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Article

Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment

by
Ziming Wang
,
Hongfei Cheng
* and
Yang Wang
*
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(6), 656; https://doi.org/10.3390/min15060656
Submission received: 29 April 2025 / Revised: 13 June 2025 / Accepted: 17 June 2025 / Published: 18 June 2025
(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)
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Abstract

:
The lacustrine mudstones and shales of the Triassic Yanchang Formation in the Ordos Basin serve as critical hydrocarbon source rocks. However, previous studies predominantly focus on individual lithologies, with comparative investigations into the sedimentary environments of dark mudstones and black shales remaining relatively limited. The study systematically compares sedimentary environment parameters (e.g., paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity characteristics) between mudstones and shales, and how these distinct environmental factors governed the differential enrichment mechanisms of organic matter within the depositional aquatic system has been elucidated. Geochemical proxies (e.g., CIA, Sr/Cu, Rb/Sr, Sr/Ba, V/Ni, U/Th, V/Cr, Rb/Zr, P/Ti, Cu/Ti) reveal marked contrasts: In comparison with the Chang 7 and Chang 8 dark mudstones, the Chang 7 black shales exhibit (1) warmer–humid paleoclimatic regimes, (2) higher paleosalinity, (3) intensely anoxic conditions, (4) deeper paleowater depth, and (5) elevated paleoproductivity. These environmental divergences directly govern the significant total organic carbon content disparity between black shales and dark mudstones. Organic enrichment in the Chang 7 dark mudstones and black shales is primarily controlled by paleoproductivity and paleoredox conditions, with secondary influences from paleoclimate and paleowater depth. Based on the above studies, this research established a differential organic matter enrichment model. This research is of significant importance for guiding oil and gas exploration and development in the Ordos Basin.

1. Introduction

Fine-grained sedimentary rocks, composed mainly of minerals such as clay and silt, are defined as rocks with a particle size smaller than 0.0625 mm and a particle content greater than 50% [1,2]. These rocks are mainly mudstones and shales, which account for approximately two-thirds of the stratigraphic record and are the most common rock types [3]. Generally, mudstones are fine-grained rocks with poorly developed lamination, whereas shales are fine-grained rocks with well-developed lamination. Most of the high-quality source rocks discovered so far are fine-grained sedimentary rocks [4,5]. With the development of continuous hydrocarbon exploration, the study of organic matter enrichment in fine-grained shales and mudstones, a complex geological process dominated by factors such as paleoenvironment and paleoclimate, has become increasingly important [6,7,8]. Consequently, reconstructing and restoring the paleoenvironment and paleoclimate not only provides a deeper understanding of the organic matter enrichment process in fine-grained shales and mudstones but also offers a scientific basis for subsequent oil and gas exploration and development.
Geochemical methods, such as major and trace elements, have become crucial tools for interpreting paleoenvironment and paleoclimate conditions [9]. For instance, the chemical index of alteration (CIA), the SiO2/Al2O3 ratios of major elements, and the Sr/Cu and Rb/Sr ratios of trace elements can be used for paleoclimatic reconstructions [10,11]. Paleosalinity can be inferred from the concentrations of elements such as B, Sr, Ba, Rb, and K [12]. Paleoredox conditions can be assessed using a range of redox-sensitive elements, including U, V, and Cr, and also through the analysis of rare earth element anomalies [13]. Elements that are prone to differentiation during sedimentary processes, such as Rb, can provide insights into paleowater depth variations [14]. Additionally, paleoproductivity levels can be judged using biological Ba, as well as elemental ratios like P/Ti and Cu/Ti [15,16]. In summary, geochemical methods play a crucial role in reconstructing paleoenvironment and paleoclimate. However, the applicability of these indicators varies depending on the water body environment and basin type. Therefore, although there are many methods, their applicability and differences in lacustrine basins need further research.
The organic-rich black shale and dark mudstone of the Chang 7 Member in the Triassic Yanchang Formation are the most widely distributed and important set of source rocks within the basin [17]. Recent studies show that the Chang 8 dark mudstone can also serve as effective hydrocarbon source rocks [18,19]. Previous studies have extensively investigated the paleoenvironment and organic matter enrichment mechanisms in this interval. Specifically, for the dark mudstone in Chang 7 Member, Li et al. used biomarkers and major and trace elements to conclude that the paleoclimate was cold and dry [20], while other studies suggested that the paleoclimate was warm and humid [21,22]. For the black shale in Chang 7 Member, Qiao et al. conducted biomarker research and found that the primary organic matter source came from coniferous trees and other terrestrial higher plants [23]. In contrast, Zhang et al. investigated the impact of volcanic activity on the paleoclimate and paleoenvironment, suggesting that the extreme high-temperature anoxic environments induced by volcanic activity were conducive to organic matter accumulation [24]. However, there are relatively few comparative studies on the paleoenvironment and paleoclimate of dark mudstone and black shale at this layer, and the relevant research on the Chang 8 dark mudstone is also relatively scarce. Most scholars have only focused on the research of the Chang 7 black shale, resulting in a significant lack of understanding of the paleoenvironment and organic matter enrichment of the dark mudstone from Chang 7 and Chang 8 Members [15,25]. Some scholars have generally classified both as a “mud shale” system for research, which may lead to biased conclusions. Therefore, it is necessary to conduct comparative studies on the dark mudstone from the Chang 7 and Chang 8 Members and the Chang 7 black shale to reveal the differences in sedimentary environments and organic matter enrichment patterns, providing theoretical support for differentializing exploration.
This contribution analyzes the total organic carbon (TOC) contents and geochemical characteristics of the Chang 7 dark mudstone and black shale and the Chang 8 dark mudstone samples. The main objectives are to (1) analyze the sedimentary environments of the Chang 7 dark mudstone and black shale and the Chang 8 dark mudstone, (2) conduct comparative analysis of the paleoenvironmental differences between the Chang 8 dark mudstone, Chang 7 dark mudstone, and Chang 7 black shale, and (3) analyze the main controlling factors of organic matter enrichment. In this way, the gap in the comparative study of mudstone and shale sedimentary environments in this region can be addressed, providing valuable guidance for oil and gas exploration and development.

2. Geological Background

The Ordos Basin, situated upon the Paleozoic craton platform of North China, is a large Meso–Cenozoic intracontinental superimposed basin, covering an area of approximately 37 × 104 km2. The basin contains six secondary tectonic units, namely the Yimeng Uplift, Shaanbei Slope, Tianhuan Depression, Western Thrust-fault Zone, Jinxi Fault-fold Zone, and Weibei Uplift. From the edge of the basin to the interior of the basin, the structural morphology changes from complex to simple (Figure 1a). The present landform has been shaped by multi-stage tectonic cycles, such as Luliang, Jinning, Caledonian, Hercynian, Indosinian, Yanshan and Himalayan. Therefore, it has experienced various sedimentary evolution stages and has retained the sedimentary stratum since the Proterozoic, among which the Yanchang Formation of the Triassic system is widely regarded as one of the crucial source rocks for hydrocarbon resources in the Ordos Basin [26].
The Yanchang Formation can be divided into 10 sub-members, from Chang 10 to Chang 1, from the bottom to the top accordingly, which represents the entire lacustrine basin evolution (Figure 1b). Throughout the evolutionary history of the basin, the Chang 8 Member was deposited during the initial expansion phase of the lacustrine basin, while the overlying Chang 7 Member formed during the peak developmental stage of the lake system, characterized by the accumulation of extensive thick black shales and dark mudstones [28]. The rocks from the Chang 7 Member are regarded as the most crucial source rocks in the Ordos Basin. This member can be further divided into Chang 73, Chang 72 and Chang 71 [22]. Thick black shales and dark mudstones of the Yanchang Formation are widely distributed in the Ordos Basin. Generally speaking, the dark mudstones were mainly deposited in shallow to semi-deep lakes, while the black shales represent the deep lake facies [29].

3. Methodology

A total of fifteen core samples, including eight black shale samples and seven dark mudstone samples with a burial depth from 1678.9 to 1782.2 m from Well A, were collected from the Triassic Yanchang Formation, southwest Ordos Basin (Figure 1a). Within this collection, all eight black shale samples are derived exclusively from the Chang 7 Member, while the seven dark mudstone specimens comprise strata from both the Chang 7 (n = 4) and Chang 8 Members (n = 3). These fifteen core samples, labeled N1 to N15 from top to bottom, were tested for TOC content, as well as major and trace elements. The dark mudstone samples exhibit a gray-black color, homogeneous fine-grained texture, and massive structure with poorly developed lamination (Figure 2a,b). The black shale samples are gray in color, occur in horizontal layers, have well-developed fissility, and display horizontal lamination with visible pyrite (Figure 2c,d).
The sample analysis was conducted at the Laboratory of Mineralization and Dynamics, Chang’an University, Xi’an, China. TOC content analysis was performed using a LECO C-S analyzer. Prior to analysis, crushed samples (passing through 200 mesh) underwent decarbonation treatment with a 1:7 (v/v) HCl/H2O solution to eliminate carbonate minerals. The X-ray fluorescence (XRF) spectrometry was utilized for major element quantification. The fusion process incorporated 0.5 g aliquots of powdered samples (about 200 mesh) with Li2B4O7 flux (5.0 g) and NH4NO3 crucible protectant (0.3 g). The relative error for major oxide measurements was maintained below 2%. The loss on ignition (LOI) measurements were acquired through controlled calcination at 1100 °C for 60 min in muffle furnaces. Trace element concentration was tested by using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Sample digestion protocols followed optimized high-pressure acid decomposition methodologies adapted from Tan et al. [30], with all dissolution procedures conducted in Class 1000 cleanroom environments. Method validation using certified reference materials demonstrated analytical reproducibility with relative standard deviations typically below 10%.

4. Results

4.1. Total Organic Carbon Content

Organic matter abundance is the fundamental indicator for evaluating source rocks, and the TOC content is an important parameter for assessing it [7,21]. Although the TOC of the samples will be lost during the thermal maturation process, if they have undergone similar thermal histories, this loss will not significantly affect the relative values between the samples. In this study, the TOC contents of the Chang 7 dark mudstone and black shale samples range from 0.5 wt.% to 6.2 wt.% (average of 2.1 wt.%) and 4.2% to 24.7% (average of 11.9 wt.%), respectively (Table 1). The TOC contents of the Chang 8 dark mudstone range from 1.9 wt.% to 2.8 wt.%, with an average of 2.3 wt.% (Table 1). It is worth noting that the TOC value of the Chang 7 dark mudstone sample N-1 exceeds 5%, indicating that it has a good ability to preserve organic matter. According to the quality evaluation table for source rocks [31], the dark mudstone of the Chang 7 and Chang 8 Members can be classified as fair-to-good source rocks, while the black shale is considered high-quality source rock and can be referred to as organic-rich shale.

4.2. Elemental Geochemistry

4.2.1. Major Elements

According to the core test results of Well A in the study area, the dark mudstone of the Chang 7 Member is mainly composed of SiO2, Al2O3, and TFe2O3 (Figure 1; Table 1). The content of SiO2 ranges from 42.93% to 57.81% (an average of 42.97%), followed by Al2O3 of 15.75%–18.84% (an average of 17.28%), and TFe2O3 of 5.06%–14.17% (an average of 7.68%). For the black shale samples from the Chang 7 Member, the SiO2 content ranges from 18.29% to 59.30%, with an average of 43.24%. Al2O3 content ranks second, ranging from 4.26% to 20.40% (an average of 12.29%), while TFe2O3 content ranks third, varying between 2.57% and 18.74% (an average of 10.65%) (Table 1). The dark mudstone of the Chang 8 Member is mainly composed of SiO2, Al2O3, and TFe2O3 (Table 1). The SiO2 content ranges from 55.39% to 60.30% (an average of 57.83%), followed by Al2O3 of 16.19%–18.38% (an average of 17.42%), and TFe2O3 of 6.18%–9.56% (an average of 7.32%). The major element values of the N-1 sample are close to those of shale. From the values of SiO2, Al2O3, and others, it can be seen that the sample is not significantly influenced by terrestrial detritus and exhibits environmental differences compared to other mudstone samples. Compared to dark mudstones, black shales exhibit lower Al2O3 and higher TFe2O3 contents, indicating distinct depositional environments.

4.2.2. Trace Elements

Table 2 shows the trace element results for 15 studied samples. In general, trace element concentrations are different for dark mudstone and black shale. The Chang 7 and Chang 8 dark mudstone units (stratigraphically grouped as dark mudstone members) exhibit elevated concentrations of Cr, Co, Ni, Rb, Sr, and Ba, contrasting with significantly depleted levels of V, Mo, Pb, and U relative to the organic-rich Chang 7 black shale beds. The systematic variation in geochemical behavior of these elements (e.g., redox-sensitive U/Th ratios and V/Cr ratios) allows for clear differentiation between black shale lithofacies and their corresponding paleo-depositional environments. Similarly, the N-1 sample is also highly distinctive in terms of trace elements compared to other mudstone samples (e.g., Rb/Zr and U/Th). Overall, dark mudstones are characterized by higher concentrations of transition metals (such as Cr and Ni) and alkali/alkaline earth elements (e.g., Rb, Sr, and Ba), while black shales show significant enrichment in redox-sensitive elements (such as V, Mo, and U) and organic-associated metals (e.g., Pb and Cu). These results provide a foundation for further interpretation of depositional environments of the Yanchang Formation.

4.2.3. Rare Earth Elements

The abundances of rare earth elements and related parameters are presented in Table 3. The REE characteristics of the Chang 7 and Chang 8 mudstones and shales in the study area show minimal variation (Table 3; Figure 3). The total REE (∑REE) of the mudstone samples from these members ranges between 134.0 μg/g and 284.4 μg/g, with an average of 218.1. For the shale samples, ∑REEs vary from 78.5 μg/g to 261.6 μg/g, averaging 186.1 μg/g, which is higher than the UCC (146.0 μg/g) [32]. The chondrite-normalized REE distribution patterns of the samples are relatively consistent, showing a right-leaning distribution, which indicates a consistent source of REEs. The light rare earth elements (LREE) to heavy rare earth elements (HREE) concentration ratio (LREE/HREE) ranges from 6.2 to 9.5, with an average of 8.1. The (La/Yb)N values range from 5.2 to 11.8, with an average of 8.8, indicating enrichment of LREEs. The δCe values range from 0.9 to 1.2 (average 1.0), showing no anomalies, whereas the δEu values range from 0.4 to 0.7 (average 0.6), indicating a significant negative Eu anomaly.

5. Discussion

5.1. Sedimentary Environments Differential Between Shale and Mudstone

5.1.1. Paleoclimate

The paleoclimatic regime exerts fundamental controls on sedimentary processes through its regulation of weathering intensity, detrital flux, and biogeochemical cycles. In sedimentary environment analysis and paleogeographic reconstruction, paleoclimate serves as a critical boundary condition. Geochemical proxies, including major element (CIA) and trace element (elemental ratio Sr/Cu, Rb/Sr), have been widely employed as robust indicators for paleoclimate characterization [34,35].
CIA is commonly used to assess the degree of chemical weathering in the provenance area, with implications for paleoclimate reconstruction [10,11,36]. CIA is calculated as follows:
CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100,
where all variables represent the molar amounts of major-element oxides, and CaO* represents the fraction of CaO in silicate minerals [10]. Previous studies have shown that when the CIA is less than or equal to 50, it indicates that the parent rock has experienced little to no weathering. A CIA value between 50 and 65 suggests weak weathering under cold and arid conditions. A CIA value between 65 and 85 reflects moderate weathering under warm and moist conditions, while a value between 85 and 100 indicates strong weathering under hot and humid conditions [37]. The CIA values of all 15 samples fall within the range of 65.36 to 82.92 (Table 2). Specifically, the mean CIA value for the Chang 7 dark mudstone is 73.93, for the Chang 7 black shale is 75.90, and for the Chang 8 dark mudstone is 75.07 (Figure 4a and Figure 5). The CIA values suggest that the study area experienced warm and humid climatic conditions during the deposition of the Chang 7 and Chang 8 Members.
Furthermore, Sr is an element that typically favors dry conditions, while Cu is more abundant in wet conditions. Therefore, the Sr/Cu ratio can be used to distinguish different paleoclimate conditions [38]. A low Sr/Cu ratio (<5.0) indicates hot and humid climate conditions, and a moderate ratio (ranging from 5.0 to 10.0) suggests warm and moist climate conditions, while a high Sr/Cu ratio (>10.0) indicates cold and arid climate conditions [21]. The Sr/Cu ratios of these studied samples are relatively similar, most falling below 5.0 (Figure 5). The average Sr/Cu ratio of the Chang 7 mudstones and shales are 3.07 and 2.35, respectively. Comparatively, the Chang 8 mudstone is 4.14, which is higher than the average ratio of the Chang 7 samples (Figure 5). From the Sr/Cu ratio and Figure 4a, it can be inferred that the paleoclimate during the deposition of the Chang 7 black shale was warmer and more humid compared to that during the deposition of the dark mudstone of the Chang 7 and Chang 8 Members. Compared with the dark mudstone of Chang 7 Member, the paleoclimate during the dark mudstone deposition of Chang 8 Member appears to be somewhat dry and cold.
Additionally, the Rb/Sr ratio can also be used for constraining paleoclimate [39]. During the weathering process, Rb remains relatively stable, while Sr is water-soluble and is leached out under humid conditions [40]. As a result, a high Rb/Sr ratio indicates a warm and humid climate, whereas a low ratio suggests a cold and arid climate [41]. The Rb/Sr ratios of the Chang 7 dark mudstone, Chang 7 black shale, and Chang 8 dark mudstone are 0.61–0.85 (avg. 0.75), 0.18–1.74 (avg. 0.82), and 0.22–0.38 (avg. 0.30), respectively (Figure 4b and Figure 5). The Rb/Sr ratios exhibit distinct stratigraphic variations, with maximum values recorded in the Chang 7 shale and minimum values in the Chang 8 mudstone (Figure 5). These Rb/Sr ratios indicate that the climate of the Chang 7 shales was the most humid, followed by the Chang 7 mudstones, and finally the Chang 8 mudstones. This is consistent with the results derived from the Sr/Cu ratio analysis.
Based on the geochemistry of the elements, the variations in paleoclimate proxies, and crossplots of these ratios (Figure 4 and Figure 5), it was comprehensively concluded that all the samples are overall in warm and moist climate conditions. The paleoclimate during the deposition of the black shale was warmer and more humid than that of the dark mudstone.

5.1.2. Paleosalinity

Paleosalinity reflects past water salinity preserved in sedimentary deposits and serves as a vital indicator for analyzing sedimentary environment characteristics. It plays a crucial role in paleoenvironmental and paleoecological research. Sr has a relatively high concentration in seawater and is minimally influenced by biological activities. Its high solubility in seawater allows it to become increasingly enriched as salinity rises. In contrast, Ba concentrations are higher in freshwater environments due to particle adsorption [42]. As salinity increases, Ba tends to precipitate as BaSO4 through interaction with sulfate ions, resulting in a significant reduction in the concentration of Ba in body water [43]. Moreover, SrSO4 precipitates only at significantly higher salinities. In low-salinity freshwater settings, both Ba and Sr remain predominantly dissolved. As salinity rises and Ba begins to precipitate, the Sr/Ba ratio in sediments gradually increases. This relationship makes the Sr/Ba ratio a reliable paleosalinity indicator in sedimentary environments [44]. Commonly, an Sr/Ba ratio below 0.2 indicates a freshwater environment, a ratio between 0.2 and 0.5 suggests a brackish environment, and a ratio above 0.5 points to a saline water environment [12]. The Sr/Ba ratio of the Chang 7 dark mudstone ranges from 0.21 to 0.24, with an average of 0.24, while the ratio for the Chang 7 black shale ranges from 0.21 to 0.87, with an average of 0.39 (Figure 5). In comparison, the Chang 8 dark mudstone displays moderately higher Sr/Ba ratios (0.33–0.45; mean: 0.38), yet remains significantly lower than those of the Chang 7 black shales (Figure 5). The curve exhibits a low-high-low trend (Figure 5). The data indicate that during the Chang 7 and Chang 8 Member sedimentary periods, although some sediments were deposited in saline water environments, the overall water body was under brackish water conditions, representing a terrestrial aquatic environment (Figure 6). Moreover, during the sedimentary period of the Chang 7 Member, the salinity of the black shales was higher than that of the dark mudstones. Additionally, the salinity of the water body during the deposition of the Chang 8 dark mudstone was higher than that of the dark mudstone of the Chang 7 Member.

5.1.3. Paleoredox Conditions

Elements sensitive to redox conditions are more likely to dissolve in an oxidizing environment and tend to migrate and undergo self-enrichment in minerals within a reducing environment [45]. This characteristic makes elements such as U, V, Mo and Cr reliable indicators of redox conditions [13]. Furthermore, certain metallic elements, such as Ni, Cd, and Zn, tend to form precipitates under anoxic conditions, which differ from their dissolution under aerobic conditions [46]. Therefore, the ratios of these elements can reflect the redox conditions of the water body. In this study, V/Ni, U/Th, and V/Cr are primarily used as redox indicators for the water body.
Under anoxic conditions, V tends to combine with organic matter (such as porphyrin complexes) and accumulates in sulfidic environments. In contrast, Ni is more stable in oxidizing conditions. However, due to its susceptibility to edge debris and microorganisms, Ni is less sensitive to redox conditions than V [9,44]. In an oxidizing environment, the enrichment of V is restricted. Ni may be relatively enriched in such conditions because of its adsorption onto various surfaces. This results in a decrease in the V/Ni ratio, as the enrichment of Ni outpaces that of V. Conversely, in an anoxic (reduced) environment, V tends to be more enriched due to reduction processes, which facilitate its accumulation. In contrast, the enrichment of Ni is relatively lower in an anoxic environment, resulting in an increase in the V/Ni ratio, as V becomes more concentrated relative to Ni [47]. Therefore, V/Ni is a good indicator of redox conditions. A V/Ni ratio exceeding 3 indicates that organic matter was deposited in an anoxic environment, whereas a ratio between 1.9 and 3 suggests deposition under dysoxic to oxic conditions [48,49]. As documented in Table 2, the V/Ni ratios of the Chang 7 dark mudstone range from 1.66 to 3.21 (average = 2.48), whereas coeval black shales show markedly elevated values (average = 5.18; range: 3.58–8.29). For the dark mudstone of the Chang 8 Member, the V/Ni ratios range from 2.04 to 2.36 (average = 2.20), slightly lower than its Chang 7 counterpart (Figure 5). Based on this, it can be concluded that the redox environments of the Chang 7 dark mudstone and the Chang 8 dark mudstone are generally similar, both reflecting dysoxic-oxic conditions. However, the Chang 7 black shale is characterized by a strongly reducing environment. Therefore, there is a significant difference in the redox conditions during the formation of dark mudstone and black shale.
As mentioned earlier, V is easily soluble in oxidized water bodies, where it forms stable vanadates. However, in anoxic water bodies, it tends to combine with organic matter or form oxide precipitates [50]. Cr is commonly found in the detrital components of sediments. Under reducing conditions, it is reduced to Cr3+ and is easily adsorbed by clay minerals [51]. Therefore, the V/Cr ratio can be used to determine the paleoredox environment [52]. According to previous studies, a V/Cr ratio of less than 2 indicates an oxic condition; a ratio between 2 and 4.25 indicates a dysoxic condition; and a ratio greater than 4.25 suggests an anoxic environment [53,54]. The V/Cr ratios of the Chang 7 dark mudstone range from 1.43 to 1.90 (with an average of 1.58; Figure 7). In contrast, the V/Cr ratios of the Chang 7 black shale range from 2.61 to 12.95 (average = 6.38; Figure 7). The Chang 8 dark mudstone shares comparable V/Cr characteristics with the Chang 7 mudstone, ranging from 1.22 to 1.51 (with an average of 1.35; Figure 7). From this, it can be concluded that the Chang 7 black shale was deposited in a strongly reducing environment, while the redox environments of the Chang 7 and Chang 8 mudstones are generally similar, both reflecting dysoxic-oxic conditions.
Apart from the above two methods, the U/Th ratio can also reflect the paleoredox environment. The chemical form of U varies significantly with the redox conditions. In an oxidizing environment, U exists in the form of U6+, forming soluble substances (such as UO22+), which can be easily transported and migrated by water bodies. However, in a reducing environment, U is easily reduced to insoluble U4+, facilitating the enrichment of U in sediments [47,55]. The chemical property of Th is generally more stable than those of U. Th’s solubility is less influenced by the redox environment, typically existing in a stable Th4+ form. During the sedimentation process, it is deposited together with detrital minerals. In oxic water bodies, dissolved U6+ can persist for extended periods, and a large amount of U is lost. In contrast, Th4+ is quickly adsorbed by particulate matter and deposits quickly. This results in the loss of U in an oxic condition, leading to a low U/Th ratio. Conversely, under anoxic conditions, U4+ precipitates early, resulting in a marked increase in the U/Th ratio. In fact, some scholars have discovered that when the U/Th ratio is less than 0.75, it indicates oxic conditions; when the U/Th ratio ranges from 0.75 to 1.25, it suggests dysoxic conditions; while when the U/Th ratio is greater than 1.25, it indicates anoxic conditions [56,57]. The U/Th ratios of the Chang 7 dark mudstone range from 0.19 to 1.99, with an average of 0.66 (Figure 5). The U/Th ratios of the Chang 7 black shale range from 0.41 to 14.21 (average = 5.50; Figure 5). The Chang 8 dark mudstone displays stabilized U/Th ratios (ranging from 0.24 to 0.38; average = 0.29), systematically lower than those in the Chang 7 mudstone. Based on the U/Th ratio, the ratio of the dark mudstone sample N-1 from the Chang 7 Member is 1.99 (Table 2). Combined with Figure 7, it can be determined that sample N-1 was formed in an anoxic environment. As this sample was formed in an anoxic environment, which is conducive to the preservation of organic matter, its TOC value is relatively high. Therefore, during the deposition of the Chang 7 and Chang 8 dark mudstones, the redox environment of the water body was mainly oxic, whereas the redox environment during the deposition of the Chang 7 black shale was characterized as anoxic.
In general, the trends of the U/Th, V/Ni, and V/Cr curves are generally consistent, all showing significant high values in the Chang 7 shale, demonstrating a strong correlation (Figure 5). Based on the elemental characteristics and the elemental crossplots (Figure 7), it can be concluded that dark mudstones are predominantly formed in oxic conditions, with the exception of Chang 7 black shales, which are mostly found in anoxic conditions. This observation further supports the idea that the black shale formed under anoxic conditions, while the dark mudstone primarily developed in dysoxic-oxic conditions.

5.1.4. Paleowater Depth

In general, determining ancient water depth is an important step in reconstructing paleoenvironments, as it can provide indirect insights into the hydrodynamic and redox conditions of the sedimentary environment. Typically, due to the influence of aquatic organisms, light intensity diminishes as water depth increases. As a result, deeper water is more favorable for the preservation of organic matter, which in turn leads to the formation of distinct rock types and varying organic matter content. Zr exhibits relatively stable chemical properties and primarily precipitates as heavy minerals in shallow water environments. In contrast, Rb has more active chemical properties and is more likely to precipitate in deeper water bodies [58]. During the sedimentation process, Zr tends to accumulate in coarse-grained form, while Rb accumulates in fine-grained form. Therefore, the Rb/Zr ratio can be used to infer the paleowater depth [14,59]. Higher ratios indicate deeper water and weaker water flow intensity, while lower ratios suggest shallower water and stronger water flow intensity [58]. According to Table 2, the Rb/Zr ratios of the Chang 7 dark mudstone range from 0.55 to 1.10 (with an average of 0.75), while those of the Chang 7 black shale exhibit a wider variation between 0.12 and 1.50 (mean value of 1.08). Comparatively, the Chang 8 dark mudstone exhibits systematically depressed Rb/Zr ratios (range from 0.40 to 0.72; average = 0.55) relative to both Chang 7 lithologies. The Rb/Zr curve shows a distinct peak in the Chang 7 shale (Figure 5). From these, it can be inferred that compared with dark mudstone from the Chang 7 and Chang 8 Members, the Chang 7 black shale was in a relatively deeper water environment when deposited. However, as shown in Table 2, the Rb/Zr ratio of sample N-1 is 1.10, significantly higher than that of the other dark mudstone samples, indicating a relatively greater water depth. This conclusion further supports the interpretation in Section 5.1.3 that the depositional environment of this sample was anoxic. It can be seen that from the Chang 8 to the Chang 7 sedimentary period, the water body depth in the study area first increased and then decreased. Due to the shallower water depth during mudstone deposition, the material composition of the mudstone is more susceptible to the influence of terrigenous clastics.
From Figure 2, the mudstone of the Chang 7 Member shows no obvious layering, and the sedimentation process was slightly disturbed (Figure 2a,b). In contrast, the shale samples from the Chang 7 Member exhibit well-developed horizontal layering and visible pyrite, suggesting a deep-water depositional environment (Figure 2c,d). These observations are consistent with the values of the Rb/Zr indicator.

5.1.5. Paleoproductivity

Paleoproductivity refers to the rate at which organisms fix energy during the energy cycle; that is, the amount of organic matter produced per unit of time and area [6]. It has significant implications for organic matter accumulation and the formation of hydrocarbon-rich source rocks. Generally, the organic matter in dark mudstone and black shale come primarily from plankton, bacteria, and a small amount of terrigenous detritus, and the magnitude of the primary productivity of lakes is mainly influenced by these factors as well [7,15]. Due to the relative stability of certain nutrient elements (like P, Ba, Cu, Zn) during thermal evolution, which retain original information, they can be used to assess the magnitude of paleoproductivity.
P is a crucial nutrient element for the growth and development of plankton in water bodies [60]. The concentration of nutrients is directly correlated with biological productivity; thus, P is considered a key determinant of paleoproductivity. Ti is primarily found in marginal minerals and exhibits stable chemical properties. It can also serve to mitigate the dilution effects of marginal substances [61]. Consequently, the P/Ti ratio can be effectively utilized as a proxy for assessing paleoproductivity. The P/Ti ratios of the Chang 7 black shale range from 0.19 to 2.78 (average = 0.79; Figure 8), significantly higher than those for Post-Archean Australian Shale (PAAS) (average = 0.13; McLennan, 2018) [62], indicating relatively high paleoproductivity. For the Chang 7 and Chang 8 dark mudstone, the ratios range from 0.13 to 0.57 (average = 0.22) and 0.14 to 0.49 (average = 0.27), suggesting relatively low paleoproductivity (Figure 5; Figure 8). The relatively high average ratio suggests that the black shale of Chang 7 Member likely experienced higher primary productivity during its deposition. Compared with the black shale, the P/Ti ratio of the dark mudstone from the Chang 7 and Chang 8 Members is lower, indicating that the paleoproductivity during the deposition of dark mudstone was relatively reduced.
When paleoproductivity was high, the adsorption of Cu by plankton increased, leading to the enrichment of authigenic Cu in the sediments. Cu can also serve as a reliable indicator of paleoproductivity after excluding the influence of terrigenous detritus [63]. Generally, as the Cu/Ti ratio increases, the paleoproductivity becomes higher [15]. The Cu/Ti ratios of the Chang 7 black shale range from 0.009 to 0.225 (with an average of 0.085; Table 2; Figure 5). The Cu/Ti ratios of the Chang 7 and Chang 8 dark mudstone range from 0.008 to 0.039 (with an average of 0.017) and 0.009 to 0.059 (with an average of 0.027), respectively (Table 2; Figure 5). The two curves of paleoproductivity exhibit a similar trend, both indicating high values in the Chang 7 shale (Figure 5). These results are also consistent with the P/Ti ratio data, indicating that the paleoproductivity of the Chang 7 black shale is substantially higher than that of the dark mudstone in both the Chang 7 and Chang 8 Members.

5.1.6. Tectonic Setting and Sedimentary Provenance

Provenance determination constitutes a fundamental prerequisite for reconstructing paleogeographic patterns and guiding hydrocarbon exploration strategies. The chemical properties of REEs are highly similar, with low concentrations in water bodies, allowing them to rapidly enter sediments without significant division and making them remarkably resistant to weathering alteration [64]. As a result, the REEs present in sedimentary rocks are significant to trace the geochemical feature and tectonic setting of provenance areas.
The crossplot of REE vs. La/Yb is particularly useful for identifying the provenances. As shown in Figure 9, the majority of the samples fall within the intersection zone of sedimentary rock and granite, with one sample each from the Chang 7 shale and Chang 8 mudstone falling into the sedimentary rock area. The results suggest that the provenance of mudstones and shales in Chang 7 and Chang 8 Members was upper crustal felsic provenance, mixed with contributions from sedimentary rocks.
The La–Th–Sc diagram serves as an effective discriminative tool for characterizing tectonic settings. For the Chang 7 shale, samples are distributed in continental arc, active continental arc margin and passive margin (Figure 10). This indicates that the Chang 7 shale has a mixed-source origin [65]. Most of the Chang 7 and Chang 8 mudstone samples fall into the continental arc area, which is related to the fact of the tectonic activity of Yinshan in the Late Triassic. Therefore, the terrigenous detritus within the Chang 7 and Chang 8 mudstones primarily originates from the northern Yinshan uplift, which is consistent with previous studies [66].

5.2. Insight for Organic Matter Enrichment

5.2.1. The Main Controlling Factors for Organic Matter Enrichment

Organic matter is a key source material for hydrocarbon generation in dark mudstones and black shales. Its origin, type, and enrichment processes directly determine the potential for oil and gas generation. There are various factors controlling the enrichment of organic matter, including paleoproductivity, paleoredox conditions, sedimentation rates, and et al. [28]. Currently, three main depositional enrichment models are recognized: the production model, the preservation model, and the production-preservation synergistic model [67]. In this study, TOC was adopted as an indicator for organic matter enrichment. By investigating the relationships between TOC and geochemical parameters of paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity, the primary controlling factors of organic matter enrichment in the Chang 7 Member will be revealed.
In terms of paleoclimate, Sr/Cu is used as the representative element. The plot shows significantly negative correlations with correlation coefficients of R2 values of 0.8218 in the Chang 7 dark mudstone, indicating a strong influence of paleoclimate on the enrichment of organic matter in mudstone (Figure 11). The plot shows slight negative correlations with R2 values of 0.3991 in the Chang 7 black shale, suggesting that the weak effect of paleoclimate on the organic-matter enrichment in shale (Figure 11). Based on the discussion in Section 5.1.1, it can be seen that during the Chang 7 stage, both dark mudstone and black shale were in a relatively warm and humid climate, which was favorable for the growth of terrestrial organisms. This relationship suggests that terrestrial organisms had a smaller impact on the organic matter enrichment in black shale but a greater impact on the organic matter enrichment in dark mudstone. The reason why TOC in mudstone remains relatively low under warm and humid climate conditions may be that the organic matter in mudstone is diluted, leading to a lower abundance of organic matter [22].
The correlation between TOC and paleosalinity indicators is weak, with R2 values of 0.1350 for dark mudstone and 0.0842 for black shale of the Chang 7 Member (Figure 12). This suggests that under freshwater to slightly brackish water conditions, small variations in water salinity have a minimal effect on organic matter enrichment.
As shown in Figure 13, the U/Th vs. TOC and V/Cr vs. TOC in dark mudstone exhibit pronounced correlations with R2 values of 0.9928 and 0.9436, respectively. Similar relationships are observed in the Chang 7 black shale, where the correlations of U/Th vs. TOC and V/Cr vs. TOC remain statistically significant, yielding R2 values of 0.7341 and 0.6647, respectively (Figure 13). As mentioned before, larger U/Th and V/Cr ratios indicate a strongly reducing environment. It can be seen that the organic matter enrichment in dark mudstone and black shale of the Chang 7 Member is greatly influenced by redox conditions. However, since shale forms in anoxic environments, while mudstone forms in dysoxic-oxic environments, anoxic conditions are more favorable for organic matter enrichment. This is one of the main reasons why the TOC values of dark mudstone and black shale in the Chang 7 Member differ so greatly.
As illustrated in Figure 14, Rb/Zr ratios exhibit positive correlations with TOC content in both dark mudstones (R2 = 0.9032) and black shales (R2 = 0.4097) from Chang 7 Member. As the interpretation in Section 5.1.4, larger Rb/Zr ratios indicate deeper water and weaker water flow intensity. These correlations demonstrate a stratigraphy-dependent relationship between paleowater depth and organic matter accumulation. Specifically, the greater the water depth, the more conducive it was to the enrichment of organic matter.
The Chang 7 dark mudstone demonstrated an exceptionally strong positive correlation between TOC values and both P/Ti (R2 = 0.9977) and Cu/Ti ratios (R2 = 0.9999), as illustrated in Figure 15. Similarly, for the Chang 7 black shale, the TOC values also showed a positive correlation with the P/Ti and Cu/Ti ratios, with R2 values of 0.3883 and 0.8115, respectively. Therefore, paleoproductivity was also an important factor contributing to the enrichment of organic matter. Because the paleoproductivity during the black shale deposition period was significantly higher than that during the dark mudstone deposition period, that is also one of the important factors contributing to the higher TOC content in shale compared to mudstone.
Above all, warm and humid climate, anoxic environment, deep water depth and high paleoproductivity are important conditions for the development of high-quality source rocks in the Chang 7 Member. However, paleoclimate indirectly affects organic matter accumulation by influencing vegetation development and other factors, while paleowater depth indirectly affects organic matter accumulation by influencing redox conditions. Therefore, the key controlling factors are paleoredox conditions and paleoproductivity, followed by paleoclimate and paleowater depth. In conclusion, it is proposed that the organic matter enrichment in the Chang 7 dark mudstone and black shale is mainly controlled by paleoproductivity at medium to high levels, and the sedimentary process is influenced by the preservation of redox conditions, which belongs to the production model. The TOC values of the Chang 7 black shale are significantly higher than those of the Chang 7 dark mudstone due to the anoxic environment, deeper water depth, and higher paleoproductivity associated with the shale. This difference in the depositional environment between black shale and dark mudstone is the main reason for the different degrees of organic matter enrichment in these source rocks.
Based on the current exploration and development situation in the Ordos Basin, dark mudstones and black shales of the Chang 7 Member are both high-quality source rocks, with the shale exhibiting superior hydrocarbon generation potential compared to the mudstones [68]. Although the TOC of the mudstones of the Chang 7 Member has strong heterogeneity, they can still be used as good source rocks in many areas [7,25]. In this study, the TOC, paleosalinity, paleoredox conditions, and paleoproductivity indicators of the Chang 8 mudstones are quite similar to those of the Chang 7 mudstones, with only little differences in paleoclimate and paleowater depth (Figure 5). Based on this, it can be inferred that the organic-rich dark mudstones of the Chang 8 Member likely have significant hydrocarbon generation potential, a conclusion supported by the research of numerous scholars [18,69]. The Chang 8 Member therefore represents an ideal stratigraphic interval for focus in the next phase of exploration and development.

5.2.2. The Model of Organic Matter Enrichment

Based on the comprehensive analysis of the main controlling factors for organic matter enrichment, this study has established sedimentary environment and organic matter enrichment patterns for the three lithological units in the study area (Figure 16).
During the depositional stage of Chang 8 mudstone (Figure 16a), multiple deltaic systems developed in the Ordos Basin, with the study area primarily characterized by a delta front to shallow lake environment [28]. The climate was relatively warm and humid, which promoted the generation of abundant organic matter from plants and plankton. The water was slightly brackish and featured an oxic to dyoxic environment. Although it was under a relatively warm and humid climate, its poor preservation conditions, low paleoproductivity, and strong clastic influx led to a low TOC content.
During the depositional stage of Chang 7 shale (Figure 16b), the inland lacustrine basin of the Ordos Basin expanded significantly, developing semi-deep to deep lake facies sediments [28]. The climate became increasingly warm and humid, promoting the formation of organic matter. The black shale deposition occurred in slightly brackish water, with significant water depth. Due to the deep-water environment, the influence of clastic influx on its deposition was limited, and the water dynamics were weak, which was conducive to the preservation of organic matter. Overall, high paleoproductivity provided a large amount of organic matter, and the anoxic aquatic environment offered favorable conditions for the preservation of organic matter. These key factors contributed to the higher TOC content.
During the depositional stage of Chang 7 mudstone (Figure 16c), tectonic activity led to the formation of semi-deep lake facies [28]. Although the climate changed during this period, the climate remained relatively warm and humid. As the water became shallower, the influence of clastic influx increased, leading to a relative decrease in organic matter. Similarly to Chang 8 mudstone, lower paleoproductivity and poor preservation conditions resulted in a lower TOC content. Meanwhile, the dilution effect exacerbated this outcome [22].

6. Conclusions

This study focuses on the dark mudstone and black shale of the Chang 7 Member, as well as the dark mudstone of the Chang 8 Member in the Yanchang Formation of the Ordos Basin. By applying geochemical methods, this study reconstructed the paleoenvironment of the dark mudstone and black shale, explored the mechanisms of organic matter enrichment, and established the model of organic matter enrichment. The main conclusions of this study are summarized as follows:
(1) The Chang 7 black shales exhibited a relatively high TOC content, which could be considered as organic-rich shale. The TOC content in the dark mudstones of both the Chang 7 and Chang 8 Members was lower than that of the black shale; however, they still represented high-quality source rocks.
(2) In terms of paleoclimate, the climate of the Chang 7 shales was the warmest and most humid, followed by the Chang 7 mudstones, and finally the Chang 8 mudstones. The Chang 7 dark mudstones, Chang 7 black shales, and Chang 8 dark mudstones were mainly developed in a freshwater to slightly brackish water environment.
(3) The black shales were deposited in anoxic conditions, while the dark mudstones were formed in dysoxic-oxic conditions. The paleowater depth of the Chang 7 black shales was greater than that of the dark mudstone in both the Chang 7 and Chang 8 Members, with weaker hydrodynamic conditions. In terms of paleoproductivity, the Chang 7 black shales were substantially higher than the dark mudstones in both the Chang 7 and Chang 8 Members. The provenance of the samples within the study area is mainly from the upper crustal felsic provenance.
(4) The enrichment of organic matter in the Chang 7 dark mudstones and black shales is primarily controlled by paleoredox conditions and paleoproductivity, with secondary influences from paleoclimate and paleowater depth. The disparity in the depositional environments of black shale versus dark mudstone governed their distinct capacities for organic matter enrichment. By analogy, it can be inferred that the Chang 8 dark mudstone could also serve as an effective source rock.

Author Contributions

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

Funding

This research was funded by the National Natural Science Foundation of China (42202148) and the Fundamental Research Funds for the Central Universities, Chang’an University (300102273203).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Picard, M.D. Classification of fine-grained sedimentary rocks. J. Sediment. Res. 1971, 41, 179–195. [Google Scholar] [CrossRef]
  2. Peng, J.; Yao, Z.; Yang, Y.M.; Yu, L.D.; Xu, T.Y. Discussion on classification and naming scheme of fine-grained sedimentary rocks. Petrol. Explor. Dev. 2022, 49, 121–132. [Google Scholar] [CrossRef]
  3. Pang, X.J.; Wang, G.W.; Kuang, L.C.; Lai, J.; Gao, Y.; Zhao, Y.D.; Li, H.B.; Wang, S.; Bao, M.; Liu, S.C.; et al. Prediction of multiscale laminae structure and reservoir quality in fine-grained sedimentary rocks: The Permian Lucaogou Formation in Jimusar Sag, Junggar Basin. Pet. Sci. 2022, 19, 2549–2571. [Google Scholar] [CrossRef]
  4. Jiao, X.; Liu, Y.Q.; Yang, W.; Zhou, D.W.; Bai, B.; Zhang, T.S.; Zhao, M.R.; Li, Z.X.; Meng, Z.Y.; Yang, Y.Y.; et al. Fine-grained volcanic-hydrothermal sedimentary rocks in Permian Lucaogou Formation, Santanghu Basin, NW China: Implications on hydrocarbon source rocks and accumulation in lacustrine rift basins. Mar. Pet. Geol. 2020, 114, 104201. [Google Scholar] [CrossRef]
  5. Wang, F.; Chen, R.; Yu, W.; Tian, J.; Liang, X.; Tan, X.; Gong, L. Characteristics of lacustrine deepwater fine-grained lithofacies and source-reservoir combination of tight oil in the triassic chang 7 member in Ordos Basin, China. J. Pet. Sci. Eng. 2021, 202, 108429. [Google Scholar] [CrossRef]
  6. Pan, X.; Wang, Z.; Li, Q.; Gao, J.; Zhu, L.; Liu, W. Sedimentary environments and mechanism of organic matter enrichment of dark shales with low TOC in the Mesoproterozoic Cuizhuang Formation of the Ordos Basin: Evidence from petrology, organic geochemistry, and major and trace elements. Mar. Pet. Geol. 2020, 122, 104695. [Google Scholar] [CrossRef]
  7. Yu, W.; Tian, J.; Wang, F.; Liang, Q.; Yang, T.; Kneller, B.; Liang, X. Sedimentary environment and organic matter enrichment of black mudstones from the upper Triassic Chang-7 member in the Ordos Basin, Northern China. J. Asian. Earth. Sci. 2022, 224, 105009. [Google Scholar] [CrossRef]
  8. Luo, Q.Y.; Goodarzi, F.; Zhong, N.M.; Qiu, N.S.; Wang, X.M.; Suchý, V.; Khan, I.; Zheng, X.W.; Liu, B.; Ardakani, Q.H.; et al. Dispersed organic matter from pre-Devonian marine shales: A review on its composition, origin, evolution, and potential for hydrocarbon prospecting. Earth Sci. Rev. 2024, 261, 105027. [Google Scholar] [CrossRef]
  9. Adegoke, A.K.; Abdullah, W.H.; Hakimi, M.H.; Yandoka, B.M.S.; Mustapha, K.A.; Aturamu, A.O. Trace elements geochemistry of kerogen in Upper Cretaceous sediments, Chad (Bornu) Basin, northeastern Nigeria: Origin and paleo-redox conditions. J. Afr. Earth Sci. 2014, 100, 675–683. [Google Scholar] [CrossRef]
  10. Nesbitt, H.; Young, G. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 1982, 299, 715–717. [Google Scholar] [CrossRef]
  11. Cao, Y.; Song, H.; Algeo, T.J.; Chu, D.; Du, Y.; Tian, L.; Wang, Y.; Tong, J. Intensified chemical weathering during the Permian-Triassic transition recorded in terrestrial and marine successions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2019, 519, 166–177. [Google Scholar] [CrossRef]
  12. Wei, W.E.I.; Algeo, T.J. Elemental proxies for paleosalinity analysis of ancient shales and mudrocks. Geochim. Cosmochim. Acta 2020, 287, 341–366. [Google Scholar] [CrossRef]
  13. Xiao, B.; Liu, S.; Li, Z.; Ran, B.; Ye, Y.; Yang, D.; Li, J. Geochemical characteristics of marine shale in the Wufeng Formation–Longmaxi Formation in the northern Sichuan Basin, South China and its implications for depositional controls on organic matter. J. Pet. Sci. Eng. 2021, 203, 108618. [Google Scholar] [CrossRef]
  14. Syaeful, H.; Bakhri, S.; Muljana, B.; Sumaryanto, A.; Sukadana, I.G.; Pratama, H.A.; Muhammad, A.G.; Indrastomo, F.D.; Ciputra, R.C.; Widodo, S.; et al. Elemental Geochemistry on Paleoenvironment Reconstruction: Proxies on Miocene-Pliocene of Marine to Fluvial Sediment in Serpong, Banten, Indonesia. Geosciences 2024, 14, 189. [Google Scholar] [CrossRef]
  15. Li, Q.; Wu, S.; Xia, D.; You, X.; Zhang, H.; Lu, H. Major and trace element geochemistry of the lacustrine organic-rich shales from the Upper Triassic Chang 7 Member in the southwestern Ordos Basin, China: Implications for paleoenvironment and organic matter accumulation. Mar. Pet. Geol. 2020, 111, 852–867. [Google Scholar] [CrossRef]
  16. Al-Taee, N.T.; Al-Juboury, A.I.; Ghafor, I.M.; Zanoni, G.; Rowe, H. Depositional environment of the late Paleocene-early Eocene Sinjar Formation, Iraq: Implications from facies analysis, mineralogical and geochemical proxies. Heliyon 2024, 10, e25657. [Google Scholar] [CrossRef]
  17. Hou, L.; Pang, Z.; Luo, X.; Lin, S. Sedimentary features of the gravity flow influenced shale deposit: A case study of Chang 7 member in the Ordos Basin, China. Mar. Pet. Geol. 2023, 153, 106277. [Google Scholar] [CrossRef]
  18. Cui, J.; Zhang, Z.; Liu, J.; Liu, G.; Huang, X.; Qi, Y.; Mao, Z.; Li, Y. Hydrocarbon generation and expulsion quantification and contribution of multiple source rocks to hydrocarbon accumulation in Yanchang Formation, Ordos Basin, China. J. Nat. Gas Geosci. 2021, 6, 375–391. [Google Scholar] [CrossRef]
  19. Jiang, Y.; Gong, H.J.; Pu, R.H. Geochemical characteristics of source rocks in the Chang 9–Chang 7 members and their correlation with the tight oil sources of Chang 8 member in the Ganquan-Fuxian area, Ordos Basin. Sediment. Geol. Tethyan Geol. 2024, 44, 656–670, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  20. Li, D.; Shi, Q.; Mi, N.; Xu, Y.; Wang, X.; Tao, W. The type, origin and preservation of organic matter of the fine-grain sediments in Triassic Yanhe Profile, Ordos Basin, and their relation to paleoenvironment condition. J. Pet. Sci. Eng. 2020, 188, 106875. [Google Scholar] [CrossRef]
  21. Fu, J.H.; Li, S.X.; Xu, L.M.; Niu, X.B. Paleo-sedimentary environmental restoration and its significance of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China. Petrol. Explor. Dev. 2018, 45, 998–1008. [Google Scholar] [CrossRef]
  22. Zheng, R.; Zeng, W.; Li, Z.; Chen, X.; Man, K.; Zhang, Z.; Wang, G.; Shi, S. Differential enrichment mechanisms of organic matter in the Chang 7 Member mudstone and shale in Ordos Basin, China: Constraints from organic geochemistry and element geochemistry. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2022, 601, 111126. [Google Scholar] [CrossRef]
  23. Qiao, J.; Baniasad, A.; Zieger, L.; Zhang, C.; Luo, Q.; Littke, R. Paleo-depositional environment, origin and characteristics of organic matter of the Triassic Chang 7 Member of the Yanchang Formation throughout the mid-western part of the Ordos Basin, China. Int. J. Coal Geol. 2021, 237, 103636. [Google Scholar] [CrossRef]
  24. Zhang, B.; Mao, Z.; Zhang, Z.; Yuan, Y.; Chen, X.; Shi, Y.; Liu, G.; Shao, X. Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China. Petrol. Explor. Dev. 2021, 48, 1304–1314. [Google Scholar] [CrossRef]
  25. Yuan, W.; Liu, G.; Zhou, X.; Xu, L.; Li, C. Palaeoproductivity and organic matter accumulation during the deposition of the Chang 7 organic-rich shale of the Upper Triassic Yanchang Formation, Ordos Basin, China. Geol. J. 2020, 55, 3139–3156. [Google Scholar] [CrossRef]
  26. Loucks, R.G.; Ruppel, S.C.; Wang, X.; Ko, L.; Peng, S.; Zhang, T.; Rowe, H.; Smith, P. Pore types, pore-network analysis, and pore quantification of the lacustrine shale-hydrocarbon system in the Late Triassic Yanchang Formation in the southeastern Ordos Basin, China. Interpretation 2017, 5, 63–79. [Google Scholar] [CrossRef]
  27. Wang, Y.; Liu, L.; Cheng, H. Pore structure of Triassic Yanchang mudstone, Ordos Basin: Insights into the impact of solvent extraction on porosity in lacustrine mudstone within the oil window. J. Pet. Sci. Eng. 2020, 195, 107944. [Google Scholar] [CrossRef]
  28. Guo, H.; He, R.; Jia, W.; Peng, P.; Lei, Y.; Luo, X.; Wang, X.; Zhang, L.; Jiang, C. Pore characteristics of lacustrine shale within the oil window in the Upper Triassic Yanchang Formation, southeastern Ordos Basin, China. Mar. Pet. Geol. 2018, 91, 279–296. [Google Scholar] [CrossRef]
  29. Zhang, K.; Liu, R.; Liu, Z. Sedimentary sequence evolution and organic matter accumulation characteristics of the Chang 8–Chang 7 members in the Upper Triassic Yanchang Formation, southwest Ordos Basin, central China. J. Pet. Sci. Eng. 2021, 196, 107751. [Google Scholar] [CrossRef]
  30. Tan, X.; Wang, Z. General high-pressure closed acidic decomposition method of rock samples for trace element determination using inductively coupled plasma mass spectrometry. J. Anal. Chem. 2020, 75, 1295–1303. [Google Scholar] [CrossRef]
  31. McCarthy, K.; Rojas, K.; Niemann, M.; Palmowski, D.; Peters, K.; Stankiewicz, A. Basic petroleum geochemistry for source rock evaluation. Oilfield. Rev. 2011, 23, 32–43. [Google Scholar]
  32. McLennan, S.M. Relationships between the trace element composition of sedimentary rocks and upper continental crust. Geochem. Geophys. Geosyst. 2001, 2, 4. [Google Scholar] [CrossRef]
  33. Boynton, W.V. Cosmochemistry of the rare earth elements: Meteorite studies. Developments in geochemistry. Dev. Geochem. 1984, 2, 63–114. [Google Scholar] [CrossRef]
  34. Xu, C.; Shan, X.; He, W.; Zhang, K.; Rexiti, Y.; Su, S.; Liang, C.; Zou, X. The influence of paleoclimate and a marine transgression event on organic matter accumulation in lacustrine black shales from the Late Cretaceous, southern Songliao Basin, Northeast China. Int. J. Coal Geol. 2021, 246, 103842. [Google Scholar] [CrossRef]
  35. Qadrouh, A.N.; Alajmi, M.S.; Alotaibi, A.M.; Baioumy, H.; Almalki, M.A.; Alyousif, M.M.; Salim, A.M.A.; Rogaib, A.M.B. Mineralogical and geochemical imprints to determine the provenance, depositional environment, and tectonic setting of the Early Silurian source rock of the Qusaiba shale, Saudi Arabia. Mar. Pet. Geol. 2021, 130, 105131. [Google Scholar] [CrossRef]
  36. Wang, P.; Du, Y.; Yu, W.; Algeo, T.J.; Zhou, Q.; Xu, Y.; Qi, L.; Yuan, L.; Pan, W. The chemical index of alteration (CIA) as a proxy for climate change during glacial-interglacial transitions in Earth history. Earth Sci. Rev. 2020, 201, 103032. [Google Scholar] [CrossRef]
  37. Guo, Y.M.; Xiang, F.; Huang, H.X.; Yang, Q.; Ding, L.; Yang, K.M. Paleoclimate implications of Quaternary river sediments in Yibin area, Sichuan Basin, China. Geomorphology 2022, 413, 108369. [Google Scholar] [CrossRef]
  38. Zhang, T.; Cheng, X.; Wang, S.; Miao, P.; Ao, C. Middle Jurassic-Early Cretaceous drastic paleoenvironmental changes in the Ordos Basin: Constraints on sandstone-type uranium mineralization. Ore Geol. Rev. 2022, 142, 104652. [Google Scholar] [CrossRef]
  39. Zhang, Y.; Liao, Z.; Cao, J.; Lash, G.G.; Wei, Y.; Shi, Q.; Zhang, B.; Kuang, H.; Liu, Y.; Huang, Q. Climate variability during the late Ediacaran: Insights from episodic deposition of black shale-hosted Mn-carbonates in South China. Chem. Geol. 2024, 646, 121910. [Google Scholar] [CrossRef]
  40. Jeong, G.Y.; Cheong, C.S.; Kim, J. Rb-Sr and K-Ar systems of biotite in surface environments regulated by weathering processes with implications for isotopic dating and hydrological cycles of Sr isotopes. Geochim. Cosmochim. Acta 2006, 70, 4734–4749. [Google Scholar] [CrossRef]
  41. Zou, C.; Mao, L.; Tan, Z.; Zhou, L.; Liu, L. Geochemistry of major and trace elements in sediments from the Lubei Plain, China: Constraints for paleoclimate, paleosalinity, and paleoredox environment. J. Asian Earth Sci. 2021, 6, 100071. [Google Scholar] [CrossRef]
  42. Vetter, L.; Spero, H.J.; Eggins, S.M.; Williams, C.; Flower, B.P. Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I. Quat. Sci. Rev. 2017, 178, 102–117. [Google Scholar] [CrossRef]
  43. Sun, L.; Zhang, J.; Zhang, T.; Yan, X.; Chen, T.; Liu, J. Paleosalinity reconstruction for the paleocene sequence of lishui sag in the east China sea shelf basin. Arab. J. Sci. Eng. 2022, 47, 7433–7448. [Google Scholar] [CrossRef]
  44. Khan, D.; Zijun, L.; Qiu, L.; Kuiyuan, L.; Yongqiang, Y.; Cong, N.; Bin, L.; Li, X.; Habulashenmu, Y. Mineralogical and geochemical characterization of lacustrine calcareous shale in Dongying Depression, Bohai Bay Basin: Implications for paleosalinity, paleoclimate, and paleoredox conditions. Geochemistry 2023, 83, 125978. [Google Scholar] [CrossRef]
  45. Russell, A.D.; Morford, J.L. The behavior of redox-sensitive metals across a laminated–massive–laminated transition in Saanich Inlet, British Columbia. Mar. Geol. 2001, 174, 341–354. [Google Scholar] [CrossRef]
  46. Algeo, T.J.; Maynard, J.B. Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems. Chem. Geol. 2004, 206, 289–318. [Google Scholar] [CrossRef]
  47. Lewan, M.D. Factors controlling the proportionality of vanadium to nickel in crude oils. Geochim. Cosmochim. Acta 1984, 48, 2231–2238. [Google Scholar] [CrossRef]
  48. Liang, Q.; Tian, J.; Zhang, X.; Sun, X.; Yang, C. Elemental geochemical characteristics of Lower—Middle Permian mudstones in Taikang Uplift, southern North China Basin: Implications for the FOUR-PALEO conditions. Geosci. J. 2020, 24, 17–33. [Google Scholar] [CrossRef]
  49. AlGhamdi, F.; AlQuraishi, A.; Amao, A.; Laboun, A.B.; Fattah, K.A.; Kahal, A.; Lashin, A. Depositional setting, mineralogical and diagenetic implication on petrophysical properties of unconventional gas reservoir of the Silurian Qusaiba Formation, northwestern Arabian Peninsula. Geoenergy Sci. Eng. 2023, 223, 211563. [Google Scholar] [CrossRef]
  50. Morford, J.L.; Russell, A.D.; Emerson, S. Trace metal evidence for changes in the redox environment associated with the transition from terrigenous clay to diatomaceous sediment, Saanich Inlet, BC. Mar. Geol. 2001, 174, 355–369. [Google Scholar] [CrossRef]
  51. Bjorlykke, K. Geochemical and mineralogical influence of Ordovician island arcs on epicontinental clastic sedimentation: A study of Lower Palaeozoic sedimentation in the Oslo region, Norway. Sedimentology 1974, 21, 251–272. [Google Scholar] [CrossRef]
  52. Sun, Y.; Zhang, Y.; Xi, A.; Tang, Y.; Zhou, R.; Liu, D.; Chen, S. Petrographic and geochemical characteristics of oncolites in the ediacaran Dengying Formation of the upper Yangtze area, China: Implications for their paleo-environment. Sediment. Geol. 2022, 442, 106296. [Google Scholar] [CrossRef]
  53. Overare, B.; Azmy, K.; Garzanti, E.; Osokpor, J.; Ogbe, O.B.; Avwenagha, E.O. Decrypting geochemical signatures in subsurface Niger delta sediments: Implication for provenance, weathering, and paleo-environmental conditions. Mar. Pet. Geol. 2021, 126, 104879. [Google Scholar] [CrossRef]
  54. Zhao, H.; Wu, Z.; Zhang, S.; Zhou, X.; Wang, Y.; Cheng, H. Geochemical features of lithium–rich bauxite from the Benxi Formation in Qinyuan County, Shanxi, China: Insights into their depositional environment and lithium enrichment. Ore Geol. Rev. 2023, 163, 105780. [Google Scholar] [CrossRef]
  55. Xiao, W.; Cao, J.; Luo, B.; He, Y.; Zhou, G.; Zuo, Z.; Xiao, D.; Hu, K. Marinoan glacial aftermath in South China: Paleo-environmental evolution and organic carbon accumulation in the Doushantuo shales. Chem. Geol. 2020, 555, 119838. [Google Scholar] [CrossRef]
  56. Nath, B.N.; Bau, M.; Rao, B.R.; Rao, C.M. Trace and rare earth elemental variation in Arabian Sea sediments through a transect across the oxygen minimum zone. Geochim. Cosmochim. Acta 1997, 61, 2375–2388. [Google Scholar] [CrossRef]
  57. Ai, Y.; Zhu, G.; Li, T.; Zhu, Z. Copper and Zinc isotopes trace the evolution of the Ediacara-Early Cambrian paleo-ocean redox condition in the Tarim Basin, China. Appl. Geochem. 2023, 150, 105588. [Google Scholar] [CrossRef]
  58. Zhang, T.; Wang, L.L.; Liao, H.H.; Zou, M.; Liang, R.; Wang, P.; Su, Z.T. Methods and research progress of Paleo-water depth reconstruction in Sedimentary Basin. Sediment. Geol. Tethyan Geol. 2023, 44, 582–599, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  59. Dypvik, H.; Harris, N.B. Geochemical facies analysis of fine-grained siliciclastics using Th/U, Zr/Rb and (Zr+Rb)/Sr ratios. Chem. Geol. 2001, 181, 131–146. [Google Scholar] [CrossRef]
  60. Yu, Y.; Li, P.; Guo, R.; Zhao, Y.; Li, S.; Zou, H. Upwelling-induced organic matter enrichment of the Upper Permian Dalong Formation in the Sichuan Basin, SW China and its paleoenvironmental implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2021, 576, 110510. [Google Scholar] [CrossRef]
  61. Sageman, B.B.; Murphy, A.E.; Werne, J.P.; Ver Straeten, C.A.; Hollander, D.J.; Lyons, T.W. A tale of shales: The relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian basin. Chem. Geol. 2003, 195, 229–273. [Google Scholar] [CrossRef]
  62. Mclennan, S.M. Rare earth elements in sedimentary rocks: Influence of provenance and sedimentary processes. Rev. Miner. Geochem. 1989, 21, 169–200. [Google Scholar] [CrossRef]
  63. Tribovillard, N.; Algeo, T.J.; Lyons, T.; Riboulleau, A. Trace metals as paleoredox and paleoproductivity proxies: An update. Chem. Geol. 2006, 232, 12–32. [Google Scholar] [CrossRef]
  64. Chen, M.; Nie, F.; Xia, F.; Yan, Z.; Yang, D. Provenance, Sedimentary Environment, Tectonic Setting, and Uranium Mineralization Implications of the Yaojia Formation, SW Songliao Basin, NE China. Minerals 2023, 13, 1053. [Google Scholar] [CrossRef]
  65. Liu, H.L.; Zou, C.N.; Qiu, Z.; Yin, S.; Yang, Z.; Wu, S.T.; Zhang, G.S.; Chen, Y.P.; Ma, F.; Li, S.X.; et al. Sedimentary Depositional Environment and Organic Matter Enrichment Mechanism of Lacustrine Black Shales: A case study of the Chang 7 member in the Ordos Basin. Acta Sedimentol. Sin. 2023, 41, 1810–1829, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  66. Qiu, X.W.; Liu, C.Y.; Wang, F.F.; Deng, Y.; Mao, G.Z. Trace and rare earth element geochemistry of the Upper Triassic mudstones in the southern Ordos Basin, Central China. Geol. J. 2015, 50, 399–413. [Google Scholar] [CrossRef]
  67. Calvert, S.E.; Fontugne, M.R. On the late Pleistocene-Holocene sapropel record of climatic and oceanographic variability in the eastern Mediterranean. Paleoceanography 2001, 16, 78–94. [Google Scholar] [CrossRef]
  68. Xu, Z.; Liu, L.; Liu, B.; Wang, T.; Zhang, Z.; Wu, K.; Feng, C.; Dou, W.; Wang, Y.; Shu, Y. Geochemical characteristics of the Triassic Chang 7 lacustrine source rocks, Ordos Basin, China: Implications for paleoenvironment, petroleum potential and tight oil occurrence. J. Asian Earth. Sci. 2019, 178, 112–138. [Google Scholar] [CrossRef]
  69. Zhang, T.; Wang, X.; Zhang, J.; Sun, X.; Milliken, K.L.; Ruppel, S.C.; Enriquez, D. Geochemical evidence for oil and gas expulsion in Triassic lacustrine organic-rich mudstone, Ordos Basin, China. Interpret.-A J. Subsurf. Charact. 2017, 5, 41–61. [Google Scholar] [CrossRef]
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Figure 1. (a) Tectonic divisions of the Ordos Basin showing the sampling location and (b) lithology section of the Yanchang Formation (modified after Wang et al. [27]).
Figure 1. (a) Tectonic divisions of the Ordos Basin showing the sampling location and (b) lithology section of the Yanchang Formation (modified after Wang et al. [27]).
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Figure 2. Representative rock photographs of mudstone and shale from the Chang 7 Member. (a) Chang 7 dark mudstone, N-1; (b) Chang 7 dark mudstone, N-4; (c) Chang 7 black shale, N-7; (d) Chang 7 black shale, N-12.
Figure 2. Representative rock photographs of mudstone and shale from the Chang 7 Member. (a) Chang 7 dark mudstone, N-1; (b) Chang 7 dark mudstone, N-4; (c) Chang 7 black shale, N-7; (d) Chang 7 black shale, N-12.
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Figure 3. Chondrite-normalized REE distribution patterns of the samples (chondrite-normalized values from Boynton, 1984 [33]).
Figure 3. Chondrite-normalized REE distribution patterns of the samples (chondrite-normalized values from Boynton, 1984 [33]).
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Figure 4. (a) Plots of Sr/Cu vs. CIA (modified after Yu et al. [7]); (b) Plots of Sr/Cu vs. Rb/Sr (modified after Xu et al. [34]).
Figure 4. (a) Plots of Sr/Cu vs. CIA (modified after Yu et al. [7]); (b) Plots of Sr/Cu vs. Rb/Sr (modified after Xu et al. [34]).
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Figure 5. Variations in the paleosedimentary environment proxies.
Figure 5. Variations in the paleosedimentary environment proxies.
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Figure 6. Crossplots of Ba (ppm) vs. Sr (ppm).
Figure 6. Crossplots of Ba (ppm) vs. Sr (ppm).
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Figure 7. Crossplots of U/Th vs. V/Cr.
Figure 7. Crossplots of U/Th vs. V/Cr.
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Figure 8. Crossplots of P/Ti vs. Cu/Ti.
Figure 8. Crossplots of P/Ti vs. Cu/Ti.
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Figure 9. Provenance identification plot (REE vs. La/Yb).
Figure 9. Provenance identification plot (REE vs. La/Yb).
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Figure 10. Diagram of La–Th–Sc identifying tectonic setting.
Figure 10. Diagram of La–Th–Sc identifying tectonic setting.
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Figure 11. Correlations between paleoclimate proxy (Sr/Cu) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
Figure 11. Correlations between paleoclimate proxy (Sr/Cu) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
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Figure 12. Correlations between paleosalinity proxy (Sr/Ba) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
Figure 12. Correlations between paleosalinity proxy (Sr/Ba) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
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Figure 13. Correlations between paleoredox proxies and TOC content in the Chang 7 Member shale and mudstone samples ((a,b) U/Th vs. TOC; (c,d) V/Cr vs. TOC).
Figure 13. Correlations between paleoredox proxies and TOC content in the Chang 7 Member shale and mudstone samples ((a,b) U/Th vs. TOC; (c,d) V/Cr vs. TOC).
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Figure 14. Correlations between paleowater depth proxy (Rb/Zr) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
Figure 14. Correlations between paleowater depth proxy (Rb/Zr) and TOC content in (a) Chang 7 mudstone and (b) Chang 7 shale.
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Figure 15. Correlations between paleoproductivity proxies and TOC content in the Chang 7 Member shale and mudstone samples ((a,b) P/Ti vs. TOC content; (c,d) Cu/Ti vs. TOC content).
Figure 15. Correlations between paleoproductivity proxies and TOC content in the Chang 7 Member shale and mudstone samples ((a,b) P/Ti vs. TOC content; (c,d) Cu/Ti vs. TOC content).
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Figure 16. Model of differential organic matter enrichment in the Yanchang Formation, Ordos Basin ((a) Chang 8 dark mudstone; (b) Chang 7 black shale; (c) Chang 7 dark mudstone).
Figure 16. Model of differential organic matter enrichment in the Yanchang Formation, Ordos Basin ((a) Chang 8 dark mudstone; (b) Chang 7 black shale; (c) Chang 7 dark mudstone).
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Table 1. Major element concentrations (wt.%), TOC (wt.%), and main element ratios for studied samples.
Table 1. Major element concentrations (wt.%), TOC (wt.%), and main element ratios for studied samples.
SamplesDepth (m)UnitLithologySiO2TiO2Al2O3TFe2O3MnOMgOCaONa2OK2OP2O5LOITOCP/TiCIA
N-11678.85Chang 7Dark mudstone42.930.5617.0314.170.031.730.960.913.730.4518.066.160.5780.60
N-21712.80Chang 7Dark mudstone56.440.8518.845.060.063.452.201.474.260.157.600.650.1372.35
N-31738.65Chang 7Dark mudstone57.810.7917.515.130.063.042.121.713.820.157.960.500.1471.01
N-41740.90Chang 7Dark mudstone57.680.7315.756.350.093.142.681.313.570.188.801.180.1871.79
N-51753.55Chang 7Black shale57.650.3114.037.480.061.281.521.122.570.1414.305.450.3274.73
N-61754.20Chang 7Black shale33.980.288.0413.290.060.511.220.461.980.4640.5020.521.1779.08
N-71755.55Chang 7Black shale37.290.5111.6117.640.050.791.120.762.610.1827.7210.070.2574.90
N-81757.10Chang 7Black shale18.290.244.2615.580.060.461.130.491.390.3958.4024.671.1671.36
N-91757.90Chang 7Black shale28.750.287.4918.740.090.592.331.101.961.0938.2616.052.7881.00
N-101758.10Chang 7Black shale54.850.4912.386.890.131.621.961.164.240.1716.508.240.2565.36
N-111759.30Chang 7Black shale55.810.6520.123.020.021.041.373.141.780.1712.905.620.1977.85
N-121762.85Chang 7Black shale59.300.1520.402.570.021.950.600.613.160.0511.904.220.2482.92
N-131778.40Chang 8Dark mudstone55.390.6116.199.560.212.491.651.503.530.428.582.150.4975.41
N-141779.20Chang 8Dark mudstone57.810.7118.386.180.072.360.721.564.260.177.882.780.1775.47
N-151782.20Chang 8Dark mudstone60.300.8917.686.210.052.600.742.253.680.175.941.850.1474.34
Table 2. Trace element contents (ppm) and main element ratios for studied samples.
Table 2. Trace element contents (ppm) and main element ratios for studied samples.
SamplesVCrCoNiCuRbSrZrMoBaPbThUSr/CuRb/SrSr/BaV/NiU/ThV/CrRb/ZrCu/Ti
N-1203.06106.6341.0887.71130.93168.25199.06152.4376.49815.6650.9421.3442.361.520.850.242.321.991.901.100.039
N-2148.36103.4525.7189.3245.17107.51143.97191.081.83671.8727.9919.994.263.190.750.211.660.211.430.560.009
N-3131.6286.1820.0441.0137.92101.45166.44183.381.12682.9828.9820.163.774.390.610.243.210.191.530.550.008
N-4131.4589.5119.3248.0749.86125.05158.57159.735.82649.4930.1621.635.253.180.790.242.730.241.470.780.011
N-5103.5639.7418.0526.2366.12134.16130.90111.2334.13631.8837.6831.6826.791.981.020.213.950.852.611.210.036
N-6393.9041.7033.5167.46178.78170.49191.75124.37186.28491.0040.1723.08143.101.070.890.395.846.209.451.370.107
N-7223.5276.3030.1662.43196.83166.4795.51129.84140.27485.6130.2613.38112.510.491.740.203.588.412.931.280.064
N-8726.3856.0742.1287.67323.41140.99175.8494.16344.12463.2930.5014.68208.560.540.800.388.2914.2112.951.500.225
N-9437.1653.8537.1073.75307.85134.29420.97101.45211.34484.0953.3713.07152.571.370.320.875.9311.688.121.320.183
N-10178.1067.5822.5343.2384.73184.05148.18132.7851.98509.5634.6118.6431.681.751.240.294.121.702.641.390.029
N-1160.0311.349.8011.2635.8126.11144.66219.9013.64433.1133.4321.188.744.040.180.335.330.415.290.120.009
N-1230.524.302.946.9521.9962.48165.61130.1216.32338.5841.2042.2622.117.530.380.494.390.527.090.480.024
N-13121.2780.3617.2555.09213.9777.29344.49144.441.05757.9827.9810.784.091.610.220.452.200.381.510.540.059
N-14120.0490.2423.9558.7162.12112.27295.42154.931.79892.7443.6812.422.964.760.380.332.040.241.330.720.015
N-15120.1098.6022.1450.9949.3085.38298.66213.031.28861.4627.1612.683.176.060.290.352.360.251.220.400.009
Table 3. Rare earth element (REE) abundances (μg/g) and relevant geochemical indexes for studied samples.
Table 3. Rare earth element (REE) abundances (μg/g) and relevant geochemical indexes for studied samples.
SamplesLaCePrNdSmEuGdTbDyHoErTmYbLuδCeδEu∑REELREEHREEL/H(La/Yb)NLa/Yb
N-158.00112.1512.9053.2810.342.3911.081.639.331.885.090.764.850.741.00.7284.4249.135.47.08.111.9
N-242.3886.899.0933.225.921.185.720.845.071.073.160.513.460.531.10.6199.1178.720.48.88.212.2
N-344.3492.3810.0239.316.961.396.610.965.451.103.130.493.220.481.10.6215.8194.421.49.19.313.8
N-446.8295.4710.4640.957.471.457.461.096.181.233.440.523.440.511.00.6226.5202.623.98.59.213.6
N-541.2879.808.6231.025.990.975.930.895.151.043.020.493.370.521.00.5188.1167.720.48.28.312.2
N-659.01106.6512.0848.348.611.548.531.186.501.273.470.523.380.511.00.5261.6236.225.49.311.817.5
N-734.7167.838.0230.556.361.406.410.925.241.032.780.422.690.391.00.7168.7148.919.97.58.712.9
N-843.1776.389.2037.667.211.547.141.025.611.102.960.442.850.420.90.7196.7175.221.58.110.215.2
N-948.1185.7210.2843.178.331.949.041.307.441.524.230.634.220.660.90.7226.6197.629.06.87.711.4
N-1039.8878.228.7731.826.211.276.100.874.880.972.700.412.690.401.00.6185.2166.219.08.710.014.8
N-1135.3977.359.0936.136.870.936.140.884.720.872.330.342.250.331.00.4183.6165.817.99.310.615.7
N-1213.1432.993.5914.023.480.413.220.512.870.551.510.241.690.261.20.478.567.610.86.25.27.8
N-1335.9289.8312.1555.3712.612.7911.971.769.201.684.020.543.250.471.00.7241.6208.732.96.37.411.0
N-1427.3656.036.3322.944.711.054.390.663.850.842.430.402.650.411.00.7134.0118.415.67.67.010.3
N-1545.2593.9210.8243.858.281.747.210.995.301.062.970.462.990.451.00.7225.3203.921.49.510.215.2
∑REE = LREE + HREE; LREE = La + Ce + Pr + Nd + Sm + Eu; HREE = Gd + Tb + Dy + Ho + Er + Tm + Yb + Lu; L/H = LREE/HREE; δEu = EuN/(SmN × GdN)0.5; δCe = CeN/(LaN × PrN)0.5. Subscript N values present chondrite-normalized values; chondrite-normalized values derived from Boynton, 1984 [33].
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Wang, Z.; Cheng, H.; Wang, Y. Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment. Minerals 2025, 15, 656. https://doi.org/10.3390/min15060656

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Wang Z, Cheng H, Wang Y. Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment. Minerals. 2025; 15(6):656. https://doi.org/10.3390/min15060656

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Wang, Ziming, Hongfei Cheng, and Yang Wang. 2025. "Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment" Minerals 15, no. 6: 656. https://doi.org/10.3390/min15060656

APA Style

Wang, Z., Cheng, H., & Wang, Y. (2025). Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment. Minerals, 15(6), 656. https://doi.org/10.3390/min15060656

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