## 4 Results  
### Mineral Composition, TOC Content, and Lithofacies Classification  
The mineral composition of the Da'anzhai Member in the study area is depicted in Table 1 and Figure 2. Predominantly, clay minerals constitute the mineral composition, with a distribution range of 36.8% to 74.7% (mean of 58.1%). Siliceous minerals follow, with a content range of 20.0% to 46.8% (mean of 32.6%). Quartz is the primary siliceous mineral, with a distribution range of 11.7% to 39.1% (mean of 28.6%). Feldspar content is relatively low, ranging from 1.6% to 8.3% (mean of 4.4%), predominantly composed of plagioclase, while potassium feldspar generally constitutes less than 1%. Carbonate minerals have the lowest content, ranging from 0% to 24.7% (mean of 7.7%), primarily dominated by calcite (mean of 7.3%), with low levels of dolomite and siderite. Simultaneously, the samples exhibit generally low levels of pyrite, ranging from trace amounts (less than 0.5%) to 3.9%, with a mean of 1.5%. The TOC content in the Da'anzhai Member samples from the study area has a distribution range of 0.4% to 2.3%, with a mean of 1.1%. Lithofacies serve as indicators reflecting sedimentary environments and rock geological characteristics [14], providing a reliable basis for detailed shale studies [5; 13]. Shale, defined as fine-grained sedimentary rocks with a grain size of \(<\)0.0039 mm [13; 20; 42], has been a subject of study since early times, but its investigation is still in the initial stages [43; 44], lacking a uniform standard. In this study, we refer to prior research outcomes and primarily conduct lithofacies division based on two criteria: mineral composition and organic matter content [12; 13]. It is crucial to note that we adjusted the criteria for determining TOC content. In a previous study [5], TOC content greater than 2% was considered as organic-rich shale. However, their focus was on marine Longmaxi Formation shale, which generally exhibits high TOC content. In this paper, we set TOC = 1% as the threshold for determining shale organic matter content. When TOC content exceeds 1%, the sample is classified as organic-rich shale, and when TOC content is below 1%, the sample falls into the category of organic-poor shale. Considering the geological characteristics outlined above, three types of lithofacies are identified in the study area: organic-rich argillaceous shale (ORA), organic-poor argillaceous shale (OPA), and organic-rich mixed shale (ORM) (Table 1, Figures 2 and 3). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Depth** & **Quartz** & \begin{tabular}{c} **Polestimate** \\ **Feldspar** \\ \end{tabular} & \begin{tabular}{c} **Polestimate** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Calcite** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Formula** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Calcite** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Formula** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Solienta** \\ **Felspaper** \\ \end{tabular} & \begin{tabular}{c} **Plosimetric** \\ **Mineral** \\ \end{tabular} & \begin{tabular}{c} **Clary** \\ **Mineral** \\ \end{tabular} & \
\begin{tabular}{c} **Carbonate** \\ **Mineral** \\ \end{tabular} &
\begin{tabular}{c} **TOC** \\ **Unification** \\ \end{tabular} \\ \hline
**(9)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** & **(5)** \\ \hline  
& & 41.4 & 51.7 & 6.9 & 0.6 & OPA \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mineral composition and TOC content of Da’anzhai Member. Figure 3: Microscopic characteristics of different lithofacies in the Da’anzhai Member: (**a**) 2959.78 m, organic-rich argillaceous shale (ORA); (**b**) 2972.5 m, organic-rich argillaceous shale (ORA); (**c**) 2995.1 m, organic-poor argillaceous shale (OPA); (**d**) 3004.11 m, organic-poor argillaceous shale (OPA); (**e**) 2961.13 m, organic-rich mixed shale (ORM); (**f**) 2986.3 m, organic-rich mixed shale (ORM).