Lithofacies Characteristics and Sedimentary Evolution of the Lianggaoshan Formation in the Southeastern Sichuan Basin

Abstract

The Lower Submember of the Second Member of the Lianggaoshan Formation (LGS2-LS) in the Fuling area, southeastern Sichuan Basin, represents the deepest lacustrine depositional stage of the formation and constitutes an important target for shale oil and gas exploration. Based on core observations, thin-section petrography, X-ray diffraction, geochemical analyses, and sedimentary facies interpretation from representative wells, this study characterizes the lithofacies types, sedimentary environments, and depositional evolution of the LGS2-LS. Results show that the LGS2-LS is dominated by clay–quartz assemblages, with average clay mineral and quartz contents of 44.6% and 38.8%, respectively, and can be subdivided into shallow and semi-deep lacustrine subfacies comprising eight microfacies. Geochemical proxies indicate alternating warm-humid and hot-arid paleoclimatic phases, predominantly freshwater conditions, variable redox states, and fluctuations in paleoproductivity. Sedimentary evolution reveals multiple transgressive–regressive cycles, with Sub-layer 6 recording the maximum water depth and deposition of thick organic-rich shales under strongly reducing conditions. The proposed sedimentary model outlines a terrigenous clastic lacustrine system controlled by lake-level fluctuations, transitioning from littoral to shallow-lake to semi-deep-lake environments. The distribution of high-quality organic-rich shales interbedded with sandstones highlights the LGS2-LS as a favorable interval for shale oil and gas accumulation, providing a geological basis for further hydrocarbon exploration in the southeastern Sichuan Basin.

1. Introduction

The Lianggaoshan Formation, particularly the Lower Subsection of the Second Member (LGS2-LS), is a vital geological unit in the southeastern Sichuan Basin, China (30°30′ N 105°30′ E) [1,2]. As an organic-rich interval, it has gained significant attention for its potential as a source rock for shale oil and gas, driving extensive research into its sedimentary characteristics, lithofacies distribution, and reservoir properties [3,4,5]. The LGS2-LS is situated within delta-lake sedimentary systems, encompassing delta front and lacustrine facies, which directly influence the formation and spatial distribution of high-quality source rocks. Understanding these facies relationships is essential for evaluating its hydrocarbon exploration potential [6,7,8].

The depositional environment of the LGS2-LS is primarily influenced by factors such as water depth, salinity, and organic productivity. These parameters play a critical role in the accumulation and preservation of organic matter, directly impacting the formation’s hydrocarbon generation potential [9,10]. The shale exhibits favorable reservoir properties, with porosities ranging from 1% to 5.2% and an average total organic carbon (TOC) content exceeding 2%, thereby meeting the criteria for effective hydrocarbon generation and storage [11,12,13]. Furthermore, the formation’s thermal maturity falls within the oil window, which enhances its hydrocarbon-generating capability and underscores its significance in shale resource exploration [14,15,16]. Previous studies have suggested that the LGS2-LS possesses excellent hydrocarbon generation potential, which is further enhanced by a well-developed network of fractures and pores that improves both storage capacity and fluid mobility [8,17].

Exploration practice has demonstrated that the Lianggaoshan Formation shale in the Fuling area possesses significant exploration potential [18]. The mineral composition and sedimentary structural characteristics of terrestrial mud-shale are complex, with rapid shifts in lacustrine basin centers during different depositional periods. The generation of shale oil and gas is directly influenced by the sedimentary environment [19,20]. Previous studies on the Lianggaoshan Formation shale have primarily focused on fundamental geological conditions, reservoir types and characteristics, and pore effectiveness. For example, Nie et al. (2017) [21] used scanning electron microscopy, mercury intrusion, and gas adsorption to suggest that the black shale reservoirs of the Lianggaoshan Formation in eastern Sichuan are dominated by mineral intergranular residual pores, with limited development of microfractures and organic pores. Fang et al. (2023) [22] concluded that the storage spaces of different shale assemblages mainly consist of microfractures and inorganic pores, with silty shale assemblages exhibiting wider throats and better pore connectivity compared to pure shale and bioclastic shale assemblages.

Currently, systematic research on lithofacies classification and sedimentary environment evolution of the Lianggaoshan mud-shale in the Fuling area remains lacking, which significantly restricts large-scale exploration and development in the region [23]. With the refinement of shale lithofacies and assemblage characterization, studying shale stratigraphic depositional facies by adopting single depositional facies or purely shale lithofacies approaches can no longer adequately reflect sedimentary environmental differences among stratigraphic units, falling short of the demands of modern shale gas exploration and development.

Liu et al. (2019) [24] identified 6 categories and 20 types of shale lithofacies in the Lower Jurassic of the Sichuan Basin through a novel lithofacies classification method based on mineral composition, TOC grading, rock fabric, and sedimentary structures. Fu and Liu (2023) [25] classified the Lianggaoshan mud-shale in the Fuling area into six lithofacies assemblages based on mineral composition. Zhong Quancheng (2021) [26] established a lithoelectric facies model using sedimentological markers and analyzed depositional facies characteristics, dividing the Lianggaoshan Formation into littoral lake, shallow lake, and semi-deep lake subfacies.

In summary, previous studies on the Lianggaoshan Formation have primarily focused on the reservoir sections, with research on lithofacies characteristics and sedimentary environment evolution mostly conducted at the formation level. Studies at the finer submember scale are relatively limited. Additionally, the lack of unified standards and schemes for lithofacies classification of terrestrial lacustrine basin shales has made it difficult to accurately distinguish sedimentary environmental differences between stratigraphic units.

Against this backdrop, this study primarily focuses on the representative Well TY1 within the study area. Based on detailed core observations, in combination with thin-section identification, macroscopic petrographic analysis, and trace-element analysis, and integrated with regional drilling data, the research investigates the regional lithofacies characteristics and their lateral correlations. The depositional environments of different submembers are reconstructed, and the distribution of favorable reservoir intervals is delineated, thereby providing a scientific basis for the further exploration and development of high-quality shales in the Lianggaoshan Formation of the Fuling area.

2. Regional Geological Background

The Sichuan Basin, located in southwestern China, is an important oil and gas-bearing basin with a long and complex tectonic history [3,27]. It is bounded by the Longmenshan Fault to the northwest, the Yunnan-Guizhou Plateau to the southwest, and the Yangtze Platform to the east and southeast [28,29,30]. This intra-cratonic depression has experienced a series of tectonic processes, including rifting, subsidence, and compressional deformation, which have significantly influenced its stratigraphic architecture and resource potential. These tectonic activities laid the foundation for the deposition of the Lianggaoshan Formation, which records the transition from a shallow marine to a lacustrine environment during the Jurassic period [29,31].

During the Jurassic, the Sichuan basin underwent extensive extensional tectonics, which promoted regional subsidence and the development of large-scale lacustrine systems [32,33]. In the southeastern part of the basin, where the LGS2-LS is located, delta-lake sedimentary systems dominated the depositional framework. The delta front facies were marked by fine-grained sandstone deposition, while semi-deep lacustrine environments accumulated organic-rich mudstones and shales. These shales are now recognized as important source rocks for hydrocarbons, with their distribution and quality heavily influenced by tectonic subsidence and fluctuating lake levels [33,34].

The LGS2-LS represents a transgressive phase within the Lianggaoshan Formation, characterized by deepening lake basins and thick shale deposits [7,35]. These organic-rich shales exhibit high TOC content, making them ideal for hydrocarbon generation [36,37,38]. The thermal evolution of the basin further enhanced their maturity, placing the shales within the oil window, which is critical for their hydrocarbon production potential (Figure 1). Understanding the depositional environment and regional tectonic influences on the LGS2-LS provides a solid foundation for evaluating its hydrocarbon potential and guiding future exploration strategies.

3. Samples and Methods

This study comprehensively analyzes the lithofacies characteristics, sedimentary evolution, and hydrocarbon potential of the LGS2-LS in the southeastern Sichuan Basin. To achieve this, a multidisciplinary approach integrating core sampling, laboratory analysis, and sedimentary facies interpretation was adopted.

3.1. Sample Collection

Core samples were collected from the TY-1 wells in the Fuling area, representing a depth range of 1000 to 2000 m. Sampling intervals were maintained at 10 to 15 m, ensuring a diverse representation of lithological variations within the LGS2-LS. A total of 220 samples were selected, covering key lithologies such as shale, siltstone, sandstone, and mudstone. Among them, 54 samples were subjected to whole-rock mineral X-ray diffraction (XRD) analysis; over 100 specimens were used for petrographic thin-section identification and total organic carbon (TOC) analysis; and 30 specimens were analyzed for metallic element concentrations in rocks.

3.2. Core Analysis

Macroscopic observations of the cores were conducted to classify lithofacies and identify sedimentary structures such as lamination, cross-bedding, and bioturbation. Thin section analysis, performed under a polarizing microscope, further revealed the mineralogical composition and distribution of organic matter. Lithofacies classification was based on visual features, texture, and sedimentary structures, highlighting the dominance of delta front and lacustrine facies in the LGS2-LS depositional system.

3.3. Laboratory Testing

Several laboratory analyses were performed on the samples to evaluate their geochemical, mineralogical, and petrophysical properties. Petrographic thin-section identification was carried out at Chengdu University of Technology. Whole-rock mineral XRD analysis, and major/trace element analyses were all performed at the Testing Center of SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms.

Thin-section analyses were performed using a polarizing microscope (Nikon E600 POL, made in Tokyo, Japan) to examine the mineral composition, textures, and sedimentary structures of the samples.

XRD was applied to determine the mineralogical composition of the samples. By exposing the powdered samples to X-rays, diffraction patterns were collected and analyzed to identify the mineral content, including clay minerals, quartz, and carbonates. This information is crucial for understanding the reservoir properties and how mineral composition may affect hydrocarbon accumulation.

The whole-rock geochemical analyses were conducted in accordance with the SY/T 6404-2018 standard [39], employing inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) for the determination of metallic elements in rocks. The analytical instruments included a VISTA MPX ICP-AES spectrometer (YQ4-04-05, Melbourne, Australia) and an X II ICP-MS spectrometer (YQ4-12-14, Bremen, Germany). Analytical weighing was performed using precision balances (TP4-07-03 and TP4-07-06).

3.4. Sedimentary Facies Interpretation

Based on lithological characteristics, sedimentary structures, and organic matter distribution, two primary subfacies—shallow lake and semi-deep lake—were identified. These subfacies were further divided into eight sedimentary microfacies, such as mud-rich lacustrine deposits, silty interbeds, and deltaic sand layers [40].

4. Results

4.1. Lithofacies Classification

XRD mineral composition analysis was conducted on 54 samples from the LGS2-LS. The results indicate that this stratigraphic unit is generally dominated by a clay mineral–quartz assemblage characteristic of clayey–siliceous clastic rocks (Figure 2). The average clay mineral content is 44.6%, while quartz averages 38.8%. However, significant variations exist among different stratigraphic levels. Layers 7 and 6 are characterized by high clay content, reflecting a low-energy deep-lake depositional environment; in contrast, Layers 1 and 5 exhibit higher quartz contents indicative of higher-energy hydrodynamic conditions. Notably, Layer 3 shows an anomalous enrichment of calcite (

Table 1. Classification of mineral components in the LGS2-LS of TY1.

No.	Layer No.	Depth/m	Clay Minerals/%	Detrital Minerals/%			Carbonate Minerals/%			Other Minerals/%
			Clay	Quartz	Potassium Feldspar	Plagioclase	Calcite	Dolomite	Siderite	Pyrite	Barite	Amphibole	Gypsum	Anhydrite	Pyroxene
1	Layer 8	2536.53	26.0	42.0	13.9	0.2	0.3	15.5		0.5	0.6				0.1
2		2538.46	45.6	37.8	2.7	0.8	0.2	10.7		0.3	0.4				0.1
3	Layer 7	2541.66	51.6	36.3	1.3	0.2	0.3	3.1		1.2	0.8			1.3	0.2
4		2543.14	50.0	38.8	1.3		0.2	3.9		1.2	0.2			1.1	0.1
5		2544.97	52.6	29.7	9.5	0.1	0.2	2.5		1.2	0.9			0.8	0.2
6		2545.87	57.9	29.7	1.2	0.5	0.4	2.4		1.6	0.8			1.3	0.1
7		2546.77	46.3	33.4	7.8	0.2	0.3	3.9		1.8	1.9			0.4
8		2548.08	44.9	33.4	6.9	1.0		9.3		1.3	1.2				0.1
9	Layer 6	2548.95	33.3	48.2	2.5	0.1	0.4	10.7		0.9	0.4			1.1
10		2549.88	46.1	41.1	2.2	0.1		6.1		1.0	0.7
11		2550.94	55.7	33.6	0.1	0.1	0.2	3.8		1.2	0.6			0.9	0.1
12		2552.03	52.0	30.2	2.6	2.8	0.4	3.4		1.6	0.6			1.4	0.2
13		2552.48	57.3	32.7	0.3	0.0		4.0		0.7	0.9				0.2
14		2553.52	42.5	40.6	6.5	0.3	0.3	3.7		1.8	1.7			0.4	0.3
15		2555.56	49.7	39.2	0.1	0.3	0.2	3.8		1.0	0.5		0.6	0.4	0.2
16		2556.18	51.1	37.2	0.4	1.1	0.2	4.7		1.1	0.3			0.9
17		2557.35	52.5	36.4	1.0	0.4	0.3	4.4		0.6	0.8				0.1
18		2558.10	55.5	31.4	1.1	0.8	0.4	4.2		1.3	0.2			0.6	0.0
19		2559.10	50.1	30.1	9.3	0.1	0.2	2.2		2.5	1.5		0.5	1.2	0.2
20		2560.41	54.9	33.0	0.8	0.8	0.3	4.1		1.0	0.5		0.2	0.3	0.2
21		2561.28	51.4	38.4	0.2	3.5	0.9	0.4	0.6	0.6			0.2	1.2	2.6
22	Layer 5	2562.27	52.3	35.4	0.2	4.6	0.4	0.1	1.5	0.9	0.2	0.3	0.2	1.2	2.7
23		2563.20	42.7	41.1		8.6	0.2	1.4	1.2	0.2		1.5		1.2	1.9
24		2565.32	40.4	47.4	0.2	7.2	0.6	0.6	0.6	0.1			0.1	0.9	1.9
25	Layer 4	2566.94	46.9	40.7	0.4	7.5	0.4	0.5	0.7	0.3			0.1	0.7	1.8
26		2568.35	55.7	32.4	0.5	4.3	0.8	0.3	1.1	0.3	0.4	0.7	0.1	1.1	2.3
27		2568.70	40.0	44.6	0.8	6.6	0.1	0.1	1.3			1.4		1.6	3.5
28		2569.70	52.2	36.0		3.8	2.5	0.2	1.4	0.2		1.0		1.0	1.7
29		2571.05	50.5	35.1	0.1	3.7	3.4	1.0	1.0	0.7		1.0	0.2	1.2	2.1
30		2571.44	50.9	37.7	0.1	3.5	0.9	0.6	1.2	0.5		1.1		1.2	2.3
31		2572.20	42.8	41.3	0.2	6.6	1.5	1.2	0.7	1.3		1.3		1.0	2.1
32		2573.23	45.8	39.5	0.3	7.6	1.7	0.7	0.6	0.9			0.2	1.2	1.5
33		2574.45	53.9	35.6	0.2	4.6	0.4	0.4	1.2	0.3				1.2	2.2
34		2575.25	50.8	37.0		6.0	0.5		1.0	0.9		0.6	0.1	0.7	2.4
35		2576.34	51.7	34.7	0.2	5.7	0.9		1.2	0.7	0.7	0.5	0.1	1.2	2.4
36		2577.67	52.4	37.0	0.2	3.4	2.3	0.9	0.9	0.2			0.1	1.0	1.6
37		2579.14	54.3	34.8	0.3	2.9	2.5		1.6	0.8			0.2	1.1	1.5
38		2579.92	56.4	34.1	0.2	4.3	0.2	0.1	1.5	1.0				1.2	1.0
39	Layer 3	2581.10	46.5	41.2		6.7	0.9		0.7	0.2		0.9	0.1	0.8	2.0
40		2582.13	45.0	41.1	0.2	7.0	0.4	1.5	0.8	0.4		0.8	0.1	1.1	1.6
41		2584.43	17.3	34.8	0.3	16.1	29.1	0.5	0.2	0.4		0.7		0.6
42	Layer 2	2585.34	24.1	35.8	0.2	35.7	1.3	0.0	0.1	0.2		0.7	0.1	0.7	1.1
43		2586.73	49.3	40.3	0.2	3.8	0.3	0.3	0.7	0.5	0.3	0.8	0.1	0.9	2.5
44		2587.97	56.3	32.8	0.4	3.1	1.2	1.1	0.4	0.7		1.0	0.2	0.9	1.9
45		2589.61	50.5	37.9	0.5	5.4	0.8	0.8	0.7	1.0		0.6		0.9	0.9
46		2590.49	46.9	39.2	0.3	4.6	0.5		2.6	2.1		0.7	0.1	0.8	2.2
47		2591.28	47.7	40.1	0.4	5.0	0.4	0.2	1.1	0.5			0.2	1.3	3.1
48		2592.25	46.0	41.0	0.5	3.3	1.5	1.0	1.4	0.7	0.4	0.7		1.2	2.3
49		2593.74	48.3	41.3	0.4	3.4	0.3	0.2	1.4	0.4			0.1	1.1	3.1
50		2594.36	53.0	37.9	0.5	3.5	0.2	0.1	0.6	0.5	0.5		0.1	1.1	2.0
51		2595.02	56.5	33.7	0.3	3.8	0.3	0.3	1.1	0.3		0.8		1.0	1.9
52		2596.35	42.2	49.3	0.2	3.9	0.4		0.6	0.4	0.1	0.7	0.1	0.8	1.3
53	Layer 1	2598.15	50.2	41.6	0.2	1.7	0.1	0.3	1.0	0.2	0.2	0.6	0.1	1.1	2.7
54		2599.96	22.0	63.4	0.2	5.2	6.7	0.3	0.1	0.5		0.6	0.1	0.3	0.6

).

Based on the identified mineral composition, the lithofacies classification of the LGS2-LS is determined according to the following criteria: (1) lithology, established through core observations and mineral composition analysis; (2) sedimentary structures, including lamination, bioturbation, and cross-bedding; (3) texture and grain size, ranging from clay to fine sandstone; and (4) organic content, evaluated by TOC measurements and hydrocarbon generation potential. These parameters together provide a systematic framework for defining lithofacies types and interpreting their depositional environments [1,16,17,41].

4.2. Sedimentary Facies Characteristics of the LGS2-LS

The sedimentary facies of the LGS2-LS were classified based on lithological combinations, sedimentary structures, grain size distribution, and vertical facies successions observed in core samples. Integrating these parameters with logging and geochemical data, the LGS2-LS was divided into two primary sedimentary subfacies: shallow lacustrine and semi-deep lacustrine. These were further subdivided into eight sedimentary microfacies (

Table 2. Classification table of sedimentary facies in the LGS2-LS of TY1.

Facies Type	Subfacies Type	Microfacies Type	Lithology	Sedimentary Structures	Logging Response Characteristics
Continental Lacustrine	Shallow Lake Subfacies	Gray Massive Sandstone Microfacies	Gray fine sandstone, gray lithic sandstone	Horizontal bedding, wavy bedding, occasional mud clasts, calcite veins	Low GR, Low KTH GR values range from 50.61 to 86.30 API, with an average of 62.17 API; KTH values range from 32.45 to 53.64 API, with an average of 42.35 API
		Gray-White Dense Shell Limestone Microfacies	Shell limestone, argillaceous limestone	Liquefaction deformation, convolute bedding, developed bioclastic layers	Medium to High GR, Medium to High KTH GR values range from 98.08 to 107.36 API, with an average of 103.21 API; KTH values range from 81.05 to 86.97 API, with an average of 84.49 API.
		Gray Sand-Wavy Bedded Siltstone Microfacies	Gray silty mudstone, gray siltstone	Horizontal bedding, slump structures	Low GR, Low KTH GR values range from 86.78 to 104.62 API, with an average of 90.65 API; KTH values range from 60.85 to 78.34 API, with an average of 67.29 API.
		Dark Gray Low-Carbon Silty Shell-Laminated Mixed Shale Microfacies	Dark gray mudstone, gray-white massive mud-rich shell limestone	Developed bedding, horizontal bedding, bioclastic layers	Medium to High GR, Low KTH GR values range from 87.54 to 116.68 API, with an average of 103.10 API; KTH values range from 63.95 to 80.77 API, with an average of 73.14 API.
		Gray to Dark Gray Liquefied Deformed Sand-Mud Interbedded Microfacies	Gray silty mudstone, dark gray muddy siltstone interlayers	Developed lamination, sand wave lamination, lenticular bedding, liquefaction structures	High GR, Medium to High KTH GR values range from 98.11 to 113.36 API, with an average of 105.06 API; KTH values range from 74.91 to 85.18 API, with an average of 79.21 API.
	Semi-Deep Lake Subfacies	Gray-Black Medium-Carbon Silty Laminated Clay Shale Microfacies	Gray-black mudstone	Developed bedding, horizontal bedding, occasional bioclasts	Medium to High GR, Medium to High KTH GR values range from 89.84 to 110.00 API, with an average of 100.85 API; KTH values range from 68.89 to 89.10 API, with an average of 76.64 API.
		Gray-Black High-Carbon Silty Laminated Clay Shale Microfacies	Gray-black mudstone	Bedding fractures, high-angle fractures filled with calcite, horizontal bedding	High GR, Medium to High KTH GR values range from 103.89 to 110.82 API, with an average of 106.93 API; KTH values range from 68.89 to 89.10 API, with an average of 76.64 API.
		Dark Gray Medium-Carbon Interbedded Silty Laminated Clay Shale Microfacies	Dark gray mudstone	Horizontal bedding, muddy lamination, lenticular structures, convolute structures	High GR, High KTH GR values range from 99.76 to 121.62 API, with an average of 109.70 API; KTH values range from 77.92 to 94.28 API, with an average of 86.85 API.

).

The shallow lacustrine subfacies lies between the outer edge of the lakeshore subfacies and above the wave base. It is characterized by deeper water compared to the lakeshore zone, with sediments strongly influenced by wave and lake currents (Table 2). This subfacies is mainly developed in Sub-layer 1, the base and top of Sub-layers 2 and 3, the sandy-muddy interbedded section of the middle part of Sub-layer 4, Sub-layer 5, the base of Sub-layer 7, and the top of Sub-layer 8 of the LGS2-LS. The sediments primarily consist of sand and mud, with localized development of carbonate rocks. During the shallow lacustrine stage, sandy sediments were often transformed into sand bars due to the effects of storm flows, seismic activity, and gravitational deformation. These processes predominantly resulted in the formation of sandy shoals and shell-fragment shoals, with occasional occurrences of shell fossils.

Based on the sedimentary characteristics, five sedimentary microfacies have been identified within the shallow lacustrine subfacies (Figure 3): Gray massive sandstone microfacies; Gray-white densely shelled limestone microfacies; Gray sandy laminae siltstone microfacies; Dark gray low-carbon silty shelled laminated mixed shale microfacies; Gray to dark gray liquefaction-deformed sandy-muddy interbedded microfacies.

The semi-deep lacustrine subfacies is situated below the wave base in relatively deeper water. It represents a low-energy environment under suboxic to reducing conditions, distant from the lakeshore and shallow lacustrine zones. In this setting, sedimentation is dominated by still-water deposition, with finer-grained and darker-colored sediments exhibiting horizontal bedding and silty laminae. Despite the overall low-energy conditions, wave and bottom current reworking still influenced this subfacies (Figure 3). The semi-deep lacustrine subfacies is mainly developed at the top of Sub-layer 2, the base and top of Sub-layer 4, Sub-layer 6, the middle to upper parts of Sub-layer 7, and the lower part of Sub-layer 8.

Based on sedimentary characteristics, the semi-deep lacustrine subfacies can be further divided into the following microfacies (Figure 3): Gray-black medium-carbon silty laminated clay shale microfacies; Gray-black carbon-rich silty laminated clay shale microfacies; Dark gray medium-carbon silty laminated clay shale microfacies with silt intercalations.

5. Discussion

5.1. Analysis of Factors Controlling the Depositional Environment

Geochemical element ratios provide valuable insights into the depositional environment of the LGS2-LS. Geochemical analysis is a commonly employed method for reconstructing depositional environments during geological history [42,43]. Ratios such as Sr/Cu and Sr/Cr, involving strontium, copper, and chromium are effective paleoclimate proxies, where high values indicate dry and hot conditions, and low values suggest warm and humid climates [44,45,46,47]. The Sr/Ba and V/Ni ratios, which incorporate strontium, barium, vanadium, and nickel, are commonly used to infer paleosalinity, allowing differentiation among saline, brackish, and freshwater environment [48,49,50]. Redox conditions are evaluated using Cu/Zn, V/(V + Ni), and Ni/Co ratios, which involve copper, zinc, vanadium, and nickel; variations in these ratios reflect changes between oxidizing and reducing depositional environments [51,52]. Additionally, biogenic barium (Ba_bio), derived by excluding terrigenous contributions from total Ba content, serves as a reliable indicator of paleoproductivity [53,54,55].

Based on the geochemical data from Well TY-1 (

Table 3. Statistics of elements in the sedimentary environment of the LGS2-LS (×10⁻⁶).

Layer No.	Sr/Cu	Sr/Cr	Sr/Ba	V/Ni	Cu/Zn	V/(V + Ni)	Ni/Co	Babio
Layer 8	3.39~14.71 9.05	2.61~3.33 2.97	0.26~0.81 0.53	2.15~2.39 2.27	0.27~0.50 0.39	0.68~0.71 0.69	1.96~2.38 2.17	−10.16~345.32 167.58
Layer 7	2.55~6.93 4.99	1.87~2.74 2.43	0.23~0.39 0.30	2.61~3.16 2.86	0.30~0.54 0.38	0.72~0.76 0.74	2.54~3.36 2.82	48.70~143.59 78.23
Layer 6	3.23~6.99 4.89	2.20~3.162.51	0.29~0.37 0.32	2.84~3.15 3.02	0.29~0.49 0.38	0.74~0.76 0.75	2.62~3.11 2.84	30.51~131.99 111.55
Layer 5	5.55~6.54 6.04	1.64~2.28 1.96	0.40~0.44 0.42	2.60~3.59 3.10	0.31~0.33 0.32	0.72~0.78 0.75	2.35~3.24 2.79	−142.11~−44.98 −93.71
Layer 4	3.68~4.69 4.20	1.67~2.24 1.92	0.28~0.40 0.31	2.59~3.29 3.10	0.31~0.39 0.35	0.72~0.77 0.76	2.50~2.87 2.69	−155.58~123.07 −23.95
Layer 3	4.52~7.97 6.24	1.11~1.681.39	0.17~0.45 0.31	2.98~3.05 3.02	0.26~0.33 0.30	0.74~0.75 0.75	2.44~2.82 2.63	−210.77~399.16 94.20
Layer 2	3.92~9.25 4.93	1.13~2.17 1.70	0.26~0.47 0.34	1.87~3.80 2.98	0.19~0.38 0.31	0.65~0.79 0.74	1.72~2.98 2.51	−237.62~30.71 −81.03
Layer 1	3.89	0.74	0.27	2.72	0.37	0.73	2.22	−14.52

Note: Data in the table are expressed as (${\displaystyle \frac{\mathrm{m}\mathrm{i}\mathrm{n}~\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}$).

), the following interpretations are made for the LGS2-LS: The average Sr/Cu ratios in sublayers 3, 5, and 8 exceed 5, indicating a hot and arid climate, while sublayers 1, 2, 4, 6, and 7 have average Sr/Cu ratios below 5, suggesting a warm and humid climate (Figure 4a). The mean Sr/Ba ratios across all eight sublayers are below 0.6, indicating that the water bodies were predominantly freshwater within a continental lacustrine environment (Figure 4b). The average Cu/Zn ratios are less than 0.35 in sublayers 2, 3, 4, and 5, whereas sublayers 1, 6, 7, and 8 exhibit ratios greater than 0.35. The V/(V + Ni) ratios range between 0.6 and 0.84. The average Ni/Co ratios in sublayers 1 and 8 are 2.17 and 2.22, respectively, indicative of oxic conditions, while the other sublayers have average Ni/Co ratios between 2.5 and 3.0, reflecting an overall sub-oxic to sub-reducing depositional environment (Figure 4c,d). The Ba_bio ratios are low in sublayers 1, 2, 4, and 5, with averages below zero, indicating low paleoproductivity during their deposition, whereas higher values in sublayers 3, 6, 7, and 8 reflect elevated paleoproductivity during those intervals.

Between the top of Sub-layer 2 and the base of Layer Sub-layer 3, Sr/Cu peaks while Sr/Cr, Sr/Ba, V/Ni, Cu/Zn, V/(V + Ni), and Ni/Co ratios decline, marking a transition from a warm and humid climate to a hot and dry one. During this period, Ba_bio values reach their highest, reflecting enhanced paleoproductivity. In the middle shale section of Sub-layer 4 and throughout Sub-layer 6, Ba_bio values show the second and third peaks, indicating a consistent increase in paleoproductivity (Figure 5).

The combined analysis demonstrates that shale sections in the LGS2-LS exhibit higher paleoproductivity than sandstone sections. These findings highlight the role of climate, salinity, and redox conditions in influencing sediment deposition, providing crucial insights into the depositional evolution and resource potential of the Lianggaoshan Formation.

5.2. Depositional Evolution of the LGS2-LS

The LGS2-LS represents the deepest water stage of the Lianggaoshan Formation, dominated by shallow-to-semi-deep lacustrine environments with significant shale development [1,22,56]. During Sub-layer 1, a lake regression event led to shallower water, faster sedimentation, and oxidizing conditions, transitioning from black mudstone to gray sandstone (Figure 6a). Sub-layer 2 experienced a lake transgression, with deepening water and reduced sedimentation, forming semi-deep lacustrine carbonaceous shales (Figure 6b).

Sub-layer 3 marked another regression, with shallower water and oxidation, while Sub-layer 5 saw lake expansion, deeper water, and increased reducing conditions (Figure 6c–e). Sub-layer 6 represented the maximum water depth, with thick organic-rich shales deposited under low sedimentation rates and strong reducing conditions (Figure 6f). Sub-layers 7 and 8 showed regression and renewed transgression, reflecting fluctuating water levels and evolving depositional environments (Figure 6g,h).

5.3. Sedimentary Model and Petroleum Geological Significance

Based on the above research findings, we have established the sedimentary model for the LGS2-LS. This model illustrates a systematically zoned lacustrine sedimentary system controlled by variations in water depth and depositional energy. The primary controlling factor is the fluctuation of lake level [57], and the model records a continuous depositional evolution from the terrigenous clastic supply zone to the low-energy lacustrine center (Figure 7).

The provenance and proximal high-energy zone are characterized by the transport of clastic materials from the source area into the lake basin [59,60,61,62,63]. Immediately adjacent to the provenance, a fan delta system develops, marking the flow-energy reducing environment, where fluvial energy begins to significantly decrease [64,65]. Further lakeward, sandy beach facies and sand–mud interbedding appear, representing a littoral lake environment influenced by strong wave action and relatively high hydrodynamic energy [66,67]. The sediments in this zone are dominated by sand or show frequent alternations of sand and mud.

The flooding surface serves as a critical stratigraphic boundary, recording episodes of relatively rapid lake-level rise [68,69]. This surface often separates shallow-water or subaerial deposits above from relatively deeper-water deposits below, indicating a landward shift in sedimentary environments due to transgression.

Beyond the flooding surface, moving further lakeward, lies the clastic shallow lake environment, which is located above the wave base. Here, wave influence diminishes, and sedimentation is dominated by suspended-load deposition of silt and mud, reflecting further reduced energy conditions. The wave base acts as an important interface distinguishing shallow lacustrine from deeper-water environments [70,71].

Beneath the wave base is the semi-deep lake environment, where water depth is sufficient to prevent wave action from reaching the lake bottom [72,73]. This zone represents a persistently low-energy, reducing environment. Sedimentation is primarily governed by suspension settling, forming fine-grained, dark-colored mudstone and shale with relatively high organic matter content, indicating a favorable setting for potential hydrocarbon source rock development.

In summary, the sedimentary model of the LGS2-LS clearly depicts a terrigenous clastic lacustrine system controlled by lake-level fluctuations. Clastic material is transported from the source area through fan deltas into the lake, and under the combined influence of lake-level changes and the wave base, the sedimentary system transitions from littoral to clastic shallow lake to semi-deep lake environments, forming a regular spatial distribution of sedimentary facies belts. The sedimentary evolution of the LGS2-LS provides critical insights into the region’s petroleum potential. The deep-water conditions during Sub-layer 6, characterized by thick organic-rich mudstones and favorable reducing environments, indicate strong potential for shale oil and gas generation. The shallow and semi-deep lacustrine environments, alternating oxidizing and reducing conditions, promote the formation of reservoir-quality rocks, particularly in the mudstone intervals.

The distribution of high-quality source rocks, combined with favorable sedimentary facies, suggests that the LGS2-LS is a promising target for future exploration. The variations in water depth, sedimentation rate, and organic matter preservation further enhance the potential for oil and gas accumulation, particularly in areas with well-preserved organic-rich shale layers. These findings provide a solid foundation for the exploration and development of hydrocarbon resources in the Lianggaoshan Formation.

6. Conclusions

The LGS2-LS in the Fuling area exhibits a clear sedimentary evolution from shallow lacustrine to semi-deep lacustrine environments, accompanied by significant variations in lithofacies. The interval is characterized by the development of organic-rich mudstones interbedded with sandstones, reflecting changes in water depth, sedimentary energy, and provenance supply. These lithofacies patterns record alternating transgressive–regressive cycles and provide key evidence for reconstructing the paleoenvironmental evolution of the southeastern Sichuan Basin during the Jurassic.

Geochemical proxies such as Sr/Cu, Sr/Cr, and Sr/Ba reveal multiple shifts between warm-humid and hot-arid climates, persistent freshwater conditions, and variations in redox states from oxic to sub-reducing. Ba_bio variations indicate significant differences in paleoproductivity among sublayers, with shale-dominated intervals generally exhibiting higher productivity. These geochemical patterns demonstrate that climate change, lake-level fluctuations, and redox conditions jointly controlled the accumulation and preservation of organic matter.

The LGS2-LS contains thick, organic-rich shale intervals—particularly in Sub-layer 6—deposited under low-energy, reducing conditions, offering favorable hydrocarbon generation potential. The alternation of shale and sandstone enhances both reservoir and source rock potential, making this submember a promising target for shale oil and gas exploration. The sedimentary model established in this study provides a robust geological basis for predicting the distribution of high-quality reservoirs and source rocks in future exploration of the Lianggaoshan Formation.