Paleoenvironmental characteristics and organic matter enrichment mechanisms of the upper Ordovician-lower Silurian organic-rich black shales in the Yangtze foreland basin, South China

The Wufeng Formation-Longmaxi Formation (O₃w-S₁l₁) black shales within the Yangtze foreland basin (South China) hold significant potential as unconventional oil and gas resources. However, several challenges, including sluggish sedimentation rates, thin sedimentary layers, and notable homogeneity, hinder a comprehensive grasp of the organic matter enrichment (OME) mechanism within these shales. These challenges impede the exploration of unconventional oil and gas resources. This study aims to identify the O₃w-S₁l₁ black shales in the Weiyuan area through a combined approach of organic and inorganic geochemical analyses. By doing so, it delves into the controlling mechanism behind OME in the black shales of the Yangtze foreland basin. The findings of this research reveal that the O₃w-S₁l₁ black shales primarily consist of sapropelic kerogen and exhibit signs of over-maturation, with TOC content ranging from 0.43% to 8.21%. These shales are classified as organic-rich, mainly composed mixed and siliceous lithofacies. The presence of silica in the shales originates mainly from biogenic sources, and the highest TOC content coincides with a Si_xs contribution of around 30% to the total silica content. During the late Katian, a global sea level drop led to an oxic paleoenvironment, transitioning into a euxinic paleoenvironment as sea levels rose in the early Rhuddanian. Notably, TOC content exhibited a significant correlation with paleoceanographic conditions (e.g., V/Cr, Ni/Co) and paleoproductivity levels (e.g., P/Al, Ba_bio), while displaying a negative correlation with paleoclimate conditions (e.g., CIA, C-value), terrigenous detrital input intensity (e.g., Si_xs, Ti/Al), and sedimentation rate ((La/Yb)_N). Specifically, favourable conditions for OME encompass robust reducing seawater conditions, high paleoproductivity, a humid climate, reduced influx of terrigenous debris, and relatively elevated sedimentation rates. Regional tectonic movements (Guangxi movement) and fluctuations in relative sea levels exert influence over the deposition of O₃w-S₁l₁ black shales. This study provides a valuable case study for exploring and developing shale gas resources within the Yangtze foreland basin.

1 Introduction

The increasing global energy demand and declining conventional oil and gas resources have emphasized the significance of developing unconventional gas reservoirs, such as shale gas, tight gas, and coalbed methane. The marine organic-rich black shales at the O₃w-S₁l₁ in the Sichuan Basin is considered the most promising layer for shale gas production and the primary target for shale gas commercial development in China (Zou et al., 2019; Yan et al., 2021; Guo et al., 2022). The extensive development of medium and large shale gas fields, including Weiyuan, Changning, and Fuling, has affirmed the immense exploration potential of shale gas reservoirs as “self-generated and self-stored” gas reservoirs (Liang et al., 2009; Nie et al., 2017; Qiu and Zou, 2020a; Ding et al., 2023; Feng et al., 2023).

Numerous studies have been conducted by domestic and foreign scholars on the sedimentology, sequence stratigraphy, paleontology, and geochemistry of the O₃w-S₁l₁ (Zou et al., 2018; Rong and Huang, 2019; Dahl et al., 2021). They believe black shale is an organic-rich sedimentary rock with a close positive correlation between gas and total organic carbon (TOC) content (Liu et al., 2011; Cai et al., 2020). Furthermore, its organic matter enrichment (OME) is closely related to significant global or regional geological and environmental events (Qiu and Zou, 2020b). The O₃w-S₁l₁ were formed during a turbulent geological period (Late Ordovician-Early Silurian transition), experiencing the Hirnantian glaciation (Delabroye and Vecoli, 2010; Melchin et al., 2013; Pohl et al., 2017), biotic extinction (Zou et al., 2018; Dahl et al., 2021; Kozik et al., 2022), ocean anoxia (Melchin et al., 2013), sea level changes (Haq and Schutter, 2008; Lu et al., 2020), and volcanic eruptions (Su et al., 2009; Chen et al., 2014; Xu et al., 2016; Shu, 2021). The complex sedimentary processes result in significant heterogeneity of organic matter (OM) content under different sedimentary environments. As confirmed by considerable research, OME in black shales is regulated by various factors such as paleoredox conditions, paleoproductivity levels, and terrigenous detrital input (Tyson and Pearson, 1991; Feng et al., 2023; Li et al., 2023). Paleoredox conditions are crucial for preserving and accumulating OM; paleoproductivity levels serve as the bedrock for OME; terrigenous detritus input can diminish the abundance of OM (Bornemann et al., 2005).

However, previous studies on OME have mainly concentrated on the relationship between organic-rich shales and paleoenvironments (Li et al., 2023). In contrast, research on regionality paleogeography and paleoenvironments has been limited. The Weiyuan area, influenced by the Zigong uplift, exhibits distinct spatial and temporal variations in OME compared to open water areas such as Changning. This study focuses on the black shales of the O₃w-S₁l₁ in the Weiyuan of the Yangtze foreland basin, investigating the primary factors influencing OME and developing an OME mechanism. Our analysis and research provide a foundation for studying tectonic and paleogeographic changes during the Late Ordovician-Early Silurian in the Yangtze foreland basin, offering scientific support for efficient shale gas development and case studies for identifying new exploration areas.

2 Geological setting

The collision between the Yangtze Block and Cathaysia Block during the Neoproterozoic era led to the formation of the South China Plate. In the Ordovician-Silurian transition period (OST), the South China Plate was situated in a tectonically active region near the ancient equator on the northern edge of the Gondwana continent. During this period, the South China Plate underwent a counterclockwise rotation of approximately 90° from its present position, as depicted in Figure 1A (Zou et al., 2018).

FIGURE 1

FIGURE 1. (A) The Paleogeographic of the South China plate in the Late Ordovician (445 Ma) (Zou et al., 2018); (B) The Yangtze foreland basin system and well location (Guo, 2014); (C) Sedimentary evolution model of organic-rich shales in the Weiyuan area (Liang et al., 2017); (D) The schematic stratigraphic histogram and sampling location of WY1.

During the OST, the South China Plate mainly experienced intracontinental orogeny deformation, forming a large-scale intracontinental foreland basin system (Shu et al., 2015; Zhang J. et al., 2017; Shu, 2021). The geological structure of the Yangtze foreland basin system mainly comprises forebulge, foreland ramp, foredeep, and orogenic belt (Figure 1B). The Chuanzhong uplift, a flexural uplift associated with the tectonic load of the Cathaysia orogenic belt, is situated to the northwest of the Yangtze foreland basin. To the southeast lies the foreland ramp (Upper Yangtze Basin) and the foredeep (Xiangzhong Basin), which resulted from flexural subsidence. The Cathaysia orogenic belt, extending northeastward like a mountain or island chain, represents the primary source area of the foreland basin (He, 2022). The Yangtze foreland basin is surrounded by the Motianling uplift, Hannan uplift, Chuanzhong uplift, Qianzhong uplift, Zigong uplift, and Yichang uplift, forming a semi-closed marine bay facing northward (Wang et al., 2016; Liang et al., 2017; Wang et al., 2018). Figure 1C demonstrates that the Zigong uplift significantly influences the deposition of organic-rich shales in the Weiyuan area.

The shale sequence of the OST in the Yangtze foreland basin comprises three formations in ascending order: Wufeng Formation (Katian), Guanyinqiao Formation (Hirnantian), and Longmaxi Formation (Rhuddanian). During the post-glacial epoch, the Rhuddanian stage deposited Longmaxi-1 (S₁l₁) under the influence of rising sea levels. S₁l₁ composes of black shales and grey muddy siltstone and can be divided into subunits S₁l₁¹ and S₁l₁². The thickest gas-bearing layer is S₁l₁¹, which has a thickness of 54.3 m. Logging data indicates apparent stratification in the O₃w-S₁l₁. The comparison of natural gamma-ray logging (GR) and gamma-ray without uranium (KTH) shows higher uranium (U) content in S₁l₁¹. Additionally, there is a significant positive correlation between mud and U content in the sedimentary rocks, indicating higher mud content in this section. Comparison of the compensated neutron logging (CNL) reflects a higher porosity in this section, as shown in Figure 1D. Considering the TOC content, S₁l₁¹ presents a promising target for the exploration and development of shale gas in the Weiyuan area.

3 Sampling and methods

3.1 Sampling

The Weiyuan area is a well-preserved specimen of the O-S foreland ramp in the Yangtze foreland basin, retaining stratigraphic and tectonic characteristics. The stratum sequence shows almost no deformation or displacement, remaining in situ with complete structural integrity. Therefore, the output state of shale basically reflects the ancient tectonic state at that time and the current preservation state. The high-quality coring from Well WY1 provides essential information on the lithofacies types and sedimentary environmental evolution of the O₃w-S₁l₁ organic-rich black shales in the study area. To prevent weathering from influencing the samples, this study selected twenty samples of the O₃w-S₁l₁ formation from the Well WY1 core at an average sampling interval of approximately 3–4 m, including two samples of O₃w and eighteen samples of S₁l₁. For detailed information on the sample numbers, stratigraphic positions, and lithological characteristics, as shown in Figure 1D. All test analysis is completed in the Analysis Experiment Center of Exploration and Development Research Institute of Petrochina Southwest Oil and Gasfield Company. Before submission, samples are ground to 200 mesh for mineralogical and geochemical analysis.

3.2 Analytical methods

Selected core samples were meticulously prepared for analysis through rough crushing, rinsing with deionised water, drying, and pulverisation to obtain a fine powder. The powder was then passed through a 200-mesh sieve and stored in polypropylene bags for subsequent testing.

To determine the mineralogical compositions, the samples were ground to a powder with a grain size of less than 40 µm and analysed using X-ray diffraction (XRD, X’Pert Pro MPD, Nalytical, Netherlands). The analysis operated Cu Kα radiation at 40 kV and 40 mA and a scan over a range of 5°–45° (2θ). The experimental procedures were by SY/T 5163-2010. The WPF module of JADE 9.0 software was used to determine semi-quantitative phase abundances through Rietveld refinement by identifying mineral species using characteristic mineral diffraction peaks.

Mix polyvinyl alcohol and distilled water in a 1:9 to 1:11 ratio to prepare a polyvinyl alcohol solution through heating and slow stirring. Mix the shale sample with the polyvinyl alcohol solution to make a thin film, and use a polarising microscope (AxioScope A1 type) to identify and measure macerals and component content.

After drying the samples, send the shale sample to a rock pyrolysis analyser for thermal analysis of the rock. The experimental procedures were following GB/T 18602-2012.

To remove inorganic carbon and carbonate minerals, samples were soaked in a hydrochloric acid and water mixture at 1:9. The total organic carbon content was calculated by testing the samples using a Leco CS-230 carbon sulfur analyser. The experimental procedures followed by GB/T 19145-2003.

Put the sample into the muffle furnace, burn it at 1,000°C, and weigh it again after cooling. The difference in weight before and after heating the residues is the LOI (Loss on Ignition). Major element analysis was performed by X-ray fluorescence spectrometry (XRF, Malvern Panalytical PAxios-mAx, Netherlands). After the samples were digested with a mixture of NaOH, and HNO₃, trace elements were measured using an inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer NexION 350X, United States). The experimental procedures were following GB/T 14506-2010. Quality control was performed using blanks, replicates, and reference materials. For all samples, the analytical results of the reference material were within±10% of the certified value, and the relative standard deviation (RSD) value of the replicates was within 5%.

3.3 Data presentation

(1) The Enrichment Factor (X_EF) is a common metric for expressing the degree of enrichment of major and trace elements in shale. Standardizing and calibrating trace elements using Al is essential to eliminate the influence of detrital material derived from the land (Wedepohl, 1971; Tribovillard et al., 2006). The calculation formula of the X_EF is as follows:

${\mathrm{X}}_{\text{EF}}={\left(X/\text{Al}\right)}_{\text{sample}}/{\left(\mathrm{X}/\text{Al}\right)}_{\text{average}\text{shale}}(1)$

In the formula, X_EF stands for the enrichment factor of an element with no units. (X/Al)_{average shale} is the ratio of the average element content to Al in shale. An X_EF value greater than 1 indicates relative enrichment of the element in shale, while an X_EF value less than 1 suggests relative deficiency in comparison to the average shale.

(2) Excess silicon or biogenic silicon (Si_xs, Si_bio) refers to the residuals obtained by subtracting the estimated silica content of terrigenous detrital input from the total siliceous content. It is mainly of biogenic origin and serves as a standard proxy for paleoproductivity in sediments (Tribovillard et al., 2006; Ross and Bustin, 2009). The calculation formula of the Si_xs is as follows:

$\mathrm{S}{\mathrm{i}}_{\text{xs}}=\mathrm{S}{\mathrm{i}}_{\text{sample}}-\left[\mathrm{A}{\mathrm{l}}_{\text{sample}}\times {\left(\text{Si}/\text{Al}\right)}_{\text{background}}\right](2)$

In the formula, Si_xs represents the excess silicon content in the sample, Si_sample and Al_sample refer to the SiO₂ and Al₂O₃, respectively, and (Si/Al)_background is 3.11 (Wedepohl, 1971; Adachi et al., 1986).

(3) Barium (Ba) is a widely used paleoproductivity indicator with multiple sources, including biological sources (Ba_bio) and input of terrigenous debris. Ba_bio is the only parameter that reflects the magnitude of paleoproductivity (Wedepohl, 1971; Dehairs et al., 1987; Paytan et al., 1996; Tribovillard et al., 2006). Modern ocean research suggests that high primary productivity in the sedimentary environment is represented by a Ba_bio content ranging between 1000 and 5000 μg/g (Pi et al., 2013; Schoepfer et al., 2015). The calculation formula of the Ba_bio is as follows:

${\text{Ba}}_{\text{bio}}={\text{Ba}}_{\text{sample}}-\left[{\mathrm{A}\mathrm{l}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\times {\left(\text{Ba}/\text{Al}\right)}_{\text{background}}\right](3)$

In the formula, Ba_sample and Al_sample refer to the Ba and Al₂O₃ contents measured in the samples. The (Ba/Al) _background is the ratio of the two elements in the post-Archean Australian shale (PAAS), and the (Ba/Al) _background is 0.0077 (Taylor and McLennan, 1985).

(4) Cerium (Ce) has low solubility in seawater but a high concentration in terrigenous sediments. Therefore, sedimentary material of this type frequently notable Ce anomalies (δCe) influenced by terrigenous detrital input. This anomaly can reflect the degree of seawater submergence in the sediment and the intensity of terrigenous detrital input, providing essential indicators of sea level change. A δCe value greater than 1 indicates a positive anomaly signifying euxinic conditions in the water. Conversely, a δCe value less than 1 indicates oxic conditions in the water and is considered a negative anomaly (Elderfield and Greaves, 1982; Baar et al., 1985; Murray et al., 1991; Wilde et al., 1996). The calculation formula of the δCe is as follows:

$\delta \text{Ce}={\text{Ce}}_{\mathrm{N}}/{\left({\text{La}}_{\mathrm{N}}\times {\mathrm{Pr}}_{\mathrm{N}}\right)}^{1/2}(4)$

In the formula, n is the average shale normalized value.

(5) The Chemical Index of Alteration (CIA) is widely used to determine the intensity of paleochemical weathering in the provenance areas and the characteristics of paleoclimate (Nesbitt and Young, 1984). A CIA value within the range of 85–100 indicates intense chemical weathering, suggesting a hot and humid paleoclimate. A value between 65–85 signifies moderate chemical weathering, representing a warm and humid paleoclimate. CIA values between 50-65 points to initial chemical weathering in the provenance area, reflecting cold and dry paleoclimate. The calculation formula of the CIA is as follows:

$\text{CIA}=\left[\mathrm{n}\left(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)/\left(\mathrm{n}\left(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)+\mathrm{n}\left({\mathrm{K}}_2\mathrm{O}\right)+\mathrm{n}\left(\mathrm{N}{\mathrm{a}}_2\mathrm{O}\right)+\mathrm{n}\left(\text{Ca}{\mathrm{O}}^{*}\right)\right)\right]\times 100(5)$

In addition, CaO in non-silicate minerals needs to be removed when calculating the weathering index. CaO in apatite is removed by P₂O_5, and then CaO^* is calculated (McLennan et al., 1993; Nesbitt et al., 1996). The calculation formula of the CaO^* is as follows:

$\mathrm{n}\left(\text{Ca}{\mathrm{O}}_{\text{residue}}\right)=\mathrm{n}\left(\text{CaO}\right)-\mathrm{n}\left({\mathrm{P}}_2{\mathrm{O}}_5\right)\times 10/3(6)$

In this formula, if $\mathrm{n}\left(\text{Ca}{\mathrm{O}}_{\text{remaining}}\right)<\mathrm{n}\left(\mathrm{N}{\mathrm{a}}_2\mathrm{O}\right)$, then $\left(\text{Ca}{\mathrm{O}}^{*}\right)=\mathrm{n}\left(\text{Ca}{\mathrm{O}}_{\text{remaining}}\right)$, whereas if $\left(\text{Ca}{\mathrm{O}}_{\text{remaining}}\right)>\mathrm{n}\left(\mathrm{N}{\mathrm{a}}_2\mathrm{O}\right)$, then $\left(\text{Ca}{\mathrm{O}}^{*}\right)=\mathrm{n}\left(\mathrm{N}{\mathrm{a}}_2\mathrm{O}\right)$.

(6) The C-value is a critical indicator for assessing the paleoclimate conditions of sedimentary rocks (Getaneh, 2002; Zhao et al., 2007; Cao et al., 2012). Generally, when C-value is more greater than 0.8, it indicates a humid climate; when C-value is between 0.6 and 0.8, it means a semi-humid environment; when C-value is between 0.4 and 0.6, it shows a semi-arid/semi-humid climate; when C-value is between 0.2 and 0.4, it indicates a semi-arid environment, and when C-value is less than 0.2, it means an arid climate. The calculation formula of the C-value is as follows:

$\mathrm{C}-\text{value}=\sum \left(\text{Fe}+\text{Mn}+\text{Cr}+\text{Ni}+\mathrm{V}+\text{Co}\right)/\sum \left(\text{Ca}+\text{Mg}+\text{Sr}+\text{Ba}+\text{Na}\right)(7)$

(7) The mineral composition significantly influences fracturing behaviour during development. The brittleness index (Br, %) directly measures the rock’s brittleness characteristics (Jarvie et al., 2007; Zhang et al., 2019). It is conventionally considered that a shale exhibits favourable fracturing properties when its brittleness index reaches 40% (Zhang et al., 2018). The calculation formula of the Br is as follows:

$\text{Br}=\frac{{\mathrm{\omega }}_{\text{qtz}}+{\mathrm{\omega }}_{\mathrm{f}}+{\mathrm{\omega }}_{\text{cal}}+{\mathrm{\omega }}_{\text{py}}}{{\mathrm{\omega }}_{\text{qtz}}+{\mathrm{\omega }}_{\mathrm{f}}+{\mathrm{\omega }}_{\mathrm{c}}+{\mathrm{\omega }}_{\text{py}}+{\mathrm{\omega }}_{\text{cl}}}\times 100\%(8)$

In the formula, qtz, f, c, py and cl are the mass fraction (%) of quartz, feldspar, carbonate minerals, pyrite and clay, respectively.

4 Results

4.1 Geochemistry characteristics

SiO₂ and Al₂O₃ constitute the predominant elements within O₃w-S₁l₁ black shales. The SiO₂ content in the O₃w ranges from 52.11% to 53.75% (avg. 52.93%), while the Al₂O₃ content falls between 12.19% and 21.11% (avg. 16.65%). The SiO₂ content in the S₁l₁ ranges from 42.59% to 77.53% (avg. 57.33%), and the Al₂O₃ content ranges from 9.31% to 18.34% (avg. 14.30%). In O₃w-S₁l₁ black shales, there is clear vertical differentiation in the content of the main elements. As the depth is Shallow, the content of SiO₂ and Na₂O gradually increases while the range of MgO, TiO₂, and MnO decreases. Al₂O₃, Fe₂O₃, and K₂O exhibit a trend of first decreasing and then growing with depth, while CaO and P₂O₅ show a trend of first increasing and then declining (Table 1). Redox-sensitive elements (RSEs), including V, Cr, Th, U, and Mo, are commonly used to reconstruct ancient marine environments (Tribovillard et al., 2006). Most trace elements in O₃w-S₁l₁ black shales are enriched compared to the PAAS, except for Sr and Zr, which are depleted (Table 2 and Table 4). V, Mo, U, and other elements enrich significantly (EF>5).

TABLE 1

TABLE 1. Content of major elements (wt%) in the O₃w-S₁l₁ black shales.

TABLE 2

TABLE 2. Content of trace elements (ug/g) in the O₃w-S₁l₁ black shales.

Rare earth elements (REE) divergence is influenced by variations in the depositional seawater conditions and sedimentation rates (Elderfield and Greaves, 1982; Shields and Stille, 2001; Rimmer et al., 2004; Li et al., 2008). Table 3 and Table 4 list the rare earth element concentrations in the O₃w- S₁l₁ black shales in the study area. The O₃w and S₁l₁ shales exhibit ΣREE values ranging from 189.93 μg/g to 266.94 μg/g (avg. 228.44 μg/g) and 135.68 μg/g to 271.92 μg/g (avg. 173.77 μg/g), all of which surpass the PAAS reference value. For the O₃w shales, the light rare earth elements (LREE), heavy rare earth elements (HREE), and LREE/HREE ratio values range from 172.36 μg/g to 241.88 μg/g (avg. 207.12 μg/g), 17.57 μg/g to 25.06 μg/g (avg. 21.32 μg/g), and 9.81 to 9.60 (avg. 9.71), respectively. Correspondingly, the S₁l₁ shales display these values at 120.69 μg/g to 245.10 μg/g (avg. 155.59 μg/g), 13.50 μg/g to 28.11 μg/g (avg. 18.18 μg/g), and 4.97 to 10.51 (avg. 8.74). Negative anomalies in Eu and Ce abundance are observed compared with the chondrite. For the O₃w shales, δEu and δCe values fall within the range of 0.56 to 0.56 (avg. 0.56) and 1.17 to 1.23 (avg. 1.20), respectively. As for the S₁l₁ shales, these values vary from 0.56 to 0.89 (avg. 0.64) and 0.78 to 0.93 (avg. 0.88) (Table 4).

TABLE 3

TABLE 3. Content of rare earth elements (ug/g) in the O3w-S1l1 black shales.

TABLE 4

TABLE 4. Content of geochemical indexes in the O₃w-S₁l₁ black shales.

The TOC content of O₃w and S₁l₁ black shales ranges from 0.43% to 0.99% (avg. 0.71%) and 0.75%–8.21% (avg. 2.17%), respectively. The hydrocarbon potential (S₁+S₂) ranges from 0.03 mg/g to 0.05 mg/g (avg. 0.04 mg/g) and 0.04 mg/g to 0.14 mg/g (avg. 0.09 mg/g), respectively. The organic macerals in O₃w-S₁l₁ black shales are predominantly sapropelic, with a TI index ranging from 84.70 to 93.20 (avg. 88.95), 86.40 to 96.60 (avg. 91.50), respectively. This indicates that the shale has sapropelic kerogen (Type I), primarily derived from low-level aquatic organisms like graptolites, with the potential for crude oil generation. Vitrinite reflectance (Ro) is a suitable parameter for assessing the maturity of sedimentary hydrocarbon source rocks. In cases where vitrinite is scarce, the bitumen reflectance (Rb) can serve as a substitute parameter. The Ro values of the WY1 well, all exceeding 2.00% (Table 5), were calculated using a formula based on the empirical knowledge of different researchers (Jacob, 1985; 1989). The Ro values and the T_max (avg. 535.50°C) indicate that the O₃w-S₁l₁ black shales have reached the gas dryness stage and are dominated by over-maturity (Table 5).

TABLE 5

TABLE 5. Content of organic geochemistry indicators in the O₃w-S₁l₁ black shales.

4.2 Mineral compositions and lithofacies types

The mineral composition of the O₃w-S₁l₁ black shales in the study area is primarily composed of quartz, clay mineral, feldspar, calcite, and pyrite, in descending order. The quartz content in the black shales decreases gradually with increasing depth, accompanied by a corresponding increase in clay mineral content. The changes in other mineral contents are insignificant, as shown in Figure 2A and Table 6. The brittleness index (Br, %) of the O₃w and S₁l₁ black shales range from 63.64% to 65.67% (64.66%) and 32.30%–86.29% (avg. 62.76%), respectively. These values suggest a susceptibility of the mineral composition in these shales to undergo hydraulic fracturing transformations during subsequent stages (Huo et al., 2018).

FIGURE 2

FIGURE 2. Lithological characteristics of O₃w-S₁l₁ black shales (Wang et al., 2018). (A) Vertical variations in mineral content and lithofacies types; (B) Distribution of lithofacies types in shale across various layers; (C) Distribution of lithofacies types in shale with varying TOC content.

TABLE 6

TABLE 6. Mineral composition (%) in O₃w-S₁l₁ black shales.

Choose three groups of mineral components for shale lithofacies classification: siliceous minerals (QFM, quartz+feldspar), clay minerals, and carbonate minerals (calcite+dolomite). The O₃w-S₁l₁ black shales are classified into three types of shale lithofacies: siliceous, clay shale, mixed shale, which further comprise seven subfacies. The dominant lithofacies is the mixed shale lithofaciess (M-7, 30%), followed by the clayey siliceous shale lithofacies (S-4, 20%) and the siliceous clay shale lithofacies (CM-2, 20%). The minor lithofacies include the calcium and clay siliceous shale lithofacies (S-3, 10%), the calcium clay siliceous mixed shale lithofacies (M-5, 10%), the calcium clay siliceous shale lithofacies (S-8, 5%), and the clay calcium siliceous mixed shale lithofacies (M-4, 5%) (Figure 2B; Table 6).

5 Discussion

5.1 Data validity analysis

5.1.1 Applicability of geochemical element evaluation index

In order to evaluate the likelihood of an event, this study utilizes significance tests from statistical theory to calculate the p-value among sedimentary geochemical parameters, as depicted in Figure 3. The RSEs, such as V, Mo, U, Ni and Cr, demonstrate variations in sedimentary rocks in response to different redox conditions. These elements are frequently utilized as indicators of paleoredox conditions within the sedimentary environment (Tribovillard et al., 2006). Multiple parameters are necessary to collectively constrain paleoenvironmental conditions and mitigate potential analytical errors in the determination of single-element concentrations (Qiu et al., 2021).

FIGURE 3

FIGURE 3. The correlation coefficient of paleoenvironmental geochemical indicators. Red circles indicate a positive correlation, and blue circles indicate a negative correlation. Darker colours indicate a stronger correlation.

In this study, paleoredox conditions, represented by V/Cr, Ni/Co, and U/Th ratios, were utilized to confirm the correlations among sedimentary geochemical parameters. The resulting correlation coefficients were 0.96, 0.94, and 0.35, respectively. Furthermore, consistent explanatory outcomes observed across diverse paleoredox conditions in Section 5.3.1 offer additional support for the reliability of the data.

5.1.2 Late diagenesis

When performing a quantitative analysis of sedimentary geochemical characteristics in a paleoenvironment, it is crucial to recognize the complex composition of sediments in the source area, as well as the effects of potassium metasomatism and recycling during diagenesis on these characteristics (Gaillardet et al., 1999; Garzanti et al., 2013; Abedini and Calagari, 2017a; Perri, 2018).

The characteristics of REE content in clastic sedimentary rocks can be used as parameters to assess diagenetic intensity. Stronger diagenesis is associated with a stronger correlation between δCe and ΣREE, as well as Dy_S/Sm_S (Shields and Stille, 2001). The δCe of O₃w-S₁l₁ black shales shows a weak correlation with Dy_S/Sm_S and ΣREE, indicating a limited influence of diagenesis on these elements (Figures 4A,B). Additionally, the comparison of the correlation between CIA, K-co CIA, and CIW suggests that potassium metasomatism has had a negligible impact on the O₃w-S₁l₁ black shales (Nesbitt and Young, 1984; Fedo et al., 1995; Zhang X. S. et al., 2017) (Figures 4C,D). The majority of black shale samples have ICV values greater than 1, ranging from 0.87 to 2.51, with an average value of 1.41, suggesting limited recycled sedimentation and low maturity of the clastic rock composition (Van De Kamp and Leake, 1985; Cox et al., 1995). These samples can be used to assess the paleoenvironmental in the study area.

FIGURE 4

FIGURE 4. Indicator for late diagenesis. (A) Dy_S/Sm_S-δCe; (B) ΣREE-δCe; (C) CIA-Correction CIA; (D) CIA-CIW.

5.2 Origin of silica

Previous studies have indicated that silica in sedimentary rocks primarily originates from three sources: siliceous organisms, hydrothermal fluids, and silica-rich terrigenous debris (Van Den Boorn et al., 2007). During the Early Silurian, the Yangtze foreland basin represented a paleoenvironment with an epicontinental sea, accompanied by frequent volcanic eruptions in multiple phases. As a result, a complex composition of siliceous genesis influenced by various factors emerged (Cai et al., 2022b; Lu et al., 2022). The Al/(Al+Fe+Mn) of Si_bio is usually >0.60, with a δEu value <1, whereas the Al/(Al+Fe+Mn) of hydrothermal silica is generally <0.30, with a δEu value >1 (Adachi et al., 1986; Murray, 1994; Khan et al., 2019). The Si/(Si+Al+Fe) of O₃w-S₁l₁ black shales in the study area ranges from 0.72 to 0.79 (avg. 0.76), and the δEu value ranges from 0.56 to 0.89 (avg. 0.63). Based on the ∑REE-Cu+Co+Ni, Th-U, Al/Al+Fe+Mn-LogFe/Ti, and Al₂O₃-Fe₂O₃-MnO plots (Figures 5A–D), the O₃w-S₁l₁ black shales are primarily associated with non-hydrothermal deposits, showing limited influence from seafloor hydrothermal deposition. These findings indicate that the siliceous component of O₃w-S₁l₁ is predominantly a typical biogenic deposit. Tuff layers, widely distributed in the Yangtze foreland basin during the O-S, are relatively thin, with an average thickness of 2–10 cm, and are interpreted as deposits transported from a distant source (Yang et al., 2022). Hence, these volcanic deposits cannot prove the existence of a heat source in the Yangtze foreland basin and surrounding areas but they suggest indicate a limited influence of volcanogenic silica on siliceous minerals (Wang et al., 2017).

FIGURE 5

FIGURE 5. Discrimination chart of the paleoenvironmental geochemical index in the Yangtze foreland basin from Late Ordovician to Early Silurian. Plates A-D show origin of sillica, E-I show paleoredox conditions and seawater restriction, J-L show paleoclimate.

5.3 Reconstruction of paleoenvironment

5.3.1 Paleoredox conditions and paleoseawater restriction

Paleoredox conditions refer to the equilibrium between oxygen and reducing agents in the sedimentary environment, which significantly influence the type and preservation of OM. An elevated oxidant content accelerates the degradation of OM, while a high reducing agent content inhibits it (Zhang et al., 2016). Marine sedimentary environments are commonly reconstructed using RSEs in marine deposits, such as V, Cr, Th, U, and Mo, owing to their distinct behaviours under seawater oxidation-reduction conditions (Tribovillard et al., 2006; Algeo and Rowe, 2012; Scott and Lyons, 2012; Abedini et al., 2019). V is typically taken up by OM and becomes enriched in reducing environments. Sediments deposited in a low-oxygen environment exhibit high U values (Abedini and Calagari, 2017b).

The redox classifications (O₂ concentration in bottom waters, ml O₂/H₂O) in this research adhere to the standards established by Tyson and Pearson (1991), which include oxic: [O₂]>2, suboxic: 2>[O₂]>0.2, anoxic(no free H₂S in the water column): [O₂]<0.2, and euxinic(free H₂S present in the water column): [O₂]=0. Hatch and Leventhal (1992), Jones and Manning (1994), and Wignall and Twitchett (1996) identified that V/Cr<2, Ni/Co<5, and U/Th<0.75 indicate an oxygen-rich environment, while V/Cr indicate 2-4.25, 5 <Ni/Co<7, and 0.75 <U/Th<1.25 indicate an oxygen-depleted suboxic-anoxic environment. Finally, V/Cr>4.25, Ni/Co>7, and U/Th>1.25 show a euxinic environment. Based on the results presented in Figures 6A–C, the O₃w black shales indicate an oxic-anoxic environment, while the S₁l1 1 black shales exhibit a strongly reducing sedimentary environment with a decreasing reduction trend vertically.

FIGURE 6

FIGURE 6. Vertical variation of the paleoenvironmental geochemical index in the Yangtze foreland basin from Late Ordovician to Early Silurian. Plates A-C show paleoredox conditions, D shows upwelling, E-G show paleoproductivity, H-I show paleosalinity, J-L show paleoclimate, M shows paleoseawater temperature, N-P show terrigenous influx, Q shows sedimentation rate and R shows sea level.

The retention of seawater in a basin has a significant impact on the enrichment and preservation of OM. OM in water is more likely to be enriched and preserved under conditions of slower water exchange and renewal (Algeo and Rowe, 2012). U_EF-Mo_EF and TOC-Mo are effective indicators of the oxygen conditions and the extent of sediment retention in the environment (Algeo and Tribovillard, 2009; Tribovillard et al., 2012). Mo/TOC can determine seawater retention degree under reducing paleoenvironmental conditions. Under euxinic environmental conditions, the degree of seawater retention can be determined using Mo/TOC, whereas, in oxic environments, the values of Mo and Mo/TOC are determined by the redox environment rather than the degree of seawater retention (Zheng et al., 2002; Algeo and Rowe, 2012; Li et al., 2015). The low TOC and Mo content in the O₃w black shales make it unsuitable for using Mo/TOC to determine its seawater retention degree. In contrast, the S₁l₁ black shales were deposited in an anoxic-euxinic environment with high seawater retention (Figures 5E,F).

Upwelling sediments tend to have higher enrichment of Cd and Mo, while Co and Mn are relatively depleted. Co×Mn<0.4, Co_EF×Mn_EF<0.5, and Cd/Mo>0.1 are characteristic indicators of upwelling sediments. Stronger upwelling activity corresponds to lower values of Co×Mn or Co_EF×Mn_EF in the sediment and higher Cd/Mo ratios. Sediments in seasonal upwelling environments typically exhibit Cd/Mo ratios ranging from 0.1 to 0.6, while sediments in perennial upwelling environments have Cd/Mo ratios >0.6 (Crusius et al., 1996; Algeo and Tribovillard, 2009; Nakagawa et al., 2012; Scott and Lyons, 2012; Sweere et al., 2016). Figures 5G–I and Figure 6D illustrate the enhanced retention of O₃w black shales during its sedimentation period, while S₁l₁ black shales were subjected to seasonal upwelling influences.

During the Hirnantian stage (late Ordovician), the cold climate and expanding ice cover may have led to the emergence of high paleo-topography (Zigong uplift) above sea level. As a result, the accommodation space and relative paleoseawater depth of the Weiyuan area may have been affected. The periodic closure of the environment resulted in interrupted exchange between the basin and seawater, leading to a weaker upwelling intensity as indicated by the geochemical elements (Wang et al., 2018; Liang et al., 2019; Qiu et al., 2020b). In contrast, the relatively high sea level during the Rhuddanian stage (early Silurian) promoted seawater exchange and nutrient supply between the basin and the vast oceanic currents (Haq and Schutter, 2008; Cai et al., 2022a).

5.3.2 Paleoproductivity

Paleoproductivity refers to the primary productivity of paleoseawater ecosystems during a geological period, encompassing plant and phytoplanktonic primary productivity, among others (Tribovillard et al., 2006; Wei, H., 2012). Barium (Ba) is a well-established element utilized as an indicator of paleoproductivity (Wedepohl, 1971). The average Ba_bio content in the O₃w-S₁l₁ black shales of the Yangtze foreland basin is 497.79 μg/g and 683.40 μg/g, respectively. Notably, the Ba_bio content of high TOC samples in the S₁l₁ black shales are significantly lower than that of other samples, suggesting reduced overall productivity (Figure 6F). This is due to the decomposition of barite (the primary occurrence form of Ba by sulfur-reducing bacteria and other microorganisms through bacterial sulfate reduction (BSR) in a relatively restricted environment, leading to a relatively low Ba_bio content in organic-rich black shales. Consequently, employing Ba as an indicator of productivity, may produce distorted results, and relatively low Ba_bio content does not necessarily signify low levels of paleoproductivity during the deposition of the O₃w-S₁l₁ black shales (Dymond et al., 1992; Murray and Leinen, 1993; Yan et al., 2015). Previous studies have demonstrated that the biogenic component Si_bio (Si_xs) is a valuable parameter for estimating paleoseawater productivity (Tribovillard et al., 2006; Khan et al., 2019). According to Chapter 5.2, the siliceous content in the O₃w-S₁l₁ black shales are primarily biogenic, which can be used to evaluate paleoproductivity. The vertical variation of Si_xs content in the O₃w-S₁l₁ black shales suggests a higher paleoproductivity level for the S₁l₁ shales than the O₃w shales.

Wang et al. (2002) reported that the Hirnantia fauna thrived as a result of increased seawater oxygen content in the Yangtze Block. Rong and Huang, 2019 proposed that the Yangtze Block was located in the equatorial-tropical zone during the Late Ordovician and experienced minimal impact from the Gondwana glaciation. As a result, although the biological extinction rate was high, the ecological impact was minimal, and the level of paleoproductivity remained stable. The Zigong uplift in the Weiyuan area restricted the nutrient supply from upwelling. Additionally, the decline in global sea levels during the Hirnantian glacial epoch led to an elevation in oxygen content within the stratified water column. These changes resulted in an unfavourable sedimentary environment for preserving OM during the Late Ordovician (Wang et al., 2018). In the Early Silurian, the Yangtze foreland basin obtained nutrient supply from the open ocean, such as the Qinling Ocean to the north (Wang et al., 2017). Seasonal upwelling had a significant impact on seawater exchange and nutrient supply, consequently affecting paleoproductivity. Planktonic organisms, such as radiolarians and sponge spicules, exhibited their highest paleoproductivity levels during the Early Silurian (Figures 6E–G). The paleoproductivity levels of the O₃w-S₁l₁ black shales in the Weiyuan area exhibit substantial variations, primarily attributed to the influence of the regional oceanic environment and nutrient supply (Guo, 2017).

5.3.3 Paleoclimate and paleosalinity

Paleoclimate and paleosalinity conditions exert a significant influence on the provenance, abundance, and type of OM. Humid climate conditions may increase the deposition of OM, while high salinity water bodies can preserve it (Jin et al., 2008). The abundance of specific major and trace elements in shale, such as the CIA, C-value, and Fe/Mn ratios, can serve as indicators of paleoclimate conditions (Nesbitt and Young, 1982; Wang et al., 1997; Getaneh, 2002). Elevated Fe/Mn ratios in sediment are indicative of warm and humid climatic conditions, whereas low ratios suggest a hot and arid environment (Wang et al., 1997).

The CIA and K-co CIA values suggest that the O₃w-S₁l₁ black shales in the Yangtze foreland basin were deposited during warm and humid paleoclimatic conditions (Figures 5J,K; Figue 6J). The C-value and Fe/Mn trends suggest that the O₃w-S₁l₁ black shales experienced a shift in climatic conditions from humid to dry and then back to humid during the deposition process (Figures 6K,L). The paleosalinity indicators, Ca/(Fe+Ca) and Rb/K in Figures 6H–I, indicate that seawater salinity increased during the late Ordovician because of global climate change and the stagnant water habitat in the basin. In contrast, during the early Silurian, water circulation increased as the glacial epoch ended, reducing seawater salinity. Dong et al. (2022) states that this change is attributed to regional climate conditions and the oceanic environment in the Yangtze foreland basin during the early Silurian.

5.3.4 Paleoseawater temperature

During the Early to Late Ordovician, the South China Plate migrated northward from the subtropical climate zone to the tropical climate zone near the equator, resembling the modern-day Galápagos Islands in Ecuador(Figure 7) (Torsvik and Cocks, 2013). This northward migration was expected to lead to higher paleoseawater temperature. However, evidence from stable oxygen isotope data of conodonts and apatites indicates that the South China Plate underwent a transition from warm to cold water during the Early to Late Ordovician, contradicting the results of paleogeographic reconstructions (Jin et al., 2018). The Sr element estimation of paleoseawater temperature in the Yangtze foreland basin during the O-S interval ranged from 10.64°C to 26.08°C (17.92°C). This value is considerably lower than the average equatorial temperature of 27°C recorded during that period (Figure 6M). The substantial global temperature decrease of approximately 9°C observed during the same period could account for the apparent contradiction and imply that the Hirnantian glacial event might have impacted the marine environment of the Yangtze foreland basin (Melchin et al., 2013).

FIGURE 7

FIGURE 7. (A) Paleogeographic map of the South China Plate in Ordovician (Torsvik and Cocks, 2013; Jin et al., 2018); (B) Sea surface temperature chart around the modern equator. Data are from the National Oceanic and Atmospheric Administration: http://www.ospo.noaa.gov/Products/ocean/sst.html.

5.3.5 Terrigenous detrital input

Terrigenous detrital input particularly impacts sedimentary environments, as it can enrich sediment with nutrients and dilute the abundance of OM. Al and Ti serve as reliable indicators for assessing the intensity of terrigenous detrital input, as they are minimally affected by weathering and diagenesis, thereby preserving the signature of the source region (Nesbitt et al., 1996). A strong correlation between Al and Ti, Th, Nb, Zr, or Hf indicates that Al is derived from typical silicate detrital input (Tribovillard et al., 2006; Tribovillard et al., 2012). Ti/Al and Zr/Al ratios provide insights into the intensity of terrigenous detrital input.

The elevated Ti/Al and Zr/Al ratios observed during the S₁l₁¹ in comparison to the O₃w (Figures 6O, P) indicate a relatively more significant input of terrigenous detritus in the Yangtze foreland basin during this period. This can be attributed to the intensified tectonic activity (Guangxi movement), which supplied a substantial detrital input starting from the late O₃w. Furthermore, the sea level decrease during the Hennantian glaciation led to the exposure and weathering of certain portions of the original seafloor material. Subsequently, these materials were transported to the seabed after the glaciation, resulting in a comparatively higher input of detrital material during this period. Following the sea level rise to a certain threshold, the water environment stabilizes, leading to a gradual decline in the terrigenous detritus (Ti/Al) content, albeit the change is not significant in its entirety. During the S₁l₁², a global decrease in sea level occurred, resulting in an upward trend in Ti/Al ratios, signifying an augmentation in the input of terrigenous detritus.

Moreover, the variation in shale lithofacies indicates alterations in the intensity of terrigenous detritus input. Chapter 4.3 demonstrates that the lowermost portion in S₁l₁ comprises siliceous shale, which gradually transitions vertically to mixed shale and clay shale with a higher clay mineral content. This vertical change signifies an augmentation in the intensity of terrigenous detrital input.

5.3.6 Sedimentation rate

The sedimentation rate (SR) notably impacts the preservation duration and type of OM (Ding et al., 2021; Khaled et al., 2022). In marine sedimentary environments, sediment rich in OM tends to accumulate in areas with low oxygen levels and slow SR (Wignall and Twitchett, 1996; Schulte et al., 2000; Chen et al., 2006). According to Ibach (1982), an increase in SR can enhance the burial efficiency of OM when the SR of black shale falls below 3.2 m/Myr. Differential sediment particle SR induces REE fractionation (Murray et al., 1991). Under slow SR, REE are adsorbed by clay, complexed with OM, and subjected to pertinent chemical reactions. This process leads to the depletion (or enrichment) of light (or heavy) REE, resulting in significant shift in the (La/Yb)_N (with N representing the chondrite-normalized parameter). An approaching (La/Yb)_N of 1 indicates minimal or weak fractionation of REE, implying a higher SR. Conversely, substantial deviation from 1 signifies notable REE fractionation, suggesting reduced SR (Elderfield and Greaves, 1982). The (La/Yb)_N for the O₃w-S₁l₁ black shales are S₁l₁ (4.48–11.33, avg. 9.47)>O₃w (7.13–9.32, avg. 8.23), suggesting SR is lower for S₁l₁ than O₃w (Table 4; Figure 6Q). Guan (2020) calculated average SR in the graptolite zone in the Yangtze foreland basin: LM4 was 6.78 m/Myr; LM1-LM3 was 1.5 m/Myr; WF2-WF4 is 2.67 m/Myr, which is similar to the result of (La/Yb)_N indication in this paper.

Variations in SR are closely linked to the paleogeographic conditions of the era, with a stable tectonic setting being a crucial factor for the low SR development (Zhang et al., 2005; Li et al., 2021). From the late Katian to Early Rhuddanian, the Yangtze foreland ramp experienced relatively minor and later tectonic influence from the southeastern margin of the Yangtze Block (Cathaysia orogenic belt). More substantial influence emerged during the Telychian (Llandovery, Silurian) due to heightened orogenic activity in the southeast (He, 2022; Zhao et al., 2023). The foreland ramp, distanced from uplifting and erosional regions, has slow SR, favoring OM enrichment and preservation. The black shales with high TOC content at the base of S₁l₁ was deposited in an environment with relatively high SR (Sample S-1, (La/Yb)_N=4.48, TOC=8.21%). OM may have been quickly buried after early diagenesis, efficiently bypassing regions with intense degradation during sedimentation. This likely contributed to a higher proportion of preserved OM (Zhang et al., 2023).

5.3.7 Sea level change

The δCe values in the black shales of O₃w-S₁l₁ range from 0.78 to 1.17. The presence of a substantial negative δCe anomaly in the black shales at the bottom of S₁l₁ indicates a significant sea level drop during this period, reaching its lowest point compared to the reference plane. This drop corresponds to the Hirnantian glacial event that took place during the O-S (Figure 6R). The development of the Antarctic Gondwana ice sheet resulted in a swift and substantial global sea level decrease of approximately 100 m over a brief interval (Rong, 1984; Chen et al., 2001; Zhu et al., 2021). After the glacial period ended, the global sea level experienced frequent fluctuations. Nevertheless, the δCe value gradually increased and maintained stability within the range of 0.78–0.91. This pattern indicates a contemporaneous rise in sea level that aligns with trend depicted by the previously established global sea level curve (Liu et al., 2017). However, in the early Silurian, the regional Guangxi movement restricted the Yangtze Sea, impacting the water depth of the Yangtze Block due to the influence of surrounding uplifts. The magnitude of sea level rise was smaller than the uplift magnitude of the Zigong underwater highlands, resulting in a gradual or delayed increase in relative sea level (Li et al., 2023). The rise in relative sea level decreases the mixing efficiency in the water column, ultimately promoting the accumulation and preservation of OM in oxygen-depleted bottom water conditions. Additionally, this phenomenon creates more space for the burial of OM (Peng, 2021).

5.4 Controlling factors and enrichment mechanisms of OM

5.4.1 Lithofacies types influencing OME

Considering the influence of lithofacies types on TOC, the TOC content is classified into five groups based on the respective reference values: lean carbon (<1%), low carbon (1%–2%), medium carbon (2%–3%), high carbon (3%–4%), and rich carbon (>4%) (Wu et al., 2020). Organic-rich shale with TOC content exceeding 2% consists primarily composed of mixed shale (M-4, M-5, M-7) and siliceous shale (S-3, S-4, S-8) lithofacies (Figure 2C). These two shale lithofacies dominate primary gas-bearing interval of O₃w-S₁l₁, spanning from O-1 to S-12. The lithofacies thickness ratio between them is approximately 67.99% (41.17 m).

Commonly used proxies, such as quartz, QFM, SiO₂, and Si_xs (Si_bio), reflect silicon content characteristics in shale (Cai et al., 2020; Zhang et al., 2022). However, the correlation between silica content and TOC is nonlinear (Qiu et al., 2020a). Previous studies have demonstrated that shale reaches its highest TOC content when a specific balance between terrigenous silicon and biogenic silicon (Si_bio, Si_xs) is achieved. However, exceeding a threshold for Si_xs content leads to dilution and disaggregation effects that reduce TOC conten (Qiu et al., 2020a; Cai et al., 2020). Based on findings presented in Figure 8, the highest TOC content is reached when the quartz, QFM, and SiO₂ content around 55%, or when the proportion of Si_xs is approximately 30%.

FIGURE 8

FIGURE 8. Correlation analysis between indicators of siliceous content (quartz, QFM, SiO₂, Si_xs) and TOC. The red line and equation indicate a linear relationship, and the red band represents the linear equation’s confidence interval. Both correlation coefficients between TOC and siliceous content are less than 0.5, demonstrating either a weak negative correlation or no correlation with siliceous content. The blue line and equation indicate a nonlinear relationship, displaying a high degree of correlation between TOC and siliceous content and possessing characteristics of a standard distribution curve. This implies that the corresponding OM reaches its optimum state only when the siliceous content within the rock falls within a specific range rather than displaying a simple positive linear relationship.

5.4.2 The paleoenvironment influencing OME

Mechanisms influencing OME in marine sediments encompass diverse factors, classified into productivity and preservation mechanisms (Demaison, 1980; Pedersen. and Calvert, 1990; Murphy et al., 2000). The productivity mechanism underscores the pivotal role of primary productivity, proposing that heightened primary productivity and sufficient nutrient availability elevate oxygen consumption in the water column. Consequently, this fosters anoxic conditions, culminating in OME. Stated differently, anoxic bottom water conditions ensue as an outcome, not a cause, of OME (Pedersen. and Calvert, 1990; Sageman et al., 2003; Gallego Torres et al., 2007). The preservation mechanism indicates that anoxic water bodies formed in restricted basins can preserve OM.

TOC serves as a representative parameter to assess the extent of OME (Ma et al., 2018; Qiu et al., 2020a). We explore the correlation between TOC and eight sedimentary paleoenvironmental conditions, encompassing paleoredox conditions, paleoproductivity, paleoclimate, paleosalinity, paleowater temperature, terrigenous detrital input, sedimentation rate, and sea level change. Correlation analysis reveals the most substantial impact on OME by paleoredox conditions (e.g., V/Cr, Ni/Co) and paleoproductivity indicators (e.g., P/Al, Ba_bio), demonstrating positive correlations with TOC. Close behind are the paleosalinity indicators (e.g., Rb/K). Conversely, paleoclimatic conditions (e.g., CIA, C -value), the degree of terrigenous detrital input (e.g., Si_xs, Ti/Al), and sedimentation rate ((La/Yb)_N) show negative correlations with OME. In essence, a heightened OME concentration corresponds to reduced humid climate, terrigenous detrital input, and a relatively high SR (Figure 3). Overall, the OME observed in O₃w-S₁l₁ black shales from the Yangtze foreland basin reflects the influence of numerous factors. Thus, relying solely on individual mechanisms such as the productivity mechanism or preservation mechanism proves insufficient in comprehensively elucidating the enrichment and preservation of OM in black shales (Khaled et al., 2022; Li et al., 2022; Hu et al., 2023; Xu et al., 2023).

5.4.3 Mechanism of OME

The tectonic-sedimentary evolution of the early Paleozoic Yangtze foreland basin divided in two stages, each marked by subordinate tectonic loading: the Katian stage and the Rhuddanian stage (Su et al., 2007; Zhang et al., 2020). O₃w and S₁l₁, representative of two distinct sets of black shales, emerge as products of disparate tectonic movements, revealing varying paleogeographic origins (Ge et al., 2021). By assimilating the aforementioned paleoenvironmental geochemical indicators and regional geological context, we can summarize an OME mechanism for O₃w and S₁l₁ black shales within the Yangtze foreland basin (Figure 9).

FIGURE 9

FIGURE 9. Organic matter enrichment mechanisms of the Upper Ordovician-Lower Silurian organic-rich black shales in the Yangtze foreland basin. (A) The sedimentary environment of the O₃w-S₁l₁ black shales; (B) The sedimentary processes of the O₃w-S₁l₁ black shales.

During the late Katian, the development of the Gondwana ice sheet triggered a worldwide decline in sea levels, isolating the Yangtze foreland basin from the Yangtze Sea and impeding upwelling (Delabroye and Vecoli, 2010; Elrick et al., 2013; Algeo et al., 2016; Lu et al., 2019). Amidst the interplay of global sea level fluctuations and regional tectonic shifts (Guangxi movement), the Yangtze foreland basin transformed, fostering a seawater setting characterized by retention and high oxygen content (oxic) (Yan et al., 2009). Despite the flourishing of the Hirnantian fauna within the global cold water environment (Wang et al., 2002; Rong and Huang, 2019), the preservation of OM encountered hindrances within the oxic paleoenvironment.

During the Early Rhuddanian, a shift from a cold and arid to a warm and humid climate triggered the rapid melting of glaciers. The glacial meltwater substantially surged sea levels and relatively high SR (Haq and Schutter, 2008; Munnecke et al., 2010). Nevertheless, the continued compression associated with the Guangxi movement brought about a delayed or gradual relative sea-level rise for the Yangtze foreland basin (Li et al., 2023). These rising sea levels fostered euxinic conditions conducive to OM preservation. Furthermore, the Yangtze foreland basin experienced reduced terrestrial detrital input and diminished OM dilution compared to the source region (Cathaysia orogenic belt), augmenting OME (Zhao et al., 2023). Consequently, the OME observed in S₁l₁ black shales of the Yangtze foreland basin is rooted in regional tectonic dynamics and climatic factors. Subsequently, during the late Rhuddanian, the ongoing uplift of the forebulge brought about shallowing of the seawater on the foreland ramp. This period had a substantial volume of terrigenous coarse debris, disrupting the anoxic bottom environment. Consequently, the preservation conditions essential for OME deteriorated, leading to the dilution of OM abundance and a decline in TOC content.

6 Conclusion

Through our investigation of the organic geochemical characteristics, mineral composition, and sedimentary geochemical characteristics of the O₃w-S₁l₁ black shales in the Yangtze foreland basin, we have able to analyze the siliceous genesis, reconstruct the paleoenvironment, and examine the organic matter enrichment (OME) mechanism in the black shales. It provides valuable information for exploring and developing shale gas in the O₃w-S₁l₁ under different sedimentary locations. Our findings suggest that:

(1) The O₃w-S₁l₁ black shales exhibit a TOC content ranging from 0.43% to 8.21% and possess a sapropelic kerogen type (type I). The Ro values and T_max suggest that the shales have undergone the high-over mature stage, with a primary occurrence in the over-mature stage.

(2) The mineral composition of O₃w-S₁l₁ black shales reveals the identification of seven lithofacies: mixed shale lithofacies (M-7), clayey siliceous shale lithofacies (S-4), siliceous clay shale lithofacies (CM-2), calcium and clay siliceous shale lithofacies (S-3), calcium clay siliceous mixed shale lithofacies (M-5), calcium clay siliceous shale lithofacies (S-8), and clay calcium siliceous mixed shale lithofacies (M-4). The organic-rich shale, characterized by TOC >2%, is primarily found within the mixed shale lithofacies (M-4, M-5, M-7) and siliceous shale lithofacies (S-3, S-4, S-8).

(3) Biogenic silica (Si_xs, Si_bio) serves as the primary source of high silicon content in O₃w-S₁l₁ black shales. The highest TOC content is observed when Si_xs constitutes approximately 30% of the total silica content.

(4) The correlation between TOC content and paleoenvironment indicators reveals that the OME in O₃w-S₁l₁ black shales are controlled by regional tectonic movements and sea level changes. These changes result from the combined outcome of four essential factors: paleoredox conditions, paleoproductivity, paleoclimate, terrigenous detrital input and sedimentation rate. The Zigong Uplift may have a certain influence on the OME and shale gas accumulation potential in the Yangtze foreland basin.

(5) Despite the high level of paleoproductivity during the Late Ordovician, the oxygen-rich seawater conditions in the Yangtze foreland basin resulting from the global sea level decline were unfavourable for OME. Subsequently, during the early Rhuddanian period, a thick layer of organic-rich black shales were deposited in a relatively enclosed and reduced paleoenvironment. During the late Rhuddanian period, tectonic uplift disrupted the anoxic seawater conditions, resulting in a dilution of OM abundance due to an increase in terrigenous debris input.

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.

Author contributions

LZ formulated the research question and drafted the manuscript. YL contributed to the methodology. LZ and SZ conducted the statistical analysis. CZ supervised and edited the manuscript. CW was responsible for proofreading and editing the manuscript for grammar. All authors contributed to the article and approved the submitted version.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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