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Article

Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales

by
Weibing Shen
1,
Weibin Shen
2,*,
Xiao Xiao
1 and
Shihao Shen
3
1
National Key Laboratory of Petroleum Resources and Engineering, College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
2
Production Organization Department, Tianjin Anton Petroleum Machinery Manufacturing Co., Ltd., Anton Oilfield Services (Group) Ltd., Tianjin 300450, China
3
Basic Geological Survey Center, Geological Survey of Anhui Province, Hefei 230088, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(3), 296; https://doi.org/10.3390/min15030296
Submission received: 9 January 2025 / Revised: 24 February 2025 / Accepted: 24 February 2025 / Published: 14 March 2025
(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)
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Abstract

:
Permian black shale, as a potential target for marine shale gas exploration in South China, is characterized by its great thickness and organic matter (OM) content. To understand the constraints on the heterogeneous accumulation of OM in Permian black shale, high-resolution geochemical data related to paleoenvironment variations are collected on the Gufeng and Dalong Formations of the Putaoling area, the Anhui province, and the Lower Yangtze area. The OM was heterogeneously enriched in the Permian shales, as shown by the highly organic-matter-rich Gufeng Formation and the moderately organic-matter-rich Dalong Formation. The distribution patterns of rare earth elements (REEs) indicate a stably high sedimentary rate throughout the shale deposition. Redox indexes, including MoEF, UEF, V/Sc, and U/Th, indicate anoxic conditions for the deposition of the Gufeng and Dalong Formations, and that seawater oxygenation has occurred. The stratigraphic decreases in the (Fe+Mn)/Ti ratios, the index of chemical alteration (CIA), and the content of nutrient elements demonstrate the upward weakening patterns of hydrothermal activity and chemical weathering, which result in a reduction in the primary production. The redox state combined with the primary production jointly control the heterogeneous accumulation of OM in the Permian shales. Our paleoenvironmental evolution model for OM accumulation in the black shales indicates that the Gufeng Formation might be the priority object for the exploration of shale gases in the Permian strata within the Lower Yangtze area.

1. Introduction

Shale gas is an important alternative resource in the energy field and has become an integral part of the global diversified energy landscape [1,2]. In China, shale gas is abundant and its exploration to date has dominantly occurred in the Upper Yangtze area, South China [3,4,5]. Nevertheless, many studies have shown that the Lower Yangtze area is a potential area for shale gas exploration [6,7,8]. Well Gangdi 1 and Well WWD1 in the Anhui province of the Lower Yangtze area recently made breakthroughs in shale gas exploration within Permian sea–land transition deposits [9], indicating Permian black shale as a potential target for shale gas exploration in South China [3,5,10]. Hence, Permian shales from the Lower Yangtze area have attracted more and more attention [10,11].
In the Lower Yangtze area, the previous studies showed that Permian black shales, with a large OM abundance, were stably distributed and shallowly buried [11,12]. However, the OM distribution and accumulation mechanisms to date remain unclear. This is especially the case when there are significant heterogeneities within the OM distribution and the related causes of the potential heterogeneities in the OM distribution are unclear [13]. The enrichment of OM in the shale is controlled by the evolution of the paleoenvironment, mainly including the biological productivity, redox state, and sedimentary rate [14]. Generally, a low total organic carbon (TOC) value may reflect low productivity, low sedimentary rate, or oxic conditions. Thus, an analysis of the paleoenvironment is key to investigating OM accumulation in the deposits. The elemental geochemistry of siliclastic sediments have great potential to track the paleoenvironment evolution [15,16,17,18]. A decrease in nutrient elements of the siliclastic sediments, such as P, may indicate a weakening of biological productivity [19,20]. Redox-sensitive elements significantly change during seawater oxygenation, significantly increasing oxide content [21,22,23]. The sedimentary rate controls OM preservation and can be qualitatively characterized by the distribution pattern of rare earth elements, such as the La/Yb ratio [24,25].
In this paper, we carry out element and TOC analysis on the Permian deposits, namely the Gufeng and Dalong Formations, from a shelf setting in the Lower Yangtze area, South China. In combination with the petrological study, we attempt to elucidate the constraints on the Permian environment evolution and show their impact on OM accumulation in the Permian shales.

2. Geological Setting and Section Description

The Lower Yangtze area, located on the northeastern Yangtze Block, is composed of the Suwan Tectonic belt and the Jiangnan uplift (Figure 1; [26]). The Permian strata, including the Qixia Formation, the Gufeng Formation, the Longtan Formation and the Dalong Formation, are well developed in this area (Figure 2a; [13,26]). The Gufeng/Dalong Formations are characterized by their organic mud shale layers. When the Gufeng Formation was developed, a land shelf developed from the southern, from the central to the northern parts, respectively [27]. Toward the northern and southeastern lands, the facies changed from a shelf to a delta (Figure 1c). During the Dalong Formation deposition, the sea level fell and the sedimentary facies were different, although the land in the northern and southeastern areas still existed [13,27]. In the central part, there was a deep-water shelf and a pre-delta area, which gradually transitioned into a shallow-sea shelf, a platform margin, an isolated platform, a shelf, a pre-delta area, a near-coastal region, and a tidal area toward the southeast. Toward the north, the deep-water shelf and pre-delta area directly transitioned into the fluvial zone and a swamp (Figure 1d).
The Putaoling section, as the study object, is situated in the Suwan tectonic belt in the Lower Yangtze area. Carboniferous–Permian–Triassic deposits are observed in the section, and the Permian strata includes the Qixia/Gufeng/Longtan/Dalong Formations (Figure 2a). The Gufeng Formation, ca. 33 m, is mainly composed of black shales (Figure 2b). The shale is dense and hard, and weak laminas are developed. The siliceous clastic particles are common and show weak horizontal laminas, indicating a deep-water environment. The Dalong Formation is about 32 m (Figure 2c). The lower and middle members of this formation are black shales, and the upper member of this formation comprises gray-black siliceous/mud mixed shales (Figure 2c).

3. Sample and Methods

A total of 38 shale samples from the Gufeng/Dalong Formations was obtained, with an average interval of ca. 2 m, including 22 pieces of the Gufeng Formation and 16 pieces of the Dalong Formation. Surface sample number, depth, and lithology are shown in Figure 2. The major, trace, and rare earth elements and the TOC were measured at the National Geological Experimental Testing Center in China.

3.1. Sampling

To ensure sampling accuracy, a surface layer with a thickness of approximately 0.3 m was removed to avoid interference from weathering. The surface layers of the 38 samples were removed in the laboratory, and the samples were powdered to <74 µm.

3.2. TOC Analysis

To remove the carbonate minerals, an appropriate amount of 10% HCl was used to dissolve the powdered sample (about 0.5 g). The residue was washed with ultrapure water and then dried in a desiccator for 10 h at 60 °C, followed by combustion at 900 °C to oxidize the organic carbon in a pure oxygen atmosphere. The resultant carbon dioxide was subsequently measured to provide the TOC data, using a LECO CS-230 carbon analyzer (National Geological Experimental Testing Center in Beijing, China) with an accuracy < 0.5%.

3.3. Elemental Analysis

About 30 mg and 25 mg of the powdered samples were accurately weighed for the major element analysis and trace/rare element analysis, respectively. The sample preparation and testing process were taken from the study by Shen et al. [15]. The testing methods for the major element analysis and the trace/rare element analysis were inductively coupled plasma–atomic emission spectrometry (ICP-AES, National Geological Experimental Testing Center in Beijing, China) and inductively coupled plasma–mass spectrometry (ICP-MS, National Geological Experimental Testing Center in Beijing, China), respectively. Analytical precision and accuracy were <2–8% and less than 10%.

3.4. Data Processing

Element accumulation factors (EFs) were calculated from XEF = [(X/Al)sample/(X/Al)PAAS] [31], where the element content of the standard value of North American shale (PAAS) was adapted from the study by Taylor and McLennan [32]. (La/Yb)N was calculated from (La/Yb)N = [(X/Al)sample/(X/Al)N] [33], where the subscripts of N denoted the elemental concentrations which were normalized to PAAS [32]. Europium (Eu) anomalies were calculated from Eu/Eu* = EuN/(SmN × GdN)0.5 [34], where the subscripts of N denoted the elemental concentrations which were normalized to PAAS [32].

4. Results

4.1. TOC

Based on the previous evaluation criteria for OM abundance of marine shales, shales can be divided into the following three categories: (1) organic-matter-poor shales (TOC < 2%), (2) organic-matter-rich shales (TOC: 2% to 4%), and (3) highly organic-matter-rich shales (TOC > 4%). The TOC values in the Gufeng Formation have a range of 1.11–18.87% (average of 7.28%). Some data regarding the Gufeng Formation are from the study by Shen et al. [13]. Vertically, the Lower Gufeng Formation has smaller TOC values than those of the Upper Gufeng Formation. Notably, the TOC values at the top of the Gufeng Formation are generally above 5% and corresponds to highly organic-matter-rich shale. The TOC values of the Dalong Formation have a range of 0.52–9.44% (average of 3.33%) and the formation is characterized by organic-matter-rich shale. Specifically, the TOC values in the Lower Dalong Formation are larger, while the TOC values in the Upper Dalong Formation decrease to less than 2%, corresponding to organic-matter-poor shale.

4.2. Elemental Compositions

The w(SiO2), w(Al2O3), w(CaO), w(TFe2O3), and w(P2O5) content within the shales of the Gufeng Formation range from 53.6% to 92.9% (average 77.94%), from 1.0% to 9.7% (average 4.6%), from 0.02% to 9.9% (average 1.08%), from 0.5% to 5.1% (average 2.16%), and from 0.01% to 7.9% (average 0.66%), respectively. Vertically, the w(P2O5) content increases gradually from the bottom to the top and reaches a maximum of 7.9% at the top. The distribution of the other elements is relatively stable. Some data regarding the Gufeng Formation are from the study by Shen et al. [13]. The w(SiO2), w(Al2O3), w(CaO), w(TFe2O3), and w(P2O5) content within the shales of the Dalong Formation range from 31.6% to 91.6% (average 66.18%), from 1.9% to 15.6% (average 9.14%), from 0.02% to 29.8% (average 5.07%), from 0.95% to 4.8% (average 2.35%), and from 0.02% to 0.25% (average 0.085%), respectively. Overall, both the formations are enriched with SiO2 and are composed of siliceous rocks and argillaceous siliceous shales. Notably, the P2O5 content in the Gufeng Formation is higher than in the Dalong Formation.
Redox-sensitive elements can reflect the seawater redox state. Compared to PAAS [32], V, Mo, and U are highly enriched in the black shales of the Gufeng and Dalong Formations and vertically change from the bottom to the top. Their average contents are as follows: (1) the average contents of V are 902.1 ppm in the Gufeng Formation and 852.6 ppm in the Dalong Formation; (2) the average contents of Mo are 89.5 ppm in the Gufeng Formation and 113.2 ppm in the Dalong Formation; and (3) the average contents of U are 16.9 ppm in the Gufeng Formation and 11.0 ppm in the Dalong Formation. Some data regarding the Gufeng Formation are from the study by Shen et al. [13]. In addition, the Fe, Ni, and Zn contents also show vertical variations and are as follows: (1) the average content of Fe in the Gufeng Formation is 1.48 ppm, which is similar to 1.59 in the Dalong Formation; (2) the average content of Ni in the Gufeng Formation and the Dalong Formation are 84.8 ppm and 71.5 ppm, respectively; (3) the average content of Zn in the Gufeng Formation (50.3 ppm) and the Dalong Formation (52.8 ppm) are similar.
REE in the Gufeng Formation and the Dalong Formation range from 33.2 μg/g to 556.54 μg/g (average 131.14 μg/g) and from 43.21 μg/g to 212.16 μg/g (average 126.49 μg/g), respectively. Some data regarding the Gufeng Formation are from the study by Shen et al. [13]. Average values for the two formations are obviously smaller than the value (185 μg/g) of PAAS [32]. The rare earth element distribution pattern of the samples in the Gufeng Formation show a slight Ce-negative anomaly, a slight Eu anomaly, and an obvious Y anomaly. Similarly, the rare earth element distribution pattern of the samples in the Dalong Formation also shows an obvious positive Y anomaly and a slight Eu anomaly (Figure 3). At the top of the Gufeng Formation, the rare earth element distribution pattern of Sample GF21 shows a ‘hat’ distribution, which is similar to that of a rock of hydrothermal origin. Similarly, Sample DL1 at the bottom of the Dalong Formation also has a ‘hat’ distribution of rare earth elements and a positive Eu anomaly.

5. Discussion

5.1. Sedimentary Rate

OM enrichment has a certain relationship with sedimentary rate [15,24]. Under suitable conditions, a high sedimentary rate can reduce the oxidative decomposition of OM and the consumption of benthic organisms, leading to OM enrichment [35,36]. The REE+Y differentiation can evaluate the sedimentary rate [24]. REE+Y exist in water and are adsorbed on minerals, and the different duration times of such REE+Y adsorption on minerals can lead to REE differentiation [24,25]. If the sedimentary rate is high, the REE+Y have little time to adsorb on minerals, leading to weak REE+Y differentiation and a flat distribution pattern of the REE+Y. On the contrary, if the sedimentary rate is low, the REE+Y have enough time to adsorb on minerals, leading to significant differentiation and an oblique distribution pattern of the REE+Y. Thus, the distribution pattern of the REE+Y can be used to trace the sedimentary rate [15,24]. The (La/Yb)N ratio is a quantitative characterization index for the distribution pattern of the REE+Y, as follows: (1) (La/Yb)N ratios of close to 1.0 represent a flat distribution pattern of the REE+Y, showing a high sedimentary rate; (2) (La/Yb)N ratios of much less than or greater than 1.0 represent an oblique distribution pattern of the REE+Y, showing a low sedimentary rate [24,25].
The distribution pattern of the REE+Y of the samples from the Gufeng and Dalong Formations in the Putaoling section are generally flat with low REE differentiation, indicating a relatively high sedimentary rate throughout shale deposition (Figure 3). Meanwhile, the (La/Yb)N ratios, respectively, range from 0.45 to 2.35 (average 0.99) in the Gufeng Formation and from 0.59 to 1.60 (average 0.98) in the Dalong Formation (Figure 4), also showing a relatively high sedimentary rate. The distribution patterns of the REE+Y in the shales from the Dalong Formation are flatter than those from the Gufeng Formation. Compared to the Dalong Formation, stronger (La/Yb)N ratio dispersions are observed in the Gufeng Formation. The maximum (La/Yb)N ratio in the Lower Gufeng Formation is 2.35, and the minimum (La/Yb)N ratio is 0.45. The proportion of (La/Yb)N ratios within the range of 0.9–1.0 accounts for 75% of all the (La/Yb)N ratios in the Dalong Formation. These show a higher sedimentary rate during the Dalong Formation deposition.

5.2. Paleoclimate

Paleoclimate, related to the weathering of rocks and the erosion of sediments, controls the terrigenous nutrient influx of the ocean [37]. CIA is a commonly used paleoclimate index. CIA data from 80 to 100 represent intense weathering under tropical climate conditions. CIA values from 60 to 80 indicate moderate weathering under warm and humid climate conditions, and CIA values of less than 60 represent moraine clay formed in cold, dry climate conditions [38].
For the Gufeng Formation, the average CIA value from the black shales is 81.2, reflecting an overall warm and humid environment (Figure 4). From the bottom to the top, the CIA values increase, and they range from 60.4 to 86.1 (average 74.0) in the Lower Gufeng Formation, indicating a typical warm and wet climate. The CIA values in the Upper Gufeng Formation range from 78.6 to 89.4 (average 84.2), indicating a hot and humid environment. The CIA values from the black shales in the Dalong Formation range from 69.4 to 76.8, with an average of 72.4, reflecting a warm and humid environment. The CIA values show that the chemical weathering intensity recorded in the Dalong Formation is weaker than that in the Gufeng Formation (Figure 4). Similarly, the Sr/Cu ratio is also commonly used to trace paleoclimate [38,39], where Sr/Cu ratios of 1.3–5 and >5 reflect a warm/humid environment and an arid/hot environment, respectively. Most of Sr/Cu ratios of the Gufeng and Dalong Formations in the Putaoling section are smaller than 5, indicating a warm/humid environment (Figure 4). The warm climate is likely to occur near the equator. Interestingly, the South China craton was located near the equator during the Permian (Figure 1a). Similarly, the CIA values of the Dalong Formation shales in the South Anhui province vary from 56.3 to 79.3 (average 70.3), also indicating a warm paleoclimate with medium chemical weathering [40]. In addition, the CIA values of the black shales investigated in the Duwi Formation from El Sebaiya, Nile Valley, Egypt, varied from 80.78 to 84.61, with an average of 82.86%. This value suggested that these rocks possibly accumulated during a period of strong intensity chemical weathering, which supports our interpretation [41,42].

5.3. Hydrothermal Activity

Seafloor hydrothermal fluid is enriched with nutrients, which can provide an abundance of nutrients for the flourishing of microbial communities, such as algae [43]. The Permian strata in the southern Anhui area developed multiple layers of widely distributed volcanic ash and volcanic porphyries [44,45], which indicates that intense volcanic activity had occurred and hydrothermal fluid was present in the Lower Yangtze area. The (Fe+Mn)/Ti ratios can be used to distinguish the hydrothermal activities. When the (Fe+Mn)/Ti ratio is larger than 20 ± 5, hydrothermal fluids may be present [46]. As can be seen from Figure 4, the (Fe+Mn)/Ti ratios of the Gufeng Formation were mainly higher than 20, indicating that hydrothermal activity was frequent during the deposition of the Gufeng Formation. Specifically, (Fe+Mn)/Ti ratios in the Lower Gufeng Formation were between 8.7 and 39.4 (average 21.4), showing that hydrothermal activities were likely episodic (Figure 4). The (Fe+Mn)/Ti ratios in the Upper Gufeng Formation were between 4.3 and 69.6 (average of 32.6), indicating intense hydrothermal activity. Except for Sample DL1, the (Fe+Mn)/Ti ratios of the Dalong Formation rock samples were less than 20, obviously indicating that there was no hydrothermal activity during the Dalong Formation deposition (Figure 4).
The REE are stable in diagenetic process, which is why their distribution pattern can be used to interpret paleoenvironmental information [47,48]. Generally, deposits formed by hydrothermal fluids can reflect LREE enrichment, low ∑REE content, no obvious Ce/Ce* anomaly, significant positive Eu/Eu* anomaly, etc. The ∑REE contents of the Gufeng Formation and the Dalong Formation are low, and heavy and light rare earth elements are severely depleted (Figure 3). Most of the samples present a slight Ce-negative anomaly and a weak Eu- positive anomaly. The distribution pattern of the REE+Y is similar to that of a hydrothermal plume, suggesting that the Gufeng and Dalong siliceous rocks might be the product of hydrothermal depositions. Notably, the distribution patterns of the REE+Y of some samples in the Gufeng Formation show obvious characteristics of hydrothermal plume products. For example, Samples GF21 and GF19, with large (Fe+Mn)/Ti ratios, have significantly low ∑REE contents; the ∑REE contents of Sample GF21 is only 48.6 (Figure 3). The distribution patterns of the REE+Y are typically a ‘hat’ shape, and there is a significant positive Eu/Eu* (1.16) anomaly, which reflected an influence of hydrothermal activity during the deposition of the Upper Gufeng Formation. Hydrothermal proxies, such as Al/(Al+Fe+Mn), Eu/Eu*, Al-Fe-Mn diagram, and LuN/LaN, also indicated a significant hydrothermal contribution for the Gufeng Formation in the Tongling areas (South China), which supported our interpretation [49].

5.4. Paleoproductivity

When the Gufeng and Dalong Formations were deposited, a chemical weathering of the parent rocks occurred due to the warm/humid environment, and frequent volcanic activities were accompanied by considerable hydrothermal activities, which provided plenty of nutrients to the ocean, promoting microbial prosperity within the surface water and thus improving paleoproductivity. Compared to that of the Dalong Formation, the climate for the Gufeng Formation deposition was more favorable for chemical weathering, and the volcanic activity was stronger, which controlled the vertical variation in Permian biological productivity. Some nutrients for life, such as P and Zn, can be used to evaluate the paleo-productivity [50,51].
Phosphorus plays a fundamental role in many metabolic processes and serves as an essential structural and functional component of all forms of life on Earth [52] and, ultimately, limits the net marine primary productivity on geological timescales [53]. Phosphorus, as an important nutrient for biological metabolism and an organism’s skeleton, is usually buried in sediments after biological death, and thus P/Al can be used to characterize biological productivity [14,54]. The P/Al values of the samples from the Gufeng Formation and the Dalong Formation were 0.0814 and 0.0153, respectively, which were significantly higher than that of PAAS (average 0.0070), indicating higher biological productivity (Figure 5). Compared to P and the other major nutrients, V, Ni, Fe, Zn, and other trace metal micronutrients have low concentrations in seawater and low requirements for biological activities. However, they are indispensable for the photosynthesis of marine phytoplankton. FeEF, ZnEF, VEF, and NiEF are used to evaluate productivity [14]. The FeEF of the samples from the Gufeng Formation and the Dalong Formation in the Putaoling section were 1.61 and 0.86; the ZnEF were 2.48 and 1.42; the VEF were 31.31 and 14.34; and the NiEF were 8.10 and 3.67, respectively, showing an upward decreasing trend (Figure 5). The vertical evolution characteristics of Permian P/Al, FeEF, ZnEF, VEF, and NiEF show that biological productivity decreased significantly. Specially, the abnormal high values appeared in the topmost of the Gufeng Formation, which reflected the highest paleoproductivity (Figure 5).

5.5. Redox Conditions

In order to trace the seawater redox state, the connectivity of water in the study area was investigated. A Mo-TOC diagram can be used to judge the retention degree of the seawater [55,56]. According to Figure 6, the water retention degree during the deposition of the Dalong Formation was weaker than during the deposition of the Gufeng Formation. Samples from the Gufeng Formation mainly fell in the middle retention area, as the lower 7 samples were scattered and mainly fell in the middle retention area, while the upper 15 samples mostly fell in the middle–weak retention environment, indicating that the water retention degree during the Gufeng Formation deposition gradually weakened (Figure 6). Samples from the Dalong Formation were mainly distributed in the moderate–slight retention area, in which the lower 10 samples mainly fell in the slight retention area, and the upper 6 samples mostly fell in the moderate–weak retention environment, indicating that the water retention degree gradually increased from the early to late depositional time.
Redox-sensitive metallic elements (RSMs) are usually reduced and activated in anoxic water and transferred to sediments, resulting in high concentrations within deposits [21,22]. Shown by the relative enrichment of Mo and U, the Gufeng and Dalong Formations were deposited in an anoxic environment. According to the UEF–MoEF correlation graphs [22], the MoEF from the Lower Gufeng Formation were between 65.2 and 340.8 (average 146.3), and the UEF ratios were between 19.4 and 102.6 (average 53.2). The MoEF in the Upper Gufeng Formation ranged from 41.2 to 1070.2, with an average of 493.1, while the UEF ranged from 25.7 to 207.6, with an average of 69.6, and all fell in the sulfide region. The MoEF and UEF of the Lower Dalong Formation ranged from 98.0 to 1140.0 and from 12.4 to 71.4, with mean values of 423.4 and 33.7, respectively, indicating an anoxic state (Figure 7). The MoEF and UEF from the Upper Dalong Formation ranged from 9.9 to 471.0 and from 5.5 to 40.9, with mean values of 124.5 and 17.2 respectively, indicating an anoxic environment. It can be seen that water oxygen fugacity fluctuated and evolved during the different depositional periods of the Gufeng Formation and the Dalong Formation. Nevertheless, compared to that of the Gufeng Formation deposition, the water oxygen fugacity was higher during the Dalong Formation deposition (Figure 7).
The U/Th and V/Sc ratios can also be used to indicate seawater redox state. Generally, the smaller the U/Th and V/Sc ratios, the higher the seawater oxygen fugacity [21,22,57]. V/Sc ratios of less than 9 show an oxic environment; V/Sc ratios between 9 and 30 show an oxygen-poor environment; and V/Sc ratios of greater than 30 show an anoxic environment. U/Th ratios of less than 0.75 show an oxic environment; U/Th ratios between 0.75 and 1.25 show an oxygen-poor environment; and U/Th ratios of greater than 1.25 show an anoxic environment [58]. The V/Sc and U/Th ratios from the Lower Gufeng Formation were between 54.3 and 136.2, as well as between 0.69 and 0.95, with mean values of 87.3 and 0.83, respectively, indicating that the seawater was low in oxygen (Figure 7). The V/Sc and U/Th ratios of the Upper Gufeng Formation ranged from 93.9 to 208.2 and from 0.79 to 0.96, with mean values of 163.8 and 0.92, respectively, showing an anoxic environment. The V/Sc and U/Th ratios from the Lower Dalong Formation were between 81.9 and 269.7, as well as between 1.02 and 4.75, with mean values of 183.6 and 2.37, respectively, showing that the environment was anoxic (Figure 7). The V/Sc and U/Th ratios from the Upper Dalong Formation were between 12.6 and 101.8, as well as between 0.36 and 1.65, with mean values of 38.8 and 0.89, respectively, showing that the environment was low in oxygen. Compared to the Gufeng Formation, the U/Th and V/Sc ratios also indicate that the seawater oxygen fugacity was larger during the Dalong Formation deposition. Similarly, an anoxic environment was also reflected by the organic-rich deposits in the oil shales of the Duwi Formation from the Safaga–Qussier area in Egypt. Abou El-Anwarthe et al. [42,59] indicated that high organic enrichment in terms of quantity and quality was accompanied by enriched metal composition including trace elements that signify anoxic conditions. This composition pointed to prominent anoxic conditions that were stabilized by a slow sea level rise, inhibiting water circulation and favoring water column stagnation in a restricted basin.

5.6. Paleoenvironmental Model and OM Enrichment

During the deposition of the Permian shales in southern Anhui, the climate was warm and humid, and the hydrothermal activities were frequent, leading to the flourishing of microbial life. Meanwhile, the South China sea, with a high sedimentation rate, was dominated by an anoxic state and was a favorable place for OM enrichment.

5.6.1. OM Enrichment in the Gufeng Formation

During the deposition of the Lower Gufeng Formation, the warm climate in the Lower Yangtze area was advantageous for rock weathering, resulting in a significant nutrient influx in the shelf. The Lower Yangtze area was near the equator, and upwelling from high altitude carried the sea nutrients into the slope and onto the edge of the basin, and these nutrients contributed to the algae, radiolarian siliceous organisms [60]. After the death of an organism, the debris would settle to the seafloor and provide plenty of OM for black shale deposition [60]. In addition, our data indicated that seawater oxygen was significantly consumed by biological respiration, producing seawater stratification. The stratified ocean also provided benefits for OM preservation and OM-rich shale deposition. During the deposition of the Upper Gufeng Formation, rock weathering in the study area was further enhanced, and the terrigenous nutrient influx was further increased. Moreover, the hydrothermal activity in the study area was intense and brought many nutrients to the slope and basin margin. The nutrient influx promoted further organismal flourishing and an increase in productivity. Our results show that P, Fe, Zn, Ni, V, and the other elements in the Upper Gufeng Formation siliceous shale were obviously more enriched than those in the Lower Gufeng Formation. Moreover, with the sea level falling, the living space for organisms decreased. The death and decomposition of marine life would lead to the expansion of anoxic conditions and even the formation of euxinic conditions, which was beneficial to OM preservation and organic-matter-rich siliceous mudstone deposition. This was shown by the high TOC values (average 8.2%; Figure 8a).

5.6.2. OM Enrichment in the Dalong Formation

During the deposition of the Lower Dalong Formation, the Lower Yangtze area underwent a sea transgression, and the sea level rose. Nevertheless, the sea level was shallower than during the Gufeng Formation deposition, and the living space for the organisms decreased. Although small-scale upwelling developed in the local areas [45], promoting biological flourishing to a certain extent, silicate chemical weathering and volcanic hydrothermal activity were obviously weakened. This led to an obvious decrease in nutrient input into the ocean and limited improvement in biological productivity. Compared to the Gufeng Formation, the contents of P, Fe, Zn, Ni, and V in the Lower Dalong Formation were significantly lower. In addition, with the falling sea level, the redox boundary moved down, and seawater oxidation ranges increased. Thus, a seawater oxygenation event occurred. Shown by the decrease in the MoEF, UEF, U/Th and V/Sc ratios, the seawater oxygenation accelerated OM decomposition, which did not contribute to the organic-matter-rich shale deposition. During the deposition of the Upper Dalong Formation, although chemical weathering and volcanic hydrothermal intensity did not change significantly, the continuous shallowing of sea level further caused the oxidation of surface water, which was demonstrated by the decreasing redox indexes, such as MoEF, UEF, U/Th and V/Sc. As a result, a further decrease in TOC values in the Upper Dalong Formation was observed (Figure 8b).

5.6.3. Model of Heterogeneous Enrichment

The differential enrichment of OM in the Permian black shales within the Gufeng and Dalong Formations was not controlled by one single factor but by the interaction and coupling of the paleoclimate, redox state, biological productivity, and sedimentary rate. The Gufeng Formation deposition took place during a period when the regional sea level rose, during which the deposit depth grew while the seawater retention degree increased. The regional sea level rise brought deep-water nutrients to shallow marine euphotic zones, and the strong weathering and frequent hydrothermal activity brought plenty of nutrients to the ocean, promoting microbial growth in the surface waters and enhancing biological productivity. The respiration of the organisms accelerated the consumption of oxygen in the seawater, and euxinic water was beneficial for the OM preservation, forming a highly enriched form of OM. During the period of the Dalong Formation deposition, the regional sea level fell, the seawater became shallow, and the retention degree reduced. The water oxygen fugacity was higher than during the Gufeng Formation deposition. The seawater oxygenation was detrimental to OM preservation. At the same time, the chemical weathering and volcanic activity decreased, and the nutrient input to the sea was limited, thereby reducing the biological productivity and the source of the OM. Nevertheless, a higher sedimentary rate shortened the time of seawater OM decomposition, which led to the rapid deposition and burial of OM. As a result, the medium degree of enrichment of the OM under the condition of an oxygen-poor environment occurred.
Although the sedimentary rate of the Gufeng Formation was lower than that of the Dalong Formation, the conditions for the formation and preservation of OM in the Gufeng Formation were better, resulting in a heterogeneous accumulation of OM in the black shales of the Gufeng Formation and the Dalong Formation (Figure 8 and Figure 9).

6. Conclusion

In the Lower Yangtze area, the Permian black shales act as a record of the complex paleoenvironmental evolution, including a relatively stable sedimentary rate, dynamic seawater redox state, paroxysmal hydrothermal activities, and changing intensity of continental weathering. Our findings are as follows: (1) During deposition of the Gufeng Formation, strong continental weathering and intense hydrothermal activity brought a large amount of nutrients to the Lower Yangtze area, greatly improving the primary productivity and OM supply. Meanwhile, anoxic conditions and high sedimentary rate were beneficial to OM preservation. (2) During deposition of the Dalong Formation, the nutrient influxes decreased with the decreasing hydrothermal activity and weakening of the continental weathering, resulting in biodepletion. Meanwhile, seawater oxygenation accelerated OM degradation, although an increasing sedimentary rate reduced the OM exposure duration. This dynamic evolution of the oceanic environment controlled the heterogeneous accumulation of OM in the Permian shales, as shown by the highly organic-matter-rich Gufeng Formation and the moderately organic-matter-rich Dalong Formation.

Author Contributions

Methodology, X.X., W.S. (Weibing Shen) and S.S.; software, X.X. and S.S.; formal analysis, X.X. and S.S.; investigation, X.X., W.S. (Weibin Shen), W.S. (Weibing Shen) and S.S.; writing—original draft preparation, W.S. (Weibing Shen) and W.S. (Weibin Shen); visualization, X.X., W.S. (Weibing Shen) and S.S.; supervision, W.S. (Weibin Shen); project administration, W.S. (Weibing Shen) and W.S. (Weibin Shen); funding acquisition, W.S. (Weibing Shen) and W.S. (Weibin Shen). All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (U2344211), the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (No. JB2322) and Science Foundation of China University of Petroleum, Beijing (No. 2462025QNXZ002).

Data Availability Statement

The original contributions presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Weibin Shen was employed by the Tianjin Anton Petroleum Machinery Manufacturing Co., Ltd., Anton Oilfield Services (Group) Ltd. The remaining 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.

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Figure 1. (a) Permian paleogeography and the relative location of South China (modified from Liu and Selby [28]). (b) Tectonic division map of South China and the location of the Lower Yangtze area. (c,d) Tectonic units and facies distribution of the Lower Yangtze area during the Gufeng Formation and Dalong Formation deposition, respectively (modified from Bai et al. [26] and Ding et al. [29]). The sampled Putaoling section can be seen.
Figure 1. (a) Permian paleogeography and the relative location of South China (modified from Liu and Selby [28]). (b) Tectonic division map of South China and the location of the Lower Yangtze area. (c,d) Tectonic units and facies distribution of the Lower Yangtze area during the Gufeng Formation and Dalong Formation deposition, respectively (modified from Bai et al. [26] and Ding et al. [29]). The sampled Putaoling section can be seen.
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Figure 2. (a) Stratigraphic log of the Permian in the Lower Yangtze area; (b,c) are stratigraphic column of the Gufeng Formation and Dalong Formation at the Putaoling section. (a) is modified from Shen et al. [30]. HL: Huanglong Formation; QX: Qixia Formation; GF: Gufeng Formation; LT: Longtan Formation; DL: Dalong Formation; YK: Yinkeng Formation.
Figure 2. (a) Stratigraphic log of the Permian in the Lower Yangtze area; (b,c) are stratigraphic column of the Gufeng Formation and Dalong Formation at the Putaoling section. (a) is modified from Shen et al. [30]. HL: Huanglong Formation; QX: Qixia Formation; GF: Gufeng Formation; LT: Longtan Formation; DL: Dalong Formation; YK: Yinkeng Formation.
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Figure 3. PAAS-normalized REE+Y patterns of Permian shales at the Putaoling section. (a,b) are the Gufeng Formation and the Dalong Formation, respectively. Data of the Gufeng Formation are from Shen et al. [13]. The red lines reflect rock samples of hydrothermal origin.
Figure 3. PAAS-normalized REE+Y patterns of Permian shales at the Putaoling section. (a,b) are the Gufeng Formation and the Dalong Formation, respectively. Data of the Gufeng Formation are from Shen et al. [13]. The red lines reflect rock samples of hydrothermal origin.
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Figure 4. Profiles of TOC, (La/Yb)N, CIA, Sr/Cu, and (Fe+Mn)/Ti from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from the study by Shen et al. [13]. The red and green circles in the stratigraphic column represent pyrite and phosphorus nodules, respectively.
Figure 4. Profiles of TOC, (La/Yb)N, CIA, Sr/Cu, and (Fe+Mn)/Ti from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from the study by Shen et al. [13]. The red and green circles in the stratigraphic column represent pyrite and phosphorus nodules, respectively.
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Figure 5. Profiles of TOC, FeEF, ZnEF, NiEF, and VEF from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from a study by Shen et al. [13].
Figure 5. Profiles of TOC, FeEF, ZnEF, NiEF, and VEF from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from a study by Shen et al. [13].
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Figure 6. (a) Plot of the Mo concentration versus TOC from Permian shales at the Putaoling section. The SW represents the seawater. (b) Plot of the MoEF versus UEF from Permian shales at the Putaoling section. Data on the Gufeng Formation are from a study by Shen et al. [13].
Figure 6. (a) Plot of the Mo concentration versus TOC from Permian shales at the Putaoling section. The SW represents the seawater. (b) Plot of the MoEF versus UEF from Permian shales at the Putaoling section. Data on the Gufeng Formation are from a study by Shen et al. [13].
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Figure 7. Profiles of TOC, MoEF, UEF, U/Th, and V/SC from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from a study by Shen et al. [13].
Figure 7. Profiles of TOC, MoEF, UEF, U/Th, and V/SC from Permian shales at the Putaoling section. QX: Qixia Formation; LT: Longtan Formation; HL: Huanglong Formation. Data on the Gufeng Formation are from a study by Shen et al. [13].
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Figure 8. Schematic diagram showing Permian climate, seawater redox state, distribution of black shale, hydrothermal activities, terrigenous input, and sedimentary rate in South China during (a) deposition of the Gufeng Formation and (b) deposition of the Dalong Formation. (a) Anoxic conditions and high primary productivity (intense hydrothermal activities and enhanced chemical weathering) led to OM enrichment of the Gufeng Formation. (b) Shrinking of anoxic conditions and low primary productivity (incongruent chemical weathering, limited hydrothermal activities, and high sedimentary rate) resulted in OM being moderately enriched in the Dalong Formation.
Figure 8. Schematic diagram showing Permian climate, seawater redox state, distribution of black shale, hydrothermal activities, terrigenous input, and sedimentary rate in South China during (a) deposition of the Gufeng Formation and (b) deposition of the Dalong Formation. (a) Anoxic conditions and high primary productivity (intense hydrothermal activities and enhanced chemical weathering) led to OM enrichment of the Gufeng Formation. (b) Shrinking of anoxic conditions and low primary productivity (incongruent chemical weathering, limited hydrothermal activities, and high sedimentary rate) resulted in OM being moderately enriched in the Dalong Formation.
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Figure 9. Plot of element indexes versus TOC from Permian shales at the Putaoling section. (a) (La/Yb)N versus TOC, (b) MoEF versus TOC, (c) V/Sc versus TOC, and (d) VEF versus TOC.
Figure 9. Plot of element indexes versus TOC from Permian shales at the Putaoling section. (a) (La/Yb)N versus TOC, (b) MoEF versus TOC, (c) V/Sc versus TOC, and (d) VEF versus TOC.
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Shen, W.; Shen, W.; Xiao, X.; Shen, S. Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales. Minerals 2025, 15, 296. https://doi.org/10.3390/min15030296

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Shen W, Shen W, Xiao X, Shen S. Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales. Minerals. 2025; 15(3):296. https://doi.org/10.3390/min15030296

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Shen, Weibing, Weibin Shen, Xiao Xiao, and Shihao Shen. 2025. "Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales" Minerals 15, no. 3: 296. https://doi.org/10.3390/min15030296

APA Style

Shen, W., Shen, W., Xiao, X., & Shen, S. (2025). Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales. Minerals, 15(3), 296. https://doi.org/10.3390/min15030296

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