4.1.2. Terpanes

Tricyclic terpanes are abundant in the P2l shale of the study area, mainly in the range of C19–C28 (Figure 3), indicating that their formation was closely related to the input of an algae source [29]. The baseline of the GC–MS spectra is nearly horizontal, with no UCM "hump", suggesting that there was nearly no influence of biodegradation. The source rocks are rich in hopanes. The C30αβ-hopane/C29ααα,(R + S)-sterane ratios are 1.26–19.28, with an average of 6.12 (Figure 3, Table 1), suggesting that bacteria reproduced rapidly in a lake environment and provided a large amount of hopane precursors. The ratios of some individual samples were relatively low, which may be related to a certain amount of terrestrial organic matter input. C31–C35 homohopanes are closely related to prokaryotic microorganisms, and high abundance of C35 homohopane often indicates a strongly reducing depositional environment [25,30]. The C35 homohopane index (C35/(C31–C35) homohopane%) of the P2l shale is relatively high, with a range of 1.46–7.19 and an average of 3.17, indicating that the sedimentary water body had strong reducibility (Figure 3, Table 1). The C31R homohopane/C30 hopane ratios are 0.08–0.25, with an average of 0.17, also exhibiting the characteristics of lacustrine crude oil (Figure 3, Table 1). Gammacerane can effectively reflect the stratification of sedimentary water bodies, which in turn is related to the salinity of the water body [31]. The abundance of gammacerane in coal-measure source rocks formed in a swamp environment is very low, and their gammacerane index (Gammacerane/(Gammacerane + C30αβ-hopane)) is usually less than 0.05 (or even less than 0.01). The abundance of gammacerane in lacustrine sedimentary mudstone is relatively high, and the gammacerane index is greater than 0.05 in most cases [31,32]. The gammacerane index values of P2l shale source rocks in the study area were 0.12–0.61, with an average of 0.30 (Figure 3, Table 1), indicating that evaporation was very strong and the sedimentary water body had a fairly high salinity. In other words, in the Middle Permian, the Jimsar Depression was a saline lake basin.



**Table 2.** *Cont.*


**Figure 3.** Gas chromatography–mass spectroscopy (GC–MS) spectra of the biomarkers of the (**<sup>a</sup>**,**b**) terpanes (*m*/*z* 191) and (**<sup>c</sup>**,**d**) steranes (*m*/*z* 217) in the P2l source rocks in the Jimsar Depression (the compounds corresponding to all of the labels are shown in Table 2).

## 4.1.3. Sterane

The data sugges<sup>t</sup> that the Lucaogou source rocks were rich in C27 and C28 steranes, implying the existence of large quantities of algae organic matter [33]. The contents of the C27, C28, and C29 regular steranes in the P2l shale in the study area were 13.1–42.2% (average 22.9%), 17.4–57.3% (average 33.0%), and 28.1–57.3% (average 44.1%), respectively (Table 1, Figures 3 and 4). The total content of the C27 and C28 regular steranes in the source rock samples exceeded the content of C29 regular steranes. This indicates that algae organic matter was dominant; however, it also indicates that there may have been some contribution from terrestrial organic matter to source rocks in the study area. The relatively high C29 sterane content was not just related to the source of the organic matter; it was also affected by the depositional environment of the source rocks. Generally, lacustrine source rocks dominated by input of algae sources have higher relative C29 sterane content than marine carbonate/shale [34–36]. Overall, the P2l shale had relatively high C27 and C28 sterane content (Figure 4).

**Figure 4.** Ternary diagram of the relative percentages of the C27, C28, and C29 regular steranes in the P2l source rocks in the Jimsar Depression.

#### *4.2. Type and Abundance of Source Rocks*

#### 4.2.1. Types of Organic Matter

The elemental analysis revealed that the H/C atomic ratios of the P2l<sup>1</sup> member in the study area were 0.79–1.33 (average 1.06); the O/C atomic ratios were 0.04–0.08 (average 0.06); the H/C atomic ratios of the P2l<sup>2</sup> member were 0.82–1.60 (average 1.23); the O/C atomic ratios were 0.03–0.30 (average 0.08); and the two sets of source rocks did not exhibit significant differences, mainly exhibiting the characteristics of type II kerogen (Figure 5a). Hunt (1996) [37] pointed out that when the H/C atomic ratio is >0.8, organic matter starts to have the capability to generate oil. According to this standard, the P2l shale had a relatively high H/C atomic ratio and was a set of oil-prone source rocks. It should be noted that large differences in the H/C atomic ratios of the different samples indicated that this set of source rocks was strongly heterogeneous.

Based on the pyrolysis HI values of >600, 300–600, 200–300, 50–200, and <50 mg hydrocarbons/g TOC, the organic matter can be divided into five types: type I (extremely oilprone type), type II (oil-prone type), II/III type (oil/gas-prone type), type III (gas-prone type), and type IV (non-source rock) (Figure 5b) [38,39]. The HI values of the P2l<sup>1</sup> member in the study area were 12–781 mg hydrocarbons/g TOC (average of 367 mg hydrocarbons/g TOC), the HI values of the P2l<sup>2</sup> member were 72–808 mg hydrocarbons/g TOC (average of 390 mg hydrocarbons/g TOC), and there was no large difference between the two sets of source rocks (Figure 5b). The type I, type II, type II/III, type III, and type IV samples accounted for 14.5%, 47.1%, 21.0%, 15.2%, and 2.0% of the total samples, respectively, indicating that the P2l source rocks were dominated by type II kerogen, and that type I kerogen did not account for a large proportion. In addition, this set of source rocks was very strongly heterogeneous, and the types of parent materials involved were very different. This heterogeneity was very obvious even in samples from the same well. As an example, for Well Ji174, from which a large number of samples were collected, the HI values of the P2l<sup>1</sup> and P2l<sup>2</sup> members were 48–294 and 72–808 mg hydrocarbons/g TOC, respectively.

**Figure 5.** Plots of (**a**) O/C vs. H/C and (**b**) atomic ratio of HI vs. Tmax showing the organic matter types of the P2l source rocks in the Jimsar Depression (H/C, atomic ratio; O/C, atomic ratio; immature; mature; oil generation window; highly mature; condensate oil-wet gas; over-mature; dry gas generation).

The analysis of the organic microscopic compositions of whole-rock samples revealed that the contents of the vitrinite group and exinite group of the P2l source rocks were low (<20%), while the contents of the sapropelinite group + exinite group were very high (62.3–78.7%). In general, the sapropelinite group component mainly generated crude oil, the vitrinite group component mainly generated natural gas, the exinite group component generated both, and the inertinite group component had almost no potential for hydrocarbon generation. Therefore, the P2l source rocks in the study area should mainly be oil-generating. Figure 6 shows the results of the whole-rock organic microscopic composition analysis of a core sample (2321.4 m) of P2l<sup>1</sup> from Well Ji15. Under reflected fluorescence, the clay (Cl) substrate exhibited a strong fluorescence, and, among them, the parallel-distributed microsporinite (MiS), resinite (R), liptodetrinite (Ld), and vitrodetrinite (Cd) were also widely distributed (Figure 6). Under reflected plane-polarized light (oil immersion), the clay (Cl) minerals exhibited a granular structure in which Cd and semifusinite (SF) fragments were distributed and the latter's structure was broken. Pyrite (Py) was widely distributed, reflecting the strong reducibility of the sedimentary environment and the good organic matter preservation conditions (Figure 6).

**Figure 6.** Analysis of the organic microscopic composition of the whole-rock P2l<sup>1</sup> sample (2321.4 m) from Well Ji15 (left image: reflected fluorescence; right image: reflected plane-polarized light). Cl is clay; MiS is microsporinite; R is resinite (R); Ld is liptodetrinite; Cd is vitrodetrinite; SF is semifusinite; MB is mineral bitumen; Py is pyrite.

4.2.2. Abundance of Organic Matter

Huang et al. (1982) [40] summarized the abundance of organic matter in the source rocks of the main petroliferous basins in China, and proposed corresponding evaluation criteria for lacustrine source rocks deposited in freshwater and brackish water environments (Figure 7). The TOC values of the P2l 1 and P2l 2 source rocks were 0.38–7.55% (average 3.30%) and 0.34–12.45% (average 3.80%), respectively. From the perspective of organic carbon, the proportions of good and extremely good source rocks in the P2l 1 source rocks were 15.0% and 70.0%, respectively, while the proportions of good and extremely good source rocks in the P2l 2 source rocks were 14.3% and 75.5%, respectively. The source rock S1 + S2 values of the P2l 1 and P2l 2 members were 0.26–39.44 mg hydrocarbons/g rock (average of 15.21 mg hydrocarbons/g rock) and 0.47–78.96 mg hydrocarbons/g rock (average of 17.60 mg hydrocarbons/g rock), respectively (Figure 7). In terms of S1 + S2, the proportions of the good and extremely good source rocks in the P2l 1 member were 40% and 30%, respectively, while the proportions of the good and extremely good source rocks in the P2l 2 source rocks were 37.8% and 34.7%, respectively (Figure 7).

**Figure 7.** TOC vs. S1 + S2 diagram showing the abundance of organic matter in the P2l source rocks in the Jimsar Depression (extremely good source rock; good source rock; medium-grade source rock; poor-non-source rock).

From the perspective of the TOC and S1 + S2, the P2l source rocks were generally good to extremely good source rocks, but they also exhibited a strong heterogeneity (Figure 7). It should be noted that when the TOC was used as the evaluation standard, the quality of the source rocks was considerably better than the results obtained using the hydrocarbon generation potential as the standard. For example, when TOC was used as the evaluation criterion, the proportion of extremely good source rocks out of the total number of samples was twice as high as the results obtained based on S1 + S2. This is mainly due to the difference in the hydrogen content of the source rocks. In thermochemical reactions, carbon must be combined with hydrogen to generate hydrocarbons. If the hydrogen content is low, even if the organic carbon content is high, the result is often ineffective. The heterogeneity of the hydrogen content of the source rocks, which is reflected in the analysis of organic elements and the pyrolysis hydrogen index, also confirms this view (Figure 5). When

evaluating the P2l source rocks, the results obtained using the pyrolysis hydrocarbon generation potential as the standard may be more reliable.

#### *4.3. Thermal Evolution of the Source Rocks*

#### 4.3.1. Maturity of the Source Rocks

The vitrinite reflectance values of the P2l source rocks in the study area were 0.52–1.24%, with an average of 0.71%, meaning they were within the main oil generation window. It is generally believed that when S2 is less than 0.2 mg hydrocarbons/g rock, Tmax is unreliable [41]. Except for the two samples from Well Ji5, the S2 values of the samples from the study area were greater than 0.2 mg hydrocarbons/g rock. The Tmax values obtained based on these values were generally reliable, ranging from 428 ◦C to 454 ◦C, with an average of 445 ◦C. The samples were essentially in a mature stage, which is consistent with the situation reflected by the vitrinite reflectance.

The gas chromatographic analysis of the extracted source-rock bitumen revealed that the carbon preference index (CPI) values of the P2l source rocks in the Jimsar Depression were 1.18–2.0 (average 1.41), and the odd-to-even preference (OEP) was 1.14–1.71 (average 1.30), exhibiting a significant odd–even predominance, which indicates that the maturity of the source rocks was not high. The C2920S/(S + R)-sterane and C29ββ/(αββ + ααα)- sterane ratios of the source rock extracts were 0.09–0.46 and 0.11–0.45, respectively, making them lower than the equilibrium values (0.67–0.71 and 0.52–0.55), though they were still within the oil generative window (Figure 8, Table 1) [25].

**Figure 8.** Use of sterane isomer maturity parameters to determine the organic matter maturity of the P2l source rocks in the Jimsar Depression.

#### 4.3.2. Thermal Evolution of the Source Rocks

Based on a two-dimensional seismic profile crossing the Jimsar Depression in the east– west direction, we restored the hydrocarbon generation and expulsion history and burial history of the P2l source rocks (Figure 9; the location of the profile is shown in Figure 1c). At the end of the Triassic, the maturity of the P2l shale was low, EasyRo < 0.5%, and it had not ye<sup>t</sup> reached the oil generation threshold (Figure 9a). The maturity of the P2l shale increased significantly at the end of the Cretaceous, with an EasyRo > 0.7%, and it entered the main oil generation window, producing a large amount of normal crude oil. The highly mature stage (1.0–1.3% EasyRo) was reached in the deep part of the Depression, and certain amounts of highly mature crude oil and condensate gas were generated (Figure 9b). After the Cretaceous deposition, the formation was uplifted and denuded, and the hydrocarbon generation process in the P2l source rocks stalled (Figure 9c). In the Paleogene, P2l continued to settle (Figure 9d). Currently, the deep part of the depression has generally reached the high-maturity stage, producing a large amount of highly mature oil and condensate gas (1.0–1.3% Ro). The shallow part had relatively low maturity and could only generate a large amount of normal crude oil (0.7–1.0% EasyRo) (Figure 9e).

#### *4.4. Thermal Simulation Experiment on Source Rocks in a Closed System*

In this study, a P2l calcareous and siliceous black shale sample (core, 2321.4 m; sampling location is shown in Figure 1c) from Well Ji15 in the eastern margin of the Jimsar Depression was selected. Its kerogen was extracted, and a gold-tube thermal simulation experiment was carried out in a closed system. The total organic carbon content of this sample was 2.46%, the hydrocarbon generation potential (S1 + S2) was 13.36 mg hydrocarbons/g rock, and the HI was 491 mg/g TOC. Generally speaking, the abundance index was in the middle of the range of the P2l shales in the Jimsar Depression, and was representative. In addition, the maturity of the sample was relatively low, the measured vitrinite reflectance was 0.54%, and the sample was determined to be an ideal thermal simulation sample.

When the gas leaves the gold tube under the internal pressure of 1 × 10<sup>2</sup> Pa and enters the vacuum equipment for gaseous compound analysis, large quantities of C6 and C7 compounds evaporate and are lost. Therefore, when the total amount of liquid hydrocarbon compounds produced was determined, only the amount of C8+ compounds was measured (Figure 10). At 2 ◦C/h and 20 ◦C/h, the maximum amounts of oil generated by the P2l shale in the experiment were 209.2 mg/g (0.96EASY%Ro) and 172.3 mg/g (1.08EASY%Ro), respectively (Figure 10). There was no large difference in the maximum amount of oil generated by the sample at the heating rates of 2 ◦C/h and 20 ◦C/h. When EASY%Ro > 1.0, the maximum amount of oil generated at a heating rate of 2 ◦C/h was slightly higher than that generated at a heating rate of 20 ◦C/h (Figure 10). The experimental results show that the heating rate was not the key factor controlling the crude oil yield in this study. During the thermal simulation process, when the kerogen was cracked to produce crude oil for the first time, part of the crude oil was cracked to generate natural gas at the same time. Therefore, we could assume that the measured maximum amount of oil generated represented a conversion rate of 95% for the kerogen sample [42]. Based on this assumption, the maximum amount of oil generated by the sample in this study was 220.2 mg/g TOC (209.2 mg/g TOC/0.95).

**Figure 9.** Two-dimensional simulation profile of the thermal evolution and burial history of the P2l source rocks in the Jimsar Depression (Late Triassic (**a**); Late Cretaceous (**b**); after Cretaceous deposition, uplift and denudation of formations (**c**); Late Paleogene (**d**); present day(**e**)). The location of this section can be found in Figure 1c.

**Figure 10.** Simulation results of the oil production rate under different heating rates of the P2l source rocks in the Jimsar Depression.

The organic adsorption model proposed by Pepper (1992) and Sandvik et al. (1992) [43,44] can be used to estimate the amount of oil expelled from the source rocks. We defined R as the amount of residual oil in g/g TOC, and we defined M and W as the final amount of oil generated and the final amount of oil expelled per gram of initial organic carbon in the source rocks, respectively [42]. Assuming that the carbon content of the expelled crude oil was 80% after the oil expulsion was completed, the amount of residual organic carbon and residual oil per gram of original organic carbon in the source rocks were (1 − 0. 8 × W) and R × (1 − 0.8 × W), respectively; thus, the following equations could be established [42]:

$$\mathbf{M} = \mathbf{W} + \mathbf{R} \times (1 - 0.8 \times \mathbf{W}),\tag{1}$$

$$\mathbf{W} = (\mathbf{M} - \mathbf{R})/(1 - 0.8 \times \mathbf{R}),\tag{2}$$

where the units of M and W are g/g TOC. As previously mentioned, according to the pyrolysis experiment conducted in the closed system, the maximum amount of oil generated (M) by the sample was 220.2 mg/g TOC. The content (R) of bitumen extracted from the P2l source rocks in the Jimsar Depression was mainly in the range of 60–100 mg/g, the maximum amount of expelled crude oil (W) was 130.7–168.2 mg/g TOC, and the maximum hydrocarbon expulsion efficiency was 59.3–76.4% [42,43,45].

## **5. Conclusions**

Based on data from the hydrocarbon pyrolysis, the contents of organic carbon and soluble organic matter, the biomarkers, the organic microscopic composition, the vitrinite reflectance, the basin simulation and hydrocarbon generation experiments, the sedimentary environment, hydrocarbon generation capacity, quality, and maturity of the Lucaogou Formation were discussed. Four main conclusions could be drawn.


rocks. Most of them were rated as good and extremely good, but their heterogeneity was strong.


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

**Funding:** This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA14010301), National Natural Science Foundation of China (Grant No. 41702138).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We want to thank the Xinjiang Oilfield Company for the support in preparing this manuscript. We also thank the editors and anonymous reviewers for their helpful suggestions and comments.

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