https://orcid.org/0009-0001-0307-2072
Figures
Abstract
Shale samples from source rocks of the Upper Triassic Yanchang Formation (Chang 9 member) in the Ansai area, Ordos Basin, North China, were analyzed using gas chromatography – mass spectrometry (GC-MS) to investigate the distribution, abundance, and enrichment mechanisms of rearranged hopanes. Four rearranged hopane series were detected, with all four present simultaneously in individual samples. Analysis of the C₃₀ hopane series (regular C₃₀H, diahopane C₃₀D, and neohopane C₃₀E) using a ternary diagram revealed a distinct linear trend, demonstrating a systematic, inverse relationship between the abundance of regular hopane and the combined abundance of its rearranged counterparts. These results provide strong evidence that C₃₀D and C₃₀E in the Chang 9 shales are diagenetic products derived from C₃₀H, sharing a common biological precursor. Both diasteranes and regular steranes with the ββ configuration were correlated positively in abundance with rearranged hopanes, further supporting a common origin linked to specific organism assemblages rather than widespread organisms. Samples deposited under highly saline, suboxic sedimentary environments displayed relatively high abundances of rearranged hopanes, indicating the critical role of depositional conditions in their enrichment. Multi-proxy analysis revealed a complex, non-linear control of thermal maturity on rearranged hopane abundance. The C₃₀ Rearranged Hopane Index showed statistically significant positive correlations with multiple maturity parameters (including sterane and hopane isomerization ratios), indicating maturity as a primary driver in the early oil window. However, this trend diverged at higher maturity levels, suggesting that other factors, such as the catalytic activity of the mineral matrix, become dominant. Our findings establish a robust biomarker-based framework for interpreting oil-source correlations and informing petroleum exploration in the Ordos Basin, particularly for the Chang 9 member source rocks.
Citation: Yue H, Liu Y, Li R, Liu X (2025) Distribution and characteristics of rearranged hopanes in the black shale of the Chang 9 member, the Upper Triassic Yanchang Formation in the Ansai area, Ordos Basin, North China. PLoS One 20(12): e0337076. https://doi.org/10.1371/journal.pone.0337076
Editor: Timothy Omara, Makerere University College of Natural Sciences, UGANDA
Received: July 25, 2025; Accepted: November 4, 2025; Published: December 1, 2025
Copyright: © 2025 Yue et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the manuscript and its Supporting Information files.
Funding: The research is funded by the National Natural Science Foundation of China (Grant No. 41772118, 42273064), the Shaanxi Natural Science Foundation Project (Grant No. 2023-JC-QN-0289), and the Fundamental Research Funds for the Central Universities (Grant No. 300102121107). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
1 Introduction
Regular 17α(H), 21β(H) hopanes, with carbon numbers ranging from C27 to C35, are abundant in sediments and crude oils from many basins worldwide [1–7]. Rearranged hopanes are a class of biomarker compounds with the same carbon skeleton as regular hopanes but differing substitution patterns along methyl side chain. Four series of rearranged hopanes have been reported and identified in sediments and crude oils, including 17α(H)-diahopanes (D series), early-eluting series (E series), 18α(H)-neohopanes (Ts series), and 21-methyl-28-norhopanes (Nsp series). 18α(H)-22,29,30-Trisnorneohopane (Ts) of the Ts-series was the first compound to be structurally identified by X-ray crystallography, having a rearranged methyl group at C-17 [8,9]. The other known compound of the Ts series identified was 18α(H)-30-neohopane (C29Ts) using nuclear magnetic resonance (NMR) techniques [10]. Moldowan et al. also discovered a novel rearranged hopane series, the D-series, with carbon numbers C29–C34, using gas chromatography-mass spectrometry (GC-MS) [10]. The early-eluting rearranged hopane series (E-series) has been observed in lacustrine oils and detected using ion monitoring [2,11]. The E-series, C27E and C29E–C35E, are characterized by elution about two carbon numbers earlier than the corresponding regular hopanes of the same carbon number [3]. Huang et al. first reported the C29 28-nor-spergulane (C29Nsp), a member of the Nsp series, in the western Pearl River Basin off the coast of South China [12]. Subsequently, Nytoft et al. identified the Nsp series ranging from C29 to at least C34 using NMR techniques [13].
Although the origin and geochemical significance of rearranged hopanes have been widely studied since their identification, their formation mechanisms remain controversial. 17α(H)-Diahopanes, first observed in terrigenous oils and coals, are considered terrestrial biomarkers [14]. Killops and Howell postulated that rearranged hopanes terrigenous organic matter that was bacterially reworked [2]. It was then suggested that their biological precursors were bacterial in origin because of their isotopic similarity to regular hopanes [10]. However, Zhang et al. suggested that some specific algae (e.g., rhodophytes) may also have been sources of rearranged hopanes [4]. Crucially, the conversion from biological precursors to rearranged hopanes is now widely understood to be a diagenetic, acid-catalyzed process rather than a direct inheritance from organisms [15–17]. The catalytic effect of acidic clay minerals in the sediment matrix is considered a primary driver of this molecular rearrangement. For instance, studies on Chinese lacustrine and coal measure rocks have demonstrated that acidic clays, such as kaolinite and smectite, facilitate the formation of rearranged hopanes, with their abundances sometimes showing a positive correlation with clay content [17]. However, this relationship is not always straightforward; other factors within the depositional environment, lithology, redox and/or pH conditions, and rock fabrics have been considered to be the most important factors influencing the relative abundance of rearranged hopanes [3,14]. The potential applications of rearranged hopanes as geological markers in oils and sediments are becoming increasingly important, including oil maturity assessment, oil family classification, and oil-source correlation studies [3,6,9,12,18,19]. Consequently, various ratios based on the increasing thermodynamic stability of the rearranged products with thermal maturation have been developed as widely used maturity indicators. These include the classic Ts/(Ts + Tm) and C₂₉Ts/(C₂₉Ts + C₂₉H) ratios, as well as ratios involving C₃₀ 17α(H)-diahopane [5,20–22].
Despite these advances, the interplay between controlling factors (e.g., organic matter source, clay catalysis, depositional environment, and thermal maturity) can vary significantly across geological settings, leading to regional variations in rearranged hopane distributions. The Chang 9 member in the Ordos Basin is a critical source rock, yet the distribution patterns and controlling mechanisms of rearranged hopanes within it have not been systematically investigated. A lack of understanding of these controls has hindered precise oil-source rock correlations in the study area, particularly for tight oil exploration where migration distances are short. Therefore, this study: (1) systematically characterizes the distribution of four series of rearranged hopanes in the Chang 9 source rocks from the Ansai area; (2) investigates the potential roles of organic matter input, depositional environment, and thermal maturity in controlling their abundances; and (3) establishes a more robust biomarker framework to support petroleum exploration in the Ordos Basin.
2 Geological background
The Ordos Basin is the second-largest sedimentary basin in China and the largest oil and gas production base (Fig 1a). The Mesozoic strata of the basin host substantial quantities of oil and gas resources, with reservoirs exhibiting low-to-ultralow porosity and permeability. The Upper Triassic Yanchang Formation represents a series of fluvial-lacustrine terrestrial clastic sedimentary systems deposited during a period of continuous depression and stable subsidence within the basin. The Yanchang Formation has been subdivided into 10 oil-bearing formations, designated from top to bottom as Chang 1 to Chang 10, based on sedimentary cycles and lithological characteristics [21]. These formations are regarded as the principal oil-bearing layers [22]. Previous studies have identified the Chang 7 member as the primary source rock in the Ordos Basin due to its extensive distribution and high-grade lacustrine source rocks [22,23]. Recently, shale source rocks have been identified in the Chang 9 member [4,24]. Additionally, the Chang 8–10 reservoirs adjacent to the Chang 9 source rock have demonstrated promising oil and gas potential. Furthermore, industrial oil flow has been successfully obtained [25], suggesting that the Chang 9 source rock may play a critical role in oil and gas generation.
(a) Location of Ordos basin, China. (b) Structural division of the Ordos Basin and the studied area. (c) Sedimentary facies of the Chang 9 section, Upper Triassic Yanchang Formation. The base map for panel (a) was generated using QGIS software with public domain data (Projection: WGS84 (EPSG:4326)) from Natural Earth (www.naturalearthdata.com). The geological maps in panels (b) and (c) were compiled based on previous studies and internal data.
The sedimentary period of the Chang 9 member represents the initial phase of lake basin formation, characterized by the accumulation of delta-lake facies deposits. The deposition of these sediments corresponds to the first lake invasion during the Late Triassic. At the end of the Chang 9 period, the lake basin expanded to its maximum extent along the Wuqi–Zhidan–Ganquan–Huangling section, resulting in the deposition of a narrow and semi-deep lake sediment commonly known as the Lijiapan Shale [26]. Previous studies have demonstrated that the Chang 9 shale is an effective source rock with proven hydrocarbon generation and expulsion potential [24]. The organic matter content of the Chang 9 shales is relatively high, ranging from 0.53% to 10.5% and averaging at approximately 5.0%. The organic matter is derived from both lower aquatic organisms and higher terrestrial plants. The organic matter types are predominantly Types I and II1, which are in the mature stage. The hydrocarbon generation and transformation rates of source rocks in the Zhidan–Ansai region are high, indicating a significant capacity for hydrocarbon generation and migration [27,28].
3 Materials and methods
3.1 Materials
Eight core samples were collected from the Chang 9 member of the Yanchang Formation (Fig 1c). Following common practice [29,30], we used TOC > 1 wt% to designate black shale. Accordingly, our dataset included both black shales (TOC > 1 wt. %) and lower‑TOC shales (TOC < 1 wt. %). TOC ranged from 0.12 to 2.0 wt% (Table 1) and average extractability was 0.22 mg/g. All samples contained Type I–II kerogens and were within the peak oil window of thermal maturity based on Rock‑Eval parameters.
Permits and approvals: No field sampling was conducted for this study. All samples were obtained by subsampling archived drilling cores owned and curated by Shaanxi Yanchang Petroleum (Group) Co., Ltd. (Xingzichuan Oil Production Plant), at the core repository in Yan’an, Shaanxi Province, China. Access to the cores and permission to sample were granted in writing by the Exploration and Development Department of the Xingzichuan Oil Production Plant (authorization dated 30 October 2022). No governmental permits were required because the work relied exclusively on existing industry-owned cores housed outside protected areas and did not involve living organisms, human participants, or endangered species.
3.2 Solvent extraction and fractionation
The core shale samples were washed and cut off to remove the external surfaces before crushing. The samples were then crushed to a particle size of less than 100 mesh. The crushed powder was Soxhlet-extracted using a dichloromethane and methanol solution (9:1 v:v) for 72 h to obtain the extractable organic matter (EOM). The resulting EOM was filtered to remove any impurities, de-asphalted with cold n-hexane, and the precipitated asphaltenes were weighed. Subsequently, the maltene fraction was separated into subfractions of saturated hydrocarbons, aromatic hydrocarbons, and resins using activated silica gel and alumina column chromatography, with sequential elution using n-hexane, dichloromethane:methanol (1:1 v:v), and anhydrous ethanol:dichloromethane (3:1 v:v).
3.3 Gas chromatography-mass spectrometry
Analyses of the saturated and aromatic hydrocarbon fractions were performed using an Agilent 6890N gas chromatograph coupled to a 5975 mass-selective detector (MSD). The GC was equipped with an HP-5MS fused-silica capillary column (60 m × 0.25 mm i.d., 0.25 μm film). The oven temperature program started at 50 °C, increased at 3 °C· min−1 to 310 °C, and was held for 15 min. Helium was used as the carrier gas at a constant flow rate of 1.0 mL·min−1. Samples were introduced via an autosampler in split mode; 1 μL of sample solution (extracted fractions diluted with n-hexane to a final analyte concentration of 10–100 μg·mL−1) was injected. The mass spectrometer operated under electron ionization (70 eV), with the ion source and quadrupole temperatures set at 230 °C and 150 °C, respectively. Data were acquired in full-scan mode over m/z 50–580 and, where appropriate, by selected-ion monitoring (SIM) to enhance the detection of hopanes and rearranged hopanes.
For quantification, each sample was spiked with D50-n-tetracosane (n-C24D50) to determine n-alkane concentrations and with 5α-androstane for sterane and hopane biomarkers; the final concentrations of the internal standards in the injection solution were 0.4701 μg·μL−1 and 0.1049 μg·μL−1, respectively. Concentrations and molecular parameters were calculated from the peak-area ratios relative to the corresponding internal standard.
3.4 Statistical analysis
All statistical analyses were performed using Microsoft Excel 365 (Microsoft Corporation, Redmond, WA, USA) with the Real Statistics Resource Pack add-in (version 9.5.5). To avoid the statistical closure problem associated with compositional data, a ternary diagram was used to visualize the relative proportions of C₃₀ hopanes and a C₃₀ Rearranged Hopane Index (RHI) for quantitative analysis.
Spearman’s rank correlation analysis (rs), a non-parametric method that is robust to small sample sizes and non-normally distributed data, was employed to assess monotonic relationships between key biomarker parameters. This analysis was applied to investigate the relationships between various rearranged hopane ratios (e.g., C₃₀D/C₃₀H, C₃₀E/C₃₀H) and between the RHI and multiple thermal maturity proxies, including the calculated vitrinite reflectance (Rc; derived from MPI-1), Ts/Tm, and C₂₉ sterane isomerization ratios. For maturity trend analysis, the statistical outlier sample X762 was excluded to better evaluate the underlying trend of the main sample population.
The two-tailed significance level was set at α = 0.05. Correlation strength was interpreted using conventional thresholds for the absolute value of the Spearman’s coefficient (|rs|): strong (≥0.7), moderate (0.5–0.7), or weak (<0.5). Linear trend lines and coefficients of determination (R2) were generated in the plots for visual purposes only, to illustrate the general trends. All detailed statistical results, including correlation coefficients and p-values, are provided in the Supplementary Information (S2 Table).
4 Results and discussion
4.1 n-Alkane and acyclic isoprenoid characteristics
The results revealed the presence of n-alkanes ranging from n-C14 to n-C37 in all the shale samples from the Chang 9 member (Fig 2). The n-alkanes displayed a smooth unimodal distribution of C14–37 with maxima centered around n-C19 to n-C21 [31,32]. This distribution pattern is indicative of a relatively mature stage [33]. Sample PE312 exhibited a slightly elevated maxima at n-C23. The odd–even preference (OEP) and the carbon preference index (CPI) were close to 1 for the majority of the samples (Table 1), indicating a minimal OEP for n-alkane. However, sample X1107 exhibited notably elevated OEP (1.41) and CPI (1.5) values (Table 1), indicative of an odd-over-even n-alkane preference for n-alkanes spanning n-C19 to n-C30 (Fig 2). This result indicates that sample X1107 was relatively less mature. Notably, samples X1080 and X1107 exhibited medium-heavy molecular weight ratios of n-alkanes, as indicated by the ratio of ((n-C21 + n-C22)/(n-C28 + n-C29)) and ∑n- C21-/∑n-C22+ (0.67–0.76 and 0.98–1.38) compared to others (0.82–2.0 and 1.7–2.6) (Table 1). Furthermore, the terrigenous to aquatic n-alkane ratios [34] for samples X1080 and X1107 were 1.06 and 0.87, respectively, considerably higher than the average ratio of 0.54 observed for the remaining samples (Table 1). It can thus be posited that samples X1080 and X1107 likely contained a greater proportion of land-derived organic matter during the process of sedimentation than the remaining samples, which exhibited a combination of aquatic plants and higher organism contributions.
Pristane (Pr) and phytane (Ph) are important acyclic isoprenoids, and their ratios, such as Pr/Ph, Pr/n-C17, and Ph/n-C18, are widely used as indicators of redox conditions, organic matter input, and thermal maturity [31,35–38]. The Pr/Ph ratios of the Chang 9 samples ranged from 0.76 to 3.4 (Table 1). As shown in Fig 3a, most samples fell within the suboxic–dysoxic range (Pr/Ph = 0.5–3.0), while one sample (PE312) showed plots in the oxic field (Pr/Ph > 3.0), suggesting variable degrees of terrestrial influence among samples.
(a) Pr/Ph ratios, showing that most samples were deposited under suboxic-to-anoxic conditions, with the notable exception of sample PE312 suggesting an oxic or terrestrial-influenced setting. (b) Cross-plot of Pr/n-C17 versus Ph/n-C18, showing a transitional trend where samples plot within the ‘Mixed organic sources’ field and extend into the ‘Terrestrial organic matter’ field. No samples fall within the ‘Marine organic matter’ field. Crucially, sample PE312, which appeared anomalous in Fig 3a, now plots unequivocally within the ‘Terrestrial organic matter’ field. This provides definitive evidence that its high Pr/Ph ratio is primarily controlled by the dominant input of terrestrial organic matter rather than reflecting a truly oxic depositional environment. Furthermore, the data points spread across the oxidizing-to-reducing trend lines, indicating that this organic matter was deposited under fluctuating, oxygen-limited conditions. Interpretive bands modified after Shanmugam (1985) [38] and Peters et al. (2005) [33]. Sample PE312 is explicitly labeled for clarity, and sample X762 (red symbol) is highlighted.
However, a more precise interpretation is provided by the Pr/n-C17 versus Ph/n-C18 cross-plot (Fig 3b), a diagnostic tool widely used for its ability to effectively deconvolve the interconnected effects of organic matter source and redox conditions [38]. The plot shows a clear transitional trend for the Chang 9 samples, which originate from a ‘Mixed organic sources’ setting, characterized by a significant aquatic contribution (as evidenced by n-alkane maxima at n-C19–C21 and TAR ≤ 1; Table 1), and extend clearly into the ‘Terrestrial organic matter’ field. Importantly, this source trend corresponds well with a redox gradient, where samples in the ‘Mixed’ field represent suboxic-dysoxic conditions, while the progression towards the ‘Terrestrial’ field aligns with increasingly oxic conditions. This framework helps to explain the characteristics of the “anomalous” sample PE312, which appeared as a strongly oxic outlier in Fig 3a. In Fig 3b, this ambiguity is clearly resolved, as PE312 is located at the end of this trend, plotting clearly within the ‘Terrestrial organic matter’ field and in the ‘Oxidizing’ position. This suggests that its high Pr/Ph ratio can be primarily attributed to the combined effects of both a dominant terrestrial organic matter input and a more oxic depositional environment. While other factors such as thermal maturity or biodegradation can influence these parameters (as indicated in Fig 3b), the consistent alignment of the data along the source-redox trend indicates that this interplay is the principal control for the Chang 9 sample suite. Consequently, the apparent contradictions in the dataset are best explained by a source-redox system, where increased terrestrial input correlates with more oxic conditions.
4.2 Sterane and diasterane characteristics
Steranes are widely distributed in source rocks and crude oils. The relative contents of C27, C28, and C29 steranes from different sources are commonly used to determine the nature of the parent organic materials [20,31,39]. The m/z 217 chromatographic analysis of representative Chang 9 source rock samples revealed differences in distribution patterns and relative abundances (Fig 4, Table 2). The proportions of C27, C28, and C29 regular steranes are presented in the ternary diagram in Fig 5. In general, the ααα 20R C27–C28–C29 distribution exhibited a slight preference for C27 > C28 < C29, resulting in a “V” shaped profile in the m/z 217 mass chromatograms (Fig 4). Sample X1107 exhibited a distinctive inverted “L” type distribution, accompanied by an elevated C29 contribution of 55.1%, situated in the upper region of the ternary diagram.
The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols); note its position within the main data cluster.
This distribution infers a greater contribution from terrigenous plants (Figs 4–5, Table 2). The C29 steranes exhibited a slightly higher relative abundance (38.8–55.1%) in the majority of samples, followed by C27 (21.7–37.1%) and C28 (19.1–36.7%). This indicates mixed organism input, and a slightly higher abundance of C27 steranes (36.9%) than that of C29 steranes (35.5%) was observed in sample X762 (Fig 5, Table 2). The ratio of diasteranes to regular steranes is generally consistent across samples; however, their relative abundances vary. Diasterane abundance in the samples was relatively low, with a diasterane/sterane value of 0.15. Nevertheless, diasteranes constitute significant components in some samples, with the highest diasterane/sterane ratio of 0.51 observed in sample X762 (Fig 4, Table 2).
The sterane maturity parameters, C29 20S/(20S + 20R) and C29 ββ/(αα + ββ), exhibited ranges of 0.43–0.58 and 0.31–0.59, respectively, in the studied samples (Table 2). The ratio values were close to or at their thermal equilibrium values (Fig 6), indicating that the shale samples of the Chang 9 member had reached the thermal maturity of the peak oil window [39].
The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols), plotting within the mature end of the sample distribution.
4.3 Hopane characteristics
4.3.1 Occurrence of rearranged hopanes.
In m/z 191 mass chromatograms of saturated hydrocarbons derived from shale samples of the Chang 9 source rocks in the Ansai area, both regular and rearranged hopanes were identified, exhibiting notable variations in distribution between samples. Four series of rearranged hopanes were identified, including the 17α(H)-diahopanes (D series), 18α(H)-neohopanes (Ts series), early-eluting rearranged hopanes (E series), and relatively small amounts of 28-nor-spergulanes (Nsp series) (Fig 7) [40]. These biomarkers were identified through comparison of their relative retention times and mass spectra with those reported in the literature [10]. In general, the most abundant hopanes are typically the D-series, which have a similar distribution to regular hopanes but consistently elute earlier at the same carbon number. D-series biomarkers were present in the carbon number range C27–C35, except C28. Paired peaks of C31–C35D, representing both S and R isomers, were observed in the shale samples of the Chang 9 source rock (Fig 7c).
However, only C29D and C30D were consistently detected in all samples. The Chang 9 source rock extracts showed lower abundance of 18α(H)-neohopanes than 17α(H)-diahopanes. The former were primarily composed of C27, C29 and C30 homologues (Ts, C29Ts, and C30Ts, respectively). The early-eluting rearranged hopanes eluted two carbon numbers earlier than regular hopanes as a consequence of their shorter and more compact molecular structures. However, the E-series had low relative abundances and absolute concentrations, and only the C30E homolog was clearly detected. Its peak position was between Ts and Tm in the m/z 191 mass chromatograms and much closer to Tm. The Nsp series, with the exception of C29Nsp, were rarely detected in this study and were relatively low in abundance.
Nevertheless, the occurrence of all four hopane rearrangements in a single geological sample is extremely rare. Furthermore, it is also unusual for the abundance of C30E to exceed that of its neighbors, including Ts and Tm, in the m/z 191 mass chromatograms [32]. Sample X762 displayed the most anomalous composition and distribution of pentacyclic triterpenes, with very few normal 17α(H)-hopane series and a prevalence of rearranged hopanes. Notably, all four series were detected (Fig 7).
4.3.2 Distribution of rearranged hopanes.
The mass chromatograms for m/z 191 demonstrated a notable variation in the distribution of rearranged hopanes among the samples within the Chang 9 member. Based on this observation, the Chang 9 source rocks were classified into three main types. The first type, exemplified by X1107, was characterized by a predominance of C30 hopanes (C30H) and a normal distribution pattern of C31–C35 homohopane compounds, which decreased in abundance with increasing carbon number. This sample type exhibited a lower relative abundance of Ts than Tm. The rearranged hopanes were primarily composed of low-abundance C29D, C29Ts, C30D, and C30M (Fig 7a). The ratios of C30D/C30H and Ts/Tm were relatively low, with values of 0.06–0.07 and 0.11–0.76, respectively (Table 2). In contrast, sample X762 was characterized by strong dominance of C30D and a relatively complete D series. The ratio of C30D to C30H was as high as 4.1 (Fig 7c, Table 2). Furthermore, the samples contained a substantial amount of Ts, C29D, C30E, and diahopanes (C31–C34), which exhibited markedly higher abundances than the corresponding 17α(H)-homohopanes (Fig 7c). In particular, the C30E isomer was more abundant than C30H in sample X762, with a C30E/C30H ratio of 1.46 (Table 2). Notably, this sample also exhibited elevated abundances of diasteranes (Fig 4, Table 2). To avoid the leverage effect, X762 was plotted as a separate group (in red) in all the figures. The third source rock type, exemplified by W474, demonstrated a pronounced increase in the diversity and relative abundance of rearranged hopane compounds detected, compared to the first sample type. Additionally, Ts series and C30E were found to have high relative abundances (Fig 7b).
The distribution relationships of the rearranged hopanes from the Chang 9 source rocks were further analyzed. The associated hopane parameters were calculated and are presented in Table 2 for purposes of analysis. As illustrated in Fig 8a, the ratio of C30E/C30H exhibited a significant positive monotonic correlation with that of C30D/C30H (Spearman’s rs = 1.0000, p < 0.0001), with a linear trend line showing a high coefficient of determination (R2 = 0.9956) and an intercept of less than 0.0209, indicating similar biological origins and formation conditions for these two series. Sample X762 exhibited considerably higher ratios than other samples across multiple hopane parameters; for example, the exclusion of X762 from the analysis results in a strong monotonic correlation between C29D/C29H versus C30D/C30H (Spearman’s rs = 0.9286, p = 8.63 × 10−4) with a moderate linear fit (R2 = 0.735) (Fig 8b) and a slightly higher value was observed in sample X281. Furthermore, the plots of C29Ts/C29H and C29D/C29H demonstrated a robust monotonic correlation (Spearman’s rs = 0.9643, p = 0.0001), supported by a strong linear fit (R2 = 0.984) (Fig 8c). However, including X762 significantly weakened the linear fit (R2 = 0.531, Spearman’s rs = 0.8571, p = 0.0065) (Fig 8d), prompting a detailed examination of the geochemical drivers behind this anomaly.
(a) C30D/C30H vs. C30E/C30H. (b) C29D/C29H vs. C30E/C30H. (c) C29Ts/C29H vs. C29D/C29H, with the linear regression excluding X762. (d) The same plot as (c), but with the regression including X762 to illustrate its strong leverage effect. The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols).
Sample X762 exhibited exceptionally high rearranged-hopane ratios relative to the rest of the Chang 9 dataset, with C30D/C30H = 4.08, C30E/C30H = 1.46, and C29D/C30H = 0.42 (Table 2). We attributed this anomaly primarily to enhanced mineral acid-catalyzed rearrangement during early diagenesis in a clay-rich lacustrine microenvironment, with a secondary contribution from differences in organic-matter input and water-column stratification. This interpretation was supported by three lines of evidence. (1) Clay-catalysis proxies were markedly elevated in X762: the diasterane/sterane ratio was the highest of the set (Dia/St = 0.51), Ts/Tm was extremely high (6.7, comparable to X281 = 6.5), and C29Ts/C30H was also high (0.42). These features were classically associated with acid-catalyzed rearrangement on clay mineral surfaces [31,33]. (2) The thermal maturity was not a dominant control. Although X762 showed relatively high maturity based on several proxies—including MPI1 (0.85), C₂₉ 20S/(20S+20R) sterane isomerization (0.59), and its calculated vitrinite reflectance (Rc) of approximately 0.91 (derived from MPI-1 using the equation Rc = 0.6 × MPI-1 + 0.4 [41])—this alone cannot account for the anomaly. Crucially, a sample with similar or even greater maturity signatures (X281: C29 20S/(20S+20R) = 0.62; Ts/Tm = 6.5) displayed a much lower C₃₀D/C₃₀H ratio of 1.14. This sharp contrast indicates that maturity was not the primary driver of the extraordinary enrichment of rearranged hopanes observed in sample X762. (3) Facies and input indicators point to a stratified lacustrine setting with a comparatively stronger algal/bacterial contribution, which provides hopanoid precursors and favors preservation: X762 had the highest gammacerane content relative to C30 hopane (Ga/C30H = 0.72), the lowest or near-lowest Pr/n-C17 (0.19) and Ph/n-C18 (0.09), and a relatively low TAR (0.51). Taken together, these observations favor mineral-catalyzed rearrangement as the dominant driver of the anomaly, amplified by facies and organic-matter input effects.
4.4 Primary influencing factors of rearranged hopanes
4.4.1 Organic matter/acid catalysis.
The conventional view holds that rearranged hopanes share identical biological precursors with regular hopanes, originating primarily from bacteriohopanetetrol [10,42]. This implies a precursor–product relationship in which regular hopanes are converted into rearranged hopanes during diagenesis [10,43]. To investigate this relationship in the Chang 9 samples while avoiding the statistical pitfalls of compositional data closure, we visualized the relative proportions of C₃₀ diahopane (C₃₀D), C₃₀ neohopane (C₃₀E), and regular C₃₀ hopane (C₃₀H) using a ternary diagram (Fig 9a).
(a) Ternary plot showing the fractional contributions of C30 hopane (C30H), C30 diahopane (C30D), and C30 neohopane (C30E), normalized to C30H + C30D + C30E = 1. (b) Relationship between the C30 Rearranged Hopane Index (RHI = (C30D + C30E)/(C30D + C30E + C30H)) and the relative abundance of C30 hopane (C30H/(C30D + C30E + C30H)); the dashed line represents the theoretical mass-balance complement (y = 1 – x). Values are derived from m/z 191 mass chromatograms. The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols).
The data points form a clear linear trend extending from the C₃₀H apex towards the C₃₀D–C₃₀E baseline. This distribution strongly supports a systematic conversion process in which the abundance of C₃₀H decreases as those of C₃₀D and C₃₀E increase. To quantify the extent of this conversion, we defined a C₃₀ Rearranged Hopane Index (RHI) as (C₃₀D + C₃₀E)/ (C₃₀D + C₃₀E + C₃₀H). As shown in Fig 9b, there was a mathematically defined inverse relationship between the RHI and the relative proportion of regular C₃₀ hopane (%C₃₀H). This robust relationship confirms that the formation of rearranged hopanes in the Chang 9 source rocks occurred at the expense of regular hopanes. This establishes the crucial premise that this transformation is driven by geological factors, such as clay-mediated acid catalysis.
High diasterane/sterane ratios are typical of petroleum generated from clay-rich source rocks, and the diasterane/sterane ratio shows a good relationship with the diahopane/hopane or early-eluting series/hopane ratio [3,31]. Similarly, it was found that the samples with high abundances of rearranged hopanes also had high abundances of diasteranes (Figs 4 and 7, Table 2). The same feature was reported by Ruble TE [42]. Furthermore, samples with high abundances of regular sterane isomers with ββ configuration also had relatively high abundances of rearranged hopanes (Figs 4 and 7). As shown in Fig 10, the C30D/C30H ratios increased with increasing Dia/St and C27-29 ββ/αα ratios. Previous studies have reported that sterane isomerization is controlled not only by thermal maturity, but probably also by the catalysis of active clay (e.g., kaolinite), mineral matrix (e.g., gypsum), and hypersaline environments [44–46].
The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols).
Previous reports suggest that the high abundance of 17α(H)-diahopanes in oils from the Korea Bay Basin may be closely related to the input of terrigenous land plant material [14] and/or may be generated from bacterially reworked terrigenous organic matter [2,32]. However, most of the Chang 9 samples have mixed sources, and the contribution of terrigenous land plant material appears to be insignificant. Sample X1107 had the lowest abundance of rearranged hopanes, despite having the highest input of terrigenous organic matter (Table 2). This suggests a limited role of terrigenous organic matter in the formation of rearranged hopanes in the Ansai area.
4.4.2 Sedimentary environment.
Sedimentary environments can contain different assemblages of organisms, resulting in different biomarkers in sediments [31]. The redox conditions and salinity of sedimentary and diagenetic waters influence the relative abundance of C30D [10,14,31]. According to Moldowan et al. [14], the four rearranged hopane series are formed by clay-mediated acid catalysis of bacteriohopanoid precursors under oxic or suboxic depositional conditions. Xiao et al. [17] concluded that high-salinity water conditions would inhibit the proliferation of rearranged hopane precursors. The environments reflected by Ga/C30H and Pr/Ph varied greatly in this study. The Chang 9 member was mainly deposited in fresh to slightly saline water and suboxic sedimentary environments. In contrast, samples from X762 and X281 were formed in brackish to saline water environments (Fig 11).
The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols).
Zhang et al. [47] reported a positive relationship between C30D/C30H and Pr/Ph in Yanchang source rocks from the Wuqi–Zhidan area. They suggested that a high to very high abundance of C30D indicates a shallow-oxic environment, while a low abundance of C30D may indicate an anoxic sedimentary environment. The C30D/C30H ratios in the Upper Triassic Chang 9 member show good correlation with the gammacerane index but poor correlation with Pr/Ph (Fig 12, Table 2). Samples X762 and X281, which were deposited in saline to brackish and suboxic sedimentary environments, respectively, showed relatively high abundances of rearranged hopanes. This suggests a positive correlation between the water salinity of the depositional environment and the high content of rearranged hopanes. Other studies [42,48] have also found that a saline water environment promotes the formation of rearranged hopanes. However, the redox state of the sedimentary environment appears to have had little effect on the rearranged hopanes in the Chang 9 member. Both oxic and anoxic sedimentary environments will inhibit the enrichment of rearranged hopanes. For example, sample PE312 was deposited under more oxic conditions than X762 and X281 (Fig 11), but contained fewer rearranged hopanes relative to hopanes. The Ga/C30H and C30D/C30H values for sample X762 showed a substantially different range than that of the other samples. It is suggested that the rearranged hopanes in sample X762 are predominantly derived from a specific sedimentary environment in which the presence of organisms rich in precursor molecules favored their synthesis. A strong positive trend between C30D/C30H and Ga/C30H (Spearman’s rs = 0.6904, p = 0.0579, R2 = 0.861) indicates that hopane rearrangement was influenced by water salinity, and the lack of correlation between C30D/C30H and Pr/Ph (Spearman’s rs = 0.6369, p = 0.0895, R2 = 0.0548) means that redox conditions did not determine diahopane content (Fig 12).
(a) C30D/C30H vs. Ga/C30H. (b) C30D/C30H vs. Pr/Ph. The anomalous sample X762 (red symbol) is highlighted to distinguish it from the main group (blue symbols).
4.4.3 Thermal maturity of organic matter.
Thermal maturity is a critical factor influencing the rearrangement of hopanes and steranes during diagenesis. In our samples, a positive correlation was observed between the abundances of diasteranes and rearranged hopanes (Figs 4 and 7, Table 2), suggesting similarities in their formation controls. Generally, increasing thermal maturity is thought to favor isomerization and rearrangement reactions, as the rearranged products are often thermodynamically more stable [3,46]. Molecular mechanics calculations support this, predicting a stability order of 17α(H)-diahopanes > 18α(H)-neohopanes > 17α(H)-hopanes [10,18].
To robustly evaluate the impact of maturity, and in response to the valid concern that using single maturity proxies or burial depth across different wells can be misleading, we conducted a multi-proxy analysis. We assessed the relationship between the C₃₀ RHI and four different maturity parameters derived from aromatic, triterpane, and sterane compounds (Fig 13). The outlier sample X762, which exhibited the highest RHI, was excluded from the correlation analysis to assess the general trend of the majority of the samples.
(a) RHI versus Rc (Ro-equivalent calculated from MPI-1; (b) RHI versus Ts/Tm; (c) RHI versus C29 sterane 20S/(20S+20R); (d) RHI versus C29 sterane ββ/(αα + ββ). RHI = (C30D + C30E)/(C30D + C30E + C30H). Sample X762 is highlighted as a red diamond. Dashed lines show linear fits computed after excluding X762; R2 values refer to these fits. Note that Ts belongs to the rearranged hopane family and is included here for comparison only.
The results revealed a clear and consistent positive trend between the RHI and all four maturity proxies. Spearman’s rank correlation analysis confirmed that these relationships were statistically significant. The strongest correlation was observed between RHI and the Ts/Tm ratio (Fig 13b; Spearman’s rs = 0.964, p < 0.001), followed by the C₂₉ 20S/(20S+20R) sterane isomerization ratio (Fig 13c; Spearman’s rs = 0.893, p = 0.003). Significant positive correlations were also found with the C₂₉ ββ/(ββ + αα) sterane ratio (Fig 13d; Spearman’s rs = 0.714, p = 0.047) and the calculated vitrinite reflectance, Rc (Fig 13a; Spearman’s rs = 0.714, p = 0.047). This consistent trend across multiple, independent maturity indicators provides strong evidence that, for the main population of Chang 9 shales studied, increasing thermal maturity is a primary factor driving the formation of rearranged hopanes.
However, the non-linear nature of this control becomes evident when considering the full dataset. While the RHI generally increased with maturity, the trend was not uniform, and samples with similar high maturity levels can exhibit vastly different RHI values. For example, sample X281 and the outlier X762 had comparable high maturities (e.g., Rc ≈ 0.86–0.91), but their RHI values differed significantly (0.60 vs. 0.84, respectively, Table 2). This divergence at higher maturities suggests that once a certain thermal threshold is reached, the influence of thermal maturity may become secondary to other geological factors. At that stage, variations in the catalytic activity of the mineral matrix (i.e., clay content and type) or specific characteristics of the depositional environment likely become the dominant controls on the final abundance of rearranged hopanes, leading to the observed scatter in the data.
5 Conclusion
The Chang 9 shale samples from the Ansai area of the Ordos Basin were deposited under a spectrum of redox conditions (representing a transition from suboxic to oxic states), with a mixed organic matter input characterized by an initial significant aquatic contribution that shows a progressively increasing terrestrial influence. Comprehensive analysis identified all four major series of rearranged hopanes. Based on their distinct distribution patterns, the Chang 9 source rocks can be classified into three types: a low-maturity type dominated by regular hopanes (e.g., X1107), a highly rearranged type with an exceptionally high abundance of C₃₀ diahopane (e.g., X762), and an intermediate type. A robust precursor–product relationship was confirmed through ternary diagram analysis, which demonstrated that rearranged hopanes (C₃₀D and C₃₀E) were diagenetic products formed at the expense of regular C₃₀ hopanes (C₃₀H). The enrichment of these compounds was controlled by a complex interplay of multiple factors. The depositional conditions were critical, with higher abundances observed in more saline and suboxic environments. Thermal maturity acted as a primary driver in the early oil window, as evidenced by the significant positive correlations with multiple, independent maturity proxies. However, its influence diminished at higher maturity levels, where the catalytic properties of the mineral matrix likely became the dominant control. The comprehensive analysis of the biomarker characteristics in this study provides robust insights into the geochemical features of the Chang 9 shale. However, we acknowledge that this study is geographically focused. To verify the universality of these findings, further research incorporating data from other areas of the Ordos Basin is recommended.
Supporting information
S1 Table. Hopane isomeric alkane data and figures.
https://doi.org/10.1371/journal.pone.0337076.s001
(XLSX)
S2 Table. Hopane ratio normality and correlation tests.
https://doi.org/10.1371/journal.pone.0337076.s002
(XLSX)
S3 File. Mass Spectral Identification of C30E and C30D.
https://doi.org/10.1371/journal.pone.0337076.s003
(PDF)
Acknowledgments
We would like to thank the staff of Yanchang Oilfield for their invaluable assistance in sample screening and provision. We also extend our gratitude to the personnel of the Key Laboratory of Metallogenic Dynamics, Chang’an University, for their technical support with the GC-MS analysis.
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