Effect of Sedimentary Environment on Mudrock Lithofacies and Organic Matter Enrichment in a Freshwater Lacustrine Basin: Insight from the Triassic Chang 7 Member in the Ordos Basin, China

Abstract

Gradually replacing fossil fuels with renewable energy constitutes a long-term strategy for achieving sustainable development. In the short term, it is necessary to explore unconventional oil and gas resources to support current economic sustainability and to secure essential time for the energy transition. With the continuous growth in global energy demand, unconventional resources such as shale oil and shale gas have become important alternative energy sources. Lacustrine mudrock successions demonstrate significant potential for unconventional oil and gas resources. However, the unclear understanding of how paleoenvironmental evolution influences lithofacies and organic matter enrichment restricts the optimization of shale oil reservoirs and evaluation of shale oil resources, thereby hindering the progress of lacustrine shale oil exploration and development. The mudrocks in the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin, were deposited in a pro-delta to a deep lacustrine environment and are rich in shale oil resources. Through petrographic, sedimentological, sequence stratigraphic, and geochemical analyses, this study reveals how the evolution of the paleoenvironment controlled the development of mudrocks and the enrichment of organic matter, and establishes a sedimentary model for freshwater lacustrine systems. Six lithofacies have been identified within the mudrock interval of the Chang 7 Member. According to the T-R (transgressive–regressive) sequence model, the Chang 7 Member can be subdivided into three fourth-order sequences, termed Parasequence Set 1–3 (PPS1–3). Mudrock is predominantly developed in the fourth-order sequences PSS1 and PSS2. The PSS1 and the lower part of PSS2 consist of lithofacies 1–4, representing semi-deep to deep lacustrine deposits. The upper part of PSS2 develops lithofacies 5, representing shallow lacustrine to pro-delta deposits. Fluctuations of the lake level controlled the vertical stacking of lithofacies and the transition in depositional mechanisms. During lake-level rise, bottom currents shifted to suspension settling, whereas the opposite occurred during lake-level fall. The organic matter is derived from algae, and its enrichment is jointly controlled by productivity and the redox conditions. Volcanic–hydrothermal activity and a humid climate promoted high productivity in the water body. This high productivity promotes dyoxic conditions in the bottom water. Fourth-order relative lake-level fluctuations also influence organic matter enrichment. During lake-level rise, increased productivity coupled with reduced consumption and dilution favors organic matter enrichment. Conversely, organic matter accumulation is inhibited during lake-level fall. Ultimately, a depositional model for a freshwater lacustrine basin under a humid to semi-humid climatic background was established. This paper elucidates the influence of sedimentary environment on mudrock lithofacies and organic matter enrichment, providing a theoretical basis for optimizing shale oil reservoir selection and resource assessment, thereby promoting efficient exploration and low-carbon development of shale oil in lacustrine basins.

1. Introduction

The international community has established an energy transition strategy to address the global climate crisis, which involves progressively replacing fossil fuels with renewable energy sources such as wind, solar, and hydropower [1,2,3]. To implement this long-term strategy, sustained technological research is essential for overcoming challenges associated with renewable energy, such as wind power intermittency and uneven solar energy distribution. Meanwhile, in the short term, advancing the exploration of unconventional oil and gas resources can not only support current economic sustainability but also buy critical transition time for technological breakthroughs in renewable energy [4,5]. In recent years, a significant increase in unconventional hydrocarbon production (including shale gas and shale oil) in the United States has reshaped the global energy landscape. Asian countries such as China, India, and Pakistan are also advancing the development of unconventional hydrocarbon resources, positioning them as crucial alternative energy sources [6,7,8].

In the United States, shale oil is predominantly sourced from marine basins. In China, however, shale oil primarily occurs in lacustrine mudrock successions and demonstrates significant resource potential [9,10]. However, the efficient exploration and low-carbon development of lacustrine shale oil currently face multiple bottlenecks. One major constraint is the insufficient understanding of the genesis of lacustrine mudrocks and the mechanisms of organic matter enrichment, which hinders the identification of high-quality shale oil reservoirs and the evaluation of shale oil resources.

High-quality shale oil reservoirs are typically mudrocks with high porosity, high hydrocarbon mobility, and high brittleness. These three characteristics are controlled by the lithofacies of the mudrocks [9,10,11,12,13]. Organic-rich intervals are the target zones in shale oil development, and understanding the mechanisms of organic matter enrichment is fundamental for selecting favorable reservoirs [14,15,16,17,18,19,20]. The mudrock lithofacies and the enrichment of organic matter are influenced by the sedimentary environment [21,22,23,24,25,26]. Clarifying the specific mechanisms of this influence is essential for evaluating and optimizing shale oil reservoirs, thereby advancing efficient exploration and low-carbon development of shale oil.

Mudrocks can be formed in diverse sedimentary environments, ranging from marine to continental settings. In recent years, the results of flume experiments and observations of sediment have provided new explanations for the sedimentary genesis of mudrocks. The sedimentary processes of marine mudrocks are diverse, including turbidity currents, hyperpycnal flows, storm flows, and bottom currents [27,28,29]. However, compared to marine basins, the study of mudrocks in lacustrine basins remains less advanced. Key aspects of lacustrine mudrocks, including lithofacies classification, sedimentary mechanisms, and sedimentary settings, are still poorly constrained.

Organic matter in mudrocks serves not only as the foundation for hydrocarbon generation but also provides essential pore space for hydrocarbon storage [30]. The deposition of organic matter in sediments is a complex process, involving both sedimentology and biogeochemistry [31,32]. This process can be simply summarized as the carbon fixed from the surface water being partially degraded by microorganisms and diluted by debris during the sedimentation process in the water column, and eventually retained and buried in the sediments [33,34,35]. Organic matter enrichment in marine or lacustrine sediments is controlled by three main factors: production, destruction, and dilution [19,20,36,37,38]. Lacustrine basins can be classified into freshwater and saline lacustrine basins, which exhibit significant differences in water conditions and organic matter enrichment mechanisms [18,21,22,25,33]. Understanding the organic matter enrichment mechanisms in freshwater lacustrine basins requires specific analysis of the sedimentary environment and water conditions.

Following the retreat of seawater from the southeastern direction, the Ordos Basin transitioned into a continental basin during the Mesozoic Era. The Chang 7 Member of the Middle–Upper Triassic Yanchang Formation developed a set of lacustrine mudrocks [39,40]. This interval is a significant source rock and a target for shale oil exploration. A key characteristic of this member is its exceptionally high organic matter content, with an average Total Organic Carbon (TOC) reaching 13.5% [41]. Geochemical evidence suggests that abundant nutrients supplied by volcanic–hydrothermal materials promoted algal blooms, which enhanced high primary productivity and subsequently led to the enrichment of organic matter [39,40,42,43]. However, the influence of sedimentary environment evolution on lithofacies and organic matter enrichment of mudrocks in the Chang 7 Member needs further study. This issue constrains the understanding of fine-grained sedimentation in the basin and hinders the progress of shale oil exploration.

This study presents an analysis of the sedimentary characteristics of the Chang 7 mudrocks in the Ordos Basin, along with their organic and elemental geochemical properties. The objectives are to (1) classify the lithofacies of the mudrocks and interpret their depositional mechanisms; (2) reconstruct the sedimentary environment using geochemical proxies; (3) elucidate the effect of the sedimentary environment on lithofacies development and organic matter enrichment within a fourth-order sequence-stratigraphic framework. These findings can provide theoretical support for identifying favorable shale oil reservoirs and evaluating shale oil resources, thereby facilitating efficient exploration and low-carbon development.

2. Geological Settings

2.1. Tectonic Setting and Sedimentary Characteristics

The Ordos Basin is located in the western part of the North China Block, with an area of 320,000 km² (Figure 1A,B). The tectonic–sedimentary evolution of the basin can be divided into six stages: the aulacogen in the Middle–Late Proterozoic Era (shallow marine carbonate, clastic rock), the pericratonic basin in the Early Paleozoic Era (shallow marine carbonate platform), the uplift and erosion in the Late Ordovician–Early Carboniferous Period, the intracratonic basin in the Late Carboniferous–Late Permian Period (marine–continental transitional deposition), the intracontinental lacustrine basin in the Mesozoic Era (river, delta and lake deposition), and the Cenozoic Era rift basin (continental deposition) [44].

During the Middle to Late Triassic Period, the basin experienced continuous subsidence with sufficient sediment supply (Figure 1C). During this period, a set of fluvial–lacustrine–deltaic sediments was formed and named the Yanchang Formation. Based on lithology, sedimentary facies, and oil-bearing properties, the Yanchang Formation can be divided into 10 members (Chang 1 to Chang 10) from bottom to top (Figure 2A,B) [16]. The Chang 7 period represents the maximum lake expansion stage of the basin. Prior to this period, lacustrine deposits were progressively expanding, while afterwards, they gradually shrank.

The Chang 7 Member, with a thickness of approximately 80–120 m, can be subdivided into three units from bottom to top: Chang 7₃, Chang 7₂, and Chang 7₁. The Chang 7₃ and lower Chang 7₂ units are primarily composed of dark gray to black mudstone and shale, interbedded with thin layers of tuff and siltstone. These deposits are interpreted as deep-lacustrine sediments formed during a lake transgression period. The upper Chang 7₂ and Chang 7₁ units were deposited during a lake level drop. The sandstones developed along the lacustrine basin margin are interpreted as deltaic deposits. The sandstones in the central part of the lacustrine basin are interpreted as gravity flow deposits [46,47]. Organic-rich fine-grained rocks of the Chang 7 Member are widely distributed in the whole basin and are considered to be the main source rock and hydrocarbon reservoir, which is the target of this study.

Figure 2. (A) The seismic profile showing the sequence division of the Yanchang Formation. (B) Lithostratigraphy, sedimentary facies, and lake level change in the Yanchang Formation (modified after [16]). (C) INPEFA (integrated prediction error filter analysis) curve showing the sequence division of the Yanchang Formation (modified after [48]). (D) Lithology and INPEFA curves showing the sequence division of the Chang 7 Member (modified after [49]).

Figure 2. (A) The seismic profile showing the sequence division of the Yanchang Formation. (B) Lithostratigraphy, sedimentary facies, and lake level change in the Yanchang Formation (modified after [16]). (C) INPEFA (integrated prediction error filter analysis) curve showing the sequence division of the Yanchang Formation (modified after [48]). (D) Lithology and INPEFA curves showing the sequence division of the Chang 7 Member (modified after [49]).

2.2. High-Resolution Sequence-Stratigraphic Framework

The Yanchang Formation corresponds to a second-order sequence, which can be further subdivided into five third-order sequences (SQ1~SQ5) (Figure 2A,B) [48]. The Chang 7 Member is situated in the middle part of the third-order sequence (SQ3). The seismic profile indicates that the bottom boundary of the SQ3 (SB3) is in the middle of the Chang 8 Member. And the top boundary of the SQ3 is SB4, which is in the middle of the Chang 6 Member (Figure 2A). After applying MESA (maximum entropy spectral analysis) and integration processing to the GR curve, the resulting INPEFA (integrated prediction error filter analysis) curve is often used for identifying sequence boundaries [50,51,52]. The locations of SB3 and SB4 identified on the INPEFA curve correspond with those on the seismic profile (Figure 2C) [48]. The organic-rich mudrocks of the Chang 7 Member mark the position of the maximum flooding surface.

Based on lithology, the INPEFA curve, and seismic reflection characteristics, the Chang 7 Member can be further subdivided into three fourth-order sequences (Figure 2D) [49,53]. Researchers utilized gamma ray (GR) and density (DEN) log curves for cyclo-stratigraphic analysis, establishing a geochronological framework and depositional timeline for the Chang 7 Member [54]. In terms of temporal span, the sub-members Chang 7₁, Chang 7₂, and Chang 7₃ essentially correspond to three fourth-order sequences [55].

3. Data and Methods

Three continuously cored wells (Y66, Y56, and H269) are located in the northeastern part of the basin (Figure 1C). Covering a transect from shallow to deep lake environments, these wells provide fundamental data for studying the continuous variations in lithofacies and paleoenvironments. This study involved a detailed core description of the three wells, with a cumulative core length of 319 m. A total of 101 samples were collected.

3.1. Thin Sections and Electron Microscopy

101 samples were stabilized with epoxy resin and made into thin sections, in order to observe the sedimentary structure, texture, and mineral composition using a microscope (Olympus BX51). The samples were observed under a field emission scanning electron microscope (FESEM) (FEI Quanta200F) in the Laboratory of Energy Materials Microstructure, China University of Petroleum (Beijing).

3.2. Total Organic Carbon and Rock-Eval Pyrolysis Analysis

A total of 21 samples from Well H269 were analyzed for Total Organic Carbon (TOC) and Rock-Eval pyrolysis.

Total Organic Carbon (TOC) was determined using a LECO CS230 carbon–sulfur analyzer following the standard GB/T 19145-2003 [56]. The procedure was as follows: first, the samples were crushed to pass through an 80-mesh sieve. Second, the powdered samples were mixed with an excess of dilute hydrochloric acid (prepared in a 1:7 volume ratio) and heated on a hot plate at 60–80 °C for over two hours. Third, the samples were rinsed with distilled water to remove excess acid. Fourth, the samples were dried in an oven at 60–80 °C. Finally, the organic carbon content of the processed samples was measured using the carbon analyzer.

Rock-Eval pyrolysis was performed using an OGE-IV Rock-Eval analyzer according to the standard GB/T 18602-2012 [57]. The rock samples were ground to pass through an 80-mesh sieve. Helium and nitrogen, both with a purity of 99.999%, were used as carrier gases, with working pressures ranging from 0.2 to 0.4 MPa. An FID detector was employed for detection. Hydrogen was supplied by a hydrogen generator at a working pressure of 0.2–0.3 MPa, and compressed air was provided by an air compressor at a working pressure of 0.3–0.4 MPa.

These two experiments were conducted at the National Key Laboratory of Oil and Gas Resources and Engineering at China University of Petroleum, Beijing. The analytical results are presented in Figure 3 and

Table 1. Total Organic Carbon (TOC) and Hydrogen Index (HI) of mudrocks in the Chang 7 Member.

Sample NO.	Well	Depth (m)	Fourth-Order Sequence	TOC (%)	Tmax (°C)	HI (mgHC/gTOC)
1	H269	2492.8	PSS2	0.88	441.00	65.77
2	H269	2494.3	PSS2	3.92	450.00	240.32
3	H269	2496.2	PSS2	4.17	450.00	199.14
4	H269	2498.2	PSS2	5.17	445.00	198.34
5	H269	2500.0	PSS2	3.90	441.00	158.15
6	H269	2502.1	PSS2	7.37	442.00	224.09
7	H269	2503.1	PSS2	10.53	448.00	294.78
8	H269	2506.6	PSS2	10.89	450.00	265.01
9	H269	2507.8	PSS2	4.71	446.00	289.78
10	H269	2512.7	PSS2	5.20	442.00	269.41
11	H269	2515.6	PSS2	3.65	439.00	233.63
12	H269	2517.5	PSS2	3.23	436.00	229.41
13	H269	2519.8	PSS1	7.90	449.00	293.14
14	H269	2521.2	PSS1	7.74	449.00	282.95
15	H269	2523.7	PSS1	7.84	450.00	300.32
16	H269	2526.5	PSS1	5.34	439.00	262.12
17	H269	2528.5	PSS1	3.99	444.00	293.26
18	H269	2530.8	PSS1	8.18	448.00	267.33
19	H269	2532.8	PSS1	10.04	450.00	291.04
20	H269	2534.0	PSS1	6.23	446.00	304.91
21	H269	2536.8	PSS1	4.88	446.00	319.60

.

The free hydrocarbon content (S1), pyrolyzable hydrocarbon content (S2), and maximum pyrolysis temperature (T_max) were determined based on Rock-Eval pyrolysis experiments [38,58]. Furthermore, the Hydrogen Index (HI) can be calculated using the formula HI = (S2/TOC) × 100.

3.3. Elements Analysis

A total of 21 samples from Well H269 were analyzed for major and trace elements.

Major element analysis was performed using an Axios^MAX X-ray fluorescence spectrometer, following the standard GB/T 14506.28-2010 [59]. The procedure was as follows: First, the samples were dried at 105 °C. Second, the samples were placed in a porcelain crucible, mixed with a flux (5.2 g of lithium tetraborate, 0.4 g of lithium fluoride, and 0.3 g of ammonium nitrate), and stirred thoroughly before being transferred to a platinum-gold alloy crucible. Third, a mold release agent (1 mL of 15 g/L lithium bromide solution) was added to the crucible and dried. Fourth, the crucible was placed on a fusion machine and melted at 1150–1250 °C for 10–15 min. Fifth, the molten sample was poured into a mold and air-cooled for 3 min, after which the sample disk automatically separated from the mold. Finally, the sample disk was retrieved and analyzed for major elements using the instrument. The oxides analyzed included SiO₂, Al₂O₃, TiO₂, CaO, MgO, K₂O, Na₂O, MnO, P₂O₅, and TFe₂O₃ (T denotes the total).

Trace element analysis was conducted using an X SERIES2 inductively coupled plasma mass spectrometer (ICP-MS) following the standard GB/T 14506.30-2010 [60]. The procedure was as follows: First, 50 mg of the sample was accurately weighed into the inner vessel of a sealed digestion container, moistened with water, and then 1 mL of hydrofluoric acid and 0.5 mL of nitric acid were added before sealing. Second, the container was placed in an oven and heated at 185 °C for 24 h. After cooling, the inner vessel was removed and heated on a hot plate until nearly dry. Then, 0.5 mL of nitric acid was added and evaporated to near dryness; this step was repeated once. Third, 5 mL of nitric acid (1 + 1) was added, and the vessel was resealed and placed in the oven at 130 °C for 3 h. Fourth, after cooling, the inner vessel was removed, and the solution was quantitatively transferred to a glass tube, diluted with water, and made up to a volume of 50 mL. The mixture was shaken thoroughly. Fifth, under the selected conditions, the rare earth elements and trace elements (Ba, Co, Cr, Cu, Ni, Pb, Sr, V, Ga, Rb, Nb, Ta, Th, U, Hf, and Zr) were measured.

The analyses of major and trace elements were carried out at the Laboratory of the Hebei Provincial Institute of Regional Geology and Mineral Resources Exploration. The analysis results, after further calculation, are presented in

Table 2. Paleoenvironmental proxies of the Chang 7 Member.

Sample NO.	Well	Depth	Sequence	C-Value	Sr/Cu	Sr/Ba	V/Cr	Ni/Co	U/Th	DOPT	P/Ti	P/Al	Ni + Cu + Zn (ppm)	Ti/Al	Zr/Al (ppm/%)
S1	H269	2492.8	PSS2	0.73	7.30	0.29	1.78	1.44	0.22	0.01	0.20	0.01	176.24	0.04	8.95
S2	H269	2494.3	PSS2	1.00	4.07	0.32	2.48	1.80	0.44	0.05	0.23	0.01	226.72	0.04	9.55
S3	H269	2496.2	PSS2	0.80	8.56	0.45	1.78	1.57	0.31	0.02	0.65	0.03	211.58	0.05	13.17
S4	H269	2498.2	PSS2	1.02	7.62	0.45	1.82	2.03	0.39	0.05	0.25	0.01	219.80	0.05	13.09
S5	H269	2500.0	PSS2	1.28	12.84	0.56	1.75	1.70	0.36	0.01	1.20	0.06	181.82	0.05	17.73
S6	H269	2502.1	PSS2	0.64	19.34	0.91	2.26	1.86	0.60	0.44	3.62	0.15	193.82	0.04	17.55
S7	H269	2503.3	PSS2	1.12	5.74	0.54	2.53	2.28	1.02	0.48	0.52	0.02	223.07	0.04	12.41
S8	H269	2506.6	PSS2	1.21	4.93	0.55	2.72	1.74	0.93	0.56	0.51	0.02	257.74	0.05	13.26
S9	H269	2507.8	PSS2	0.89	6.35	0.57	1.65	4.45	0.43	0.31	0.29	0.01	241.19	0.04	15.69
S10	H269	2512.7	PSS2	1.00	5.59	0.52	1.89	2.23	0.31	0.21	0.25	0.01	166.22	0.05	18.16
S11	H269	2515.6	PSS2	0.95	10.63	0.54	2.00	1.74	0.47	0.05	0.66	0.04	173.32	0.05	22.40
S12	H269	2517.5	PSS2	1.10	8.76	0.48	1.96	1.53	0.34	0.04	0.53	0.03	200.04	0.05	15.75
S13	H269	2519.8	PSS1	1.17	3.73	0.44	2.54	1.99	0.69	0.44	0.52	0.02	204.49	0.04	12.34
S14	H269	2521.2	PSS1	0.99	4.80	0.50	2.49	2.05	0.41	0.32	0.60	0.03	216.27	0.04	10.40
S15	H269	2523.7	PSS1	0.80	7.19	0.57	2.86	2.92	0.88	0.15	2.48	0.09	223.59	0.04	13.08
S16	H269	2526.5	PSS1	1.55	4.22	0.55	2.35	2.19	0.72	0.06	0.66	0.03	217.44	0.04	16.26
S17	H269	2528.5	PSS1	0.79	5.05	0.45	1.77	1.93	0.38	0.06	0.30	0.01	192.47	0.05	15.85
S18	H269	2530.8	PSS1	1.21	2.92	0.50	2.29	2.24	0.84	0.46	0.51	0.02	234.43	0.05	13.52
S19	H269	2532.8	PSS1	1.01	3.46	0.76	2.26	2.34	1.31	0.49	1.25	0.05	239.44	0.04	13.62
S20	H269	2534.0	PSS1	0.90	2.68	0.42	2.11	2.21	0.49	0.22	0.18	0.01	198.81	0.05	12.89
S21	H269	2536.8	PSS1	0.38	3.68	0.37	3.21	2.06	0.93	0.34	0.40	0.01	154.60	0.03	11.44

.

Some scholars take the C-values as an indicator of paleoclimate [39,61,62]. The C-values are defined as follows: C-value = Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na).

3.4. Sequence-Stratigraphic Division

Based on the description of lithofacies and well-log responses, this study identifies fourth-order sequences. Firstly, lithofacies incorporate information on texture (grain size), mineral composition, and sedimentary structures, whose stacking patterns can reflect cyclical changes in sedimentary processes and depositional environments. Secondly, in shale sequences, well-logs such as total gamma ray (GR), computed gamma ray (CGR), and individual uranium (U), thorium (Th), and potassium (K) are relatively sensitive to lake-level fluctuations. The log curves can be used to identify sequence boundaries and delineate sequences [63,64]. The deposition of two radioactive elements (potassium and thorium) is mainly related to the adsorption of rocks. The finer the particles, the more radioactive materials are adsorbed. Uranium deposition is closely related to redox conditions and the enrichment of organic matter.

4. Results

4.1. Description and Interpretation of Lithofacies

Based on previously proposed nomenclature schemes, six lithofacies were identified in the Chang 7 Member [28]. Information on grain size, mineral composition, sedimentary structures, and bioturbation contained within mudstone can be used to interpret sedimentary mechanisms.

4.1.1. Organic-Rich Parallel-Laminated Fine Mudstone

Description

Lithofacies 1 (LF-1) is characterized by black, continuous, planar-parallel laminated, argillaceous fine mudstone to argillaceous–siliceous fine mudstone. The dark, planar-parallel laminae are organic-rich layers. The light-colored laminae primarily consist of a mixture of clay minerals and silt (Figure 4A–C). Additionally, common to abundant framboidal pyrite is observed (Figure 4C). Within this lithofacies, scattered fish bone fragments appear to “float” within the laminae, causing local disruption of the lamination. These fish bone fragments and spherical algae (Tasmanites?) are filled with phosphate (Figure 4D–F). LF-1 is undisturbed, with a bioturbation index of 0 [28,65].

Interpretation

LF-1 was formed in a distal setting (semi-deep to deep lake), with its depositional mechanism being suspension settling. The laminated structure developed within this lithofacies is a product of seasonal variations in the water body. Among them, the dark organic-rich laminae are formed during the summer, while the clay-silt laminae are developed in the winter (Figure 4A–C) [66]. In deep-water areas, terrigenous input is minimal and the water column remains tranquil. Under such conditions, suspended bioclastic particles (e.g., fish bones and algae) gradually settle and accumulate at the lake bottom (Figure 4D,E).

4.1.2. Phosphatic Fine Mudstone

Description

Lithofacies 2 (LF-2) is characterized by a black, discontinuous, planar-parallel laminated, phosphatic fine mudstone. Collophane is an amorphous phosphate mineral, often occurring as ellipsoidal nodules (Figure 4G–I). Under plane-polarized light, it typically appears brown. In some nodules, apatite crystals formed by recrystallization can be observed along the edges (Figure 4H).

Interpretation

LF-2 exhibits planar-parallel laminations, formed in a distal environment (semi-deep to deep lake). The formation of phosphate nodules is associated with early diagenesis and organic matter degradation. In surface waters, phosphorus is incorporated into organisms and settles into the bottom waters as organic phosphorus. During early diagenesis, these organic materials undergo decomposition and degradation, releasing large amounts of elements such as phosphorus. When phosphorus becomes enriched in pore water, authigenic phosphate minerals form [31,67,68].

4.1.3. Massive to Faintly Laminated Medium–Fine Mudstone

Description

Lithofacies 3 (LF-3) is a grayish-black, discontinuous, massive to faintly wavy-parallel laminated, siliceous–argillaceous or siliceous–argillaceous medium to fine mudstone with a scour surface, bioturbation layer, and silt lag. This lithofacies exhibits distinct scour surfaces, along with irregular horizontal bedding disruptions interpreted as cryptic bioturbation (Figure 4J). The base of the silty layers shows erosional scouring structures, overlain by discontinuous silt lag deposits (Figure 4K). Some clay clasts are visible in this lithofacies (Figure 4L). The bioturbation index typically ranges between 0 and 1.

Interpretation

Lithofacies 3 was deposited in a distal environment, yet it contains higher concentrations of terrigenous detrital grains (quartz and feldspar) compared to LF-1 and LF-2. Scour surfaces and lag deposits were formed by bottom currents in the distal setting.

4.1.4. Current Ripple Coarse to Medium Mudstone

Description

Lithofacies 4 (LF-4) is a gray to grayish-black, discontinuous, wavy-parallel, and wavy-nonparallel-laminated siliceous coarse–medium mudstone. Abundant scour surfaces, current ripple with low-angle cross-lamination, and silty lenses are observed in LF-4 (Figure 5A–C). The silty lens is mainly composed of detrital quartz and feldspar grains, which are cemented by early diagenetic calcite (Figure 5B). The granular clay clasts are squeezed and deformed weakly between siliceous grains (Figure 5C). The bioturbation index typically varies between 0 and 1.

Interpretation

Lithofacies 4 is interpreted as the product of wind-driven bottom currents in a distal depositional environment.

In marine shelf environments, the generation of bottom currents is typically induced by storm activity [32]. In lacustrine basins, the mechanism behind bottom currents is more likely driven by wind. Wind-driven bottom currents generally develop below the storm wave base and transport detrital sediments from shallow-water areas into the deep lacustrine zone below the wave base [69].

4.1.5. Wavy Ripple Coarse Mudstone Interbedded Fine Mudstone

Description

Lithofacies 5 (LF-5) is a light-gray to gray, continuous to discontinuous, curved-nonparallel-laminated coarse mudstone interbedded in fine mudstone. Scour surfaces, wavy-ripple with low-angle cross-lamination, are common in LF-5 (Figure 5G,H). The bioturbation index typically ranges from 1 to 2.

Interpretation

Lithofacies 5 was deposited in a pro-delta to shallow lacustrine environment. The presence of wave ripples indicates extensive wave reworking above the wave base. Significant accumulation of silty particles suggests high terrigenous input, rapid sedimentation rates, and relatively strong hydrodynamic conditions (Figure 5D,E). The occurrence of current ripples and wave ripples with arcuate scalloped topography demonstrates substantial wave reworking above the wave base during deposition. Strong bioturbation disrupts primary depositional beds, indicating high oxygen levels in the bottom water.

4.1.6. Massive to Layered Muddy Tuff/Tuff

Description

The tuff and muddy tuff occur as brown thin beds, with thickness ranging from 0.2 to 2.5 cm (Figure 5G). Lithofacies (LF-6) is mainly composed of volcanic particles, including vitrics, felsic grains, and lithic grains (Figure 5H,I). Vitrics are angular and shardlike, while the silty felsic grains are subangular to angular. These grains are scattered and unoriented. The matrix of the rock is microcrystalline quartz and a small amount of clay.

Interpretation

LF-6, developed in the shale formation of the Chang 7 Member, represents multi-period volcanic events (Figure 5). Tuff is composed mainly of pyroclastic material less than 2 mm in diameter, with a single-layer thickness of 0.5 to 5 cm. Based on geochemical evidence, it is believed that the tuff of the Chang 7 Member is intermediate-acidic, and the crater was located in the Qinling orogenic belt in the south of the basin [40,70]. The fine-grained lapilli and ash could have been ejected into the air. They can be transported by wind over distances ranging from several kilometers to even thousands of kilometers from the volcanic vent. When volcanic ash gradually settled on the lake bottom, it formed massive to layered muddy tuff/tuff.

The tuff at the base of the Chang 7 Member was used for dating the formation. Zircon U-Pb dating by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) yielded an age of 239.1 ± 1.4 Ma [71]. Zircon U-Pb ages determined by ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry) range from 241.06 ± 0.12 Ma to 241.36 ± 0.12 Ma [72,73]. Zircon U-Pb dating by SHRIMP (Sensitive High-Resolution Ion Microprobe) yielded ages ranging from 238.3 ± 2.3 Ma to 241.8 ± 2.5 Ma [74]. The depositional age of the lower Chang 7 Member determined by these methods ranges from 238 to 242 Ma, corresponding with the Ladinian.

4.2. Sequence Stratigraphy

The three 4th-order sequences PSS1–PPS3 (Parasequence Set) can be identified in the Chang 7 Member in this study.

High-frequency sequences (fourth-order sequences) are classified based on the transgressive–regressive (T-R) sequence scheme proposed by Embry [75,76,77]. The thickness of each fourth-order sequence ranges from 28m to 35m. Each sequence contains a transgressive systems tract (TST) and a regressive systems tract (RST), which are divided by the maximum flooding surface (MFS) and maximum regressive surface (MRS) (Figure 6). This subdivision of fourth-order sequences is consistent with previous results established based on the INPEFA curve and seismic reflection characteristics [49,53,54,55]. Taking Well H269 as an example, mudrocks were primarily deposited in the middle and lower parts of the Chang 7 Member, corresponding to the fourth-order sequences PSS1 and PSS2.

4.3. TOC and Rock-Eval Pyrolysis

The TOC of Well H269 ranges from 0.88% to 10.89% (average 5.99%). Among these, the TOC of the fourth-order sequence PSS1 ranges from 3.99% to 10.04% (average 6.90%), while that of PSS2 ranges from 0.88% to 10.89% (average 5.30%). The HI of Well H269 ranges from 65.77 to 319.60 mgHC/gTOC (average 251.55 mgHC/gTOC), and T_max ranges from 436 to 450 °C (average 445 °C). Among these, the HI of the fourth-order sequence PSS1 ranges from 262.12 to 319.60 mgHC/gTOC (average 290.52 mgHC/gTOC), and Tmax ranges from 439 to 450 °C (average 447 °C). For PSS2, HI ranges from 65.77 to 294.78 mgHC/gTOC (average 290.52 mgHC/gTOC), and Tmax ranges from 436 to 450 °C (average 445 °C).

The relationship between Tmax and Hydrogen Index (HI) indicates the type of organic matter. Three samples plot in the Type I field, fifteen in the Type II₁ field, and three in the Type II₂ field (Figure 3B). These results suggest that the organic matter in the Yanchang Formation Member 7 is primarily derived from plankton such as algae.

4.4. Paleoenvironmental Proxies

Several characteristic elemental ratios have been used to reconstruct paleoenvironmental and water column conditions, including paleoclimate, salinity, productivity, redox conditions, and terrigenous input. The paleoclimate proxies include the C-value and Sr/Cu ratios. The paleosalinity proxy is Sr/Ba. Paleoproductivity proxies include P/Ti, P/Al, and Ni + Cu + Zn. Redox condition proxies are V/Cr, Ni/Co, U/Th, and DOP_T. Terrigenous input proxies are Ti/Al and Zr/Al ratios. The calculated values of these indicators for 21 samples are presented (Figure 7; Table 2). The application principles of these indicators and the environmental changes they reflect will be emphasized in the discussion section of this paper.

5. Discussion

5.1. Reconstruction of Paleoenvironment

Sedimentary rocks are the product of sedimentary environments and depositional processes. Geochemical indicators can be utilized for paleoenvironmental reconstruction.

5.1.1. Paleoclimate

The C-value integrates 12 elements to reconstruct the paleoclimate during the depositional period [39,61,62]. The C-values > 0.8, 0.6–0.8, 0.4–0.6, 0.2–0.4, and <0.2 suggest a humid, semi-humid, semi-arid to semi-humid, semi-arid, and arid paleoclimate, respectively.

The C-value of Chang 7 Member ranges from 0.38 to 1.55 (0.98 on average). Among these, the C-value of the fourth-order sequence PSS1 ranges from 0.38 to 1.55 (0.98 on average), while that of PSS2 ranges from 0.64 to 1.28 (0.98 on average). These results indicate a semi-humid to humid paleoclimate for Member 7 (Figure 7A; Table 2). The climate shows a decreasing trend in humidity from the bottom upwards.

The Sr/Cu ratio can also be used for paleoclimate reconstruction [39,62,78]. Sr/Cu ratios of 1.3–5, 5–10, and >10 indicate warm-humid, semi-humid, and arid climates, respectively.

The Sr/Cu ratio of Member 7 ranges from 2.68 to 19.34 (6.64 on average). The Sr/Cu ratio of PSS1 ranges from 2.68 to 7.19 (average 4.19), while that of PSS2 ranges from 4.07 to 19.34 (8.48 on average) (Figure 7B; Table 2). This indicates a humid to semi-humid paleoclimate. From bottom to top, the climatic humidity shows a decreasing trend.

The C-value and Sr/Cu ratio indicate a humid to semi-humid paleoclimate during the Chang 7 period, which aligns with the findings of several previous studies. First, fluoranthene (Flu) and pyrene (Py) are considered to be the products of original organic matter and biomass combustion, and biomass combustion generally occurs in arid climates. The Flu/(Flu + Pyr) ratio in the Chang 7 Member ranges from 0.04 to 0.26 (0.12 on average). The low ratio suggests a humid climate during the depositional period [79]. Second, the palynological assemblage of the Chang 7 Member in the southern part of the basin is mainly Asseretospora–Walchiites, indicating a warm and humid climate characteristic of temperate to subtropical zones [80,81].

5.1.2. Salinity

The Sr/Ba ratios are a common and effective indicator to evaluate the salinity of water bodies in ancient lakes. When Sr/Ba < 0.5, fresh water is indicated, while Sr/Ba between 0.5 and 1.0 suggests brackish water, and Sr/Ba > 1.0 indicates salt water [39,79,82].

The Sr/Ba ratio of the Chang 7 Member ranges from 0.29 to 0.91 (0.51 on average), indicating freshwater to brackish water conditions during the depositional period (Figure 7C; Table 2).

This is consistent with previous results obtained from other elemental ratios and biomarker compounds. First, B/Ga ratios of <3, 3–6, and >6 indicate freshwater, brackish water, and saline water environments, respectively. Twenty-four samples from the Maquan section in the southern basin have B/Ga ratios ranging from 0.24 to 3.22 (1.43 on average), indicating a freshwater environment [82]. Second, seventy-four samples from the Yanchi area in the northern basin have gammacerane/C30 hopane ratios ranging from 0.05 to 0.43 (0.15 on average), indicating a freshwater environment [83]. Third, forty-seven samples from the central-southern basin have C26-20S-TAS values ranging from 0.24 to 0.62 (0.17 on average), indicating a freshwater environment [79].

5.1.3. Paleoproductivity

Phosphorus (P) is an essential nutrient for the survival and reproduction of plankton in the water column [39,84,85,86]. In general, element P is modified with the content of element Ti or Al to eliminate the dilution effect of detrital input [24,25,26,34,35].

The P/Ti ratio of Well H269 ranges from 0.18 to 3.62 (0.75 on average) (Figure 7D; Table 2). The average P/Ti ratio of the Ubara Shale is 0.79 ± 1.63, often used as a reference for high productivity. The average P/Ti ratio of the Gujo-Hachiman Shale is 0.17 ± 0.23, commonly used as a reference for moderate productivity [87]. The P/Ti ratio of the Chang Member 7 is higher than that of the Gujo-Hachiman Shale and close to that of the Ubara Shale, indicating an overall high productivity level.

In Well H269, the P/Ti ratios in PSS1 range from 0.18 to 2.48 (0.77 on average), while the P/Al ratios range from 0.01 to 0.09 (0.03 on average). In PSS2, the P/Ti ratios range from 0.20 to 3.62 (0.74 on average), and the P/Al ratios range from 0.01 to 0.15 (0.03 on average) (Figure 7D,E; Table 2). The consistent mean values of these productivity proxies suggest a slight decrease in productivity from the base to the top.

Some elements (Ni, Cu, and Zn) are delivered to the sediment mainly in association with organic matter. Therefore, some scholars suggested that Ni, Cu, Zn, or their sum can be used to qualitatively evaluate the productivity of the water column [23,84].

The Ni + Cu + Zn values of the Chang 7 Member range from 154.6 to 257.74 (average 207.29). In Well H269, the Ni + Cu + Zn values of PSS1 range from 154.60 to 239.44 (average 209.06), while those of PSS2 range from 166.22 to 257.74 (average 205.96) (Figure 7F; Table 2). From bottom to top, the productivity of the Chang 7 Member decreases.

5.1.4. Redox Condition

The redox conditions of water column mainly control the enrichment of organic matter by affecting the preservation conditions [34,35]. Paleo-redox conditions can be evaluated according to various element ratios such as V/Cr, Ni/Co, and U/Th [88]. Degree of pyritization (DOP) is the ratio of pyritic iron to total reactive iron (pyritic iron plus HCl-soluble iron), which was proposed to determine redox conditions. DOP_T calculated from pyritic iron/total iron can be used to approximate DOP [89,90]. (V/Cr < 2.00: oxic; 2.00 < V/Cr < 4.25: dysoxic; V/Cr > 4.25: suboxic and anoxic; Ni/Co < 5.00: oxic; 5.00 < Ni/Co < 7.00: dysoxic; Ni/Co > 7.00: suboxic and anoxic; U/Th < 0.75: oxic; 0.75 < U/Th < 1.25: dysoxic; U/Th > 1.25: suboxic and anoxic; DOP < 0.42: oxic; 0.42 < DOP < 0.75: dysoxic; DOP > 1.25: suboxic and anoxic).

The V/Cr ratios of the Chang 7 Member range from 1.65 to 3.21 (2.21 on average), with U/Th ratios ranging from 0.22 to 1.31 (average 0.59). DOP_T values range from 0.01 to 0.56 (average 0.23 on average) (Figure 7G–I; Table 2). These three indicators suggest that the water column was oxic to dysoxic. The Ni/Co ratios (1.44–4.45, 2.11 on average) indicate that the Chang 7 Member was deposited under oxic conditions (Figure 7J; Table 2). Thresholds of these indicators should not be applied strictly when redox conditions are reconstructed [91]. Based on the integration of multiple proxies, the bottom water was dysoxic to oxic during the accumulation of the Chang 7 Member.

The V/Cr ratio in PPS1 ranges from 1.65 to 3.21 (average 2.21), and the Ni/Co ratio ranges from 1.93 to 2.92 (average 2.22). The U/Th ratio in PPS1 ranges from 0.22 to 1.31 (average 0.59), and the DOP_T value ranges from 0.06 to 0.49 (average 0.28). The V/Cr ratio in PPS2 ranges from 1.65 to 2.72 (average 2.05), and the Ni/Co ratio ranges from 1.44 to 4.45 (average 2.03). The U/Th ratio in PPS2 ranges from 0.22 to 1.02 (average 0.48), and the DOP_T value ranges from 0.01 to 0.56 (average 0.18). Compared to PPS1, all four redox condition indicators in PPS2 show a decreasing trend. From bottom to top, the reducing conditions of the water column weaken.

5.1.5. Detrital Input

Detrital input can influence burial efficiency and adsorption of organic matter on aluminosilicate to control organic matter enrichment [24,25,26,34,35]. Ti is primarily derived from clay minerals and heavy minerals (such as rutile and ilmenite). Zr is typically hosted in clay minerals and zircon. Al is mainly sourced from aluminosilicate clay minerals. The Ti/Al and Zr/Al ratios are commonly used to indicate terrigenous input [34,35,92,93].

The detrital input indicator Ti/Al ratio ranges from 0.03 to 0.05 (0.04 on average), and the Zr/Al ratio ranges from 8.95 to 22.40 ppm/% (14.15 ppm/% on average). In PPS1, the Ti/Al ratio ranges from 0.03 to 0.05 (0.04 on average), and the Zr/Al ratio ranges from 10.40 to 16.26 ppm/% (average 13.27 ppm/%). In PPS2, the Ti/Al ratio ranges from 0.04 to 0.05 (0.05 on average), and the Zr/Al ratio ranges from 8.95 to 22.40 ppm/% (14.81 ppm/% on average) (Figure 7K,L; Table 2). From bottom to top, the detrital input indicators Ti/Al and Zr/Al show a decreasing trend.

5.2. The Effect of Sedimentary Environment on Lithofacies

The evolution of sedimentary environments controlled the development of lithofacies. The fluctuation of lake levels associated with fourth-order sequences governed the vertical stacking pattern of lithofacies.

5.2.1. Fourth-Order Sequence PSS1

The fourth-order sequence PSS1 develops near the maximum flooding surface of the third-order sequence and is primarily composed of LF-1 and LF-2, which were deposited in a distal setting (deep lake) (Figure 6).

During the fourth-order lake-level rise (TST), sandstone grades into LF-2 and LF-1 sharply. The log response of the TST is characterized by upward-increasing trends in the gamma ray (GR), Compensated Carbon-Gamma (CCG), and Uranium (U) curves (Figure 6). The depositional mechanism is suspension sedimentation, accompanied by a weakening of hydrodynamic conditions and an enhancement of redox conditions. The fourth-order TST form is in the stage of transgression of the third-order sequence, characterized by low clastic input and high organic flux to the floor. Both the low bioturbation indicates an overall low sedimentation rate, which is mainly due to the increase in water depth during this period.

With the fall of the fourth-order lake level (RST), LF-1 and LF-2 transition into LF-3 and LF-4. The grain size of the mudstone coarsens upward overall. The depositional mechanism shifts from suspension settling to bottom current. The log response characteristics are manifested as decreasing upward trends in the GR curve, CNL curve, and U curve (Figure 6). The increase in both bioturbation intensity and trace fossil diversity indicates a relative fall in lake level, accompanied by an increase in bottom-water oxygenation.

5.2.2. Fourth-Order Sequence PSS2

Compared to PSS1, the fourth-order sequence PSS2 developed during the regressive phase of the overall third-order sequence. It features a generally shallower water depth, with depositional environments ranging from deep-lake to shallow-lake settings (Figure 6).

During the transgressive period of the fourth-order sequence PSS2, LF-1, LF-2, and LF-3 dominate, with occasional occurrences of LF-4. The water body remains primarily a distal setting (deep lake). Compared to the RST period of PSS1, the water depth increased again. Upward fining of sediment and lower bioturbation intensity are accompanied by upward-increasing trends in the GR, CNL, and U curves.

The lower part of the fourth-order regression system domain (RST) of sequence 2 is formed in the distal setting (deep lake), while the upper part is formed in the proximal (pro-delta to shallow lake). The upward coarsening of lithofacies grain size shows that wavy cross-laminated coarse mudstone (LF-5) and sandstone dominate in the upper part. Abundant wavy ripples and current ripples in LF-4 indicate that the reworking of sediments by waves is common. In the proximal setting, the intensity of bioturbation increased significantly to level 3. This indicates that the water column is relatively shallow and the oxygen content of the bottom water is increased, which provides the basis for the reproduction of benthic organisms.

5.3. The Effect of Sedimentary Environment on Organic Matter Enrichment

The accumulation of organic matter is controlled by complex, nonlinear interactions of three main variables: rates of production, destruction, and dilution [19,20,30,36,37,38]. By controlling the changes in these three variables, the evolution of the depositional environment ultimately drives the enrichment of organic matter.

5.3.1. Production

The cross plot of T_max and Hydrogen Index (HI) indicates that the organic matter i is primarily derived from algae and plankton (Figure 3B). A large number of algae fossils can be observed, especially in LF-1 and LF-2 (Figure 4). BSE Image photos show that, in addition to some algae with a diameter greater than 1 mm (Figure 3B), there are also abundant Leiosphaeridia with a diameter of about 10–30 μm in the mudrocks (Figure 3C).

The enrichment of organic matter is controlled by water column productivity. First, correlation analysis shows a weak positive correlation between P/Ti and P/Al ratios and TOC in Well H269, with correlation coefficients of 0.08 and 0.07, respectively (Figure 8A,B). Second, a significant positive correlation exists between Ni + Zn + Cu and TOC, with a correlation coefficient of 0.36 (Figure 8C). The overall weaker correlation between phosphorus-related indicators (P/Ti and P/Al) and TOC compared to that of Ni + Cu + Zn can be attributed to the enrichment mechanisms of phosphorus in sediments. Phosphorus in water bodies is primarily hosted in biological debris, such as phytoplankton, zooplankton, fish scales, and bones. This debris settles and deposits at the sediment–water interface. Under anoxic–euxinic bottom water conditions, most phosphorus is released back into the water column in the form of PO₄³⁻ from the organic matter in sediments, preventing its accumulation in the sediments. In oxic to dyoxic bottom waters, the redox cycling of iron within the sediments restricts the diffusion of phosphorus into the overlying water column, thereby promoting phosphorus enrichment in the sediments. Consequently, due to influences such as redox conditions, the abundance of phosphorus is not always strongly correlated with organic matter flux and productivity levels.

High productivity is the basis for organic matter enrichment. Volcanic–hydrothermal activity and a humid climate fostered this high productivity during the deposition of the Chang 7 Member.

Firstly, the volcanic and hydrothermal activities accompanying the Chang 7 sedimentation provided a vast supply of nutrients to the lake basin. During the Chang 7 period, the South China Block was subducted beneath the Qinling Orogenic Belt and North China Craton, and it melted [44]. The molten materials in the deep arched upward with heat, causing the base of the basin to stretch and frequent volcanic hydrothermal activity. Volcanic ash was transported by wind and subsequently deposited into the lake water, where it decomposed and released abundant elements such as phosphorus (P), iron (Fe), and calcium (Ca) [40]. Concurrently, deep-sourced hydrothermal fluids migrated upward along faults to the lake, supplying substantial nutrients including phosphorus (P), nitrogen (N), copper (Cu), iron (Fe), molybdenum (Mo), and vanadium (V) [42,43]. The influx of these nutrient elements from volcanic and hydrothermal activities triggered algal blooms in the water column, resulting in a massive production of organic matter.

Secondly, favorable climate conditions promoted the recovery of the lake ecosystem and enhanced productivity. The Permian–Triassic Mass Extinction (PTME; ~252 Ma) was followed by a prolonged period of hot global climate during the Early Triassic Period. It was not until the Ladinian and Carnian stages that global temperatures gradually declined to moderate levels, a period known as the Ladinian–Carnian Cooling (242–233 Ma), which facilitated the recovery of the ecosystem in the Ordos Lake [15,94]. The primary productivity of organic matter in lakes is a function of multiple variables, including solar radiation, wind, precipitation, water chemistry, and temperature [95]. Palaeoclimatic indicators reveal a correlation between the palaeoproductivity proxy (Ni + Zn + Cu) and the palaeoclimate proxies (C-value and Sr/Cu) during the deposition of the Chang 7 Member (Figure 8D,E). As humidity increased, water column productivity rose. Under the warm and humid climatic conditions, ample solar radiation promoted photosynthesis in algae and other aquatic plants, thereby favoring the production of organic matter.

5.3.2. Destruction

The enrichment of organic matter in the Chang 7 Member is controlled by redox conditions. All four redox proxies show clear positive correlations with TOC. The correlations of V/Cr and Ni/Co with TOC are 0.31 and 0.33, respectively, while U/Th and DOP_T exhibit stronger correlations with TOC, at 0.63 and 0.74 (Figure 9A–D). The bottom water conditions during the deposition of the Chang 7 Member were oxic to dysoxic. The dysoxic conditions are more favorable for the enrichment of organic matter.

Suboxic conditions are associated with two factors: high paleoproductivity and hydrothermal activity. Firstly, redox conditions were influenced by productivity. The four redox proxies show positive correlations with the productivity indicator Ni + Cu + Zn (Figure 9E–H). Due to the high productivity during the deposition of the Chang 7 Member, a large amount of dissolved organic matter settled and accumulated in the bottom water. The microbial decomposition of this organic matter consumed part of it, a process which consumed oxygen in the bottom water, thereby enhancing reducing conditions. Secondly, the hydrothermal activity accompanying the deposition of the Chang 7 Member introduced reducing elements into the lacustrine basin, such as sulfur. This process promotes the enhancement of reducing conditions in the bottom water [41].

5.3.3. Dilution

The dilution of organic matter primarily comes from detrital material. This material can be terrigenous clastics in proximal environments or bioclastic material in distal environments [36]. In the Chang 7 Member, the detrital material is mainly silt-sized particles, which are transported into the lacustrine basin by deltaic or wind processes. Cross-plots of TOC versus detrital input indicators (Ti/Al and Zr/Al) show that TOC decreases with increasing terrigenous input (Figure 9I,J). Terrigenous input primarily plays a diluting role in the enrichment of organic matter.

5.3.4. Influence of Lake Level Fluctuation on Organic Matter

The accumulation of organic matter is controlled by productivity, redox conditions, and clastic dilution, all of which are closely linked to fourth-order lake-level fluctuations.

In both fourth-order sequences PSS1 and PSS2, organic matter shows an upward-increasing trend during the transgressive stage. Paleoproductivity proxies indicate a rising trend coinciding with the relative lake-level rise. Redox proxies reveal a decrease in bottom-water oxygen content upward, with redox conditions transitioning from oxic to dyoxic. Simultaneously, terrigenous input exhibits a declining trend, resulting in a reduced dilution effect on organic matter. As the relative lake level rose, enhanced productivity and increasingly reducing conditions of the water column, coupled with decreased terrigenous dilution, collectively promoted the enrichment of organic matter.

Conversely, within the fourth-order sequence-stratigraphic framework, the fall in relative lake level corresponds to a reduction in organic matter content. Multiple indicators suggest a decline in both productivity levels and redox conditions in the bottom water, alongside increased terrigenous input, all of which are unfavorable for organic matter enrichment (Figure 10).

5.4. A Depositional Model for a Freshwater Lacustrine Basin

Palaeoclimatic and palaeosalinity indicators indicate that the Ordos Basin was a freshwater lacustrine basin with a humid to semi-humid climate during the Chang 7 period (Figure 7A–C). This interpretation is which is also confirmed by paleontological evidence. The diverse fossils in the fine-grained sediments of Member Chang 7 include plants, Antarctic algae, ostracods, insects, fish, and fish coprolites, recording a multileveled freshwater lake ecosystem [15,94]. The spherical algae commonly found in LF-1 are the primary producers in this ecosystem (Figure 4F). Under humid climatic conditions, heavy rainfall and surface runoff provide sufficient water, which is conducive to maintaining fresh water in the lake.

During the lacustrine transgression, the humid climate and intense rainfall, along with surface runoff, supplied ample water to the lake basin (Figure 11A). This not only helped maintain freshwater conditions but also contributed to the rise in lake level. Due to the significant increase in water depth, semi-deep to deep lacustrine deposits (distal environments) predominantly developed around the maximum flooding surface of the fourth-order sequence. As the lake level rose, productivity and the reducing conditions of the bottom water strengthened, while terrigenous input weakened. The coupling of these three factors led to an increase in organic matter content.

During the lowstand period following fourth-order lacustrine regression, the climate became relatively arid, resulting in reduced fluvial input and an overall lower lake level (Figure 11B). During the lacustrine regression of the fourth-order sequence PPS1, the depositional process transitioned from suspension sedimentation to bottom current transport. In the fourth-order sequence PPS2, the depositional process shifted from suspension sedimentation to a combination of bottom currents (in deep-lacustrine facies) and traction currents (in shallow-lacustrine and deltaic facies). From the bottom upward, the productivity and reducing conditions weakened, while terrigenous input increased. Accompanying this process, organic matter was degraded and diluted, leading to a decrease in its content.

6. Conclusions

The mudrocks of the Triassic Chang 7 Member in the Ordos Basin represent a suite of pro-delta to deep-lacustrine deposits within a freshwater lake basin. This unit demonstrates significant unconventional oil resource potential. Based on detailed core descriptions, six lithofacies are identified in the Chang 7 member: parallel organic-rich laminated fine mudstone (LF-1), phosphatic fine mudstone (LF-2), which is massive compared to faintly laminated medium–fine mudstone (LF-3), current ripple cross-laminated coarse to medium mudstone (LF-4), wavy ripple cross-laminated coarse mudstone interbedded fine mudstone (LF-5), which too is massive compared to thin layered muddy tuff/tuff (LF-6). According to the T-R sequence model (transgressive–regressive), the Chang 7 Member was subdivided into three fourth-order sequences. Mudrocks are predominantly developed in the fourth-order sequences PSS1 and PSS2. PSS1, characterized by a greater water depth, is dominated by lithofacies 1–4, representing semi-deep to deep lacustrine deposits. PSS2, with a relatively shallower water depth, contains LF-5 in its upper part, indicating pro-delta to shallow lacustrine sedimentation. Fluctuations in the lake level associated with these fourth-order sequences controlled the vertical stacking of lithofacies and the transition in depositional mechanisms. A shift from bottom currents to suspension settling occurred during lake-level rise, with the reverse trend during lake-level fall. The organic matter in the Chang 7 Member was primarily derived from aquatic algae, and its enrichment was jointly controlled by primary productivity and the redox conditions of the water column. Volcanic–hydrothermal activity and a humid climate promoted high primary productivity, which in turn contributed to dyoxic conditions in the water body. Within the fourth-order sequence-stratigraphic framework, the rising relative lake level led to enhanced productivity and increased water column reducibility, accompanied by decreased terrigenous dilution. These three factors acted in concert to promote organic matter enrichment. Conversely, during the lake-level stage, these conditions became unfavorable for organic matter accumulation. Paleoclimate and paleosalinity indicators suggest that the Ordos Basin was a freshwater lake under a humid to semi-humid climate during the Chang 7 period. Consequently, a model for lithofacies development and organic matter enrichment in a freshwater lacustrine basin was established. This study elucidates the controlling effects of sedimentary environment evolution on lithofacies and organic matter enrichment. These research outcomes provide theoretical support for optimizing the selection of lacustrine shale oil reservoirs and assessing their resources, thereby facilitating efficient and low-carbon development of shale oil and gas.