# 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 Carbon Isotopes  
The carbon isotope characteristics of natural gas are the most important identification indicators and methods for determining the genesis of natural gas [11].  
The carbon isotope of ethane is mainly influenced by the type of hydrocarbon source rock parent material [12; 13]. It is generally believed that the ethane carbon isotope value \(\delta^{13}\)C\({}_{2}=-28\)% is used as the boundary value for identifying oil type gas and coal type gas. If it is lower than this value, it indicates oil type gas, and if it is higher, it indicates coal type gas [14]. The majority of ethane carbon isotope \(\delta^{13}\)C\({}_{2}\) in the central and western parts of the southern margin of the Junggar Basin ranges from \(-28\)% to \(-22\)%, which is characterized by coal type gas and mainly comes from Jurassic coal source rocks. However, the ethane carbon isotope \(\delta^{13}\)C\({}_{2}\) of natural gas from Well Xyd1 in the Yongfeng sub-sag is \(-28.45\)% (Table 2), which belongs to oil type gas associated with the parent material type of the Lucaogou Formation hydrocarbon source rock, and is different from the natural gas in the central and western regions of the southern margin of the Junggar Basin.  
The methane carbon isotope is generally influenced by the degree of thermal evolution of the source rocks. Generally speaking, as the maturity of the source rocks increases, the methane carbon isotope of natural gas also increases [15]. The thermal methane carbon isotope value \(\delta^{13}\)C\({}_{1}\) is distributed between \(-30\)% and \(-45\)% [16], and the higher the degree of evolution, the larger the methane carbon isotope value. However, the biogenic methane carbon isotope value is relatively depleted, generally distributed between \(-75\)% and \(-45\)%, and usually less than \(-55\)% [17; 18]. Most natural gas methane carbon isotope \(\delta^{13}\)C\({}_{1}\) values in Anjihai, Khorgos, Manasi, and other areas in the central and western part of the southern margin of the Junggar Basin range from \(-35\)% to \(-33\)%. The methane carbon isotope \(\delta^{13}\)C\({}_{1}\) of natural gas in Well Xyd1 in the Yongfeng sub-sag ranges from \(-58.62\)% to \(-59.42\)% (Table 2). The methane carbon isotope of natural gas is the lightest, which is significantly different from that of natural gas in the surrounding areas.  
In addition, the distribution sequence of natural gas carbon isotope composition is also a method to identify organic origin natural gas [19; 20]. In general, the different components of natural gas carbon isotopes usually show a positive sequence of becoming gradually heavier, that is, \(\delta^{13}\)C\({}_{1}<\delta^{13}\)C\({}_{2}<\delta^{13}\)C\({}_{3}\). From the distribution of natural gas carbon isotope components in different regions in the southern margin of the Junggar Basin, most of them show a positive sequence distribution, indicating that these natural gases are normal organic origin natural gas (Figure 4). However, the carbon isotope composition of natural gas in the Yongfeng sub-sag is lighter than that in other regions. From the perspective of ethane and propane carbon isotopes, the natural gas type in the Yongfeng sub-sag is oil type gas, which is different from that in the central and western regions of the southern margin of the Junggar Basin. However, most natural gas in the central and western regions of the southern margin of the Junggar Basin belongs to coal type gas or mixed gas (Figure 5) [9].