## 4 Results and Discussion
### Controlling Factors of Source Rock Development  
The development environment of coal-measure source rocks is different from that of deep-semideep lacustrine facies source rocks. Deep-semideep lacustrine facies source rocks are mainly developed in an anoxic reducing environment with good preservation conditions, and the organic matter mainly comes from phytoplankton and algae. However, coal-measure source rocks are mainly developed in an oxidational shallow water environment, and the organic matter is mainly from terrestrial higher plants (Figure 12). Therefore, the factors that control the development of coal-measure source rocks cannot simply be equal to lacustrine source rocks.  
Since the organic matter of coal-measure source rocks mainly comes from terrestrial higher plants, there should be a certain correlation between the biomarker parameters that characterize higher plants and TOC. C\({}_{29}\) sterane and C\({}_{24}\)Te/C\({}_{26}\)TT can both characterize terrestrial higher plants, and make their own cross plots with TOC. It is found that in the coal, carbonaceous mudstone and low TOC (TOC \(<\) 3%) dark mudstone samples, C\({}_{29}\) sterane content demonstrates good correlation with TOC (Figures 13 and 14). However, in dark mudstone samples with high TOC (3 wt.% \(<\) TOC \(<\) 6 wt.%), C\({}_{29}\) sterane content has poor correlation with TOC (Figure 13). The specific reasons need to be studied. The cross plot of the C\({}_{24}\)Te/C\({}_{26}\)TT ratios to TOC for dark mudstone also have a similar law (Figure 15). In general, the abundance of organic matter in coal-measure source rocks is positively correlated with the input of terrestrial higher plant organic matter.  
Figure 11: Relationship between C\({}_{24}\)Te/C\({}_{26}\)TT and Pr/Ph for coal-measure source rock.  
Figure 12: Schematic diagram of shallow and deep water environment.  
Because the TOC of carbonaceous mudstone and coal is large and that of dark mudstone is small, the distance between them when they appear on the linear coordinate axis at the same time is very different. In addition, the number of samples of carbonaceous mudstone and coal is very small, so it is difficult to find systematic rules in the figure. Therefore, we only discuss the factors that affect the development of dark mudstone source rocks in the following paragraphs.  
Figure 14: Relationship between C\({}_{29}\) sterane content and TOC for carbonaceous mudstone and coal.  
Figure 13: Relationship between C\({}_{29}\) sterane content and TOC for dark mudstone.  
Since the organic matter of coal-measure source rocks mainly comes from terrestrial higher plants, we have reason to believe that the environment suitable for the development of higher plants is also suitable for coal-measure source rocks. The environment suitable for the development of higher plants is the shallow water environment with low salinity. The gammacerane index of dark mudstone is cross plotted with TOC, and it is found that there is a good negative correlation between them when TOC is low (TOC \(<\) 3 wt.%) (Figure 16), indicating that the low salinity environment controls the development of coal-measures source rocks.  
The shallow water environment is turbulent, with high oxygen content and strong oxidation (Figure 12), which seems to be detrimental to the preservation and enrichment  
Figure 16: Relationship between Ga/C\({}_{30}\)H and TOC for coal-measure source rocks.  
Figure 15: Relationship between C\({}_{24}\)Te/C\({}_{26}\)TT and TOC for dark mudstone.  
of organic matter. However, after drawing the cross plot of Pr/Ph to TOC, we found that the TOC of most dark mudstone increases with the increase in Pr/Ph value (Figure 17), indicating that the partial oxidation environment is more advantageous to the enrichment of organic matter in coal-measure source rocks. This demonstrates that the factors that influence the development of deep-semideep lacustrine facies source rocks and coal-measure source rocks are distinct.