## 4 Results and Discussion
### Test Results of Volume Fraction and Carbon Isotope Value
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Volume Fraction**} & \multicolumn{2}{c}{**Isotope Value**} & \multicolumn{2}{c}{**Volume Fraction**} & \multicolumn{2}{c}{**Isotope Value**} \\
**Specimens** & **(\%)** & & **(\%)** & & **Specimens** & **(\%)** & **(\%)** & **(\%)** \\ \cline{2-10}
**No.** & **CH\({}_{4}\)** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)** & **No.** & **CH\({}_{4}\)** & **CO\({}_{2}\)** & **CH\({}_{4}\)** & **CO\({}_{2}\)** \\ \hline
1–1 & 83.32 & 0.28 & \(-34.4\) & \(-3.5\) & 4–6 & 89.77 & 0.29 & \(-32.56\) & \\
1–2 & 82.63 & 0.24 & \(-34.86\) & \(-3.93\) & 4–8 & 91.82 & 0.27 & \(-33.2\) & \\
1–3 & 83.57 & 0.33 & \(-34.12\) & \(-3.48\) & 4–10 & 82.60 & 0.26 & \(-35.01\) & \\
1–6 & 90.55 & 0.28 & \(-33.98\) & \(\backslash\) & 4–b5 & 8.09 & 0.07 & \(-33.1\) & \(-4.75\) \\
1–b8 & 92.09 & 0.28 & \(-34.04\) & \(-7.06\) & 7d–7–10 & 31.29 & 0.32 & \(-35.27\) & \\
2–2 & 91.59 & 0.30 & \(-35\) & \(-4.9\) & 5d–b14 & 41.40 & 0.33 & \(-34.79\) & \\
2–3 & 63.00 & 0.48 & \(-35.07\) & \(-0.7\) & 6m–5 & 46.23 & 0.34 & \(-35.02\) & \\
2–b7 & 76.98 & 0.27 & \(-34.09\) & \(-5.21\) & 6m–6 & 13.26 & 0.28 & \(-34.62\) & \\
2–5 & 63.28 & 0.30 & \(-32.2\) & \(\backslash\) & 6m–7 & 15.05 & 0.16 & \(-33.29\) & \\
2–10–b2 & 75.95 & 0.30 & \(-34.33\) & \(-4.52\) & 6u–2 & 21.18 & 0.18 & \(-35.94\) & \\
3–1 & 85.22 & 0.21 & \(-34.18\) & \(\backslash\) & 6u–3 & 24.76 & 0.24 & \(-36.02\) & \\
3–2 & 85.28 & 0.25 & \(-33.87\) & \(-7.03\) & 6u–4 & 24.39 & 0.28 & \(-36.01\) & \(-8.37\) \\
3–3 & 84.03 & 0.26 & \(-33.9\) & \(-5.72\) & 6u–5 & 0.72 & 0.08 & \(-32.97\) & \\
3–6 & 32.79 & 0.15 & \(-34.44\) & \(-4.05\) & 6u–6 & 37.47 & 0.19 & \(-34.79\) & \\
3–b8 & 86.88 & 0.31 & \(-32.97\) & \(-2.15\) & 6u–8 & 59.88 & 0.04 & \(-33.8\) & \\
4–5 & 56.46 & 0.18 & \(-33.55\) & \(\backslash\) & \(\backslash\) & \(\backslash\) & \(\backslash\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test results of volume fraction and carbon isotope value. It is generally believed that the origin of CBM is divided into biological origin and thermal origin, and the carbon isotope value of CBM is less than \(-55\%\) for biological origin, and greater than \(-55\%\) for thermal origin. The carbon isotope value of CBM in the Xiaotun Coal mine is \(-37\%\)\(\circ\)\(\bullet\)\(-32\%\), which belongs to thermogenic gas. The degree of thermal evolution of coal and rock is an important factor to control the carbon isotope composition and change in CBM; especially in the stage of higher maturity, this control is more obvious. The coalbeds of Xiaotun coal mine are all high gas content, high maturity of high metamorphic anthracite; from the upper coabled to the lower coabled there is an increasing trend. It is generally believed that the origin of carbon dioxide is divided into organic (biological) and inorganic (abiotic). The carbon isotope values of CO\({}_{2}\) from organic origin generally range from \(-39\%\) to \(-8\%\), and those from humus organic matter generally range from \(-25\%\) to \(-5\%\). The carbon isotope value of CO\({}_{2}\) from the Xiaotun Coal mine is \(-9\%\)\(\circ\)\(-4\%\), which belongs to inorganic gas. At the same time, the negative value of the CO\({}_{2}\) carbon isotope is small, indicating that the CO\({}_{2}\) carbon isotope is closer to the original CBM in the deep mantle, and there is no fractionation effect of the CO\({}_{2}\) carbon isotope in the process of coalification. In the process of CBM extraction, the CBM pressure is gradually released, which creates the desorption and diffusion of CBM, accompanied by isotopic fractionation. With the  
Figure 9: Comparison of gas components and carbon isotope values in different coalbeds, the rhomboids in the figure indicate the tested values of gas components and carbon isotopes. (**a**) CH\({}_{4}\) gas component; (**b**) CO\({}_{2}\) gas component; (**c**) CH\({}_{4}\) carbon isotope values; (**d**) CO\({}_{2}\) carbon isotope values.