## 5 Discussion
### Kerogen Content vs. TOC  
To assess the kerogen content, we estimated the surface porosity of the selected sample on Maps images, based on the grayscale threshold. The pores of the shale sample mainly included clay mineral intercrystalline pores, organic pores/fractures, intergranular pores/fractures, and intragranular pores (Figure 7). The sample had a high clay mineral content with a large number of clay mineral intercrystalline pores, which represented the majority of the total porosity. The sum of surface porosity of the organic pores/fractures and the intergranular pores/fractures was only about 0.3%. Because of the wide range of grayscale variation of pores in clay minerals and the large difference of pore content in different clay minerals, the surface porosity of clay mineral intercrystalline pores needs to be estimated by region. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mineral/Component**} & \multicolumn{2}{c}{**Content by**} & \multirow{2}{*}{**Corrected**} & \multirow{2}{*}{**XRD (vol.\%)**} & \multirow{2}{*}{**XRD (wt.\%)**} & \multirow{2}{*}{**Density**} \\  & **QEMSCAN** & & & & \\  & **(\%)** & & & & \\ \hline Pores/Kerogen & 1.18 & 17.22 & / & / & \\ Unclassified & 4.69 & 2.98 & / & / & \\ Quartz & 18.31 & 21.63 & 31.09 & 30.8 & \\ Non-clay & K-Feldspar & 4.41 & 5.23 & 9.7 & 9.9 & 2.57 \\ minerals\({}^{*}\) & Albite & 6.92 & 8.96 & 8.49 & 8.6 & 2.63 \\  & Pyrite & 1.65 & 1.04 & 0.62 & 1.2 & 5.00 \\  & Biotite & 8.01 & 5.25 & / & / & \\  & Muscovite & 1.83 & 1.58 & / & / & \\  & Illite & 32.19 & 33.35 & 9.81 & 10.4 & 2.75 \\ Clay minerals & Kaolinite & 8.96 & 7.56 & 6.37 & 6.44 & 2.62 \\  & Chlorite & 9.35 & 8.1 & 7.53 & 8.42 & 2.90 \\  & Smetictic & 1.24 & 0.29 & / & / & 2.35 \\  & I/Smix & / & / & 26.38 & 24.26 & 2.39 \\ Total \({}^{**}\) & Non-Clay & 38.42 & 43.86 & 49.9 & \\  & Clay & 61.58 & 56.14 & 50.1 & \\ \hline \hline \end{tabular}  
* Due to low content and non-detection by XRD, some non-clay minerals of QEMSCAN are not listed here, including pyrophyllite, apatite, siderite, oligoclase, rutile, dolomite, and calcite. \({}^{**}\) The content results of identified minerals are normalized to 100 percent, after pore/kerogen and unclassified components are removed.  
\end{table} 
Table 3: Comparison among the mineral/component surface content values by QEMSCAN-MAPS correction and volume content values by XRD. The surface porosity of different clay minerals was estimated based on the automatic identification of MAPS images in combination with previous research results [22]. The surface porosity of illite, chlorite, and kaolinite are about 20%, 10%, and 10%, respectively. Therefore, the equation for calculating the surface porosity of the samples is simplified as:  
\[\varnothing_{2D}=\sum_{i=1}^{n}C_{i}\times\varnothing_{i}+\varnothing_{others} \tag{3}\]  
where \(\varnothing_{2D}\) is the surface porosity of the whole sample, \(C_{i}\) and \(\varnothing_{i}\) are the 2D area share of clay minerals (illite, chlorite, and kaolinite) and their porosity, respectively, and \(\varnothing_{others}\) is the 2D area share of non-clay pores, such as organic pores and intergranular pores. The calculation yields a surface porosity of about 6.86% for the whole sample. According to the area ratio of pore/kerogen (Table 3), the surface content of kerogen \(C_{kerogen}\) can be deduced to be about 10.56%. Considering the thermal maturity [23] and the type of kerogen, the kerogen density of the sample was about 1.3 g/cm\({}^{3}\)[24]. The density of the  
Figure 7: Main pore/fracture types of the shale sample. shale sample was 2.3 g/cm\({}^{3}\). Therefore, we calculated the kerogen content of the sample to be about 6.26 wt.%, using the following equation. \[TOC=\frac{C_{krogen}\times\gamma}{C_{krogen}\times\gamma+\left(1-C_{krogen} \right)\times\rho_{b}}*100\% \tag{4}\]  
where \(TOC\) is the total organic carbon content, %; \(C_{krogen}\) is the kerogen surface content, %, \(\gamma\) is the kerogen density, g/cm\({}^{3}\), and \(\rho_{b}\) is the shale density, g/cm\({}^{3}\).