Sedimentary Environment and Organic Matter Enrichment of the First Member in the Upper Triassic Xujiahe Formation, Southeastern Sichuan Basin

Abstract

The Xujiahe Formation (FM) is a significant source rock layer in the Sichuan Basin. In recent years, a growing number of scholars believe that the shale gas potential of the Xujiahe Formation is equally substantial, with the first member of the formation being the richest resource. The deposition of Member (Mbr) 1 of Xujiahe FM represents the first and most extensive transgression event within the entire Xujiahe Formation. This study investigates the sedimentary environment and organic matter (OM) enrichment mechanisms of the dark mud shales in the Mbr1 of Xujiahe FM on the southeastern margin of the Sichuan Basin, utilizing methods such as elemental geochemistry and organic geochemistry analyses. The results indicate that these dark mud shales possess a relatively high OM abundance, averaging 2.20% and reaching a maximum of 6.22%. The OM is primarily Type II₂ to Type III. Furthermore, the paleoclimate during the Mbr1 period in the study area was warm and humid with lush aquatic vegetation. Intense weathering and ample precipitation transported large amounts of nutrients into the lacustrine/marine basin, promoting the growth and reproduction of algae and terrestrial plants. Correlation analysis between the Total Organic Carbon (TOC) content and various geochemical proxies in the Mbr1 mud shales suggests that OM enrichment in the study area was primarily controlled by the climate and sedimentation rate; substantial OM accumulation occurred only with abundant terrigenous OM input and a relatively high sedimentation rate. Redox conditions, primarily productivity, and terrigenous detrital input acted as secondary factors, collectively modulating OM enrichment. Event-driven transgressions also played an important role in creating conditions favorable for OM preservation. Synthesizing the influence of these multiple factors on OM enrichment, this study proposes two distinct composite models for OM enrichment, dominated by climate and sedimentation rate.

1. Introduction

With the continuous advancement of exploration technologies, our understanding of oil and gas resources is constantly being updated. In particular, the discovery of shale gas resources has challenged the traditional theory of source–reservoir–cap rock combinations. Many formations once considered solely as source rocks are now becoming promising shale gas production layers and have achieved successful commercial exploitation. For instance, the Marcellus Shale in the Appalachian Basin of North America [1] and the Roseneath and Murteree Shales in the Cooper Basin of Australia represent typical marine shale gas examples. The Irati Shale in the Paraná Basin of Brazil and the Longtan/Wujiaping Formation shale in the Sichuan Basin of China exemplify transitional marine–continental facies shale gas [2]; the two exhibit significant differences in OM content and type. Therefore, understanding the mechanisms of OM enrichment under different sedimentary environments is crucial, and the geochemical composition of clastic rocks holds the key to unlocking the mechanisms behind organic matter enrichment. The geochemical composition of clastic rocks retains valuable information on the nature of the source rock as well as the paleoenvironmental conditions during sedimentation. Its significance is primarily reflected in two aspects: (1) The chemical composition of clastic rocks is directly inherited from source rocks. By analyzing specific major, trace, and rare earth elements, we can identify parent rock types, trace tectonic settings, and reveal weathering intensity and sedimentary recycling processes. (2) During deposition and diagenesis, the distribution and enrichment of certain elements are highly sensitive to ambient conditions, making them effective proxies for reconstructing redox conditions, paleoproductivity, paleosalinity, and other environmental parameters. The importance of this research lies in its ability to improve paleogeographic and paleoclimatic reconstructions, deepen our understanding of organic matter enrichment mechanisms, guide shale gas exploration, and contribute to global comparisons.

OM accumulation is a prolonged and complex process influenced by numerous factors. Common factors affecting OM enrichment include water salinity, redox conditions, primary productivity, terrigenous clastic input, sedimentation rate, and climatic conditions [3,4]. The factors controlling organic matter enrichment are primarily explained by two distinct models: the strong preservation model, which focuses on environmental conditions that protect OM from degradation, and the productivity model, which emphasizes the initial biological output as the key factor [5,6,7]. The productivity model posits that the enrichment of organic matter is primarily controlled by the initial biological productivity in the surface layer of the water body, while the influence of redox conditions in depositional water is relatively limited [8]. In this model, the rate of oxidative degradation of organic matter is roughly equivalent to the rate of its decomposition by sulfate-reducing bacteria. Continental margin upwelling zones serve as typical representatives of the productivity model [5,9,10]. Conversely, the strong preservation model emphasizes that reducing environments and sedimentation rate are key factors influencing OM enrichment [11]. Beyond these conditions, terrigenous clastic input, sedimentation rate, and adsorption by clay mineral particles also contribute to OM accumulation [12,13]. Beyond the established paradigms of preservation and productivity, a growing body of research posits that an optimal sedimentation rate acts as a third, critical factor controlling organic matter enrichment [14]. An excessively low sedimentation rate may lead to the oxidative degradation of OM in oxygenated waters, while an excessively high rate can enhance the dilution effect on OM. A more widely accepted view is that the preservation and enrichment of OM results from the interplay and mutual constraints of multiple factors. However, the relative contribution and specific role of each factor in OM enrichment still require detailed analysis tailored to different environmental contexts [15].

The Sichuan Basin is rich in widely distributed shale gas resources. Marine shale gas formations include the Cambrian Qiongzhusi Formation and the Lower Silurian Longmaxi Formation, both of which have seen numerous successful cases of commercial shale gas development [16]. The most promising transitional marine–continental facies shale gas formations are the Upper Triassic Xujiahe Formation and the Upper Permian Longtan/Wujiaping Formation [17,18]. Among these, the Xujiahe Formation has consistently attracted significant attention due to its significant total thickness, high OM content, and extensive, continuous distribution [19,20]. While the mud shales of Xujiahe FM were long studied primarily as source rocks, a growing number of scholars are now focusing on their shale gas potential [21].

This study focuses on the dark fine-grained sedimentary rocks of Mbr1 from Xujiahe FM in the Yongchuan area of the Southeastern Sichuan Basin. Through field observations, organic geochemistry, elemental geochemistry, and research on sedimentary system characteristics, it aims to reconstruct the paleoenvironmental conditions of fine-grained sedimentary rocks under different sedimentary environments. Subsequently, it analyzes the primary and secondary controlling factors of OM enrichment and discusses the response of OM enrichment factors to different sedimentary environments. This research provides an important theoretical support for the exploration of transitional marine–continental facies shale gas in the Sichuan Basin and also serves as a reference for studies on OM enrichment mechanisms under similar conditions globally.

2. Geological Setting

During the Triassic period, the Sichuan Basin was situated on the passive continental margin of the Eastern Tethys Ocean. Its geological setting was closely related to global plate tectonics and paleoenvironmental changes [22,23]. In the Early–Middle Triassic, while Pangea was still amalgamating, the South China Block, where the Sichuan Basin is located, was positioned in the eastern part of Pangea, adjacent to the Tethys Ocean (Figure 1a). The Paleo-Tethys Ocean gradually closed during the Early–Middle Triassic, while the Neo-Tethys Ocean began to expand. This led to the southern margin of the South China Block, including Sichuan Basin, becoming a passive continental margin environment, receiving marine sedimentation [24,25].

During the Late Triassic, the initial uplift of the Longmen Mountains along the western basin margin occurred as a result of collision between the South China Block and the North China Block, driven by the Indosinian Orogeny and the early stages of Pangea breakup (Figure 1b). Seawater gradually retreated from the Sichuan Basin, and the deposition transitioned from marine to marine–continental transitional facies, eventually becoming entirely continental fluvial–lacustrine deposits. This trend is consistent with the widespread continentalization observed globally during the Late Triassic [26]. The Xujiahe Formation is a key stratigraphic unit that records this marine-to-continental transition within Sichuan Basin [27,28].

The Xujiahe Formation is widely and continuously distributed within the Sichuan Basin, though its thickness varies significantly across different regions. Overall, the formation exhibits a general trend of thinning from west to east. It can reach over 2000 m in thickness in the western part of the basin, while in the southeastern study area, it thins to approximately 300 m at its minimum [29]. The Xujiahe Formation is subdivided from bottom to top into six lithological members, designated Mbr1 to Mbr6. In the southeastern basin, these six members display a typical “sandwich-like” sedimentary structure of interbedded sand and mud. Mbr1, Mbr3, and Mbr5 are predominantly composed of fine-grained sediments such as mudstone and shale, whereas Members Mbr2, Mbr4, and Mbr6 are dominated by sandstone and siltstone deposits (Figure 1c).

Figure 1. (a) Late Triassic paleogeographic pattern with the location of the Xindianzi section (by Blakey, R.) [30]; (b) essential geological map of the present-day Sichuan Basin with the location of the Xindianzi section; (c) lithologic section of Xujiahe FM and an enlarged view of Mbr1, showing sampling locations.

Figure 1. (a) Late Triassic paleogeographic pattern with the location of the Xindianzi section (by Blakey, R.) [30]; (b) essential geological map of the present-day Sichuan Basin with the location of the Xindianzi section; (c) lithologic section of Xujiahe FM and an enlarged view of Mbr1, showing sampling locations.

The first member of Xujiahe formation (Mbr1 from Xujiahe FM) is the target interval of this study. This member can be further divided into two SubMbrs: from bottom to top, SubMbr1 and SubMbr2. SubMbr1 consists mainly of sandstone and siltstone, with mudstone at its base. SubMbr2 is primarily composed of mudstone and shale, locally interbedded with siltstone [31].

Previous studies suggest that the southeastern part of the basin received detrital material mainly from the Qianzhong Oldland to the southwest and the Xuefengshan Old Uplift to the southeast [31]. The lithological variations between members and the “sandwich-like” structure are primarily interpreted to be controlled by the paleo-depositional environment. Overall, the Xujiahe Formation in the Southeastern Sichuan Basin was deposited in a peri-marine, lacustrine–deltaic, or fluvial–lacustrine swamp environment under a tropical to subtropical warm and humid climate [32,33,34,35,36,37,38]. By the late Norian, deposition was initially dominated by lacustrine–deltaic facies, later shifting to marine tidal flat facies. During the Rhaetian, it was primarily characterized by continental lacustrine–deltaic deposition [32].

3. Samples and Methods

The samples for this study were collected from outcrops of Xujiahe FM located on the Southeastern flank of the Xindianzi anticline in Honglu Town, Yongchuan District, Chongqing City, in Southeastern Sichuan Basin. All samples were taken from beneath the surface weathered layer to ensure they were fresh and unaffected by weathering. A total of 35 fine-grained sedimentary rock samples, primarily shale, mudstone, and silty mudstone with a grain size of 0.01–0.15 mm, were collected from this section for TOC content and elemental geochemical analysis. The sample locations are shown in Figure 1c. To avoid contamination, veins within the samples were surely removed, and the samples were stored in sealed bags. All analyses were conducted at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation at Southwest Petroleum University.

The Total Organic Carbon (TOC) content was determined using a carbon/sulfur analyzer (LECO, CS-230, San Jose, CA, USA), following the industry standard Determination of Total Organic Carbon in Sedimentary Rocks (GB/T 19145-2003) [39]. We subjected each finely powdered sample (200 mesh) to 10% HCl at 65 °C for 20 h to eliminate inorganic carbon. We then rinsed the residue repeatedly with distilled water and dried it in an oven at 55 °C for more than 30 h.

Rock-Eval pyrolysis was performed in accordance with the Chinese National Standard of Rock pyrolysis analysis (GB/T 18602-2012) [40], using a YQ-VII Hydrocarbon Display Evaluator (HaiCheng Petro–Chemical Instrument Factory, Haicheng, China). The analysis involves the programmed heating of rock samples in a pyrolysis oven to release hydrocarbons, which are detected by a Flame Ionization Detector (FID) (Agilent, Intuvo 8890, BeiJing, China). Subsequently, carbon monoxide (CO) and carbon dioxide (CO₂) generated from the pyrolysis of organic matter, along with the CO₂ produced from the oxidation of residual organic carbon, are quantified by a Thermal Conductivity Detector (TCD) or an Infrared (IR) detector.

The mineral compositions of both whole-rock samples and clay fractions were determined by X-ray diffraction (XRD) analysis, performed on a Rigaku Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan). The analytical procedure followed the Chinese oil and gas industry standard of Analytical Method for Clay Minerals and Ordinary Non-clay Minerals in Sedimentary Rocks by X-Ray Diffraction (SY/T 5163-2010) [41]. Prior to analysis, all samples were crushed and dry-sieved to pass through a 200-mesh screen.

Major element analysis followed the standards Inductively Coupled Plasma Atomic Emission Spectrometry: Quantitative Analysis of 27 Elements Including Calcium Oxide (DZ/T 0279.2-2016) [42] and Chemical Analysis Methods for Silicate Rocks: Determination of 16 Major and Trace Components—Part 28 (GB/T 14506.28-2010) [43], using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) (PE, 5300V, Waltham, MA, USA), analytical precision and accuracy attained were better than 5% on average. Loss on Ignition (LOI) was determined by weighing 1 g of dried powdered sample into a porcelain crucible, placing it in a muffle furnace at 950 °C ± 25 °C for 60 min, then cooling it in a desiccator to room temperature before weighing and recording the weight lost during heating.

Trace element analysis followed the standards Determination of 15 Elements Including Barium, Beryllium, and Bismuth: Inductively Coupled Plasma Mass Spectrometry (DZ/T 0279.3-2016) [44] and Determination of 15 Rare Earth Elements Including Lanthanum and Cerium: Sealed Acid Digestion-Inductively Coupled Plasma Mass Spectrometry (DZ/T 0279.32-2016) [45], using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (Agilent, 7700, Santa Clara, CA, USA), analytical precision and accuracy attained were better than 10% on average.

The Enrichment Factor (EF), a widely adopted metric for assessing elemental enrichment, was calculated using the Post-Archean Australian Shale (PAAS) as the geochemical normalizing reference [46]. The calculation formula is: X_EF = (X/Al)_sample/(X/Al)_PAAS (where PAAS refers to the Post-Archean Australian Shale, and X represents the content of the target element).

4. Results

4.1. Organic Geochemical Characteristics

Rock-Eval pyrolysis provides key parameters such as the free hydrocarbon content (S₁), the pyrolyzable hydrocarbon content (S₂), and the maximum pyrolysis temperature (Tmax). The two most common indicators for evaluating source rock potential are the Total Organic Carbon content (TOC) and the Hydrocarbon Generation Potential (S₁ + S₂) [47]. Analysis of 20 samples from the fine-grained sedimentary rocks of Mbr1 from Xujiahe FM shows that the TOC content exhibits a wide range, from 1.10% to 7.59%, and averages 2.79%. Among these, 9 samples between 1% and 2%, classifying them as fair source rocks; 8 samples between 2% and 4%, classifying them as good source rocks; and 3 samples greater than 4%, which, combined with their actual lithology, are identified as coal. The measured S₁ + S₂ varies between 0.61 and 4.11 mg/g, averaging 1.80 mg/g (

Table 1. Thermal decomposition parameters and total organic carbon content of source rocks in Mbr1 from Xujiahe FM.

Parameters	Sample Quantity	Maximum	Minimum	Average Value
TOC (%)	20	7.59	1.1	2.2
S1 (mg/g)	20	0.92	0.04	0.38
S2 (mg/g)	20	3.73	0.52	1.42
S1 + S2 (mg/g)	20	4.11	0.61	1.8
Tmax (°C)	20	488	433	457.85

), which classifies the rocks as fair to good source rocks. Figure 2 shows nearly all samples in the TOC − (S₁ + S₂) cross-plot fall within the good–excellent source rock field.

4.2. Mineral Composition

Mineralogical analysis of three fine-grained sedimentary rock samples from the Mbr1 of the Xujiahe FM in the study area was conducted using X-ray diffraction (XRD). Two samples were collected from SubMbr1, and one from SubMbr2. The experimental results reveal that the two fine-grained sedimentary rock samples from SubMbr1 are overwhelmingly dominated by quartz, with contents of 61.0% and 77.2% (averaging 69.1%), followed by clay minerals (20.2% and 26.9%, averaging 23.6%). Additionally, the samples contain moderate amounts of K-feldspar and plagioclase, neither of which exceeds 10%. The fine-grained sedimentary rock from SubMbr 2 contains 36.8% quartz. Carbonate minerals are the second most abundant group at 32.0%, comprising 14.7% calcite and 17.3% dolomite. Clay minerals account for 29.4%, along with a minor plagioclase content of 1.8%. Data for individual samples are provided in

Table 2. Mineral composition of fine-grained sedimentary rock from Mbr1 of Xujiahe FM in the Xindianzi section.

Sample Number	Quartz	Potassium Feldspar	Plagioclase Feldspar	Calcite	Dolomite	Siderite	Pyrite	Analcime	Clay Minerals
XDZ-33	36.8	0	1.8	14.7	17.3	0	0	0	29.4
XDZ-12	61.0	8.4	3.7	0	0	0	0	0	26.9
XDZ-11	77.2	1.9	0.7	0	0	0	0	0	20.2

.

By comparison, the fine-grained sedimentary rocks of SubMbr2 exhibit significantly higher clay mineral content than those of SubMbr1, along with the presence of carbonate minerals. Due to the limited number of samples, we can only speculate that this discrepancy is likely attributable to differences in depositional environment. SubMbr2 was deposited in a tidal flat setting characterized by relatively high hydrodynamic energy. Although such environments generally undergo winnowing that reduces clay content, the preserved fine-grained sediments in SubMbr2 were likely accumulated in lower-energy, sheltered micro-environments within the tidal system (e.g., mud-rich zones or protected tidal flats), facilitating greater retention and accumulation of clay minerals. In contrast, SubMbr1 represents the distal front of a continental delta. Despite the generally low flow energy in such settings, persistent suspension and differential transport processes may have led to relative clay depletion or dilution by other siliciclastic inputs, resulting in a lower overall clay content compared to SubMbr2.

4.3. Elemental Geochemical Characteristics

4.3.1. Major Element Characteristics

The results for the major elements are listed in

Table 3. Major element content (%) data of Xujiahe FM Mbr1 fine-grained sedimentary rock.

Sample Number	Thickness (m)	SiO2	Al2O3	Fe2O3	K2O	CaO	MgO	TiO2	Na2O	MnO	P2O5	LOI
XDZ-35	477.3	65.6	22.3	2.9	5.9	0.3	1.5	1.1	0.1	0.0	0.1	7.4
XDZ-34	480.8	62.8	15.6	5.0	4.0	6.7	4.4	0.7	0.2	0.2	0.2	12.0
XDZ-33	482.3	52.8	14.5	5.4	3.7	17.3	4.8	0.7	0.2	0.2	0.1	18.6
XDZ-32	483.9	53.1	14.2	5.2	3.6	18.2	4.3	0.7	0.2	0.2	0.2	18.3
XDZ-31	485.5	58.2	15.0	5.1	3.7	12.2	4.4	0.7	0.2	0.2	0.2	15.2
XDZ-30	487.2	55.7	17.0	6.4	4.3	10.6	4.5	0.8	0.2	0.2	0.2	14.8
XDZ-29	488.7	59.6	18.9	6.9	4.7	4.7	3.8	0.8	0.2	0.1	0.2	10.7
XDZ-28	490.3	59.5	18.0	7.2	4.4	5.2	4.2	0.9	0.2	0.1	0.2	11.0
XDZ-27	492.0	64.4	16.8	6.6	4.2	2.9	3.6	0.8	0.2	0.1	0.2	9.2
XDZ-26	493.5	67.9	17.0	6.4	4.3	0.6	2.4	0.9	0.2	0.0	0.2	6.5
XDZ-25	495.2	65.2	16.7	5.9	4.3	2.7	3.6	0.9	0.2	0.1	0.2	9.3
XDZ-24	496.7	72.5	15.2	4.7	3.9	0.3	1.7	0.7	0.6	0.0	0.2	5.7
XDZ-23	498.6	73.5	12.9	4.7	3.4	1.6	2.3	0.6	0.8	0.0	0.2	5.8
XDZ-22	500.0	76.1	12.1	4.9	3.3	0.4	1.4	0.5	1.0	0.0	0.2	3.5
XDZ-21	508.3	73.1	17.6	1.8	4.8	0.1	1.3	0.9	0.2	0.0	0.0	5.9
XDZ-20	509.6	67.4	17.8	5.9	4.5	0.4	2.5	0.9	0.2	0.0	0.2	6.3
XDZ-19	510.9	71.4	15.2	5.6	4.0	0.4	2.0	0.8	0.2	0.0	0.2	4.8
XDZ-18	512.2	64.2	20.7	5.3	5.2	0.4	2.6	1.0	0.2	0.0	0.2	7.8
XDZ-17	513.5	65.7	19.1	5.8	4.8	0.4	2.6	0.9	0.2	0.0	0.2	7.1
XDZ-16	514.8	63.6	20.8	5.8	5.2	0.5	2.7	0.9	0.2	0.0	0.2	7.9
XDZ-15	516.1	67.1	16.9	5.0	4.4	1.8	3.2	0.9	0.2	0.1	0.2	8.5
XDZ-14	517.4	67.6	15.5	6.1	4.0	2.1	3.3	0.8	0.2	0.1	0.2	7.7
XDZ-13	518.7	70.1	14.8	7.4	3.6	0.4	2.2	0.6	0.4	0.0	0.2	5.5
XDZ-12	525.4	73.3	13.9	5.6	3.5	0.5	1.4	0.7	0.6	0.0	0.3	5.6
XDZ-11	552.9	84.1	10.2	1.5	2.6	0.1	0.6	0.6	0.1	0.0	0.0	3.8
XDZ-10	554.7	84.0	8.9	3.3	2.1	0.1	0.7	0.5	0.1	0.0	0.1	4.0
XDZ-9	556.9	81.4	10.9	2.8	2.9	0.1	0.9	0.7	0.1	0.0	0.1	4.2
XDZ-8	558.3	74.5	15.8	3.1	3.7	0.1	1.4	0.9	0.1	0.0	0.1	6.0
XDZ-7	560.0	80.7	11.1	3.1	2.8	0.2	1.1	0.6	0.1	0.0	0.1	3.9
XDZ-6	562.1	77.3	13.1	3.7	3.2	0.1	1.3	0.8	0.1	0.0	0.1	5.5
XDZ-5	563.3	79.0	11.7	3.7	3.0	0.1	1.3	0.7	0.1	0.0	0.1	4.8
XDZ-4	564.2	83.2	9.5	2.9	2.5	0.2	1.0	0.6	0.1	0.0	0.1	3.6
XDZ-3	565.6	57.0	29.9	4.8	4.0	0.9	1.4	1.4	0.2	0.0	0.1	67.4
XDZ-2	566.6	62.0	20.2	9.7	5.1	0.1	1.3	1.1	0.2	0.0	0.1	8.9
XDZ-1	568.0	71.4	17.7	3.0	4.7	0.1	1.6	1.2	0.1	0.0	0.1	6.3

. The most abundant oxides in the studied samples are SiO₂ (52.85–84.12%, avg. 68.71%) and Al₂O₃ (8.88–29.94%, avg. 15.93%), indicating high contents of Si and Al. These are followed by Fe₂O₃ (1.46–9.71%, avg. 4.95%), K₂O (2.13–5.90%, avg. 3.95%), CaO (0.08–18.15%, avg. 2.65%), and MgO (0.64–4.81%, avg. 2.38%). All other oxides, including TiO₂, Na₂O, P₂O₅, and MnO, are present in concentrations less than 1.0%.

The Chemical Index of Alteration (CIA), proposed by Nesbitt and Young [48], serves as a key proxy for paleoenvironmental reconstruction. CIA values below 60 reflect cold, arid conditions associated with limited chemical weathering; values ranging from 60 to 80 indicate warm, humid climates accompanied by intermediate weathering; while values exceeding 80 are characteristic of hot, humid environments with intense chemical weathering. The geochemical signature of sedimentary rocks is shaped by both chemical weathering and sedimentary recycling, necessitating an evaluation of their impact on the samples to determine data reliability [49,50]. In the A-CN-K diagram (Figure 3), the weathering trend of the samples is essentially parallel to the A-CN line, indicating weak K-metasomatism and eliminating the need for K correction [51,52,53,54]. Furthermore, scientists have proposed the Index of Compositional Variability (ICV) as a quantitative indicator for analyzing weathering intensity, sediment sorting, and recycling processes. An ICV > 1 suggests the sediment contains minimal clay minerals and represents initial deposition in a tectonically active setting; an ICV < 1 indicates the sediment contains abundant clay minerals, representing either intense weathering under initial depositional conditions or recycled deposition [55,56].

The calculated indices for weathering and sorting, namely the CIA and ICV, range from 71.2 to 85.7 and 0.45 to 0.99 in SubMbr1, and from 67.3 to 77.3 and 0.62 to 1.47 in SubMbr2, respectively. The calculated indices for terrigenous detrital input, including Si/Al, Ti/Al, and Ti/Zr, show ranges of 1.68–8.35, 0.05–0.07, and 11.28–38.26 for SubMbr1, and 2.60–5.53, 0.043–0.059, and 13.00–18.31 for SubMbr2, respectively.

The Si-Al-Ti elemental combination is commonly used to reflect provenance nature, weathering intensity, and tectonic setting [57]. The Si/Al ratio generally reflects the relative proportion of siliceous to clay minerals in a sample. The Ti/Al ratio can effectively distinguish between felsic and basic source regions; compared to the Al₂O₃/TiO₂ ratio, which can be artificially elevated due to weathering and leaching, the Ti/Al value is less affected by weathering. The Ti/Zr ratio can also effectively indicate source rock properties and is resistant to weathering interference [58,59].

The elemental ratio Al/(Al + Fe + Mn) is widely used to assess the influence of hydrothermal activity. For the fine-grained sedimentary rock samples from SubMbr1 of Mbr1 from Xujiahe FM in Xindianzi section, the Al/(Al + Fe + Mn) values range from 0.61 to 0.84 (avg. 0.74). For SubMbr2, the values range from 0.60 to 0.88 (avg. 0.69) (Table 3, Figure 4).

4.3.2. Trace Element Characteristics

The results for the trace elements are listed in

Table 4. Trace element content (ppm) data of Xujiahe FM Mbr1 fine-grained sedimentary rock.

Sample Number	Thickness (m)	Ba	Zr	Rb	V	Sr	Ce	Zn	Li	Cr	La	Th	U	Yb	Mo
XDZ-35	477.3	588.3	450.7	219.4	162.7	90.8	113.1	71.1	21.4	76.2	68.0	15.9	5.3	5.7	2.5
XDZ-34	480.8	365.7	282.3	131.5	91.9	82.4	58.6	71.4	43.1	55.2	32.1	10.7	3.9	2.8	1.0
XDZ-33	482.3	379.9	234.5	126.2	107.5	171.1	49.6	66.4	47.3	50.5	29.1	10.1	3.2	2.4	1.5
XDZ-32	483.9	321.8	250.2	120.7	91.0	182.2	56.9	64.5	43.9	46.6	32.2	10.2	3.1	2.7	1.0
XDZ-31	485.5	343.8	274.3	119.2	91.4	124.5	60.5	76.8	42.2	43.9	33.0	10.2	3.1	2.9	1.3
XDZ-30	487.2	436.8	283.8	158.6	146.7	148.7	71.5	86.3	59.7	66.1	40.3	14.0	4.5	3.4	2.2
XDZ-29	488.7	520.9	292.7	182.0	123.0	100.6	85.0	95.8	69.9	95.9	46.1	17.0	3.0	3.3	0.7
XDZ-28	490.3	491.0	316.4	168.8	121.6	100.7	86.6	98.1	70.3	88.1	47.2	16.5	3.4	3.4	1.1
XDZ-27	492.0	509.5	294.5	162.9	105.5	81.3	89.2	89.1	67.3	79.9	48.9	16.1	3.3	3.2	1.3
XDZ-26	493.5	554.5	319.6	169.6	110.7	74.6	99.6	108.9	69.6	89.0	54.2	17.0	3.4	3.2	0.6
XDZ-25	495.2	528.8	314.9	168.0	106.6	78.1	91.8	96.8	61.8	87.3	50.9	17.0	3.3	3.0	0.8
XDZ-24	496.7	532.9	284.2	136.4	85.9	75.4	96.4	82.5	58.6	53.9	51.8	15.3	3.0	2.9	1.0
XDZ-23	498.6	514.8	258.4	112.9	65.3	78.4	81.4	89.0	58.6	67.6	44.4	13.1	2.5	2.4	0.6
XDZ-22	500.0	563.6	210.2	105.2	52.9	83.0	67.8	58.8	59.9	45.3	37.8	10.3	1.9	1.9	0.5
XDZ-21	508.3	567.5	301.4	185.9	98.9	66.8	83.8	60.1	36.2	51.4	47.6	15.3	3.5	2.6	0.3
XDZ-20	509.6	573.6	311.0	181.4	118.6	74.8	99.3	146.1	65.5	80.1	52.9	17.2	3.9	3.7	0.5
XDZ-19	510.9	598.2	297.8	146.0	87.4	70.1	93.5	119.0	67.6	59.1	50.1	16.4	3.3	3.0	1.0
XDZ-18	512.2	559.7	342.5	205.9	131.3	81.3	108.0	73.0	74.1	92.4	60.0	20.5	4.1	3.3	0.5
XDZ-17	513.5	559.5	327.6	196.1	126.9	78.3	103.2	94.9	72.5	78.1	57.6	18.9	3.9	3.3	0.8
XDZ-16	514.8	569.9	305.2	211.0	126.2	77.4	104.5	86.9	73.0	82.6	56.6	18.6	3.8	3.4	0.4
XDZ-15	516.1	579.2	304.2	173.3	105.4	76.4	100.6	71.6	62.3	81.4	55.8	17.4	3.4	3.2	0.5
XDZ-14	517.4	565.2	305.6	152.7	97.2	77.2	98.3	86.2	69.1	67.1	55.3	16.2	3.1	3.0	0.5
XDZ-13	518.7	677.9	228.1	125.0	79.1	69.9	89.3	89.0	86.9	61.7	48.5	14.1	2.5	2.5	0.4
XDZ-12	525.4	863.6	318.9	116.6	77.2	79.3	108.8	45.3	69.2	57.9	63.2	15.9	3.2	3.1	0.5
XDZ-11	552.9	281.9	298.3	107.6	59.6	57.6	71.9	16.7	34.8	35.5	41.1	12.4	2.8	2.1	0.6
XDZ-10	554.7	349.8	233.9	82.9	48.9	67.2	58.4	50.9	48.3	36.2	30.4	10.6	2.4	1.7	0.3
XDZ-9	556.9	353.8	312.3	121.4	68.5	65.4	75.5	61.3	63.7	47.5	41.1	14.2	3.5	2.3	0.5
XDZ-8	558.3	407.4	329.4	161.0	98.6	85.9	91.5	65.2	92.3	59.8	47.4	15.8	4.0	3.0	0.6
XDZ-7	560.0	354.3	320.1	119.9	69.4	67.4	77.7	91.9	75.3	45.4	42.0	14.0	3.3	2.3	0.5
XDZ-6	562.1	444.3	332.2	142.9	82.5	82.8	75.2	71.7	88.4	50.7	39.9	15.5	3.8	2.7	0.5
XDZ-5	563.3	856.9	323.1	133.4	75.5	92.4	78.6	80.3	82.1	48.8	42.9	14.8	3.6	2.7	0.6
XDZ-4	564.2	363.2	306.3	98.0	55.1	76.5	69.5	52.2	48.5	36.3	37.5	13.5	3.1	2.2	0.3
XDZ-3	565.6	277.5	225.3	56.3	63.0	38.2	45.3	79.0	57.8	29.5	23.6	19.8	9.5	5.2	1.0
XDZ-2	566.6	579.8	339.6	168.6	214.7	96.5	137.6	159.4	67.7	111.5	71.5	20.3	4.0	4.8	0.9
XDZ-1	568.0	234.5	486.9	175.9	132.6	81.5	97.6	50.5	135.9	74.2	54.8	19.9	4.6	4.3	0.4

. The most enriched trace elements in the studied samples are Ba (493 ppm), Zr (303 ppm), Rb (146 ppm), and V (99 ppm). These are followed by Sr (86 ppm), Ce (85 ppm), Zn (80 ppm), Li (64 ppm), and Cr (63 ppm). Other than the trace elements already mentioned, no other trace element has an average concentration above 50 ppm.

Ba in sediments is primarily derived from four sources: biogenic Ba, Ba in terrigenous aluminosilicates, Ba from submarine hydrothermal activity, and Ba from benthic organism secretions [60]. Among these, only biogenic Ba effectively indicates paleo-primary productivity levels. Since no evidence of hydrothermal activity or benthic Ba input was identified in this study, the following formula was applied to accurately separate the biogenic Ba from Ba associated with terrigenous aluminosilicate detritus: Ba_bio = Ba_total − Ba_alusilicate = Ba_sample − Al_sample × (Ba/Al)_alusilicate. where Al_sample and Ba_sample represent the measured Al and Ba contents of the sample, respectively. (Ba/Al)_alusilicate serves as a correction factor to exclude the influence of barium derived from terrigenous aluminosilicates. It has also been proposed that in settings where terrigenous input is dominated by heavy minerals and includes volcanic material, Ti may be more suitable than Al as the correction factor, particularly when conventional Al-based corrections yield unsatisfactory results, such as negative values [61]. The (Ba/Ti)_{aluminosilicate} was calculated as 0.03 using the average value method for the study area, and this ratio was subsequently applied to calculate the Ba_bio content. The Ba_bio content in SubMbr1 ranges from 19.00 to 741.63 ppm, with an average of 302.65 ppm, while in SubMbr2, it ranges from 198.06 to 563.90 ppm, with an average of 373.71 ppm.

Mo, U, and V are redox-sensitive elements. The enrichment factor (X_EF) is used to assess the degree of elemental enrichment in rocks. Using a threshold of X_EF = 1, trace elements can be classified into two main categories: enriched and depleted. An X_EF > 1 indicates that the target element is enriched relative to the background value. This enrichment typically results from non-terrigenous processes. Conversely, an X_EF < 1 signifies that the element is depleted compared to the background, which may be due to its mobility during weathering or sedimentation [62]. The enrichment factors (X_EF) for trace elements of Xujiahe FM Mbr1 were calculated against PAAS values. For SubMbr1, Mo_EF values range from 0.22 to 0.60 (avg. 0.40); for SubMbr2, Mo_EF values range from 0.19 to 1.29 (avg. 0.56). U_EF values for SubMbr1 range from 1.07 to 1.77 (avg. 1.51); for SubMbr2, they range from 0.87 to 1.43 (avg. 1.10). V_EF values for SubMbr1 range from 0.30 to 1.51 (avg. 0.89); for SubMbr2, they also range from 0.30 to 1.51 (avg. 0.89).

When using enrichment factors of redox-sensitive elements, it is important to note that high clay mineral content may dilute the signals of these sensitive elements. The Ti/Al ratio is commonly used to account for the dilution effect of clay minerals. Titanium (Ti) is primarily hosted in heavy minerals and is strongly correlated with terrigenous detrital input, being nearly immobile during diagenesis. In contrast, Aluminum (Al) is mainly present in clay minerals and is a typical representative of terrigenous detritus. If the proportion of clay minerals in a sample is high, the Al content increases significantly, while Ti remains relatively stable, leading to a decrease in the Ti/Al ratio [58,63]. In the Xindianzi section, the Ti/Al values for SubMbr1 of Mbr1 from Xujiahe FM range from 0.054 to 0.074 (avg. 0.064), and for SubMbr2, they range from 0.043 to 0.059 (avg. 0.054). These values are close to the PAAS reference value of 0.05, indicating normal terrigenous detrital input. Therefore, the Mo_EF, U_EF, and V_EF values are considered reliable and require no correction.

Elemental ratios such as Th/U and V/Cr are important proxies for evaluating paleo-redox conditions. The Th/U ratio increases with the degree of water oxygenation, while the V/Cr ratio shows the opposite trend. Th/U < 2 indicates an anoxic water environment, a value between 2 and 6 indicates a suboxic to weakly anoxic water environment, and a value > 8 indicates a fully oxic water environment [64,65]. Generally, V/Cr > 4.25 represents an anoxic water environment, a ratio < 2.0 represents an oxic water environment, and a ratio between 2 and 4.25 represents a suboxic to weakly oxic water environment [66]. In the Xindianzi section, the Th/U values for SubMbr1 range from 2.08 to 5.09 (avg. 4.19), and for SubMbr2, they range from 2.77 to 5.64 (avg. 4.53), representing a suboxic to weakly anoxic water environment. The V/Cr values for SubMbr1 range from 1.33 to 2.14 (avg. 1.63), and for SubMbr2, they range from 0.97 to 2.22 (avg. 1.56), indicating a suboxic to weakly oxic water environment (Figure 4).

The europium anomaly (δEu) serves as a widely used proxy for evaluating the impact of hydrothermal processes. For SubMbr1, δEu values range from 0.54 to 0.67 (avg. 0.61). For SubMbr2, δEu values range from 0.61 to 0.70 (avg. 0.64).

Previous studies indicate that the (La/Yb)_N ratio and REE distribution patterns of shales can be used to qualitatively assess sedimentation rates. In depositional water bodies, REEs are primarily adsorbed onto detrital or suspended particles [67]; their varying residence times in the water column result in differential fractionation patterns [68]. Specifically, rapid sedimentation and brief residence times inhibit fractionation, yielding (La/Yb)_N values approximating 1.0. Conversely, a slow sedimentation rate and long residence time enhance fractionation, causing (La/Yb)_N values to be significantly higher or lower than 1.0. This principle provides an important basis for using REE fractionation characteristics to reconstruct paleo-depositional environments [69]. After PAAS normalization, the fine-grained sedimentary rock samples from the first member of Xujiahe FM at the Xindianzi section show: 6 samples exhibit a heavy REE-enriched PAAS-normalized REE pattern with (La/Yb)_N values ranging from 0.33 to 0.89 (avg. 0.78); 7 samples show a flat PAAS-normalized REE pattern with (La/Yb)_N values from 0.91 to 1.09 (avg. 1.02); and 22 samples show a light REE-enriched PAAS-normalized REE pattern with (La/Yb)_N values from 1.13 to 1.50 (avg. 1.31). This indicates significant fractionation between heavy and light REE in most samples, with weak fractionation in a minority.

5. Discussion

5.1. Reconstruction of the Paleo-Depositional Environment

5.1.1. Paleo-Redox Conditions

The redox environment in transitional marine–continental facies is influenced by multiple factors such as salinity fluctuations, terrigenous input, and tidal action, requiring comprehensive discrimination using a combination of various elemental geochemical proxies. Commonly used indicators include enrichment factors of redox-sensitive elements, pyrite morphology, and rare earth element (REE) patterns.

The behavior of redox-sensitive elements in sediments—such as Mo, U, V, and Cr—including their precipitation, accumulation, migration, and loss, is governed predominantly by the prevailing redox conditions [63,70]. These elements form soluble high-valence compounds in oxidizing waters (e.g., MoO₄²⁻), whereas they precipitate as low-valence sulfides under reducing conditions. Therefore, the enrichment factors (EF) of Mo, U, and V, as well as elemental ratios such as Th/U and V/Cr, are often used as important proxies for reconstructing the redox conditions of the water body [62,63].

For the fine-grained sedimentary rock samples from SubMbr1 of Mbr1 from Xujiahe FM in the Xindianzi section, these enrichment factor values show both depletion and weak enrichment (

Table 5. Paleoproductivity Proxies of Xujiahe FM Mbr1 fine-grained sedimentary rock.

Paleoproductivity Proxy		SubMbr1	SubMbr2
Ba	Range	234.54–863.55	321.84–677.93
	Average	447.26	517.52
Babio	Range (ppm)	19.00–741.63	198.06–563.90
	Average	302.65	373.71

, Figure 4), reflecting that these fine-grained sedimentary rocks likely formed in a suboxic to oxic transitional environment (Figure 4). In the Mo_EF—U_EF cross-plot (Figure 5), most data points from SubMbr1 fall below 0.1 times the seawater value, reflecting a weakly oxic water environment. Data points from SubMbr2 fall between 0.1 and 0.3 times the seawater value, indicating a suboxic to weakly oxic water environment [71].

Regarding trace element ratios, the mean Th/U value in SubMbr1 is 4.19, which falls within the 2–6 range indicative of a dysoxic environment. In contrast, its mean V/Cr ratio is 1.63, below the threshold of 2, suggesting an oxic setting. SubMbr2 shows a similar pattern, with a mean Th/U value of 4.53, consistently within the dysoxic range and a mean V/Cr ratio of 1.56, also below 2 and pointing to oxic conditions. This presents an apparent contradiction between the proxies: Th/U indicates dysoxia, whereas V/Cr leans toward oxic conditions. Closer inspection reveals that the V/Cr values lie near the oxic–dysoxic boundary, with their upper range exceeding the lower limit for dysoxia (2.0), while the Th/U ratios remain steadily within the dysoxic range. This discrepancy typically implies an unstable, dynamic depositional environment, most likely representing a transitional zone between weakly oxic and weakly reducing (dysoxic) conditions.

5.1.2. Hydrothermal Activity

Hydrothermal activity is commonly preserved in synsedimentary deposits and displays diagnostic geochemical anomalies. A notable example is the frequent enrichment of Fe and Mn oxides, which serves as a key indicator of hydrothermal influence [72,73]. Typical hydrothermal sediments have Al/(Al + Fe + Mn) values usually less than 0.4, and this value decreases as hydrothermal processes intensify [73]. All measured values exceed the typical hydrothermal sediment threshold (0.4) and approach the Post-Archean Australian Shale (PAAS) reference value of 0.66 [74]. This indicates that the sedimentation period of these fine-grained sedimentary rocks was predominantly influenced by substantial terrigenous detrital input, with weak influence from hydrothermal activity.

Furthermore, the Europium (Eu) anomaly in rare earth elements (REE) can also serve to assess the impact of hydrothermal activity and the redox conditions of the water body [75]. Eu anomalies serve as diagnostic indicators: δEu > 1 implies hydrothermal or reducing origins, whereas δEu < 1 points to felsic sources or oxidizing environments [76]. For the fine-grained sedimentary rock samples from Mbr1 in the Xindianzi section, the vast majority of δEu values are close to the PAAS standard of 0.66 and indicate a characteristic negative Eu anomaly. This suggests that the deposition of Mbr1 from Xujiahe FM in Southeastern Sichuan Basin was essentially unaffected by hydrothermal activity.

5.1.3. Paleoclimate

Paleoclimate exerted a control on both aquatic organism development and watershed vegetation growth via temperature and precipitation, which in turn governed primary productivity and regulated the supply of terrigenous detrital material [77]. In warm, humid climates, pronounced chemical weathering promotes the leaching of soluble cations (K⁺, Na⁺, Ca²⁺), whereas less soluble cations (Al³⁺, Ti⁴⁺) become enriched in the residual sediments.

The CIA values of fine-grained sedimentary rock samples from SubMbr1 of Mbr1 from Xujiahe FM yield an average value of 76.78, reflecting a warm and humid climate with relatively strong weathering during this period. The ICV values yield an average value of 0.80, suggesting the sediments may have undergone intense weathering or long-distance transport. Considering the delta front sedimentary environment of SubMbr1 and the CIA values below 80, it is inferred that long-distance transport was the likely cause, with weak influence from recycling, indicating high data reliability. The CIA values of fine-grained sedimentary rock samples from SubMbr2 yield an average value of 75.24, also indicating a warm and humid climate with relatively strong weathering during this period. The ICV values yield an average value of 1.08, showing the samples were largely unaffected by recycling processes, indicating high data reliability. On the CIA-ICV diagram, the majority of samples plot within the domain indicative of moderate chemical weathering (Figure 6).

Previous research on the Late Triassic climate of Sichuan Basin suggests that the Early–Late Triassic climate was relatively warm and humid, with a general trend towards cooling and drying from the late Norian to the Rhaetian [78]. Other scholars propose that the Late Triassic paleoclimate was generally warm and humid, with a brief cool and dry episode around the Norian-Rhaetian boundary [79]. Fine-grained sedimentary rocks in Mbr1 of Xujiahe FM display a subtly decreasing trend from bottom to top in Chemical Index of Alteration (CIA) values (Figure 7), indicating a climatic shift from warm and humid to cooler and drier, resulting in higher precipitation during SubMbr1 period compared to SubMbr2. These findings align with previous research conclusions. It can be inferred that during SubMbr1 period, the Southeastern Sichuan Basin was a deltaic depositional environment where ample precipitation promoted the growth of terrestrial plants. This not only led to the in situ burial of substantial OM but also surface runoff transported large amounts of terrigenous organic material into the lake basin. During SubMbr2 period, reduced precipitation led to a decrease in the input of terrigenous OM. Concurrently, as the sea level rose and overtopped the Luzhou paleo-uplift, the area became partially connected to the sea, transitioning to a coastal depositional environment.

5.1.4. Paleoproductivity Conditions

Paleoproductivity generally refers to the primary productivity, predominantly by plankton, during geological history. Its study is crucial for understanding the carbon cycle, hydrocarbon source rock formation, and paleoclimate change. Due to the varying distribution and preservation of plankton in ancient marine sediments, reliable alternative geochemical proxies are commonly used to assess marine paleoproductivity levels, and these proxies are also applicable in marine–terrestrial transitional environments [80,81,82]. Typical indicators for reconstructing paleoproductivity include biogenic silica (Si_bio), biogenic phosphorus (P_bio), and biogenic barium (Ba_bio). For the Xujiahe FM fine-grained sedimentary rock, the application of Si_bio is problematic because a significant portion of the SiO₂ is derived from non-biogenic silicate minerals, making the biogenic signal ambiguous. The proxy P_bio is also unsuitable due to the lack of a direct calculation method from total P₂O₅ content. Based on these considerations, Ba_bio was selected as the preferred proxy in this study for assessing paleoproductivity.

Compared to marine shales, which often exhibit Ba_bio values exceeding 1000 ppm, the Ba_bio content of the fine-grained sedimentary rock from Xujiahe FM is less than half. Consequently, from the perspective of paleoproductivity alone, it is certain that the paleoproductivity level of the Upper Triassic Xujiahe FM Mbr1 in the Southeastern Sichuan Basin was low to moderate.

Given the weakly oxidizing depositional waters and low Ba_bio levels during the deposition of Xujiahe FM, maintaining TOC between 1% and 3% would have required a substantial input of organic matter. As shown in Table 5, the total Ba content of the fine-grained sedimentary rock in Xujiahe FM is significantly higher than the Ba_bio content. Moreover, both Ba and Ba_bio exhibit consistent vertical trends from SubMbr1 to SubMbr2. The two observations mentioned above suggest that a considerable amount of terrigenous Ba is present in both submembers, thereby pointing to a high flux of terrigenous organic material.

5.1.5. Terrigenous Clastic Input and Sedimentation Rate

Clastic input and sedimentation rate exert interdependent constraints on the preservation of OM [83]. Typically, in oxic environments, a higher sedimentation rate is conducive to OM preservation because rapid burial effectively shortens the exposure time of OM to oxygen, accelerates its accumulation, and facilitates its rapid burial and subsequent preservation. However, an excessively high sedimentation rate can also lead to the dilution of OM by a large influx of clastic material. Therefore, OM enrichment is favored only within a specific range of sedimentation rates [82]. Similarly, the composition of clastic input also influences OM enrichment. A substantial input of felsic clastics tends to dilute OM [84], while input of mafic clastics can promote biological growth and reproduction [85]. Furthermore, an increase in clay minerals can adsorb more OM [86].

The lower (0.33–0.89) and higher (1.13–1.50) (La/Yb)_N values suggest relatively slow sedimentation rates during the deposition of these fine-grained sedimentary rocks, while the intermediate values (0.91–1.09) indicate faster sedimentation rates. Vertically, the sedimentation rate shows considerable fluctuation, with two distinct cycles of first decreasing and then increasing (Figure 7). These fluctuations in sedimentation rate may be related to frequent sea-level changes in the study area.

The high Si/Al values of the samples from Mbr1 of Xujiahe FM suggest a felsic source, but Ti-related ratios show an intermediate character. However, given the pronounced negative Eu anomaly observed in the Xujiahe Formation samples, the anomalously high Ti content is interpreted to be derived from volcanic ash input, as intermediate-acidic volcanic ash often exhibits significant negative Eu anomalies. To summarize, the fine-grained sedimentary rocks of Xujiahe FM were primarily derived from a felsic provenance that underwent intense chemical weathering, with an input of volcanic material.

5.2. OM Enrichment Mechanisms

5.2.1. Main Controlling Factors for OM Enrichment

Enrichment of OM is essentially a process where its burial rate exceeds its oxidation or degradation rate. However, this is a highly complex process resulting from the interplay of multiple influencing factors. To reveal the relationships between OM enrichment and these factors, we conducted correlation analyses between TOC content and each factor separately. Considering the depositional environment of the study area, we excluded data from 3 samples with anomalously high TOC contents, ultimately retaining TOC values from 17 samples for correlation with the different factors.

Redox conditions are a crucial factor in controlling the OM protection [82,87]. The correlations between redox proxies and TOC content in the fine-grained sedimentary rocks of SubMbr1 of Mbr1 from Xujiahe FM at the Xindianzi section are highly complex. For instance, Mo_EF shows no correlation with TOC content (Figure 8a), a statistically significant positive correlation is observed between U_EF and TOC (Figure 8b), V_EF shows a weak negative correlation with TOC (Figure 8c), Th/U shows a strong negative correlation with TOC (Figure 8d), and V/Cr shows a very strong positive correlation with TOC (Figure 8e). This complex pattern of correlations suggests that OM preservation in SubMbr1 may not be primarily controlled by redox conditions. In contrast, the TOC content in SubMbr2 shows varying degrees of positive correlation with Mo_EF, U_EF, V_EF, and V/Cr (Figure 8a–c,e), and a moderate negative correlation with Th/U (Figure 8d), indicating that OM preservation in this SubMbr was somewhat influenced by redox conditions. Observation reveals that the correlations of Mo_EF, U_EF, and V_EF with TOC are weaker in SubMbr1 than in the second, while the correlations of Th/U and V/Cr (also redox proxies) with TOC are stronger in SubMbr1, opposite to the trend for Mo_EF, U_EF, and V_EF. This discrepancy is likely related to the different depositional environments of the two SubMbrs. SubMbr1 was deposited in a relatively oxic deltaic environment. In oxic conditions, the concentrations of elements like U and Cr are more dynamic (e.g., Cr exists as soluble Cr⁶⁺ under oxic conditions), making their ratios (V/Cr, Th/U) more sensitive indicators of redox conditions and thus leading to more significant correlations with TOC. SubMbr2 was deposited in a dysoxic, weakly oxidizing tidal flat-lagoonal environment. Here, TOC was likely influenced more dominantly by primary productivity, sedimentation rate, or other factors, while the effect of redox conditions like Th/U and V/Cr was masked. Furthermore, enrichment factors like Mo_EF, U_EF and V_EF reflect the absolute degree of elemental enrichment and can directly and quantitatively indicate the relationship with TOC content. In contrast, elemental ratios like Th/U and V/Cr reflect redox status based on statistically established criteria and lack a direct quantitative relationship with TOC content. This explains why, when studying relationships with TOC, elemental enrichment factors correctly reflect the quantitative differences between the two SubMbrs, whereas elemental ratios yield opposite results.

CIA is an indicator of chemical weathering intensity in the source area, and its correlation with TOC content can reveal the influence of paleoclimate conditions and weathering on OM enrichment. TOC shows a very strong positive correlation with CIA in SubMbr1 and a strong positive correlation in SubMbr2 (Figure 8f). This indicates that OM enrichment in both SubMbrs was significantly influenced by paleoclimate conditions. This influence primarily manifests as follows: under warm and humid climates, intense chemical weathering (high CIA) leads to the substantial release of nutrient elements like P and K from terrigenous clastics. Coupled with the warm, humid climate and ample rainfall, this promoted the growth of terrestrial plants. The in situ burial of deceased plants became a major source of OM. Simultaneously, abundant surface runoff transported large quantities of nutrients and terrigenous OM into lakes or oceans, stimulating primary productivity, such as algae, and increasing the sources of OM. This conclusion is also consistent with the finding that the main types of OM in Mbr1 from Xujiahe FM in the study area are Type II₂ and Type III [32].

Sedimentation rate is also a significant factor influencing the preservation of OM. In the fine-grained sedimentary rocks of Mbr1 from Xujiahe FM at the Xindianzi section, the TOC content shows a strong negative correlation with high (La/Yb)_N values, which reflect slow sedimentation rates (Figure 8g). This indicates that in dysoxic–oxic environments, faster sedimentation rates are more conducive to OM preservation. Since only one TOC data point corresponds to the low (La/Yb)_N values (also indicative of slow sedimentation), their correlation is not discussed here. Furthermore, the TOC content also exhibits a strong negative correlation with intermediate (La/Yb)_N values, which reflect faster sedimentation rates (Figure 8g). However, in reality, within the intermediate (La/Yb)_N value range (0.8–1.2), the relationship between (La/Yb)_N and TOC content is not purely linear. Instead, two thresholds close to 1 (one slightly below and one slightly above) exist, where OM preservation conditions are optimal. Before reaching these thresholds, faster sedimentation favors OM preservation. Beyond these thresholds, further increases in sedimentation rate lead to the dilution of OM by a large influx of clastic material. Therefore, the linear correlation observed within the intermediate (La/Yb)_N range (0.8–1.2) is not considered meaningful for reference. Figure 8h shows strong negative correlations between (La/Yb)_N values and TOC content for both the first and second SubMbrs, with a stronger correlation in SubMbr1. This suggests that sedimentation rate significantly influenced OM preservation throughout Mbr1 of Xujiahe FM.

The correlation between TOC and the paleoproductivity proxy Ba_bio also exhibits significant differences between the two SubMbrs (Figure 8i).

In SubMbr1, a significant negative correlation is observed between TOC and Ba_bio. Given that the depositional environment of SubMbr1 is interpreted as a delta, it is inferred that this interval potentially experienced high paleoproductivity. However, due to presumably poor preservation conditions, the produced organic matter was almost entirely oxidized, preventing significant TOC enrichment. Consequently, the pronounced negative correlation, where high Ba_bio values correspond to low TOC contents, primarily reflects these adverse preservation conditions rather than low productivity.

In contrast, SubMbr2 shows a moderate negative correlation between TOC and Ba_bio. Notably, the average Ba_bio concentration in SubMbr2 is significantly higher than in SubMbr1, indicating an elevated level of paleoproductivity. Considering that SubMbr2 was deposited in a tidal flat environment, which generally offers more conducive conditions for organic matter preservation, the moderate negative correlation suggests that while preservation conditions still exerted an influence, they were no longer the sole dominant factor controlling TOC accumulation.

The correlation analysis between the Si-Al-Ti composite indicators and TOC content in the Xujiahe Formation Member 1 at the Xindianzi Section reveals distinct patterns between its two SubMbrs.

In SubMbr1, TOC shows a weak negative correlation with the Si/Al ratio (Figure 8j), suggesting that increased siliciclastic input (e.g., quartz) exerted only a minor dilution effect on organic matter. TOC exhibits a weak positive correlation with Ti/Al (Figure 8k) and a moderate positive correlation with Ti/Zr (Figure 8l). Given that the anomalously high Ti content in the Xujiahe Formation samples is influenced by volcanic ash, the weak to moderate positive correlations of Ti/Al and Ti/Zr with TOC in Sub-member 1 suggest that the influence of normal terrigenous detrital input on TOC accumulation was minimal. In SubMbr2, TOC displays a significant negative correlation with Si/Al (Figure 8j), indicating a strong dilution effect on organic matter by substantial siliciclastic input during this period. TOC shows moderate positive correlations with both Ti/Al and Ti/Zr (Figure 8k,l). Similar to SubMbr1, the anomalously high Ti content in SubMbr 2 is also attributed to the influence of volcanic ash. This Ti-rich ash is inferred to have supplied nutrients, boosting productivity and thereby positively influencing TOC content, which counteracted the dilution effect to some extent.

Based on the above analysis, paleoclimate and sedimentation rate were the main factors controlling OM enrichment in the fine-grained sedimentary rocks of SubMbr1 of Mbr1 from Xujiahe FM in the Southeastern Sichuan Basin. In contrast, redox conditions, paleoclimate, sedimentation rate, and the adsorption by clay minerals were the primary controlling factors for OM enrichment in SubMbr2.

5.2.2. OM Enrichment Models

OM enrichment is typically the result of multiple interacting factors. Current enrichment models are primarily categorized into two types: the high productivity model and the strong preservation model [88,89].

The mudstones and shales of the Upper Triassic first member of Xujiahe FM in the Southeastern Sichuan Basin were deposited in two distinct environments: a deltaic environment, represented by SubMbr1, and a tidal flat (coastal plain)—lagoon (shallow basin) environment, represented by SubMbr2 [32]. Both environments featured ample water sources, fertile land, and lush vegetation. The relatively high depositional hydrodynamic energy and sedimentation rates were conducive to the continuous accumulation of large amounts of plant-derived detrital OM, making these settings ideal for OM enrichment.

During the deposition of SubMbr1 in the deltaic setting, the paleoclimate was warm and humid, favoring the growth and accumulation of terrestrial higher plants. Debris from these higher plants became the primary source of OM for the organic-rich shales. Although the weakly oxic water column was unfavorable for OM preservation, the high sedimentation rate shortened its exposure time to oxygenated waters. This allowed OM to be buried before significant oxidation or degradation could occur, forming a “strong preservation model” under relatively oxic conditions (Figure 9a). While this process did not significantly alter the redox conditions of the water bottom and anoxic conditions were not established, it achieved preservation through protection on a temporal scale.

During the deposition of SubMbr2 in the tidal flat-lagoon environment, the paleoclimate was slightly drier and cooler compared to SubMbr1 but remained generally warm and humid [3,79,90]. Intense weathering released nutrient elements like potassium and phosphorus from terrigenous clastics, which were carried into the water body by rainfall. This provided ample nutrients for blooms of algae and other aquatic organisms. Consequently, both lower aquatic organisms and higher plant debris contributed to abundant OM for the deposition of organic-rich shales. Along with these nutrients, a significant amount of clay mineral particles entered the water. The adsorption of OM onto these clay particles further enhanced its preservation. Compared to the relatively oxic environment of SubMbr1, the dysoxic, weakly oxic water conditions of SubMbr2 were more favorable for OM preservation. Coupled with the relatively fast sedimentation rate and the adsorptive capacity of clay minerals, these three factors worked together to promote OM enrichment. This formed a “strong preservation–productivity” composite model, dominated by excellent preservation conditions with a secondary contribution from productivity (Figure 9b).

6. Conclusions

• The fine-grained sedimentary rocks of Mbr1 from Xujiahe FM in the Southeastern Sichuan Basin were deposited in two distinct sedimentary environments. SubMbr1 was formed in a lacustrine–deltaic environment with weakly oxic to oxic water conditions and a predominantly felsic provenance. SubMbr2 was deposited in a semi-restricted coastal environment with dysoxic to weak oxic water conditions and a felsic provenance. Both submembers experienced moderate to intense chemical weathering. The paleoclimate during shale deposition was warm and semi-humid to humid, characterized by high sedimentation rates.
• OM enrichment resulted from the interplay and mutual influence of multiple factors, including paleoclimate, paleoproductivity, water column conditions, sedimentation rate, terrigenous input, and sea-level fluctuations. The primary controlling factors for OM enrichment in SubMbr1 of Mbr1 from Xujiahe FM were paleoclimate and sedimentation rate. In contrast, the main controlling factors for SubMbr2 were redox conditions, climate, sedimentation rate, and the adsorption by clay minerals.
• This study reveals the OM enrichment mechanisms for different periods within the first member of Xujiahe FM, establishing two distinct models: a “strong preservation model” under relatively oxic conditions for SubMbr1 and a “strong preservation–productivity composite model” dominated by excellent preservation conditions with a secondary contribution from productivity for SubMbr2. The fundamental cause of OM enrichment is a burial rate that exceeds the decomposition rate. However, the influence of climate and water column conditions led to the formation of these two different models. The fine-grained sedimentary rocks of SubMbr1 were deposited in a weakly oxic lacustrine–deltaic environment. The warm, humid climate and ample precipitation caused intense weathering, which delivered terrigenous OM into the lake basin but also diluted it with concurrent input of felsic terrigenous clastics. Nevertheless, high sedimentation rates compensated for this dilution by rapidly burying OM before significant decomposition could occur. The fine-grained sedimentary rocks of SubMbr2 were formed in a complex marine–terrestrial transitional environment with dynamic hydrographic conditions. As the climate became relatively drier with reduced precipitation, the water body evolved into a dysoxic, weakly oxic, semi-restricted setting where aquatic organisms began to flourish, leading to a more significant increase in the influence of paleoproductivity. However, the dysoxic weakly oxic water conditions, high sedimentation rate, and adsorption of OM by clay minerals remained the dominant factors controlling the OM enrichment model.