# Characteristics and Origin of Natural Gas in Yongfeng Sub-Sag of Bogda Mountain Front Belt
## 4 Characteristics of Source Rocks  
The Permian Lucaogou Formation around Bogda Mountain is a set of deep to semi-deep lacustrine sediments, whose thickness gradually increases near Bogda Mountain. The lithology is mainly rich in organic black to gray-black mudstone, interbedded withsilty mudstone, cloudy mudstone, oil shale, etc. The thickness is relatively large, reaching more than 300 m, and it is the most important set of source rocks in the study area [2; 5; 6; 7; 8]. According to the evaluation criteria for organic matter abundance and the comprehensive evaluation of organic matter type and maturity of lacustrine mudstone source rocks, the Lucaogou Formation in the Yongfeng sub-sag is generally a high-quality lacustrine source rock. According to the testing and analysis of the shale in the Lucaogou Formation of Well Xyd1 (Table 1), the total organic carbon content (TOC) ranges from 0.95% to 6.94%, with an average of 3.49%; the S\({}_{1}\) ranges from 0.11 mg/g to 2.03 mg/g; the S\({}_{2}\) ranges from 1.03 mg/g to 29.11 mg/g; and the oil generation potential (S\({}_{1}\) + S\({}_{2}\)) ranges from 1.42 mg/g to 29.11 mg/g, with an average of 11.16 mg/g. The chloroform bitumen A ranges from 0.0653% to 0.4937%, with an average of 0.2802%, indicating a relatively high organic matter abundance overall (Figure 3). The microscopic components of kerogen in the Lucaogou Formation source rocks are mainly composed of sapropelic material, which accounts for 60% to 80%, and the overall evaluation of organic matter types is mainly I-II\({}_{1}\). The T\({}_{\rm max}\) value is between 433 \({}^{\circ}\)C and 448 \({}^{\circ}\)C, and the R\({}_{\rm o}\) is between 1.15% and 1.39%, reaching the mature stage that is dominated by oil and wet gas generation. The dark mudstone of the Lucaogou Formation in the Hongyanchi area near Yongfeng has a thickness of over 400 m, with the TOC ranging from 0.90% to 5.46%, with an average of 4.52%. The chloroform bitumen A ranges from 0.0611% to 0.2203%, averaging 0.15%. The S\({}_{1}\) + S\({}_{2}\) ranges from 1.42 mg/g to 36.62 mg/g, averaging 8.7 mg/g. The Lucaogou Formation in the area of Yaomo Mountain is mainly composed of dark gray, gray-black, black, and gray-brown mudstone; shale; oil shale; and limestone. The TOC ranges from \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **Depth/m** & **TOC/\%** &
\begin{tabular}{c} **Chloroform** \\ **Bitumen A/\%** \\ \end{tabular} & **S\({}_{1}\)/mg/g** & **S\({}_{2}\)/mg/g** & **S\({}_{1}\) + S\({}_{2}\)/mg/g** & **T\({}_{\rm max}\)/\({}^{\circ}\)C** & **R\({}_{o}\)/\%** \\ \hline Xyd1-03 & 1874 & 0.95 & 0.0653 & 0.11 & 0.92 & 1.03 & 433 & 1.15 \\ \hline Xyd1-04 & 1981 & 1.91 & 0.153 & 0.47 & 3.7 & 4.17 & 445 & 1.2 \\ \hline Xyd1-05 & 2015 & 1.81 & 0.1576 & 0.69 & 4 & 4.69 & 444 & 1.19 \\ \hline Xyd1-06 & 2034 & 3.73 & / & / & / & / & / \\ \hline Xyd1-07 & 2043 & 6.54 & 0.3158 & 0.98 & 28.13 & 29.11 & 446 & 1.2 \\ \hline Xyd1-08 & 2067 & 4.2 & / & / & / & / & / \\ \hline Xyd1-09 & 2079 & 6.94 & 0.1976 & 0.99 & 25.33 & 26.32 & 445 & 1.15 \\ \hline Xyd1-10 & 2089 & 5.68 & / & / & / & / & / \\ \hline Xyd1-11 & 2107 & 3.07 & 0.1804 & 0.51 & 7.99 & 8.5 & 446 & 1.19 \\ \hline Xyd1-12 & 2111 & 2.16 & / & / & / & / & / \\ \hline Xyd1-13 & 2131 & 4.63 & 0.3247 & 1.05 & 12.52 & 13.57 & 448 & 1.33 \\ \hline Xyd1-14 & 2156 & 3.48 & 0.2601 & 0.91 & 11.28 & 12.19 & 445 & 1.23 \\ \hline Xyd1-15 & 2175 & 3.91 & 0.2874 & 0.83 & 12.32 & 13.15 & 447 & 1.36 \\ \hline Xyd1-16 & 2246 & 3.4 & 0.4937 & 2.03 & 10.63 & 12.66 & 443 & 1.39 \\ \hline Xyd1-17 & 2253 & 3.73 & / & / & / & / & / \\ \hline Xyd1-18 & 2309 & 2.35 & 0.425 & 1.74 & 5.21 & 6.95 & 443 & 1.39 \\ \hline Xyd1-19 & 2319 & 2.07 & 0.4461 & 1.73 & 4.5 & 6.23 & 442 & 1.34 \\ \hline Xyd1-20 & 2323 & 2.26 & 0.3357 & 1.76 & 4.8 & 6.56 & 439 & 1.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Organic geochemical characteristics of source rocks in the Lucaogou Formation of Well Xyd1. 7.49% to 7.84%, with an average of 7.2%.