Petrology and Geochemistry Features of the Middle Triassic Anisian Shale in Sichuan Basin, South China: Implications for Climatic and Environmental Condition Change

Abstract

The Middle Triassic Anisian stage records a critical phase of marine ecosystem recovery after the Permian-Triassic mass extinction. The recent discovery of the Lei3 shale member within the predominantly carbonate Leikoupo Formation in the Sichuan Basin presents a unique opportunity to investigate this recovery period. However, the depositional model, the primary controls on organic matter accumulation, and the hydrocarbon potential of this newly identified shale unit remain poorly understood. This study integrates petrological, elemental geochemical, and isotopic (δ¹³C_org, δ¹⁵N_org) data to reconstruct the depositional environment and to establish a genetic model for organic matter enrichment. Our results reveal a vertical evolution from organic-rich mixed shale at the base to clay shale and finally to calcareous shale at the top. This facies transition corresponded to decreasing chemical weathering (CIA values from ~80 to ~70), a shift from anoxic to dysoxic conditions, and declining paleoproductivity, driven by regional tectonic uplift and a climatic shift from warm-humid to sub-humid. We propose a novel ‘tectonically induced organic matter enrichment model’ where tectonic uplift, rather than climate or productivity alone, was the primary driver by modulating sea level, nutrient supply, and preservation conditions. The mixed shale, with the highest TOC content and type II₁ kerogen, represents the most promising hydrocarbon-prone interval. This work not only clarifies the intricate tectonic–climate–productivity interplay during the Anisian but also provides critical insights for the exploration of shale gas in this emerging target.

1. Introduction

The biological mass extinction event, one of the most serious prior to the Phanerozoic, occurred during the Permian–Triassic periods. Restoration of the marine ecosystem was very slow, taking about 5 million years from the early Triassic [1]. However, the recovery of marine diversity and ecosystems suddenly accelerated from the Anisian Stage of the Middle Triassic [2]. Therefore, the Anisian Stage is the key to environmental change after the Permian-Triassic mass extinction.

According to conodont biostratigraphy [3,4] and magnetostratigraphy, in combination with zircon U/Pb ages, it is believed that the age of the Early-Middle Triassic World Line is 247.21 ± 0.1 Ma [5,6]. In recent years, Li et al. (2021) conducted U-Pb geochronology on the claystone at the base of the Leikoupo Formation, yielding a depositional age of 247.6 ± 1.1 Ma [7]. After that, the Anisian Stage began. The transition from the Early Triassic to the Middle Triassic was marked by a layer of altered acid tuff. This can be identified in most outcrops in the Sichuan Basin and its neighboring provinces (e.g., Guizhou) [8]. Shallow water carbonates were generally deposited during the Anisian period; however, a set of shale was suddenly deposited in the 3rd member of the Leikoupo Formation in the Middle Triassic (Abbreviated as Lei3 member). The reason for the deposition of this set of shale is still unclear.

Previously, Sun et al. (1989) proposed that the Yangtze Plate is not only the last marine sedimentary system but also the only potential reservoir that contains potential source rocks (Middle Triassic shales) [8]. Reservoirs in the Leikoupo Formation from the Anisian Period were discovered at the end of the last century, but the shale source proposed by scholars was not found at that time [9,10]. In recent years, there has only been a small amount of evidence that carbonate source rocks existed in this period [11]. It was not until 2021 that shale was discovered in the Lei3 member of the Leikoupo Formation [12], which not only verifies the statement that shale was deposited in the Mesozoic Leikoupo Formation in the Sichuan Basin but also provided new ideas on the source–reservoir relationship and for the next steps in the oil and gas exploration and development process for Leikoupo Formation.

While the recent identification of the Lei3 shale is a significant breakthrough [12], key aspects of this unit are still poorly constrained, creating critical knowledge gaps. First, a detailed depositional model that explains the lateral and vertical heterogeneity of the Lei3 shale is lacking. Second, the relative importance of tectonic activity, paleoclimate, ocean productivity, and redox conditions in controlling organic matter enrichment has not been systematically evaluated. Is the primary novelty the detailed reconstruction of the depositional model for this newly identified unit, the investigation of the tectonic–climate–productivity interplay, or its specific hydrocarbon potential? Third, the hydrocarbon generation potential of the different shale lithofacies (mixed, clay, calcareous) remains to be quantitatively assessed.

To address these gaps, this study employs a multi-proxy dataset—including petrology, major and trace elements, stable isotopes (δ¹³C_org, δ¹⁵N_org), and organic geochemistry—from the Lei3 member. The specific objectives are to: (1) reconstruct the paleoenvironmental conditions (redox, productivity, climate) during the deposition of the Lei3 shale; (2) establish a comprehensive depositional model that explains the observed lithofacies succession and organic matter accumulation; and (3) evaluate the hydrocarbon potential of the different shale lithofacies. By doing so, this work moves beyond a standard geochemical characterization to define the unique sedimentary and tectonic controls on this emerging shale unit, with implications for both understanding the Anisian recovery and for guiding future petroleum exploration in the Sichuan Basin.

2. Geological Setting and Stratigraphy

The Sichuan Basin is located in the northwestern part of the plate, on the eastern margin of the Paleo-Tethys Ocean [13,14,15,16]. Numerous paleomagnetic measurements limit the paleolatitude of the Sichuan Basin in the Middle Triassic to 14.7–23.3° [17,18,19,20,21].

The Sichuan Basin is a multi-cycle superimposed basin, which evolved from a passive continental margin in the Pre–Middle Triassic to a late foreland basin [22,23,24]. It contains the oldest record of marine sediments in China [25,26,27].

In the Middle Triassic, the South China Block and the North China Block collided, and the Mianlüe Ocean gradually closed from east to west [28,29,30,31]. A strong orogenic movement began in southern China [32,33,34,35], which is characterised by NW-trending folds, thrust faults, and post-orogenic granitic intrusions. The formation of the Xuefengshan tectonic belt was caused by the eastward intracontinental subduction of the South China Plate [32,33,36], which was also squeezed to the west to form the Luzhou paleo-uplift [37]. These tectonic events greatly changed the sedimentary environment of the Sichuan Basin [22,23,38].

There was a large-scale transgression in the Lei3 sedimentary period, which was larger than the transgression at the end of the Early Triassic (Olenekian) [10]. During this period of transgression, seawater mainly entered the Upper Yangtze River Basin through the Kaiyuan Strait between West Sichuan Central Yunnan oldland and North Vietnam oldland, then invaded the Sichuan Oceanic Basin from the southwest by crossing the Qiannan barrier reef. The seawater in the secondary direction of transgression passed through the channel in the Longmen Mountain island chain via the Songpan-Garze trough or overflowed the underwater seawall into the Sichuan Oceanic Basin (Figure 1) [18,38]. Subsequently, the Ladinia Regression occurred in the later stage of the Middle Triassic, and the sedimentary facies converted into continental facies [39].

3. Samples and Methods

Thirty-one samples were taken from HC125 well (106°28′35″ E, 30°26′47″ N) of the Lei3 member of the upper Yangtze Platform in southern China. All samples were identified under a microscope (Nikon LV100POL produced by Nikon in the Tokyo, Japan), including by alizarin red and potassium ferricyanide staining and casting thin section analysis. Each sample was plated with gold powders to facilitate electron microscopy. The gold-plated samples were scanned by Quanta 250 FEG scanning electron microscope (SEM) produced by Thermo Fisher in Waltham, MA, USA at 5 kV acceleration voltage. Samples for geochemical analysis were carefully selected under a binocular microscope and were analysed for major and trace elements, total organic carbon (TOC), X-Ray Diffraction (XRD) and δ¹³C_org isotopes. Only 18 samples were suitable for δ¹⁵N_org isotope analysis.

3.1. XRD

To evaluate their mineralogy, thirty-one samples were ground using a mortar and pestle into powder (200 mesh). The samples were analyzed with X-rays using Panalytical X’Pert PRO X instrument produced by Panalytical in the Almelo, The Netherlands. according to the requirements of Chinese National Standard SY/T5163-2010 [41], and step-scanned at a 5–60° 2θ interval.

3.2. Major and Trace Elemental Analysis

Major and trace elements of 31 powder samples (about 200 mesh) from the Leikoupo Formation were analyzed in the Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration (CEA), Beijing. According to the glass melting method, X-ray fluorescence (XRF produced by Bruker in Bill Ricard, MA, USA; AXIOS) was used to determine the content of main elements. Prior to the experiments, powdered shale samples were calcined in a high-temperature furnace (700 °C) to completely remove organic matter. Trace elements were measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS produced by Agilent in Santa Clara, CA, USA; X-series II). Before the experiments, shale samples were placed into a polytetrafluoroethylene (PTFE) vessel with a mixed solution of HClO₄, HF and HNO₃ to dissolve powdered samples.

Uncertainty reports for individual major elemental oxide analyses were better than 5% (2σ). The major elemental oxides of all the samples were determined according to Chinese National Standard SamplesGSR-3 and GSR-11 (referring to basalts and rhyolites, respectively). The accuracy of trace and rare earth elemental analysis was monitored by Chinese National Standard Samples GSR-1, GSR-2, GSR-10 and GSR-16, representing granites, andesites, gabbros and dolerites, respectively [42].

3.3. TOC

The preparation of samples for TOC analysis followed the steps provided by Chinese National Standard GB/T 19145-2003 [43]. Sample (10 g) was crushed to 80 mesh using a pestle and agate mortar. The sample was digested in HCl (5% concentration) for 2 h to remove carbonate minerals, and the acid was removed by washing with distilled water. The sample was then dried in an oven at 60 °C for 48 h. A LECO CS-230 carbon and sulfur analyzer produced by Leco in St. Joseph, MI, USA was used to measure the TOC of the sample, and the precision was within 0.5%.

3.4. Nitrogen and Organic Carbon Isotopes

To analyze the nitrogen and carbon isotope ratios, fresh parts from Anisian (Middle Triassic) shale were cut to eliminate the influence of the weathered surface of the sample on the experimental results. Rock powder was then prepared from suitable fragments of 1–2 cm in diameter using a polycarbonate tube and agate mortar. The powder (1–2 g) was first digested overnight at 70 °C with HCl (~10 M, 12–18 mL) to eliminate any carbonate minerals. When the reaction was complete, the residue was separated from the acid and rinsed with ethanol. The samples were then dried overnight at 80 °C and stored in glass tubes. Total nitrogen (TN) and total organic carbon (TOC) concentrations and isotope ratios were determined using an elemental analyzer–isotope ratio monitoring mass spectrometer (EA-IRMS; Thermo Finnigan DELTA plus Advantage mass spectrometer combined with EA 1112 series FLASH elemental analyzer, produced by Thermo Fisher in Waltham, MA, USA). The analysis process was as described by Kikumoto et al. [44]. The analytical reproducibility of TN and TOC concentrations was evaluated by replicate analysis of internal standard materials and samples; relative standard deviations were better than ±1.8 and 1.6% when N and C content in rock samples exceeded 1 ppm, respectively. Nitrogen and carbon isotopic compositions (δ¹⁵N and δ¹³C) are reported in ‰ relative to atmospheric N₂ and Vienna Peedee Belemnite (V-PDB) standards, respectively, based on (R_sample/R_standard − 1) × 1000, where R_sample and R_standard are the isotopic compositions of rock samples and reference materials, respectively. The δ¹⁵N and δ¹³C isotopic compositions of the rock samples and reference materials are denoted ¹⁵N/¹⁴N and ¹³C/¹²C, respectively. The analytical reproducibility of the δ¹⁵N and δ¹³C values determined from the replicate analyses of the internal standard materials was better than ±0.1‰ and ±0.2‰, respectively.

3.5. Proxy Calculations

Before using the trace element substitute index to indicate the deposition environment, Ti levels are usually used to correct the trace elements [45,46]. This enables calculation of the non-detrital or excess elements and the corresponding excess values, which eliminates the interference of terrigenous materials [47,48].

The equation is as follows:

Element_xs = Element_total − Ti_total × (Element/Ti) _PAAS

Element_xs and Element_total correspond to the content of elements from non-detrital sources and the total elemental content, respectively. Element/Ti is the content ratio of the element to Ti in Post-Archean Australian Shale (PAAS) [49,50].

To determine the degree of restriction and the redox conditions of the water mass, the element enrichment factor was calculated. First, the concentration of Mo and U needed to be transformed into the form EF_X (enrichment factor of X element, X refers to some element), which was calculated according to the following equation [48]:

EF_X = (X/Al)_sample/(X/Al)_PASS

Here, X refers to some element. X and Al are the weight concentrations of X and Al elements, respectively.

Index of compositional variability (ICV) was used to assess the extent to which sediment recycling altered the composition of fine-grained clastic rocks [51]. The IVC was calculated according to the following equation:

ICV = (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO + TiO₂)/Al₂O₃

The measured chemical index of alteration (CIA) values were used to estimate the paleoclimate and degree of weathering by excluding the sedimentary alterations after circulation and sorting [52]. The CIA formula was calculated as follows:

CIA = mole [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] × 100

Here, the measured values are expressed as molar proportions, and CaO* represents the CaO in silicate minerals. CaO* can be calculated and corrected as follows:

CaO* = mole CaO − mole P₂O₅ × 10/3

This method for correcting the CaO* value was proposed by McLennan [53]. If the corrected number of moles is less than that of Na₂O, the CaO value is adopted as the CaO* value. Otherwise, the CaO* was assumed to be equivalent to the moles of Na₂O.

4. Results

4.1. Lithology and Mineralogy

At present, using siliceous minerals (quartz + feldspar), carbonate minerals (calcite + dolomite) and clay minerals as the three terminal elements, the shale facies of the Lei3 Member can be divided into three types: mixed shale, clay shale and calcareous shale (Figure 2).

There were few organisms in the shale, and lamellation could be seen. Through microscopic observation and XRD analysis, it was found that the shale deposited in the Lei3 Member contained more carbonate, clay and quartz minerals than other minerals. The carbonate minerals were mainly calcite (22.3–74.7 wt%; mean: 47.3 wt%), the quartz content was 9.2–31.2 wt% (mean: 19.3 wt%) and the clay mineral content was 14.2–42.3 wt% (mean: 33.4 wt%).

The calcareous shale was mainly deposited in the upper part of the Lei3 member. It was light-gray to gray, mostly homogeneous and massive, lacking bedding (Figure 3A,D). The content of quartz and clay minerals was low, with an average of 15.4 and 16.8 wt%, respectively, while the content of carbonate was the highest, reaching 67.7 wt% (Figure 4). The clay shale was mainly deposited in the middle of the Lei3 member. It was gray-black, had low quartz content and the highest clay mineral content (Figure 3B,E), with mean values of 18.8 and 51.8 wt%, respectively. The carbonate minerals were mainly calcite (mean: 29.4 wt%). The mixed shale was mainly deposited at the bottom of the Lei3 member. It was gray-black and contained a mean quartz and clay mineral (Figure 3C,F) content of 24.2 and 41.2 wt%, respectively. The carbonate content was relatively low (mean: 34.6 wt%).

Overall, the quartz content showed a decreasing upward trend, the clay mineral content increased and then decreased, the carbonate minerals showed an increasing trend, and the TOC showed a decreasing trend (Figure 4).

4.2. Isotopic and Elemental Geochemistry

The distribution range of δ¹³C_org values was −27.07‰ to 31.07‰ (mean: −28.92‰). Upward, a weak increasing trend was exhibited. The δ¹⁵N_org isotope values ranged from 2.55‰ to 4.85‰ (mean: 3.79‰) and showed a similar trend to δ¹³C_org, with a slight increase in the upward direction (Figure 5).

The V/Cr, Ni/Co, V/(V + Ni), U/Th, Mo_XS, U_XS and Sr_XS/Ba_XS values were used as proxies for water mass redox conditions [55,56,57,58,59]. Upward, the V/Cr, Ni/Co, V/(V + Ni), U/Th, U_XS and Mo_XS profiles exhibited similar decreasing trends (Figure 6).

The V/Cr value distribution range was 2.04–7.56 (mean: 4.21), with the distribution ranges of mixed shale, clay shale and calcareous shale being 4.6–7.56, 3.8–4.51, and 2.04–3.11, respectively (mean values of 6.18, 4.08, and 2.62, respectively). The distribution range of Ni/Co values was 5.06–10.73 (mean: 6.84) with the ranges of mixed shale, clay shale and calcareous shale being 7.05–10.73, 6.08–7.18, and 5.06–6.54 (with mean values of 8.24, 6.59, and 5.81, respectively). The V/(V + Ni) values ranged from 0.66 to 0.83 (mean: 0.74), with the ranges of mixed shale, clay shale, and calcareous shale being 0.78–0.83, 0.74–0.77, and 0.66–0.73, respectively (with mean values of 0.8, 0.75, and 0.69, respectively). The distribution range of U/Th values was 0.79–3.39 (mean: 1.33) with the ranges of mixed shale, clay shale and calcareous shale being 0.79–1.02, 1.02–1.28, and 1.36–3.39 (with mean values of 0.95, 1.11, and 1.94, respectively). The range of Mo_XS values was from 8.38 to 54.78 (mean: 29.25). The distribution range of mixed shale, clay shale and calcareous shale was 32.12–54.78, 26.7–38.37, and 8.38–29.73, respectively (with mean values of 40.93, 33.13, and 17.29, respectively). The U_XS values ranged from 3.84 to 17.74 (mean: 10.66). The distribution range of mixed shale, clay shale and calcareous shale was 9.45–17.74, 9.02–12.77, and 3.84–11.27 (with mean values of 14.09, 11.16, and 7.49, respectively). The range of Sr/Ba values was from 0.54 to 1.52 (mean: 0.88).

4.3. Terrigenous Fluxes

The concentrations of Al and Ti can be used as representative of terrigenous debris input. The distribution range of Al values was 4.07%–8.02% (mean: 5.92%) with the ranges of mixed shale, clay shale and calcareous shale being 4.07%–5.78%, 4.36%–7.16%, and 5.61%–8.02% (with mean values of 4.86%, 6.01%, and 6.76%, respectively). The Ti values ranged from 0.19% to 0.52% (mean: 0.34%), with the ranges of mixed shale, clay shale, and calcareous shale being 0.25%–0.31%, 0.29%–0.35%, and 0.38%–0.52%, respectively (with mean values of 0.25%, 0.34%, and 0.43%, respectively).

4.4. Organic Geochemistry

4.4.1. Organic Matter Abundance and Maturity

Total Organic Carbon (TOC) in Lei3 samples from HC125 ranges from 0.11 to 4.01, with an average of 1.32. Stratigraphically, shale types ranked by TOC from highest to lowest are mixed shale > clay shale > calcareous shale > carbonate rock, with TOC values of 2.77%, 1.54%, 0.76%, respectively. TOC shows a decreasing trend vertically. Carbonate rock TOC is low and not discussed in this paper.

Lei3 shale samples in HC125 exhibit high Tmax values: 424–592 °C (average: 528.25 °C). Tmax shows a slight increase with depth. All shale samples display very low S2 values (0.06–0.60), averaging 0.21, and a Production Index (PI) ranging from 0.14 to 0.35, with an average of 0.20.

4.4.2. Kerogen Components

Through vitrinite reflectance microscopy, microcharacteristics and component contents of kerogen within Lei3 shale have been observed. Based on these microscopic differences, vitrinite, inertinite, exinite, and huminite can be identified in Lei3 samples. Across all samples, huminite predominates with amorphous humic matter (52%–68%). Following this is the exinite (29%–41%). The kerogen type index (TI) is calculated using the formula (TI = (huminite × 100 + exinite × 50 − vitrinite × 75 − inertinite × 100)/100), which is used to classify kerogen types based on this index.

Huminite is characterized by amorphous humic matter, appearing orange-brown to brown-orange under microscopic examination, dispersed without specific shapes. Exinite kerogen predominantly consists of amorphous humic matter, retaining remnants of original plant morphology (Figure 3G–I).

4.4.3. Kerogen Carbon Isotopes

In HC125, Lei3-2 shale samples exhibit kerogen isotopic values ranging from −27.7‰ to −30.8‰, with an average of −28.79‰. The isotopic range for kerogen in shale samples spans from −28.1‰ to −30.8‰, averaging −28.87‰.

4.4.4. Biomarkers

Normal Alkanes

The UCM peak on the saturated hydrocarbon chromatogram was flat, indicating weak degradation. Therefore, biomarker compounds are highly reliable in their indication of the environment. HC125 samples show dominance of short-chain normal alkanes (Figure 7), with maximum abundance between carbon numbers C₁₇–C₂₀. Using the Carbon Preference Index (CPI) formula: 22(nC₂₃ + nC₂₅ + nC₂₇ + nC₂₉)/(nC₂₂ + 2(nC₂₄ + nC₂₆ + nC₂₈) + nC₃₀) and the Odd-Even Preference (OEP) formula: (nC₂₁ + 6nC₂₃ + nC₂₅)/(4nC₂₂ + 4nC₂₄), average CPI and OEP values are 0.94 and 0.99, respectively, close to the equilibrium value of 1. The ratio of C₂₁₊₂₂/C₂₈₊₂₉ ranges from 1.33 to 4.45, averaging 2.29. Specifically, clay shale averages 2.55, calcareous shale averages 1.71, and mixed shale averages 3.32.

Sesquiterpanes

In HC125 samples, the proportion of Pr and Ph in sesquiterpanes is lower compared to adjacent normal alkanes, with Pr content being less than Ph. The Pr/Ph ratio in shale samples ranges from 0.52 to 1.12, averaging 0.82 (Figure 7). Pr/nC₁₇ and Ph/nC₁₈ are used to create cross-plots to reflect the source of organic matter (Figure 8).

Terpanes

In the ion mass spectrum (m/z191) (Figure 9A,C), similar features are observed in all samples: high content of C₃₀ hopanes, followed by tricyclic terpanes. In HC125, the distribution of Ts/(Ts + Tm) values ranges from 0.45 to 0.58, with an average of 0.51. Additionally, the samples contain elevated levels of gammacerane.

Steroids

In the ion mass spectrum (m/z217) (Figure 9B,D), significant amounts of pregnane compounds are present in the HC125 shale samples, particularly C215α-(H)-pregnane. However, there are variations in the content of cholestane compounds, with cholestane content exceeding that of pregnane compounds. C₂₇ and C₂₉ regular steranes are associated with algae and plants, respectively [62]. All samples contain C₂₇–C₂₈–C₂₉ regular steranes and exhibit an “L-shaped” carbon number distribution pattern.

5. Discussion

5.1. Organic Geochemical Characteristics

5.1.1. Hydrocarbon Generation Potential

The total organic carbon (TOC) content in HC125 samples ranges from 0.11 to 4.01%, with an average of 1.32%. In the study by Huo et al. (2022), the TOC content of the Anisian shale of the Leikoupo Formation from wells CT1 and JY1 ranges from 0.13 to 1.78, averaging 0.81 [42]. HC125, CT1, and JY1 samples exhibit similar TOC distribution characteristics, with TOC values ranking as follows: mixed shale > clay shale > calcareous shale > carbonate rock, and showing a decreasing trend vertically. The C₂₉αα20S/(20S + 20R) and C₂₉ββ/(ββ + αα) ratios are effective indicators for maturity assessment [63,64]. In HC125 samples, the C₂₉αα20S/(20S + 20R) ranges from 0.40 to 0.41, averaging 0.405; CT1 ranges from 0.47 to 0.58; and JY1 ranges from 0.47 to 0.54 [42], all indicating high maturity to overmaturity of Anisian shale. Additionally, maturity can be identified using the Ts/(Ts + Tm) ratio, typically indicating high maturity when Ts/(Ts + Tm) ranges from 0.4 to 0.6 [65]. In HC125 samples, the Ts/(Ts + Tm) ratio ranges from 0.45 to 0.58, averaging 0.51; CT1 ranges from 0.45 to 0.58 (mean: 0.51); and JY1 ranges from 0.43 to 0.56 (mean: 0.5) (Huo et al., 2022) [42], indicating high maturity of Anisian shale in the Sichuan Basin. The relationship between T_max and PI values in pyrolysis results also reflects maturity characteristics, indicating that the samples are mainly in the wet gas and dry gas/condensate oil stages [66,67]. In the study by Huo et al. (2022) [42], T_max values for CT1 samples range from 452 to 576 °C (mean: 498 °C) and for JY1 samples from 442 to 528 °C (mean: 494.5 °C), both lower than HC125 samples (424–592 °C, mean: 528.25 °C). This suggests higher maturity of HC125 potentially due to its closer proximity to the sedimentation and hydrocarbon generation center. The type of organic matter is determined by the TI: Type I when TI > 80; Type II1 when TI is between 40 and 80; Type II2 when TI ranges from 0 to 40; and Type III when TI is less than 0 (referenced). Based on the results of vitrinite reflectance (Figure 3G–I), calculated TI indices (66.5–80.75), and the distribution of δ13C values of kerogens in the samples (−29‰ to −26‰), the predominant kerogen types in the Anisian sample are Type II1, with some Type I. Compared to Type III kerogens, Type II1 and Type I kerogens are richer in hydrogen and lipid components, indicating a significant hydrocarbon generation potential [68]. It is important to note that high thermal maturity alters the optical properties of macerals, making visual kerogen typing and the calculated Type Index (TI) less reliable. Therefore, the results of cheese root microscopy should be used cautiously as auxiliary evidence.

Despite the high maturity characteristics of the Anisian shale, parameters such as S1, S2, HI, and OI from pyrolysis lose their ability to assess original hydrocarbon generation potential. Indeed, Tmax results from rocks hosting black shale gas reservoirs in other basins around the world indicate highly evolved shale gas production (

Table 1. Comparison of Residual TOC and Tmax of the Lei3 Shale with Selected Paleozoic Shale Gas Rocks Worldwide.

Formation	Age	TOC (wt%)	S1 (mgHC/g Rock)	S2 (mgHC/g Rock)	Tmax (°C)	HI (mg HC/g TOC)	OI (mg CO2/g TOC	Reference
Longmaxi fm. (China)	Lower Silurian	0.34–4	0.02–0.05	0.26–0.48	313–601	8–38	8–69	[69]
Niutitang fm. (China)	Lower Cambrian	0.39–10.2	0.01–0.05	0.09–0.62	281–600	3–86	4–240	[69]
Muskwa, Besa & Fort Simpson Fm. (Canada)	Devonian- Mississippian	0.18–4.72	0.01–0.04	0.04–0.24	369–611	1–9	4–12	[58]
Barnett Shale (USA)	Mississippian	2.62–11.47	0.26–3.6	0.59–54.53	425–544	14–475		[68]
Gufeng Fm. (China)	Lower Permian	0.04–22.1	0–0.31	0–7.95	339–554	0–60	3–429	[70]
Montney Fm. (Canada)	Lower Triassic	0.03–8.2	0.02–0.81	0–1.83	347–526	1–367	0–933	[71]
Baling and Bendang Riang Fm.	Silurian-Devonian	0.73–24.6	0.01–0.08	0.02–0.37	322–502	0.28–25.18	0.89–40.89	[72]
Timah Tasoh and Sanai Fm.	Devonian	0.1–9.1	0.01–0.07	0.02–0.14	330–502	0.55–110	2.79–900	[72]
Kubang Pasu (Perlis), Singa, Batu Gajah Fm.	Carboniferous	0.06–279	0.01–0.06	0.02–0.14	301–608	15–115.94	2.27–598.43	[72]
Kubang Pasu (Kedah) Fm.	L. Permian	1.01–19.65	0.01–0.05	0.02–0.09	328–502	0.21–3.33	4.27–15	[42]
Leikoupo fm. (China)	Middle Triassic	0.11–4.01	0.01–0.11	0.02–0.6	424–592			This study

Note: Due to the high maturity of the Lei3 samples, Rock-Eval parameters S1, S2, HI, and OI are not comparable. The comparison is focused on TOC and thermal maturity (Tmax).

). High maturity hinders the identification of original hydrocarbon generation potential but indicates a more thorough and complete process of oil and gas generation. Under conditions of high maturity and with the expulsion of most hydrocarbon products, the original organic abundance in Anisian shale should theoretically be much higher than modern residual organic matter. Therefore, the hydrocarbon generation potential of Anisian shale is considerable. Furthermore, based on residual TOC comparisons, mixed shale exhibits greater hydrocarbon generation potential than clay shale, while clay shale surpasses calcareous shale and carbonate rocks in this regard.

5.1.2. Organic Matter Sources

The distribution characteristics of n-alkanes in saturated hydrocarbon chromatograms reveal a predominance of short-chain n-alkanes (peak abundance between carbon numbers C₁₇–C₂₀) in the samples. These short-chain n-alkanes are primarily derived from lower marine planktonic organisms. Detection of n-alkanes in marine organic matter shows dominance of (C₂₁ + C₂₂) hydrocarbons, while terrestrial plant organic matter predominantly contains (C₂₈ + C₂₉) n-alkanes [73]. In asphaltene samples from shale, the average C₂₁₊₂₂/C₂₈₊₂₉ ratio is 2.29, indicating dominance of C₂₁ and C₂₂, suggesting a marine origin. The C₂₁₊₂₂/C₂₈₊₂₉ ratio decreases vertically from mixed shale (3.32) to clay shale (2.55) to calcareous shale (1.71), reflecting a relative decrease in marine organic matter sources, similar to the TOC variation trend, possibly related to regression phases and indirectly proving the marine origin of the organic matter.

Isoprenoid alkanes such as pristane (Pr) and phytane (Ph), derived from chlorophyll phytol side chains in photosynthetic organisms, are sensitive indicators of redox conditions. The Pr/Ph ratio, with ranges indicating anoxic, suboxic, and oxic conditions [74], in HC125 samples (average 0.82) indicates a reducing environment for the Anisian shale deposition, corroborated by Pr/nC₁₇ and Ph/nC₁₈ diagrams (Figure 8). Through characteristics of n-alkanes and isoprenoid alkanes, the depositional environment is conclusively determined as a reducing marine water environment.

Hopanes like gammacerane are typically associated with cyanobacteria (blue-green algae) and other prokaryotes [75]. And the tricyclic terpanes primarily occur in marine source rocks as products of original bacteria and prokaryotes post-diagenesis, largely independent of terrestrial sources. This suggests that primary producers in the depositional water bodies were likely marine plankton—cyanobacteria. Gamacerane, a reduction product of tetramethylated triterpenoids in foraminifera, correlates closely with their living conditions, typically indicating vertical stratification in saline and anoxic conditions [76]. The GI index (GI = gammacerane/αβC₃₀ hopane) ranges from 0.22 to 0.28 in HC125, averaging 0.25, reflecting the degree of water column stratification.

C₂₇ and C₂₉ regular steranes are associated respectively with algae and plants [62]. Furthermore, the significant presence of C₂₇αααR > C₂₉αααR > C₂₈αααR in HC125 samples exhibits an asymmetric anti-“L-type” distribution pattern. This confirms the organic matter in the shale originates from marine algae. Similar characteristics are observed in CT1 and JY1 samples [42].

In conclusion, the deposition and organic matter enrichment processes of Anisian shale occurred in a reducing marine environment, exhibiting vertical water column stratification in salinity and redox conditions.

5.2. Depositional Environment

The above discussion indicates that the tectonic movement had a significant controlling effect on black shale sedimentation in the study area, but the influence of the sedimentary environment on the shale could not be ignored.

5.2.1. Paleo-Weathering and Recycling Processes

Chemical weathering is closely related to paleoclimate and is an effective method for tracking paleoclimate changes [77]. Various geochemical indicators have been used for paleoclimate reconstruction, such as the chemical index of alteration (CIA) to infer the intensity of weathering [78,79,80]. Before calculating CIA values, it is necessary to exclude sedimentary recycling and post-sedimentary diagenesis alterations [51], so that the measured CIA value can be used to estimate the paleoclimate and degree of weathering.

(1)

Sediment recycling and sorting

Polycyclic reworking processes tend to accumulate stable minerals like quartz [53]. The coarse-grained sediments are rich in minerals such as quartz and zircon, and the fine sediments are rich in clay [81,82,83,84,85,86]. Analysis of the shale deposited in the Lei3 Member revealed that the influence of sedimentary differentiation could be excluded and that clay minerals were relatively enriched.

Generally, for the study of older strata, the index of compositional variability (ICV) can be used to estimate the degree of change in the composition of fine-grained clastic rocks by sediment recycling [51]. Mature sediments from the stable continental crust display lower ICV values (<1.0), whereas immature sediments from arc-related volcanic and plutonic rocks exhibit higher values [51].

Most of the ICV values of the Lei3 shale samples were greater than 1 (mean: 1.15). This meant that the mineral maturity was low, so Lei3 shale samples were weakly affected by recirculation.

(2)

Post-depositional diagenetic alteration

The vertical decrease in CIA values (Figure 5) may have been due to tectonic movements and changes in the sedimentary environment, paleoclimate, and the composition of source rocks. The CIA value is sensitive to the composition of source rocks, especially K metasomatism [78]. Therefore, the effect of K metasomatism should be excluded when calculating CIA values [87].

In the A–CN–K diagram, the trend of weathering was simulated by plotting the values of the samples on a line parallel to the A–CN axis (Figure 10). The weathering trend from the source rocks was parallel to the A–CN axis, and the CIA values were relatively concentrated and not tilted toward the K apex. Therefore, the involvement of K was not the main factor leading to the decrease in CIA values [78]. This further supported the decrease in CIA being affected by tectonic movements and paleoclimate changes. The previous analysis showed that regional tectonic uplift in the study area was related to physical weathering.

In the deposition period of mixed shale, the paleoclimate was relatively warm and humid with the CIA values of 77.91–83.22 (mean: 80.59). Vertically, the CIA values were relatively stable and gradually increased upwards in the Lei3 member. The range of CIA values of clay shale during the deposition period was 75.25–80.05 (mean: 77.51) changed to 66.81–76.05 (mean: 70.89) during the deposition period of calcareous shale.

Based on analysis of the A–CN–K diagram, it was concluded that the paleoclimate changed from warm and humid to semi-humid during the deposition period of shale in the Lei3 member. Under humid conditions, values of Fe, Mn, Cr, V, Ni, Co, and other elements in sedimentary rocks are high. However, under arid conditions, values of Ca, Mg, K, Na, Sr, and Ba are high because these elements separate out and catalyze the deposition of various salts at the bottom of the water body due to the evaporation of water and the enhanced alkalinity. The climate index was calculated from the following ratio:

C = ∑ (Fe + Mn + Cr + V + Ni + Co)/(Ca + Mg + K + Na + Sr + Ba)

According to the classification criteria, values changing from 0 to 0.2 represents an arid climate; from 0.2 to 0.4 represent a semi-arid climate; from 0.4 to 0.6 represent a semi-arid to semi-humid climate; from 0.6 to 0.8 represent a semi-humid climate; and more than 0.8 represents a humid climate [77,89,90].

In this study, the range of climate index C values of the sample is 0.63–1.01 (mean: 0.81), indicating a humid climate. The C values decreased from the mixed shale at the bottom of Lei 3 member to the calcareous shale at the top, indicating that the climate changed from warm humid to a semi-humid climate (Figure 11).

According to this analysis, the paleoclimate during the deposition of the Lei3 member changed from warm and humid to semi-humid. The longitudinal change in climate to semi-arid was not conducive to the reproduction of aquatic microorganisms, so their productivity gradually decreased. As a result, the TOC content corresponded to a slight downward trend as time progressed.

5.2.2. Marine Redox Conditions

The marine redox condition is a key factor for the sedimentary environment. It even affected the recovery of ecosystems after the P-T mass extinction.

(1)

Organic carbon isotope δ¹³C_org

δ¹³C_org values mainly reflect photosynthesis, carbon assimilation, and the isotopic composition of carbon sources [91]. The δ¹³C_org values of sedimentary organic matter are closely related to the source and sedimentary environment of the original organic matter. However, as thermal maturation does not markedly change the carbon isotope composition, the change in δ¹³C_org in the sedimentary organic matter reflects the change in δ¹³C_org of the original organic matter—in most cases.

Freudenthal et al. (2000) and Lehmann et al. (2002) believe that the widespread distribution of a hypoxic environment has a significant effect on the original isotope balance of δ¹³C_org: when environmental anoxic conditions are dominant, δ¹³C_org tends to be lighter [4,92]. The organic matter input of the Lei3 shale was dominated by algae and plankton. For marine algae, the δ¹³C_org value is usually −20‰ to −22‰ [93]; the δ¹³C_org value of the Lei3 shale was generally lower than −28‰ (Figure 5). The lighter δ ¹³C_org value is indicative of a generally hypoxic environment [94]. In our study, the co-occurrence of the most negative δ¹³Corg with the highest values of productivity proxies (P_xs, Ba_xs) and the strongest anoxia indicators (e.g., high V/Cr, Mo_EF) suggests that both processes were likely operating in concert. The high productivity fueled organic flux to the seafloor, promoting anoxia, which in turn enhanced the preservation of this ¹²C-enriched organic matter. This synergistic effect makes it difficult to isolate the primary driver, but the multi-proxy evidence strongly supports a high-productivity anoxic basin model.

(2)

δ¹⁵N_org isotope

According to previous studies, changes in marine redox and productivity and regional climate conditions are closely related to the marine N-cycle balance [95]. There are four biogeochemical processes of N cycling in the ocean: biological nitrogen fixation and nitrification mainly occur near the ocean surface, while biological denitrification and anaerobic oxidation of ammonia occur in deep water [95,96,97,98]. During the Lei3 shale deposition period, the sea level rose rapidly, and the extensive transgression resulted in a stratified and anoxic marine environment. At the same time, denitrification and biological nitrogen fixation were strengthened under high productivity, which led to the enrichment of δ¹⁴N in biological organic matter and its negative deviation in sediments. During the middle and later periods, due to the long-term anoxic reduction conditions, the remaining nitrate in the seawater inorganic N pool was relatively enriched by ¹⁵N. Tectonic uplift led to a decline in sea level, promoted water-gas exchange, and changed the seawater redox conditions. On the one hand, these factors limited the process of denitrification; on the other hand, organisms absorbed the remaining ¹⁵N-rich nitrate ions in seawater, resulting in the positive deviation of δ¹⁵N isotopes (Figure 5).

(3)

Elemental geochemistry

To define the redox environment, the three-part classification of oxic, dysoxic, and anoxic was first proposed by Rhodes and Morse [99]. For reconstruction of the paleoenvironment, selected trace elements that have few sources and relative stability after deposition are used as the geochemical indexes of paleo-oxygen facies [48]. Vxs, Crxs, Nixs, Coxs, Uxs, and Thxs were chosen as the trace element substitution indexes for determining the paleo-redox environment.

All these elements dissolve in seawater as stable, high-valence ions [47]. In a weak reduction environment, these high-valence ions are reduced to lower valences and deposited. Under strongly reducing conditions, these high-valence ions are reduced to a lower-valence state and deposited as oxides or hydroxides [47]. Therefore, the redox conditions can be determined.

The scatter diagram shows that there was a positive correlation between V/Cr, Ni/Co, V/(V + Ni), U/Th and TOC (Figure 6) and that from mixed shale to clay shale to calcareous shale, the degree of hypoxia gradually decreased. That is, mixed shale was mainly deposited in an anoxic environment, clay shale was mostly deposited in an anoxic environment, with some in a dysoxic environment, and calcareous shale was mainly deposited in a dysoxic environment (Figure 12).

The oxygen content in seawater varies over time and depends to a large extent on the geographic environment and prevailing climate [100,101]. It has been determined that the tectonic movement gradually increased during the sedimentary period of the Lei3 shale. With the gradual decrease in sea level, the degree of hypoxia did not decrease significantly. A simple sea-level fall model would predict improved ventilation and more oxic conditions. However, we propose a more nuanced mechanism linked to the interplay between tectonic uplift and basin geometry. On the one hand, regional tectonic uplift intensified physical weathering, increasing the flux of terrigenous nutrients (e.g., phosphorus) into the basin. This could have sustained high surface productivity, consuming oxygen in the water column through the degradation of organic matter [100,102]. On the other hand, the same tectonic event (e.g., the formation of the Luzhou paleo-uplift) likely altered basin topography, increasing its restriction. This is supported by the increasing Mo/TOC and Mo_EF-U_EF trends in the clay and calcareous shales. A more restricted basin, even if shallower, would have impeded the renewal of oxygenated open-ocean water, trapping organic matter and maintaining stratification and bottom-water anoxia [42]. Although the mechanisms differ, they act in concert and ultimately modulate the redox conditions of the water body through water column stratification.

Figure 12. Cross plots of redox proxies of Lei3 shale in HC125. (A) Vxs/Crxs vs. Nixs/Coxs, (B) Nixs/Coxs vs. Uxs/Thxs, (C) Nixs/Coxs vs. Vxs/(Vxs + Nixs). The empirical values of redox proxies are derived from [48,103,104,105].

Figure 12. Cross plots of redox proxies of Lei3 shale in HC125. (A) Vxs/Crxs vs. Nixs/Coxs, (B) Nixs/Coxs vs. Uxs/Thxs, (C) Nixs/Coxs vs. Vxs/(Vxs + Nixs). The empirical values of redox proxies are derived from [48,103,104,105].

5.2.3. Paleoproductivity

(1)

Organic carbon isotope (δ¹³C_org)

Higher ocean productivity leads to higher organic flux and more oxygen consumption, which is beneficial to maintain or even prolong anoxic conditions in the water column [48,56]. δ ¹³C_org values are closely related to the source of organic matter. The relationship is as follows: plankton preferentially absorbs atmospheric ¹²CO₂ as the carbon source for photosynthesis [106,107]. As a result, the generated organic matter contains more ¹²C, resulting in lighter δ¹³C_org values in mudstone. Therefore, more negative δ ¹³C_org values reflect higher productivity and the presence of more organic matter [108]. As the δ¹³C_org values were lowest in the lower part of the Lei3 member and gradually increased upward, this indicated a gradual decrease in productivity.

The δ¹³C_org values in the study area were relatively low, which indicated algae to be the source of organic matter. The increasing δ¹³C_org trend in the upward direction may have been caused by the weakening contribution of algae to the organic matter. At the initial stage of shale deposition (the mixed shale deposition period), the marine productivity was the highest and more ¹²C entered organisms, resulting in the negative deviation of δ¹³C_org [106,107]. Subsequently, marine productivity gradually decreased, with less ¹²C entering organisms, leading to an increase in the ¹³C_org isotope values upward.

Hein and Sand proposed that increased atmospheric CO₂ concentrations could improve the primary productivity of marine phytoplankton in 1997 [109]. And many studies have confirmed that changes in atmospheric CO₂ concentrations are negatively correlated with the evolution of oceanic organic carbon isotope δ¹³C_org [110]. Therefore, according to the longitudinal distribution trend shown in Figure 5, the δ¹³C_org values of the Lei3 member shale in the study area during the deposition period were positively biased from the lower mixed shale to the upper calcareous shale, which was related to the gradual decrease in CO₂ concentration and productivity. In addition, the vertical decrease in CO₂ concentration led to an increase in the CaCO₃ production rate, meaning that the calcification effect is negatively correlated with ocean acidification [111,112]. Therefore, the vertical increase of carbonate content in the study area means that calcification is enhanced, and it has evolved into calcareous shale in the upper part of the study area, corresponding to relatively low TOC and weakened paleoproductivity (2) N isotope δ¹⁵N_org.

Vertically, the excursion of N isotopes and C isotopes was similar, with a slight increase in the upwards direction in the Lei3 member. The sea level during the deposition period of the Lei3 shale was relatively high, so denitrification and biological nitrogen fixation were strengthened against the background of higher productivity, resulting in negative excursion of δ¹⁵N_org in the sediment. Additionally, the water mass in the study area was restricted, and nitrate could not be fully supplemented. Nitrogen-fixing microorganisms absorbed atmospheric N₂ and increased the concentration of ¹⁴N (biological organic matter), resulting in the relative decrease in N isotopes in the early stage of shale deposition.

However, long-term anoxic reductive conditions resulted in the relative enrichment of the remaining nitrate (NO³⁻) in the seawater inorganic nitrogen pool (DIN) by ¹⁵N. During the middle and late stages of the Lei3 shale deposition, the decrease in sea level promoted exchange between seawater and the atmosphere. On the one hand, the process of denitrification was restricted and decreased productivity. On the other hand, the biological absorption of residual ¹⁵N nitrate ions in seawater, which affected the N isotopic composition in the middle and late stages of shale deposition, eventually led to the positive excursion of δ¹⁵N_org.

(2)

Elemental geochemistry

P and Ba are the main elements used to assess productivity [48]. The content of P_org and Ba_bio in sediments and sedimentary rocks is positively correlated with TOC, which can be a good indicator of productivity.

In the present study, biogenic P and biogenic Ba can be represented by P_xs and Ba_xs, respectively. In this study, the P_xs and Ba_xs values had a good correlation, implying that the accumulation of P was less disturbed by redox conditions and could be used as an effective paleoproductivity indicator. Moreover, in the upward direction, both of them showed a gradually decreasing trend, indicating that the ancient productivity gradually declined. The overall productivity of the HC125 well showed the following vertical trend: mixed shale > clay shale > calcareous shale.

Previous studies have shown that warm and wet climates are conducive to biological development, which increases primary productivity, while dry and cold climates decrease productivity [113,114]. There was a correlation between productivity and climate in the Sichuan Basin (Figure 5). The values of CIA, P/Ti, and Ba all showed slight decreasing trends, upwards, indicating that the climatic conditions and primary productivity were stable.

The elemental indexes were consistent with organic carbon isotope values and the CO₂ curve, both of which showed a gradually decreasing trend in paleoproductivity in the vertical direction. The overall ¹³C_org values in the study area were relatively light, which was the source of algae. Marine productivity was the highest during the early stage of the shale deposition period of the Lei3 member (the mixed shale deposition period). Nevertheless, as the CIA value decreased vertically, which suggested that chemical weathering abated, the sea level dropped, and the overall salinity increased, which was unfavorable to algae breeding and led to a relative decline in ancient productivity, which was consistent with the ¹³C_org results. However, due to the gradual tectonic uplift and longitudinal physical weathering enhancement, the nutrients carried by terrigenous matter contributed some of the nutrients required by marine organisms; However, as these nutrient levels were lower than those brought by relatively strengthened chemical weathering, there was an overall downward trend in TOC in the vertical direction. So, there was no sudden decrease in organic matter content, which resulted in no significant positive excursions of organic carbon isotopes. Therefore, regional tectonic uplift had a certain impact on productivity, and climate change was the dominant factor affecting productivity.

5.2.4. Water Salinity

Typically, the depositional environment (marine or terrestrial) and water salinity can be inferred using the SrXS/BaXS ratio [115]. Variations in SrXS/BaXS ratios are generally positively correlated with paleosalinity, with high Sr content often associated with arid climates. Longitudinally, Lei S3 shale samples exhibit an overall increasing trend in SrXS/BaXS ratio, indicating a rise in salinity. This is interpreted as seawater concentration during marine regression, gradual aridification, and localization processes, leading to increased salt concentrations. Among the shale types, sedimentary water salinity increases in the following order: mixed shale, clay shale, and calcareous shale. Paleosalinity shows a clear negative correlation with TOC (Figure 6). This provides a reference for determining whether salt reduction can serve as an indicator of organic-rich accumulation in polymictic shales.

5.3. Water Column Restriction

The Mo/TOC ratio and the Mo-U covariant model are usually used to evaluate the restriction of a water mass [59,116].

5.3.1. Mo/TOC Pattern

The study area is in the Yangtze Platform, surrounded by uplifts or ancient land, and is connected to the open sea through a relatively narrow strait. Therefore, the study area is subject to certain restrictions. Mo is more enriched in anoxic environments than in dysoxic and oxygenated environments [59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117].

There was a strong positive correlation between Mo and TOC in the Lei3 member shale (Figure 6), indicating that the enrichment of Mo was closely related to the organic matter content and that it formed in an anoxic-dysoxic environment [48]. The shale of the Lei3 member was characterized by Mo enrichment and relatively low TOC, resulting in high Mo/TOC values. The Mo/TOC distribution range was 15–46.5 with a mean of 32.5; the Mo/TOC values of the Cariaco Basin and the Saanich Inlet were 25 and 45, respectively (Figure 13A).

The higher Mo content in the sedimentary period mixed shale may be due to the low degree of restriction, high sea level and smooth water exchange at that time. By the middle and late stages of shale deposition (clay shale to calcareous shale), a few samples fell into the Framvarent Fjord area, exhibiting a more confined character with significantly decreased elemental Mo. However, there was still a positive correlation between Mo and TOC, which may have been caused by regional tectonic uplift, the sea level decrease and the increased restriction of the water mass, resulting in insufficient Mo supplementation. These results revealed that the Lei3 shale in the study area was formed in neither an extremely confined environment nor a completely open environment.

5.3.2. Mo_EF-U_EF Covariant Model

The Mo-U covariant model can be used to identify the restriction of a water mass and redox conditions [57,59,116]. There have been multiple authigenic Mo-U covariation models reported for the sedimentary period of the Lei3 number in the study area. Common to all these models are the positive covariant characteristics of Mo_auth (authigenic Mo) and U_auth (authigenic). As the TOC content increased, the degree of reduction in the water body increased, and the enrichment coefficients of Mo and U both increased with the same amplitude.

The Mo_EF/U_EF values of most samples in the study area were between those of current seawater (Mo-EF/U-EF = 1*SW) and the “particulate shuttle” (Figure 13B). There was no downward trend as Mo/U increased until the particulate shuttle was reached (e.g., Mn-oxides particulate) [59]. This indicates that the sedimentary environment was not strongly restricted because the Mo supply rate in the water exceeded the Mo consumption rate in the sediment. When the water column is strongly reducing, the absorption of Mo increases faster than the absorption of U. Therefore, this result reflected the transition from anoxic to dysoxic conditions. The degree of restriction from clay shale to calcareous shale increases [57,59], especially as mixed shale samples showed higher Mo_EF and U_EF values. Additionally, the Mo_EF and U_EF values in upward sedimentary clay shale and calcareous shale gradually decreased, reflecting the decrease in oxygen minimum zone (OMZ), weakening reduction conditions and the decreased Mo and U in the water body [116,118,119].

The trends in U_EF and Mo_EF were similar to those for Mo/TOC. This may have been because the sea level was generally high during the Lei3 shale deposition period, then gradually decreased over time. The regional tectonic movement led to the uplift of the study area, resulting in an increased degree of restriction.

5.4. Depositional Model for Organic Matter Accumulation

During the sedimentary period of Lei3 shale, the organic carbon isotope ¹³C_org in the lower part suddenly increased, and the corresponding elements changed significantly. For example, the paleoproductivity indexes (P, Ba, P/Ti) suddenly decreased, as did the terrigenous clastic indexes (Al and Ti), and the corresponding N isotopes showed an overall increasing trend. Redox indexes (V, Ni, U, Mo, Ni/Co, V/Cr, U/Th, V (V + Ni)) suddenly decreased. In fact, the correlation between TOC values and elemental geochemical indicators is used to identify the controlling factors of organic matter enrichment. TOC values and water salinity (Figure 14A) suggest that the products of halophilic organisms’ life activities and the input of terrestrial nutrients may not be the primary sources of organic matter. Indicators of reducing conditions in depositional waters, such as V/Cr, Ni/Co, and U/Th, show a positive correlation with TOC values (Figure 14B–D), highlighting the importance of reducing environments for the preservation of organic matter in shale. The positive correlation between PXS/Ti and TOC indicates the control of ancient water productivity on organic matter abundance (Figure 14E). Additionally, the negative correlation between terrestrial input and organic matter abundance suggests that terrestrial organic matter is not the main source of organic matter in Lei3 shale (Figure 14F,G). Enhanced chemical weathering and rising C-index both promote the enrichment of organic matter (Figure 14H,I).

In terms of the mineral content, the quartz content decreased, the clay content increased, and there was no significant change in the carbonate content. The sedimentary environment changed during this period, and clay shale began to be deposited. Then, upon entering the calcareous shale deposition period, the paleoproductivity index and redox environment index showed overall downward trends. However, in the middle of the calcareous shale deposition period, the redox index suddenly increased, and the paleoproductivity index also correspondingly increased. There may have been a short period of sea-level rise during this period, following which the sea level fell to the lowest value.

During the Lei3 shale deposition period, the lithofacies associations transitioned from mixed shale to clay shale and then changed to lithofacies dominated by calcareous shale. It was apparent that the lithofacies transformations represented changes in the sedimentary environment. Accordingly, based on the structure and sedimentary background of the Lei3 member shale as determined in this study, a sedimentary model of shale was established to explain the organic matter enrichment mechanism in different lithofacies associations.

In the early depositional stage of the Lei3 member, there was an extensive transgression [120]—even larger than the Olenekian transgression [10]—and mixed shale was deposited under this background. The ¹³C_org and ¹⁵N isotopes showed a negative bias during this stage, indicating that the sea level was high, the seawater was anoxic and organic matter was enriched. The CIA value was high, reflecting strong chemical weathering that provided high levels of nutrient minerals for plankton in the surface seawater. Additionally, with the warm and humid climate and the high levels of marine primary productivity, marine organisms absorbed atmospheric CO₂, and their subsequent burial and storage in the seabed resulted in a decrease in CO₂ partial pressure. This in turn promoted the gradual decline in temperature during the middle and later stages of Lei3 member formation, which was also consistent with the transition from a warm-humid climate to a semi-humid climate during the Lei3 deposition period.

Although the Sichuan Basin was in a shallow sea environment during this period, the water body was not as smooth as the open sea. This formed an anoxic-dysoxic or even a sulfidic bottom seawater environment. Such an environment resulted in the death of many benthic organisms, but it was highly conducive to the preservation of organic matter. Therefore, the TOC content of mixed shale was also the highest in the Lei3 shale. Vertically, the clay shale to calcareous shale deposition periods were increasingly affected by regional tectonic uplift, and the sea level gradually decreased. ¹³C_org and ¹⁵N isotopes both increased, and the climate changed from a warm-humid climate to a semi-humid climate.

The gradual decrease in seawater CO₂ concentration resulted in a weakening of ocean acidification accompanied by an increase in the production rate of CaCO₃ [111,112]. Therefore, in the study area, the longitudinal calcification was enhanced and the carbonate content increased, resulting in the deposition of mainly calcareous shale in the upper part of the shale. During this period, chemical weathering gradually weakened (but the CIA value was still high, generally greater than 70), and the input of nutrients decreased, which limited plankton reproduction. Compared with the early stage, the primary productivity gradually decreased.

This is also consistent with the paleo-sedimentary background at that time. After the P-T mass extinction event, the sedimentary environment did not improve in a direction that is obviously conducive to biological reproduction. Until entering the Anisian period, marine algae and other small organisms began to recover and change with the times. As the times changed, the paleoenvironment in the early Anisian period did not improve enough for large organisms to thrive. Subsequently, the temperature decreased from 36° to 30°. As the temperature became moderate [121], the oxygen content gradually increased and the CO₂ concentration gradually decreased, suggesting that “calcification feedback” occurred during this period [122]. This negative feedback greatly promoted the gradual recovery of macrofauna and plants, and the development of macrofauna and plants began in the middle and late Anisian period.

Vertically, there was an increasing flux of debris (Al and Ti), and the TOC decreased (Figure 5), indicating that there was an organic matter dilution effect from debris inputs. Therefore, the input of terrigenous clastic materials was not the dominant factor of organic matter enrichment in the study area. It is certain that the sea level decreased during the Middle and Late Mesozoic (Anisian period) and the Sichuan Basin was in a relatively active tectonic environment. In tectonically active areas, the physical dissolution rate is usually higher than the chemical weathering rate and non-steady-state weathering often occurs [123]. Therefore, longitudinally, chemical weathering (CIA) weakened, but terrigenous debris increased.

While changes in paleoclimate, productivity, and redox conditions all played a role, the regional tectonic activity served as the overarching driver that orchestrated these changes. The argument for a tectonic driver is built upon a cascade of interconnected evidence. First, the observed increase in terrigenous input (Al, Ti) coincides with a decrease in chemical weathering intensity (CIA). This decoupling is a key indicator. A simple progradation or change in oceanic currents would likely increase terrigenous input, but it would not necessarily suppress chemical weathering. In contrast, regional tectonic uplift provides a coherent mechanism for both: it enhances physical erosion and sediment supply (increasing Al, Ti) while simultaneously creating topographic barriers that reduce hydrological connectivity and shift the climate towards a more sub-humid state (lowering CIA).

Furthermore, previous systematic paleogeographic and tectonic reconstructions of the Middle Triassic Sichuan Basin have delineated a distinctive sedimentary configuration in the study area. Over time, continental cracking of Pangea influenced the uplift of the overall structure of the Sichuan Basin and the gradual closure of the Paleo-Tethys Ocean basin [124]. The continental collision stage represented by the Indosinian movement began in the Middle and Late Triassic. The final collision and assembly of the South China Block and the North China Block arose from the combined effect of the stress from the Northwest and the reaction force of the Central Guizhou Uplift and the Jiangnan Uplift [125] under the background of the convergent stress field. The Jiangnan-Xuefeng orogenic belt on the southeastern margin of the Yangtze block gradually expanded and uplifted during the Middle Triassic. The Luzhou paleo-uplift [37] was formed during the process of compression and migration to the west. Due to the enhancement of physical erosion caused by uplifting, it has become an important provenance area (terrigenous fluxes) in the study area [37]. By the way, the tectonic model does not rely solely on Al and Ti trends. Instead, it integrates them with independent proxies for climate (CIA), basin restriction (Mo/TOC), and redox conditions (V/Cr, etc.), all of which show synchronous changes that are best explained by a common tectonic trigger.

The enhancement of physical weathering leads to an increase in terrigenous debris input but results in fewer nutrient inputs than from chemical weathering. Additionally, many terrigenous debris inputs dilute the proportion of organic matter in rock. Therefore, the input of terrigenous debris increased longitudinally, but the TOC decreased (Figure 5).

In summary, the global sea level and paleoclimate controlled the material basis of shale deposition, while the regional tectonic uplift promoted shale deposition. The study area has formed a large-scale starvation-type and relatively blocked retention environment, surrounded by a semi-restricted retention basin. Therefore, the Lei3 shale was formed under semi-closed conditions. The enrichment mechanism of organic matter in Lei3 shale is summarized as follows: the preservation mechanism was driven by phased regional tectonic movement and climate change. This is shown in Figure 15, below.

6. Conclusions

This integrated study of the Middle Triassic Lei3 shale in the Sichuan Basin yields the following principal conclusions:

• The depositional environment evolved from a productive, anoxic system (mixed shale) to a less productive, dysoxic one (calcareous shale), concurrent with a climatic shift from warm-humid to sub-humid conditions. This transition is consistently recorded by geochemical proxies for redox (e.g., V/Cr, U/Th), productivity (P_xs, Ba_xs), and weathering (CIA).
• Regional tectonic uplift, associated with the Indosinian Orogeny, is identified as the primary driver of this evolution. It induced a sea-level fall, enhanced physical weathering, increased basin restriction, and moderated the climate, thereby orchestrating the observed facies succession. This finding supports a tectonically modulated depositional model for organic matter accumulation.
• The mixed shale facies, characterized by the highest residual TOC content and deposition under the most favorable environmental conditions, is identified as the most promising interval for shale gas exploration.

In summary, this work reconstructs the paleoenvironmental evolution of the Lei3 shale, establishes tectonic activity as its fundamental control, and defines the mixed shale as the primary exploration target, thereby contributing to both the understanding of Anisian ecosystem recovery and the evaluation of hydrocarbon resources in the Sichuan Basin.