*Article* **Lipid Biomarker and Stable Isotopic Profiles through Late Carboniferous–Early Triassic of the Deepest Well MS-1 in the Junggar Basin, Northwest China**

**Shuncun Zhang 1,3, Tao Wang 1,3, Hui Guo 1,3, Shengyin Zhang 1,3 and Bo Chen 2,\***


**Abstract:** The Carboniferous–Triassic period was an important stage of global sea–land transformation, with coal formation in the Carboniferous, biological extinction at the end of the Permian, and global drought in the Triassic. The MS-1 well in the Mosuowan High of the Junggar Basin is the deepest well drilled in Northwestern China. In this paper, we investigate the sedimentary environment and climate evolution of the Mosuowan area in the central Junggar Basin during the Late Carboniferous–Early Permian by the petrothermal, lipid biomarker, and isotopic composition of mud shale core samples, and explore the tectonic–climatic events and Central Asian orogenic belt evolution driving the sedimentary environment. The study shows that the organic matter from the Upper Carboniferous to the Lower Permian is at a mature stage, but biomarkers maintained the primary information although the organic matter was subjected to thermal evolution. In the late Carboniferous period (Tamugan Formation), the study area was a closed remnant sea with a relatively humid climate, triggering lush terrestrial vegetation and high organic carbon content in the sediments, which had the potential to evolve into natural gas. During the Xiazijie Formation of the Middle Permian, tectonic activity shifted to the subsidence period, and the salinity of the water decreased after a large input of fresh water. The lake basin area expanded, and the content of aquatic organisms continued to increase. As the Lower Permian stratigraphy is missing, this sea–land transition seems to jump. The low and upper Urho Formations of the Middle–Upper Permian are a deltaic foreland deposit, and geochemical indicators show an overall lake retreat process with a continuous increase in organic matter content of terrestrial origin. The lithologic assemblage of the Triassic Baikouquan Formation is braided river deltaic sedimentation with migration of deposition centers of the lake basin. In conclusion, the Late Carboniferous–Early Permian period was influenced by global changes, Paleo-Asian Ocean subduction, and continental splicing, which resulted in a continuous increase in terrestrial organic matter, water desalination, and oxidation-rich sediments in the Mosuowan region, but the P–T biological mass extinction event was not recorded.

**Keywords:** geochemistry; environment; tectonic evolution; Carboniferous–Triassic; Junggar Basin

## **1. Introduction**

Between the Carboniferous and Triassic periods, north China experienced global events such as coal-forming movements, paleo-oceanic closures and land emergence, and the Permian–Triassic biological mass extinction [1,2]. The Junggar Basin is located at the southwest edge of the Central Asian orogenic belt, where it is the key part of the intersection of the Siberian plate, the Kazakhstan plate, and the Tarim plate. The Carboniferous of Junggar was characterized by the subduction of the ancient Asian Ocean and regional volcanic activity. The Paleozoic–Mesozoic period experienced collisional basin

**Citation:** Zhang, S.; Wang, T.; Guo, H.; Zhang, S.; Chen, B. Lipid Biomarker and Stable Isotopic Profiles through Late Carboniferous–Early Triassic of the Deepest Well MS-1 in the Junggar Basin, Northwest China. *Minerals* **2022**, *12*, 1299. https://doi.org/ 10.3390/min12101299

Academic Editors: Marianna Kulkova and Dmitry Subetto

Received: 14 September 2022 Accepted: 14 October 2022 Published: 15 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

formation, faulting, subsidence, and regenerated foreland basin stages. Meanwhile, the Paleozoic–Mesozoic was also an important stage for hydrocarbon source rock and reservoir development in the Junggar Basin, and several Permian–Triassic hydrocarbon-bearing sags are located in the central part of the basin, such as Mahu Sag, Jimsar Sag, Shawan Sag, etc. [3–5]. At present, sand and conglomerate, dense sandstone, and oil shale of the Permian–Triassic in the northwest margin of the Junggar Basin have become an important, prospective oil and gas reservoir [6,7].

The sedimentation evolution of the Junggar Basin was not only influenced by global tectonic–climatic events but also deeply constrained by the accretionary and extension of the Central Asian orogenic belt. The evolution of the Junggar Basin from the Paleozoic to the Mesozoic was closely related to the subduction and subsidence of the Paleo-Asian Ocean in the northern Xinjiang region. During the Carboniferous period, the Junggar Basin developed several ophiolite belts and regional volcanic clastic rocks. The central part of the basin mainly experienced mechanical subsidence in the Early Permian, thermal subsidence with post-fracture in the Middle Permian, and tectonic inversion during the Late Permian–Early Triassic [8–10]. Paleozoic calc-alkaline volcanic rocks on the perimeter of the basin are the main source of sediments, and the braided, river-fan, deltaic, shallow lake phase sediments were developed from the mountain front region to the lake. The paleontological data show that the northern part of the Junggar Basin experienced four important paleoclimatic events from the Carboniferous to the Triassic period: the Late Carboniferous cooling event, the Middle Permian greenhouse event, the Late Permian– Early Triassic drought event, and the Middle to Late Triassic humid event [11–14].

A large number of geochemical studies have been carried out on volcanic and sedimentary rocks in the Junggar Basin, mainly focusing on tectonic environment discrimination, hydrocarbon source rock evaluation, and sedimentary environment reconstruction in the Carboniferous and Permian systems [15–17]. Geophysical data and sedimentary rock records from the drilled strata have also been used to map the depositional environment [18–20]. However, there is still a lack of regional, continuous, geochemical data for reconstructing the environmental changes from the Carboniferous to Triassic period. In this study, core samples from the deepest well, MS-1, in the Junggar Basin were analyzed for pyrolysis, lipid biomarkers, and carbon isotopic compositions. We studied the composition of the sedimentary organic matter, depositional environment, and climatic evolution of Mosuowan High in the central Junggar Basin during the Carboniferous–Triassic period. The global, tectonic–climatic events and the evolution of the Central Asian orogenic belt which drove the depositional environment of the Junggar Basin are further discussed.

#### **2. Geological Setting**

The Junggar Basin is located between the Altai and Tian Shan mountains in Xinjiang, China (92◦000–95◦300 E, 43◦200–45◦000 N) (Figure 1). It is a Late Carboniferous–Quaternary sedimentary basin with a total area of about 130,000 km<sup>2</sup> . The Junggar Basin is one of the largest, superimposed, hydrocarbon-bearing basins in northwest China, and its geotectonic position is located at the intersection of three major plates: the Siberian Plate, the Kazakhstan Plate, and the Tarim Plate [21,22]. The formation and evolution of the Junggar Basin were closely related to the subduction and subsidence of the ancient Asian Ocean in the northern Xinjiang region [23–25]. Numerous ophiolite belts are exposed in the West Junggar region, including many ophiolite belts such as Dalabut, Baijiantan, Barrek, Tangbal, and Mayil, which are key evidence of the extinction of the multi-island arc oceanic crust suture [22,26–28]. Based on the Permian tectonic features and late tectonic transformation in the basin, the Junggar Basin is divided into six primary tectonic units, including the Western Uplift, the Eastern Uplift, the Luliang Uplift, the Tianshan Foreland Depression, the Central Depression, and the Ulungu Depression (Figure 1) [29,30].

**Figure 1.** (**A**) Map of sampling location in Junggar Basin, China. (**B**) Seismic profile of well P6, MS-1, F2. (**C**) Detailed stratigraphic column of the sampled interval. **Figure 1.** (**A**) Map of sampling location in Junggar Basin, China. (**B**) Seismic profile of well P6, MS-1, F2. (**C**) Detailed stratigraphic column of the sampled interval.

The MS-1 well is located in the Mosuowan Uplift in the Junggar Basin Central Depression, which is a typical basement uplift. The two flanks in the middle of and to the east of the Mosuowan Uplift have developed northwest-trending, geodesic, spreading reverse faults (Figure 1), which have opposite tendencies and make the Mosuowan Uplift have the morphology of a back-breaking block [31]. The stratigraphy drilled in the Carboniferous–Triassic MS-1 well is the Baikouquan Formation (T1*b*), Permian Upper Urho Formation (P3*w*), Lower Urho Formation (P2*w*), Xiazijie Formation (P2*x*), and Carboniferous Tamugang (C2*t*) [32]. In the Pre-Carboniferous–Carboniferous period, the central and western part of the Junggar Basin showed an island arc uplift area with calc-alkaline volcanic eruptions. At the beginning of the Permian, it was an island arc uplift zone with a northwest–west trend. With the intensification of denudation and sea level change, the island arc uplift was finally covered during the deposition of the Lower Permian Fengcheng Formation (P1*f*). At the end of deposition of the P2*x*, fold uplift occurred under the influence of north–south extrusion, and the P2*x* was locally erased, and the magnitude of the Mosuowan Uplift increased. The P2*w*–P3*w* were buried again during the depositional period. The Triassic was the period of stable deposition of the Mosuowan High [33–35]. The MS-1 well is located in the Mosuowan Uplift in the Junggar Basin Central Depression, which is a typical basement uplift. The two flanks in the middle of and to the east of the Mosuowan Uplift have developed northwest-trending, geodesic, spreading reverse faults (Figure 1), which have opposite tendencies and make the Mosuowan Uplift have the morphology of a back-breaking block [31]. The stratigraphy drilled in the Carboniferous– Triassic MS-1 well is the Baikouquan Formation (T1*b*), Permian Upper Urho Formation (P3*w*), Lower Urho Formation (P2*w*), Xiazijie Formation (P2*x*), and Carboniferous Tamugang (C2*t*) [32]. In the Pre-Carboniferous–Carboniferous period, the central and western part of the Junggar Basin showed an island arc uplift area with calc-alkaline volcanic eruptions. At the beginning of the Permian, it was an island arc uplift zone with a northwest–west trend. With the intensification of denudation and sea level change, the island arc uplift was finally covered during the deposition of the Lower Permian Fengcheng Formation (P1*f*). At the end of deposition of the P2*x*, fold uplift occurred under the influence of north–south extrusion, and the P2*x* was locally erased, and the magnitude of the Mosuowan Uplift increased. The P2*w*–P3*w* were buried again during the depositional period. The Triassic was the period of stable deposition of the Mosuowan High [33–35].

**3. Sample and Experiment** 

*3.1. Sample Collection* 

#### **3. Sample and Experiment**

#### *3.1. Sample Collection*

In this study, 73 samples were collected from the MS-1 well, including 32 samples from the C2*t*, 9 samples from the P2*x*, 16 samples from the P2*w*, 7 samples from the P3*w*, and 9 samples from the T1*b*. All of the samples were prepared for the determination of total organic carbon and rock pyrolysis parameters. A total of 64 samples were prepared for gas chromatography mass spectrometry (GC-MS) analysis, and 59 samples were selected for chloroform asphalt "A" stable carbon isotope analysis (Tables S1 and S2).

#### *3.2. Experimental Analysis*

Extraction and analysis of saturated hydrocarbons: Powder samples were placed in a Soxhlet extractor and extracted with chloroform for 72 h. The extract was concentrated and purified by passing through a silica gel/alumina (1:1) chromatographic column, and the saturated hydrocarbon fractions were collected by washing with 20 mL of n-hexane four times. The saturated hydrocarbon fractions were run in full scan and selected ion monitoring (SIM) mode by a GC-MS Thermal Scientific DSQ II quadrupole mass spectrometer equipped with a 30 m × 0.25 mm × 0.25 µm HP-5 MS column with helium as the carrier gas. The oven temperature was initially set to increase from 80 ◦C to 320 ◦C at a rate of 10 ◦C/ min and held at 320 ◦C for 20 min. The oven was held at 35 ◦C for 1 min, then the temperature was increased from 35 ◦C to 120 ◦C at a rate of 10 ◦C/min, then from 120 ◦C to 300 ◦C at a rate of 3 ◦C/min, and, finally, held at 300 ◦C for 30 min. Detection of the biomarkers, hopanes and steranes, was carried out by monitoring the respective, typical ions: *m*/*z* 191 for hopanes, *m*/*z* 217 for 5α,14α,17α-steranes, and *m*/*z* 218 for 5α,14β,17β-steranes.

Total organic carbon (TOC) and rock pyrolysis: The samples (about 100 mg) were crushed, sieved through a 120 mesh sieve, pretreated with 1 mol/L HCL to remove carbonates from the samples, and then heated to 1200 ◦C in a pyrolysis oven to obtain the TOC. The sample pyrolysis data were collected using a Rock-Eval II instrument according to the theory of Peter et al. [36]. Where S<sup>1</sup> represents the number of liquid hydrocarbons per unit mass of raw oil rock detected at 300 ◦C, S<sup>2</sup> represents the yield of caseinate cracked per unit mass of raw oil rock detected at 300–600 ◦C by heating. If the sediment is still in the mature evolution stage in the buried state, the S<sup>2</sup> peak represents the existing potential of the rock to produce oil. S<sup>1</sup> + S<sup>2</sup> characterizes the sum of the number of hydrocarbons already produced and potentially able to be produced in the hydrocarbon source rock, which is called hydrocarbon generation potential, i.e., the total number of hydrocarbons that may be produced by the rock, and it gradually decreases with the depletion of the organic matter hydrocarbon generation potential and the hydrocarbon removal process. The parameter *T*max ( ◦C) is the Rock-Eval pyrolyzer temperature at which the hydrocarbon compounds are observed to reach their maximum degree of pyrolysis, and this parameter can also indicate the maturity of the hydrocarbon compounds.

Chloroform asphalt "A" isotopic analysis: The carbon isotopic composition of chloroform asphalt "A" was determined offline by MAT 253 gas stable isotope mass spectrometer. The results of the carbon isotope analysis are reported as δ sign relative to the Pee Dee Belemnite (PDB) standard, and the analytical precision of the method was better than ±0.2‰ with reproducibility better than 0.2‰.

#### **4. Results and Discussion**

#### *4.1. TOC and Rock Pyrolysis Parameters*

The TOC of the gray mudstone, tuff, and dark-gray tuff of C2*t* was almost less than 0.50% for all, while the TOC of gray-black tuff and coal was relatively high, with ranges of 0.61%–4.64% and 3.26%–5.53%, respectively. The hydrocarbon potential (S<sup>1</sup> + S2) of the samples in C2*t* was 0.25–10.18 mg/g, and the maximum pyrolysis temperature (*T*max) was 441–502 ◦C. The TOC of P2*x* ranged from 0.05% to 1.16%, with S<sup>1</sup> + S<sup>2</sup> and *T*max of 0.14–0.96 mg/g and 425–468 ◦C, respectively. The TOC of P2*w* and P3*w* mudstone ranged

from 0.44% to 1.21%, and S<sup>1</sup> + S<sup>2</sup> from 0.29 to 3.41 mg/g, while the *T*max values were 425–471 ◦C. The T1*b* samples were divided into two groups, the brown-gray, mud-bearing siltstone and the gray mudstone. The TOC of the brown-gray siltstone of the T1*b* was very low, 0.15%–0.17%, and the TOC of the gray mudstone section was 0.50%–0.77%, with S<sup>1</sup> + S<sup>2</sup> ranging from 1.26 to 2.83 mg/g and *T*max values of 430–441 ◦C (Table S1, Figure 2). In general, only samples from the Late Carboniferous had high organic matter content, hydrocarbon generation potential, and thermal evolution degree. If considered on a regional scale, they can be used as a set of effective hydrocarbon source rocks. T1*b* samples were divided into two groups, the brown-gray, mud-bearing siltstone and the gray mudstone. The TOC of the brown-gray siltstone of the T1*b* was very low, 0.15%– 0.17%, and the TOC of the gray mudstone section was 0.50%–0.77%, with S1 + S2 ranging from 1.26 to 2.83 mg/g and *T*max values of 430–441 °C (Table S1, Figure 2). In general, only samples from the Late Carboniferous had high organic matter content, hydrocarbon generation potential, and thermal evolution degree. If considered on a regional scale, they can be used as a set of effective hydrocarbon source rocks.

*Minerals* **2022**, *12*, x FOR PEER REVIEW 5 of 19

**Figure 2.** Profiles of rock pyrolysis and organic geochemical parameters in MS-1 well. **Figure 2.** Profiles of rock pyrolysis and organic geochemical parameters in MS-1 well.

#### *4.2. Molecular Geochemistry of Organic Matter 4.2. Molecular Geochemistry of Organic Matter*

#### 4.2.1. n-Alkanes, Pr/Ph, β-Carotene, and δ13C of Chloroform Asphalts 4.2.1. n-Alkanes, Pr/Ph, β-Carotene, and δ <sup>13</sup>C of Chloroform Asphalts

The contents and variations of n-alkanes and acyclic isoprene hydrocarbons in the core samples from the C2*t*, P2*x*, P2*w*, P3w, and T1*b* of the MS-1 well were analyzed by gas chromatography. Table 1 summarizes the parameters such as carbon dominance index (CPI1 and CPI2) [37], Pr, Ph, β-carotene, and δ13C of the chloroform asphalt. The n-alkanes of the MS-1 well have single-peak and double-peak distribution patterns, in which the main carbon numbers of the front carbon peak are C17 and C19, and the main carbon numbers of the back carbon peak are C23 and C27. The n-alkanes of the C2*t* are mainly bimodal, with the main carbon peak being C19 and C23. The n-alkanes of the P2*x*, P2*w*, P3*w*, and T1*b* are mainly single-peaked, with the main carbon peaks of C17 and C19 (Figure 2). The LMW/HMW of samples in C2*t* ranged from 0.4 to 2.36, with a mean value of 1.04. The LMW/HMW of the P2*x*, P2*w*, and P3*w* samples ranged from 1.29 to 1.81, 1.15 to 11.64, and 0.37 to 11.43, respectively, and the LMW/HMW of T1*b* ranged from 1.39 to 8.69 with a mean value of 1.04. The CPI1 and CPI2 of the C2*t* were 0.95–1.31 and 1.04–2.0, respectively, while the CPI1 and CPI2 of the P2*x* were 1.01–1.1 and 0.82–1.65, respectively, and those of the T1*b* were 1.05–1.06 and 1.14–1.19, respectively. The ratio of terrestrial organic hydrocarbons (C27, C29, C31, C33) from higher vascular plant waxes and conifer-derived lipid detritus to the total sum of organic matter is known as the terrestrial index (Alkterr) [38–40], and the Alkterr values of Upper Carboniferous, Permian, and Lower Triassic samples were 0.02–0.19, 0.01–0.21, and 0.01–0.06, respectively. The contents and variations of n-alkanes and acyclic isoprene hydrocarbons in the core samples from the C2*t*, P2*x*, P2*w*, P3w, and T1*b* of the MS-1 well were analyzed by gas chromatography. Table 1 summarizes the parameters such as carbon dominance index (CPI<sup>1</sup> and CPI2) [37], Pr, Ph, β-carotene, and δ <sup>13</sup>C of the chloroform asphalt. The n-alkanes of the MS-1 well have single-peak and double-peak distribution patterns, in which the main carbon numbers of the front carbon peak are C<sup>17</sup> and C19, and the main carbon numbers of the back carbon peak are C<sup>23</sup> and C27. The n-alkanes of the C2*t* are mainly bimodal, with the main carbon peak being C<sup>19</sup> and C23. The n-alkanes of the P2*x*, P2*w*, P3*w*, and T1*b* are mainly single-peaked, with the main carbon peaks of C<sup>17</sup> and C<sup>19</sup> (Figure 2). The LMW/HMW of samples in C2*t* ranged from 0.4 to 2.36, with a mean value of 1.04. The LMW/HMW of the P2*x*, P2*w*, and P3*w* samples ranged from 1.29 to 1.81, 1.15 to 11.64, and 0.37 to 11.43, respectively, and the LMW/HMW of T1*b* ranged from 1.39 to 8.69 with a mean value of 1.04. The CPI<sup>1</sup> and CPI<sup>2</sup> of the C2*t* were 0.95–1.31 and 1.04–2.0, respectively, while the CPI<sup>1</sup> and CPI<sup>2</sup> of the P2*x* were 1.01–1.1 and 0.82–1.65, respectively, and those of the T1*b* were 1.05–1.06 and 1.14–1.19, respectively. The ratio of terrestrial organic hydrocarbons (C27, C29, C31, C33) from higher vascular plant waxes and conifer-derived lipid detritus to the total sum of organic matter is known as the terrestrial index (Alkterr) [38–40], and the Alkterr values of Upper Carboniferous, Permian, and Lower Triassic samples were 0.02–0.19, 0.01–0.21, and 0.01–0.06, respectively.


**Table 1.** Parameters of δ 13C values of n-alkanes, Pr, Ph, β-carotene, and chloroform asphalts from MS-1 well of the Late Carboniferous–Early Triassic.

<sup>a</sup> Cmax: maximum peak carbon of n-alkanes; <sup>b</sup> CPI1: 1/2[ΣC15–<sup>21</sup> (odd carbon)/ΣC14–20 (even carbon) + ΣC15–21 (odd carbon)/ΣC16–22 (even carbon)]; <sup>c</sup> CPI2: 1/2[ΣC25–35 (odd carbon)/ΣC24–34 (even carbon) + ΣC25–35 (odd carbon)/ΣC26–36 (even carbon)]; <sup>d</sup> Alkterr: (C<sup>27</sup> + C<sup>29</sup> + C<sup>31</sup> + C33)/ΣC14–38; <sup>e</sup> LMW/HMW: ΣnC21-/ΣnC22+; f δ <sup>13</sup>C = δ <sup>13</sup>C of chloroform asphalts.

Organisms have different carbon isotope compositions, and the δ <sup>13</sup>C values range from −22‰ to −31‰ for C3 plants and from −10‰ to −16‰ for C4 plants [1,2]. HCO<sup>3</sup> − in lake water provides carbon for photosynthesis of aquatic plants, and, hence, they have high δ <sup>13</sup>C values. The δ <sup>13</sup>C of phytoplankton in warm seawater is close to <sup>−</sup>20‰, while that of phytoplankton in cold seawater is around −30‰ [1,3]. Seagrasses have δ <sup>13</sup>C values ranging from −10‰ to −16‰, and marine algae have δ <sup>13</sup>C values around <sup>−</sup>18‰ [4]. The carbon isotopic composition of non-marine aquatic plants and algae is generally in the range of −12‰ to −26‰ [5]. In general, high δ <sup>13</sup>C values correspond to a warm period and low values to a cold period [6,7]. The δ <sup>13</sup>C (PDB) of the chloroform asphalts of the samples from the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from −29.52 to −27‰, −29.76 to −28.32‰, −30.14 to −23.93‰, −32.17 to −29.15‰, and −30.81 to approximately −29.28‰, respectively (Table 1 and Table S2, Figures 2 and 3).

The Pr/Ph ratio is a useful indicator of the oxidation-reduction conditions of the sediment environment [41–44]. The Pr/Ph values of the samples from the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.29 to 0.67, 0.57 to 0.67, 0.49 to 0.83, 0.56 to 0.8, and 0.67 to 0.74 (Table 1 and Table S2, Figures 2–4). B-carotene is the most prominent compound in the carbon skeleton of carotenoids preserved in lakes and highly restricted marine environments, and its higher content indicates a higher reduction in waters [45,46]. B-carotene was detected in only a few samples from the C2*t*, with a highly variable content. The ratio of β-carotene/ΣC12-35 values ranged from 0.56 to 2.7 and had a mean value of 1.42. The two samples from the P2*x* contained β-carotene, with β-carotene/ΣC12-35 values ranging from 1.0 to 3.85 and a mean value of 2.42. The mean values of β-carotene/ΣC12-35 in the P2*x*, P2*w*, P3*w*, and T1*b* were 0.51, 0.64, and 0.42. (Table 1 and Table S2, Figures 2 and 3).

**Figure 3.** Chromatograms and mass fragmentogram of *m*/*z* = 191, 217, in the samples from MS-1 well of the Late Carboniferous–Early Triassic. **Figure 3.** Chromatograms and mass fragmentogram of *m*/*z* = 191, 217, in the samples from MS-1 well of the Late Carboniferous–Early Triassic. *Minerals* **2022**, *12*, x FOR PEER REVIEW 8 of 19

**Figure 4.** The correlation diagram of Ph/n-C18 and Pr/n-C17 in MS-1 well of the Late Carboniferous– Early Triassic. **Figure 4.** The correlation diagram of Ph/n-C<sup>18</sup> and Pr/n-C<sup>17</sup> in MS-1 well of the Late Carboniferous– Early Triassic.

#### Terpenes (including hopanes) are biomarker compounds, including tricyclic terpenes 4.2.2. Terpenoids and Steroids

4.2.2. Terpenoids and Steroids

2 and S2, Figures 3, 5, and 6).

**Strata C31αβ-22S/** 

T1*b*

and pentacyclic triterpenes. The tricyclic terpenes of these samples in terms of tricyclic terpene abundance were distributed C19 < C20 < C21 < C23 in an "ascending" pattern, and the tetracyclic C24 terpene abundance was higher. The C3117β,21α(H)-hopane αβ-22S/(S + R) values for the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.38 to 0.67, 0.58 to 0.64, 0.51 to 0.64, 0.53 to 0.6, and 0.57 to 0.65 (Table 2). The values of C29/C30-hopane in the Carboniferous, Permian, and Triassic samples were 0.34–0.72, 0.43–0.89, and 0.42–0.82, respectively. Gammacerane is C30 pentacyclic triterpenoid, and high levels of gammacerane represent strongly reducing hypersaline environments [47–49], which were detected in all samples from the MS-1 well (Figure 3). The gammacerane indices (gammacerane /C31-hopane) of the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.28 to 2.47, 0.51 to 0.73, 0.42 to 1.3, 0.21 to 0.67, and 0.21 to 0.83, respectively. The ratios of C27-17α(H)-tridecane (Tm) to C27-18α(H)-tridecane (Ts) effectively reflect the depositional environment and organic matter maturity [50–53]. Ts/Tm values in the Carboniferous, Permian, and Triassic samples ranged from 0.27 to 1.18, 0.28 to 0.91, and 0.22 to 0.79 (Tables 2 and S2, Figures 3 and 5). Steroids are another group of biomarker compounds, which are tetracyclic compounds with alkyl side chains, and the tetracyclic structures are collectively called steroid nuclei. The MS-1 well Terpenes (including hopanes) are biomarker compounds, including tricyclic terpenes and pentacyclic triterpenes. The tricyclic terpenes of these samples in terms of tricyclic terpene abundance were distributed C<sup>19</sup> < C<sup>20</sup> < C<sup>21</sup> < C<sup>23</sup> in an "ascending" pattern, and the tetracyclic C<sup>24</sup> terpene abundance was higher. The C3117β,21α(H)-hopane αβ-22S/(S + R) values for the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.38 to 0.67, 0.58 to 0.64, 0.51 to 0.64, 0.53 to 0.6, and 0.57 to 0.65 (Table 2). The values of C29/C30-hopane in the Carboniferous, Permian, and Triassic samples were 0.34–0.72, 0.43–0.89, and 0.42–0.82, respectively. Gammacerane is C<sup>30</sup> pentacyclic triterpenoid, and high levels of gammacerane represent strongly reducing hypersaline environments [47–49], which were detected in all samples from the MS-1 well (Figure 3). The gammacerane indices (gammacerane/C31 hopane) of the C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.28 to 2.47, 0.51 to 0.73, 0.42 to

**C27 C28 C29** 

**Table 2.** Parameters of hopane and sterane in Late Carboniferous–Early Triassic of MS-1 well.

**(S + R) Gama a C29/C30 Ts/Tm C29-S b C29-<sup>α</sup> <sup>c</sup> ααα20R** 

Min. 0.57 0.21 0.42 0.22 0.43 0.47 11.37 19.33 43.04 Max. 0.65 0.83 0.82 0.79 0.5 0.57 34.27 33.16 55.47 Mean 0.59 0.41 0.63 0.54 0.46 0.53 27.5 21.15 50.57 P3*w* Min. 0.53 0.21 0.54 0.28 0.42 0.47 16.74 24.04 43.11

samples were rich in steroids, including C27–C29 regular steroids and rearranged steroid series, mainly regular steroids (Figure 4). The C29αααα-20S/20(S + R) values in the samples of C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.19 to 0.49, 0.42 to 0.49, 0.42 to 0.52, 0.42 to 0.49, 1.3, 0.21 to 0.67, and 0.21 to 0.83, respectively. The ratios of C27-17α(H)-tridecane (Tm) to C27-18α(H)-tridecane (Ts) effectively reflect the depositional environment and organic matter maturity [50–53]. Ts/Tm values in the Carboniferous, Permian, and Triassic samples ranged from 0.27 to 1.18, 0.28 to 0.91, and 0.22 to 0.79 (Tables 2 and S2, Figures 3 and 5). Steroids are another group of biomarker compounds, which are tetracyclic compounds with alkyl side chains, and the tetracyclic structures are collectively called steroid nuclei. The MS-1 well samples were rich in steroids, including C27–C<sup>29</sup> regular steroids and rearranged steroid series, mainly regular steroids (Figure 4). The C29αααα-20S/20(S + R) values in the samples of C2*t*, P2*x*, P2*w*, P3*w*, and T1*b* ranged from 0.19 to 0.49, 0.42 to 0.49, 0.42 to 0.52, 0.42 to 0.49, and 0.43 to 0.5, respectively, while the C29-αββ/(αββ + αααα) values ranged from 0.23 to 0.51, 0.47 to 0.51, 0.44 to 0.53, 0.47 to 0.53, 0.47 to 0.53, and 0.47 to 0.57, respectively (Tables 2 and S2, Figures 3, 5 and 6).

**Table 2.** Parameters of hopane and sterane in Late Carboniferous–Early Triassic of MS-1 well.


<sup>a</sup> Gama. = gammacerane/C31-hopane. <sup>b</sup> C29-S = ααα-20S/20(S + R). <sup>c</sup> C29-α = C29cαββ/(αββ + ααα). Mean 0.585 0.635 0.57 0.725 0.43 0.45 28.92 23.63 46.57 a Gama. = gammacerane/C31-hopane. b C29-S = ααα-20S/20(S + R). c C29-α = C29c αββ/(αββ + ααα).

**Figure 5.** Profile for parameters from hopanoids and steranes of samples from the MS-1 well of the Late Carboniferous–Early Triassic. **Figure 5.** Profile for parameters from hopanoids and steranes of samples from the MS-1 well of the Late Carboniferous–Early Triassic.

**Figure 6.** Ternary plot showing the distribution of regular steranes (C27–C28–C29) of samples from the MS-1 well of the Late Carboniferous–Early Triassic. **Figure 6.** Ternary plot showing the distribution of regular steranes (C27–C28–C29) of samples from the MS-1 well of the Late Carboniferous–Early Triassic.

#### 4.2.3. Organic Matter Maturity

4.2.3. Organic Matter Maturity The maximum pyrolysis temperature (*T*max) in the Carboniferous samples from the MS-1 well was mainly 441–502 °C, indicating that the hydrocarbon source rocks are in the stage of high maturity–overmaturity evolution. *T*max values of 425–468 °C for hydrocarbon source rocks of the Xiazijie Formation from the Permian indicated that these rocks have also reached a high-maturity evolutionary stage. The samples from the Lower Permian Urho Formation had a Tmax range of 425–471 °C, which shows that the hydrocarbon source rocks have reached the mature–high-maturity evolutionary stage. The samples from the Urho Formation of the Upper Permian also showed a mature evolutionary stage with *T*max The maximum pyrolysis temperature (*T*max) in the Carboniferous samples from the MS-1 well was mainly 441–502 ◦C, indicating that the hydrocarbon source rocks are in the stage of high maturity–overmaturity evolution. *T*max values of 425–468 ◦C for hydrocarbon source rocks of the Xiazijie Formation from the Permian indicated that these rocks have also reached a high-maturity evolutionary stage. The samples from the Lower Permian Urho Formation had a Tmax range of 425–471 ◦C, which shows that the hydrocarbon source rocks have reached the mature–high-maturity evolutionary stage. The samples from the Urho Formation of the Upper Permian also showed a mature evolutionary stage with *T*max values of 428–443 ◦C. The *T*max values of the Baikouquan Formation of the Triassic were mainly 430–441 ◦C, indicating that these rocks have just entered the mature evolutionary stage (Table 1, Figure 2).

values of 428–443 °C. The *T*max values of the Baikouquan Formation of the Triassic were mainly 430–441 °C, indicating that these rocks have just entered the mature evolutionary stage (Table 1, Figure 2). Biomarker compound parameters are another tool for evaluating the maturity of organic matter in sedimental rocks [51,54,55]. During the heating of hopanes and steranes under geological conditions, the R conformation is converted to the S conformation, forming a mixed conformation of R + S. In general, a higher 22S/(S + R) ratio represents higher maturity [52]. In addition, C29αββ has higher stability than C29ααα, and the value of C29 Biomarker compound parameters are another tool for evaluating the maturity of organic matter in sedimental rocks [51,54,55]. During the heating of hopanes and steranes under geological conditions, the R conformation is converted to the S conformation, forming a mixed conformation of R + S. In general, a higher 22S/(S + R) ratio represents higher maturity [52]. In addition, C29αββ has higher stability than C29ααα, and the value of C<sup>29</sup> αββ/(αααα + αββ) increases abruptly in the first half of a raw oil zone. The C3117β, 21α(H)-hopane, and 31αβ-22S/(S + R) values, sterane C29ααα-20S/20(S + R) values, and C29-αββ/(αββ + αααα) values from the MS-1 well all indicated that the Late Carboniferous–Early Triassic organic matter of MS-1 has reached the mature stage in general.

αββ/(αααα + αββ) increases abruptly in the first half of a raw oil zone. The C3117β, 21α(H) hopane, and 31αβ-22S/(S + R) values, sterane C29ααα-20S/20(S + R) values, and C29 αββ/(αββ + αααα) values from the MS-1 well all indicated that the Late Carboniferous– Early Triassic organic matter of MS-1 has reached the mature stage in general. In general, the ratio of C27–17α(H)-triacontane (Tm) to C27-18α(H)-triacontane (Ts) can effectively reflect the organic matter maturity and depositional environment, and, in the In general, the ratio of C27–17α(H)-triacontane (Tm) to C27-18α(H)-triacontane (Ts) can effectively reflect the organic matter maturity and depositional environment, and, in the post-rock stage, C27-17α(H)-trisnorhopane (Tm) is less stable than C27-18α(H) trisnorhopane (Ts), and, as the maturity of crude oil increases, the Ts/Tm values also increase with increasing maturity of crude oil. and for immature samples. The Ts/Tm values of samples from the C2*t* similarly indicated that the organic matter has reached the

post-rock stage, C27-17α(H)-trisnorhopane (Tm) is less stable than C27-18α(H)-trisnorho-

from the C2*t* similarly indicated that the organic matter has reached the maturity stage. Ts/Tm can also reflect the redox conditions of the depositional environment, with low Ts/Tm values for oxidizing environments and high values for reducing environments [52]; however, the samples from the MS-1well did not exhibit significant environmental indi-

cator features.

maturity stage. Ts/Tm can also reflect the redox conditions of the depositional environment, with low Ts/Tm values for oxidizing environments and high values for reducing environments [52]; however, the samples from the MS-1well did not exhibit significant environmental indicator features.

The Rock-Eval indicated that the organic matter of the MS-1 well is in the mature stage, but did not show an increase in the parameters of organic matter maturity with increasing depth of the formation. Ts/Tm, C29αααα-20S/20(S + R), and C29-αββ/(αββ + αααα) even showed a decreasing trend with organic matter type and depositional environment, indicating that the biomarkers in the study area are not significantly affected by maturity, and, on the other hand, supporting that MS-1 organic geochemical parameters can be used to reconstruct the paleoclimate and paleoenvironment.

#### **5. Sedimentary Environment**

*5.1. Carboniferous*

#### The Telegula Formation (C2*t*)

The C2*t* stratigraphic logs can be divided into three lithologic zones (Figures 1 and 7): the depth of 6806.0–6952.0 m is mainly gray-brown, conglomeratic mudstone with gray, conglomeratic mud siltstone in unequal thickness interbedded and gray tuff with tuff siltstone interbedded; the depth of 6952.0–7265.0 m is dominated by gray and greenishgray tuff, interbedded with black coal, gray-black sunken tuff, tuffaceous mudstone, and andesite; and the depth of 7265.0–7500.0 m is dominated by gray, dark-gray, and grayblack tuffs, interbedded with gray-black tuffaceous siltstone. The lithologic assemblage indicates that the C2*t* was frequently volcanic, the climate was relatively warm and humid, terrestrial higher plants were relatively developed, and large amounts of terrestrial organic matter were usually deposited in nearshore or shallow water along with tuffaceous debris. C29αααα20R and isotopes of organic matter (−29.52 to −27‰) also indicated a high content of terrestrial organic matter mixed with mixed aquatic plants in the sediments of the MS-1 well during this period. Meanwhile, the abundance of short-chain carbon number n-alkanes, Pr/Ph, and gammacerane indices indicates high salinity and reduction in the water column [52]. Additionally, intermittent volcanic activities can carry the terrestrial organic matter to the water column, causing a dramatic increase in organic matter and a rise in the salinity and reducibility of the water column [56]. In terms of biomarker trends, the aquatic life content tends to increase while the salinity and oxidation of the water column increases. Considering that the geochemistry of the Carboniferous volcanic rocks of the MS-1 well shows an island arc setting, while the complete closure of the western ocean of the Junggar Basin was delayed until the Early Permian, we believe that the MS-1 well was in a relatively closed residual sea environment, and the relatively humid climate made terrestrial plants flourish. Two recessions and one flooding event can be identified by the chloroform asphalts of δ <sup>13</sup>C (PDB), showing two decreasing trends of δ <sup>13</sup>C and an abrupt rise at 7300 m depth.

#### *5.2. Permian*

5.2.1. The Xiazijie Formation (P2*x*)

The P2*x* is directly unconformable, overlying the Carboniferous (Figures 1 and 7); 6641.0–6655.0 m is a set of dark-gray mudstone. It is mainly gray-white, muddy siltstone from the depth of 6655.0 to 6700.0 m, interbedded with gray mudstone and siltstone; an area of gray-white, muddy siltstone from the depth of 6700.0 to 6800.0 m, interbedded with gray mudstone and siltstone with varying thickness; and gray, gravelly siltstone from a depth of 6800.0 to 6806.0 m. The lithology of the P2*x* period is typical of lacustrine sedimentation. Since the stratigraphy of the Fengcheng Formation is missing [57–59], we speculate that the super-lake basin of the Fengcheng Formation gradually shrunk with stratigraphic uplift, and the P2*x* turned into a period of subsidence. During P2*x*, several alluvial fans developed in the western part of the Junggar Basin, and the sediments were characterized by coarsegrained sandstone and conglomerate. The Mosuowan area served as the sedimentary

center, and the gray, fine sandstone and mudstone were mainly deposited there. Biomarkers and δ <sup>13</sup>C (PDB) of the chloroform asphalts (−29.76 to <sup>−</sup>28.32‰) indicated a sustained increase in aquatic life content, a decrease in water column salinity, and an increase in water column oxidation. *Minerals* **2022**, *12*, x FOR PEER REVIEW 12 of 19

**Figure 7.** Scanning core photos of samples from the MS-1 well: (**a**,**b**), tuff in C2*t*; (**c**), siltstone mudstone in P2*w*; (**d**), grayish-white siltstone and black mudstone in P3*w*, with scouring surface between them; (**e**,**f**), siltstone mudstone in T1*b.* **Figure 7.** Scanning core photos of samples from the MS-1 well: (**a**,**b**), tuff in C<sup>2</sup> *t*; (**c**), siltstone mudstone in P2*w*; (**d**), grayish-white siltstone and black mudstone in P3*w*, with scouring surface between them; (**e**,**f**), siltstone mudstone in T1*b*.

#### *5.2. Permian*  5.2.2. The Lower and Upper Urho Formation (P2*w*, P3*w*)

5.2.1. The Xiazijie Formation (P2*x*) The P2*x* is directly unconformable, overlying the Carboniferous (Figures 1 and 7); 6641.0–6655.0 m is a set of dark-gray mudstone. It is mainly gray-white, muddy siltstone from the depth of 6655.0 to 6700.0 m, interbedded with gray mudstone and siltstone; an area of gray-white, muddy siltstone from the depth of 6700.0 to 6800.0 m, interbedded with gray mudstone and siltstone with varying thickness; and gray, gravelly siltstone from a depth of 6800.0 to 6806.0 m. The lithology of the P2*x* period is typical of lacustrine sedimentation. Since the stratigraphy of the Fengcheng Formation is missing [57–59], we speculate that the super-lake basin of the Fengcheng Formation gradually shrunk with stratigraphic uplift, and the P2*x* turned into a period of subsidence. During P2*x*, several alluvial fans developed in the western part of the Junggar Basin, and the sediments were characterized by coarse-grained sandstone and conglomerate. The Mosuowan area served as the sedimentary center, and the gray, fine sandstone and mudstone were mainly deposited there. Biomarkers and δ13C (PDB) of the chloroform asphalts (−29.76 to −28.32‰) indicated a sustained increase in aquatic life content, a decrease in water column salinity, and an increase in water column oxidation. 5.2.2. The Lower and Upper Urho Formation (P2*w*, P3*w*) The lithology of the Middle Permian–Upper Permian Urho Formation (Lower and Upper Urho Formation) is characterized as follows (Figures 1 and 7): It consists of grayish-The lithology of the Middle Permian–Upper Permian Urho Formation (Lower and Upper Urho Formation) is characterized as follows (Figures 1 and 7): It consists of grayishwhite siltstone, muddy siltstone, and conglomeratic siltstone, interbedded with grayishbrown, brownish-gray, gray, and greenish-gray mudstone, platy mudstone, and siltstone; from the depth of 6455.0–6548.0 m, it consists of greenish-gray, gray, brownish-gray, and dark-gray mudstone and siltstone, interbedded with grayish-white and muddy siltstone and siltstone; from 6548.0 to 6641.0 m, it consists of gray siltstone, muddy siltstone, and conglomeratic siltstone, interbedded with gray and dark-gray mudstone and siltstone; an interval (depth of 6178.0–6256.0 m) consists of brownish-gray, grayish-brown, beige, and gray, muddy siltstone, siltstone, and conglomerate, interbedded with brownish-gray and grayish-brown mudstone in unequal thickness; and the depth of 6256.0–6374.0 m is gray siltstone, sandstone, and muddy siltstone, interbedded with brownish-gray, grayishbrown, brown mudstone, and silt mudstone. The lithological assemblage of the P2*w* and P3*w* is a deltaic foreland deposit. Combined with geochemical indicators, it shows an overall lake retreat process with a continuous increase in terrestrial organic matter content and small changes in water salinity and reducibility. The δ <sup>13</sup>C (PDB) of the chloroform asphalts at P2w ranged from −30.14 to −23.93‰, during which the maximum flooding surface occurred with δ <sup>13</sup>C reaching <sup>−</sup>24‰. In contrast, P3*<sup>w</sup>* had the largest lake retreat event during C2*t*–T1*b*, reflecting the climate transition from wet to dry. β-carotene/ΣC12-35 showed that the salinity of P3*w* was higher than that of P2*w*, which might have been caused by the repeated lake level fluctuations.

white siltstone, muddy siltstone, and conglomeratic siltstone, interbedded with grayishbrown, brownish-gray, gray, and greenish-gray mudstone, platy mudstone, and siltstone; from the depth of 6455.0–6548.0 m, it consists of greenish-gray, gray, brownish-gray, and dark-gray mudstone and siltstone, interbedded with grayish-white and muddy siltstone

conglomeratic siltstone, interbedded with gray and dark-gray mudstone and siltstone; an

## 5.2.3. The Baikouquan Formation (T1*b*)

The lithology of T1*b* is divided into two sections (Figures 1 and 7), the first section (depth of 5939.0–5991.0 m) is gray, brownish-gray, off-white, muddy siltstone, siltstone, sand conglomerate, and gravelly, fine sandstone interbedded with brownish-gray, gray mudstone, and coarse siltstone, while the second section (depth of 5991.0–6178.0 m) is brownish-gray, off-white, gray, muddy siltstone, siltstone, gravelly, muddy siltstone, conglomeratic siltstone, and sand conglomerate, interbedded with gray and brown-gray mudstone. The lithologic assemblage of the T1*b* is braided river deltaic sedimentation with migration of deposition centers of the lake basin and reduced water salinity. A transient flooding event was found in the middle of the Baikouquan Formation, which increased the organic matter content of terrestrial sources and increased water salinity. The δ <sup>13</sup>C (PDB) of the chloroform asphalts ranged from −30.81 to −29.28‰, with obvious higher plant carbon isotope characteristics. Pr/Ph had been increasing continuously since P3*w* and reached its highest point, indicating a significant change in the depositional environment invention, i.e., from shallow lakeside to braided river deposition.

#### **6. Global Tectonics and Environmental Change Drives**

Large-scale sea retreat events during the Carboniferous period exposed the seafloor at the land margin, expanded the land area globally, and created fertile swamps and wetlands as freshwater rivers flowed through the new continental mass [57,60,61]. At the same time, the warm and humid climate made Lycopodium grow densely, and the tall ferns provided raw materials for coal formation, and the European continent and Siberia in the north, as well as the land of north China, which are the major coal-gathering areas in the world [62]. The C2*t* coal formation and higher, plant-rich tuffs drilled in the Carboniferous MS-1 well also reflect the global climatic characteristics of this period, but the strong volcanic activity in the Late Carboniferous Junggar Basin may have affected the growth of Carboniferous fern plants and, thus, the regional development of the coal formation. The most important global event of the Permian was the Permian–Triassic extinction that occurred at the end of the Permian, an extinction event that can be divided into one to three stages [61,63–65]. The first mini-peak may have been due to a gradual change in the environment due to sea level changes, oceanic hypoxia, and the arid climate due to the formation of the Pangaea continent. The later peaks were rapid and dramatic, probably due to impact events, super volcanic eruptions, or abrupt sea level changes that caused a massive release of methane hydrates that lasted about 60,000 years [65–67]. However, this event has no obvious carbonate record in the Junggar Basin. Since the Permian sediments of the MS-1 well are dominated by terrestrial organic matter, the negative organic carbon isotope drift is not significant. The core samples show that the Urho Formation samples are darker in color than the Baikouquan Formation, indicating that its depositional environment may have dealt with a global anoxic background. Influenced by the global aridification triggered by the Pangaea, the drought in the Junggar Basin reached its peak in the Late Permian– Early Triassic. The drought in Junggar began in the Middle Permian, and the lakes on the southern edge of Junggar dried up in its late stage, with evaporation exceeding recharge, and the lake extent south of Karameri in the Early Triassic also gradually decreased until it was replaced by the red layer, further confirming the persistence and development of the arid climate, a fact also illustrated by the gradual increase in the oxidation of the sedimentary environment of the MS-1 well (Figure 8).

**Figure 8.** Correlation between depth and δC of chloroform bitumen "A" (**a**), Alkterr (**b**), γ-gammacerane index (**c**), Pr/Ph (**d**). **Figure 8.** Correlation between depth and δC of chloroform bitumen "A" (**a**), Alkterr (**b**), γ-gammacerane index (**c**), Pr/Ph (**d**).

global aridification triggered by the Pangaea, the drought in the Junggar Basin reached its peak in the Late Permian–Early Triassic. The drought in Junggar began in the Middle Permian, and the lakes on the southern edge of Junggar dried up in its late stage, with evaporation exceeding recharge, and the lake extent south of Karameri in the Early Triassic also gradually decreased until it was replaced by the red layer, further confirming the persistence and development of the arid climate, a fact also illustrated by the gradual increase in the oxidation of the sedimentary environment of the MS-1 well (Figure 8).

The Late Paleozoic orogenic belt in northern Xinjiang and adjacent areas developed a series of successive, southward accretionary belts along the broad South Siberian active margin, forming the Alaska–Aleutian arc system (Kokchetav–Morth Tianshan), the Japanese arc system (Altai, Chinese Middle Tianshan), and the Mariana arc system (Balkhash, West Junggar, East Junggar) [4,9] (Figure 9). Paleomagnetic reconstructions indicate that complex nappes occurred in the Early Paleozoic. In the Late Devonian, a complex Kokchetav–North Tianshan arc system with a straight extension of the main axis was formed under growth and fusion of the circum-microcontinental type. Approximately corresponding to the late Early Carboniferous, the Junggar paleo sheet formed a rift trough along the Bogda area, separating the Junggar-occupied sheet. The rift trough did not reach the level of the emergence of oceanic crust, so it is also called the Ora Valley. After the middle of the Early Carboniferous, the Siberian plate and the Junggar paleolithic sheet underwent continental collisional orogeny, and the ocean basin disappeared. The ocean The Late Paleozoic orogenic belt in northern Xinjiang and adjacent areas developed a series of successive, southward accretionary belts along the broad South Siberian active margin, forming the Alaska–Aleutian arc system (Kokchetav–Morth Tianshan), the Japanese arc system (Altai, Chinese Middle Tianshan), and the Mariana arc system (Balkhash, West Junggar, East Junggar) [4,9] (Figure 9). Paleomagnetic reconstructions indicate that complex nappes occurred in the Early Paleozoic. In the Late Devonian, a complex Kokchetav–North Tianshan arc system with a straight extension of the main axis was formed under growth and fusion of the circum-microcontinental type. Approximately corresponding to the late Early Carboniferous, the Junggar paleo sheet formed a rift trough along the Bogda area, separating the Junggar-occupied sheet. The rift trough did not reach the level of the emergence of oceanic crust, so it is also called the Ora Valley. After the middle of the Early Carboniferous, the Siberian plate and the Junggar paleolithic sheet underwent continental collisional orogeny, and the ocean basin disappeared. The ocean between the Junggar paleo sheet and Kazakhstan finally closed at the end of the Carboniferous or the beginning of the Permian, while the ocean south of the basin closed in the Late Carboniferous and entered the intra-land orogenic stage, and the relatively closed residual marine environment in the Late Carboniferous study area coincided with it. In the Permian period, geosyncline closed, and folded mountain series were formed successively, and the paleo-plates were gradually linked to form a joint paleo-continent (Pangaea). With the further expansion of the land area, gymnosperms began to decline on land, true and seed ferns flourished, and the first gymnosperms appeared. From the Permian to the Middle Triassic, the Junggar Basin was in the foreland basin tectonic development stage, and the sediment evolution of the Xiazijie, Urho, and Baikouquan Formations also reflected the gradual disappearance of marine fingerprints and the increasingly obvious characteristics of continental sedimentation; the organic matter of terrestrial origin continued to increase, and the salinity of water bodies became increasingly low. In conclusion, regional tectonic evolution constrained the variability of lake waters in the MS-1 well, while the Carboniferous global warming and wetting event, Permian anoxic event, and Late Permian–Early Triassic drought event influenced the organic matter composition and redox conditions of the lake waters. stage, and the sediment evolution of the Xiazijie, Urho, and Baikouquan Formations also reflected the gradual disappearance of marine fingerprints and the increasingly obvious characteristics of continental sedimentation; the organic matter of terrestrial origin continued to increase, and the salinity of water bodies became increasingly low. In conclusion, regional tectonic evolution constrained the variability of lake waters in the MS-1 well, while the Carboniferous global warming and wetting event, Permian anoxic event, and Late Permian–Early Triassic drought event influenced the organic matter composition and redox conditions of the lake waters.

between the Junggar paleo sheet and Kazakhstan finally closed at the end of the Carboniferous or the beginning of the Permian, while the ocean south of the basin closed in the Late Carboniferous and entered the intra-land orogenic stage, and the relatively closed residual marine environment in the Late Carboniferous study area coincided with it. In the Permian period, geosyncline closed, and folded mountain series were formed successively, and the paleo-plates were gradually linked to form a joint paleo-continent (Pangaea). With the further expansion of the land area, gymnosperms began to decline on land, true and seed ferns flourished, and the first gymnosperms appeared. From the Permian to the Middle Triassic, the Junggar Basin was in the foreland basin tectonic development

*Minerals* **2022**, *12*, x FOR PEER REVIEW 15 of 19

**Figure 9.** Paleogeographic reconstruction for the bending of the Kazakhstan oroclines. Modified after [68,69]. **Figure 9.** Paleogeographic reconstruction for the bending of the Kazakhstan oroclines. Modified after [68,69].

#### **7. Conclusions 7. Conclusions**

The organic carbon content in the Carboniferous–Lower Permian sedimentary rocks drilled from MS-1 in the Mosuowan area of the Junggar Basin is low, but the Carboniferous tuffs and coal-bearing rocks are high in organic matter and have potential hydrocarbon generation capabilities. The organic matter evolution is all at a mature stage, but biomarker parameters were not significantly subjected to thermal evolution. During the Late Carboniferous, the Mosuowan area was located in a relatively enclosed remnant marine environment with a relatively humid climate that allowed for vigorous terrestrial vegetation, abundant rivers flowing into the water, and reduced water salinity yet increased depth. The Xiazijie Formation of the Middle Permian was a period of basin subsidence during which the basin area expanded significantly, and organic geochemical information indicates a continuous increase in aquatic organisms, a decrease in lake water salinity, and an increase in water oxidation. The lithologic assemblage of the Urho Formation in Permian was developed on the deltaic foreland. The biomarkers and isotopes of organic matter show an overall lake retreat process with a continuous increase in terrestrial organic matter and little change in water salinity and reduction, during which significant flooding events showed a significant decrease in terrestrial organic matter, an increase in aquatic organisms, and an increase in water salinity and reduction. The The organic carbon content in the Carboniferous–Lower Permian sedimentary rocks drilled from MS-1 in the Mosuowan area of the Junggar Basin is low, but the Carboniferous tuffs and coal-bearing rocks are high in organic matter and have potential hydrocarbon generation capabilities. The organic matter evolution is all at a mature stage, but biomarker parameters were not significantly subjected to thermal evolution. During the Late Carboniferous, the Mosuowan area was located in a relatively enclosed remnant marine environment with a relatively humid climate that allowed for vigorous terrestrial vegetation, abundant rivers flowing into the water, and reduced water salinity yet increased depth. The Xiazijie Formation of the Middle Permian was a period of basin subsidence during which the basin area expanded significantly, and organic geochemical information indicates a continuous increase in aquatic organisms, a decrease in lake water salinity, and an increase in water oxidation. The lithologic assemblage of the Urho Formation in Permian was developed on the deltaic foreland. The biomarkers and isotopes of organic matter show an overall lake retreat process with a continuous increase in terrestrial organic matter and little change in water salinity and reduction, during which significant flooding events showed a significant decrease in terrestrial organic matter, an increase in aquatic organisms, and an increase in water salinity and reduction. The Baikouquan Formation of the Early Triassic was deposited as a braided river delta, with a migration of deposition centers of the lake basin and a decrease in water salinity, during which there was a brief flooding event, an increase in organic matter of terrestrial origin, and an increase in water salinity. Paleo-Asian ocean subduction and continental collision constrained the changes of waters in the MS-1 well during the Late Carboniferous–Early Permian, while the continuous increase in sediment, terrestrial organic matter, water desalination, and oxidation enrichment in the Mosuowan area, and the global warming and wetting events of the Carboniferous, Permian anoxic events, and Late Permian–Early Triassic drought events affected the lipid biomarkers and stable isotopes of the organic matter composition; however, the P–T biological mass extinction events were not clearly recorded.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/min12101299/s1, Table S1: TOC and rock pyrolysis parameters of the Late Carboniferous-Early Triassic rocks in MS-1 well, Table S2: Parameters of δ 13C values of n-alkanes, Pr, Ph, β-carotene and chloroform extracts from MS1 wells of the Late Carboniferous-Early, Table S3: Parameters of hopance and sterane in Late Carboniferous-Early Triassic of MS1 well.

**Author Contributions:** Conceptualization: S.Z. (Shuncun Zhang). Data curation: T.W. and H.G. Investigation: T.W., H.G. and S.Z. (Shengyin Zhang). Methodology: B.C. Writing—original draft: S.Z. (Shuncun Zhang). Writing—review and editing: S.Z. (Shengyin Zhang). All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the Guangxi Natural Science Foundation, China (2020GXNSFBA297128); the Special Talent Project of Guangxi Science and Technology Base, China (Guike AD20238041); the National Natural Science Foundation of China (grant no. 41872145).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are contained within the article.

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

#### **References**

