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18 March 2025

Paleoenvironmental Controls on Organic Matter Enrichment in Member 4 of the Yingcheng Formation Source Rocks, Xujiaweizi Fault Depression

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1
School of Earth Science, Northeast Petroleum University, Daqing 163318, China
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No. 9 Oil Production Company, Daqing Oilfield, China National Petroleum Corporation, Daqing 163458, China
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Exploration and Production Research Institute, Daqing Oilfield, China National Petroleum Corporation, Daqing 163712, China
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Authors to whom correspondence should be addressed.
Appl. Sci.2025, 15(6), 3321;https://doi.org/10.3390/app15063321
This article belongs to the Section Earth Sciences
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Abstract

In recent years, substantial hydrocarbon discoveries have been documented in Member 4 of the Yingcheng Formation (Ying IV) in the northern Songliao Basin. To fully understand the developmental characteristics of the Yingcheng Formation source rocks and clarify the main controlling factors of their formation, this study focuses on typical wells in the Xujiaweizi Fault Depression. By integrating core data, organic geochemistry, elemental geochemistry, and paleoenvironmental parameter reconstruction, we determined the paleoenvironment, key controls, and organic matter enrichment model during the deposition of Ying IV. The results show that the total organic carbon (TOC) content of the source rocks ranges from 1.46% to 4.34% (average 2.65%). The paleoclimate during deposition was predominantly warm and humid, with low oxygen, reduced water conditions, freshwater-to-brackish salinity, and a deep-lake environment. Paleoproductivity was moderate to high. Relationship analysis indicates that TOC content was jointly controlled by paleoclimate (warm and humid conditions promoting biological proliferation) and paleoproductivity (nutrient supply from volcanic activity and terrigenous clastics). The positive feedback between elevated productivity under warm-humid conditions and deep-lake reducing environments led to organic matter enrichment in the source rocks of Ying IV, following a productivity-controlled model. This study provides critical geological insights for deep natural gas exploration in the Songliao Basin.

1. Introduction

The Cretaceous period has emerged as a pivotal focus in paleoclimatology and hydrocarbon exploration research, distinguished by its hallmark features of elevated CO2 levels, high global sea levels, elevated temperatures, and prolific hydrocarbon resources [1]. Continental sediments dominate China’s Cretaceous geological record, hosting numerous lacustrine basins of this age. Prominently, the Songliao Basin in Northeast China stands out as one of the world’s largest Cretaceous lacustrine systems. Its sedimentary succession is characterized by continuous lacustrine clastic deposits, which, unlike other continental depositional settings, provide uniquely comprehensive, uninterrupted, and high-resolution records [2]. These lacustrine archives preserve critical paleoclimate proxies and evidence of major geological events in exceptional detail. Through geochemical analysis of lacustrine sediments, researchers can effectively reconstruct paleoenvironmental and paleoclimatic conditions with high fidelity.
Source rocks represent the geological basis for hydrocarbon formation, with their development governed by tectonic architecture, sedimentary facies zonation, and depositional environments [3,4]. In small fault-depression lacustrine basins characterized by rapid lateral facies transitions, environmental fluctuations exert intensified controls on source rock development [5]. Emerging research linking source rock deposition to paleoenvironmental conditions highlights how cyclic changes in paleoclimatic moisture balance, water column salinity, redox states, paleodepth, and lake productivity collectively modulate vegetation assemblages, parent rock weathering processes, sediment transport dynamics, and depositional cyclicity [6]. Elemental behaviors vary systematically with environmental parameters: some elements are preferentially preserved in sediments, while others are prone to diagenetic mobilization. As such, the elemental composition, abundance patterns, and geochemical ratios archived in sediments serve as critical proxies for reconstructing depositional paleoenvironments. Given the fundamental role of depositional systems in controlling source rock quality, reconstructing paleoenvironmental frameworks, establishing high-quality source rock development models, and conducting fine-scale resource potential assessments are pivotal for identifying prospective hydrocarbon plays [7].
The Xujiweizi Fault Depression serves as the principal gas-rich fault depression in the deep strata of northern Songliao Basin [8,9]. Characterized by intricate depositional environments, strong lithological heterogeneity, diverse source rock lithologies, and complex petroleum systems, this structural unit supports multiple gas accumulation types governed by overlapping hydrocarbon supply systems, variable source rock quality, and spatial distribution patterns. Previous investigations have comprehensively addressed its tectonic framework, sedimentary architecture, and reservoir properties, with some studies examining the depositional paleoenvironments of source rock development [10,11]. However, these efforts have been primarily confined to the Shahezi Formation [12,13], leaving the paleoenvironmental controls on source rock formation and enrichment mechanisms in the Yingcheng Formation poorly understood.
To bridge this knowledge gap, this study focuses on Member 4 of the Yingcheng Formation (Ying IV). Systematic sampling was conducted from dark mudstone intervals across key wells, covering both deeper and shallower stratigraphic units. Analyses included total organic carbon (TOC) measurements and major/trace element geochemical characterization. This study integrates source rock evaluation with paleoclimate and paleoenvironmental reconstruction, systematically analyzing the controlling factors of source rock development environments. An organic matter enrichment model for Ying IV is established, providing strong evidence for the research direction of targeting gas reservoirs using these primary hydrocarbon kitchens. This work lays a solid geological foundation for subsequent deep natural gas exploration and favorable target selection in the northern Songliao Basin.

2. Geological Setting

The Xujiaweizi Fault Depression, located in the northern Songliao Basin, covers an area of approximately 3700 km2 with a north–south extension spanning 115 km. Its structure varies regionally: a half-graben in the north, a composite half-graben in the center, and an asymmetric double-faulted depression in the south [14]. Controlled by the Xuxi, Xuzhong, Xudong, and Songxi fault zones, the depression underwent multi-stage tectonic evolution, including extension, compression, strike-slip, and inversion. Sequence stratigraphic analysis identifies five syn-rift to post-rift units in descending order: Huoshiling Formation (J3h, fluvial-lacustrine), Shahezi Formation (K1sh, volcaniclastics), Yingcheng Formation (K1yc, pyroclastic-deltaic), followed by Denglouku (K1d) and Quantou (K1q) Formations marking thermal subsidence phases (Figure 1). Particularly, the volcanogenic Yingcheng Formation contains a fourth member (K1yc4) subdivided into transgressive-regressive sequences (T-R cycles): the lower sequence (Sq1) in the depression center (semi-deep lake to fan-delta front facies, dominated by dark mudstone) and the upper sequence (Sq2) (braided river and deltaic coarse clastics) [15].
Figure 1. (a) The location of the Xujiaweizi Fault Depression; (b) the composite stratigraphic column of deep layers in Xujiaweizi Fault Depression; (c) typical seismic profile of the deep layer in Xujiaweizi Fault Depression (from basement to Qingshankou Formation).

3. Sample and Method

Core samples were strategically collected from Well X22 within the source rock kitchen of Ying IV in the Xujiaweizi Fault Depression. A total of 30 representative dark mudstone cuttings samples were selected every 4 m from the 3864–3984 m interval of Ying IV. These samples were subjected to organic carbon analysis and major and trace element analysis. All analyses were completed at the National Key Laboratory of Unconventional Oil and Gas Accumulation and Exploitation, a joint provincial and ministerial laboratory at Northeast Petroleum University. For the organic carbon analysis, the crushed samples were first weighed and transferred into pre-cleaned quartz crucibles. Dilute hydrochloric acid solution (with a ratio of hydrochloric acid to water of 1:7) was slowly added until the reaction was complete, and no gas bubbles were produced. The crucible was then repeatedly rinsed with distilled water until the filtrate was neutral. The samples were subsequently dried in an oven and prepared for further use. The samples were then combusted in a high-temperature oxygen stream using the LECO CS744 carbon and sulfur analyzer from the United States, converting the organic carbon in the samples to CO2. The data were obtained through infrared detection in accordance with the Chinese National Standard GB/T 19145-2022 [16].
Sample preparation workflow comprised the following: (1) Comminution: Jaw crusher → ring mill pulverization to 80% passing 75 μm sieve. (2) Drying phase: Convection oven stabilization (Memmert UN55, 105 °C/12 h). (3) Combustion cycle: Stepwise heating program (RT → 600 °C/2 h; 600 → 1000 °C/1 h; 1000 °C/4 h) in oxidizing atmosphere. The LOI value reflecting combustible constituents was quantified via gravimetric mass balance. Then, the flux, oxidizing agent, and samples were placed in a platinum crucible and fused at a constant temperature of 1150 °C for 15 min. After complete cooling, the bulk element analysis was completed using a ZSX Primus II X-ray fluorescence spectrometer, conducted in accordance with the Chinese National Standard GB/T 14506.28-2010 [17]. The trace element analysis began with weighing 0.5 g of the dried sample and placing it into a digestion bomb. One milliliter of concentrated HNO3 and 1 mL of HF were added, respectively. The sample was then placed in an oven and heated at a constant temperature of 150 °C for more than 48 h. After evaporating three times, 1 mL of HNO3 and 1 mL of deionized water were added. The solution was then heated in a drying oven for more than 12 h. The solution was transferred to a polyethylene plastic bottle and diluted to 100 g with 2% HNO3. The analysis was performed using the NexION300D inductively coupled plasma mass spectrometer (ICP-MS) with a detection limit of 10−6, conducted in accordance with the Chinese National Standard GB/T 14506.30-2010 [18].

4. Result

Based on the results of organic carbon analysis, major element analysis, and trace element analysis of the 30 selected mudstone samples, the measured TOC (Total Organic Carbon) content of Ying IV mudstone in the Xujiaweizi Depression of the Songliao Basin during the Cretaceous period exhibits a range of 1.46–4.34 wt% (mean = 2.65 wt%, n = 30). The Loss on Ignition (LOI) values range from 5.07% to 10.27%, mean = 7.26% (Figure 2). By correlating the LOI with TOC content, a statistically significant positive relationship (R2 = 0.903, p < 0.01) was demonstrated (Figure 3). This suggests that higher organic matter content leads to higher LOI values. This relationship can be used as an initial indicator of lake productivity changes, which in turn reflects the impact of regional climate change on lake ecology [19,20].
Figure 2. Relationship between LOI and depth.
Figure 3. Relationship between LOI and TOC.
The enrichment index of major and trace elements in the Yingcheng Formation mudstone is characterized by the Enrichment Factor (EF), which is used to quantify the enrichment degree of each element and its oxides. The average content of major elements in global average shale (AS) [21] is used as the reference value to determine the relative enrichment of elements, and this method can eliminate the interference of other factors. The EF is defined as
EFx = (X/Al)Sample/(X/Al)AS
where wAl/wX represents the ratio of the mass fraction of element X to that of Al in the sample, and (wAl/wX)AS represents the ratio of the mass fraction of element X to that of Al in the global average shale. When the EFx value is greater than 1, it indicates enrichment, while a value less than 1 indicates depletion. Through comparison, it can be found (Figure 4 and Table 1) that the mudstone of Ying IV generally shows enrichment in Fe, P, and Mn, and depletion in Al, Mg, K, and Ti. The elements Si, Ca, and Na are enriched in the upper part of Ying IV mudstone but depleted in the lower part.
Figure 4. Enrichment map of major elements in Ying IV.
Table 1. Major element content of Ying IV mudstone (%).
Trace elements are highly diverse in the Earth’s crust and have a wide range of concentrations. Compared to major elements, they are more sensitive to environmental changes and can provide rich geochemical and geological information. Among the 30 tested mudstone samples, as shown in Figure 5 and Table 2, Ying IV mudstone in the Xujiaweizi area is significantly enriched in Zn and Ba, while it is depleted in V, Co, and Ti. The elements Cr, Ni, Cu, Rb, and Sr exhibit slight concentration fluctuations with changes in depth.
Figure 5. Enrichment map of selected trace elements in Ying IV.
Table 2. Partial trace element content of Ying IV mudstone (μg/g).

5. Discussion

5.1. Paleoclimate

The term paleoclimate denotes climatic regimes in Earth’s geological past. Sedimentary elemental enrichment patterns are controlled by depositional environmental dynamics, particularly climatic conditions [22]. Major element oxide compositions in sedimentary sequences serve as classical proxies for paleoclimatic reconstructions. For example, under arid climatic conditions, ancient lakes may become more alkaline due to evaporation, leading to authigenic precipitation of MgO and CaO in palustrine facies. Thermodynamic partitioning favors MgO precipitation over CaO under elevated temperatures, thus, driving a systematic increase in Mg/Ca ratios with progressive aridity. Low values of Mg/Ca indicate warm climates, while high values indicate arid climates [23]. Sr enrichment is preferentially associated with evaporative regimes, though its concentration may be modulated by aqueous carbonate ion activity. According to the relationship diagram between Sr/Cu and CaO (Figure 6), the relationship between the two is poor, with a relationship coefficient R2 of only 0.0859. After excluding the influence of carbonate rocks, Sr/Cu can be used as a good proxy indicator for paleoclimate reconstruction. Generally, an Sr/Cu ratio greater than 10 indicates arid climate conditions, while a ratio between 1 and 10 indicates warm and humid climates [24,25]. The Sr/Cu ratios of Ying IV mudstone in the study area range from 4.25 to 11.92, with an average value of 7.48, indicating an overall transitional climate environment from semi-warm and humid to semi-arid. The Si/Al ratio in sedimentary rocks can also reflect the climatic characteristics of the deposition period. If the climate is humid, intense chemical weathering can cause the transport and migration of SiO2, leading to relative enrichment of Al2O3 and a corresponding decrease in the Si/Al ratio. Generally, an Si/Al ratio of less than 4 indicates warm and humid climates, while a ratio greater than 4 indicates arid climates. The Si/Al ratios of Ying IV mudstone in the study area range from 3.16 to 5.14, with an average value of 3.98, indicating an overall transitional climate from semi-warm and humid to semi-arid (Figure 7).
Figure 6. Relationship between CaO and Sr/Cu.
Figure 7. Relationship between Sr/Cu and Si/Al.
Climatic perturbations impose significant stresses on peri-basin terrestrial ecosystems. Enhanced humidity and elevated temperatures promote accelerated hydrolysis reactions in weathering profiles, particularly affecting lithologies susceptible to chemical breakdown. Detritus-rich, fine-grained mudstones serve as sensitive archives for reconstructing source-area weathering intensities and associated paleoclimatic regimes [26]. During progressive weathering, labile alkali ions (K⁺, Ca2⁺, Na⁺) are selectively removed through aqueous transport, whereas Al2O3 accumulates residually due to its thermodynamic stability in low-pH weathering fronts. Therefore, the Chemical Index of Alteration (CIA) is used to reflect the degree of paleoclimatic changes during sedimentation. The calculation formula is as follows:
CIA = Al2O3/(Al2O3 + K2O + Na2O + CaO*) × 100
CaO* = min(CaO − 10/3 × P2O5, N2O)
where CaO* refers only to Ca in silicates, not in phosphates or carbonates. A CIA value between 50 and 70 indicates a low degree of weathering, corresponding to a cold and dry paleoclimate during sedimentation. A CIA value between 70 and 85 indicates a moderate degree of weathering, corresponding to a warm and humid paleoclimate. A CIA value between 85 and 100 indicates a high degree of weathering, corresponding to a hot and arid paleoclimate. In the formula, all oxides are expressed in moles. CaO* refers specifically to CaO in silicate minerals, avoiding interference from CaO in carbonate and phosphate minerals. During diagenesis, potassium (K) may become abnormally enriched through metasomatic processes. The A-CN-K ternary diagram (Al2O3 − (CaO* − Na2O) − K2O) is commonly used to determine whether potassium correction is necessary. As shown in Figure 8, the CIA values of the samples correlate well with the ideal weathering trend line, indicating that the samples have not been affected by potassium metasomatism. The CIA values of Ying IV mudstone in the study area range from 65.38 to 80.23, with an average value of 76.55, indicating moderate weathering. This suggests that the overall climate in the study area was warm and humid. Additionally, the (Al2O3 + K2O + Na2O) − SiO2 cross-plot also indicates that the paleoclimate of Ying IV in the Xujiaweizi area was generally warm and humid (Figure 9).
Figure 8. A-CN-K diagram of the source rocks of Ying IV.
Figure 9. Source rocks (Al2O3 + K2O + Na2O) − SiO2 cross-plot.

5.2. Paleoredox Conditions

The paleoredox conditions of the water directly affect the preservation of organic matter [27]. In an oxidizing environment, organic matter is oxidized and decomposed, while a stable reducing environment is conducive to the preservation of organic matter. Among trace elements, V (Vanadium), Zn (Zinc), Cu (Copper), and Ni (Nickel) are common redox-sensitive elements. These elements share similar chemical properties, as they tend to precipitate under reducing conditions and dissolve in water under oxidizing conditions. Once deposited, they are unlikely to migrate and, thus, serve as excellent indicators of redox environments.
In this study, based on previous research methods, the V/(V + Ni) and Cu/Zn ratios are selected as discriminative parameters for determining the redox environment. When the V/(V + Ni) ratio is less than 0.46, it indicates an oxidizing environment; when the ratio is between 0.46 and 0.54, it indicates a weak oxidizing-weak reducing environment; and when the ratio is greater than 0.54, it indicates a reducing environment. The discriminative standard of Cu/Zn is more refined than that of V/(V + Ni). When the Cu/Zn ratio is less than 0.21, the sedimentary water body is considered to be anaerobic; between 0.21 and 0.38, it indicates a weak reducing environment; between 0.38 and 0.5, it indicates a weak oxidizing-weak reducing environment; between 0.5 and 0.63, it indicates a weak oxidizing environment; and when greater than 0.63, it indicates an oxygen-rich environment [28].
Using the above indicators to analyze the paleoredox conditions of Ying IV in the study area, the V/(V + Ni) ratios of the selected samples range from 0.41 to 0.75, with an average value of 0.56. Most samples have ratios greater than 0.46, accounting for 86.7% of the total samples, and the remaining samples have ratios close to 0.46, indicating that the water body in the study area is predominantly in a weakly oxidizing to reducing environment. Among the selected samples, only one sample has a Cu/Zn ratio of 0.22, while the rest have ratios less than 0.21, reflecting that the water body environment in the study area is mainly reducing. A comprehensive judgment based on the relationship diagram of the two ratios (Figure 10) shows that the water body of Ying IV in the Xujiaweizi area is generally characterized by reducing conditions.
Figure 10. Relationship between V/(V + Ni) and Cu/Zn.

5.3. Paleosalinity

Paleosalinity is the most direct manifestation of changes in paleoclimate and paleoenvironment, influencing the types and abundance of organisms that thrive in different salinities, which in turn affects the formation of source rocks. Sr (Strontium) and Ba (Barium) are two similar alkaline earth elements among trace elements, but they have different solubility products. Barium precipitates as barite (BaSO4), while strontium tends to precipitate under higher salinity conditions. Therefore, the Sr/Ba ratio recorded in sediments can reflect changes in salinity and has a strong positive relationship with paleosalinity. Based on previous studies [29], an Sr/Ba ratio greater than 1 indicates a marine environment, a ratio between 0.5 and 1 indicates a brackish environment, and a ratio less than 0.5 indicates a freshwater to slightly brackish environment. In the study area, the Sr/Ba ratios of the mudstone samples range from 0.01 to 0.16, with an average value of 0.056, preliminarily indicating a freshwater-to-slightly-brackish environment.
The migration capabilities of Ca (Calcium) and Fe (Iron) elements also differ in waters of varying salinities, and the Ca/(Ca + Fe) ratio can be used to judge the salinity of the water body. When the ratio is less than 0.4, it indicates a slightly brackish environment; a ratio between 0.4 and 0.6 indicates a brackish environment; and a ratio greater than 0.6 indicates a marine environment. The Ca/(Ca + Fe) ratio of the mudstone samples in the study area ranges from 0.1 to 0.44, with an average value of 0.23. A total of 93% of the samples have values less than 0.4. Combining the relationship between the Sr/Ba ratio and the Ca/(Ca + Fe) ratio (Figure 11), the overall paleosalinity of the water body in Ying IV of the Xujiaweizi area is comprehensively analyzed to be a freshwater-to-slightly-brackish environment.
Figure 11. Relationship between Ca/(Ca + Fe) and Sr/Ba.

5.4. Paleoproductivity and Paleowater Depth

Productivity refers to the ability of organisms to produce organic matter within a unit of time and area. Phosphorus (P) is one of the essential elements for biological growth [30]. Organisms in the water body enrich P through metabolic processes and store it within their bodies. When these organisms die and are buried in sediments, P becomes an effective indicator for evaluating paleoproductivity. Nickel (Ni), Copper (Cu), and Zinc (Zn) exist in sediments in the form of organic–metal ligands and are released during the decomposition of organic matter in sulfidic environments. They are released during organic matter decomposition and can be preserved in sediments through reactions with pyrite in a sulfide environment. Therefore, these three heavy metals can not only indicate the reducing nature of the environment but can also be used to assess productivity [31]. Titanium (Ti) is difficult to transport, and its content mainly originates from the lake itself. To eliminate errors caused by evaluating with a single indicator, the ratios of P/Ti, Ni/Ti, Cu/Ti, and Zn/Ti (where X represents P, Ni, Cu, or Zn) are more reliable for evaluating paleoproductivity. The higher the X/Ti ratio, the higher the paleoproductivity in the region.
Statistical analysis and organization of the geochemical data from the dark mudstone of Ying IV in the Xujiaweizi area show that the P/Ti ratio ranges from 0.15 to 1.34, with an average value of 0.60; the Ni/Ti ratio ranges from 115.4 to 514.3, with an average value of 213.2; the Cu/Ti ratio ranges from 47.05 to 181.2, with an average value of 96.05; and the Zn/Ti ratio ranges from 435.98 to 2547.55, with an average value of 909.51 (Figure 12). Compared with the average values of these four ratios in Ying IV mudstone from the Shuangchengnan area of the same basin, the values from the Xujiaweizi area are relatively higher. Therefore, it is concluded that the paleoproductivity of the water body during the Ying IV period in the Xujiaweizi area was of moderate-to-high productivity.
Figure 12. Evolution of ancient productivity and paleowater depth in Ying IV of the Xujiaweizi Fault Depression.
The aggregation and dispersion of elements are correlated with paleowater depth to some extent. Many transition elements and trace elements are good indicators of paleowater depth. Compared to the mass fraction of a single element, the ratios between major elements or trace elements are more indicative. The Fe/Mn ratio is selected to quantitatively distinguish between deep-water and shallow-water environments. A ratio less than 100 indicates a deep-lake environment, a ratio between 100 and 150 indicates a semi-deep lake environment, and a ratio greater than 150 indicates a shallow-water environment. The (Fe + Al)/(Ca + Mg) ratio is chosen as a qualitative indicator of paleowater depth. Fe and Al have higher contents in terrigenous clastics, while Ca and Mg are mainly present in carbonates. The contents of Ca and Mg have a negative relationship with those of Fe and Al, that is, the greater the water depth, the smaller the (Fe + Al)/(Ca + Mg) ratio [32]. In the study area, the Fe/Mn ratio of Ying IV ranges from 11.9 to 63.7, with an average value of 34.3, all indicating a deep-water environment. The (Fe + Al)/(Ca + Mg) ratio ranges from 1.18 to 9.95, with an average value of 5.4. Based on a comprehensive judgment, the paleowater body of Ying IV in the Xujiaweizi area is considered to be a deep-lake environment.

5.5. Main Controlling Factors and Enrichment Patterns of Organic Matter

To clarify the controlling effects of the paleodepositional environment on the enrichment of organic matter in the study area, this study fitted the indicators of paleoclimate, redox conditions, paleosalinity, paleoproductivity, and paleowater depth mentioned earlier with TOC to determine which factors control the enrichment of organic matter (Figure 13).
Figure 13. Relationship between TOC and paleoclimate (a–c), redox conditions (d,e), paleosalinity (f,g), paleoproductivity (h), and paleowater depth (i) parameters.
In terms of paleoclimate, the TOC (Total Organic Carbon) content in the study area shows a certain negative relationship with the Sr/Cu ratio. There is no significant relationship with the CIA (Chemical Index of Alteration). Higher Sr/Cu values are associated with more pronounced decreases in TOC values. This indicates that during the deposition of Ying IV, when the climate was relatively arid, precipitation decreased, leading to lower levels of biological activity and less favorable conditions for the enrichment of organic matter. In contrast, when the climate was warm and humid, it was more conducive to the proliferation of life, especially with increased rainfall, which led to higher reproduction rates of aquatic organisms. As a result, the conditions were more favorable for the enrichment of organic matter, leading to higher organic carbon content. Regarding redox indicators, TOC shows a certain positive relationship with the V/(V + Ni) ratio. This suggests that the enrichment of organic matter is partly controlled by the redox conditions of the water body. Better reducing conditions in the water body are more favorable for the enrichment and preservation of organic matter. Using the Cu/Zn ratio as an indicator, the water body environment is determined to be reduced, with no significant differences in the degree of reduction. Therefore, there is no clear relationship with TOC. Paleosalinity is also an important factor affecting the enrichment of organic matter. When salinity increases, the vertical salinity gradient in the lake rises, leading to stratification of the water body. In such conditions, organic matter is less likely to be consumed and decomposed in the high-salinity water, thus, being preserved. The TOC content in Ying IV mudstone of the study area shows a certain negative relationship with the Sr/Ba ratio. There is no significant relationship with the Ca/(Ca + Fe) ratio. This indicates that although higher salinity improves the preservation conditions for organic matter, the high salinity also inhibits the survival and reproduction of organisms in the lake. As a result, the organic matter content does not increase significantly and may even decrease slightly. Since the overall water depth of the paleolake in the study area is characterized by a deep-lake environment, there is no significant relationship between water depth fluctuations and TOC content.
Paleoproductivity is the foundation for the enrichment of organic matter. Since the four indicators used to assess paleoproductivity (P/Ti, Ni/Ti, Cu/Ti, and Zn/Ti) show consistent vertical variations, P/Ti is selected as the representative indicator of paleoproductivity. The positive relationship between P/Ti and TOC indicates that the organic matter content in sediments increases with higher paleoproductivity, providing a rich source of organic carbon. Volcanic activity also occurred during the sedimentary period of Ying IV. The volcanic hydrothermal fluids and volcanic ash generated by the volcanic activity provided abundant nutrients for the lake, thereby increasing the lake’s paleoproductivity. Meanwhile, the deep lake environment, which was predominantly reducing, facilitated the preservation of organic matter. As a result, organic-rich source rocks were formed through sedimentation, with the enrichment pattern being a productivity-dominated model (Figure 14).
Figure 14. Model showing the influence of volcanic activities on the preservation of the organic matter (modified from [7]).

6. Conclusions

(1) Member 4 of the Yingcheng Formation source rocks in the Xujiaweizi Depression of the northern Songliao Basin have a total organic carbon (TOC) content ranging from 1.46% to 4.34%, with an average value of 2.65%. The elements Fe, P, Mn, Zn, and Ba are relatively enriched, while Al, Mg, K, Ti, V, Co, and Ti are relatively depleted.
(2) Member 4 of the Yingcheng Formation source rocks in the Xujiaweizi Depression were formed under warm and humid climatic conditions, with moderate to high paleoproductivity. The reducing conditions of the deep-lake environment inhibited the oxidative decomposition of organic matter, thereby ensuring its long-term enrichment.
(3) The main controlling factors for the enrichment of organic matter in Member 4 of the Yingcheng Formation source rocks are paleoclimate and paleoproductivity. The enrichment pattern follows a productivity-driven model, where high productivity under warm and humid climatic conditions provides the material basis, while deep lacustrine reducing environments facilitate effective preservation. This process is jointly driven by volcanic activity and fluvial input.

Author Contributions

Conceptualization, Y.Z. and X.Y.; Funding acquisition, Y.Z. and Y.L.; Resources, L.S., L.Y. and Y.H.; Formal analysis, J.X. and Z.W.; Project administration, X.Y.; Visualization and Writing—original draft, Z.W.; Writing—review and editing, Z.W., X.Y. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (Grant No. 42372150), China Postdoctoral Science Foundation (Grant No. 2022MD723760) and Heilongjiang Province Postdoctoral Special Funding (Grant No. LBH-TZ2308).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The author does not have permission to share data due to internal policy.

Conflicts of Interest

Author Yanhua Hou was employed by No.9 Oil Production Company, Daqing Oilfield, China National Petroleum Corporation. Author Lidong Sun, Liang Yang and Jinshuang Xu were employed by Exploration and Production Research Institute, Daqing Oilfield, China National Petroleum Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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