Bicyclic sesquiterpanes are widely distributed in terrigenous sedimentary organic matter and are serve as valuable biomarker in the field of organic geochemistry. However, their formation and evolution, especially in geological conditions, are still poorly understood. In the present study, a series of bicyclic sesquiterpanes, including C15–C16 rearranged and regular compounds with generally dominant isomers of 4,4,8,8,9-pentamethyl-trans-decalin, 8(β)H-drimane, and 8(β)H-homodrimane, have been detected in approximately 230-m-thick Upper Triassic source rocks from the Yinan 2 well in the Kuqa Depression of Tarim Basin, NW China. The ratios of the ∑C15/∑C16 drimanes, 8(β)H-drimane/8(β)H-homodrimane, and ∑C15 − 16 rearranged/∑C15 − 16 drimanes generally decrease from the bottom to the top of the source rocks. The decreasing of ∑C15 − 16 rearranged/∑C15 − 16 drimane follows the decrease of the illite content in the source rock, showing a strong correlation with R2 = 0.82. The formation and evolution of bicyclic sesquiterpanes are restrained by multiple factors, including thermal maturity, depositional environment, parent input type, and clay minerals. Specifically, terrigenous higher plant input and the presence of clay minerals (such as illite) may facilitate the formation and rearrangement of bicyclic sesquiterpanes, respectively. For example, 8(β)H-drimane may undergo rearrangement through 1,3-methyl migration to form 4,4,8,8,9-pentamethyl-trans-decalin via clay mineral catalysis. These findings will significantly advance geochemical evaluations of the crude oils depleted in conventional biomarkers.
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Introduction
Bicyclic sesquiterpanes are a class of biomarkers with various types of chemical structures on their carbon skeleton, such as drimane, cadinane and eudesmane types1,2,3. Prior studies have revealed their widespread presence in terrestrially sourced organic matter, such as modern sediment4, coal5, source rock6 and crude oil7(Table 1). As it shows that, C14–C16 bicyclic sesquiterpanes are frequently detected in crude oils15,16,17, and a series of C15–C24 sesquiterpanes are found in oil sand bitumen reported by Dimmler et al.1. Bicyclic sesquiterpanes have high thermal stability18,19,20 and are widely used in oil-source correlation8,21,22,23, identification of thermal maturity17, sedimentary environment and paleovegetation reconstruction8,11.
Bendoraitis is the earliest to discover bicyclic sesquiterpanes in crude oil17. The initial understanding of bicyclic sesquiterpanes is obtained from the laboratory synthesis of 8(β)H-drimane by Alexander et al.19. Some hypotheses have been proposed to explain the origin of bicyclic sesquiterpanes, including: (1) They derive from bacteria or terrigenous higher plants resin3,12,22; (2) They are the pyrolysis products of tricyclic terpanes24; (3) They are the biodegradation16 or thermal degradation products of tetracyclic terpanes (such as oleanane or pentacyclic triterpane hopane)3,16,19,25. It appears that the main pathway to form bicyclic sesquiterpanes is the degradation of resin or higher terpanes. Although it has been recommended that terrigenous higher plants are the main source of bicyclic sesquiterpanes, the formation mechanism of these compounds remains controversial. In addition, the thermal evolution of bicyclic sesquiterpanes is poorly studied at present17,26, except for a study of pyrolysis simulation experiments on immature organic‑rich mudstone by Yan et al.27.
In the present work, a series of bicyclic sesquiterpanes have been detected in the Upper Triassic source rocks of Yinan 2 well in the Kuqa Depression of Tarim Basin, NW China. The composition, distribution and evolution of these sesquiterpanes and other biomarkers such as steranes, terpanes and aromatics, are studied to evaluate the thermal maturity, parent material, and sedimentary environment of the source rocks. The genetic mechanism and geochemical significance of the bicyclic sesquiterpanes in the Upper Triassic source rocks of Yinan 2 well are particularly discussed.
Geological settings
The Tarim Basin is rich in oil and gas sources28, is located in the south of the Xinjiang Uygur Autonomous Region, covering an area of 5.6 × 106 km2. It is surrounded by the Tianshan Orogenic Belt in the north, the Kunlun Orogenic Belt in the south, and the Altun Orogenic Belt in the southeast29. The Tarim Basin can be divided into seven tectonic units including the Kuqa Depression, Tarim Uplift, Northern Depression, Central Uplift, Southeast Depression, Tanan Uplift and Southern Depression. Among these units, the Kuqa Depression, is located in the south of the Tianshan Mountains and adjacent to the Northern Tarim Uplift, covering an area of approximately 16,000 km2 (Fig. 1)30. The tectonic evolution of the Kuqa Depression can be divided into three stages: a peripheral foreland basin stage, an extensional rift basin stage and a rejuvenated foreland basin stage31,32.
The Kuqa Depression Unit Zoning and Yinan 2 well position.
The depositional cycle of the Upper Triassic strata is identified from the stratigraphic column of the Kuqa Depression, comprising the Kelamayi (T2 − 3k), Huangshanjie (T3h), and Taliqike (T3t) formations33. The structural style of the depositional cycle shifted from a deep and steep faulted boundary in the north to a shallow, gentle overlapping, structurally stable, and topographically flat depression in the south34.
There are mainly two sets of source rocks in the Upper Triassic in the Kuqa Depression, including Taliqike Formation coaly source rocks and Huangshanjie Formation lacustrine source rocks35. The Huangshanjie Formation, in the eastern part of the Kuqa Depression has a total thickness of approximately 200 m (Fig. 1). Its upper part is mainly composed of gray and dark gray mudstone, while its lower part is mainly composed of black shales (Fig. 1). The Taliqike Formation has a total thickness of approximately 100 m, with sand, mudstone and coal36,37.
The Huangshanjie Formation, enriched with sapropel organic matter, is mainly deposited in the environment with semi-deep lake-deep lacustrine facies. In contrast, the Taliqike Formation, which enriched with humic organic matter, was deposited in a swamp facies environment. Palynological analysis of the Huangshanjie Formation has revealed the presence of fossils of eupteridaceae, dictyopteraceae, lycopoidea, cycads, and ferns. Meanwhile, the Taliqike Formation is abundant in palynological fossils of pinaceae, pododendraceae and arieaceae from the conifers, as well as osmundae of ferns are abundant in the Taliqike Formation38,39.
Experimental
Samples and pre-treatment
Nine source rock samples were collected from the Upper Triassic source rocks (including six samples from Huangshanjie Formation samples and three samples from Taliqike Formation) at depth of 5,070–5,302 m in Yinan 2 well of the Kuqa Depression in the Tarim Basin, NW China. The samples were carefully treated with ultrasonic extraction of dichloromethane to remove any possible organic contamination on their surfaces. Subsequently, the samples were crushed into 100-mesh grains and subjected to extraction using a dichloromethane/methanol mixture (93:7, v/v) for 72 h. The extracts were concentrated via rotary evaporation and transferred into 4-ml cell bottles. For further fractionation, the extracts were processed using silica–alumina column chromatography. A mixture of silica and alumina in a 3:1 (v/v) ratio were successively added to the column. The saturated, aromatic, and nonhydrocarbon fractions were eluted with n-hexane, dichloromethane/n-hexane (3:1, v/v), and methanol, respectively.
Instrument pyrolysis
Gas chromatographic–mass spectrometric (GC–MS) analyses of the saturated and aromatic hydrocarbon fractions were carried out using an Agilent 5977B MSD coupled with an Agilent 8890GC, equipped with an HP-1MS capillary column (30 m × 0.32 mm i.d. × 0.25 μm phase thickness, the column size:19091 S-916). Samples were injected in splitless mode, with helium as the carrier gas in the constant flow mode (1.1 mL/min). The injector and ion source temperatures were set at 285 °C and 230 °C, respectively. The MS was operated in electron ionization mode at 70 eV, with acquisition modes of selected ion monitoring (SIM) and full scan (mass range: 50–550 amu). Individual compounds were identified based on retention time and characteristic ion peaks.
For saturated hydrocarbon analysis, the initial GC oven temperature was initially set at 60 °C for 2 min, followed by a ramp to 120 °C at 10 °C/min and maintained for 15 min. Subsequently, the temperature increased to 305 °C at 3 °C/min and maintained for 20 min. The injection volume was 1.0 µL. For aromatic hydrocarbon analysis, the initial GC oven temperature was initially set at 80 °C for 4 min, followed by a ramp to 295 °C at 2 °C/min and maintained at 295 °C for 30 min. The injection volume was 1.0 µL.
Rock pyrolysis analysis of the samples was performed using the Rock-EVAL 6.0 standard pyrolysis analyzer. The mineral composition of the samples was determined using an OLYMPUS Innova-X BTX X-ray diffractometer (XRD).
Results
Geochemical data
The Rock-Eval data from previous studies36,40,41 and this study indicate that the kerogen types of the Upper Triassic source rocks in the Yinan 2 well are predominantly types I–III for the Huangshanjie Formation and types I–II for the Taliqike Formation (Fig. 2a). These source rocks are featured with total organic carbon (TOC) values of 0.72–31.87%, hydrogen index (HI) values of 79–206 mg/g, and Tmax values of 455 °C–467 °C (Table 2), indicating relatively high thermal maturity. Based on the evaluation plot of source rocks42, both general and excellent source rocks were found in the two formations (Fig. 2b). In addition, the contents of the saturated and aromatic fractions in the source rocks are 0.11–0.51 mg/g and 0.24–0.71 mg/g, respectively, with the former being generally lower than the latter.
The mineral composition of these samples is dominated by quartz and clay minerals, including illite, chlorite, epidote, anorthite, calcite and montmorillonite. Among the clay minerals, illite exhibits a relatively high in terms of relative content (Fig. 2c). In the Huangshanjie Formation, the relative content of illite and quartz exceeds 10% and 35%, respectively. In the Taliqike Formation, quartz, illite, and chlorite are the primary minerals with quartz being the most abundant, followed by illite. Compared to the Taliqike Formation, carbonate minerals such as anorthite and calcite are more developed in the Huangshanjie Formation (Table 2).
(modified from Wu et al.43), and (c) Mineral composition. Note: In the Fig. 2a and b, hollow symbols represent data from Guo et al.36, Wang et al.40 and Liu et al.41, while solid symbols denote data obtained in this study.
Basic geochemical characteristics of source rocks from the Huangshanjie and Taliqike formations in Yinan 2 well (a) Kerogen type classification diagram, (b) Scatter plot of TOC versus (S1 + S2) values.
Saturated biomarkers
n-Alkanes
The saturated hydrocarbons extracted from Upper Triassic source rocks are predominantly composed of low-molecular-weight n-alkanes, with a carbon number distribution ranging from C10 to C35 and exhibit a unimodal distribution pattern (Fig. 3a). Specifically, in the Huangshanjie Formation, the saturated hydrocarbons are characterized by dominant peak carbons of C15 and C17, along with pristane/phytane (Pr/Ph) ratio ranging from 0.88 to 1.45. In contrast, the Taliqike Formation exhibits saturated hydrocarbons with dominant peak carbons of C16–C18 and the Pr/Ph ratio of 1.08–2.22. As the depth increases from the Huangshanjie to Taliqike formations, the dominant peak carbon of the n-alkanes undergoes a transition, initially shifting towards higher molecular weights and subsequently reverting to lower molecular weights.
Distribution characteristics of (a) saturated hydrocarbons, (b) terpanes, and (c) steranes in the Upper Triassic source rocks from the Yinan 2 well of the Kuqa Depression in Tarim Basin. The organic compounds corresponding to spectral peaks in the chromatograms are shown in Table 3.
Terpanes and steranes
Figure 3b and c show the chromatograms of terpanes and steranes in the extracts of the Upper Triassic source rocks, with a relatively low content of terpanes, which is consistent with the observations from the previou studies36,44,45. In the Huangshanjie Formation, the Ts/Tm ratios range from 0.4 to 1.0, with an average value of 0.63 (Table 4). The C27–C29 regular steranes inthis formation exhibit an inverse “L” or “V” type distribution pattern (Fig. 3c), and the ratios of C2920S/(20R + 20 S), C29αββ/(αββ + ααα), (C21 + C22)/C29 steranes, and C27 rearranged/regular steranes are 0.38–0.48, 0.45–0.54, 0.10–0.57, and 0.44–0.91, respectively.
In the Taliqike Formation, the Ts/Tm ratios range from 0.30 to 0.71, with an average value of 0.57. The C27–C29 regular steranes in this formation predominantly display an inverse “L” type distribution (Fig. 3c). The corresponding ratios of C2920S/(20R + 20 S), C29αββ/(αββ + ααα), (C21 + C22)/C29 steranes, and C27 rearranged/regular steranes in this formation are 0.39–0.56, 0.48–0.61, 0.01–0.20, and 0.80–2.17, respectively. In addition, the relative abundance of C29 steranes increases, while that of the C28 steranes decreases with decreasing depth from the Huangshanjie to Taliqike formations.
Sesquiterpanes and diterpanes
A series of bicyclic sesquiterpanes have been identified in the Upper Triassic source rocks, encompassing drimane-type sesquiterpanes such as 8(α)H-drimane and 8(β)H-homodrimane, as well as rearranged drimane sesquiterpanes including 4,4,8,8,9-pentamethyl-trans-decalin, 4,4,8,9,9-pentamethyl-trans-decalin, and 4,4,8,10-tetramethyl-9-ethyl-decalin. These compounds characterized by a drimane-skeleton compounds are displayed in the Appendix A. Among these bicyclic sesquiterpanes, C15 drimanes are generally dominant compounds, followed by C16 drimanes (Fig. 4). Regular drimanes are predominant in the upper segment of Upper Triassic, while rearranged drimanes are more abundant in the bottom and middle segments. As the depth decreased, the relative contents of 8(β)H-drimane and 8(β)H-homodrimane to the whole bicyclic sesquiterpanes generally increased. Concurrently, the ratio of ∑C15 − 16 rearranged/∑C15 − 16 drimanes gradually decreased with a range of 0.21–0.77 (Table 4). Additaionally, the ratios of 8(β)H-drimane /8(β)H-homodrimane range from 0.09 to 2.74 (Table 4).
In addition to bicyclic sesquiterpanes, tricyclic and tetracyclic diterpanes are also identified in the Upper Triassic source rocks, with the tricyclic diterpanes being dominant (Fig. 4). The tricyclic terpanes included pimarane, abietane, and beyerane, while the tetracyclic diterpanes contained kauronane, rimuane, and phyllocladane. The diterpanes in these source rocks have similar distribution characteristics. The tetracyclic diterpanes are characterized by relative high contents of 4β(H)-19-norisopimarane, 8α(H)-labdane and C21-bicyclan. From the Huangshanjie to Taliqike formations, the relative contents of norpimarane, 4α(H)-18-norisoparane, and rimuane exhibit a slightly increasing trend.
Distribution characteristics of bicyclic sesquiterpanes and diterpanes in the m/z 123 chromatogram in the Upper Triassic source rocks of Yinan 2 well in the Kuqa Depression, Tarim Basin. The organic compounds corresponding to the spectral peaks in the chromatograms are shown in Table 3.
Aromatic hydrocarbons
Polycyclic aromatic hydrocarbons
A series of polycyclic aromatic hydrocarbons (PAHs), including bicyclic (e.g., alkylnaphthalenes), tricyclic (e.g., retene), and tetracylic (e.g., fluoranthene, pyrene, and chryene) aromatic hydrocarbons have been identified. Among these PAHs, tricyclic aromatic hydrocarbons are the dominant component, while bicyclic and tetracyclic hydrocarbons are the secondary components. The phenanthrene (P) exhibits the highest content among the tricyclic aromatic hydrocarbons (Fig. 5). In the Huangshanjie Formation, the methyl phenanthrene (MP) ratios of F1 [F1 = (3 − MP + 2 − MP)/ (3 − MP + 2 − MP + 9 − MP + 1 − MP)] and F2 [F2 = 2 − MP/ (3 − MP + 2 − MP + 9 − MP + 1 − MP)], proposed by Kvalheim et al.46, range from 0.61 to 0.66 and 0.35 to 0.38, respectively. The F1 and F2 values in the Taliqike Formation range from 0.60 to 0.65 and 0.34 to 0.36, respectively.
Distribution characteristics of polycyclic aromatic hydrocarbons under total ion chromatogram (TIC) in the Upper Triassic source rocks of Yinan 2 well, Kuqa Depression, Tarim Basin. Table 3 shows the information of organic compounds corresponding to spectral peaks.
Fluorenes, dibenzofurans, and dibenzothiophenes
Fluorenes (FLs), dibenzofurans (DBFs), and dibenzothiophenes (DBTs) are important heterocyclic aromatic compounds in sedimentary organic matter. Figure 5 illustrates the distribution characteristics of these compounds in the Upper Triassic source rocks. In the Huangshanjie Formation, the relative contents of DBFs, FLs, and DBTs range from 49.04 to 76.07%, 10.83–29.68%, and 13.10–36.43%, respectively, with DBFs being the dominant component (Table 4). In the Taliqike Formation, the relative contents of DBFs, FLs, and DBTs range from 44.39 to 54.89%, 21.84–39.53%, and 16.08–23.46%, respectively. The relative contents of DBFs gradually decrease from the Huangshanjie to Taliqike formations.
Discussion
Thermal maturity
Thermal maturity of source rock can be commonly assessed by using specific biomarker parameters such as C29 sterane ααα 20 S/ (20 S + 20R), C29 sterane αββ/ (αββ + ααα) and Ts/Tm47. In the Upper Triassic source rocks, the ratios of C29 sterane 20 S/ (20R + 20 S) and C29 steraneαββ/ (αββ + ααα) range from 0.38 to 0.56 and from 0.45 to 0.61. These values are close to the “equilibrium stage” of their respective indexes48, indicating high thermal maturity. The baselines both on the m/z 191 and 217 chromatograms change from uplift to relative flat with sample deep decrease (Fig. 3b and c), which is consistent with the minor change in thermal maturity.
The F1 parameter is commonly used to estimate the equivalent vitrinite reflectance (eq Ro) of organic matter (Ro = 2.598F1 − 0.2749)46. The calculated eq Rc (F1) values in the Huangshanjie and Taliqike formations range from 1.31 to 1.45% and 1.30–1.40%, respectively (Table 3). The eq Rc (F1) values cover the Ro range of approximately 1.40–1.43% reported in previous studies49,50,51. Therefore, the eq Rc (F1) values, i.e., 1.30–1.45%, are likely to represent the primary thermal maturity range of the Upper Triassic source rocks in the Yinan 2 well.
The high thermal maturity, as indicated by the C29 sterane parameters and equivalent vitrinite reflectance (eq Rc) values (F1) discussed above, aligns well with the elevated Tmax values observed in the Upper Triassic source rocks (Fig. 2a; Table 2). This maturity is further supported by the characteristics of the saturated hydrocarbons, which include the absence of odd-even predominance in n-alkane distributions. Additionally, the baselines of both the m/z 191 and m/z 217 mass chromatograms transition from an uplifted to a relatively flat profile as the sample depth decreases, further corroborating the high thermal maturity of these rocks. However, the Ts/Tm ratio, another thermal maturity parameter48, exhibits relatively low values (0.30–1.0) in the samples and does not match the high thermal maturity. This discrepancy suggests that Ts and Tm may be influenced by the catalysis of clay minerals6,48.
Organic parent material sources
Pr and Ph are mainly derived from the chlorophyll of photosynthetic organisms47 and the ratios of Pr/nC17 and Ph/nC18 could be used to indicate organic matter sources52. As shown in Fig. 6a, the samples from the Huangshanjie and Taliqike formations predominantly fall within the mixed organic matter (MOM) region, with only one exception. This suggests that both of the Huangshanjie and Taliqike formations are derived from the contribution of terrigenous higher plants and lower aquatic organisms, reflection deposition in a weak redox environment. According to Huang and Meinschein, the primary sources of C27, C28, and C29 regular steranes are phytoplankton and metazoan, lower organisms (such as algae), and terrigenous higher plants. Thus, the triangular chart of C27–C28–C29 regular steranes is widely used to assess the input of organic matter parent sources52. As shown in Fig. 6b, the samples are distributed across mixed source, terrigenous plant dominated source, and terrigenous plant source regions. Most samples from the Huangshanjie Formation are located in the mixed source region, which is consistent with the result of the Pr/nC17 and Ph/nC18 ratios. This suggests that the Huangshanjie Formation source rocks are derived from a combination input of lower organisms and terrigenous higher plants, while the Taliqike Formation source rocks are mainly sourced from terrigenous higher plants.
Alkyl naphthalene series compounds such as 1,2,5-Trimethylnaphthalene (1,2,5-TMN) and 1,2,7-TMN are common components of aromatic fractions53. These compounds are derived from precursors such as β-amyrine from angiosperms and hopanes, with β-amyrine originating from angiosperms and/or gymnosperm resins54,55. Retene is widely recognized as originating from diterpenoids synthesized by conifers56,57,58. In Taliqike Formation samples from the Yinan 2 well, higher relative contents of 1,2,5-TMN, 1,2,7-TMN, and retene have been detected, indicating that coniferous plants may contribute to the Upper Triassic source rocks of the Kuqa Depression.
Pimaranes and abietanes are mainly derived from the resin of conifers and ferns7,59. Tetracyclic diterpanes (e.g., phyllocladanes) and tricyclic diterpanes (e.g., abietanes) are believed to be the products of hydrogenation reduction reactions of diterpanes7,19 derived from pteridophytes and conifers59. Isopimarane precursors are mainly found in gymnospermophyta conifers60,61, while platycladane is associated with pteridophyta, lycopodium, or gymnospermophyta62. For example, higher plants such as Agathis Salisb and Lagarostrobos Quinn can produce isopimaranes during the burial process63,64. The identification of these tetracyclic diterpanes in the Huangshanjie and Taliqike formations (Fig. 4) indicates a substantial input of gymnosperms, including ferns, conifers, and cycads.
Scatter plots of biomarker parameters indicating organic matter source for the Upper Triassic source rocks in Yinan 2 well, Kuqa Depression. (a) Relationship between Pr/nC17 and Ph/nC18 ratios. (the plate modified from Shanmugam65). (b) Triangular chart of C27–C28–C29 regular sterane (the triangular plate modified from Huang and Meinschein52).
Sedimentary environment
It is generally believed that the Pr/Ph ratio < 1.0 indicates a strongly reducing environment, while the Pr/Ph ratio > 3.0 suggests an oxidizing environment66. In the Upper Triassic source rocks, the Pr/Ph ratio ranges from 0.88 to 2.22 (with an average of 1.33) (Table 4). As shown in Fig. 6a, the samples from the Huangshanjie and Taliqike formations are distributed within the region representing weak oxidation–weak reduction environments.
The FLs, DBTs, and DBFs are believed to originate from the same precursor featured with a five-membered ring with a chemically active α-C atom linked to the No.9 carbon. This chemical structure is chemically high active and easily substitutable, allowing it to undergo various transformations: oxidation to form DBFs, hydrogenation to form FLs, and vulcanization to form DBTs. These transformations occur under both oxidation and weak reduction conditions67, as well as strong reduction conditions (such as those found in saline lake or marine environment)68. Therefore, FLs, DBTs, and DBFs are widely used to indicate the redox environment of sedimentary organic matters69,70. Based on the plot of DBTs/(DBTs + FLs)–DBFs/(DBFs + FLs) in Fig. 7, all samples are located in the region of the weak oxidation environment. During the deposition transition from Huangshanjie to Taliqike formations, the relative content of DBFs gradually decreased with the ratio of DBFs/ (DBFs + DBTs + FLs) reaching 0.4 (Table 4; Fig. 8). This suggests that the Upper Triassic source rocks in the Kuqa Depression of Tarim Basin were deposited in a redox environment, with oxidizability gradually weakening from the bottom to the top.
Plot of DBTs/(DBTs + FLs)–DBFs/(DBFs + FLs) of source rocks, identifying the sedimentary environment (the plate modified from Li and He71).
Evolution of biomarker parameters in the Upper Triassic source rocks from Yinan 2 well of Kuqa Depression, Tarim Basin.
Origin, evolution and constraint conditions of bicyclic sesquiterpanes
The origin of bicyclic sesquiterpanes remains a subject of ongoing research72, with current evidence pointing to two main sources: resin of terrigenous higher plants3,12 and the thermal degradation products of tricyclic, tetracyclic, and pentacyclic terpanes3,24,25. The formation and distribution of bicyclic sesquiterpanes are influenced by a complex interplay of geological factors, including thermal maturity7,60,72, mineral catalysis14,73, depositional environment7,73, and parent material3,20,22,74. Table 1 provides a comprehensive summarizes of previous studies investigating sesquiterpanes occurrences across different basins. These studies have elucidated the significant effects of sedimentary environments, organic matter sources and thermal maturity on the formation and distribution of sesquiterpanes. Building upon this foundation, the current study aims to further explore the genetic mechanisms of sesquiterpanes through an integrated analysis of existing literature and new findings.
The thermal maturity of the sedimentary organic matter containing bicyclic sesquiterpanes detection spans low to high maturity, as evidenced by prior studies6,9,13,74. Bicyclic sesquiterpanes were even detected in the coal with the Ro = 1.76%12 which is beyond to the Rc range (1.30% ~ 1.45%) of the Upper Triassic source rocks in the Kuqa Depression in this study. This suggests that these bicyclic sesquiterpanes in the Upper Triassic source rocks are still undergoing their thermal evolution process.
Previous studies have demonstrated that the SI index (SI = [∑C15 + ∑C16 – 8β(H)-homodrimane]/8β(H)-homodrimane)26 and 8(β)H-drimane/8(β)H-homodrimane13,70 have a good synergistic relationship with maturity and have been proposed as maturity indexes. However, a good correlation between F1 and F2 (Fig. 9a), as well as between ∑C15/∑C16 drimanes and 8(β)H-drimane /8(β)H-homodrimane (Fig. 9b), was observed. There is a poor correlation between the SI and F1(Fig. 9c), as well as 8(β)H-drimane/8(β)H-homodrimane and F1 (Fig. 9d). These observations suggest that the SI index and 8(β)H-drimane/8(β)H-homodrimane ratio are influenced by other factor than thermal maturity, thereby diminishing their reliability as indicators of thermal maturity. Consequently, thermal maturity, may not be the main controlling factor for the evolution of bicyclic sesquiterpanes in the Upper Triassic source rocks of Yinan 2 well.
Bicyclic sesquiterpanes are usually determined in detected weak oxic13,21, weak oxic–weak redox76,77, and reducing environments6,78, with an increasingly reducing sedimentary environment favoring the formation C16 drimane14,45,77. From the Huangshanjie to Taliqike formations, the redox conditionsredox conditions of the deposited environment initially become less reducing and then reducibility increases, as indicated by the ratios of the DBFs/ (DBTs + DBFs + FLs) (Fig. 8). However, both the ratios of ∑C15/∑C16 drimanes and 8(β)H-drimane/8(β)H-homodrimane decrease with increasing DBFs/ (DBTs + DBFs + FLs), as shown in Fig. 9e. This suggests that the redox conditions of depositional environment may not be the dominant factor influencing the distribution of C15 and C16 bicyclic sesquiterpanes.
(a) Cross-plots of F2 versus F1, (b) 8(β)H-drimane/8(β)H-homodrimane versus ∑C15/∑C16 drimanes, (c) SI versus F1, (d) 8(β)H-drimane/8(β)H-homodrimane versus F1, and (e) 8(β)H-drimane/8(β)H-homodrimane versus DBFs/(DBTs + DBFs + FLs), (f) ∑C15-16 rearranged / ∑C15-16 drimanes versus illite.
Numerous studies have reported that the presence of bicyclic sesquiterpanes in the terrigenous organic matter, suggesting these compounds originated from higher plants4,20,72,78. Gymnosperms and angiosperms, including conifers (e.g., Podocarpaceae and Araucariaceae), and ferns were important sources of bicyclic sesquiterpanes79. Regular steranes C27 and C28 are mainly originated from lower aquatic organisms, while C29 is mainly derived from terrestrial higher plants, with a minor exception46. From the Huangshanjie to the Taliqike formations, the ratio of C29/C27 − 29 regular steranes increases (Figs. 3c and 8), indicating a greater input of terrigenous higher plants. The increase of the ∑C15/∑C16 drimanes and 8(β)H-drimane/8(β)H-homodrimane ratios correlates with the C29/C27 − 29 regular steranes ratio (Fig. 8), suggesting that these parameters may also be influenced by the material source.
A previous study proposed that the cracking of tricyclic terpanes is the primary source of sesquiterpanes17, which aligns with our findings of the coupling occurrence of bicyclic sesquiterpanes and tricyclic diterpanes (Fig. 5). Drimanes undergo isomerization to form rearranged drimanes under the catalysis of clay minerals7,25, which is likely the main pathway of sesquiterpane isomerization6,71. Cesar and Grice27 suggested that high clay content may promote the formation of rearranged drimane through acid–clay-catalyzed rearrangement. The relatively high ratios of C27 rearranged/regular steranes, and ∑C15 − 16 rearranged/∑C15 − 16 drimanes, along with the inconsistency between Ts/Tm ratio and thermal maturity, indicate that rearrangement processes of biomarkers were prevalent in the Upper Triassic source rocks. Figure 9f shows a good correlation (R2 = 0.82) between the illite content and the ratio of ∑C15 − 16 rearranged/∑C15 − 16 drimane. With the illite content decrease from the Huangshanjie to the Taliqike formations, the ratio of ∑C15 − 16 rearranged/∑C15 − 16 drimanes also gradually decreases (Fig. 8). These observations suggest that C15 − 16 rearranged sesquiterpanes may be mainly originated from the rearrangement of regular sesquiterpanes catalyzed by illite.
Genetic mechanism of bicyclic sesquiterpanes in Upper Triassic source rocks of Yinan 2 well, Kuqa Depression(From Wang et al.22, with modification).
Based on the above discussion, the formation and evolution of bicyclic sesquiterpanes are influenced by many factors, such as redox environment, thermal maturity, organic matter source, and clay mineral catalysis. Tricyclic terpanes are likely one of the precursors of bicyclic sesquiterpanes. The genetic mechanism of these sesquiterpanes in the Upper Triassic source rocks of Yinan 2 well, Kuqa Depression, Tarim Basin is illustrated in Fig. 10. During the burial process, tricyclic diterpane undergo C–C bond cleavage and C ring opening under the weak redox condition, which forming sesquiterpane precursors. These precursors then undergo branch chain cleavage under the weak redox condition, generating a series of drimane compounds. Subsequently, these drimanes are rearranged under the catalytic influence of illite to form rearranged drimanes, such as the transformation from 8(β)H-drimane to 4,4,8,8,9-pentamethyl-trans-decalin through 1,3-methyl migration.
Paleoclimate, paleoenvironment and paleovegetation evolutions coupling the sesquiterpanes formation
The Early Triassic Period, which inherited the warm climate of the Late Permian80, is often described as a “greenhouse” world, with the Earth’s poles free of ice and snow81. Previous studies have indicated that the Late Triassic experienced sustained rainfall such as the Carneian rainfall event, followed by prolonged drought periods82, During the Late Triassic, the evolution of peleovegetation progressed significantly, with ferns thriving and becoming the dominant force in the terrestrial ecosystem. Meanwhile, large lycopodiums had disappeared, while lycopodium minuta remained83. These variations were mainly resulted from the flourshing of lycopodium and ferns in the humid swamp environment84. Palynological studies reported that cycadophytes, ginkgophytes, ryophytes, and lycophytes were prosperous in the Tarim Basin during the Late Triassic period36. The determination of the bicyclic sesquiterpanes and diterpanes in the Huangshanjie and Taliqike formations is consistent with these palynological records.
The sedimentary environment of Upper Triassic in the Kuqa Depression was characterized by weak oxidation and weak reduction. The parent material sources were distinctly marked by the presence of both higher terrigenous plants (e.g. lycopodium, ferns, and cycads) and lower organisms (e.g. herbaceous plants, phytoplankton, and algae). Notably, a significant shift occurred during the deposition from Huangshanjie to Taliqike formations, as both the environmental reducibility and the input of higher plants showed a marked increase. Throughout the Late Triassic, bicyclic sesquiterpanes formed by the thermal degradation of tricyclic terpanes in a weak redox environment. Simultaneously, illite catalysis may play a role in the formation of rearranged drimane compounds. For instance,8(β)H-drimane undergoes 1,3-methyl migration to generate 4,4,8,8,9-pentamethyl-trans-decalin (Fig. 11).
Comprehensive diagram of bicyclic sesquiterpane formation in Yinan 2 well of the Kuqa Depression during the Late Triassic.
Conclusion
The Upper Triassic Huangshanjie and Taliqike formations of the Yinan 2 well in Kuqa Depression, Tarim Basin have reached a high maturity stage (Rc = 1.30% ~ 1.45%) and are generally good source rocks, mainly deposited in weak oxidation–weak reduction environments. They are mainly derived from mixed organic source (terrigenous higher plants and lower organisms) and terrigenous higher plants, respectively. From Huangshanjie to Taliqike formations, the terrestrial higher plants input increase cooperates with the sedimentary environment trend to more reduction.
Bicyclic sesquiterpanes with ten homolog compounds and general domination of 4,4,8,8,9-pentamethyl-trans-decalin, 8(β)H-drimane, and 8(β)H-homodrimane are identified in the Upper Triassic source rocks. From bottom to the top of the source rocks, the ∑C15/∑C16 drimanes, ∑C15 − 16 rearranged/∑C15 − 16 drimanes and 8(β)H-drimane /8(β)H-homodrimane generally decrease, accompanying with the reduction of the illite content. There are many factors controlling the formation of sesquiterpanes including thermal maturity, depositional environment, parent material type and clay mineral. However, weak redox environment and clay mineral catalysis (such as illite) are beneficial to the formation of bicyclic sesquiterpanes and their rearrangement, respectively. The rearranged reaction associated with bicyclic sesquiterpanes can be caused by methyl migration, such as 1,3-methyl migration.
Data availability
The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.
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This work is financially supported by the National Natural Science Foundation of China (42272144).
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Appendix A: Chemical structures of bicyclic sesquiterpanes detected in the Upper Triassic source rocks of the Yinan 2 well in the Kuqa Depression of Tarim Basin, NW China
Appendix A: Chemical structures of bicyclic sesquiterpanes detected in the Upper Triassic source rocks of the Yinan 2 well in the Kuqa Depression of Tarim Basin, NW China
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Zhang, Y., Zhang, H., Deng, H. et al. Genetic mechanism of bicyclic sesquiterpanes in Upper Triassic source rock of Yinan 2 well from the Kuqa Depression of Tarim Basin, China. Sci Rep 15, 10100 (2025). https://doi.org/10.1038/s41598-025-95112-9
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DOI: https://doi.org/10.1038/s41598-025-95112-9
