# 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 Light Hydrocarbon Composition  
The composition and content of light hydrocarbons in natural gas are influenced by geological conditions, maturity, and natural gas generation processes. Therefore, the composition of light hydrocarbons plays an important role in identifying the genetic type, maturity, gas source correlation, and natural gas accumulation of natural gas. By studying light hydrocarbons, we can better understand the generation mechanism and sources of natural gas, and provide guidance for the exploration and development of natural gas.  
The composition of C\({}_{7}\) light hydrocarbons can distinguish the parent material types of natural gas [15; 22]. Methylcyclohex mainly comes from lignin, cellulose, and sugars  
Figure 4: Distribution sequence of natural gas carbon isotope components in different areas in the southern margin of the Junggar Basin [9].  
Figure 5: Identification diagram of natural gas \(\delta^{13}\)C\({}_{1}\)—\(\delta^{13}\)C\({}_{2}\)—\(\delta^{13}\)C\({}_{3}\) in different areas in the southern margin of the Junggar Basin [19; 21].  
in higher plants, and its thermodynamic properties are relatively stable, making it a good parameter for reflecting humic parent materials. Dimethylcyclopentane mainly comes from lipid compounds of aquatic organisms, which are sensitive to maturity and are indicators of sapropelic parent material. N-heptane mainly comes from algae and bacteria, and is sensitive to maturity, making it a good indicator of maturity. The relative content of n-heptane in oil type gas is greater than 30% and the relative content of methylcyclohexane is less than 70%. The relative content of n-heptane in coal type gas is less than 35% and the relative content of methylcyclohexane is greater than 50%. The natural gas from Well Xyd1 has a high content of n-heptane and a relatively low content of methylcyclohexane (Figure 6a), mainly sourced from algae and bacteria, and belongs to oil type gas.  
The relative content triangle diagram of C5-C7 light hydrocarbons is an important parameter commonly used to identify different types of natural gas. The light hydrocarbon components derived from sapropelic parent materials are rich in n-alkanes, while those derived from humic parent materials are rich in isoparaffins and aromatic hydrocarbons. The condensate rich in cycloalkanes is also an important characteristic of terrestrial parent materials [19; 23]. Oil type gases generally have a relative content of C5-C7 n-alkanes greater than 30% [24]. The C5-C7 light hydrocarbon composition triangle diagram (Figure 6b) shows that the content of n-alkanes in the natural gas of Well Xyd1 is higher than 50%, indicating that it belongs to oil type gas.  
The relative content of C6-C7 light hydrocarbons is greatly influenced by the type of organic matter and is also commonly used to identify natural gas of different genesis types [25]. Natural gas with the advantage of alkanes is mostly related to sapropelic source rocks, whereas natural gas with high aromatic hydrocarbon content is mostly related to coal-bearing source rocks. The relative content triangle diagram of C6-C7 light hydrocarbons that the natural gas from Well Xyd1 is mostly composed of alkanes that are mainly derived from sapropelic source rocks (Figure 6c).