## 4 Results and Discussion
### Geochemistry Features as Indication for Organic Matter Input and Depositional Conditions
#### 4.2.1 Depositional Environment  
The depositional environment can be described by the total organic carbon and total reduced sulfur content in sediments [19; 20]. The sulfur content of the coal sample is 0.87 wt.%; the sulfur content of carbonaceous mudstone samples is 0.36-0.57 wt.%, with an average of 0.45 wt.%; the sulfur content of mudstone samples is 0.09-0.23 wt.%, with an average of 0.14 wt.%. Modern freshwater sediments rich in organic matter have a substantially greater C/S ratio than marine sediments with a similar organic matter concentration [21]. The S/C ratio also represents how much microbial sulfate reduction occurs during the decomposition of organic matter, and this information can be utilized to describe the redox conditions of the depositional environment [22; 23]. The S/C ratio of oxic marine sediments is generally lower than 0.36 [24], while the S/C ratio in anoxic environment is generally greater than 0.36 [25]. The S/C ratio of coal sample in this study is 0.02; the S/C ratio of carbonaceous mudstone samples is between 0.02 and 0.03, with an average of 0.02; the S/C ratio of dark mudstone samples is between 0.04 and 0.28, with an average of 0.12. It indicates that these source rock samples were deposited in the oxidation and freshwater environments. [PERSON] et al. (1997) proposed that the slope of TS-TOC scatter regression line is a more effective tool for judging depositional environment [26]. [PERSON] et al. (2012) found in the study of samples from the Yangtze River Delta that the slope of TS-TOC scatter regression line is higher in marine and lacustrine environments with high water salinity, while the slope of TS-TOC is significantly lower in estuarine areas affected by strong freshwater [27]. [PERSON] and [PERSON] (1992) calculated that the regression slope of TS-TOC in the normal marine environment was 0.36 [28], while the regression slope of TS-TOC in the sample is only 0.02 (Figure 5), indicating the freshwater depositional environment. Figure 4: Organic matter types of coal-measure source rocks. Phytane and pristane are often the two most significant acyclic isoprene alkanes [29,30]. Their combination with \(n\)C\({}_{17}\) and \(n\)C\({}_{18}\) can characterize the depositional environment. It is widely accepted that a low Pr/Ph ratio (<0.8) indicates reduction environment, medium Pr/Ph ratio (1.0\(\sim\)3.0) indicates weak oxidation environment, and high Pr/Ph ratio (>3.0) reflects terrestrial organic matter input under oxidation conditions [29, 30, 31, 32]. The Pr/Ph ratio of the coal rock sample is 2.12; the Pr/Ph value of carbonaceous mudstone samples is between 0.70 and 4.01, with an average of 1.93; the Pr/Ph value of dark mudstone samples is between 0.53 and 1.85, with an average of 0.95. Generally speaking, the depositional environment of source rocks in the study area is oxyc to slightly dysoxic. Furthermore, many researchers used the crossplot of Pr/nC\({}_{17}\) and Ph/nC\({}_{18}\) to represent oil maturity, as well as the source rock depositional environment [33, 34, 35, 36]. Compared with coal, the ratio of isopenoid to normal alkane in dark mudstone is relatively high. The Pr/nC\({}_{17}\) and Ph/nC\({}_{18}\) values of coal are 0.28 and 0.08, respectively, the average Pr/nC\({}_{17}\) and Ph/nC\({}_{18}\) values of carbonaceous mudstone are 0.69 and 0.33, respectively, and the average Pr/nC\({}_{17}\) and Ph/nC\({}_{18}\) values of dark mudstone are 1.01 and 0.64, respectively (Table 1). From this plot (Figure 6), it is evident that the source rock samples were deposited in a freshwater swamp lacustrine facies environment. The abundance ratio of sterane can also indicate the depositional environment [37,38]. [PERSON] et al. (1979) counted the sterane content of organic matter in different depositional environments and devised a plot of the C\({}_{27}\), C\({}_{28}\) and C\({}_{29}\) sterol contents [37].