# Characteristics and Origin of Natural Gas in Yongfeng Sub-Sag of Bogda Mountain Front Belt
## 5 Geochemical Characteristics of Natural Gas  
### Characteristics of Natural Gas Composition  
Composition is one of the geochemical features of natural gas. The factors that affect the variation of natural gas composition include the type and maturity of the source rock parent materials. Therefore, studying the characteristics and variation laws of natural gas composition plays an important role in the analysis of natural gas genesis and the study of gas accumulation laws.  
The natural gas in the Yongfeng sub-sag is mainly composed of hydrocarbon gases, with significant differences in hydrocarbon gas content among different testing layers, ranging from 70.29% to 95.51%, with an average of 85.66%. Other gases are mainly nitrogen, which accounts for a high proportion at 4.49% to 29.71%, followed by carbon dioxide, which accounts for 0.1% to 0.2%. In hydrocarbon gases, there are significant differences in the content of different components; however, methane is the main component, accounting for 70.06% to 94.7%, with an average of 84.32%. The content of heavy hydrocarbon components (C\({}_{2+}\)) is relatively low, ranging from 0.17% to 3.54%, with an average of 1.35%. The drying coefficients of natural gas are all greater than 0.96, ranging from 0.9628 to 0.9976, exhibiting typical dry gas characteristics (Table 2).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \multirow{2}{*}{**Formation**} & \multirow{2}{*}{**Depth/m**} & \multirow{2}{*}{\
\begin{tabular}{c} **Oil** \\ **Testing** \\ **Interval** \\ \end{tabular} } & \multicolumn{8}{c}{**Gas Composition/\%**} & \multicolumn{8}{c}{**Drying**} & \multicolumn{2}{c}{**\(\mathbf{\delta^{13}}\)C/\%**} \\ \cline{3-14}  & & & \(\mathbf{C_{1}}\) & \(\mathbf{C_{2}}\) & \(\mathbf{C_{3}}\) & \(\mathbf{nC_{4}}\) & \(\mathbf{iC_{4}}\) & \(\mathbf{nC_{5}}\) & \(\mathbf{CO_{2}}\) & \(\mathbf{N_{2}}\) & \(\mathbf{Coefficient}\) & \(\mathbf{C_{1}}\) & \(\mathbf{C_{2}}\) & \(\mathbf{C_{3}}\) \\ \hline \multirow{6}{*}{
\begin{tabular}{c} Lucaogou Formation \\ \end{tabular} } & 2045- & \multirow{2}{*}{S1} & 89.32 & 2.5 & 0.71 & 0.16 & 0.07 & 0.01 & 0.1 & 7.14 & 0.9628 & & & \\ \cline{3-14}  & & & 91.97 & 2.57 & 0.72 & 0.16 & 0.07 & 0.02 & 0 & 4.49 & 0.9629 & & & \\ \cline{3-14}  & & & 87.87 & 0.19 & 0.07 & 0.02 & 0.01 & 0.03 & 0 & 11.81 & 0.9963 & \(-\)58.62 & \(-\)28.45 & \(-\)27.18 \\ \cline{3-14}  & & 1804- & \multirow{2}{*}{S2} & 94.79 & 0.21 & 0.08 & 0.03 & 0.01 & 0.01 & 0.23 & 4.61 & 0.9963 & & & \\ \cline{3-14}  & & & 70.06 & 0.06 & 0.07 & 0.07 & 0.03 & 0 & 0 & 29.71 & 0.9967 & & & \\ \cline{3-14}  & & 1776.3 & & 0.04 & 0.06 & 0.05 & 0.02 & 0 & 0 & 27.94 & 0.9976 & \(-\)59.42 & / & / \\ \hline \end{tabular}
\end{table}
Table 2: Natural gas composition and carbon isotopes in the Lucaogou Formation of Well Xyd1.  
Figure 3: Histogram of TOC (**left**) and chloroform bitumen A (**right**) for source rocks in the Lucaogou Formation of Well Xyd1.  
The characteristics of the natural gas composition in the Yongfeng sub-sag are quite different from those in other areas in the southern margin of the Junggar Basin. The methane content of natural gas in the Anjihai, Khorgos, Manasi, Tugulu, and Hutubi areas in the central and western sections of the southern margin of the Junggar Basin generally ranges from 50% to 95%, and the natural gas is mainly wet gas [9; 10].