## 4 Results
### Petrological and Mineralogical Characteristics  
Mineral composition is an important factor that affects the nanopore structure of shale. Different inorganic minerals have distinct differences in crystal structure and physical and chemical properties, which result in variations in pore morphology and size between crystals and particles, as well as SSA and even adsorption heat of particles. Consequently, these differences lead to variations in the adsorption capacity of shale [41]. The XRD test results of Taiyuan Formation shale samples show that the mineral composition is predominantly quartz and clay minerals (Figure 3), with a significant variation in the content of clay minerals ranging from 12.4% to 60.0%, averaging 40.7%. The quartz content ranges from 19.7% to 51.3%, with an average of 37.7%. In addition to quartz and clay minerals, the Taiyuan Formation shale also contains small amounts of feldspar, carbonate, and pyrite. Among these, the feldspar content ranges from 0% to 10.3%, averaging at 2.8%; the carbonate content ranges from 0% to 57.7%, averaging at 11.7%. It exhibits obvious heterogeneous distribution vertically, where individual samples with high carbonate content may be attributed to their collection location near limestone layers. The pyrite content ranges from 0% to 18.1%, averaging at 5.2%. Pyrite shows distinct enrichment sections in the vertical direction. A small amount of siderite is also present in the Taiyuan Formation shale, with the content ranging from 0 to 7.5% and an average of 2.0%. This indicates that the Taiyuan Formation shale has a high concentration of brittle minerals, which is more conducive to shale reservoir fracturing (Figure 4). According to the analysis results of clay mineral composition in the Taiyuan Formation shale, the main clay minerals are I/S mixed layer, accounting for 19.0% to 81.0%, with an average of 55.5%. Followed by illite, its content ranges from 7% to 40%, with an average of 25.2%; only a few samples contain trace amounts of chlorite, with the highest content being at 9% (Figure 5).  
Lithofacies refer to the combination of rock types or lithologies formed in a specific sedimentary environment. The division of lithofacies mainly relies on factors such as mineral composition, sedimentary structure, organic matter abundance, and development characteristics of biology [42; 43; 44]. Based on the characteristics of shale mineral composition, the three-terminal element method is employed, with quartz + feldspar + pyrite, clay minerals, and carbonate minerals as three-terminal elements. The Taiyuan Formation shale in the SNCB can be mainly classified into siliceous shale, clay-rich shale, and mixed shale, with a small amount of siliceous shale (Figure 6).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & \multicolumn{5}{c}{**Element Content/10\({}^{-6}\)**} & \multicolumn{5}{c}{**Redox Proxies**} \\ \cline{2-9}
**ID** & **V** & **Ni** & **Cu** & **Zn** & **La** & **Ce** & **V/(V + Ni)** & **Cu/Zn** & **Ce/La** \\ \hline Z-1 & 120 & 18.4 & 16.1 & 73.3 & 49.32 & 98.56 & 0.87 & 0.22 & 2.00 \\ Z-3 & 95 & 24.0 & 21.5 & 92.4 & 50.28 & 87.98 & 0.80 & 0.23 & 1.75 \\ Z-5 & 164 & 28.0 & 22.8 & 99.2 & 47.52 & 88.32 & 0.85 & 0.23 & 1.86 \\ Z-7 & 56 & 12.1 & 19.4 & 120.2 & 58.08 & 91.31 & 0.82 & 0.16 & 1.57 \\ Z-13 & 99 & 30.5 & 14.1 & 60.4 & 26.40 & 49.80 & 0.76 & 0.23 & 1.89 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The content of some elements of the Taiyuan Formation shale and identifications for redox condition.  
Figure 4: The content of whole-rock minerals for Taiyuan Formation shale in the SNCB.  
Figure 3: The XRD patterns of Taiyuan Formation shale in the SNCB (Z-10).  
Figure 5: The content of clay minerals for Taiyuan Formation shale in the SNCB.  
Figure 6: Triple graph of shale mineral compositions of Taiyuan Formation in the SNCB.