**Table 3.** Geochemical parameters of Nongan oil shale samples at different pyrolysis stages.

**Figure 7.** TIC diagrams in each stage.

Biomarkers are mainly used to study the source, maturity and paleosedimentary environment of organic matter in sediments [36,61,62]. In this study, the composition and biomarker characteristics of n-alkanes, isoprenoids, steranes, hopanes and other organic matter in the semicoke and pyrolysis oil of Nenjiang Formation oil shale in Nongan were investigated (Figures 8 and 9; Table 3) and selected to analyze the change in thermal.

A previous study found that the maturity parameter based on the relative stability of C27 hopanes ranged from immature to mature, but it was strongly dependent on the source [36]. The stability of C27 17α-trinorhopane (Tm) is better than that of C27 18α-trinorhopane II (Ts). As organic matter matures, the Ts/(Ts + Tm) value increases. Ts/(Ts + Tm) is a relatively reliable maturity index for evaluating samples from the same source rock location [62]. It was found that the Ts/(Ts + Tm) ratio increases with the increase in pyrolysis temperature. It seems that the value from the ratio increases gradually from 20 to 425 ◦C [61,62] (Figures 8 and 9, Table 3).

The isomerization index 22S/(22S + SSR) of hopane is feasible to evaluate the maturity from the immature to low maturity [36]. The isomerization of C31–C35 17α-hopane on C-22 can be used to evaluate the maturity of crude oil or asphalt. The biogenic hopane precursor has a 22R configuration, and it gradually transforms into a mixture of 22R and 22S isomers. After the oil generation stage reaches equilibrium, the 22S/(22S + 22R) value remains unchanged, so it is impossible to obtain further maturity information from it [36,61–64]. The change in hopane C32 22S/(22S + 22R) in Nongan samples in the range of 20–520 ◦C can be divided into two stages, which increase from 0.341 to 0.447 within 0–400 ◦C. The hopane C32 22S/(22S + 22R) in samples increases suddenly to 0.52 at 425◦C and slowly from 0.52 to 0.584 within 425–520 ◦C (Figures 8 and 9).

**Figure 8.** MS at different temperatures with mass chromatogram *m*/*z* = 191.

**Figure 9.** MS at different temperatures with mass chromatogram *m*/*z* = 217.

In this study, the CPI and hopane 22S/(22S + 22R) have mutations when the temperature reaches 350–400 ◦C, indicating the kerogen has reached the maturity stage. The value of Ts/(Ts + Tm) can indicate that the organic matter has never matured to the peak of the oil generation window.

The C29 ααα20R of sterane is the biological configuration of the sterane precursor that exists only in living organisms, while C29 ααα 20S, C29 αββ 20R and C29 αββ 20S have stable chemical structures [36,65]. As the kerogen matures, the C-20 bond of the C29 ααα20R configuration is isomerized. The bond converts the C29 ααα20R configuration to a C29 ααα20S configuration and forms a mixture of 20R and 20S, which increases the ratio of 20S/20 (R + S) of sterane from 0 to 0.5 [36,65,66]. The isomerization of 20 s and 20 R regular steranes at the C-14 and C-17 sites leads to the formation of isomers ββ/(αα + ββ). The ratio increases from nearly 0 to approximately 0.7 with increasing thermal maturity [67–69]. This study found that with the increase in pyrolysis temperature, the evolution of sterane C29 20S/20(R + S) and ββ/(αα + ββ) can be subdivided into three stages. In the immature stage of the non-oil shale semicoke, the ββ/(αα + ββ) ratio sample range from 0.165 to 0.214 and from 0.071 to 0.201, respectively, when the pyrolysis temperature is 0–250 ◦C. The temperature increases gradually and rapidly in the range of 250–450 ◦C leading to ββ/(αα + ββ) values from 0.243 to 0.424 and from 0.233 to 0.390, respectively. As the pyrolysis temperature continues to increase to 520 ◦C, only C29 20S/20(R + S) and ββ/(αα + ββ) increase slowly, and this also shows that the kerogen is in the mature stage (Figures 8 and 9, Table 3).

#### *3.3. Feedback of Maturity Parameters on the Progress of the Pyrolysis Reaction*

The production and composition of organic products in each stage of oil shale pyrolysis change regularly. Therefore, it is very important to master the process of underground pyrolysis reaction in oil shale in situ conversion project for efficient cracking and maximization of economic benefits. Although the process of pyrolysis reaction can be analyzed by the maturity of oil shale kerogen, the existing theoretical research on oil shale organic matter maturity can only analyze the maturity of oil shale core samples Ro and Tmax [29–34,36]. However, drilling and coring in the process of in situ conversion of oil shale not only greatly increases the cost of the project. Moreover, it destroys the underground thermal reaction environment and causes serious disadvantages to the exploitation. In order to realize the dynamic feedback of the in situ pyrolysis process of oil shale, this time, combined with the experience of in situ conversion pilot experiments, it is proposed that oil and gas products can be obtained through production wells during project operation. By testing the maturity information of biomarker compounds of oil and gas products, the reaction process of subsurface oil shale pyrolysis can be feedback.

As shown in Figure 10, as organic matter decreased, six different maturity indicators changed differently with thermal maturity. The Tmax can define the reaction process of oil shale with pyrolysis temperatures at a temperature lower than 425 ◦C, but the small change in Tmax can be used only as an indicator to judge the maturity stage [29], and it is difficult to give feedback on the kerogen pyrolysis process. In the simulation experiment, the Ro and biomarker compounds at various temperature stages change regularly with the increase in the pyrolysis temperature, especially for the stage of rapid reduction in TOC; these thermal maturity parameters have obvious changes.

**Figure 10.** The changes in the six maturities and TOC contents of Nongan samples at different pyrolysis temperatures.

#### *3.4. Analysis of Pyrolysis Process of Nong'an Oil Shale In Situ Conversion Project*

The pyrolysis oil samples obtained on-site were geochemically tested, including group component separation and GC-MS analysis. The test results of the on-site cracking oil group components revealed that the sum of saturated hydrocarbons and aromatic hydrocarbons accounted for approximately 90%, non-hydrocarbon accounted for 10%, and asphaltene accounted for less than 1% (Table 4). The high-temperature and high-pressure simulation experiment of Nongan oil shale revealed that the proportion of non-hydrocarbon to asphaltene is more than 10% (Figure 6), but the sum of non-hydrocarbons and asphaltene in the group components of pyrolysis oil in the in situ conversion project is lower than 10%. When combined with field project analysis, it is necessary to pass through 66 m of strata from the thermal reaction strata at the bottom of the well to the wellhead of the mining well; this separation may have been due to the large molecular weight of asphaltene and non-hydrocarbon during the migration from the bottom of the well to the surface.


**Table 4.** Composition of pyrolysis oil in the Nongan in situ conversion project.

The two collected pyrolysis oils were subjected to group composition tests and GC-MS analysis together, with three parallel samples for each test and six samples in total. The results showed that the carbon number distribution range was between C11 and C33, and the main peak carbon was between C19 and C22. The CPI (the odd-even advantage index) was between 0.95 and 1.07, and the change was not significant. The average values of the two samples were 1.01 and 1.05, respectively, indicating that the organic matter in underground oil shale was mature (Figure 11).

**Figure 11.** GC-MS analysis of field oil shale pyrolysis oil and 400–425 ◦C simulated pyrolysis oil samples.

The homohopane isomerization index 22S/(22S + SSR) has high feasibility for evaluating the maturity from the immature to early oil generation stages [65,66]. According to the identification integral of the mass spectrum at *m*/*z* = 191, the ratio of the homohopane isomerization index C32 22S/(22S + SSR) of on-site pyrolysis oil samples remained between 0.41 and 0.49, and the average values were 0.45 and 0.47, respectively, reaching the hydrocarbon generation threshold [67,68]. The Ts/(Ts + Tm) ratios of the samples were maintained between 0.41 and 0.46, and the average values of the two samples were 0.43 and 0.44, respectively, which had reached the mature stage (Figures 11–13, Table 5). Additionally, this study found that sterane C29 20S/20 (R + S) and ββ/(αα + ββ) were 0.29–0.34 and 0.28–0.34, respectively, indicating that organic matter had entered the mature stage [62–66] (Table 5).

**Figure 13.** Analysis of C29 sterane 20S/(20S + 20R) and C32 hopane 22S/(22S + 22R) on maturity.

**Table 5.** Geochemical parameters of field pyrolysis oil.


By comparing the maturity parameters of biomarkers in the simulation results of Nongan oil shale, the experimental field data are similar to the simulation data at 400 and 425 ◦C. The projection plots in Figures 12 and 13 also showed that the field cracking oil projection is uniform and concentrated for the maturity indicator, including the immature to overmature sterane C29 20S/20(R + S) and ββ/(αα + ββ) projection plots. In the isomerization diagram, the input point of hopanoids is relatively dispersed because the indicating ability is only to the hydrocarbon generation threshold, but the lateral concentration is also in the range of 400–425 ◦C. The target heating formation is in the maturity stage [62–66] (Table 5, Figures 11–13).

## *3.5. Calculation and Application Feasibility of the In Situ Conversion Degree of Nongan Oil Shale*

Organic geochemistry of pyrolysis oil samples obtained in the in situ conversion project of the Nongan oil shale was carried out. In summary, the organic matter reaction process of subsurface oil shale in the in situ conversion project of Nongan oil shale is equivalent to the reaction of Nongan oil shale in the high-temperature and high-pressure simulation laboratory in the range of 400–425 ◦C. The TOC test of the Nongan subsurface oil shale reaction layer shows that the peak period of hydrocarbon generation is in the range of 400–425 ◦C, which also corresponds to the oil yield in the high-pressure thermal weightlessness experiment and high-pressure distillation experiment. Therefore, the pyrolysis oil is equivalent to the 400–425 ◦C range of the pyrolysis test samples and should be the best temperature for heating the oil shale formation to the oil production peak [35,36]. The previous paper on the area also points out that 425 ◦C is the key heating control point, and there is a turning point within 425–450 ◦C [39]. If heating continues, the input energy is wasted, and the economic benefits are reduced. Therefore, the research on the pyrolysis oil of the mining well in the in situ conversion project of Nongan oil shale reveals that the heating technology used in site construction corresponds to the original purpose of project experiments and has economic value.

In this paper, the analysis of biomarker compounds in pyrolysis oil is essential for the in situ mining of oil shale in other areas. Early resource evaluation should be undertaken, as well as tracking evaluation during the project and evaluation of surplus resource potential at the end of the project. It is necessary to conduct a detailed simulation study on the target horizon in early resource evaluation by comprehensive geochemical research. The geochemical characteristics of hydrocarbon gas and cracked oil samples discharged from mining wells were analyzed during project construction, and the reaction process of organic matter in subsurface oil shale was estimated. The total organic carbon reduction, as well as the oil and gas production rates, were calculated to aid in decision-making for the temperature increase and project process.

#### **4. Conclusions**

The results of high-pressure heating experiments show that when the pyrolysis temperature is 300–475 ◦C, the main emission products are pyrolysis oil, and the TOC of the semicoke sample decreases from 8.06% to 2.65%, and the yield is significantly improved. A transition point appeared between 425 ◦C and 450 ◦C, and the TOC of the oil shale above the temperature of the transition point decreased slowly. This temperature node is of great significance for the selection and control of the subsurface temperature during the original production of oil shale.

The research on the parameters of pyrolysis oil biomarker compounds found that the four parameters Ts/(Ts + Tm), C32 22S/(22S + 22R), C29 20S/(20S + 20R), and C29 ββ/(αα + ββ) also have a good feedback effect on the progress of Nongan oil shale pyrolysis reaction.

A comprehensive analysis of the current in situ conversion project of Nongan oil shale shows that the pyrolysis stage of the Nongan oil shale in situ conversion project is equivalent to the 400–425 ◦C simulated experiment, which is at the peak of the oil window. The heating process that is currently used corresponds to the experimental purpose and has economic value.

If the evolutionary trend of maturity parameters of pyrolysis oil biomarker compounds during in situ conversion of oil shale is studied, and accurate feedback of the maturity of kerogen in the formation is obtained, it can reveal its response principle to the organic matter pyrolysis process. The study is of great significance to the efficient, economic and stable development of oil shale in situ conversion projects.

**Author Contributions:** Conceptualization, methodology, validation, formal analysis, investigation, resources, H.Z., L.Y., J.S., X.M. and X.C.; data curation, writing—original draft preparation, writing review and editing, visualization, supervision, project administration, and funding acquisition, W.H. and W.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Sinopec "Key Technologies for In-situ Conversion and Exploitation of Oil Shale" (Grant No. P20066), and the National Key R&D Program of China (Grant No. 2019YFA0705502, Grant No. 2019YFA0705503), and the National Natural Science Fund Project of China (Grant No. 41790453, 4210020395 and 42002153), and the China Postdoctoral Science Foundation (Grant No. 2021M700053).

**Acknowledgments:** Thanks to my sweetie Chenyang Wu for her contribution and assistance with this study.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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