C20-C21-C23 tricyclic terpanes abundance patterns: Origin and application to depositional environment identification

Reconstruction of paleo-depositional environments in a sedimentary basin is often obstructed by the absence of typical environmental indicators in sedimentary rocks. Here, we propose a biomarker method using C20-C21-C23 tricyclic terpanes (TTs) as a tracer, which is simple in analysis but robust to provide reliable and detailed environmental information. Based on the analysis of 271 C20-C21-C23TT data from 32 basins in 18 countries, we observed a relationship between C20-C21-C23TT abundance patterns and depositional environments. This relationship was attributed to the control of depositional environments on the input proportions of plankton and terrigenous plants, which act as two end-member precursors for the TTs in a depositional system. The various mixing proportions between these two end-members result in different C20-C21-C23TT abundance patterns associated with different depositional environments, e.g., C20>C21>C23TT in river-lake transitional, C20<C21<C23TT in marine or saline lacustrine environments, C20<C21>C23TT in freshwater lacustrine and C20>C21<C23TT in marine-continental transitional environments. In addition, the C23/C21TT ratio increases with elevated salinity of depositional water, and the C21/C20TT ratio increases with increasing water depths. Based on these observations, a discrimination diagram using C23/C21TT vs. C21/C20TT was developed for environmental identification. The validity of this C20-C21-C23TT biomarker method is well demonstrated by the rock samples with typical environmental indicators. This method is applicable in a broad spectrum of rocks and in maturities up to 2.4%Ro. Its strength was shown by a case study of a complex depositional system in the East China Sea Basin, which has been strongly affected by eustasy.


Introduction
Reconstruction of paleo-depositional environment is critical in oil-gas exploration, paleoclimatic and paleoenvironmental studies. Conventional methods for environmental reconstruction mostly rely on sedimentary, petrological and/or mineralogical characterizations (e.g., Oskay et al., 2019;Pichat et al., 2021), paleontological record (e.g., Heard et al., 2020), and/or geochemical tracing (e.g., Govind et al., 2021). For example, lithology, sedimentary/biogenic structures, rock fabrics and texture in sedimentary rocks have been used to reconstruct sedimentary microfacies, which is further used to infer depositional environments (e.g., El-Sabbagh et al., 2017;Mtelela et al., 2017;Barrera et al., 2020). However, these sedimentological, mineralogical and petrological characterizations are sometimes limited by the availability of outcrops and drilling cores, or the lack of typical depositional indicators. Paleontological methods are highly efficient in revealing paleo-depositional environment. Fossils, bioglyphs and their assemblages have been used to classify biofacies (e.g., Laprida et al., 2007;El-Sabbagh et al., 2017;Mahfouz et al., 2021). However, well-preserved characteristic fossils are not always available in sedimentary rocks because most of organisms in sediments have been degraded during burial and diagenesis. Geochemical data are sensitive to a variety of depositional conditions, such as redox condition (e.g., V/Cr, Ni/Co, U/Th) and salinity (e.g., Sr/Ba, B/Ga, Rb/K). However, these geochemical parameters are not diagnostic to a specific depositional environment, and thus cannot be used solely for depositional environment identification. Recently, biomarkers produced from the degradation of organisms living in different environments has attracted increasing attention in depositional environment studies (e.g., Aderoju and Bend, 2018;Wendorff-Belon et al., 2021;Liu et al., 2022). Here, we demonstrate a new biomarker method using C 20 -C 21 -C 23 tricyclic terpanes (TTs) as a robust tool for the identification of depositional environment.
TTs with carbon numbers range from C 19 to C 29 are ubiquitous in crude oils and extracts of sedimentary rocks (De Grande et al., 1993). Higher carbon TTs are also present but are often masked by hopanes in the m/z 191 mass chromatogram (Samuel et al., 2010) (Figure 1). TTs have been widely applied to oil-source correlation due to their high thermal stability and resistance to biodegradation (Farrimond et al., 1999;Xiao et al., 2019a). Although the exact precursors of TTs have not been identified yet (Dutta et al., 2006;Philp et al., 2021), previous studies have noticed a close relationship between TTs and depositional environments. TTs always show a predominance of C 23 TT in marine facies and a predominance of C 21 TT in freshwater lacustrine facies, and more abundant lower than higher carbon numbers of TTs in shallow-water environments (e.g., Zumberge, 1987;Tao et al., 2015;Atoyebi et al., 2017;Xiao et al., 2019b).
However, the exact correspondences between TTs and various depositional environments have not been clearly defined, limiting their applications to environmental identification. Here, we carried out a thorough examination of new and published C 20 -C 21 -C 23 TT data from a range of known depositional environments worldwide and discovered good correspondences between C 20 -C 21 -C 23 TT abundance patterns and typical depositional environments. Based on the analysis on the origin of C 20 -C 21 -C 23 TT abundance patterns, a discriminating diagram of C 20 -C 21 -C 23 TTs was developed for environmental identification and was then demonstrated for its validity and applicability, and was finally applied to a complex depositional system in the East China Sea Basin to show its strength.

Materials and methods
A total of 232 C 20 -C 21 -C 23 TT data from a range of depositional environments in 30 basins across 18 countries were compiled from the literature (Table 1; Supplementary Table S1). The data were obtained from gas chromatography-mass spectrometry (GC-MS) analysis of crude oils and source-rock extracts, with rock ages mainly ranging from the Devonian to Neogene (Supplementary Table S1).
Additionally, new C 20 -C 21 -C 23 TT data were acquired from four source-rock samples and 35 oil samples from three basins (the Ordos, Bohai Bay and Qaidam basins) in China (Table 1 and Supplementary  Table S1). The soluble organic matter was extracted from the source rocks following the description by Philp et al. (2021). The biomarkers including C 20 -C 21 -C 23 TTs in the rock extracts and oils were analyzed by GC-MS as described by Wang et al. (2019). Subsequently, the source rocks for the oils were determined by the previous oil-source correlations in these basins (e.g., Sun, 2006;Cao et al., 2008;Zhang et al., 2009). As the depositional environments of source rocks in these three basins have been well constrained by previous studies (e.g., Zhu and Jin, 2003;Sun, 2006;Cao et al., 2008;Zhang et al., 2009;Gao et al., 2014), the correspondence between C 20 -C 21 -C 23 TT data and depositional environments were easily determined.
In order to verify the validity of our biomarker method and its applicability to thermal maturity, 13 core samples from 8 wells were collected from the Ordovician and Permian in the Ordos Basin (Table 2). These samples contain typical depositional environment indicators or were deposited in the well-defined depositional environments. All these samples are now at highly mature stages (Table 2). C 20 -C 21 -C 23 TTs in these rock extracts were also analyzed by GC-MS as described by Wang et al. (2019). Two samples (i.e., L65, 4296.3 m and L41-1, 4120 m) were selected to measure vitrinite reflectance (Ro%) following the description by Kalinowski and Gurba (2020). Two thin sections (i.e., L65, 4296.3 m and L66, 4040.45 m) were prepared for the observation of petrography and fossil.
Forty-four sets of geochemical data (including total organic carbon (TOC), Rock-Eval pyrolysis, chloroform asphalt 'A', Ro and biomarkers of rock extracts) obtained from 14 wells in the Pingbei area in the East China Sea Basin (Table 3), were collected from the SINOPEC Shanghai Offshore Oil & Gas Company for a case study.

Marine facies
The marine C 20 -C 21 -C 23 TT data were compiled from 20 basins in 11 countries (Table 1). Their C 21 /C 20 TT and C 23 /C 21 TT ratios vary from 0.98 to 2.83 and 1.20 to 3.57, respectively ( Figure 2). The relative abundances of C 20 -C 21 -C 23 TTs display a pattern of C 20 <C 21 <C 23 TT (e.g., Figure 3A).

Saline lacustrine facies
Saline lacustrine facies is represented by the source rocks from the Junggar Basin in Northwestern China (Bian et al., 2010;Yu et al., 2017) and the oil samples from the Bohai Bay Basin in Eastern China (Table 1). Their C 21 /C 20 TT and C 23 /C 21 TT ratios vary from 1.00 to 2.08 and 1.15 to 1.52, respectively (Figure 2), which slightly overlaps with samples classified as marine in origin (Zhu and Jin, 2003;Yu et al., 2017). The relative abundances of C 20 -C 21 -C 23 TTs associated with saline lacustrine facies are similar to those of marine facies.

Terrigenous source
Terrigenous organic matter can be transported by rivers and eventually deposited in lacustrine or marine environments. Terrigenous organic matter transported by rivers has been reported to be deposited in marine facies in the northern South China Sea (Li et al., 2011;Deng et al., 2019) and lacustrine facies in the Junggar Basin   . On this condition, the C 20 -C 21 -C 23 TT data are strongly controlled by the source instead of depositional environment. The C 20 -C 21 -C 23 TT data of terrigenous source in the Junggar Basin give C 21 /C 20 TT ratios of 0.50-0.91, and C 23 /C 21 TT ratios of 0.25-0.53 (Figure 2), with a relative abundance of C 20 >C 21 >C 23 TT (e.g., Figure 3C).

Transitional facies 3.5.1 Marine-continental transitional facies
Marine-continental transitional facies is represented by the C 20 -C 21 -C 23 TT data from 11 basins in nine countries collected by Zumberge (1987) and Gao et al. (2017) (Table 1). The majority of the C 21 /C 20 TT ratios are lower than 1.0, while the C 23 /C 21 TT ratios are mostly greater than 1.0 (Figure 2), showing an abundance pattern of C 20 >C 21 <C 23 TT (e.g., Figure 3D).

River-lake transitional facies
River-lake transitional facies is represented by the C 20 -C 21 -C 23 TT data from the Junggar Basin (Wang et al., 2020) and Qaidam Basin (this study and Cao et al., 2008;
Here, the analysis of a large amount of C 20 -C 21 -C 23 TT data from various depositional environments provides some insights into the origin of the observed C 20 -C 21 -C 23 TT abundance patterns. Previous studies have shown that the organic matter in typical marine and shallow-water terrestrial facies mainly originated from plankton and terrestrial plants (e.g., Tissot and Welte, 1978). Accordingly, the C 20 <C 21 <C 23 TT pattern likely corresponds to a dominant plankton input, while the C 20 >C 21 >C 23 TT pattern may correspond to a dominant contribution of terrigenous plants. The former correspondence can be evidenced by the Neoproterozoic oil shale (900-873 Ma; maturity: 0.6-0.7%Ro) in North China, in which the terrigenous plant input can be ignored (Zhang et al., 2007). This oil shale contains cyanobacteria and green algae, and shows a C 20 <C 21 <C 23 TT abundance pattern in the rock extracts, suggesting that planktons are the biological source for the TTs with C 20 <C 21 <C 23 TT abundance pattern.

FIGURE 2
Cross-plots of C 20 -C 21 -C 23 tricyclic terpane (TT) ratios in context of depositional environments. The solid boundary lines were established with confidence by the data. The dotted line is a speculated boundary between brackish and freshwater lacustrine facies.
Frontiers in Earth Science frontiersin.org C 20 <C 21 >C 23 TT patterns, which have been observed in marinecontinental transitional and brackish-freshwater lacustrine facies, respectively. As these two depositional environments commonly receive blended input of plankton and terrigenous plants, mixing from different sources is likely the cause of the C 20 >C 21 <C 23 TT and C 20 <C 21 >C 23 TT patterns. This hypothesis is well supported by the numerical modeling of a mixing between plankton and terrigenous plants at various mixing ratios (Figure 4). Representative C 20 -C 21 -C 23 TT abundance patterns in the terrigenous plants domain (i.e., Point A in Figure 4A) and the plankton domain (i.e., Points B or C in Figure 4A) determined by this study, were used as endmembers for the mixing modeling.

FIGURE 4
Numerical simulation for the origin of C 20 -C 21 -C 23 tricyclic terpane (TT) abundance patterns by mixing plankton and terrigenous plants at various mixing ratios. Points (A-C) are the representative end-member C 20 -C 21 -C 23 TT distributions for the mixing modeling. With the change of mixing ratios between A and B (C), the C 20 -C 21 -C 23 TT abundance pattern changes along Line A-B (C). Points D and E are the representative C 20 -C 21 -C 23 TT abundance patterns of A-B mixture and A-C mixture, respectively.
Frontiers in Earth Science frontiersin.org As illustrated in Figure 4, with the change of mixing ratios between A and B, the C 20 -C 21 -C 23 TT abundance pattern changes from C 20 >C 21 >C 23 TT to C 20 >C 21 <C 23 TT (e.g., D in Figures 4A, B) when the mixing ratio (expressed as A%/B% ratio) decreases from 100/0 to 75/ 25, and further to C 20 <C 21 <C 23 TT when the mixing ratio is lower than 44/56 ( Figure 4B). Similarly, mixing between A and C can generate the C 20 <C 21 >C 23 TT pattern (e.g., E in Figures 4A, C) when the mixing ratios (expressed as A%/C% ratio) are in the range of 81/19-42/58 ( Figure 4C). Notably, almost all C 20 -C 21 -C 23 TT data in this study fall into a range from the terrigenous plants domain to the plankton domain ( Figure 5), suggesting that C 20 -C 21 -C 23 TT abundance patterns are strongly controlled by the two end-member biological sources and their mixing contributions. Thus, the input mixing of plankton and terrigenous plants at different proportions should be responsible for the formation of various C 20 -C 21 -C 23 TT abundance patterns.

Environmental implication of C 20 -C 21 -C 23 TT abundance patterns
Depositional environments control the input proportions of plankton and terrigenous plants, which is proposed to control the C 20 -C 21 -C 23 TT abundance patterns in sedimentary rocks. The analysis of available C 20 -C 21 -C 23 TT data from known depositional environments clearly show that the C 20 -C 21 -C 23 TT abundance patterns in different depositional environments are distinct from each other, with C 20 <C 21 <C 23 TT in typical marine and saline lacustrine facies, C 20 <C 21 >C 23 TT in freshwater lacustrine facies, C 20 >C 21 >C 23 TT in river-lake transitional facies, and C 20 >C 21 <C 23 TT in marine-continental transitional facies. As illustrated in Figure 2, the C 23 /C 21 TT ratio not only increases progressively from freshwater lacustrine, to saline lacustrine, to marine facies, but also increases progressively from terrigenous source, to river-lake transitional, to marine-continental transitional facies. The C 23 /C 21 TT ratio appears to increase gradually with elevated salinity of depositional water. The increase in salinity usually indicates a decrease in fresh-water input and therefore a decrease in terrigenous input, which results in the C 23 /C 21 TT ratios approaching their source signatures of halophilic plankton. Accordingly, the boundary between brackish and freshwater lacustrine facies is expected to lie around a C 23 /C 21 TT ratio of 1.0 ( Figure 2). Furthermore, the C 21 /C 20 TT ratios in marine and lacustrine facies are obviously greater than those in transitional facies (Figure 2), suggesting that the C 21 /C 20 TT ratio seems to increase with elevated depths of depositional water. In general, the greater the depositional depth, the farther offshore, and the less terrigenous input. Thus, their C 21 /C 20 TT ratios approach the signatures of planktons (halophilic plankton or freshwater plankton).
Based on the good correspondences between C 20 -C 21 -C 23 TT abundance patterns and typical depositional environments, the crossplot of C 23 /C 21 TT vs. C 21 /C 20 TT in context of depositional environments (Figure 2) can be used as a discriminating diagram for environmental identification. C 23 /C 21 TT and C 21 /C 20 TT are expected to be the parameters to assess water salinity and depositional depth, respectively.

Validity of the C 20 -C 21 -C 23 TT biomarker method
The Ordos Basin undergone (1) an early Paleozoic shallow oceanicplatform stage and 2) a late Paleozoic offshore-plain stage during the Paleozoic (Li, 2004). During the Middle Ordovician, the Majiagou Formation was deposited within a semi-closed epicontinental sea environment (Li et al., 2018). From the Late Ordovician to the Early Carboniferous, the basin was uplifted by the Caledonian orogeny and underwent 130 million years of erosion (Yang et al., 2012;Xu et al., 2018). The following Hercynian orogeny caused the Late Paleozoic Ordos Basin subsidence. Large scale of transgression occurred from the east and west of the basin during the Benxi period and the seawater Cross-plot of C 20 -C 21 -C 23 tricyclic terpane (TT) ratios with two defined end-member domains of plankton and terrigenous plants. Almost all C 20 -C 21 -C 23 TT data in this study fall into a range from the terrigenous plants domain to the plankton domain.

FIGURE 6
Tectonic units and composite stratigraphic columnar section of the Ordos Basin (modified from Li et al., 2021). The samples were collected from the Yishan Slope with the ages ranging from the Ordovician to Permian.

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frontiersin.org connected together during the Late Taiyuan period. With the gradual regression during the Shanxi period, continental deposition began to dominate, resulting in the development of marine-continental transitional facies, delta facies and lacustrine facies (Sun, 2017). Thirteen core samples from the Ordovician-Permian in the Ordos Basin ( Figure 6; Table 2) and ten coal or carbonaceous mudstones from the Eocene in the East China Sea Basin (Table 3) were used to verify the validity of C 20 -C 21 -C 23 TT biomarker method.

Marine facies
The four samples including gypsum bearing mudstones, massive limestone and calcareous mudstone (Table 2) from the Majiagou Formation were all identified as marine facies (Figure 7), which is consistent with the present understanding of epicontinental sea environment (Li et al., 2018). Furthermore, the suggestion that the C 23 /C 21 TT ratio is a salinity indicator in this study is also evidenced by these samples. The Majiagou Formation is divided into six members, numbered from bottom to top as Ma1 to Ma6. The Ma1, Ma3 and Ma5 members were deposited with evaporite production during low sea level, while the Ma2, Ma4 and Ma6 members were deposited with carbonate production during high sea level (Xiao et al., 2019b). Thus, the depositional water salinity of Ma3 is higher than that of Ma4. It should be noted that the C 23 /C 21 TT ratios in Ma3 extract are indeed greater than those in Ma4 extract (Table 2; Figure 7).

Brackish-freshwater lacustrine facies
The depositional environments of the Permian Shanxi Formation in the southern Ordos basin have been determined by previous studies to be continental facies including delta and lacustrine facies (e.g., Wang et al., 2007;Sun, 2017;Li et al., 2021). Four dark mudstones from the Shanxi Formation in XY1 and XY2 wells (See locations in Figure 6) were extracted for environmental identification. As shown in Figure 7, the C 21 /C 20 TT and C 23 /C 21 TT ratios of these mudstones plot within the brackish-freshwater lacustrine facies, consistent with the regional depositional environments.

Transitional facies 4.3.3.1 Cordaitean fossil leaves
Cordaitean fossil leaves are known from early Carboniferous to early Permian deposits, representing the depositional environments including floodplains, river levees, coastal plains or swamp (Zodrow et al., 2000;Yang, 2007). The carbonaceous mudstone containing the Cordaitean fossil leaves ( Figure 8A; Table 2) was identified as riverlake transitional facies by our biomarker method (Figure 7). This is consistent with the depositional environments indicated by the Cordaitean fossil leaves.

Fuzulinid fossil
The Fuzulinid fossil with 1 mm~4 mm in size and limestone gravels ( Figures 8D, E) constitute a storm deposition which indicates a sub-tidal depositional environment. This sample was identified by the biomarker method to be deposited within a marine-continental transitional facies (Figure 7). As sub-tidal belongs to marine-continental transitional facies, the identification result of our biomarker method is thus correct.

Coal and carbonaceous mudstone
It is widely known that coal and carbonaceous mudstone are the indicators of shallow-water environment (e.g., swamp). The C 20 -C 21 -C 23 TT biomarker method not only identified the coal and carbonaceous mudstone as shallow-water environment (i.e., transitional environments defined by C 21 /C 20 TT<1.0) as expected, but further determined their specific depositional environments: marine-continental transitional or river-lake transitional environments ( Figure 9A).

Siderite and pyrite
Although siderite can occur in various depositional environments, layered nodular siderite without pyrite in mudstone ( Figures 8F, G) was considered to be deposited in delta front, where iron oxides and terrigenous organic matter transported by river water are condensed and precipitated in a large amount (Shen et al., 2017). The two siderite bearing mudstones were identified by the biomarker method to be deposited within a river-lake transitional facies (Figure 7). According to the sedimentary model of rock series containing siderite and pyrite created by Shen et al. (2017), the mudstone containing pyrite was expected to be deposited in lagoon or tidal flat. The pyrite bearing mudstone was identified by the biomarker method to be deposited within a marine-continental transitional facies (Figure 7). It is clear that these two identification results using the biomarker method are both correct.
Compared with siderite, pyrite was considered to be precipitated in a relatively deeper environment ( Figure 10). Notably, the C 23 / C 21 TT ratio of pyrite bearing mudstone is greater than those of siderite bearing mudstones (Table 2; Figure 7), indicating that the suggestion of C 23 /C 21 TT ratio as a depth indicator is reasonable. This suggestion was also evidenced by the negative correlation between pristane/ phytane (Pr/Ph) and C 21 /C 20 TT ratios ( Figure 9B). Pr/Ph ratios, which have been widely used to assess the redox of depositional environment, decrease with elevated anoxic conditions (Rashid, 1979). As shown in Figure 9B, the Pr/Ph ratios decrease along the C 21 /C 20 TT values. The increase in C 21 /C 20 TT ratios indicates an increase in depositional depths, which further indicates an increase in anoxic conditions.

FIGURE 7
Environmental identification for the samples from the Ordovician and Permian in the Ordos Basin.

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frontiersin.org 4.4 Applicability of the C 20 -C 21 -C 23 TT biomarker method The C 20 -C 21 -C 23 TT biomarker method only requires a small amount of rock extracts, depending on the lower limit of GC-MS analysis. At present, the rock extracts more than 10 μg is guaranteed to obtain high-quality m/z191 mass chromatogram. Take the mudstone samples from the Pingbei area for example, the TOC and S 1 +S 2 values of mudstone samples vary from 0.23% to 3.68% and from 0.20 mg/g to 6.03 mg/g (Table 3), which were classified as "poor" to "fair" level  Frontiers in Earth Science frontiersin.org source rocks. Although the chloroform asphalt 'A' extracted from the mudstones are as low as 0.01% (Table 3), the high-quality m/z 191 mass chromatogram with C 20 -C 21 -C 23 TT signature has been obtained from every mudstone sample (e.g., Figure 1). This indicates that our method is not necessarily restricted to hydrocarbon source rocks, but can be applied to a broad spectrum of rocks. TTs are characterized by higher thermal stability than hopanes and steroterpenes (Peters et al., 1990;Farrimond et al., 1999;Xiao et al., 2019a). Thermal evolution during maturation and high-maturation stages makes little effect on the abundance patterns of C 20 -C 21 -C 23 TT (Chen et al., 2017;Xiao et al., 2019b). As shown in Table 2, although the rocks have evolved into the maturities ranging from 1.6% to 2.4%Ro, C 20 -C 21 -C 23 TT biomarker method are still effective in environmental identification. Furthermore, C 20 -C 21 -C 23 TT distributions are stable even in the severely biodegraded oils with 25-norhopane series, showing strong resistance to biodegradation (Xiao et al., 2019a).

Case study: Environmental identification for a complex depositional system in the East China Sea Basin
To test the robustness of the C 20 -C 21 -C 23 TT biomarker method for environmental identification, it has been applied to a complex depositional system: the Pingbei area in the East China Sea Basin ( Figure 11).

Geological background of the Pingbei area
The Pingbei area was a hinged margin of a rift basin during the Eocene (Soreghan and Cohen, 1996). The petroleum system in the area is within the upper-middle Eocene Pinghu Formation, which consists of alternate sandstone, mudstone and thin coal seams. From bottom to top, the Pinghu Formation is further divided into the P1, P2, P3 and P4 members ( Figure 11A).
Due to the scarcity of wells and the lack of typical facies indicators in drill core, the depositional environments of the Pinghu Formation have been strongly debated among three possibilities: delta environment (e.g., Shen et al., 2016;Jiang et al., 2020), tidal flat environment (e.g., Zhao et al., 2008) and mixed delta and tidal flat environment (e.g., Zhang et al., 2017;Abbas et al., 2018).

Depositional environments of Pinghu Formation in the Pingbei area
The Ro values in the Pinghu Formation in the area vary from 0.51% to 0.87% (Table 3), indicating a low-mature to mature stage. The C 20 -C 21 -C 23 TT abundance patterns cannot be affected by this maturity range, and are thus available to identify depositional environments. The C 21 /C 20 TT and C 23 /C 21 TT ratios were plotted in the discriminating diagram to identify the depositional environment of each member in the Pinghu Formation ( Figure 12). Paleogeomorphologic map ( Figure 11B) and seismic inversion data  are also reported here as additional evidence to (1) support the environmental identification by the C 20 -C 21 -C 23 TT biomarker method, and (2) help characterize the spatial distribution of depositional environment ( Figures 12B, D, F, H).

P1 member
The depositional environments of P1 Member in zones A, B and C (see locations in Figure 11B) are identified as river-lake transitional, freshwater lacustrine and saline-lacustrine/marine facies ( Figure 12A), respectively. Organic matter in the carbonaceous mudstone (Table 3) in Zone D ( Figure 11B) was originated from a terrestrial source through river transportation ( Figure 12A). Based on the environmental identification and paleogeomorphologic map of this period ( Figure 11B), the depositional environment in Zone D is expected to be marine facies.
Notably, the spatial distribution of these facies is consistent with an environmental transition from continental facies in the FIGURE 10 Schematic diagram of the facies change of sedimentary iron ore deposits (modified from Qiu, 1987).

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frontiersin.org north to marine facies in the south ( Figure 12B). The Baoyun High formed in the rifting stage ( Figures 11A, B) probably isolated the northern freshwater deposition and southern saline water deposition. The interpretation of a freshwater lake in Zone B is acceptable because Zone B was located at the Wuyun Subsag during the P1 period ( Figure 11B).

P2 member
The depositional environments of P2 Member are identified as a continental depositional system including freshwater lacustrine facies in the subsag and river-lake transitional facies around the subsag ( Figures 12C, D). This environmental identification is supported by the evidence from seismic sedimentology: a large-scale fluvial-induced delta with a bird-foot shaped distribution of sand bodies occurred in the study area ( Figure 11D).

P3 member
The spatial distribution and sand body shapes revealed by seismic inversion indicate a wave-altered delta in the north and a speculative delta in the south (Figure 11E), suggesting a transitional facies dominated in the area. For comparison, the C 20 -C 21 -C 23 TT biomarker method also identified a transitional facies, but provides much more detailed depositional information: river-lake transitional facies in Zone A, marine facies in Zone B, and marine-continental transitional facies in C and D zones ( Figures 12E, F). Frontiers in Earth Science frontiersin.org

P4 member
The depositional environment of P4 Member in Zone A is identified as marine-continental transitional facies ( Figure 12G), which is consistent with the interpretation of wave-altered deltas by seismic inversion ( Figure 11F). The depositional environments in B and C zones are identified as marine/saline lacustrine and marine-continental transitional facies ( Figure 12G). These environments indicate that the Pingbei area was dominated by saline-water deposition during the P4 period ( Figure 12H).

Environment evolution of Pinghu Formation
Based on the above depositional environments identified by the C 20 -C 21 -C 23 TTs, the environmental evolution process of the Pinghu Formation was reconstructed. The P1 Member in the area is characterized by a coexistence of continental system in the north and marine system in the south which were isolated by the Baoyun High ( Figure 12B). During the P2 period, freshwater deposition range obviously expanded and the area was dominated by a continental depositional system ( Figure 12D), suggesting a

FIGURE 13
The profile of C 23 /C 21 TT ratios along depths for the rock samples in the B2 (A), C1 (B) and A1 (C) wells. The C 23 /C 21 TT ratios in these three wells vary synchronously along stratum.

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frontiersin.org regression has occurred since the P1 period. This regression can be characterized by a synchronous decrease of C 23 /C 21 TT values from the P1 to P2 members in B1 and C1 wells ( Figures 13A, B). Subsequently, the depositional environments in the area gradually evolved into a marine depositional system ( Figures  12F, H), suggesting a transgression occurred during the P3 and P4 periods in the area. This transgression was also characterized by a synchronous increase of C 23 /C 21 TT values from the P2 to P4 members ( Figure 13). In summary, the depositional environments of the Pinghu Formation in the Pingbei area were controlled by a complex marine-continental transitional system to a large extent. The C 20 -C 21 -C 23 TT biomarker method not only identified the depositional environments supported by evidence from seismic sedimentology, paleogeomorphology and other conventional methods (e.g., Shen et al., 2016), but also provided much more depositional details. Take the freshwater lacustrine facies hidden in the marine-continental transitional environment for example, it has not been recognized by previous studies but was easily identified by our method. Thus, the C 20 -C 21 -C 23 TT abundance patterns are robust for environmental identification even for the complex systems strongly affected by eustasy, which generally resulted in coexistence of and/or fast transition between diverse depositional environments.

Conclusion
Based on the analysis of a large quantity of published data and this study from a range of depositional environments worldwide, we propose that the relative abundance of C 20 -C 21 -C 23 TTs in sedimentary rocks and oils are controlled by the relative contribution of plankton and terrigenous plants. The C 20 -C 21 -C 23 TT abundance patterns in marine and saline lacustrine, freshwater lacustrine, shallow-water terrestrial, and marinecontinental transitional facies are very distinct, characterized by C 20 <C 21 <C 23 TT, C 20 <C 21 >C 23 TT, C 20 >C 21 >C 23 TT and C 20 >C 21 <C 23 TT, respectively. The C 23 /C 21 TT ratio increases with increasing salinity of depositional water, while the C 21 / C 20 TT ratio increases with increasing depth of depositional water.
A discrimination diagram has been developed in this study for environmental identification. The C 20 -C 21 -C 23 TT ratios can not only identify depositional environments, but also restore the environmental evolution through the analysis of salinity and depth variation. The effectiveness, applicability and robustness of this C 20 -C 21 -C 23 TT method have been demonstrated by the samples with typical environmental indicators and a case study in a complex depositional system in the East China Sea Basin.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.