# Characteristics and Origin of Natural Gas in Yongfeng Sub-Sag of Bogda Mountain Front Belt
## 6 Natural Gas Origin  
### Analysis of the Origin of Natural Gas  
The composition and carbon isotope of natural gas are closely related to the sedimentary environment, organic matter type, and thermal evolution degree of the gas source material. The methane carbon isotope \(\delta^{13}\)C1 and natural gas composition C1/(C2 + C3) values can be used to identify the origin of natural gas (Figure 7) [26; 27].  
Figure 6: Characteristics of natural gas light hydrocarbon composition of Well Xyd1. (**a**) Triangle diagram of natural gas C7 light hydrocarbons; (**b**) Triangle diagram of natural gas C5-7 light hydrocarbons; (**c**) Triangle diagram of natural gas C6-7 light hydrocarbons.  
The natural gas composition of the Yongfeng sub-sag shows that methane is the main hydrocarbon component, with a low, heavy hydrocarbon content and a high dryness coefficient. However, the R\({}_{\rm o}\) of the Permian Lucaogou Formation source rocks in the study area ranges from 1.1% to 1.25%. According to the thermal evolution stage, it should be in the stage of large-scale oil and wet gas generation. Obviously, there is a discrepancy between the drying coefficient of natural gas and the thermal evolution stage of the source rock. It is thought that the degree of natural gas dryness is not directly related to the thermal evolution of the source rock, and that natural gas does not come from the thermal evolution of the source rock to generate hydrocarbons. In general, the main product of natural gas generated by biological processes is methane that has very little heavy hydrocarbon content and has a drying coefficient that is generally greater than 0.95 [16; 28; 29; 30]. From Well Xyd1, the drying coefficient of natural gas is relatively high, reaching more than 0.96, and the heavy hydrocarbon content is lower, generally ranging from 0.24% to 3.72%. The natural gas composition C\({}_{1}\)/(C\({}_{2}\) + C\({}_{3}\)), with an average value of 329, is far higher than the natural gas composition C\({}_{1}\)/(C\({}_{2}\) + C\({}_{3}\)) in Hutubi, Manasi, Tugulu, and other areas on the southern edge of the Junggar Basin. In addition, the methane carbon isotope of natural gas in the Yongfeng sub-sag is also significantly lighter than that in other areas in the southern margin of the Junggar Basin. Therefore, it is determined that the natural gas in the Yongfeng sub-sag is biogenic gas.  
In addition to natural gas, a small amount of crude oil was also obtained in the Lucaogou Formation of Well Xyd1 during the oil testing process. The full analysis data of the crude oil showed that its density ranged from 0.89 to 0.90 g/cm\({}^{3}\) and its viscosity at 50 \({}^{\circ}\)C was 230-618 mPa.s. Based on these characteristics, it is classified as heavy oil according to the classification standard of crude oil. Therefore, it is thought that the natural gas of Well Xyd1 is a biological degradation gas of crude oil. This process refers to the source rock undergoing a certain degree of thermal evolution to reach the oil generation stage and generate crude oil. Due to factors such as later structural uplift, the crude oil is degraded by microorganisms to generate natural gas. During the process of crude oil  
Figure 7: Genetic types of natural gas in different areas of the southern margin of the Junggar Basin [27].  
degradation, natural gas becomes drier, with very little heavy hydrocarbon content, and the nitrogen content is usually higher than that of conventional natural gas reservoirs. In this process, residual oil will become heavier, so biodegradable gas is often associated with heavy oil [31; 32; 33].