Introduction

The Ordos Basin, the second largest sedimentary basin in China, is abundant in hydrocarbon resources and constitutes a key component of the Mesozoic petroleum system1. Previous studies have revealed that the Chang 7 member source rocks of the Triassic Yanchang formation are the main source rocks for the Mesozoic petroleum system. The organic geochemical characteristics of the Chang 7 member source rocks include high organic matter abundance, mixed biogenic organic matter types, and great degrees of thermal evolution2. In addition, analyses of paleoproductivity, organic matter preservation conditions, and sedimentary environment reconstruction have indicated that the formation of high-quality Chang 7 member source rocks was primarily controlled by the extensive volcanic hydrothermal activity resulting from the collision of the North China and Yangtze plates3,4. This hydrothermal activity, originating from deep crustal and mantle sources, provided substances rich in essential elements for life, such as iron (Fe) and phosphorus (P), which are crucial for the prosperity of primary producers in water masses5. Simultaneously, the massive amount of volcanic ash produced by volcanic activity creates severely anoxic conditions, providing an ideal environment for the preservation of organic matter6,7. The deposition of volcanic ash has led to the death of numerous primary producers, whose remains have contributed to the enrichment of primary organic matter in sediment. Moreover, volcanic hydrothermal activity not only supplies life-essential elements but also introduces transition metals, such as vanadium (V), chromium (Cr), manganese (Mn), and molybdenum (Mo), which are effective catalysts for the generation of hydrocarbon from organic matter8. Volcanic and hydrothermal activities transfer deep-seated thermal energy to the surface, heating the organic matter in source rocks and enhancing the generation of hydrocarbon from organic matter. Although extensive research has been conducted on the organic geochemical characteristics and formation mechanisms of the Chang 7 member source rocks9,10, these studies have focused predominantly on areas in the central and southern regions of the basin that are commonly affected by volcanic hydrothermal activity, with relatively less attention being given to the northern regions of the basin.

In recent years, many oil and gas resources have been discovered in the northern margin of the lake basin, particularly in the Yanchi–Dingbian area (Jiyuan Oilfield). Previous studies have confirmed that the oil originates from in situ shallow lake Chang 7 member source rocks, suggesting that the shallow lake source rocks in the northern margin of the basin have the potential to be effective source rocks11,12. However, the organic geochemical characteristics of these shallow lake source rocks remain unclear, and their hydrocarbon generation potential needs to be elucidated. Additionally, because the northern margin of the Ordos Basin is far from the region affected by volcanic hydrothermal activity, determining whether the depositional model of lake source rocks is in accordance with those in the southern basin is crucial. If these models differ, it is important to determine the controlling factors and depositional models and to evaluate their differences with the Chang 7 member source rocks in the southern margin of the basin.

In this study, a comprehensive analysis of the organic geochemical characteristics of the Chang 7 member source rocks in the Yanchi–Dingbian area is carried out to clarify the hydrocarbon generation potential of shallow lake source rocks in the Ordos Basin. Additionally, by utilizing the elemental geochemistry and molecular marker characteristics of the Chang 7 member source rocks, the formation mechanisms of shallow lake source rocks are explored and compared with those of deep lake source rocks in the southern portion of the basin. Additional objectives of this study include identifying the variations in the key factors controlling the formation of high-quality source rocks of the Chang 7 member across different sedimentary facies, and subsequently establishing distinct depositional models for these source rocks within the Ordos Basin. This research not only has significant implications for expanding oil and gas exploration areas but also contributes to the understanding of differential enrichment patterns for organic matter in large sag-type lake basins, providing a scientific basis for future hydrocarbon exploration projects and objective resource assessments.

Geological background

The Ordos Basin, situated in northern China (Fig. 1), is the country’s second largest sedimentary basin, covering an area of approximately 370,000 square kilometers12. The basin is located at the northwest edge of the North China Plain. The basin is bordered by the Qinling Mountains to the south, the Inner Mongolia Plateau to the north, the Loess Plateau to the east, and the Longdong Plateau to the west. The Ordos Basin consists of six first-level tectonic units, including the Tianhuan depression, the western margin retrograde belt, the Yimeng uplift, the Weibei uplift, the Yishan slope, and the Jinci flexure belt. The Yishan slope is the main depositional region of the upper Triassic Yanchang formation. The strata within the Yishan slope are gently inclined, with dip angles of less than 1°, and they primarily feature nose-like structures. The Yishan slope received marine deposits during the early Paleozoic and lake deposits during and after the late Paleozoic13.

Fig. 1
figure 1

(modified from Chen, 2021).

Depositional facies, well distribution, and stratigraphic lithology correlation of the Chang 7 member source rocks in the Ordos Basin.

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The Ordos Basin has undergone multiple tectonic evolution stages: a marginal ocean basin stage in the early Paleozoic, a passive continental margin basin stage in the early–late Paleozoic, a large intracontinental basin stage in the late Permian–Middle Triassic, and a foreland basin stage from the Late Triassic–Early Cretaceous13. During the Late Triassic, the southern part of the Ordos Basin evolved into a foreland basin due to regional collision tectonics (Indosinian movement), characterized primarily by lake environments14,15. This phenomenon resulted in a set of fluvial lake deltaic deposits known as the Yanchang formation16 (with Chang 7 member lake shales ranging in thickness from 15 to 100 m and burial depths of approximately 500 to 2000 m). The extensively developed Triassic Yanchang Formation is the main oil-generating rock and petroleum reservoir in the Mesozoic strata of the basin, with the large-scale high-quality source rocks of the Chang 7 member controlling the distribution of oil reservoirs. On the basis of sedimentary cycles and lithological assemblages, the Yanchang formation can be divided into 10 members: Chang 10 to Chang 1, from bottom to top (Fig. 1). The Chang 7 member can be further divided into three sub-members from bottom to top: Chang 73, Chang 72, and Chang 7117. During the deposition of the Chang 7 member, significant lake development led to the formation of semilake and deep lake conditions and extensive organic-rich deposits, making it a target for high-quality source rocks, shale oil, and shale gas18,19.

Methodology

This study involves the differential hydrocarbon generation potential and organic matter enrichment of the Chang 7 member source rocks between shallow and deep lake facies in the Ordos Basin. The data for the shallow lake facies source rocks came from Wells Y56 and YY1, whereas the data for the deep lake facies source rocks came from Wells Z40, JH-420, and YQ-121. The locations of the wells from which data were extracted are shown in Fig. 1.

The core samples of the Chang 7 member source rocks from Well Y56 were subjected to organic geochemical analyses, such as organic carbon content (TOC), rock pyrolysis, soluble organic matter extraction and separation, saturated hydrocarbon gas chromatography‒mass spectrometry (GC‒MS), and major and trace element analyses. Except for the major and trace element analyses, the tests were performed at the State Key Laboratory of Oil and Gas Resources and Exploration of the China University of Petroleum (Beijing). The major and trace element analyses were conducted at the Analysis and Testing Research Center of the Beijing Geological Institute of the Nuclear Industry.

Total organic carbon (TOC) and rock pyrolysis analyses

All selected source rock samples were ground and sieved using an 80-mesh sieve. The fine rock powder samples were then mixed with hydrochloric acid (HCl and distilled water at a volume ratio of 1:9) and reacted for 1 h to remove inorganic carbon. The samples were subsequently rinsed with distilled water for approximately 24 h to eliminate all acid ions. Iron–tungsten flux was added to the samples, and total organic carbon (TOC) analysis was conducted using a LECO CS230 carbon analyzer with 99.5% oxygen as the carrier gas at a temperature of 24 °C and a relative humidity of 48%. Rock pyrolysis analysis was performed using an OGE-VI instrument with a sample weight of 80–120 mg. The temperature program for rock pyrolysis involved rapid heating to 300 °C and holding for 3 min to measure the free hydrocarbon (S1) content. The temperature was then increased at a rate of 50 °C/min to 600 °C and held for 1 min to measure the pyrolyzed hydrocarbon (S2) content. During the S2 measurement, the Tmax values were recorded.

Soluble organic matter extraction and group fraction separation

The extraction of soluble organic components from source rock samples was conducted using the Soxhlet extraction method with a mixture of dichloromethane and methanol in a volume ratio of 97:3. After continuous extraction for 72 h, the solvent from the extract was removed using rotary evaporation techniques to obtain chloroform bitumen “A”. N-hexane was subsequently added to the chloroform bitumen “A” for precipitation, and the asphaltenes were removed through a filtration process. To further fractionate the organic components, a solid-phase chromatography column packed with a mixture of silica gel and alumina (volume ratio 2:3) was utilized. The column was eluted sequentially with n-hexane, a mixture of n-hexane and dichloromethane (volume ratio 2:1), and a mixture of dichloromethane and methanol (volume ratio 97:3), resulting in the separation of saturates, aromatics, and nonhydrocarbon fractions, respectively.

Saturated hydrocarbon gas chromatography‒mass spectrometry (GC‒MS) analysis

The saturated hydrocarbons were analyzed using an Agilent 6890/5975 GC‒MS system equipped with an HP-5 ms capillary column (30 m × 0.25 mm × 0.25 μm). The temperature program was set to hold the temperature at 50 °C for 1 min, increase it at a rate of 3 °C/min to 310 °C, and then hold it at 310 °C for 15 min. Helium was used as the carrier gas in constant flow mode at a flow rate of 1.0 mL/min. The scan range was 50–550 amu, with detection in electron ionization (EI) mode at 70 eV. Data acquisition was performed in both full-scan mode and selected ion monitoring mode.

Major and trace element analysis

The major oxides and trace elements in hydrocarbon rock samples were analyzed. By using X-ray fluorescence (XRF) and an AxiosmAX XRF spectrometer and considering the national standard GB/T 14506.28–2010, 15 fused discs were prepared for the analysis of their major oxides: SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, TiO2, and P2O5. The samples were analyzed for trace elements via ICP‒MS (Element XR) after ablation and quantified according to the national standard GB/T 14506.30–2010. The sample powder was dissolved with 30% HF and 68% HNO3 at 190 °C for 24 h. After the reaction was complete, the sample was heated with ultrapure water to evaporate the excess solvent. The samples were subsequently placed in a 2–6 mol/L HNO3 solution at 150 °C for 48 h. After the solution was evaporated to near dryness, 1–6 mol/L HNO3 was added. Finally, the samples were diluted for analysis. After sample digestion, these samples were analyzed for trace elements using ICP–MS (Element XR) according to the Chinese national standard GB/T 14506.30–2010, and the detection limits of trace elements ranged from 0.1 × 10− 12 to 9 × 10− 12.

Results

Organic geochemical characteristics

The Chang 7 source rock exhibits substantial organic matter enrichment but displays marked heterogeneity in organic abundance across stratigraphic members and depositional facies. The total organic carbon (TOC) and pyrolysis parameters (S1 + S2) serve as critical proxies for evaluating the hydrocarbon generation potential. In the shallow lake facies (Fig. 1), Well Y56 has TOC values of 0.68–8.72 wt% (hydrocarbon potential: 1.6–49.73 mg/g), with distinct vertical variations. The Chang 71+2 sub-member contains 0.68–7.17 wt% TOC (average of 4.35 wt%) and 1.6–39.2 mg/g generation potential (average of 16.02 mg/g), whereas the underlying Chang 73 sub-member has elevated values of 1.93–8.72 wt% TOC (average of 5.06 wt%) and 6.82–49.73 mg/g potential (average of 25.01 mg/g) (Table 1).

Table 1 Characteristic parameters of organic geochemistry of hydrocarbon source rocks in the study area.
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Hydrogen index (HI) analysis was performed to determine the thermal maturity gradients. The shallow lake facies of Well Y56 exhibits HI values of 100.9–499.8 mg/g (average of 280.4 mg/g) and Tmax values of 431–450 °C (average of 444 °C) in Chang 71+2, which progress to values of 245.32–576.58 mg/g for HI (average of 405.96 mg/g) and 442–452 °C for Tmax (average of 447.4 °C) in Chang 73. Deep lake facies wells present contrasting patterns: the Chang 71+2 sub-member of Well JH-4 has HIs ranging from 78.47 to 513.07 mg/g (average of 262.72 mg/g) with a Tmax ranging from 436 to 444 °C (average of 442 °C), whereas Well Z40 has enhanced thermal parameters with Tmax ranging from 441 to 452 °C (average of 447 °C) and HI ranging from 185.65 to 649.3 mg/g (average of 377.6 mg/g) alongside elevated values for TOC (0.67–16.35 wt%, average of 2.87 wt%) and hydrocarbon potential (1.57–110.08 mg/g, average of 16.62 mg/g). The Chang 73 sub-member in the deep lake facies demonstrates superior source quality, particularly in Well Z40, with TOC values ranging from 0.91 to 17.91 wt% (average of 5.62 wt%), S1 + S2 values ranging from 3.99 to 108.1 mg/g (average of 34.94 mg/g), and HI values ranging from 261.36 to 672.05 mg/g (average of 489.98 mg/g).

Shallow lake facies deposits display more constrained thermal evolution characteristics, as evidenced by the data from Well YY1: the Chang 71+2 sub-member has TOCs ranging from 0.76 to 13.49 wt% (average of 5.88 wt%), with HIs ranging from 195.49 to 295.12 mg/g (average of 238.03 mg/g), whereas the Chang 73 sub-member has reduced values (TOCs: 1.42–5.14 wt%, average of 3.31 wt%; HIs: 171.21–294.38 mg/g, average of 219.72 mg/g) and lower thermal maturity (Tmaxs: 401–450 °C, average of 432.92 °C). This facies-controlled variability highlights differential organic preservation and thermal history across depositional environments.

Biomarkers

N-Alkanes and isoprenoid alkanes

N-Alkanes are major components of petroleum hydrocarbons, which are derived from a wide range of organic sources, including microorganisms, algae, and higher-level plants12. The geochemical parameters of n-alkanes can reflect differences in biological source inputs and the characteristics of the depositional environment22. In the Chang 7 member source rocks, the distribution pattern of n-alkanes is characterized by a unimodal distribution with a predominant front peak, and nC17 serves as the main peak carbon. The carbon preference index (CPI) values range from 1.09 to 1.20, and the odd‒even predominance (OEP) values range from 1.02 to 1.07 (Table 2).

Table 2 TIC parameters of satuates in each subunit in the Chang 7 member in study area.
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Organic geochemical characterization of depositional environments was conducted through biomarker ratios, including pristane/phytane (Pr/Ph), pristane/nC17 (Pr/nC17), and phytane/nC18 (Ph/nC18) ratios. The shallow lacustrine samples (Well Y56) present Pr/Ph ratios ranging from 0.57 to 1.10 (average of 0.74), whereas their deep lacustrine counterparts (Well Z40) present lower values ranging from 0.25 to 0.94 (average of 0.61). The diagnostic parameters maintain consistent patterns across facies: the Pr/nC17 ratios range from 0.23 to 0.51 (average of 0.35), and the Ph/nC18 ratios range from 0.26 to 0.74 (average of 0.51) in shallow lacustrine systems. This systematic geochemical differentiation suggests the presence of distinct redox conditions and organic matter preservation pathways between subaqueous depositional regimes.

Steranes

Steranes, which are derived from steroids in organisms, are widely distributed in sediments. Owing to their stable structures and degradation resistance, steranes are extensively used for biogenesis, thermal evolution, and sedimentary environment analyses23. The relative concentrations of C27–C29 regular steranes represent different biological sources of organic matter in sediments24. The relative abundance distributions of the shallow lacustrine facies of the C27, C28, and C29 regular steranes of the Chang 71+2 sub-member (Well Y56) range from 0.38 to 0.53, 0.20–0.30, and 0.25–0.32, respectively. These values contrast with those of deeper lacustrine equivalents (Well Z40) in the same sub-member, which have modified distributions of 0.25–0.34 (C27), 0.19–0.33 (C28), and 0.36–0.54 (C29). The Chang 73 sub-member shows analogous facies-dependent variations: shallow lacustrine samples (Y56) maintain C27/C28/C29 proportions of 0.34–0.59/0.15–0.31/0.24–0.40, whereas their deep lacustrine counterparts (Well Z40) display systematically elevated C29 concentrations (0.40–0.47) relative to the contracted C27 (0.25–0.32) and C28 (0.28–0.33) ranges. This stratigraphically consistent divergence in sterane partitioning underscores that the depositional environment controls organic matter preservation and microbial reworking processes.

The sterane isomerization parameters C29 sterane 20 S/(20 S + 20R) and C29 sterane ββ/(αα + ββ) are effective indicators of oil maturity and hydrocarbon migration25. While equilibrium thresholds for C29 sterane 20 S/(20 S + 20R) and ββ/(αα + ββ) are established at 0.52–0.55 and 0.67–0.71, respectively, our dataset reveals that they have facies-dependent maturation patterns. The shallow lacustrine samples (Well Y56) present C29 20 S/(20 S + 20R) ratios ranging from 0.38 to 0.45 and ββ/(αα + ββ) values ranging from 0.53 to 0.60. In contrast, the deep lacustrine counterparts (Well Z40) demonstrate enhanced thermal maturation signatures with elevated C29 20 S/(20 S + 20R) ratios (0.47–0.54) but decreased ββ/(αα + ββ) ratios (0.45–0.56), revealing the complexity of the redox-controlled diagenetic pathways in subaqueous depositional systems.

Triterpanes and Hopanes

Triterpanes are highly resistant to thermal evolution and biodegradation due to their stable carbon skeletons26. In shallow lake facies source rock samples, compounds such as tricyclic terpanes (TTs), tetracyclic terpanes (TeT), C30 hopane (C30H), gammacerane (Ga), and the C31–C35 homohopane series are commonly detected (Fig. 7).

The geochemical parameters demonstrate distinct facies-controlled characteristics in terms of the organic matter composition and depositional environment. The gammacerane index (Ga/C30H) exhibits comparable ranges in the shallow lake facies of Well Y56 between the Chang 71+2 (0.05–0.16, average of 0.1) and Chang 73 sub-members (0.06–0.17, average of 0.1). The biomarker ratios reveal significant facies differentiation: the C24TeT/C26TT values in the Y56 well range from 1.26 to 2.54 (average of 1.99) for Chang 71+2 and from 1.07 to 2.47 (average of 2.48) for Chang 73, whereas the C26TT/C25TT ratios range from 1.13 to 2.04 (average of 1.52) and from 1.31 to 2.8 (average of 1.81), respectively. The corresponding C19TT/C23TT values in this shallow facies range from 0.2 to 0.47 (average of 0.32) for Chang 71+2 and 0.4–0.78 (average of 0.54) for Chang 73.

In contrast, the deep lake facies in Well Z40 has distinct biomarker profiles: the C26TT/C25TT ratios decrease to 0.43–0.75 (average of 0.58) in Chang 71+2 and to 0.43–0.74 (average of 0.59) in Chang 73, with C19TT/C23TT values ranging from 0.16 to 1.09 (average of 0.41) and 0.05–0.49 (average of 0.28), respectively. The C24TeT/C26TT ratios in the Z40 well range from 0.68 to 1.73 (average of 1.56) for Chang 71+2 and 0.75–1.73 (average of 1.14) for Chang 73. The thermal maturity parameters (C31 22 S/(22 S + 22R)) maintain consistency across facies, ranging from 0.52 to 0.57 (average of 0.53) in Chang 71+2 and from 0.52 to 0.58 (average of 0.55) in Chang 73 (Table 3) in Well Y56. This multiparameter dataset systematically documents the interplay between the depositional energy, redox, and organic matter preservation conditions across lacustrine subenvironments.

Table 3 Steranes, Hopanes biomarker parameters of saturates in each subunit of the Chang 7 member in study area.
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Elemental geochemical characteristics

The contents and distribution patterns of major and trace elements in sedimentary rocks are widely used to analyze the depositional environments of source rocks. Elements such as Si and Al can reflect the siliciclastic mineral sources of the source rocks, whereas variations in Fe, Ca, Zr, and MnO/TiO2 can indicate the input of terrigenous clastic material and the changes in the depositional environment15. Trace elements play crucial roles in reconstructing the depositional environment, with V and Ni indicating the redox conditions of source rocks and Sr and Ba indicating the paleosalinity of water bodies28. The results of major and trace element analyses for source rocks in the Chang 7 member from Wells Y56, JH-4, YQ-1, YY1, and Z40 are presented in Tables S1–S6.

Paleoproductivity trends, as indicated by P/Ti ratios, demonstrate significant facies-controlled variations with stratigraphic evolution. In the Chang 71+2 sub-member, shallow lake systems exhibit enhanced productivity, with P/Ti values of 0.24–1.93 (average of 0.57) in Well Y56, which is further substantiated by the elevated ratios of 0.6–1.87 (average of 0.96) in Well YY1. This phenomenon contrasts sharply with the deep lake facies where Wells YQ-1 and JH-4 have lower recorded values of 0.11–0.34 (average of 0.17) and 0.14–0.25 (average of 0.19), respectively; this pattern was maintained in the range of 0.002–0.96 (average of 0.22) for Well Z40.

The reversal pattern in the Chang 73 sub-member reveals that deep lacustrine systems have superior productivity: Well Z40 shows a marked increase to 0.12–2.23 (average of 0.55), whereas the Wells YQ-1 and JH-4, which are deep lake wells, exhibit values of 0.19–3.54 (average of 0.87) and 0.11–1.82 (average of 0.66), respectively. The shallow facies maintains elevated levels, with Well Y56 values ranging from 0.23 to 5.06 (average of 0.72) and Well YY1 values ranging from 0.76 to 6.41 (average of 1.76), indicating intensified nutrient cycling in shelf-margin environments. This stratigraphic inversion of productivity patterns between sub-members likely reflects basin–bathymetry changes and terrigenous nutrient flux variations.

The enrichment factor (EF) has been used to estimate the degree of element enrichment in shales and is calculated as follows:

XEF = [ \(\:(\frac{X}{Al}\))/样品\(\:(\frac{X}{Al}\)) ]PAAS,

where X and Al represent the weight% concentrations of X and Al, respectively. The samples are normalized using the post-Archean average shale (PAAS) composition from Taylor and McLennan (1985)29.

The authigenic uranium enrichment factor (UEF) reveals differential redox evolution characteristics between lacustrine subenvironments. In the Chang 71+2 sub-member, shallow lake facies exhibit comparable UEF ranges across sampling locations: Well Y56 (0.40–4.72, average of 1.09) supplemented with Well YY1 (0.71–3.16, average of 1.34) are indicative of moderately oxic conditions. Deep lake systems show transitional characteristics, with Wells YQ-1 (0.98–5.22, average of 2.12) and JH-4 (4.54–52.91, average of 20.21) displaying partial overlap with shallow facies, whereas Well Z40 has enhanced but variable signatures (0.59–31.69, average of 3.39).

The Chang 73 sub-member exhibits intensified redox stratification. The shallow facies maintains relative stability, with Well Y56 values ranging from 0.99 to 4.59 (average of 1.62) and Well YY1 values ranging from 0.45 to 1.28 (average of 0.87), contrasting sharply with those of deep lacustrine systems, where Wells JH-4 (3.54–86.15, average of 26.36) and Z40 (1.09–38.32, average of 8.87) exhibit exponential enrichment. Notably, Well YQ-1’s deep lake facies shows intermediate but elevated values (3.49–20.41, average of 8.7). This sub-member-specific amplification of deep water anoxia correlates with basin restriction events and enhanced organic matter preservation in stratified water columns.

In the Chang 7 member, the Sr/Cu ratios clearly differ between the Chang 71+2 and Chang 73 sub-members. The Sr/Cu ratios for the deep and shallow lake facies in the Chang 71+2 sub-member are 2.14–6.70 (average of 4.21) and 1.9–3.91 (average of 3.12), respectively, and those for the Chang 73 sub-member are 1.4–7.9 (average of 3.5) and 1.64–7.42 (average of 4.18), respectively. The Sr/Cu ratio is significantly lower in the deep lake facies of the Chang 73 sub-member than in the shallow lake facies.

The chemical index of alteration (CIA) has been widely used in the study of the chemical weathering of sedimentary rocks30.

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

CaO* refers to the amount of CaO present in silicates. The CIA values for the shallow lake facies in the Chang 71+2 and Chang 73 sub-members range from 70.45 to 83.45 (average of 78.53) and 71.09–83.99 (average of 80.52), respectively. For the deep lake facies, the CIA values in the Chang 71+2 and Chang 73 sub-members range from 71.31 to 78.3 (average of 73.78) and 70.27–81.97 (average of 75.60), respectively, indicating overall strengthened weathering in the shallow lake facies.

The geochemical signatures of the Chang 7 member exhibit systematic variations across lacustrine subenvironments. In the Chang 71+2 sub-member, shallow lake systems (Wells Y56 and YY1) present elevated Zr concentrations ranging from 91.9 to 211 (average of 140.23) and 149.21–204.92 (average of 171.99), respectively, with concomitant MnO/TiO₂ ratios ranging from 0.05 to 0.61 (average of 0.21) and 0.06–0.81 (average of 0.28), respectively. These values contrast with the deep lake facies (Well Z40), which has low Zr values (76.4–145, average of 123.85) and moderate MnO/TiO₂ values (0.08–0.42, average of 0.17).

The Chang 73 sub-member reveals intensified geochemical differentiation. Shallow lacustrine systems maintain high Zr concentrations (Y56: 158–291, average of 203.83; YY1: 173.15–232.87, average of 192.96) but reduced MnO/TiO₂ ratios (0.04–0.35 (average of 0.13) and 0.09–0.83 (average of 0.27)). Deep lake facies (Well Z40) exhibit notably reduced Zr magnitudes (61.7–140, average of 95.49) and significantly amplified MnO/TiO₂ ratios (0.1–3.24, average of 0.51).

The complementary Sr/Ba ratios maintain consistent patterns: shallow facies dominate in Chang 71+2 (0.22–0.58, average of 0.34) versus deep systems (0.15–0.56, average of 0.285), with both phases showing elevated values in Chang 73 (shallow: 0.26–0.83, average of 0.415; deep: 0.16–0.85, average of 0.387). This multiproxy dataset illustrates the progressive redox changes and detrital input modifications over time.

Discussion

Geochemical characteristics of the Chang 7 member source rocks

The abundance, type, and maturity of organic matter in source rocks are critical factors in evaluating hydrocarbon generation potential31. This study utilizes organic geochemical parameters, such as total organic carbon content (TOC), hydrocarbon generation potential (S1 + S2), maximum pyrolysis temperature (Tmax), and vitrinite reflectance (Ro), to evaluate the OM abundance, type, and thermal maturity of the Chang 7 member source rocks.

The TOC values and hydrocarbon generation potentials of the Chang 7 member source rock samples range from intermediate to excellent in quality. The hydrocarbon generation potential of the Chang 73 sub-member is greater than that of the Chang 71+2 sub-member. In the Chang 71+2 sub-member, the organic matter abundance in the shallow lake facies source rocks is comparable to that in the deep lake facies (Fig. 2). However, in the Chang 73 sub-member, the organic matter abundance in the source rocks is significantly greater in the deep lake facies than in the shallow lake facies.

Fig. 2
figure 2

Crossplots of the TOC and S1 + S2 contents of the Chang 7 sub-member source rocks in the Ordos Basin.

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In addition, the organic matter abundance in shallow lake facies source rocks is generally lower than that in deep lake facies. The deep lake facies source rocks demonstrate higher total organic carbon content and greater hydrocarbon generation potential, placing them overall in the good to excellent quality category. In contrast, the shallow lake facies source rocks exhibit moderate organic richness and hydrocarbon potential, rating fair to good in terms of source rock quality.

In addition to organic matter abundance, organic matter type has a decisive effect on the hydrocarbon generation capacity of source rocks. On the basis of the Tmax and HI crossplots32, the organic matter type in the Chang 73 sub-member of the source rocks is predominantly Type I–II1, whereas that in the Chang 71+2 sub-member is mainly Type II1–II2 (Fig. 3). Moreover, the organic matter type in the deep lake facies source rocks is mostly Type I–II1, and in the shallow lake facies source rocks, it is Type II1–II2 (Fig. 4). Moreover, the relative contents of C27–C29 regular steranes are effective parameters of organic matter origin33. The integration of sterane distribution patterns with bulk organic matter compositions (Fig. 5), together with well correlation cross-sections from Y56, Z40, and JH-4 wells (Fig. 1), reveals depositional environment-mediated molecular fractionation. The stratigraphic continuity demonstrated by these wells clearly illustrates that samples from shallower facies at the Y56 area predominantly contain algal organic matter, while samples from the deeper facies at Z40 show a stronger terrestrial influence. The Chang 7 member source rocks contain hybrid organic inputs from lower-level aquatic organisms and terrestrial higher-level plants with stratigraphic differentiation: the Chang 73 sub-member has an increased algal contribution, whereas the shallow intervals present elevated terrestrial signals. This fractionation pattern correlates with redox-controlled diagenesis; suboxic conditions in shallow lacustrine environments promote the oxidative degradation of labile algal components, preferentially preserving resistant terrestrial-derived C27 steranes. Conversely, anoxic deep-water settings in the Chang 73 sub-member enhance the preservation of algal-derived sterols that preferentially isomerize to C29 configurations during early diagenetic sulfurization processes. The observed disparity in the sterane profiles can quantitatively constrain the paleoenvironmental gradients depicted in Fig. 5, confirming depth-dependent organic matter partitioning within the lacustrine system.

OM abundance and type are fundamental for assessing the quality of source rocks, but a certain degree of thermal evolution is required for organic matter to undergo hydrocarbon generation. In this study, the maturity of organic matter is compared using vitrinite reflectance (Ro), pyrolysis Tmax, and biomarker parameters. In the Ordos Basin, the Ro values for shallow lake facies Chang 7 member source rocks range from 0.53 to 0.8%, while deep lake facies source rocks exhibit Ro values mainly between 0.6% and 1.3%, indicating that they are at mature stages, according to data reported by Zheng and Zhang et al.34,35,36,37,38. The Tmax values for shallow lake facies source rocks are between 431 and 452 °C, and those for deep lake facies Chang 7 member source rocks range from 436 to 446 °C, which indicates that they are in mature stages39.

Fig. 3
figure 3

Organic matter type distributions of the Chang 7 sub-member source rocks in the Ordos Basin.

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Fig. 4
figure 4

Relationships between Pr/nC17 and Ph/nC18 of the Chang 7 sub-member source rocks across different sedimentary facies in the study area.

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Fig. 5
figure 5

Triangular diagram showing the distributions of regular steranes in the shallow lake facies Chang 7 sub-member source rocks in the study area.

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In addition, biomarker characteristics in source rocks reflect the thermal changes in organic matter40. The CPI and OEP values of shallow lake facies source rocks in the Chang 7 member indicate slight thermal alteration, consistent with maturity. Isomerization parameters of hopane compounds (C31 22 S/(S + R) and C32 22 S/(S + R)) and sterane isomerization parameters (C29 ααα 20 S/(20 S + 20R) and C29 αββ/(αββ + ααα)) further confirm that the organic matter in these shallow lake facies is in a mature stage.

(Fig. 6). Similarly, deep lake facies source rocks show CPI, OEP, and sterane isomerization values consistent with maturity, albeit with slightly different parameter ranges41. Overall, both deep and shallow lake facies source rocks are established to be in the mature stage.

Fig. 6
figure 6

Crossplot of geochemical parameters of the Chang 7 sub-member source rocks in the Ordos Basin (a). Crossplot between C29 ααα 20 S/(20 S + 20R) and C29 αββ/(αβ + ααα); ; (b). crossplot between C31 22 S/(S + R) and C32 22 S/(S + R)).

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Differential organic matter enrichment mechanism of the Chang 7 member source rocks

Paleoproductivity

Paleoproductivity refers to the rate at which ancient aquatic organisms fix energy during the energy cycle, i.e., the amount of organic matter produced per unit area per unit time, which plays a crucial role in organic matter enrichment42. Elements such as phosphorus (P) and barium (Ba) are important indicators for reconstructing paleoproductivity. The systematic biomarker evidence further corroborates the facies differentiation, with deep lake environments exhibiting biomarker signatures indicative of enhanced organic matter preservation through limited biodegradation (lower C19TT/C23TT ratios) and sustained anoxic conditions (elevated C24TeT/C26TT values). These biomarker constraints align with and complement the P/Ti-derived productivity patterns.

Paleoproductivity during sediment deposition cannot be directly measured, but it can be inferred indirectly, such as by the concentration of essential nutrient elements preserved in sediments43. Phosphorus (P) is the most important nutrient element for algae growth and plays a significant role in metabolism. To evaluate paleoproductivity in the study area, the influence of terrestrial input must be eliminated, which can be indicated by the P/Ti ratio44,45. In the study area, P/Ti ratios of shallow lake facies source rocks in the Chang 7 member average around 0.5, with the Chang 71+2 sub-member showing slightly higher values than the Chang 73 sub-member. For deep lake facies source rocks, average P/Ti ratios are generally higher, around 0.65, with notable variation between the sub-members. These differences suggest variations in sedimentary or geochemical conditions between facies and sub-members.

Overall, the paleoproductivity of deep lake facies sediments is greater than that of shallow lake facies, especially in the Chang 73 sub-member, where deep lake facies sediments reach eutrophic-hypereutrophic levels, whereas shallow lake facies sediments are only mesotrophic (Fig. 9). However, in some local layers, such as the Chang 71+2 sub-member, the paleoproductivity of shallow lake facies sediments is greater than that of deep lake facies, which may be related to the essential nutrients derived from terrestrial detrital input, as evidenced by the corresponding biomarker shifts in C24TeT/C26TT (0.75–1.73) and C19TT/C23TT (0.05–0.49) ratios within these specific intervals.

Redox conditions

The redox conditions of a water body significantly affect the enrichment of organic matter. In anoxic environments, the degradation of organic matter is limited, which actually promotes the preservation of organic matter. Additionally, the redox nature of the water body not only controls the solubility, differentiation, and precipitation of chemical elements in the water but also records these changes in sedimentary rocks, serving as key clues for interpreting ancient environmental changes. The concentrations of certain elements (such as Mo, U, P, and Fe) in sediments are related to redox conditions at the time of deposition47. Therefore, the enrichment of certain elements and their ratios can be used to reconstruct ancient environments.

The concentrations of Mo and U in sediments are useful redox indicators because the uptake rates of Mo and U increase under more reducing conditions. An EF > 1.0 indicates enrichment relative to the PAAS concentration. Specifically, EF > 3 indicates detectable enrichment, and EF > 10 signifies moderate to strong enrichment. In the study area, UEF values indicate that the redox conditions in the Chang 71+2 sub-member are similar for both deep and shallow lake facies, ranging from weakly oxidizing to weakly reducing. However, in the Chang 73 sub-member, redox conditions differ markedly between facies: deep lake facies are strongly reducing, while shallow lake facies remain weakly oxidizing to weakly reducing.

Additionally, the pristane (Pr) to phytane (Ph) ratio, which is derived from isoprenoid alkanes commonly found in source rocks, can be used to evaluate redox conditions during the deposition of source rocks46. As shown in the TIC chromatogram mass spectra (Fig. 7), the shallow lacustrine facies present relatively high pristane (Pr) and phytane (Ph) concentrations, whereas the deep lacustrine facies present relatively low Pr and Ph values. These findings indicate that the deep lacustrine facies experience stronger reducing conditions than the shallow lacustrine facies. In anoxic and porphyrin- and sulfur-rich environments, the Pr/Ph ratio is usually less than 1, whereas under relatively oxidizing conditions, this ratio exceeds 1. For the high-quality source rocks of the shallow lake facies Chang 7 member (Fig. 8), the average Pr/Ph ratio is about 0.74, indicating weakly oxidizing to weakly reducing conditions. The deep lake facies show a similar average ratio around 0.76, but with more pronounced reducing conditions. As shown in Figs. 7 and 9, although the redox conditions of the deep and shallow lake facies do not differ greatly, the reducing conditions are more pronounced in the deep lake facies.

Fig. 7
figure 7

Chromatogram–mass spectra of source rocks from various sedimentary facies in sub-members of the Chang 7 member in the Ordos Basin.

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Fig. 8
figure 8

Organic geochemical parameter profile of mud shale elements in the Chang 7 member in the Ordos Basin.

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Fig. 9
figure 9

Inorganic geochemical parameter profile of mud shale elements in the Chang 7 member in the Ordos Basin.

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Sedimentary environment

Paleoclimate

Climate affects organic matter accumulation by controlling exogenic processes, terrestrial sediment flux, and biological flourishing47. The relative concentrations of certain trace elements can provide clues for reconstructing the paleoclimate conditions of sediment deposition. Different climatic conditions result in different patterns of element migration and enrichment. Copper (Cu) tends to be enriched in warm and humid climates, whereas strontium (Sr) is enriched in arid and hot climates. Therefore, the ratio of Sr/Cu, which combines a moisture-loving element (Cu) with a dry-loving element (Sr), is a common indicator for climate analysis. A Sr/Cu ratio greater than 10.0 indicates an arid and hot climate, a ratio between 5.0 and 10.0 suggests a semihumid to semiarid climate, and a ratio between 1.3 and 5.0 represents a warm and humid climate48. The Sr/Cu ratios indicate that the Chang 7 member in the study area formed under warm and humid climatic conditions in both shallow and deep lake facies. Among the sub-members, the Chang 73 sub-member reflects the most humid climate, while the climate in the subsequent Chang 71+2 sub-member gradually became drier and hotter.

The chemical index of alteration (CIA) is a parameter used to characterize the intensity of chemical weathering in clastic rocks. During the chemical weathering of upper crustal rocks, potassium feldspar is the primary parent mineral, and alkali metals such as sodium (Na), potassium (K), and calcium (Ca) contained within the crystal lattice are lost in ionic form through surface runoff. High CIA values typically indicate intense chemical weathering conditions, which usually form under warm and humid climate conditions with significant rainfall and continental runoff. CIA values of 50–70, 70–80, and 80–100 indicate weak, moderate, and strong weathering conditions, respectively.

In the study area, CIA values indicate that the shallow lake facies of the Chang 7 member underwent moderate weathering, with the Chang 73 sub-member showing slightly stronger weathering than the Chang 71+2 sub-member. This suggests that the Chang 73 sub-member experienced a more humid climate, while the Chang 71+2 sub-member was deposited under relatively arid and hot conditions. Similarly, for the deep lake facies, moderate weathering is observed overall, with the Chang 73 sub-member again showing somewhat greater weathering intensity than the Chang 71+2 sub-member. These findings imply that the Chang 73 sub-member corresponds to a relatively humid climate, whereas the Chang 71+2 sub-member reflects drier and hotter conditions with lower weathering.

The palynomorph characteristics of parent vegetation provide critical constraints for reconstructing regional paleoclimatic conditions. Two primary sporopollen assemblages have been identified in the Yanchang formation of the Ordos Basin49,50,51,52: the Aratisporites–Punctatisporites assemblage and the Asseretospora–Walchiites assemblage. The Chang 7 member is dominated by the Asseretospora‒Walchiites assemblage, characterized by abundant Duplexisporites and gymnosperm pollen, with a particular abundance of Picea-type pollen grains.

This assemblage configuration suggests warm‒humid climatic conditions during the depositional phase, potentially transitioning toward comparatively arid conditions through temporal evolution. The inferred climatic progression strongly correlates with the environmental signatures reflected in the Sr/Cu ratios and chemical index of alteration (CIA) values obtained in this study, collectively demonstrating coherent hydroclimatic evolution patterns within the lacustrine system.

Terrestrial detrital inputs

The detrital dynamics of the sedimentary system govern organic matter sequestration through coupled depositional–diagenetic processes. Zirconium (Zr) concentrations and MnO/TiO253 ratios serve as dual proxies for detrital provenance differentiation; Zr reflects heavy mineral influx from weathered terrains, whereas MnO/TiO2 tracks oxidative weathering intensities54,55. In the shallow lacustrine systems of the Chang 7 member, Zr concentrations show limited stratigraphic variation between sub-members, whereas MnO/TiO2 ratios reveal distinct facies-related patterns, with higher values in shallow facies compared to deeper ones. Corresponding biomarker evidence, manifested by elevated C26TT/C25TT ratios in shallow environments, supports the presence of hydrodynamic stratification, reflecting enhanced oxidative degradation of organic matter under higher-energy conditions (Fig. 8).

This geochemical dichotomy reflects intensified proximal weathering during the deposition of the Chang 7 member, characterized by increased physical erosion relative to chemical alteration. The observed MnO/TiO2 stratigraphic trends inversely correlate with biological productivity indicators, supporting the inference that high-energy detrital transport delivers bioessential nutrients while maintaining sedimentation rates below thresholds that would dilute organic matter accumulation. This finding further indicates a low sedimentation rate throughout the depositional period of the Chang 7 member. Therefore, the dilution effect of terrestrial detrital influx on organic matter enrichment is limited. The higher detrital input in the shallow lake facies of the Chang 71+2 sub-member provides additional nutrients, promoting increased paleoproductivity, which is in line with the previously analyzed paleoproductivity changes.

Water mass salinity

Water salinity is an essential physical property of sedimentary water bodies that significantly influences biological assemblages and organic matter preservation. Elements such as strontium (Sr) and barium (Ba) are important indicators of water salinity, and the Sr/Ba ratio is used to evaluate salinity55. The Sr/Ba ratio is effective for analyzing the paleosalinity of the Chang 7 member of the study area, given the low carbonate mineral content (generally less than 5%) in the source rocks.

The Sr/Ba ratio effectively distinguishes freshwater, brackish water, and saltwater environments. A Sr/Ba ratio > 0.5 indicates a saline or marine environment; a ratio of 0.5–0.2 indicates a brackish water environment; and a ratio < 0.2 indicates a freshwater environment55. The Sr/Ba ratios indicate a general decrease in water salinity during deposition within the shallow lake facies of the Chang 7 member, characterizing the sedimentary environment as a low-salinity brackish water phase. In contrast, the deep lake facies exhibit relatively higher Sr/Ba values, suggesting that water salinity in these settings was consistently greater than in the shallow lake facies, though still within the brackish water range.

Additionally, molecular biomarkers can be used to assess water salinity in sedimentary environments. Gammacerane can indicate water stratification, often resulting from abnormally high salinity, with high gammacerane contents indicating strongly reducing and high-salinity conditions during organic matter deposition56. As previously discussed, the generally low gammacerane/C30 hopane (Ga) ratios suggest that the shallow lake facies predominantly represent freshwater to brackish water environments (Fig. 7). The high water salinity in the deep lake facies of the Chang 73 sub-member readily leads to water stratification, reducing the bottom water conditions that facilitate organic matter enrichment.

Volcanic activity

Volcanic eruptions release lava and substantial high-temperature gases, generating voluminous volcanic ash that disperses through atmospheric circulation to distal regions57. Volcanic activity not only provides abundant nutrients for primary productivity through the dissolution of pyroclastic materials in depositional water bodies but also introduces toxic volcanic gases. Aqueous dissolution causes the mass mortality of primary producers, whose remains form substantial primitive organic matter—crucial nutrient sources in sedimentary basins58.

Our investigation of core samples from the Chang 7 member source rocks reveals abundant millimeter-scale laminated tuff layers (1–10 mm thickness, maximally 30–40 mm), which are predominantly light gray in color (Fig. 10). These tuffs are primarily distributed in the lower Chang 7 member. In the southern part of the Ordos Basin, contemporaneous source rock intervals contain exceptionally dense tuff sequences with up to 180 distinct layers, reaching maximum thicknesses of 10 cm. These concentrated tuffaceous intervals represent products of frequent volcanic activity during the Chang 7 deposition process.

Additionally, endogenic degassing represents the predominant mercury source in global Hg cycling. Mercury anomalies in sedimentary records are frequently associated with volcanism or hydrothermal processes59. Therefore, Hg concentrations coupled with isotopic signatures provide an effective tracer system for discriminating volcanic-sourced Hg anomalies in stratigraphic sequences and identifying principal pathways for incorporating volcanic Hg into sediments. TOC–Hg crossplots were used to systematically validate volcanic inputs within the deep lacustrine facies of the Chang 73 sub-member (Fig. 10).

Fig. 10
figure 10

(a): TOC–Hg crossplot of deep lacustrine facies in the Chang 7 member. (b): Yishan outcrop section, Tongchuan area, southern Ordos Basin.

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The Chang 7 member volcanic–hydrothermal eruptions in the Ordos Basin were predominantly intermediate to acidic in composition. Characterized by high-intensity, short-duration eruptions, these events exerted limited influence on paleoclimatic conditions while avoiding mass extinctions60. The volcanic-derived nutrients instead stimulated biological proliferation. Simultaneously, volcanic ash coverage on lacustrine surfaces impeded atmospheric gas exchange with water bodies, creating enhanced reducing conditions in lakes.

Differential organic matter enrichment mechanisms of the Chang 7 member source rocks

During the Middle to Late Triassic, the Indosinian movement heavily influenced North China and the Yangtze plates, causing comprehensive collision and orogenesis. This basin‒mountain coupling led to significant uplift in the southwestern part of the basin adjacent to the Qilian–Qinling orogenic belt, providing essential accommodation for the deposition of the Chang 7 member hydrocarbon source rocks.

During the deposition of the Chang 73 sub-member, intense tectonic activity spurs frequent volcanic and hydrothermal activity in the southern basin, dispersing volcanic ash throughout the lake basin via atmospheric circulation (Fig. 11). Because the shallow lake area is far from the tectonic activity, the limited volcanic ash could not cause mass extinctions, but it provided essential nutrients for primary producers, enhancing primary productivity and contributing to the formation of organic-rich hydrocarbon source rocks in shallow lake facies. However, in deep lake areas, intense volcanic activity releases gases such as hydrogen sulfide and sulfur dioxide, leading to the death and burial of primary producers and creating anoxic conditions that preserve organic matter. Simultaneously, the relatively warm and humid climate during the depositional period, combined with the high primary productivity and relatively low terrigenous input, promoted the formation of the Chang 73 sub-member organic matter-rich hydrocarbon source rocks in the deep lake area.

Fig. 11
figure 11

Organic matter enrichment patterns of source rocks in the Chang 71+2 and Chang 73 sub-members of the Yanchang formation in the Ordos Basin. Created using CorelDRAW Graphics Suite 2020 (64-bit; https://www.coreldraw.com/).

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However, in the Chang 71+2 period, as basin tectonic activity decreased, depositional accommodation decreased, resulting in a decrease in water mass depth (Fig. 11). In addition, relatively arid and hot paleoclimate conditions promoted water mass evaporation, decreasing the distribution of anoxic water masses. This phenomenon reduced organic matter preservation efficiency and reduced organic matter abundance in the Chang 71+2 sub-member relative to those in the Chang 73 sub-member. However, relatively arid and hot paleoclimate conditions intensified physical weathering, increasing terrigenous clastic input in shallow lake areas. This increase provided essential nutrients for primary producer explosion, resulting in greater organic matter abundance in shallow lake areas than in deep lake areas in the partial layers of the Chang 71+2 sub-members.

In summary, the differences in sedimentary environments between shallow and deep lake facies during various depositional periods significantly influenced paleoproductivity and organic matter preservation. During the depositional period of the Chang 73 sub-member, sediments in deep lake facies exhibited favorable organic matter preservation conditions, leading to significantly greater organic matter abundance in hydrocarbon source rocks than in shallow lake facies. However, during the Chang 71+2 sub-member period, relatively arid and hot paleoclimate conditions facilitated primary producer explosions in shallow lake facies, resulting in a higher organic matter abundance in partial layers of shallow lake facies sediments than in deep lake facies sediments.

Conclusions

  1. 1.

    (1) The Chang 7 member source rock in Ordos Basin was characterized by high organic matter abundance, mixed organic matter origin, and a low-mature to mature stage, indicating a good-excellent hydrocarbon generation potential. In addition, the overall organic matter abundance of the Chang 73 sub-member source rock in the deep lake area was greater than that in the shallow lake area, whereas in the Chang 71+2 sub-member, the organic matter abundance of the shallow lake area source rock was higher than that of the deep lake area in some layers.

  2. 2.

    (2) The warm, humid paleoclimate during Chang 71+2 sub-member deposition promoted intense chemical weathering, elevating terrestrial siliciclastic flux. Although terrestrial siliciclastic input promoted oxic conditions in the watermass—generally unfavorable for organic matter preservation—it simultaneously delivered critical nutrients that enhanced primary productivity. This detrital input acted as a vector for nutrient fertilization (e.g., P, Fe), triggering eutrophication and sustained primary producer blooms in the receiving watermass. Thus, the Chang 71+2 sub-member source rock in shallow lake facies shows higher organic matter abundance than that in deep lake facies.

  3. 3.

    (3) The Ordos Basin was predominantly influenced by tectonic collision and uplift associated with Indosinian movement during the Triassic period, which led to intense volcanic and hydrothermal activities in the deep lake area. In particular, in the Chang 73 sub-member, frequent and intense volcanic eruptions ejected vast quantities of ash that blanketed the water surface, severely restricting atmosphere-water gas exchange. The resulting anoxic to euxinic conditions in the water column promoted exceptional organic matter preservation. In addition, hydrothermal fluids derived from water-rock reactions exhibit high ionic concentrations, often leading to marked watermass salinity increases. However, volcanic ash transported by atmospheric circulation provided essential nutrients for algae blooms instead of leading to anoxic conditions.