## 5 Discussion  
### Type of Multi-Origin Natural Gas  
Accurate identification of natural gas types is a fundamental aspect of petroleum geology, as it directly influences source determination, reservoir formation model reconstruction, and resource potential evaluation [36; 37]. This task becomes particularly challenging in basins with vertically superimposed source rocks and complex accumulation histories, where coal-derived, oil-derived, and biogenic gas may coexist or be mixed. The stable carbon isotopic composition of natural gas is a key indicator for distinguishing genetic types, as it is closely related to the type of source organic matter and its thermal maturity [38; 39]. Typically, inorganic gases exhibit a \"reversed carbon isotope sequence\" \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Area**} & \multirow{2}{*}{**Well**} & \multirow{2}{*}{**Depth (m)**} & \multirow{2}{*}{**Formation**} & \multicolumn{6}{c}{**Composition of Natural Gas**} \\ \cline{5-10}  & & & & **C\({}_{\textbf{1}}\%\)** & **C\({}_{\textbf{2}}\%\)** & **C\({}_{\textbf{3}}\%\)** & **C\({}_{\textbf{1}}\)/(C\({}_{\textbf{1-5}}\))** & **N\({}_{\textbf{2}}\%\)** & **CO\({}_{\textbf{2}}\%\)** \\ \hline \multirow{9}{*}{Dongdaohaizi} & Dn8 & 3070–3084 & C\({}_{\textbf{2}}\)b & 93.35 & 5.15 & 0.74 & 0.96 & 1.37 & 0.14 \\  & Dn8 & 3132 & C\({}_{\textbf{2}}\)b & 84.71 & 10.54 & 0.68 & 0.89 & 2.91 & 0.12 \\  & Dn8 & 3650–3665 & C\({}_{\textbf{2}}\)b & 90.36 & 5.2 & 0.75 & 0.95 & 4.34 & 0.00 \\  & Dn8 & 3363 & P\({}_{\textbf{3}}\)w & 89.42 & 6.19 & 0.65 & 0.94 & 4.19 & 0.15 \\  & Dn8 & 3510 & P\({}_{\textbf{3}}\)w & 88.66 & 7.9 & 0.75 & 0.92 & 3.01 & 0.11 \\  & Dn1 & 3345 & P\({}_{\textbf{3}}\)w & 87.56 & 8.76 & 0.45 & 0.91 & 2.51 & 0.07 \\  & Dn1 & 3625 & P\({}_{\textbf{3}}\)w & 88.4 & 8.34 & 0.42 & 0.91 & 3.06 & 0.03 \\  & Dn1 & 3830 & P\({}_{\textbf{3}}\)w & 88.03 & 9.15 & 0.41 & 0.91 & 2.73 & 0.02 \\  & C31 & 3635 & J\({}_{\textbf{2}}\)t & 84.41 & 11.35 & 0.55 & 0.88 & 3.97 & 0.05 \\  & C341 & 3502 & J\({}_{\textbf{2}}\)t & 85.72 & 11.21 & 0.45 & 0.88 & 2.38 & 0.08 \\  & C34 & 2906 & J\({}_{\textbf{2}}\)t & 82.20 & 9.30 & 0.41 & 0.90 & 6.60 & / \\  & C34 & 3652 & J\({}_{\textbf{2}}\)t & 87.24 & 7.78 & 0.54 & 0.92 & 3.16 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 3: Carbon isotopic composition of methane (**a**) and ethane (**b**) in natural gas from the Dixi area, Junggar Basin. (i.e., \(\delta^{13}\)C\({}_{1}\) > \(\delta^{13}\)C\({}_{2}\) > \(\delta^{13}\)C\({}_{3}\)), while organically derived alkane gases show a \"normal carbon isotope sequence\" [38,39]. Carbon isotope analyses of natural gas from the Carboniferous, Cretaceous, Jurassic, and Permian strata in the study area all show normal isotopic ordering (Figure 3), indicating an organic origin. Furthermore, inorganic gases tend to exhibit relatively heavier carbon isotope values. According to [PERSON] et al., a \(\delta^{13}\)C\({}_{1}\) value of \(-30\)% can be used as a threshold to distinguish between organic and inorganic gases: \(\delta^{13}\)C\({}_{1}\) values > \(-30\)% are characteristic of inorganic gas, while \(\delta^{13}\)C\({}_{1}\) values < \(-30\)% indicate organic gas. In this study, the maximum \(\delta^{13}\)C\({}_{1}\) values for gases from the four stratigraphic intervals were \(-31.3\)%, \(-32.6\)%, and \(-33.6\)%, respectively, all of which fall below the \(-30\)% threshold, further confirming the organic origin of the gases in the Dixi area (Figure 4). [PERSON] and [PERSON] proposed one of the most comprehensive classification diagrams for natural gas origins based on geological and geochemical data from 20,621 natural gas samples across 76 countries and six continents. Their diagram refined the distribution fields for thermogenic gas, primary biogenic gas, secondary biogenic gas, and inorganic gas [40].