered to be derived from the hydrocarbon generation process and decomposition or erosion of carbonate rocks. Moreover, the gas from the rift basins in eastern China such as Bohai Bay Basin may have higher CO₂ content up to nearly 100%, and the CO₂ is mainly derived from volcanic-magmatic activities or being mantle-derived (Dai et al., 2017).

The correlation diagram between CH₄/³He and R/Ra ratios are commonly used to constrain natural gas from the crustal or magmatic/mantle source (Poreda et al., 1986; Jenden et al., 1993). Natural gas from cratonic basins in China (e.g., Sichuan, Ordos, and Tarim) is mainly of crustal origin with the CH₄/³He ratios of 10¹⁰–10¹² and R/Ra < 0.1 (Ni et al., 2014; Dai et al., 2017). However, the gas from rift basins (e.g., Bohai Bay and Songliao) displays the addition of mantle-derived components with CH₄/³He ratios mainly of 10⁶–10¹¹ and R/Ra > 0.1 (Ni et al., 2014; Dai et al., 2017). The CH₄/³He ratios of natural gas from different strata in the Dongpu Sag range from 3.7 × 10⁹ to 1.8263 × 10¹² (Table 1), which are mainly consistent with those from other areas in the Bohai Bay Basin and display the characteristics of crust-mantle mixing. The high CH₄/³He ratios and low R/Ra ratios for several E₃s² and E₂s³ samples are consistent with those for natural gas from cratonic basins such as Sichuan Basin (Figure 8A).

Magmatic/mantle-derived fluids generally have low CO₂/³He ratios and high R/Ra ratios, whereas the crustal fluids have high CO₂/³He ratios and low R/Ra ratios, and the gas from active continental margins displays a two-endmember mixing trend in the correlation diagram between CO₂/³He and R/Ra ratios (Poreda et al., 1988). Natural gas from typical cratonic basins such as Sichuan and Ordos is crustal gas as indicated in the correlation diagram (Wu et al., 2013), whereas the gas from rift basins such as Bohai Bay displays the contribution of mantle-derived fluids (Figure 8B). The CO₂/³He ratios of natural gas from the Dongpu Sag range from 0.05 × 10⁹ to 35.04 × 10⁹ (Table 1), which are mainly consistent with those from other areas of the Bohai Bay Basin and display the characteristics of mixing by mantle-derived gas. Several E₃s² and E₂s³ samples with relatively high CH₄/³He and CO₂/³He ratios show similar characteristics with natural gas from cratonic basins such as Sichuan, which display the characteristics of typical crustal gas (Figures 8A,B).