¹H and ²H(D) have the largest relative mass difference compared to other stable isotopes, making the hydrogen isotopes of organic matter have a wide range^[1]. The hydrogen isotope of alkane gases, combined with carbon isotopes, plays an very important role in the identification of genetic-type analysis of natural gas, parent material source, maturity, mixing and biodegradation, and sulfate thermochemical reduction (TSR)^{[2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]}. Compared with carbon isotopes, factors affecting hydrogen isotopes of alkane gases are more diverse and complex, besides the parent material type, maturity and biodegradation, and TSR, the water environment during the deposition and diagenesis of source rock (such as salinity) also plays an important role^{[20, 21]}.

Many studies have been carried out on the hydrogen isotopic characteristics of the natural gases from the Permian of Ordos Basin and the Upper Triassic Xujiahe Formation of the Sichuan Basin; the origin and source of the natural gases have also been analyzed based on the carbon and hydrogen isotopic and chemical composition characteristics of the alkane gas. It is proved that the natural gases in the above two areas are both coal-formed gas^{[22,23,24,25,26,27,28,29,30,31,32]}, and the hydrogen isotopic indexes for identifying natural gas genesis and vitrinite reflectance (R_o) value have been proposed^{[19, 29]}. Some studies on factors affecting the hydrogen isotopes of alkane gas have been conducted^{[19, 29]}, but most of them looked the composition of hydrogen isotopes and affecting factors of the Permian natural gases (methane and its homologues) and the Upper Triassic Xujiahe Formation in the two basins from a single basin perspective, a few made comparison of them^[29], but limited only to the hydrogen isotopes of methane. There were few discussions on the composition, affecting factors, maturity, and natural gas genetic identification indicators of hydrogen isotopes of heavy alkane gases, e.g., ethane and propane. The hydrogen isotopic composition and its differences of the coal-formed Permian gas in the Ordos Basin and the Upper Triassic gas in the Sichuan Basin are analyzed in this study, and the hydrogen isotopic compositions of alkane gas in different regions are compared. The affecting factors of and their influence degree on the hydrogen isotopic composition of alkane gases are examined, and the hydrogen isotope indexes of alkane gas able to identify the natural gas genetic type and R_o value of gas are proposed, in the hope to develop and improve the coal-formed gas theory and genetic identification theory of natural gas and guide natural gas exploration.

The Ordos Basin is one of the major petroliferous basins in China. It has stable geological structure and is characterized by oil in the Mesozoic, gas in the Paleozoic, oil in the shallow part, and gas in the deep^[33]. The Paleozoic approximately 25 × 10⁴ km² in area has an obvious two-layer structure. The Lower Paleozoic is composed of marine carbonate and gypsum and the Upper Paleozoic continental clastic rock and coal measures (Fig. 1). The Permian of Upper Paleozoic in the basin developed large-scale river-delta deposits, with large-scale reservoir sand bodies and coal measures, where large gas fields, Yulin, Wushenqi, Daniudi, Sulige, Zizhou, Shenmu, and Yan'an have been discovered successively, with proved reserves greater than 100 billion cubic meters and total proved coal-formed gas reserves of 5.24×10¹² m^3[34]. The coal seams are generally 10-15 m thick, and up to 40 m thick in local parts. The dark mudstone is 200 m thick cumulatively. The mudstone in the middle and eastern areas of the basin is generally 70 m thick^{[35, 36]}. The mudstone has a total organic carbon (TOC) of 2%-4% generally and the coal has an average TOC of 60%. The kerogen of the organic matter is mainly type III, and in some mudstone type II₂. The Carboniferous source rock in the north of Etuokeqi-Wushenqi-Zizhou has R_o of less than 2.0%, while that in the South area reaches over- mature stage. The source rock in the west of the Jingbian-Yichuan- Qingyang area has R_o of greater than 2.4%^[37].

The Sichuan Basin is also one of the major petroliferous basins in China, covering an area of approximately 18×10⁴ km². Since the late Indosinian Period, it has experienced many stages of strong tectonic movements in the Yanshanian and Himalayan periods, forming the current tectonic pattern^[38]. The Sinian-Middle Triassic in this basin is marine deposit, dominated by carbonate; and the Upper Triassic-Quaternary is mainly a set of clastic rock^[38]. The Upper Triassic Xujiahe Formation is foreland basin deposit. When the deposition began, the seawater gradually withdrew from the Southwestern part of the basin, and continental brackish water-freshwater and river-delta-lake clastic sediments deposited^[37]. Since 2005, several large gas fields, such as Guang'an and Hechuan, have been discovered successively in Xujiahe Formation of Sichuan Basin^[39]. In the basin, the Xujiahe Formation varies greatly in thickness, and generally thins from the Northwest to Southeast direction^[40]. From the bottom to the top, the Xujiahe Formation is divided into six members (T₃x₁-T₃x₆) (Fig. 2), in which the first member (T₃x₁) is the marine-continental transitional facies and the second to the sixth ones are continental deposits. The first, third, and fifth members mainly composed of dark mudstone and coal seams are major source-rock layers. The second, fourth, and sixth members dominated by sandstone and siltstone, are the main reservoir sections, and the six members form favorable source-reservoir combinations^[41]. The Xujiahe Formation is very rich in organic matter, with a TOC of 0.5% to 9.7%, 1.96% on average. The kerogen types of the organic matter are mainly type II₂ and type III^[42]. The R_o values of the source rock of the Xujiahe Formation are higher in the northwest and lower in the southeast. The first member source rock has a R_o value of 1.0% to 2.5% and is largely in the high to over- mature stage; the third member has a R_o of 1.0% to 1.9%, is highly mature in the area west of Wangcang-Nanchong-Suining-Yan’an, and has a R_o between 1.0% and 1.3% in the central part of the Sichuan Basin (hereinafter referred to as “central Sichuan Basin”) and the Southern part of the Sichuan Basin (hereinafter referred to as “Southern Sichuan Basin”); the fifth member has a R_o of 0.9% to 1.5%^[37].

Natural gas samples of eight wells and five wells were collected from the Ordos Basin and Sichuan Basin, respectively, and the composition and stable carbon and hydrogen isotopes of the samples were tested at the Research Institute of Petroleum Exploration and Development. The isotopic and chemical data of 175 natural gas samples published in previous literature were also collected, including 68 gas samples from the Sinian and Cambrian reservoirs in the Sichuan Basin, and the Ordovician, Silurian, and Carboniferous reservoirs in the Tarim Basin^{[16,31,43-48]}, for comprehensive comparison analysis. The natural gas composition and hydrogen isotope data are shown in Tables 1 and 2, respectively.

The gases from the Permian of the Ordos Basin and Triassic Xujiahe Formation of the Sichuan Basin contain the largest proportion of alkane gases, and a small amount of carbon dioxide and nitrogen.

The natural gas samples from Permian of the Ordos Basin have a methane content of 86.05%-96.68%, 92.86% on average; ethane content of 0.30%-8.37%, 3.43% on average; propane content of 0.02%-2.33%, 0.68% on average; and butane content of 0.01%-1.13%, 0.32% on average. In terms of dryness coefficient (C₁/C_1-4), natural gas samples from the Southern Ordos Basin (Yan'an gas field) are higher, ranging between 0.991 and 0.997 and averaging at 0.994, representing typical dry gas; while natural gas samples from the northern Ordos Basin (Sulige, Yulin, Daniudi, Zizhou, Mizhi, Dongsheng, and other gas fields) vary widely in dryness from 0.882 to 0.983 (0.946 on average), indicating most of the natural gas samples are wet gas. Except the Yan'an gas field in the south, the hydrogen isotope composition of methane and its homologues gradually becomes heavier with the increase of carbon number (δ²H_CH4<δ²H_C2H6<δ²H_C3H8) (Fig. 3a), while the hydrogen isotopes of methane and ethane in Yan'an gas field are reversed (δ²H_CH4>δ²H_C2H6) (Fig. 3b). The gas samples from this field have δ²H_CH4 of -210‰ to -163‰, -186‰ on average; δ²H_C2H6 of -197‰ to -150‰, on average -169‰; and δ²H _C3H8 of -183‰ to -134‰, on average -160‰.

The natural gas samples from the Upper Triassic Xujiahe Formation in the Sichuan Basin have a methane content of 83.86% to 96.50%, 89.99% on average; ethane content of 1.57%-10.13%, 6.06% on average; propane content of 0.12% to 3.86%, 1.70% on average; and butane content of 0.03% to 2.32%, 0.78% on average. The natural gas samples from the western Sichuan Basin have a dryness coefficient from 0.885 to 0.982, on average 0.937, indicating most gas samples are wet gas; the gas samples from central Sichuan Basin have a lower dryness coefficient of 0.853-0.964, 0.892 on average, indicating the absolute majority of them are wet gas. In the respect of hydrogen isotopes, the hydrogen isotopes of methane and its homologues gradually become heavier with the increase of carbon number (Fig. 3c and 3d). The δ²H_CH4 values range from -173‰ to -147‰, with an average of -162‰; the δ²H_C2H6 values range from -147‰ to -108‰, with an average of -129‰; the δ²H_C3H8 values range from -139‰ to -96‰, with an average of -119‰.

3.1.1. Kerogen types of organic matter of gas

Similar to carbon isotopic composition, the hydrogen isotope composition of alkane gas is affected by the source rock, showing an apparent characteristic of parental inheritance^{[4, 9, 11]}. The natural gas generated by source rocks deposited in the ocean or in high-salinity lacustrine environment has the characteristic of rich ²H^[3,43]. Therefore, the hydrogen isotope composition can be used to identify the genetic type of natural gas, and then the kerogen type of the source rock. On the δ¹³C_CH4-C₁/C₂₊₃ natural gas genetic identification plot (Fig. 4)^[10], it can be seen that the natural gas samples from the Permian of the Ordos Basin and the Triassic Xujiahe Forma- tion of the Sichuan Basin fall in the kerogen type-III region, indicating that the gases are coal-formed gas, which is consistent with previous views^[8,24-26,44]. The δ¹³C_C2H6 value of the natural gas samples from Sinian and Cambrian of the Anyue gas field in the Sichuan Basin and the Ordovician, Silurian, Silurian, and Carboniferous gas pools of the Tarim Basin are all lighter than -28.5‰, and they mostly fall in the type-II kerogen region on the δ¹³C_CH4-C₁/C₂₊₃ natural gas genetic identification plot (Fig. 4), indicating that these natural gases are oil-derived gas, which is also consistent with previous knowledge^[16,45-49]. The δ²H_CH4-C₁/C₂₊₃ genetic identification plot of natural gas was drawn using the natural gas data of Sichuan, Ordos, and Tarim basins (Tables 1 and 2) (Fig. 5). It can be seen from this plot that the humic kerogen-derived natural gas, i.e., coal-formed gas, has a δ²H_CH4 value of less than -150‰, and C₁/C₂₊₃ value of less than 1 000 generally; while the sapropelic kerogen-formed gas, i.e., oil-derived gas, has a δ²H_CH4 value of greater than -160‰ and a distribution range of C₁/C₂₊₃ wider than that of coal-formed gas (Fig. 5). With the increase of maturity, the δ²H_CH4 and C₁/C₂₊₃ values of coal- formed gas gradually become heavier or larger. In Fig. 5, the oil-derived gases are selected from the Sinian and Cambrian gases of the Sichuan Basin and the Ordovician, Silurian, and Carboniferous gases of the Tarim Basin. The sources of the gases in the two basins have significant differences. The δ²H_CH4 value reverses at the -130‰ position and turns lighter with the further increase of R_o (Fig.5). This should be due to the differences in composition and depositional environment of the source rocks in different basins. Except for a small number of natural gas samples with δ²H_CH4 value in the range -160‰ to -150‰, the plot can distinguish coal-formed gas from oil-derived gas, and the identification results are basically consistent with those shown in Fig. 4, indicating that the plot is effective and provides a new means for identifying the genetic type of natural gas in other basins. The δ²H_CH4 range of -160‰ to -150‰ is the coincidence area of coal-formed gas and oil-derived gas. If data points fall in this range, it must be cautious when identifying the genetic type of gas. It also indicates that the identification of natural gas origin requires multiple parameters. This plot, combined with other natural gas genetic identification diagrams, can effectively identify the genetic type of natural gas.

Δδ²H_C2H6-CH4 is a good index for identifying the genetic type of natural gas^[19]. The gas samples from Permian of the Ordos Basin and the Xujiahe Formation of the Sichuan Basin are coal-formed gas^[8,24-26,44], and the gas samples from the Ordovician, Silurian, and Carboniferous reservoirs of the Tarim Basin are typical oil-derived gas^[16,45-49]. The hydrogen isotope difference between the heavy alkane gases and methane, and the hydrogen isotopes of alkane gases, are plotted in Fig. 6, and it is found that the figures, e.g., (δ²H_C2H6-δ²H_CH4)- δ²H_CH4 (Fig. 6a), (δ²H_C2H6-δ²H_CH4)-δ²H_C2H6 (Fig. 6b), (δ²H_C2H6- δ²H_CH4)-δ²H_C3H8 (Fig. 6c), (δ²H_C3H8-δ²H_CH4)-δ²H_CH4 (Fig. 6d), (δ²H_C3H8-δ²H_CH4)-δ²H_C2H6 (Fig. 6e), (δ²H_C3H8-δ²H_CH4)-δ²H_C3H8 (Fig. 6f), etc., can tell the oil-derived gas from coal-formed gas. At the same time, based on Fig. 6, the hydrogen isotope composition of one alkane gas cannot effectively distinguish different types of natural gas; therefore, it is necessary to compare multiple parameters comprehensively when identifying natural gas genesis to avoid false identification conclusions.

3.1.2. Difference between kerogens of Carboniferous- Permian source rock in Ordos Basin and that of Xujiahe Formation source rock in Sichuan Basin

The microscopic composition is an important parameter for source-rock kerogen type classification. Different microscopic components have obvious differences in hydrogen isotope composition, and the general rule is δ²H_inertinite >δ²H_vitrinite> δ²H_exinite^{[9, 50]}. The coal microscopic composition of the Carboniferous-Permian coal in the Ordos Basin and the Triassic Xujiahe Formation coal in the Sichuan Basin (Fig. 7) indicate that in the inertinite-vitrinite-exinite+saproplite composition, the former has an inertinite relative content of 3.2%-93.6%, on average 49.2%; the latter has a relative inertinite content of 0.7%-80.7%, on average 28.0%^{[51, 52]}. The inertinite content in the former is much higher than that in the latter (Fig. 7). The carbon isotopic compositions of the kerogen of the Carboniferous-Permian coal in the Ordos Basin are -26.3‰ to -21.5‰, and the average value of 49 samples is -24.2‰^[53]; those of the coal of Xujiahe Formation in the Sichuan Basin are -27.2‰ to -24.7‰, on average -25.9‰^[52,53,54]. The two differ little in carbon isotopic compositions of the kerogens, indicating terrestrial high plant input contributes a large proportion to the Carboniferous-Permian kerogen in the Ordos Basin.

The source rocks of Carboniferous-Permian in the Ordos Basin have higher inertinite content than source rock of the Xujiahe Formation in the Sichuan Basin. According to the inheritance characteristic of hydrogen isotopes of alkane gas, it can be speculated the hydrogen isotope of the alkane gas generated by Carboniferous-Permian coal-bearing source rocks in the Ordos Basin should be more enriched in ²H than that generated by the Xujiahe Formation in the Sichuan Basin, excluding other affecting factors. However, the fact is opposite, indicating that the hydrogen isotope composition of the natural gas produced by the organic matter with the same or similar kerogen type may vary greatly, the composition of the original parent material has a certain influence on the hydrogen isotope of the alkane gas, but there are other more important factors affecting the hydrogen isotope composition.

The hydrogen isotopic compositions of alkane gas gradually becomes heavier with the increase of R_o value^{[3, 4 ,6]}, and the relation of δ²H_CH4-R_o has also been proposed^{[3, 29]}. For thermogenic gas, methane has the highest thermal stability, and heavy alkane gases will gradually crack into hydrocarbons with smaller carbon numbers with the rise of maturity, and will eventually turn into the most thermodynamically stable methane^[55], so the dryness coefficient (C₁/C_1-4) can reflect the maturity of natural gas^{[2, 56]}. The δ¹³C₁-R_o relationship is widely used in determining the maturity of natural gas. Based on the actual source-rock maturity range (Fig. 8), the R_o value of natural gas of the Xujiahe Formation in the Sichuan Basin was calculated by formula (1) in this study^[57], with R_o≤0.9%:

R_o value of coal-formed gases from other study areas were calculated using formula (2)^[58]:

R_o value of oil-derived gases were calculated using formula (3)^[58]:

The natural gas samples from Permian reservoirs in the northern Ordos Basin have a R_o of 0.55%-2.53%, 1.30% on average, indicating most of gas samples are in the mature-high-mature stage; those from the southern Ordos Basin have a R_o of 1.80%-3.03%, on average 2.34%, obviously higher than those from the Northern part, clearly, most of them are the products of the source rock in over-mature stage. The natural gas samples from the Xujiahe Formation in the Western Sichuan Basin have a R_o value of 0.66%-1.89%, on average 1.18%, suggesting most of them are products of mature source rock, and a small number are products of the high-mature source rock. The natural gas samples from Xujiahe Formation in the central Sichuan have lower R_o value from 0.70% to 0.87%, on average 0.76%, showing they are products of mature source rock. The above calculated R_o value of natural gas samples are consistent with the actual R_o value of the source rocks. The natural gas of the Permian reservoir in the Ordos Basin has clearly higher maturity than that of the Xujiahe Formation in the Sichuan Basin.

The hydrogen isotope of methane has a positive correlation with R_o value and C₁/C_1-4 (Fig. 8a and 8d). Through linear regression, the relationship between δ²H_CH4 and R_o of the Permian natural gas in the Ordos Basin is as follows, where R²=0.50 and n=80:

The relationship between δ²H_CH4 and R_o of natural gas of the Xujiahe Formation in the Sichuan Basin is as follows, where R²=0.66 and n=22:

The relationships between hydrogen isotope composition of heavy alkane gas and R_o value and C₁/C_1-4 of natural gas in the Ordos Basin are not obvious (Fig. 8b, 8c, and 8f). The hydrogen isotopic composition of ethane in the Ordos Basin seems to have a negative correlation with the R_o value (Fig. 8e). This is because the δ²H_C2H6 value in the Yan'an gas field in the Southern Ordos basin becomes very light under high-temperature conditions^[8,30], and the δ²H_CH4 and δ²H_C2H6 values reverse. Based on this feature, the hydrogen isotope difference between the heavy alkane gases and methane was fitted to the R_o value (Fig. 9). It is found that the two have a negative correlation, especially the (δ²H_C2H6-δ²H_CH4)-R_o plot (Fig. 9a), so it can be used as an index of maturity. The relationship between the δ²H_C2H6-δ²H_CH4 and R_o of coal-formed gas is as follows, where R²=0.57 and n=105:

Besides hydrogen isotope, the carbon isotope difference between the heavy alkane gases and methane also has a negative correlation with R_o value^[23,27], indicating that during the formation of natural gas the isotope composition of the alkane gas has subjected to Rayleigh fractionation. With gradual increase of maturity, the fractionation effect gradually reduces, and the isotope composition of heavy alkane gases and methane tend to be similar. In the over-mature stage (R_o>2.2%), the hydrogen isotopes of methane and ethane can even reverse (δ²H_CH4 >δ²H_C2H6).

The carbon and hydrogen isotopic compositions of methane and ethane in the Yan'an gas field of the Southern Ordos Basin generally reverse (i.e., δ¹³C_CH4>δ¹³C_C2H6, δ²H_CH4>δ²H_C2H6) (Table 1 and Fig. 3). Corresponding to the primary reversing carbon isotope series of inorganic alkane gas, Dai et al. named the complete carbon isotope reversal of alkane gases like that in the Yan'an gas field secondary complete reversal of carbon isotope series^[8]. There are several reasons for the formation of large-scale secondary complete reversal of carbon isotope series in commercial gas fields: (1) cracking of hydrocarbons in source rocks under high maturity condition^{[8, 59]}; (2) diffusion^{[8, 60]}; (3) the Rayleigh fractionation of ethane and propane caused by redox reaction of transition metals and aqueous medium at 250 °C-300 °C^[61]; and (4) high formation temperature greater than 200 °C^[62]. The Permian natural gases in the Yan'an gas field have a R_o from 1.80% to 3.03%, on average 2.34%, which is consistent with the R_o value of the Carboniferous-Permian source rocks in the area, indicating that the natural gas was generated under high-temperature conditions. Looking at the above reasons leading to complete reversal of the carbon isotope of alkane gas, it is found that high temperature (over-maturation) is one of the most important controlling factors, and this is also the main factor leading to the complete reversal of the hydrogen isotope of alkane gas. Under high temperature, secondary cracking, diffusion, or Rayleigh fractionation of retained hydrocarbons in the source rocks by transition metals can result in the complete reversal of hydrogen isotope of alkane gases.

The Ordovician-Carboniferous natural gas in the central Tarim Basin has a partial reversal of the hydrogen isotope series, i.e., δ²H_CH4>δ²H_C2H6<δ²H_C3H8, while the carbon isotopes of alkane gases follow the normal sequence. Many factors can cause the isotopic reversal of natural gas^[63], one of them is the mixing of natural gas at different stages of maturity. Since ¹H and ²H (D) have the largest isotope mass difference relative to other stable isotopic compositions, the hydrogen isotope fractionation effect is more pronounced than that of the carbon isotope. The mixing of the dry gas generated in high thermal maturity (such as kerogen-cracking gas) and the oil-cracking gas in the mature stage causes the reversal of the hydrogen isotope of methane and ethane. Because degree of mixing is low, the carbon isotope reversal is not triggered, so comes the normal sequence of the carbon isotope of alkane gases and the reversal of hydrogen isotope composition of Ordovician gas in the Tarim Basin. Wu et al.^[64] also believed that the hydrogen isotopic reversal of the Ordovician natural gas in the Tazhong gas field is caused by the mixing of oil-derived gases of different source rocks or gases of different mature stages of the same source rock.

It is found that water directly involved in the formation of natural gas under the experimental conditions, and exchanged hydrogen with the source rock, and consequently affected the stable hydrogen isotope of the generated alkane gas^{[29, 65-67]}. Although the alkane gas can exchange hydrogen with water^{[15, 66]}, this reaction is very slow under natural conditions. For example, δ²H_CH4 had hardly changed during several billion years when the formation temperature was over 200 °C-240 °C^{[4, 66]}. Therefore, the hydrogen isotope exchange between alkane gas and water after the formation of natural gas can be ignored^[29], and only the water environment when the source rock deposited needs to be considered.

The study of sedimentary facies shows that when the Xujiahe Formation began to deposit, seawater gradually withdrew from the Southwestern part of the Sichuan Basin^[67,68], so T₃x₁ is marine-continental transitional facies and T₃x₂ to T₃x₆ are continental deposits^[40]. But the characteristics of glauconite minerals and biomarker compounds indicate sea transgression happened during the sedimentary periods of T₃x₂ to T₃x₃^[69,70]. The Sr/Ba value is positively correlated with the paleo-salinity ^[71,72]. Comparing Sr/Ba value of the Xujiahe Formation in the Sichuan Basin and the Taiyuan Formation-Shanxi Formation in the Ordos Basin shows that the former ranges between 0.2 and 1.0, the value of the T₃x₁ reaches 1.0, and those of some layers of the T₃x₂-T₃x₃ are greater than 0.5^[73]; in contrast, in the Ordos Basin, except some layers in Taiyuan Formation with Sr/Ba of greater than 0.5, Shanxi Formation is generally less than 0.5 in Sr/Ba^[74,75]. By analyzing the contents of boron, strontium, barium, and potassium, it is confirmed that the salinity of the Xujiahe Formation gradually decreased from the bottom to the top, the first and second members were higher in paleo-salinity, with the maximum value of 37‰^[68,72]. In the Ordos Basin, the Carboniferous Benxi Formation and the Permian Taiyuan Formation deposited in epicontinental sea environment^[76,77], while the Shanxi Formation is a marine-continental transitional facies ^[74]. The paleo-salinity of the Taiyuan Formation to Shanxi Formation calculated by contents of boron and clay minerals shows a significant decrease trend. The Taiyuan Formation has a paleo-salinity of 5.79‰-44.61‰, 24.18‰ on average, while the Shanxi Formation has a paleo-salinity of 8.32‰-39.78‰, 18.45‰ on average^[74].

It can be seen from the above discussion that the salinity of water body during the deposition of T₃x₁ to T₃x₃ in the Sichuan Basin was higher than that of water body during the deposition of the Taiyuan and Shanxi formations in the Ordos Basin. The Upper Triassic Xujiahe Formation in the Sichuan Basin has lighter carbon isotope of kerogen, lower content of inertinite with heavier hydrogen isotopes in the micro-components, and lower maturity of the source rock than the Carboniferous-Permian in Ordos Basin, but the hydrogen isotope of natural gas in the former is obviously heavier than that in the latter, which indicates that the water medium in which the source rock deposit, especially the paleo-salinity, has an very important influence on the composition of hydrogen isotopes of the alkane gas.

The alkane gas from the Xujiahe Formation in the central Sichuan Basin has generally heavier hydrogen isotope composition than that from the Western Sichuan region (Fig. 3), and the former has lower R_o value, indicating that the salinity has stronger impact on the hydrogen isotope of alkane gas in the Xujiahe Formation than maturity. The higher paleo-salinity in the central Sichuan Basin is an important factor causing the heavier hydrogen isotope composition of alkane gas than that in the Western Sichuan Basin.

In the Anyue gas field of the Sichuan Basin, the Sinian natural gas samples have a R_o value of 3.35%-4.42%, on average 3.79%; Cambrian natural gas samples have a 2.98%- 4.36%, on average 3.90%, that is the natural gases of the two strata don’t differ much in maturity, but the Sinian natural gas has lighter δ²H_CH4 than the Cambrian gas (Fig. 5). The Sinian gas came from the mudstone source rocks of the Sinian Qiongzhusi Formation and the third member of Dengying Formation, while the Cambrian gas was mainly derived from the Qiongzhusi Formation^[47,48,49]. High content of gammacerane (GR) indicates strong reduction and super-saline environment during the deposition of source rock^{[78, 79]}, and GR/C₃₀ hopane ratio can reflect the salinity of water in which the source rock deposits^[78]. The GR/C₃₀ hopane value of the source rock of the Qiongzhusi Formation is lower than that of the Dengying Formation^[80]. Although the absolute content of GR of them were not compared, the GR/C₃₀ hopane ratio, to some extent, shows that the water in which Qiongzhusi Formation source rock deposited was higher in salinity than that in which the Dengying Formation deposited. The difference in the hydrogen isotopes of alkane gas between the two gas reservoirs is due to the difference in the sedimentary water environment between the third member of Dongying Formation and that of the Qiongzhusi Formation, which is consistent with previous views^{[47, 81]}.

The natural gas of Anyue gas field in the Sichuan Basin has much higher maturity than the Ordovician natural gas in the Tarim Basin, but the gas of the former, especially gas of Sinian reservoir in the former, has lighter δ²H_CH4 than the Ordovician natural gas in the Tarim Basin; that is, the δ²H_CH4 reverses at the -130‰ position (Fig. 5), as mentioned above. Searching relevant literatures shows there are few reports on the absolute content of GR and the ratio of Sr/Ba of the source rocks of Qiongzhusi, Cambrian, and the third member of Dengying Formation, the Sinian in the Sichuan Basin, and the Cambrian and Ordovician source rocks in the Tarim Basin, so it is not possible to compare the paleo-salinity of water bodies during the deposition of the source rocks in the two basins. As the source rocks in the two basins are widely different in sedimentary age and parent-material composition, the authors speculate that the obvious difference of δ²H_CH4 of natural gases in the two basins is due to the differences in parent-material composition and sedimentary water environment.

The Permian natural gas in the Ordos Basin and the Triassic Xujiahe Formation natural gas in the Sichuan Basin, are both typical coal-formed gas. The former is higher in dryness coefficient and maturity than the latter, while the latter has much heavier stable hydrogen isotope of alkane gas than the former.

The genetic identification plot of δ²H_CH4-C₁/C₂₊₃ of natural gas has been proposed, and the plots of the differences between the hydrogen isotopes of heavy alkane gases and methane versus the hydrogen isotope composition of alkane gas have been advanced to identify the genesis of natural gas. The δ²H_CH4-R_o relations of coal-formed gas for different parts of Ordos and Sichuan basins have been established, and the relation of (δ²H_C2H6-δ²H_CH4)-R_o of coal-formed gas has been proposed, providing a new index for identifying coal-formed gas maturity.

The stable hydrogen isotope composition of alkane gas is affected by multiple factors, including the parent materials, maturity, mixing, and the sedimentary water medium of the source rock, among which the salinity of water source rock deposits is one of the most important and critical factors.