1 Introduction

The Wufeng–Longmaxi Formation shale was deposited in a stable depositional environment, with good shale reservoir quality and stable distribution in the southern Sichuan Basin (Ma and Xie 2018). In addition, good shale gas preservation conditions due to the relatively small-scale faults developed in the South Sichuan Basin (Li et al. 2016), leading a significant potential for shale gas exploration in this area. Organic matter has always been the focus of scholarly research, because it is an important factor that controls shale gas enrichment. There are many factors affecting organic matter accumulation in the shale, but scholars have been in the debate of “primary productivity model” and “preservation model” (Demaison and Moore 1980; Pedersen et al. 1990; Ingall et al. 1993; Arthur and Sageman 1994). According to the current research, the organic matter accumulation model of marine shale can be mainly attributed to the ‘preservation model’ (Demaison and Moore 1980), ‘primary productivity model’ (Pedersen et al. 1990; Ingall et al. 1993; Arthur and Sageman 1994) and ‘productivity–preservation condition cooperative control model’ (Sageman et al. 2003; Mort et al. 2007).

Recent studies have also shown that sea level changes (Li et al. 2019; Liu et al. 2019a, b; Wang et al. 2019a, b; Yan et al. 2019), deposition rate and deposition processes (Loucks and Ruppel 2007; Chen et al. 2020), input of terrigenous debris (Bohacs et al. 2005; Li et al. 2017), tectonic activities (Tang et al. 2020; Han et al. 2021), climate change (Bohacs et al. 2012; Zhang et al. 2019a, b; Lu et al. 2019; Tang et al. 2020) and other environmental factors affect organic matter enrichment in shale. Therefore, the formation of high-TOC shale is controlled not by a single factor, but by a combination of multiple factors (Sageman et al. 2003; Mort et al. 2007; Lash and Blood 2014; Zeng et al. 2015; Li et al. 2017). Scholars generally believe that anoxic bottom water conditions (He et al. 2019; Ma et al. 2019; Jeon et al. 2020; Ji et al. 2020) and high productivity (Yan et al. 2008, 2019; Liang et al. 2018; Jin et al. 2020) are the two most important factors controlling organic matter enrichment in shale. Many scholars have researched organic matter enrichment and its controlling factors in the Wufeng–Longmaxi Formation shale in the Sichuan Basin and the adjacent area (Zou et al. 2015a; Jin et al. 2016; He et al. 2016; Qiu et al. 2020). It is generally believed that the redox condition is the key factor that controls organic matter accumulation in the Wufeng–Longmaxi Formation shale. Thus, a ‘preservation model’ is proposed for the organic matter enrichment (Li et al. 2017, 2019; He et al. 2019). However, the high-quality shale section of the Wufeng-Long11 submember of the Longmaxi Formation (S1l11) is often treated as a whole in most previous studies, and it is unclear how each controlling factor affects organic matter enrichment in different sublayers. Therefore, it is necessary to conduct a detailed study on organic matter accumulation and its controlling factors in each sublayer in a high-quality shale section (S1l11).

2 Geological setting

During the Late Ordovician-Early Silurian period, the Upper Yangtze Plate was compressed due to the strong Caledonian movement (Li et al. 2017; He et al. 2019). Affected by the Guangxi movement during the Late Ordovician, the collision and integration of the Cathaysian and Yangtze blocks was slowed down. Surrounded by the Chengdu Uplift in the northwest, Dianqian Uplifts in the south and Jiangnan-Xuefeng Uplifts in the southeast, and the Upper Yangtze Sea evolved into a generally restricted enclosed basin (Zou et al. 2015b) (Fig. 1A). This paleogeographic pattern was inherited during the early Silurian Longmaxi Formation period. At the same time, the combination of rapid sea level rise caused by the melting of the Hernantian Antarctic ice sheet led to a low-energy, anoxic, and under-compensated semi-restrict sedimentary environment covered most areas of the Upper Yangtze area, resulting in a set of black organic-rich black shale widely deposited in the Upper Yangtze area (Zou et al. 2015b; Zhang et al. 2016; Ma and Xie 2018) (Fig. 1B).

Fig. 1
figure 1

A Paleogeographic map of Yangtze Platform in the Late Ordovician—Early Silurian Period (After Chen et al. 2004 and Zhou et al. 2015); B Shale thickness distribution of Wufeng-Longmaxi Formation and location map of the study area (After Guo 2013; Li et al. 2017); C The stratigraphic column of the Wufeng—Longmaxi Formation of Well X2 in Changning Area (from Ma et al. 2020; Shi et al. 2022)

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During the Longmaxi Formation period, the Changning area was located in the subsidence center of the Southern Sichuan Basin, with organic-rich shale being well developed in the Long11 submember of the Longmaxi Formation (S1l11) (Fig. 1C). This shale was characterized by a large thickness, stable distribution, high-TOC content (Wang et al. 2016; Feng et al. 2016), high thermal maturity (Dong et al. 2015; Gao 2016; Liu et al. 2021), high brittle mineral content, and well-developed pores and fractures. These characteristics reveal that this shale has a good accumulation and storage conditions for shale gas enrichment (Zou et al. 2015a), which is the most important layer for shale gas exploration in the study area (Fig. 1C). Previous studies have shown that high-quality shale reservoirs are developed mainly in the Long1 member (S1l1), which can be divided into two submembers (S1l11 and S1l12). S1l11 is the most favorable section for shale gas exploitation in the Longmaxi Formation (Wang et al. 2016; Zou et al. 2015a, 2016; Jiang et al. 2020).

S1l11 is further subdivided into 4 sections: S1l11–1, S1l11–2, S1l11–3, and S1l11–4 (Ma et al. 2020; Shi et al. 2022). S1l11–1 is characterized by well-developed carbonaceous siliceous shale, high-TOC, high gas contents and high natural gamma ray (GR) values, with a thickness of 1–4 m. S1l11–2 is dominated by carbonaceous shale, with stable TOC and gas contents, that is lower than those of S1l11–1 and S1l11–3. The GR values of this section are low and flat. The thickness of this section is generally between 4 and 12 m. Carbonaceous shale or siliceous shale is well developed in the S1l11–3, with well logs showing a high acoustic time difference, low density, and GR with an average value of approximately 210 API. The thickness of this section is between 2 and 10 m. S1l11–4 is mainly composed of shale and silty shale, with a low TOC contents and gas contents. The GR values of this section are low and show a box-shaped pattern. The thickness of this section ranges from 10 to 20 m (Zhao et al. 2016) (Fig. 1C).

3 Materials and methods

3.1 Samples

The TOC content and major-trace elements of the Longmaxi Formation shale in the Changning area were systematically studied. Eighty-eight shale samples of S1l1 were collected from three drilling wells in the Changning area (well X1 in the northwest of the Changning area, well X2 in the middle, and well X3 in the southeast of the Changning area). In addition, geochemical data (TOC and major-trace elements) from 10 shale gas wells in the study area were also collected for analysis.

3.2 TOC test

The collected fresh samples were washed with deionized water, dried, and then pulverized to powders of 200 mesh size for experimental analysis. The TOC content was tested using the acid dissolution method. The samples were dissolved in hydrochloric acid at 4 mol/L(M) for half a day, deionized water was then added, and the samples were centrifuged for separation. After drying, the samples with the inorganic carbon removed were tested using a CS230 carbon and sulfur analyzer. Finally, the TOC content was calculated.

3.3 Major element and trace element tests

Major elements were tested by X-ray fluorescence (XRF) spectrometry. The samples were burned at a high temperature (920 ℃) to remove organic matter and carbonate. Subsequently, 0.5 g of the burned sample was weighed and mixed with 4 g of Li2B2O7 evenly, and 1–2 drops of cosolvent were added, poured into a platinum crucible for melting at 1150 °C, and then examined by XRF.

The trace element testing instrument was an inductively coupled plasma mass spectrometer (ICP‒MS). First, the samples were heated to remove the organic matter, 0.1 g of the sample was weighed, and HNO3 + HF + HClO4 were added to a specific container for sealing at high temperatures (140–150 ℃) to dissolve the sample (24 h). The sealed container was then opened and placed on a heating plate for high high-temperature heating to remove the sample-dissolving acid. Finally, HNO3 was added to the container to redissolve until the solution was completely clarified. The clarified solution was transferred to a volumetric flask and made up to a constant volume for testing. All the above experiments were conducted at the Analysis and Experiment Center of the Exploration and Development Research Institute of PetroChina Southwest Oil and Gas Field Company.

3.4 Data processing

Only the authigenic composition of trace elements in shale can reflect the characteristics of the paleosedimentary environment and the changes in chemical composition in seawater during geological history (Pi et al. 2011). The content of trace elements in the samples with Al was used to eliminate the interference of terrigenous components on autogenous trace elements and reduce the influence of carbonate on the dilution degree of sediments (Calvert and Pedersen 1993; Tribovillard et al. 2007).

The concentration of biogenic barium (Babio) was calculated using the following equation according to Murray and Leinen (1996):

$${\text{W}}\left( {{\text{Ba}}_{{{\mathrm{bio}}}} } \right) = {\text{W}}({\mathrm{Ba}}_{{{\mathrm{total}}}} ){-}{\mathrm{W}}\left( {{\text{Al}}_{{{\mathrm{total}}}} } \right) \times \left( {{\text{Ba}}/{\mathrm{Al}}} \right)_{{{\mathrm{PAAS}}}}$$

W(Batotal) is the mass fraction of Ba in shale samples; W(Altotal) is the mass fraction of Al2O3 in shale samples; (Ba/Al) PAAS is the ratio of Ba content to Al2O3 content in post-Archean Australian average shale, which is 0.0077 (Taylor and Mclennan 1985).

4 Results

4.1 Total organic carbon content

In the study area, the TOC content of the S1l11 shale was between 0.46% and 8.35%, with an average value of 3.75%. The TOC contents of the S1l11–1–S1l11–4 shale was 2.38–8.35%, 0.74–6%, 0.31–6.91%, and 0.46–4.51%, respectively, and the average values were 5.01%, 3.71%, 3.94%, and 2.33%, respectively (Fig. 2). According to the average TOC value, the Longmaxi Formation shale has the characteristics of double peaks with high-TOC values in S1l11–1 and S1l11–3. Vertically, the TOC content shows a trend of decreasing initially, then increasing, and then decreasing from S1l11–1 to S1l11–4.

Fig. 2
figure 2

TOC histogram of shale in organic matter-enriched interval (S1l11) in study area

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4.2 Lateral distribution characteristics of TOC

The TOC value of the S1l11–1–S1l11–2 shale showed a decreasing trend from NW to SE in the study area. The TOC value of the S1l11–3 shale is high in the middle and low on the sides. The TOC value of the S1l11–4 shale also shows a decreasing trend from the NW to SE directions (Fig. 3).

Fig. 3
figure 3

Cross section of X1-X2-Y1-X3 well showing TOC variations in S1l11 shale in study area

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Vertically, the TOC values of the S1l11–1 shale and the lower part of the S1l11–3 shale are more than 4%. The TOC values of the middle and upper parts of the S1l11–4 shale was less than 2%, while the remaining sections was dominated by shales with TOC values ranging from 2–4%. According to the TOC contents, the high-quality shale reservoirs section of the Longmaxi Formation (S1l11) shows a pattern of a ‘sandwich cake’ with a high TOC contents in the S1l11–1 and S1l11–3 shales and a low TOC contents in the S1l11–1 and the S1l11–4 shales in the study area (Fig. 3).

4.3 Spatial distribution characteristics of TOC

The TOC content of the S1l11–1 shale was between 3.5 and 10% in the study area, showing the distribution characteristics of the highest TOC in the east, second highest in the west, and lower values in the southwest and northwest (Fig. 4A). The TOC content of the S1l11–2 shale was lower than that of the S1l11–1 shale, with TOC content ranging from 3 to 5% in the whole study area, which, which showed a similar distribution pattern to that of the S1l11–1 shale. (Fig. 4B).

Fig. 4
figure 4

TOC contour maps of S1l11–1–S1l11–4 shale in Changning area. A TOC contour maps of S1l11–1 shale in Changning area; B TOC contour maps of S1l11–2 shale in Changning area; C TOC contour maps of S1l11–3 shale in Changning area; D TOC contour maps of S1l11–4 shale in Changning area

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Compared to the S1l11–1–S1l11–2, the TOC content of the S1l11–3 shale showed a slight change, with the values ranging from 3 to 5% in the study area. However, the distribution pattern was markedly changed. During this period, the high TOC value area was mainly distributed near the X2 well in the middle and decreased gradually to the surrounding area (Fig. 4C). The TOC content of S1l11–4 shale decreased to the range of 1% to 3.5% in the whole area, with the highest content in the southeast, followed by the middle and western regions, and gradually decreased to the north and south (Fig. 4D).

The TOC value of the S1l11–1–S1l11–2 shale was the highest in the southeast, followed by the X1 well area in the west, and gradually decreased in the north and southwest. The high-TOC content of the S1l11–3–S1l11–4 shale tended to migrate to the X2 well area in the middle.

4.4 Major and trace elements

The major elements in the black shale of S1l11 were mainly Si, Ca, and Al, followed by Fe, K, Mg, and S, and a small amount of Na, P, Mn, and Ti. Si was mainly derived from terrigenous clastic quartz and biogenic silicon in shale samples, with contents ranging from 8.72% to 35.3% (mean = 24.96%). Ca was mainly derived from carbonate compositions in the shale samples, with contents ranging from 0.34% to 23.47% (mean = 6.14%). Al was mainly derived from clay minerals in the shale samples with a content of 1%–13.54% (mean = 6.07%). The contents of Fe, K, Mg, and S elements were 0.9–11.36% (mean = 3.20%), 0.27–5.01% (mean = 2.57%), 0.64%–3.88% (mean = 1.74%) and 0.39–3.11% (mean = 1.41%), respectively. The contents of Na, P, Mn, and Ti contents were less than 1%.

As the terrigenous debris input index, the Al and Zr contents and the Zr/Al ratio showed similar variation characteristics in the three shale gas wells (X1, X2, and X3). They show a significant upward trend from the bottom of S1l11 to S1l12 (Fig. 5, 6, 7). The average Al and Zr contents were 4.90% and 108.34 ppm in S1l11–1, respectively. The average Al and Zr contents increased sharply to 7.87% and 158.67 ppm in S1l11–4, respectively.

Fig. 5
figure 5

Stratigraphic variations of paleoenvironment proxies in the Wufeng–Longmaxi Formation shale in X1 well (see Fig. 1 for the well location)

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Fig. 6
figure 6

Stratigraphic variations of paleoenvironment proxies in the Wufeng–Longmaxi Formation shale in X2 well (see Fig. 1 for the well location)

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Fig. 7
figure 7

Stratigraphic variations of paleoenvironment proxies in the Wufeng–Longmaxi Formation shale in X3 well (see Fig. 1 for the well location)

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Biogenic barium (Babio) is a common indicator of marine paleoproductivity (Shen et al. 2015; Schoepfer et al. 2015). In this study, the Babio content showed a gradually increasing trend vertically (Fig. 5, 6, 7). The redox indices Ni/Co, V/Cr, and U/Th ratios were consistent with the change trend in TOC, which was characterized by the gradual decrease in the upward value and redox degree of the bottom water body gradually transforming from reduction to oxidation from bottom to the top.

5 Discussion

5.1 Analysis of the main controlling factors of organic matter accumulation

5.1.1 Paleoredox conditions

Redox-sensitive elements (e.g., V, Cr, Co, Ni, and U) and their ratios are widely used widely to distinguish marine redox conditions (Tribovillard et al. 2007; Algeo and Tribovillard 2009; Algeo and Rowe 2012). Using a single index to reflect paleoredox information is unreliable because each geochemical index has its application limitations (Chen et al. 2022; Fan et al. 2022). Therefore, the Ni/Co, V/Cr, and U/Th ratios were used to comprehensively judge the depositional environment in this study.

The research revealed a logarithmic correlation of the Ni/Co, V/Cr, and U/Th ratios with TOC, with R2 reaching up to 0.54–0.61(Fig. 8). With decreasing oxygen content in the water body, the reduction degree of the water body increased, and the TOC increased rapidly initially. When the reduction degree was high, the TOC increased slowly(Fig. 8). This indicates that the TOC value is particularly affected by the depositional environment in the initial stages of oxidation‒reduction, which may be the main controlling factor of organic matter accumulation. However, the increase in TOC may be affected by other geological factors.

Fig. 8
figure 8

Correlation diagrams of redox index and TOC showing a logarithmic correlation in Longmaxi Formation shale in the study area (some data come from Gao et al. 2019). A Relationship between Ni/Co ratio and TOC; B Relationship between V/Cr ratio and TOC, C: Relationship between U/Th ratio and TOC

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5.1.2 Paleoproductivity

Marine productivity is the foundation of marine ecosystems and plays a key role in organic matter accumulation (Wei et al. 2012). The Ba is mostly biogenic-related barium (Babio) in the seabed sediments, while Ba from other sources accounts for a relatively small proportion in the sediments (Dymond et al. 1992; Gonneea and Paytan 2006). Therefore, biogenic barium (Babio) is the most widely used indicator of palaeoproductivity (Shen et al. 2015; Schoepfer et al. 2015; Li et al. 2017; Xiao et al. 2020). Under anoxic deposition conditions, bacterial sulfate reduction is enhanced in the bottom water, and sulfate in sediments such as barite and barium sulfate is reduced to H2S by sulfide bacteria, which dissolves and precipitates barium ions, resulting in a decrease in biological barium content (Babio) in sediments (Murray and Leinen 1993; Torres et al. 1996; Yan et al. 2015; Li et al. 2015; He et al. 2019). Ultimately, the accuracy of the productivity indicator Babio will be reduced (Dymond et al. 1992).

The Babio content is significantly lower at the bottom of S1l11–1–S1l11–2 than that in S1l11–4–S1l12 (Fig. 5, 6, 7). This phenomenon may be because an anoxic environment dominated the period of the S1l11–1– S1l11–2, and part of the barium sulfate was decomposed and was lost, which distorted the Ba productivity. However, the Babio content of the S1l11–1 was between 316.8–1237.9 μg/g (mean = 789.4 μg/g), and the Babio content of the S1l11–2 was between 286.7–1646.4 μg/g (mean = 746.8 μg/g), indicating medium–high paleo-productivity. Only the Babio content in the S1l11 of the X3 well was low, while the Babio content in the S1l11–1–S1l11–2 of the X2 well was between 839.04–1320.9 μg/g (mean = 1122.5 μg/g), and the Babio content in the S1l11–1–S1l11–2 of the X1 well was between 1237.9–1646.4 μg/g (mean = 1385.2 μg/g), both showing high palaeoproductivity. This may indicate that the reduction amount of barium sulfate was low during the Longmaxi Formation. Therefore, the decomposition of barium sulfate by a reducing environment was not considered in this study, and the existing Babio content approximation was treated as the original Babio content.

According to the relationship between paleoproductivity index (Babio content) and TOC, the paleoproductivity index (Babio content) shows a complex correlation with the TOC. The TOC increased with increasing paleoproductivity index (Babio content) and reached a maximum value (TOC = 8.34%) when Babio content reached 1100 μg/g. However, TOC began to decrease with the continuous increase in Babio content. When the paleoproductivity index (Babio content) reached up to 2200 μg/g, TOC did not change significantly with Babio content but remained stable with a slightly increasing trend, indicating that paleoproductivity also played an important role in TOC accumulation (Fig. 9).

Fig. 9
figure 9

Relationship between paleoproductivity index (Babio content) and TOC showing a complex correlation in the S1l11 of the study area (some data come from Li et al. 2015 and Gao et al. 2019)

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5.1.3 Terrestrial debris input

Al and Zr are mainly derived from terrigenous debris and are often preserved in clay minerals (Calvert and Pedersen 2007). In addition, Al and Zr are stable and not susceptible to late diagenesis and weathering. Therefore, Al, Zr, and the Al/Zr ratio are often used to indicate the input of terrigenous debris (Young and Nesbitt 1998; Tribovillard et al. 2007; Lézin et al. 2013). From the top of the Wufeng Formation to the bottom of the Longmaxi Formation, the Al and Zr contents and the Al/Zr ratios of wells X1, X2, and X3 decreased rapidly initially and then increased slowly upward, while the TOC value decreased upward (Fig. 5, 6, 7). The rapid decrease in Al and Zr contents indicates that the input of terrigenous materials decreased considerably from the Guanyinqiao section at the top of the Wufeng Formation to S1l11–1, which may be caused by the rapid rise in sea level in the early Silurian (Su et al. 2009; Li et al. 2017). The Al and Zr contents increased slightly in S1l11–2, increased slowly in the middle of S1l11–3, and increased significantly in S1l11–4. This suggests that the terrigenous debris gradually increased upward in S1l11, and the top was affected more by the terrigenous clasts than the lower part. In S1l11, the data revealed an negative correlation between the Al and Zr contents and the TOC content, with a correlation coefficient of R2 reaching 0.35 (Fig. 10A, B), suggesting that the input of terrigenous debris material has diluted the organic matter, which is not conducive to the preservation of organic matter and has an adverse control on the enrichment of organic matter.

Fig. 10
figure 10

Relationship between terrigenous input and TOC showing a negative correlation in S1l11 of the study area. A Relationship between Al and TOC (some data are from Li et al. 2015; Gao et al. 2019); B relationship between Zr and TOC

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5.1.4 Water restriction conditions

The degree of water retention in sedimentary basins is an important factor affecting the sedimentary environment (Algeo and Rowe 2012), which restricts the biogeochemical cycle by influencing the exchange and circulation of water in the environment. The closed and restricted paleo-marine basins are often blocked by the circulation of bottom water, resulting in a difference in the enrichment of trace elements in the sedimentary environment. In addition, the reduced anoxic water caused by the restricted environment also provides good preservation conditions for organic matter accumulation.

Algeo and Lyons (2006) proposed a Mo-TOC model to determine the degree of water restriction in basins. In an anaerobic environment, the water body has strong reducibility, which is conducive to preserving organic matter. Mo exists in the form of stable and inactive MoO42− (+ 6) because S1l11–4–S1l12 is mainly in an oxygenic sedimentary environment (Tribovillard et al., 2007). Only when MoO42−(+ 6) is reduced to + 4 can Mo enter the sediment. The enrichment of Mo in sediments is limited in an oxygenic sedimentary environment (Zheng et al. 2000). Therefore, the Mo values of S1l11–4–S1l12 were relatively low in this study, as shown in the red circle in Fig. 11, and the actual restriction degree of the water body was not determined accurately (Fig. 11). This research shows that the water body of S1l11 was moderately restricted. S1l11–1 was a strongly restricted sedimentary environment. S1l11–2–S1l11–3 were mostly moderately restricted sedimentary environments, resulting in seafloor anoxia, which is conducive to preserving organic matter and lead to a high-TOC content. S1l11–4–S1l12 were weakly restricted sedimentary environment, with high oxygen contents in the water body, which were not conducive to preserving organic matter, and the TOC value became low.

Fig. 11
figure 11

The relationship between Mo element and TOC showing a moderately to strongly restricted sedimentary environment of S1l1 in the study area (After Algeo and Lyons 2006)

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Therefore, a moderately restricted environment dominated the Changning area during the first member of the Longmaxi Formation. With increasing restriction degree, the TOC value increased from a moderately to the strongly restricted sedimentary environment, indicating that a higher the restriction degree of the water body was more conducive to the enrichment and preservation of organic matter (Fig. 11).

5.1.5 Paleoclimate conditions

With additional research, the chemical weathering index (CIA) values can also reflect changing climates (Nesbitt and Young 1984; Young and Nesbitt 1999). When the CIA value exceeds 80, it represents a hot and humid climate with a high degree of chemical weathering. When it is between 70 and 80, it reflects moderate chemical weathering and warm, humid climate conditions, while a CIA value between 50 and 70 indicates a cold and arid climate. Yan et al. (2010) successfully applied the CIA index to the study of paleoclimate in the Ordovician–Silurian transition.

This study found that the CIA of S1l1 is between 70 and 80 in the study area, with little change. This suggests that the bottom of S1l11 to S1l12 were in a warm and humid climate, which are conducive to the abundance of paleontology and provide favourable conditions for high paleoproductivity. The paleoclimatic conditions had no significant effect on the differential enrichment of the TOC in each section because the paleoclimatic conditions were all warm and humid during the period of the first member of the Longmaxi Formation in the study area (Fig. 5, 6, 7).

5.2 Influence of main controlling factors on organic matter enrichment

The relationship between the TOC and redox, palaeoproductivity, terrigenous input and other indicators showed that organic matter accumulation was controlled by many factors in the S1l1 shale of the Changning area. In particular, the TOC showed a logarithmic correlation with the redox conditions, a negative correlation with the terrigenous input, and a complex correlation with the paleoproductivity. However, it is unclear how these controlling factors affected organic matter accumulation in each section. This study used the correlation between TOC and these parameters to illustrate how these factors affected organic matter accumulation in each section (Fig. 12, 13, 14).

Fig. 12
figure 12

Relationship between redox indexes and TOC in different sections of S1l1 in the study area. A Relationship between Ni/Co ratio and TOC; B Relationship between V/Cr ratio and TOC; C Relationship between U/Th ratio and TOC

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Fig. 13
figure 13

Relationship between terrigenous input indexes and TOC in different sections of S1l1 in the study area. A Relationship between Al element and TOC; B Relationship between Zr element and TOC

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Fig. 14
figure 14

Relationship between paleoproductivity (Babio) and TOC in different sections of S1l1 in the study area

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Overall, the sedimentary environment of S1l11–1–S1l11–2 was an anoxic environment. At this time, the TOC value decreased slowly with decreasing degree of reduction, indicating that the decrease in TOC value may also be affected by other factors. From S1l11–3 to S1l12, the sedimentary environment changed from an anoxic environment to an oxygenic environment with increasing oxygen content in the water body. The TOC value decreased rapidly with the change in sedimentary environment, suggesting that the change in sedimentary environment has a noticeable controlling effect on TOC enrichment (Fig. 12).

The terrigenous input was weak in S1l11–1, and the Al and Zr contents were flat, with averages of 3.25% and 113.18 ppm, respectively. At this time, the significant increase in TOC is unrelated to the terrigenous input but may be related to the increase in paleoproductivity and the preservation conditions of the reduction environment (Fig. 13). With decreasing depth, TOC was negatively correlated with terrigenous input in S1l11–2–S1l11–4 and decreased linearly with increasing terrigenous input. The TOC of S1l12 shale was approximately 1%, and the increase in Al and Zr contents had little effect on the TOC. The TOC showed no apparent change with increasing terrigenous input (Fig. 13A, B).

Overall, when the paleoproductivity was lower (Babio < 1100 μg/g), the preservation conditions worsened from S1l11–1 to S1l11–4 (the sedimentary environment changed from an anoxic to an oxygenic sedimentary environment and the terrigenous input increased). The paleoproductivity was weakly negatively correlated with the TOC (R2 = 0.25), and the TOC decreased with decreasing paleoproductivity. When the paleoproductivity was high (1100 μg/g < Babio < 2200 μg/g), the preservation conditions deteriorated from S1l11–1 to S1l11–4.

Although the paleoproductivity was high, the paleoproductivity also showed a significant negative correlation with the TOC (R2 = 0.46), suggesting that TOC decreased with increasing paleoproductivity. This indicates that when the paleoproductivity was less than 2200 μg/g (Babio < 2200 μg/g), the preservation conditions (i.e., sedimentary environment) were the most important factors controlling the organic matter accumulation. When the paleoproductivity was greater than 2200 μg/g (Babio > 2200 μg/g), although the preservation conditions were poor at this time, the TOC remained unchanged with the increasing paleoproductivity, showing a slow increasing trend, suggesting that the ultrahigh paleoproductivity controlled the enrichment of organic matter in this stage (Fig. 14).

5.3 Analysis of main controlling factors of organic matter enrichment in each sub-layer of S1l1.1

In S1l11–1, the obvious increase in TOC was not caused by terrigenous input, and all the test data of the samples were located in an anoxic sedimentary environment with good preservation conditions (Fig. 12, 13). At this time, TOC increased rapidly with increasing paleoproductivity, indicating that paleoproductivity was the decisive controlling factor in the anoxic sedimentary environment.

S1l11–2 was still an anoxic sedimentary environment (Fig. 12), but the terrestrial input increased as the water depth became shallower (Fig. 13). At this time, TOC increased with increasing paleoproductivity, but the increase was less than that of S1l11–1 (Fig. 14). Three sampling points with a TOC of approximately 5% were selected for detailed study (Fig. 15). The test data showed that the TOC increased slowly with increasing terrestrial input and paleoproductivity. This means that the paleoproductivity was still the decisive controlling factor in the anoxic sedimentary environment during the period of S1l11–2, but it was adversely affected by terrigenous debris (Fig. 15). This also suggested that the TOC value of S1l11–1 was greater than that of S1l11–2, even though shales had been deposited under the same anoxic environment and paleoproductivity (Fig. 14).

Fig. 15
figure 15

Relationship between terrigenous input, paleoproductivity of samples with TOC ≈ 5% of S1l11–2 in the study area

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The sedimentary environment was anoxic in the lower part of S1l11–3, dysoxic in the middle and upper parts, and oxygenic at the top of the S1l11–3, which showed a variable preservation condition. The terrestrial input increased gradually from the bottom to the top. The bottom of S1l11–3 was an anoxic sedimentary environment, which provided good preservation conditions for organic matter enrichment. The TOC values were significantly different when the terrigenous inputs were the same (Fig. 16A). At this time, the TOC increased with increasing paleoproductivity, which was the decisive controlling factor of organic matter accumulation (Fig. 16B). The middle and upper parts of S1l11–3 were deposited in a dysoxic sedimentary environment. The TOC remained unchanged when the paleoproductivity was low (Fig. 16B). At this time, the TOC decreased with the increasing terrigenous input (Fig. 16A). When the paleoproductivity was high, the high paleoproductivity was accompanied by high terrigenous input (Babio = 1515.41 μg/g; Al = 5.67 wt%), which led to shale with the same TOC as that formed in a relatively low paleoproductivity and low terrigenous input (Babio = 1002.81 μg/g; Al = 3.95 wt%). These results suggest that when the paleoproductivity was low, the enrichment of organic matter in the middle part of S1l11–3 was mainly controlled by the terrigenous input under a dysoxic sedimentary environment. However, organic matter accumulation was controlled by terrigenous input and paleoproductivity, when the paleoproductivity was high. The top of S1l11–3 was dominated by an oxygenic sedimentary environment. At this time, both the terrigenous input and paleoproductivity were higher, and shales that formed with a relatively low terrigenous input and high paleoproductivity showed higher TOC values. Although the preservation conditions were poor (oxygenic sedimentary environment and high terrigenous input), the TOC value increased rapidly with the rapid increase in paleoproductivity, indicating that the TOC is mainly controlled by the paleoproductivity at the top of S1l11–3.

Fig. 16
figure 16

Relationship between terrigenous input, paleoproductivity and TOC of S1l11–3 in the study area

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In S1l11–4, the TOC, which was mostly between 2 and 3%, decreased slowly with increasing terrigenous input, indicating that terrigenous input adversely affected organic matter enrichment but was not the main controlling factor (Fig. 17A). At the same time, the TOC showed a ‘U-shaped’ correlation with paleoproductivity. When the paleoproductivity was low (Babio < 1100 μg/g), the TOC decreased rapidly with the change in sedimentary environment from anoxic to oxygenic environment. Hence, during this time, the sedimentary environment was the main controlling factor for organic matter enrichment. When the paleoproductivity was higher (1100 μg/g < Babio < 2200 μg/g), despite the increased paleoproductivity, an oxygenic sedimentary environment and high terrestrial input were not conducive to preserving organic matter. The TOC value was near constant and was mainly controlled by the sedimentary environment and paleoproductivity (Fig. 17B). At ultrahigh paleoproductivity (Babio > 2200 μg/g), although the preservation conditions were unfavorable, the TOC increased slowly, indicating that organic matter accumulation was mainly controlled by the paleoproductivity.

Fig. 17
figure 17

Relationship between terrigenous input, paleoproductivity and TOC of the S1l11–4 in the study area

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No obvious correlation was noted between the paleoproductivity and TOC in S1l12. The TOC remained stable at approximately 1% with the change in paleoproductivity, indicating that the paleoproductivity had no significant control effect on the TOC in S1l12 (Fig. 14). Some samples have lower Al contents (2–5%) in S1l12, similar to that of the S1l11–1 (Fig. 13A, Fig. 14). However, the sedimentary environment differed from S1l12 (oxic environment) to S1l11–1 (anoxic environment), which led to significant differences in the TOC contents in these two strata. Hence, the organic matter accumulation was mainly controlled by the sedimentary environment in S1l12. In S1l12, the water body was oxic, which was not conducive to preserving organic matter and led directly to the low TOC in shale.

In summary, the organic matter accumulation of S1l11 was mainly affected by the coupled superposition of the paleoenvironment, paleoproductivity, terrigenous input and other factors in the study area. The degree of water restriction affected the sedimentary environment, which in turn controlled the preservation conditions. A higher degree of water restriction was more conducive to preserving organic matter. The moderately–strongly restriction environment of S1l11 was favorable for preserving organic matter. During the period of S1l11, the paleoclimatic conditions were warm and humid, which were conducive to abundant palaeontology and provided favorable conditions for the high paleoproductivity However, the degree of water restriction and paleoclimate had no significant effect on the differential enrichment of TOC in each section of S1l11 because of the slight variation in water restriction and paleoclimate. The sedimentary environment was the decisive factor controlling organic matter enrichment in the study area, followed by paleoproductivity, which together determined the upper limit of organic matter accumulation in shale. The terrigenous debris input was not conducive to the preservation of organic matter. The main controlling factors of organic matter accumulation in shale were different in different sections.

5.4 Analysis of vertical variation of organic matter enrichment pattern

The S1l11–1–lower part of S1l11–3 was an anoxic sedimentary environment, which provided good preservation conditions for organic matter accumulation. The paleoproductivity and terrigenous input increased as the sedimentary water became shallower upward. The paleoproductivity of S1l11–1 was 316.8–1237.9 μg/g (mean = 789.4 μg/g), showing medium–high paleoproductivity. Shale deposition was not affected by terrigenous debris, with an average TOC of 5.01%. The paleoproductivity of S1l11–2 ranged from 286.31 to 1646.37 μg/g (mean = 730.2 μg/g), and the Al content was between 1.54 and 4.91% (mean = 3.36%). Terrigenous input began to influence organic matter accumulation. The TOC average value was 3.71% in the S1l11–2. The TOC value of shale in S1l11–2 was lower than that of S1l11–1 because the paleoproductivity was slightly lower than that of S1l11–1, and the terrigenous input had an adverse effect. The productivity was between 454.89 and 1368.57 μg/g (mean = 1052.9 μg/g) at the bottom of the S1l11–3, showing high paleoproductivity. The Al content was between 4.43 and 4.59% (mean value of 4.53%), and the mean TOC value was 4.45%. Although the terrigenous input of S1l11–3 was slightly higher than that of S1l11–2, the paleoproductivity was much higher than that of S1l11–2, resulting in S1l11–3 having a higher TOC than S1l11–2. The paleoproductivity of S1l11–3 was slightly higher than that of S1l11–1, but the TOC value was slightly lower than that of S1l11–1 because of the higher terrigenous input in S1l11–3. This result explains why the TOC showed bimodal characteristics of high-TOC values in S1l11–1 and S1l11–3. At this stage, the TOC was mainly controlled by good preservation conditions (anoxic sedimentary environment), followed by paleoproductivity, which was reflected by the adverse effects of the terrestrial input in S1l11–2. S1l11–1–S1l11–2 were deposited in an anoxic sedimentary environment, and paleoproductivity was the decisive controlling factor for organic matter accumulation. At this time, the paleoproductivity was distinct in different sections, resulting in the differential distribution of TOC in S1l11–1–S1l11–2.

The middle to the upper part of S1l11–3 was a dysoxic sedimentary environment. When the paleoproductivity was low, the organic matter accumulation was mainly controlled by terrigenous input. However, when the paleoproductivity was higher, the organic matter accumulation was controlled by paleoproductivity and terrigenous input. The top of S1l11–3 was an oxygenic depositional environment, and the TOC was controlled mainly by the paleoproductivity, followed by terrigenous input. S1l11–3 was dominated by a dysoxic sedimentary environment in the study area. In contrast, the area around the Well X2 was an anoxic sedimentary environment with good preservation conditions and high paleoproductivity (839.04–1371.6 μg/g, average 1118.4 μg/g), which led to the migration of high TOC values to the area around the X2 well.

Oxygenic sedimentary conditions dominated the S1l11–4. When paleoproductivity was low (Babio < 1100 μg/g), the sedimentary environment was the main controlling factor for organic matter accumulation. When the paleoproductivity was higher (1100 μg/g < Babio < 2200 μg/g), the main controlling factor of organic matter accumulation was the joint control of the sedimentary environment and palaeoproductivity. At ultrahigh paleoproductivity (Babio > 2200 μg/g), the enrichment of organic matter was mainly controlled by paleoproductivity. The difference in the distribution of paleoproductivity in S1l11–4 led to a difference in the distribution of TOC. The organic matter enrichment was mainly controlled by the sedimentary environment in the S1l12 shale. A dysoxic-oxic environment was not conducive to preserving organic matter, and the paleoproductivity was low, which resulted in a low TOC in the S1l12 shale.

6 Conclusions

S1l11 is the organic matter enrichment section in the study area, with a TOC content between 0.46 and 8.35%. The moderately-strongly restriction environment of S1l11 was favorable for preserving organic matter. During the period of S1l11, the warm and humid paleoclimatic conditions provided favorable conditions for high paleoproductivity. On the other hand, since the slight variation in water restriction and paleoclimate, the degree of water restriction and paleoclimate have no significant effect on the differential enrichment of TOC in each S1l11 layer of the S1l11.

Organic matter accumulation of S1l11 was mainly controlled by the paleoenvironment in the study area, followed by the paleoproductivity, and the terrigenous input. When the paleoproductivity was less than 2200 μg/g, the sedimentary environment was the most important factor controlling organic matter accumulation. On the contrary, ultrahigh paleoproductivity was the main factor controlling organic matter enrichment. In the S1l11, the main controlling factor of organic matter enrichment from the bottom to the top was changed from the ‘preservation mode’ dominated by an anoxic sedimentary environment (S1l11–1 to the lower part of S1l11–3) to the 'productivity preservation condition collaborative control mode’ (upper middle part of S1l11–3–bottom of S1l11–4), and then to the ‘productivity mode’ (upper middle part of S1l11–4); The enrichment mode of organic matter in S1l12 is mainly “preservation mode”.

An anoxic redox conditions was the basis of organic matter accumulation during the shale deposition of the Longmaxi Formation in the Changning area. Medium–high paleoproductivity was the key to organic matter enrichment.