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Article

Tectonic Impact on Organic Matter Enrichment in Paleozoic Marine Shales from the Yangtze Block, SW China

by
Dadong Liu
1,2,3,*,
Mingyang Xu
1,
Hui Chen
1,
Qian Cao
1,
Zhenxue Jiang
1 and
Xianglu Tang
1
1
State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China
2
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 102206, China
3
Key Laboratory of Unconventional Natural Gas Evaluation and Development in Complex Tectonic Areas, Ministry of Natural Resources, Guiyang 550004, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(6), 1028; https://doi.org/10.3390/jmse13061028 (registering DOI)
Submission received: 6 May 2025 / Revised: 22 May 2025 / Accepted: 22 May 2025 / Published: 24 May 2025
(This article belongs to the Topic Formation Mechanism and Quantitative Evaluation of Deep to Ultra-Deep High-Quality Reservoirs)
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Abstract

:
The enrichment of organic matter in marine shale is a complex process involving tectonic–sedimentary interactions. The tectonic setting exerts critical control over sediment provenance, marine biota, and subaqueous environmental conditions in shale deposition. To unravel the mechanisms and differential controls of organic matter accumulation in marine shales across distinct tectonic regimes, this study systematically examines the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation shales in the Upper Yangtze Block, SW China. Through comprehensive geochemical analyses encompassing total organic carbon (TOC) contents, as well as major and trace elements conducted on 31 shale samples from the Niutitang Formation and 30 samples from the Longmaxi Formation, we characterized their depositional environmental features and compared the distinctions between them. The results indicate that both the Cambrian Niutitang Formation and Silurian Longmaxi Formation shales exhibit high TOC contents, which range from 1.04% to 8.83% (average 4.73%) and from 0.29% to 6.14% (average 3.35%), respectively. Paleoenvironmental proxies demonstrate that the Cambrian Niutitang shales developed under suboxic–anoxic to even sulfidic conditions, with moderate water restriction and high paleoproductivity levels, while the Silurian Longmaxi Formation was deposited under suboxic–anoxic environments with strong water restriction and low-to-moderate paleoproductivity. Organic matter enrichment in the Cambrian Niutitang Formation followed a “productivity + preservation model”, whereas the Silurian Longmaxi Formation primarily adhered to a “preservation-dominated model”. The differentiation in organic enrichment mechanisms between these two marine sequences is attributed to the distinct tectonic settings during their deposition. During the Early Cambrian, the Upper Yangtze Block was in a rift trough tectonic setting influenced by upwelling currents, which triggered algal blooms and subsequent bacterial sulfate reduction (BSR) coupled with marine anoxia and sulfidation. In contrast, the Early Silurian period featured a semi-restricted marine basin with weaker upwelling activity, where organic matter enrichment was predominantly controlled by a restricted, reducing water column. Our findings demonstrate that tectonic settings exert fundamental controls on nutrient availability for algal communities and water column retention levels, serving as critical determinants for organic enrichment processes in marine shale systems.

1. Introduction

Organic-rich shales predominantly occur during specific geological periods, not only documenting the environmental evolution of paleo-oceanic conditions, paleobiota, and paleoclimatic features, but also serving as hosts for diverse hydrocarbon resources and metal deposits [1,2,3]. Organic-rich marine shales develop across diverse tectonic settings globally, such as the Mississippian Barnett Shale in Texas, USA, formed in an intra-cratonic foreland basin [4]; the Late Jurassic Haynesville Shale of northwestern Louisiana, USA, deposited in a passive continental margin setting [5]; and the Eocene Bawang Member shales of the Belaga Formation in Borneo Island, formed within an active continental margin environment [6].
The enrichment of organic matter in shales typically results from the combined effects of multiple factors, such as primary productivity levels, redox conditions of water bodies, organic matter sedimentation rates, and terrigenous clastic inputs [7,8]. These environmental conditions are highly influenced by tectonic settings and climatic conditions [9,10]. However, the climatic influence on marine organic matter is substantially diminished compared to continental lacustrine basins owing to the vast volume of oceans, whereas the tectonic settings exert critical control on the enrichment of marine organic matter [11,12]. The tectonic regime governs nutrient fluxes in marine environments, thereby regulating the development of microorganisms and modulating paleoproductivity levels. Meanwhile, it dictates the terrigenous clastic input and provenance distance, which fundamentally constrain the confinement of subaqueous depositional systems [11,13]. These interconnected processes collectively determine the spatial heterogeneity of organic matter enrichment in marine shale successions.
The geochemical composition of shales provides an effective pathway for evaluating the effectiveness of source rocks [14] and the paleoenvironmental conditions that they were deposited in [15]. Total organic carbon (TOC) content stands out as one of the most critical parameters for evaluating hydrocarbon source rock quality. In highly mature shales where hydrocarbon generation processes have significantly diminished or ceased, TOC content acts as the definitive factor governing hydrocarbon generation potential [16]. Meanwhile, the environment-sensitive elemental compositions and their ratios can be used to reconstruct the sedimentary environments during their deposition [17,18]. Through systematic correlation analyses between these parameters and TOC contents, the dominant controls on organic matter enrichment in shale systems can be elucidated [19].
The Upper Yangtze Block developed two sets of organic-rich shales during the Early Cambrian and the Early Silurian periods, which are the most critical stratigraphic units for shale gas exploration and development in southern China, such as the Lower Cambrian Qiongzhusi and Niutitang formations and the Lower Silurian Longmaxi Formation, characterized by high organic matter abundance, significant cumulative thickness, and extensive distribution [20]. Earlier studies have indicated that the two sets of shale deposits were deposited in different tectonic backgrounds. The Lower Cambrian shales were deposited in a rift trough tectonic setting, when the Yangtze Block was connected to the open ocean, whereas the Lower Silurian shales were deposited in a compressional foreland setting, with limited connectivity to the open ocean [21,22].
This study focuses on the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation shales in the Upper Yangtze Block. Through systematic geochemical analysis, we reconstructed the paleo-sedimentary environmental conditions of the two formations, including paleoproductivity, paleo-redox conditions, water column restriction, terrigenous input, and hydrothermal activities, revealing the main controlling factors and differences in organic matter enrichment between the two marine shale units. The findings provide crucial insights for advancing the understanding of organic enrichment mechanisms in marine shales.

2. Geological Setting

During the Early Cambrian, the breakup of the Gondwana continent triggered a global marine transgression [23,24]. An extensional tectonic regime prevailed between the Yangtze and Cathaysian plates, creating a rift trough tectonic setting (Figure 1a) [23]. This period saw the deposition of high-quality source rocks in the Middle–Upper Yangtze Block, such as the Qiongzhusi Formation in the Sichuan Basin and the Niutitang Formation in the Chongqing, Guizhou, and Hunan provinces [25,26]. Subsequently, water shallowing occurred, with lithology transitioning from fine-grained shales to coarser clastic rocks like siltstones and sandstones [27,28,29,30].
During the Early Silurian, due to the convergent collision between the Cathaysian and Yangtze plates, the Upper Yangtze Block formed a semi-restricted marine basin with limited connectivity to the open ocean (Figure 1b) [25,31]. This created an extensive low-energy, undercompensated, and oxygen-depleted sedimentary environment [32]. During this period, the Longmaxi Formation organic-rich shales were deposited in the Middle–Upper Yangtze Block, which currently serve as the most critical stratigraphic unit for commercial shale gas development in China [33,34].
The Lower Cambrian Niutitang Formation primarily consists of carbonaceous shale, siliceous shale, and silty mudstone, showing an upward coarsening grain size trend with gradually decreasing organic matter abundance [35,36]. The Lower Silurian Longmaxi Formation is composed of black siliceous shale, carbonaceous shale, argillaceous shale, and argillaceous siltstone, overall exhibiting an upward-increasing sandy content and decreasing graptolite abundance [37,38].

3. Sampling and Analytical Methods

3.1. Sampling and Powdering

Systematic sampling of the Cambrian Niutitang Formation and Silurian Longmaxi Formation was conducted on the TX-1 and SX-1 wells in the Upper Yangtze Block, collecting 31 samples from the Niutitang Formation and 30 samples from the Longmaxi Formation, respectively (Figure 1). To avoid impacts from volcanic deposition and paleofluid infiltration, volcanic ash intervals and calcite vein sections were excluded. After repeated ultrasonic cleaning with deionized water and drying, samples were manually ground using an agate mortar at <200 mesh and 80 mesh for chemical analyses. The 80-mesh powders were carried out for TOC analysis, while the 200-mesh powders were employed for major and trace element analyses. The analytical data of the shales from well TX-1 are referenced from Liu et al. (2024) [36].

3.2. TOC Analysis

The measurement of TOC contents for the SX-1 well was conducted at the State Key Laboratory of Petroleum Resources and Engineering at the China University of Petroleum (Beijing, China) using a LECO CS230 carbon–sulfur analyzer (St. Joseph, MI, USA). Samples were treated with excess 5% HCl to remove carbonates, then rinsed with deionized water and dried. Decarbonated residues were combusted at 950 °C in the analyzer under pure oxygen, with CO2 measured by infrared detection.

3.3. Major Elements Analysis

Major elements determination was carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (IGCAS), utilizing a PerkinElmer NexION 350X inductively coupled plasma mass spectrometer (ICP-MS) (Waltham, MA, USA). Samples were calcined at 920 °C in a high-temperature furnace to remove organic matter. Approximately 0.5 g of calcined sample was mixed with eight times its mass of Li2B4O7, homogenized, and fused with a drop of 2% LiBr-1% NH4I flux in a platinum crucible for XRF analysis. The mixture was melted at 115 °C to form glass disks, which were then analyzed using the PerkinElmer NexION 350X ICP-MS (Waltham, MA, USA).

3.4. Trace Elements Analysis

Trace elements (including rare earth elements) analysis was also performed at the IGCAS, following the acid digestion method. Dried sample powder was calcined at 700 °C for 3 h to eliminate organic matter. Approximately 0.37–0.45 mg of the calcined sample was placed in a clean polytetrafluoroethylene (PTFE) sealed dissolution bomb and dissolved using HNO3, HF, and HClO4. The diluted solutions were analyzed by the PerkinElmer NexION 350X ICP-MS (Waltham, MA, USA).

4. Results

4.1. TOC Contents

The results show that both the Niutitang and Longmaxi Formation shales in the study area exhibit high TOC contents, with the Niutitang Formation shales showing overall higher values. The TOC contents of the Niutitang Formation shales range from 1.04% to 8.83% (average 4.73%), while those of the Longmaxi Formation shales vary between 0.29% and 6.14% (average 3.35%) (Figure 2). Detailed TOC contents are reported in Supplementary Table S1.

4.2. Major and Trace Elements

Major element compositions of the shales from the two formations are listed in full in Supplementary Table S1. The Lower Cambrian Niutitang Formation shales are predominantly composed of SiO2 (46.6–80.0%, avg. 64.5%), followed by Al2O3 (4.1–11.8%, avg. 8.0%) and Fe2O3(T) (0.6–9.4%, avg. 4.1%), with subordinate concentrations of other major oxides (e.g., CaO, K2O, Na2O). The Lower Silurian Longmaxi Formation shales show comparable SiO2 dominance (53.3–75.9%, avg. 64.4%) but higher Al2O3 (6.0–17.2%, avg. 12.2%) and slightly reduced Fe2O3(T) contents (1.7–5.6%, avg. 3.8%), with other major elements also exhibiting comparably low abundances.
Trace and rare earth element (REE) concentrations of the studied shale samples are also reported in Supplementary Table S1. The total REE concentrations of the Niutitang Formation shales vary from 106.8 to 197.6 ppm, with an average of 154.0 ppm, whereas the total REE contents of the Longmaxi Formation range from 122.5 to 361.7 ppm, with an average of 212.4 ppm. Post-Archean Australian Shale (PAAS)-normalized REE plots show that the Niutitang Formation is slightly depleted in light RERs (Ce, La, and Pr) and enriched in heavy REEs (Tb, Dy, Tm, and Lu), whereas the Longmaxi Formation generally exhibit relatively flat REE patterns (Figure 3).

5. Discussion

5.1. Paleoenvironmental Conditions During Deposition of the Two Formations

5.1.1. Paleoproductivity Conditions

Marine surface paleoproductivity refers to the capacity of plankton, algae, and other organisms to produce organic matter through photosynthesis [39]. Most organic matter are oxidized and decomposed during burial, with only a small fraction ultimately preserved; however, high primary productivity implies greater initial organic matter input [40]. In marine environments, barium (Ba) is typically deposited as bio-barite, making its content in marine sediments strongly correlated with primary productivity [41]. Phosphorus (P) is primarily concentrated through biological processes, and its enrichment often indicates elevated paleoproductivity levels [42].
When using elemental proxies to assess paleoproductivity, it is essential to exclude influences from terrigenous clastic and hydrothermal origins since sediment elements derive from multiple sources, including terrigenous clastic, biogenic, and hydrothermal inputs [43]. Aluminum (Al) and titanium (Ti) serve as reliable indicators of terrigenous clastic material [44]. By calculating the ratios of element concentrations to Al or Ti, the non-terrigenous clastic elemental contributions can be evaluated [45]. Additionally, Schoepfer et al. (2015) [42] proposed the following formula to estimate element concentrations unrelated to terrigenous clastic material:
Xbio = Xsample − [Alsample × (X/Al)PAAS]
where X represents the element, Xsample denotes the total elemental concentration in the sample, Xbio signifies the biogenic component of the element, and Alsample represents the total aluminum concentration in the sample.
Schoepfer et al. (2015) [42] proposed that a Babio range of 1000–5000 ppm indicates high paleoproductivity in ancient marine environments [45]. In the study area, the Niutitang Formation shales exhibit Babio values ranging from 2013.9 to 16,492.3 ppm (mean 4608.0 ppm), suggesting consistently high paleoproductivity during their deposition [46]. In contrast, the Longmaxi Formation shales show Babio values of 144.7–645.5 ppm (mean 409.6 ppm), reflecting low-to-moderate paleoproductivity levels (Figure 4a,b) [47]. Furthermore, Algeo et al. (2011) [45] identified a P/Ti ratio > 0.79 as indicative of high paleoproductivity and 0.34–0.79 as moderate productivity. The Niutitang Formation shales in the study area display P/Ti ratios of 0.16–1.58 (mean 0.44), while the Longmaxi Formation shales exhibit lower P/Ti ratios of 0.08–0.40 (mean 0.16), confirming that the Lower Cambrian Niutitang Formation experienced significantly higher paleoproductivity compared to the Lower Silurian Longmaxi Formation during their respective depositional periods (Figure 4c,d).

5.1.2. Paleo-Redox Conditions

Redox-sensitive elements (e.g., Mo, V, U, Cu, Ni) and their ratios are among the most effective proxies for tracing water column redox conditions. These elements exist as highly soluble oxidized ions in oxic waters but precipitate as insoluble forms in reducing environments (suboxic, anoxic, euxinic). Vanadium (V) and nickel (Ni), as iron group elements, tend to adsorb onto colloids and clay minerals, making their concentrations in fine-grained sediments highly sensitive to redox conditions. A V/(V+Ni) ratio < 0.46 indicates oxic conditions, 0.46–0.54 represents transitional environments, and >0.54 signifies reducing conditions [48]. Chromium (Cr) typically associates with terrigenous clastics by substituting Al in clay minerals, whereas V predominantly binds to organic matter as tetravalent ions under reducing conditions. Thus, the V/Cr ratio serves as a key paleo-redox proxy: <2 for oxic, 2–4.25 for suboxic, and >4.25 for anoxic environments [5]. Nickel is primarily delivered to sediments through organic matter transport. During organic degradation, Ni is released and immobilized in sediments via pyrite formation under sulfate-reducing conditions. As Ni remains stable post-deposition, sedimentary Ni concentrations reliably reflect initial organic matter fluxes. The Ni/Co ratio further discriminates redox phases: <5 (oxic), 5–7 (suboxic), and >7 (reducing), with higher values indicating stronger environmental reduction [49].
The analyzed Niutitang Formation shale samples exhibit V/(V+Ni) ratios varying from 0.50 to 0.89 (average 0.73), indicating anoxic to even euxinic conditions. Additionally, the V/Cr ratios of the Niutitang Formation vary between 2.28 and 17.72 (mean 7.45), while Ni/Co ratios range from 4.58 to 31.03 (mean 11.59), collectively confirming that the depositional environment of the Niutitang Formation was strongly reducing and locally sulfidic (Figure 5) [46]. For the Longmaxi Formation shales, V/(V+Ni) ratios range from 0.56 to 0.89 (average 0.71), V/Cr ratios from 0.91 to 8.83 (mean 3.62), and Ni/Co ratios from 2.96 to 19.56 (mean 9.06), demonstrating that the Longmaxi Formation was also deposited under predominantly strongly reducing conditions (Figure 5) [47].

5.1.3. Restricted Degree of Water Column

The restricted degree of seawater influences sedimentary environments and biogeochemical cycling [50]. Strongly restricted conditions induce water column anoxia, thereby favoring organic matter preservation. Trace elements Mo and U are less likely to enter sediments in oxygenated waters but readily precipitate and accumulate under anoxic conditions [51]. Notably, U begins to enrich in weakly reducing environments, whereas Mo precipitates only under strongly reducing conditions. These geochemical distinctions enable their use in assessing water restriction intensity. Based on the distribution patterns of Mo and U across different marine basin types, marine environments can be classified as non-restricted (open), weakly restricted, or strongly restricted. Algeo and Tribovillard (2009) [52] introduced molybdenum (MoEF) and uranium (UEF) enrichment factors to evaluate water restriction, calculated using the following formula:
XEF = (X/Al)sample/(X/Al)standard
where X represents the element, EF is the enrichment factor, Xsample and Alsample denote the concentrations of element X and aluminum in the sample, and Xstandard and Alstandard represent their respective concentrations in the standard. Here, we use the upper continental crust (UCC) as the standard. The concentrations of the elements in the UCC are reported in Taylor and Mclennan (1985) [53].
As revealed by MoEF-UEF covariation diagrams and Mo-TOC correlation plots (Figure 6), the Lower Cambrian Niutitang Formation generally developed in a moderately restricted marine environment, whereas the Lower Silurian Longmaxi Formation experienced moderately to strongly restricted conditions, with the latter demonstrating significantly stronger water restriction intensity compared to the Niutitang Formation.
Additionally, Sweere et al. (2016) [43] proposed the Al-Co × Mn index to assess water restriction environments, suggesting that Co × Mn > 0.4 indicates restricted conditions, while lower values reflect open/upwelling current settings. To eliminate terrigenous clastic influences, cobalt (CoEF) and manganese (MnEF) enrichment factors were calculated using Equation (2). Both the Al-Co × Mn diagram and the Al-CoEF × MnEF diagram demonstrate that the Lower Silurian Longmaxi Formation was deposited under relatively more restricted conditions compared to the Lower Cambrian Niutitang Formation (Figure 7).

5.1.4. Terrigenous Clastic Input

The influence of terrigenous clastic input on organic matter enrichment in marine shales is complex. Under high primary productivity conditions, terrigenous clastic input typically disperses and separates organic particles, thereby reducing organic matter abundance in depositional areas; however, in low-energy sedimentary environments, terrigenous clastics can transport organic-rich materials into the water column, enhancing organic matter abundance [54]. Simultaneously, terrigenous input impacts organic preservation by introducing oxygen into the water column, disrupting reducing environments, and promoting oxidation and biodegradation processes [55].
Aluminum (Al), as a major component of the continental crust, and Al-normalized ratios of silicon (Si), titanium (Ti), and zirconium (Zr) are widely recognized as reliable tracers for terrigenous clastic input [56]. In the study area, both the Niutitang and Longmaxi formations exhibit weak correlations between their Si/Al, Ti/Al and Zr/Al ratios and total organic carbon (TOC), indicating minimal influence of terrigenous input on organic matter transport and dilution in these marine shales (Figure 8).

5.1.5. Hydrothermal Activity

Hydrothermal activity can provide the ocean with abundant nutrients, promoting biological proliferation and enhancing primary productivity in water bodies [57]. Simultaneously, hydrothermal activity carries metal ions (Ni2+, Fe2+, As3+, etc.) and reducing gases like H2S, facilitating the formation of anaerobic–sulfidic environments conducive to organic matter preservation. As hydrothermal activity often transports metallic elements such as Fe and Mn, the Al/(Al+Fe+Mn) ratio has been widely used by previous researchers to indicate the intensity of hydrothermal activity, where a lower ratio corresponds to stronger hydrothermal influence [58]. The crossplot analysis of Fe/Ti and Al/(Al+Fe+Mn) reveals low-to-moderate hydrothermal activity during the deposition of the Niutitang Formation in the study area, whereas the Longmaxi shale formation shows minimal hydrothermal involvement (Figure 9). This disparity correlates with their distinct tectonic settings. During the Early Cambrian, the Upper Yangtze Block experienced an extensional tectonic regime, while during the Early Silurian, the Upper Yangtze Block was characterized by a confined marine basin environment.

5.2. Controlling Factors for Organic Matter Enrichment

Earlier studies have shown that the formation of organic-rich shales is associated with multiple factors, including plate tectonics and glacial activity cycles on a global scale, while on the basin scale, key variables include biological productivity, water restriction degree, and bottom water oxygen levels [12,59]. Researchers have attributed the development of organic-rich black shale systems to two primary models: the “preservation model” and the “productivity model” [43,59]. The “preservation model” posits that the formation of organic-rich shale is predominantly controlled by post-depositional preservation conditions, such as paleo-redox environments and water column restriction. In contrast, the “productivity model” emphasizes that organic enrichment is mainly governed by the initial input of organic matter, driven by activities like upwelling currents and deep hydrothermal fluids, which introduce trace elements and nutrients that enhance biological proliferation in water bodies, resulting in correspondingly higher levels of primary organic material [60,61].
During the deposition of the Lower Cambrian Niutitang Formation, the overall paleoproductivity was high, with moderate water column restriction, low-to-moderate hydrothermal activity, and anoxic-to-sulfidic conditions [46]. In contrast, the Lower Silurian Longmaxi Formation exhibited low-to-moderate paleoproductivity, with anaerobic–anoxic conditions, negligible hydrothermal activity, and strong water column restriction [47,62]. Sweere et al. (2016) [43] proposed the Co × Mn-Cd/Mo diagram based on modern marine sediments from diverse settings, including upwelling-dominated, open marine basins (e.g., Namibia margin, Arabian Sea margin, Gulf of California, Peru margin) and strongly restricted basins (e.g., Black Sea, Baltic Sea, Mediterranean Sea, Cariaco Basin), to identify the dominant controls on organic matter enrichment. They suggested that when Cd/Mo > 0.1, organic enrichment is primarily productivity-driven, whereas Cd/Mo < 0.1 indicates preservation-dominated conditions. The Co × Mn-Cd/Mo plot (Figure 10) reveals that organic enrichment in the Niutitang shale was jointly controlled by paleoproductivity and preservation conditions (reducing environment and water column restriction), representing a “productivity + preservation model”. Conversely, organic enrichment in the Longmaxi shale was predominantly governed by preservation conditions with limited influence from paleoproductivity, aligning with a “preservation model”.

5.3. Enrichment Models of Organic Matter in Shales from Different Tectonic Backgrounds

Following the end of the Sinian glaciation, significant changes occurred in global paleoclimate, paleo-oceanic environments, and biological systems. During the late Sinian to Early Cambrian, climate cooling and enhanced oceanic circulation accelerated the upwelling of nutrients such as phosphorus and iron through oceanic current activity [63]. Previous studies on the Sinian Doushantuo Formation proposed a dynamic redox “sulfidic wedge” model [64], where the ocean surface remained oxygenated, shelf-to-slope areas experienced intermittent anoxic sulfidic conditions (with free H2S) influenced by terrestrial sulfate supply, and deep-sea regions exhibited ferruginous environments. In the Early Cambrian, despite low seawater sulfate concentrations and limited terrestrial clastic input, nutrients delivered by upwelling boosted primary productivity, leading to explosive growth of benthic algae, phytoplankton, and other organisms [65]. This rapid oxygen consumption in bottom waters facilitated the development of anoxic reducing conditions on the seafloor. Additionally, increased organic matter promoted bacterial sulfate reduction (BSR), inducing sulfidation in deep-water areas [66]. Consequently, the organic matter enrichment in the marine Niutitang Formation shale of the study area resulted from the combined effects of high paleoproductivity (upwelling currents + weak-to-moderate hydrothermal activity) and favorable preservation conditions (highly reducing environment and moderate water column restriction) (Figure 11a).
By the Late Ordovician, intensified convergence between the Yangtze and Cathaysia blocks formed three paleo-uplifts, namely the Xuefeng, Chuanzhong, and Qianzhong uplifts, creating a confined tectonic framework within a restricted marine basin [67]. During the Early Silurian, post-Hirnantian deglaciation triggered rapid sea-level rise, establishing widespread seafloor anoxia that provided optimal conditions for organic matter preservation in the Longmaxi Formation [30]. This period saw weak upwelling currents and hydrothermal activity, with paleoproductivity levels significantly lower than those of the Early Cambrian [68]. The Upper Yangtze Block developed a deep-water shelf environment encircled by paleo-continents, characterized by poor connectivity to open oceans and stronger restriction compared to the Early Cambrian, forming a moderate-to-intensely restricted setting [69]. Consequently, organic enrichment in the Longmaxi Formation shale resulted primarily from the synergistic effects of a strongly reducing environment and intense water column restriction, with minimal influence from paleoproductivity (Figure 11b) [70].

6. Conclusions

(1) The Lower Cambrian Niutitang Formation was deposited in an anoxic-to-sulfidic, highly reducing environment, with high paleoproductivity levels, weak-to-moderate hydrothermal activity, and moderate water column restriction. Meanwhile, the Lower Silurian Longmaxi Formation developed in an anaerobic–anoxic reducing environment, with low-to-moderate paleoproductivity, negligible hydrothermal activity, and strong water column restriction.
(2) The organic matter enrichment in the Lower Cambrian Niutitang Formation marine shale is attributed to the joint effects of high paleoproductivity (driven by upwelling currents and weak-to-moderate hydrothermal activity) and favorable preservation conditions (highly reducing environment and moderate water column restriction), representing a “productivity + preservation model”. In contrast, the organic enrichment in the Lower Silurian Longmaxi Formation shale primarily results from the synergistic effects of a strongly reducing environment and intense water column restriction, with minimal influence from paleoproductivity, thus aligning with a “preservation model”.
(3) The distinct organic matter enrichment patterns between the Lower Cambrian and the Lower Silurian marine shales in the Upper Yangtze Block are primarily attributed to their contrasting tectonic settings. During the Early Cambrian, the Upper Yangtze Block was characterized by a rift trough tectonic regime with better connectivity to open oceans and active deep-seated fluid processes, whereas during the Early Silurian, the Upper Yangtze Block featured a restricted marine basin environment with limited oceanic exchange and diminished deep-seated fluid activity.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jmse13061028/s1, Table S1: TOC, major and trace elements of the studied shale samples from the Cambrian Niutitang Formation and the Silurian Longmaxi Formation.

Author Contributions

Conceptualization, D.L., Z.J. and X.T.; methodology, software, formal analysis, and visualization, M.X., H.C. and Q.C.; writing—original draft preparation, D.L.; writing—review and editing, D.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is funded by the National Natural Science Foundation of China (No. 42472185), Sinopec Petroleum Exploration and Production Research Institute (No. 33550000-24-ZC0699-0112), PetroChina Southwest Oil and Gas Field Company (No. 2024D104-01-07), and Geological Survey Foundation of Guizhou Province, China (No. 52000024P0048BH10174M).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors have reviewed and edited the output and take full responsibility for the content of this publication. We express our great gratitude to the three anonymous referees for their critical insight and constructive comments, which have greatly improved this manuscript. We also thank Majia Zheng, Ya Wu, and Kangjun Chen for their kind help during the sampling.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Paleogeography of the Yangtze Block during (a) the Early Cambrian and (b) the Early Silurian showing the location of the studied wells [11].
Figure 1. Paleogeography of the Yangtze Block during (a) the Early Cambrian and (b) the Early Silurian showing the location of the studied wells [11].
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Figure 2. Histogram of TOC contents of the shales from (a) the Niutitang Formation and (b) the Longmaxi Formation. Fm. = Formation.
Figure 2. Histogram of TOC contents of the shales from (a) the Niutitang Formation and (b) the Longmaxi Formation. Fm. = Formation.
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Figure 3. Post-Archean Australian Shale (PAAS)-normalized REE plots of the shales from (a) the Niutitang Formation and (b) the Longmaxi Formation. Fm. = Formation.
Figure 3. Post-Archean Australian Shale (PAAS)-normalized REE plots of the shales from (a) the Niutitang Formation and (b) the Longmaxi Formation. Fm. = Formation.
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Figure 4. Correlation relationship between paleoproductivity indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) Babio vs. TOC; (c,d) P/Ti ratios vs. TOC.
Figure 4. Correlation relationship between paleoproductivity indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) Babio vs. TOC; (c,d) P/Ti ratios vs. TOC.
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Figure 5. Correlation relationship between paleo-redox indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) V/(V+Ni) vs. TOC; (c,d) V/Cr vs. TOC; (e,f) Ni/Co vs. TOC.
Figure 5. Correlation relationship between paleo-redox indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) V/(V+Ni) vs. TOC; (c,d) V/Cr vs. TOC; (e,f) Ni/Co vs. TOC.
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Figure 6. (a,b) Correlation relationship between Mo and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales; (c,d) Ccvariation of MoEF vs. UEF of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. Sw represents of the seawater (Mo/U) molar ratio (~7.5–7.9) [52].
Figure 6. (a,b) Correlation relationship between Mo and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales; (c,d) Ccvariation of MoEF vs. UEF of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. Sw represents of the seawater (Mo/U) molar ratio (~7.5–7.9) [52].
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Figure 7. (a,b) Crossplots of Co (ppm) × Mn (%) vs. Al2O3 (%) contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales; (c,d) crossplots of CoEF × MnEF vs. Al2O3 (%) contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales.
Figure 7. (a,b) Crossplots of Co (ppm) × Mn (%) vs. Al2O3 (%) contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales; (c,d) crossplots of CoEF × MnEF vs. Al2O3 (%) contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales.
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Figure 8. Correlation relationship between terrigenous clastic input indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) Si/Al vs. TOC; (c,d) Ti/Al vs. TOC; (e,f) Zr/Al vs. TOC.
Figure 8. Correlation relationship between terrigenous clastic input indices and TOC contents of the Cambrian Niutitang Formation and the Silurian Longmaxi Formation shales. (a,b) Si/Al vs. TOC; (c,d) Ti/Al vs. TOC; (e,f) Zr/Al vs. TOC.
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Figure 9. Crossplots of Fe/Al vs. Al/(Al+Fe+Mn) of (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation shales showing the degree of hydrothermal activity.
Figure 9. Crossplots of Fe/Al vs. Al/(Al+Fe+Mn) of (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation shales showing the degree of hydrothermal activity.
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Figure 10. Crossplots of Cd/Mo vs. Co (ppm) × Mn (%) of (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation shales.
Figure 10. Crossplots of Cd/Mo vs. Co (ppm) × Mn (%) of (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation shales.
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Figure 11. Schematic model showing the enrichment mechanism of organic matter in shales from (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation.
Figure 11. Schematic model showing the enrichment mechanism of organic matter in shales from (a) the Cambrian Niutitang Formation and (b) the Silurian Longmaxi Formation.
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Liu, D.; Xu, M.; Chen, H.; Cao, Q.; Jiang, Z.; Tang, X. Tectonic Impact on Organic Matter Enrichment in Paleozoic Marine Shales from the Yangtze Block, SW China. J. Mar. Sci. Eng. 2025, 13, 1028. https://doi.org/10.3390/jmse13061028

AMA Style

Liu D, Xu M, Chen H, Cao Q, Jiang Z, Tang X. Tectonic Impact on Organic Matter Enrichment in Paleozoic Marine Shales from the Yangtze Block, SW China. Journal of Marine Science and Engineering. 2025; 13(6):1028. https://doi.org/10.3390/jmse13061028

Chicago/Turabian Style

Liu, Dadong, Mingyang Xu, Hui Chen, Qian Cao, Zhenxue Jiang, and Xianglu Tang. 2025. "Tectonic Impact on Organic Matter Enrichment in Paleozoic Marine Shales from the Yangtze Block, SW China" Journal of Marine Science and Engineering 13, no. 6: 1028. https://doi.org/10.3390/jmse13061028

APA Style

Liu, D., Xu, M., Chen, H., Cao, Q., Jiang, Z., & Tang, X. (2025). Tectonic Impact on Organic Matter Enrichment in Paleozoic Marine Shales from the Yangtze Block, SW China. Journal of Marine Science and Engineering, 13(6), 1028. https://doi.org/10.3390/jmse13061028

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