Introduction

Black shales form in specific depositional environments or during oceanic anoxic events, which result in oxygen-depleted bottom water and high productivity1. A specific depositional environment can be correlated with the tectonic setting of its depositional basins2. Plate tectonics govern relationships among provenance type, source rocks, and mineral composition3,4,5,6. The distribution of black shale is related to the distribution of continental blocks7. Regional tectonic processes have led to changes in paleogeography and sedimentary environments8. Different tectonic settings affect the accumulation of organic matter by paleogeography and sedimentary environments9.

The tectonic background has a certain controlling effect on sedimentation rate and organic carbon content10. The input and preservation of organic matter are affected by the input of terrigenous debris and the degradation of organic matter11. The distribution of continental blocks (oceanic basins) controls the position of landing blocks and the connectivity of basins12,13. The depositional environment of the Barnett Shale is a foreland located in front of major thrust belts12,14. Due to the late Paleozoic orogeny, organic-rich sediments were deposited in a narrow inland seaway12,13. Anoxia was caused by the poor connection of the basin with the open ocean, which resulted in stagnation and improved preservation of organic matter12,15. Under the influence of the Caledonian movement, the Upper Yangtze Craton Basin evolved into a foreland basin in the Silurian period16. The Sichuan basin is a restricted basin, with a low energy, under-compensated, and anoxic sedimentary environment, which is conducive to the preservation and enrichment of organic matter17,18,19.

In recent years, shale gas exploration has made a breakthrough in the Wulalike Formation in the western of the North China Platform20,21. The Wulalike Formation was deposited during the Late Ordovician Darriwilian to Sandbian. Sediment is characterized by thick layers of shale22. The tectonic setting, lithological composition, and organic geochemical characteristics of this formation are different from those of the well-known Longmaxi Formation in the Sichuan Basin and the Barnett Shale in North America23,24,25. In field exploration and research, it was found that the tectonic background of the western edge of the North China Platform in the Late Ordovician was unstable.

Previous research on the Wulalike Formation has focused mainly on the sedimentary facies, reservoir characteristics, and hydrocarbon generation potential in the western basin26,27,28,29. However, there are relatively few studies on the mechanism of organic matter accumulation in the Wulalike formation under the complex tectonic background. The enrichment of organic matter of the slope facies in Wulalike Formation is controlled by paleoproductivity and redox condition30. However, compared to the deep water shelf facies, the slope facies is far from the tectonic activity zone. Therefore, this study will combine the tectonic background of the Wulalike Formation, based on field exploration and drilling data, analyze the characteristics of the input and preservation of organic matter and the accumulation models of organic matter.

Geological setting

The North China Block is one of the oldest cratons in the world31. The North China Block is bounded by the North Qilian Orogenic Belt to the west and southwest, the Central Asian Orogenic Belt to the north, and the Qinling-Dabie and Sulu High-Pressure Metamorphic Belts to the south and southeast32. The Wulalike Formation is a shale deposited on the western edge of the North China continent (Fig. 1A). The area is bounded by the Alxa continent in the northwest and the North Qilian-Qinling Orogenic Belt in the southwest, with active tectonic activity33.

Fig. 1
figure 1

Location and sedimentary background map of the study area. (A) Location of the major tectonic plates in China (adapted with permission form34. Copyright 2022American Chemical Society.). NCC: North China Craton, SCC: South China Craton, TC: Tarim Craton, QB: Qaidam Block, and QM: Qiangtang Massif. (B) Lithofacies paleogeography map of the Ordovician Wulalikee Ages on the western edge of the North China Platform (modified after22).

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The sedimentary paleogeography of the Wulalike Formation developed slope facies, deep-water shelf facies, and basin facies from east to west22,25,35. On the side near the Ordos-denuded paleoland, the Wulalike Formation is a slope facies with a relatively stable thickness of Graptolites shales, sandwiched by several layers of calcareous breccia36. To the west, the black shale-dominated deep water shelf facies and basin facies developed with the water body deepened, which are a favorable facies zone for the development of hydrocarbon source rocks26. The underlying Kelimoli Formation mainly deposits thinly bedded bioclastic limestone37. It is overlain by the Lashizhong Formation, which is interpreted as a flysch38.

The Wulalike Formation contains two graptolites zones: Didymograptus murchisoni and Nemagraptus gracilis, with conodont zones at the top of Pygodus serrus, Pygodus anserinus, and the bottom of Baltoniodus alobatus, ca. 458 to 456 Ma before present (Fig. 2)36,39.

Fig. 2
figure 2

Characteristics of the lithology and biostratigraphy of the Wulalike Formation in the study area.

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Materials and methods

Samples and testing methods

Forty-four samples were collected from three discontinuous cores (54 m in total: 22 m in W3, 28 m in W2, and 4 m in W1; where W1, W2, and W3 are defined by sedimentary processes) obtained from well R16, located in the northwest of the North China Platform (Fig. 1B). Sampling bypassed the main diagenetic features such as calcite veins and recrystallization sites to reflect the original characteristics. Each sample was analyzed for total organic carbon (TOC), rock pyrolysis, and organic carbon isotope analysis. 23 samples were analyzed for mineral composition by XRD. 27 samples were analyzed for major and trace elements. 15 samples were analyzed by scanning electron microscopy (SEM) and energy spectrum. Each sample was crushed and ground to > 200 mesh for TOC analysis, rock pyrolysis analysis, organic carbon isotope analysis, major and trace element analysis, and XRD analysis.

A 200 mg sample was weighed to remove carbonates with dilute hydrochloric acid (10% HCl). The residual sample was then washed to neutrality by repeated drenching with distilled water. Finally, the samples were dried at 40 °C for 12 h to measure TOC with a LECO CS744 carbon-sulfur analyzer with an analytical accuracy of better than 0.1%. The mineral composition was done using an X’PertPro X-ray diffractometer. Samples of 200 mesh sizes were placed in sample holders and tested with the X-ray diffractometer. Rock pyrolysis analysis was commissioned from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The analysis of major and trace elements was tested and analyzed by the State Key Laboratory of Continental Dynamics. The samples were then made into small rectangular pieces of 1 × 0.5 × 0.5 cm and kept a fresh face was kept sprayed with gold particles to increase their conductivity for SEM.

Data Analysis

The enrichment factors (XEF) of the redox-sensitive trace elements were calculated as follows40,41:

Here, X refers to the concentration of the trace elements of interest, and the (X/Al) average shaleis the ratio between X and Al in the average shale42. XEF> 1.0 or < 1.0 represents the enrichment or depletion of the element, respectively40,42.

$$X_{{EF}} = {\raise0.5ex\hbox{$\scriptstyle {\left( {X/Al} \right)_{{sample}} }$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle {\left( {X/Al} \right)_{{average{\text{ }}shale}} }$}}{\text{ }}$$
(1)

The Corg/P ratio was calculated on a molar basis:

$$C_{{org}} /P = (TOC/{\text{12}})/(P/{\text{3}}0.{\text{97}})$$
(2)

where 12 and 30.97 are the molar weights of TOC and P, respectively. Sediment Corg:P ratios are mostly < 50:1 in oxic–suboxic environments and > 100 in anoxic environments43.

The excess silica content (Siexcess) was adopted to analyze the non-terrestrial silica content and paleo-productivity. The excess silica content can be calculated as follows44,45,46,47:

$$Si_{{excess}} = Si_{S} - \left[ {\left( {Si/Al} \right)_{{PAAS}} \times Al_{S} } \right]$$
(3)

Standard values of chondrites were used to standardize the Wulalike Formation samples in the study area48.

$$X_{N} = {\text{ }}X{\text{ }}/{\text{ }}X_{{chondrite}}$$
(4)

X refers to the concentration of REE. Xchondriterefers to the value of the X element of a chondrite48.

The negative Eu anomaly is expressed by δEu and calculated using the following formula:

$$\delta Eu = Eu/Eu* = {\text{ }}2 \times Eu_{N} /{\text{ }}\left( {Sm_{N} + {\text{ }}Gd_{N} } \right)$$
(5)

Eu is the measured value of Eu, and Eu* is the calculated value at the Eu position obtained by linear interpolation based on the content of elements on both sides of Eu.

The negative Ce anomaly is expressed by δCe and is calculated using the following formula:

$$\delta {\text{Ce}} = {\text{Ce}}/{\text{Ce}}* = {\text{ 2}} \times {\text{Ce}}_{{\text{N}}} /{\text{ }}\left( {{\text{La}}_{{\text{N}}} + {\text{ Pr}}_{{\text{N}}} } \right)$$
(6)

Ce is the measured value of Ce, and Ce* is the calculated value at the Ce position obtained by linear interpolation based on the content of elements on both sides of Ce.

Results

Mineralogy and petrological characteristics

A large amount of pyrite was found in SEM, mainly of two types (Fig. 3). Type I pyrite is framboidal, which is abundant and clustered and is usually distributed along the bedding with intergrowth organic matter. The diameter of the framboidal pyrite is 8 ~ 10 μm. It is developed at the bottom of W2 and W3. Type II pyrite is automorphic with a side length of 15 μm.

Fig. 3
figure 3

SEM images and pyrite energy spectra. (A) Type I pyrite is distributed in clusters; (B) A framboidal pyrite; (C) Type II pyrite ‘floats’ in the shale matrix and has a large diameter; (D) Energy spectrum of type II pyrite.

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The XRD results of analysis of 23 samples show (Fig. 4A) that illite and quartz are the two main minerals in the marine shale of the Wulalike Formation. Illite and quartz content ranges from 39.8% to 59.7% and 23.2% to 47.4%, respectively. Clay minerals are dominated by illite. Detrital minerals, including clay minerals, quartz, and feldspar, are distributed mainly in the study interval. In the study section, W1 and W2 have a high quartz content, approximately 40% in general, and contain thin discontinuous high clay mineral segments. The quartz content of the W3 section is relatively low, at 20%. Quartz and clay minerals have a reverse trend. The contents of the carbonate minerals calcite and dolomite are 3.1%~16.4% and 3.5%~7.2%.

Fig. 4
figure 4

(A) Mineral content of marine shale of the Wulalike Formation. (B) Plot of the major element content of the Wulalike Formation.

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Elemental geochemical characteristics

Major element

The results of major elements (Table S1) show that SiO2, Al2O3, and CaO are the main components of the Wulalike Formation shale in well R16. The SiO2 content ranges from 9.38 to 75.59% with an average of 55.77%. This shows periodic variations and W1 and W2 are significantly higher than W3, and the SiO2 content of W2 is as high as 70% (Fig. 4B). As an effective representative of the total clay content, the concentration of Al2O3 varies from 0.78 to 15.08%, with an average of 10.00%, showing an increasing trend (Fig. 4B). Abrupt changes of the Al2O3 content can also be observed in W1 and W2 (Fig. 5). The CaO content in the shale samples ranges from 0.63 to 31.84% with an average of 9.18%, with higher values in the limestone samples (R-41) (Fig. 4B). The bottom of the W2 section, as well as the W3 section, has higher CaO contents. TFe2O3, K2O, and MgO are the second most abundant oxides with an average of 4.01%, 3.00%, and 2.58%. The contents of other major elements such as Na2O, MnO, P2O5, and TiO2 are generally less than 1%.

Based on the Siexcess formula listed above, the Siexcess content of the samples ranges from 3.63 to 46.38%, with an average of 25.05%. The W1 section has a high Siexcess content, except for one chert sample. The Siexcess in the W2 section shows discontinuous high values, especially in the upper W2 section, where the Siexcess content is as high as 40%. In contrast, the W3 section Siexcess is generally low (Fig. 5).

Trace element

The enrichment factors, widely used for the reconstruction of the paleomarine environment, were considered more reliable proxies than bimetal ratios40,41,49. The enrichment factors of uranium (UEF), molybdenum (MoEF), and vanadium (VEF) are redox-sensitive parameters, and their values in the shale samples of the Wulalike Formation range from 0.75 to 7.5 (average 1.92), 0.08 to 13.76 (average 1.76) and 0.95 to 3.36 (average 1.60). In the studied profile, the three elements are highly enriched at the bottom of the Wulalike Formation and gradually decrease upwards, accompanied by small fluctuations. In the studied profile, the three elements are highly enriched at the bottom of the Wulalike Formation and gradually decrease upwards, accompanied by small fluctuations. The upper part of the Wulalike Formation is deficient in Mo, U, and V, which are slightly higher than the average shale. In the limestone layer of the W3 section (at R-41), U, Mo, and V fluctuate significantly (Fig. 5).

Fig. 5
figure 5

Paleoclimates proxies, paleoproductivity proxies, and redox proxies for wells R16.

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Rare earth element

Rare earth elements (REE) are usually characterized by clay minerals and the lithology of the host rock. In general, the REE content gradually decreases from felsic to medium felsic and then to basic igneous rocks. Significant negative Eu anomalies in the normalized patterns of REE usually indicate that the host rock lithology is feldspathic42or calcium rich. The REE of the Wulalike Formation (Table S2) shows ΣREE values between 77.47 and 248.21 µg/g, with an average of 159.47 µg/g, compared to the average of 204.15 µg/g for the specific shale (North American, European, and Russian shales) values50,51, suggesting a relative deficit of total rare earth elements (ΣREE) in the region. ΣLREE values ranged from 68.05 to 224.647 µg/g, with an average of 139.11 µg/g, and ΣHREE values ranged from 9.42 to 36.21 µg/g, with an average of 20.36 µg/g. L/H values (ΣLREE/ΣHREE) ranged from 4.86 to 9.560 with an average of 6.88, which is comparable to the average shale L/H value of 6.3550. It indicates that the light and heavy rare earth element contents in this area are differentiated to a degree comparable to specific shales.

LaN/YbNcan be used to reflect the degree of differentiation between light and heavy rare earth elements. Standard chondrite values48 were used to standardize the samples of the Wulalike Formation in the study area (Fig. 6), and the results show that the LaN/YbN values in the study area range from 1.22 to 17.16, with an average of 10.93. The curves are inclined to the right, which indicates that it is characterized by the enrichment of light rare earth elements and a deficit of heavy rare earth elements. The values of LaN/SmN range from 1.07 to 6.25, with an average of 3.00; and the values of LaN/SmN range from 1.07 to 6.25, with an average of 3.00. The LaN/SmN values range from 1.07 to 6.25, with an average value of 3.44. The curve has a larger slope in the La-Eu section, indicating a relatively high degree of fractionation of light rare earth elements. Consequently, the GdN/YbN values range from 1.10 to 2.60, with an average value of 1.72. The curve is relatively flat in the Gd-Lu section, indicating a relatively low degree of fractionation of heavy rare earth elements. δEu values range from 0.48 to 1.04, with an average value of 0.55, indicating a negative anomaly of δEu, indicating a negative anomaly of δEu. The δEu values range from 0.48 to 1.04, with an average value of 0.55, which is a negative δEu anomaly. The δCe values range from 0.84 to 1.27, with an average value of 0.98, and there is no obvious peak or valley on the curves, indicating that there is no Ce anomaly.

Fig. 6
figure 6

REE distribution patterns in different layers of the Wulalike Formation. A, B, C: REE distribution mode for sections W1, W2, and W3.

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Organic geochemical characteristics

The TOC of the Wulalike Formation shale ranges from 0.06 wt% to 2.15 wt% (Table S3). Compared to other layers of shale gas production (such as the Wufeng Formation of Upper Ordovician and the Longmaxi Formation of Lower Silurian in southern China), the TOC content of the Wulalike Formation shale is relatively low18,47,52. Within the Wulalike Formation, the distribution of TOC between different layers exhibits significant heterogeneity (Fig. 7). The calcareous shale in W1 shows discontinuous high organic matter intervals with TOC ranging from 0.36 to 2.15% (average 1.39%). The TOC in W2 ranges from 0.07 to 1.42% (average 0.70%). Carbonate-bearing shale TOC in W3 is generally low, with TOC content ranging from 0.05 to 0.58% (average 0.29%). The TOC in the section shows an overall decreasing trend (Fig. 7). The thin-bedded limestone (R-41) at the bottom of the Wulalike Formation and the argillaceous rocks at the upper part of the Wulalike Formation have a low TOC. The δ13Corg ranges from − 32.76‰ to -28.49‰, with an average of -30.85‰.

Rock-Eval pyrolysis (Table S3) shows that the S1 range of the R16 well sample is 0.01 ~ 0.69 mg HC/g rock, and the average value is 0.16 mg HC/g rock. The S2 value ranges from 0.03 to 3.34 mg HC/g rock, and the average value is 0.95 mg HC/g rock. The original hydrocarbon generation (S1 + S2) values range from 0.04 to 3.87 mg HC/g rock, and the average value is 1.12 mg HC/g rock, which is the same as the trend of TOC (Fig. 7). The hydrogen index (HI) values of the Wulalike Formation shale range from 31 to 278 mg HC/g TOC, with an average of 130 mg HC/g TOC. The oxygen index (OI) values of the Wulalike Formation shale range from 5 to 229 mg HC/g TOC, with an average of 52 mg HC/g TOC (Fig. 7). Tmax ranges from 428 to 494 ℃, and PI is in the range of 0.08 to 0.32 mg/g, and the samples are mainly in the maturity stage.

The S1-TOC plot indicates the presence of indigenous hydrocarbons53,54 (Fig. 8A). The relationship between S2 and TOC is a useful indicator for evaluating the hydrocarbon generation potential of source rocks55,56,57. According to the TOC- S2 relationship diagram (Fig. 8B), the W1 section shale has relatively good hydrocarbon generation potential. Rock pyrolysis indices (HI, OI, and Tmax) can classify organic matter types58,59. The types of organic matter in the W1 and W2 sections are predominantly type I-II organic matter, whereas the W3 sections are mainly type III organic matter (Fig. 8C, D). From the Tmax -HI relationship plot (Fig. 8D), it can be seen that the Wularlik Formation shale in the study area is within the oil generation window (430 ~ 455 °C)60,61. The geochemical characteristics of the Wularlik Formation shale are heterogeneous.

Fig. 7
figure 7

Variations of Organic Geochemical Characterization in the Wulalike Formation.

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Fig. 8
figure 8

Organic geochemistry characteristics of the Wulalike shales. (A) Plot of free hydrocarbons (S1) versus total organic carbon (TOC). (B) Pyrolysis hydrocarbons cracked from kerogen (S2) versus TOC plot, showing the potential of the generative source rock (boundary lines modified after62. (C) Hydrogen index (HI) versus oxygen index (OI) of kerogen from Wulalike source rock (boundary lines modified after60. (D) Plot of HI versus the pyrolysis temperature of the maximum pyrolysate yield (Tmax) for the Wulalike source rock, showing the kerogen quality and the thermal maturity stages (boundary lines modified after61.

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Discussion

Tectonic transition in the middle and late ordovician

REE and major elements evidence

The activity continental margin-like REE features occurred in the study area, including enrichment of HREE over LREE, steep LREE and flat HREE, and negative Eu anomaly (Fig. 9A). The Middle Ordovician, on the other hand, mainly shows the characteristics of a passive margin (Fig. 9A, the data come from63).

The SiO2 content and the K2O/Na2O ratio have special chemical characteristics in sedimentary rocks with different tectonic backgrounds. In the tectonic background diagram (Fig. 9B), the Middle Ordovician is a passive marginal background (the data comes from64), while the late Ordovician the Wulalike Formation is mainly set against the background of island arcs and active continental margins. The patterns of REE and major elements together reflect the shift of the tectonic background during the middle-late Ordovician.

Fig. 9
figure 9

Comparison of the REE distribution pattern maps of the study area with the distribution pattern maps of different tectonic backgrounds (modified from4); B: Schematic diagram of the discrimination of the structural background of shale in the Wulalike Formation (modified from65). PM: passive margin; ACM: active continental margin; OIA: oceanic island arc; CIA: continental island arc; ARC: oceanic island arc margin.

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Sedimentary events

The active continental margin is embodied in a series of sedimentary events on the outcrop. Starting from the Sambian, multiple sets of interlayers of tuff were deposited on the western and southern edges of the basin (Fig. 10A, B). The nutrients released by volcanic ash promote the proliferation of plankton and algae, leading to an increase in marine productivity66,67. The ecological damage caused by volcanic activity leads to the death of a large number of organisms, which becomes another source of sedimentary organic matter68. The biological eruption caused by volcanic activity consumes a large amount of dissolved oxygen, exacerbating the degree of hypoxia of water66,67. The coverage of volcanic ash physically isolates oxygen, further increasing the reducibility of water and aiding in the preservation of organic matter. The volcanic ash in the study area is distributed in the lower section of the Wulalike Formation, and the presence of volcanic ash helps to better preserve the organic matter that has already been deposited in the early stage.

Gravity flow (Fig. 10C, D) sedimentation develops in the upper Ordovician Sangbian stage in 3 profiles (in Fig. 1B, ourcrop Laoshidan, well E7, and well E29) along the western edges of the basin. Gravity flow sedimentation occurred after volcanic activity on the Laoshidan outcrop. The presence of frequent gravity flow deposits during this period indicates active tectonic activity69,70,71,72,73.

Fig. 10
figure 10

Field photos of the Wulalike Formation. A: The yellow dashed line outlines the tuffaceous layer in the shale on the Laoshidan outcrop. B: The red arrow points to a tuffaceous layer in the shale on the Guanzhuang outcrop; C: Slump breccia in gravity flow sedimentation on the Laoshidan outcrop; D: Turbidity sedimentation in gravity flow sedimentation on the Laoshidan outcrop.

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Ordovician tectonic transition in the western North China Craton

From the late Neoproterozoic to the Cambrian, there was a rift between the Alxa Block and the western edge of the North China Block74. At this time, The North Qilian Ocean in the southwest experienced an extensional tectonic system from the Cambrian to the Early Ordovician, with a high expansion rate. The interaction between blocks forms passive continental margins at the western edge of the North China Platform.

The tectonic activity in this region reached its peak in the Middle Ordovician and began to undergo a transition from divergence to convergence31,74,75,76,77. When the tectonic system transitions from extensional to convergent, the oceanic crust begins to subduct towards the continental crust78. The friction generated by subduction leads to the melting of the oceanic crust and induces magmatic activity along some deep faults, resulting in the formation of volcanic island arcs. At the same time, the upwelling of the asthenosphere and the occurrence of thermal convection also cause the heating of the continental crust. This reduces the thickness of the continental crust and creates a series of normal faults in a tensile environment. The continuous activity of these normal faults, combined with regional subsidence, formed a foreland basin with steep slopes at the western edges of the basin. This formed a trench/island arc/back-arc basin system.

During the Late Ordovician to Early Silurian, the North Qilian Ocean gradually closed under the subduction of the oceanic crust79,80,81. Under enormous tectonic loads, the lithosphere flexed and caused significant subsidence at the proximal end, causing the western and southern edges of the basin to sink, the carbonate platform to retreat, and the sedimentary facies to suddenly deepen, resulting in a combination of slope, shelf, and basin. Landslides, mudslides, and turbidity currents may form on these slopes. At the same time, an uplift occurred at the far end of the basin, and carbonate platform sedimentation terminated38,76.

Input characteristics of organic matter

Type of organic microscopic components

The organic microscopic components of the Wulalike Formation are composed of biodetritus and algae. The abundance of the graptolite epidermis is typical of this formation. Observing hand specimens and an optical microscope, the graptolite epidermis is the main organic component in the rich graptolite shale of the Wulalike Formation82. A large amount of silicon (Fig. 11A, B, C) and carbonaceous biological structure (Fig. 11D, E, F) are important sources of biogenic silicon and organic carbon in the study area.

Fig. 11
figure 11

Biological structure and energy spectrum under SEM. (A) Siliceous algae; (B) Microsomes; (C) Energy spectrum of microsomes; (D) Chlorella vulgaris; (E) Bacterial and algal bodies; (F) Energy spectrum of bacterial and algal bodies. Hydrocarbon-forming organisms refer to some organisms in the biota that can contribute to the generation of hydrocarbons83. Algae were the most important

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Hydrocarbon-forming organisms refer to some organisms in the biota that can contribute to the generation of hydrocarbons83. Algae were the most important hydrocarbon generating organisms in the early Paleozoic84. Algae with different living habits exhibit significant differences in hydrocarbon generation and peak periods85. The characteristics of organic carbon isotopes of source rocks vary among different hydrocarbon-generating biological combinations. Previous studies have shown that hydrocarbon source rocks are mainly composed of benthic algae, δ13Corgis less than − 34‰; Source rocks are mainly composed of planktonic algae, δ13Corg is heavier than − 30‰84. The Wulalike Formation is mainly composed of planktonic algae and benthic algae, and some rocks with high carbonate mineral content are mainly composed of planktonic algae (Fig. 12A). In summary, the isotope proxy indicated by organic carbon aligns with the organic maceral compositions.

Fig. 12
figure 12

A: Cross-graphs of TOC vs. δ13Corg show the type of hydrocarbon-generating organisms of the Wulalike Formation. B: Cross plots of TOC vs. Siexcess of the black shale of the Wulalike Formation show the relationship between paleoproductivity. C, D, E, F: Cross-plots between Corg/P, VEF, UEF, and MoEF exhibit their different geochemical behavior in a redox environment. G, H, I: The cross-plots of VEF, MoEF, and UEF versus TOC show the relationship between enrichment factors and organic matter accumulation.

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TOC and δ13Corgshow a negative correlation. The reason is that planktonic algae have a high rate of conversion of hydrocarbons. More benthic algae remained in the TOC. The higher the residual TOC, the greater the proportion of residual benthic algae, and the lighter the carbon isotope. According to Pingliang Formation, the δ13C of shale oil is around − 32‰, and both floating algae and benthic algae contribute to the hydrocarbons.

The main biodetritus (graptolite epidermis) in this layer has the characteristics of hydrogen deficiency, high residual organic carbon, and high aromaticity. Its oil generation ability is poor, and its gas generation ability in the high overmature stage is approximately 20% of that of floating algae86,87,88. The organic geochemical characteristics show that the thermal evolution stage of the research area is in the oil generation window. Therefore, the hydrocarbon-generating organisms of the Wulalike Formation are mainly algae, and the contribution of animal debris is limited.

Paleoproductivity

Biogenic Si is considered a useful parameter for estimating paleoproductivity40,44,89,90. The Al-Fe-Mn diagram (Fig. 13) shows that all samples in the study area fall within the biogenic zone. The Si/(Si + Al + Fe) and Al/(Al + Fe + Mn) ratios of biogenic Si are typically higher than 0.90 and 0.60, respectively90,91,92,93. The Al/(Al + Fe + Mn) ratio of hydrothermal Si is usually less than 0.30. Sediments with low Al/(Al + Fe + Mn) ratios typically reflect stronger hydrothermal activity90,91,92,93. The range of Si/(Si + Al + Fe) values in the study area is 0.65 to 0.83, with an average of 0.75. This indicates that the siliceous components in black shale are almost biogenic and can be used for productivity evaluation.

Fig. 13
figure 13

Al–Fe–Mn ternary plot showing the siliceous origin of samples from the black shale of the Wulalike Formation (Base map was modified from91).

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The study on the relationship between Siexcess and TOC (Fig. 12B) shows a positive correlation between TOC and Siexcess in W1, while there is no significant correlation between TOC and Siexcess in W2 and W3. The Siexcess of the W2 section is significantly higher than that of the W3 section. This indicates that the main factor controlling TOC changes in the W2 and W3 sections may not be productivity. Productivity during the W2 stage is significantly higher than during the W3 stage. The TOC in the W1 section of the shale deposits is influenced by the primary productivity of marine animals, and the productivity is higher than in the W2 and W3 stages.

Terrigenous input

The organic matter content within sediments can be diluted by terrigenous input10. The Mn/Ca ratio increases with increasing terrestrial input94. This ratio was used to estimate the terrestrial input rate for the Wulalike Formation94,95 (Fig. 5). The Mn/Ca (ppm/%) between different sections of the Wulalike Formation is shown in Fig. 5. The W1 section has the lowest terrigenous input rate and can also reflect its fastest sedimentation rate. The terrigenous input rate in section W3 is moderate. Compared to W1 and W3, the terrigenous input rate of the W2 section is the highest, with great variation.

Sedimentation rate

Due to the dilution of the terrigenous input, the TOC decreases with an increasing sedimentation rate. The Na/K ratio indicates the rate of sediment accumulation93. A slow rate of sedimentation allows more time for the Na/K ratio to be reduced by cation exchange96. The increase in the Na/K ratio is due to the degradation of illite during the slow accumulation of sediments94,97. In the Wulalike Formation, the difference in Na/K ratio reflects the instability of the sedimentation rate. The deposition rate of the W1 stage and the W3 stage is fast, while the deposition rate of the W2 stage is relatively slow. The REE distribution patterns also support the same results. The distribution of REE in the W1 and W3 phases is concentrated, which indicates a single source and fast sedimentation rate. The W2 phase distribution pattern is dispersed, indicating the diversity of provenance and indirectly indicating the slow sedimentation rate.

Organic matter preservation conditions

Organic matter degradation

The sedimentation rate not only affects the input of organic matter, but also controls the preservation of organic matter by controlling the degradation process of organic matter10. Generally, when the critical value of the sedimentation rate is reached, due to the dilution of the clastic input, TOC decreases with the increase in the deposition rate. However, with an increase in the sedimentation rate, the sediment passes through the near surface area where microorganisms degrade strongly, and the efficiency of microbial degradation is decreasing11. Therefore, the hydrocarbon-generating potential of the hydrocarbon-generating parent materials continues to increase with an increase in the deposition rate. This may explain why the sedimentation rate of the W3 section in the study area is high, but the TOC is high. The dilution effect of terrestrial input is less than that of reducing microbial degradation, and more organic matter is preserved.

In the Wulalike Formation, there is a weak positive correlation between TOC and quartz content (Fig. 14). There is a weak negative correlation between TOC and the mineral content of carbonate (Fig. 14). There are differences in the diffusion limits of O2 and SO4 and the vertical limits of the oxidation zone and the sulfate reduction zone among different mineral compositions. Siliceous sediments contain more silty quartz and diatoms, whereas fine calcareous sediments contain more clay-sized nannofossils. The solubility of SiO2 in near-surface sediments is lower than that of CaCO3, and the permeability of siliceous sediments is lower than that of calcareous sediments10. The aerobic zone and the sulfate reduction zone in siliceous sediments are less distributed in the vertical direction and retain more organic matter.

Fig. 14
figure 14

Cross-plots of TOC vs. quartz content (A) and carbonate mineral content (B).

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Redox conditions

Analysis of redox conditions is a standard feature in paleoenvironmental studies. The enrichment factors, considered to be more reliable indicators than bimetallic ratios, have been widely used to reconstruct the palaeoceanic environment40,41,98. In this study, the UEF, VEF, and MoEF showed different accumulation rates due to different chemical behaviors under different redox conditions (Fig. 12D, E, F), which indicated that they are redox-sensitive parameters of black shale40,41,99. In the study section, UEF, VEF, and MoEF profiles of black shale show a gradual downward trend, and their changing trends are the same. This shows that the reducibility is strongest at the bottom of the Wulalike Formation and weakens upward.

Redox has a significant impact on the cycle of phosphorus in sediments100. P in organisms is transported to sediments in the form of organic matter, and the redox conditions of water determine the ultimate fate of P in the remineralized organic matter43. An anoxic environment is more conducive to the preservation of organic matter and the loss of remineralized organic P. The redox parameter Corg/P (Fig. 6C) shows a trend similar to UEF, VEF, and MoEF, with W1 being an anoxic-suboxic, W2 being a suboxic-oxic, and W3 being oxidative environment.

The variation profiles of UEF, VEF, MoEF, and Corg/P (Fig. 6) show the gradual evolution of the Wulalike period from a suboxic to an oxic. Anoxics are likely to form in environments with high sea levels. The sea level during the Wulalike sedimentation period showed a gradually decreasing trend37, which is usually accompanied by a weakening of the reducibility. Additionally, oxygen may increase with increasing terrestrial input. In addition, the study area is located in a shelf facies wider than the slope facies, which may also be one reason for the lower degree of restoration in the study area.

There is a positive correlation between the redox-sensitive indicators and TOC (Fig. 12C, G, H, I), indicating that the enrichment of organic carbon in the Wulalike Formation is mainly constrained by the redox environment. It is obvious that the limestone sample (R-41) in the figure jumps out of the trend. The reason for this phenomenon can be explained by the lower solubility of SiO2 in near-surface sediments compared to CaCO3, and the lower percentage of clay-sized particles (i.e., siliceous sediments contain more silt-grade quartz and diatoms, while fine-grained calcareous sediments contain more clay-sized ultrafine fossils). Therefore, the permeability of siliceous sediments is lower than that of calcareous sediments10, which hinders the entry of oxygen into water bodies. Furthermore, the aerobic oxidation zone and the sulfate reduction zone in siliceous sediments are less distributed vertically, retaining more organic matter.

Degree of restriction

The MoEF-UEF cross plot helps to analyze paleomarine environments1,98,101. This distribution pattern in the study area is consistent with the current Mo-U enrichment characteristics of the eastern tropical Pacific, which manifests itself as a larger UEF(the Mo/U value is about 0.1~0.3×SW) at low EFs while exhibiting a larger MoEF under high EFs (Mo/U value >1 × SW)98. This enrichment trend is a typical enrichment feature of the “open sea”. It can be seen from Fig. 15 that the period W1 is a suboxic to anoxic, and the period W2 is an oxic to a suboxic. However, Mo is not enriched in the W3 section, which indicates that the transition from a suboxic to an oxygen-containing environment hinders the enrichment of Mo and U, especially elemental Mo. The Sr/Ba ratio, as an indicator of paleosalinity, is usually used in paleoenvironment research102. In this study, the Sr/Ba ratios in the different sections increase from the bottom to the top (the mean of W1 is 0.36, the mean of W2 is 0.79, and the mean of W3 is 1.36). This indicates a transition from a relatively closed low salinity environment to open sea sedimentation with normal salinity. This corresponds to the redox conditions.

The restricted bottom water, which is more likely to form a reducing environment, is favourable to the preservation of organic matter1. When Sr/Ba>1.5, the TOC were all less than 0.5%; when 1.5 ≥ Sr/Ba>0.5, the TOC were mainly distributed in 0 ~ 1.1%; and when Sr/Ba ≤ 0.5, the TOC were distributed in 0.5 ~ 2.15 (Fig. 16). Therefore, the restriction of the water column was a necessary condition for organic matter accumulation. As the bottom water openness increased, the exchange of oxygen and nutrients between the study area and the outer sea increased. The increase in salinity of the water body was accompanied by an increase in the degree of oxidation of the water body, which was not favourable for the preservation of organic matter.

Fig. 15
figure 15

Cross plot of MoEF versus UEF of black shale in Wells showing the changes in the palemarine environment of different sedimentary stages. The base figure was modified from95.

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Fig. 16
figure 16

Cross plot of TOC versus Sr/Ba.

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Models of organic matter accumulation in the Wulalike formation

The tectonic setting controls the enrichment of organic matter in marine shale through the sedimentary environment. High quality source rocks are formed in foreland basins under active continental margins. High-quality marine shales are developed in both North America and southern China (Table S4). Organic-rich shale is developed with low sedimentation rates and restriction degree controlled by tectonic settings.

Within the Wulalike Formation, the deposition process can be divided into the following three stages:

During the W1 period (Fig. 17), the level of terrestrial input was relatively low. The productivity level was relatively high during this period. Although the sedimentation rate was high, the low terrestrial input prevented organic matter from diluting and accelerated its passage through the biodegradation zone, resulting in better preservation. The fast sedimentation rate, less terrestrial input, and high level of productivity ensured the abundance of original organic matter during this period. The anoxic-suboxic environment is conducive to better preservation of hydrocarbon-generating organic matter such as algae. Therefore, primitive organic matter was preserved during this period and transformed into sedimentary organic matter. In addition, the enrichment of TOC is positively correlated with the degree of redox and the productivity level. This indicates that the enrichment of organic carbon during this period is controlled by both preservation conditions and paleo-productivity.

The ancient productivity in the W2 period (Fig. 17) remains at a relatively high level. Strong tectonic activity led to an alternating increase in terrestrial inputs, resulting in the deposition of a set of lithofacies combinations with cyclic variations in carbonate content and terrestrial input content. Meanwhile, the sedimentation rate during this period was slower than that of the W1 period, but still higher than the equilibrium value between organic matter dilution by terrestrial debris and organic matter degradation in siliceous shale. At this time, the marine environment gradually changes from a suboxic environment to an oxic environment. The original organic matter is not as well preserved. TOC is positively correlated with the degree of redox, but there is no significant correlation with the level of paleoproductivity. The advantage of productivity is weakened by an increase in the oxygen content. Although the productivity level is high at this time, organic matter is also oxidized and decomposed due to poor storage conditions. The enrichment of organic matter in this stage is mainly controlled by preservation conditions.

Fig. 17
figure 17

Evolution of the geological environment and sedimentary models of the Wulalike Formation.

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In the W3 period (Fig. 17) the terrestrial input in the study area remains at a relatively high level. The paleoproductivity has also been maintained at a relatively low level. During this period, the sedimentation rate was the highest, and the high terrestrial input at high sedimentation rates diluted the organic matter. At the same time, the sea level decreased and seawater became shallower, causing the redox environment to become an oxic environment, which is not conducive to the preservation of organic matter. The unfavorable storage environment worsened the organic carbon content during this period. The level of paleoproductivity and preservation conditions jointly determine the low TOC during this period.

The organic matter enrichment model on the western edge of the North China Platform is a model controlled by regional tectonics, dominated by sedimentation rate, and preservation conditions, supplemented by primary productivity levels. The late Ordovician transgression reached its high tide. Starting from the early stages of Wulalike, it gradually retreated. The tectonic background transition caused lithospheric bending and the water column in the study area deepened abruptly, but remained connected to the outer sea and did not create restriction conditions. Sedimentation rates were erratic and terrestrial inputs were diverse. The “open sea” environment and high overall sedimentation rate lead to dilution of organic matter, resulting in a low TOC in the entire Wulalike Formation. The differences in terrestrial dilution and preservation conditions between different layers result in the best source rocks in the W1 segment.

Conclusions

The Wulalike Formation was formed during a tectonic Transition from a passive continental margin to an active continental margin. The tectonic background during the sedimentation period of the Wulalike Formation is the active continental margin.

The organic microscopic components are made up of biodetritus and algae. Hydrocarbon-generating organisms are mainly algae, and the contribution of animal debris is limited. The level of paleoproductivity gradually decreases. The Wulalike sedimentation period is characterized by high sedimentation rates and increasing terrestrial inputs. Typical open-sea features make the basin poorly restricted. The preservation conditions of organic matter are gradually disrupted by increasing oxygen content. The changes in the sedimentary environment have led to the vertical differential enrichment of organic carbon in the Wulalike Formation. The high sedimentation rate and open water body dilute the overall organic matter of the Wulalike Formation, exhibiting low TOC characteristics.

Organic-rich shale is developed with low sedimentation rates and reducing conditions controlled by tectonic settings.The organic matter enrichment model of the Wulalike Formation is a model controlled by regional tectonics, dominated by sedimentation rate and preservation conditions, supplemented by primary productivity levels.