## 3 Data Preparation
### Correlation Analysis of Logging Curves  
Correlation analysis result is an important basis for the TOC quantitative prediction model to select logging data. Through correlation analysis, logging data that are highly related with TOC sample data were chosen to establish the TOC quantitative prediction model. The most sensitive logging parameters to TOC changes were determined according to the correlation analysis between 158 TOC measured samples and logging data (Figure 5).  
Radioactivity of source rocks mainly comes from clay minerals and organic carbon. In the deposition process, radioactive elements are adsorbed by clay minerals and organic matters, thus increasing the radioactivity of source rocks. The reducing environment which is formed by kerogen in source rock is beneficial for sedimentation of the radioactive element Uranium (U). In addition, source rock is characteristic of fine particles and a large surface area. Organic matters in source rock have strong absorptivity of radioactive elements [21,22]. Hence, organic content influences the numerical value of natural gamma-ray logging. The radioactivity of source rock is strengthened with the increase of TOC content. The TOC content is positively related with natural gamma (Figure 5a).  
Organic matters in source rock belong to a medium to high interval transit time. The interval transit time increases due to the existence of organic matters. TOC content is positively related with interval transit time (Figure 5b).  
Solid organic matters are lighter than protolith and their density is close to that of water [23]. Moreover, the density of source rock is negatively related with maturity. With the increase of organic content, density of source rocks decreases (Figure 5c).  
Macerals of organic matters in source rock are rich of element hydrogen, which influence the neutron logging value and bring abnormal high values of neutron logging. Therefore, neutron porosity is positively related with TOC content (Figure 5d).  
Since kerogen and oil gas have relatively poor conductivity, the formation resistivity increases when there's organic enrichment. In practical data, resistivity is influenced by TOC slightly due to the low TOC of source rock in the study area. This implies the insignificant correlation between resistivity and TOC (Figure 5e).  
Radioactivity in source rock is mainly attributed to radioactive elements (e.g., U), which were adsorbed in the process of sedimentation. Hence, natural gamma spectrometry is positively related with TOC content (Figure 5f-h).  
Figure 5: _Cont_.  
The correlations between re-selected TOC sample data and logging curves are improved significantly. At the 99% confidence level, the T critical value is 2.3578, and the T statistic value of each log data is greater than it (Table 1), which proves that the variables are significantly correlated at the 99% confidence level. According to correlation analysis, natural gamma-ray (GR) and density (DEN) are more correlated with TOC.  
Figure 5: Correlation between TOC content and (**a**) natural gamma ray, (**b**) acoustic time difference, (**c**) organic matter density, (**d**) neutron porosity, (**e**) formation resistivity, (**f**) uranium, (**g**) kaium and (**h**) thorium.