Geochemical Characteristics of Elements and Main Controlling Factors of Organic Matter Enrichment in Shale from Wufeng Formation to Submember Long-1₁ in Southern Sichuan

Abstract

A total of 70 shale samples from seven core holes, namely, N203, N208, N209, N210, N211, N215 and N217, in Changning area, south Sichuan, were analyzed for major and trace elements. Based on an analysis of longitudinal variation characteristics of major and trace elements, the REDOX conditions, paleoproductivity, relative sea level height and terrigenous detrital injection amount of gas-bearing shale in Wufeng Formation–Submember Long-1₁ in the Changning area were studied. And, the main controlling factors of organic matter enrichment in Wufeng Formation–Submember Long-1₁ were discussed. The results show the following: (1) Trace elements Mo, U, Ba and Ni are relatively enriched in the shale of Wufeng Formation–Submember Long-1₁ in the Changning area, while the Co element is deficient. The flat distribution pattern of rare earth in Wufeng Formation–Submember Long-1₁ shows that the shale deposits are weakly influenced by terrigenous materials. The amount of terrigenous injection in Wufeng Formation is higher than that in Submember Long-1₁, and the amount of terrigenous injection increased gradually from the lower part of Bed Long-1₁¹ to the lower part of Bed Long-1₁⁴. (2) The REDOX environment discrimination indexes such as U/Th, Ni/Co, V/Sc and δU show that the water REDOX conditions of Wufeng Formation–Submember Long-1₁ in the Changning area have changed many times during the shale sedimentary period, and the overall REDOX conditions are dominated by an anaerobic environment. The REDOX indexes at the boundary between Long-1₁¹ and Wufeng Formation are relatively small and in the oxidizing environment range. The REDOX index from Bed Long-1₁¹ to Bed Long-1₁⁴ decreases gradually, and the anaerobic environment of Bed Long-1₁¹ and Bed Long-1₁² gradually changes to the oxygen-poor environment of Bed Long-1₁³; and, the REDOX index of Bed Long-1₁⁴ is the lowest, which is generally oxygen rich. (3) Organic matter enrichment in Wufeng Formation Submember Long1₁ in the Changning area is mainly controlled by REDOX conditions. Low terrigenous injection and a relatively high sea level have positive effects on organic matter enrichment, but there is no obvious relationship between the paleoproductivity and organic matter enrichment.

1. Introduction

Shale gas is a kind of unconventional energy with characteristics such as large reserve, clean energy and high efficiency. It is estimated that the amount of China’s shale gas resources reaches (30~100) × 10¹² m³ [1], which is twice the amount of conventional natural gas resources. Its development and application is of great significance to optimize China’s energy structure and promote sustainable development. Wufeng Formation–Submember Long-1₁ in the Changning area of southern Sichuan is the most favorable shale development layer series in China at present [2]. Organic matter, as the hydrocarbon generating material of shale, controls the reservoir space and adsorption capacity of shale, and it ultimately determines the output of shale gas [3]. Therefore, it is very important to study the controlling factors of organic matter enrichment for shale gas exploration and development.

The main controlling factors of organic matter enrichment in sediments have been discussed a lot in the past 20 years [4,5,6]. There are two main factors affecting organic matter enrichment: marine surface primary productivity and favorable organic matter preservation conditions. The geochemical characteristics of shale can realize the reconstruction of a paleo-marine REDOX environment [7,8]. When the sources of U, V, Mo, Cr, Ni and other elements are few, the enrichment is significantly affected by the REDOX environment during deposition, while the diagenesis has little influence, so they are often used to identify the REDOX conditions of water bodies [9,10]. The Ba element derives from terrigenous detrital and biogenic origin, and the paleoproductivity index biogenic barium (Ba_XS) of Ba content can be used to evaluate the paleoproductivity of marine basins [4,11,12]. Rare earth elements have strong stability, and the solubility of seawater to rare earth elements is very low, so the content of rare earth elements (∑REE) can indicate the supply of terrigenous materials in sediments. By studying the geochemical characteristics of shale elements, the informations relating to the sedimentary environment during deposition can be obtained, providing clues for the controlling factors of organic matter enrichment in shale [13,14].

In recent years, with the exploration and development of shale gas, many scholars have conducted detailed discussions on the enrichment of organic matter in the shale of the Wufeng Formation–Longmaxi Formation in Sichuan Basin, and made many valuable achievements [15,16]. However, the systematic studies on organic matter enrichment in Submember Long-1₁ and Wufeng Formation are relatively lacking. Based on previous work, this study takes the Changning area as the research object and adopts the systematic geochemical analysis of elements, including major and trace elements, combined with the change in TOC, to discuss the characteristics of major and trace elements in Wufeng Formation–Submember Long-1₁, and discuss the geological significance of organic matter enrichment in the shale.

2. Geological Condition

The main anticlinal structure of Changning is located at the junction of Sichuan Basin and Yunnan–Guizhou Plateau, between the low and steep fold belt of the southern Sichuan and Loushan fold belts. During the Devonian–Carboniferous period, the Wufeng Formation and Longmaxi Formation of the Tiantangong anticline core in the research area was denuded. From Late Ordovician to Early Silurian, the Longmaxi Formation was exposed to the surface and further expanded by the central Guizhou uplift, which connected with the Kangdian ancient land in the west and Xuefeng underwater paleo-uplift in the east. With the further uplift of Longnvsi underwater paleo-uplift in Leshan in the middle of Sichuan, the continental shelf deposits were formed in the Changning area of southern Sichuan, and a set of organic-rich black shale was widely developed.

The Upper Ordovician Wufeng Formation–Submember Long-1₁ of Lower Silurian Longmaxi Formation can be successively divided into the Wufeng Formation and four beds from bottom to top: Long-1₁¹, Long-1₁², Long-1₁³ and Long-1₁⁴, and each bed is in conformity contact (Figure 1).

The shale of the Wufeng Formation is integrated with the limestone of the lower Baota Formation, and the shell limestone with a thickness less than 0.5 m is developed in the upper Guanyinqiao Member. There is a clear boundary between the shale bottom boundary of Bed Long-1₁¹ and the shell limestone boundary of the Guanyinqiao Member of Wufeng Formation. Black carbonaceous graptolite shale is developed in Bed Long-1₁¹, and the GR (natural gamma ray) value at the bottom of shale has a maximum value between 200 and 500 (API). Bed Long-1₁² is composed of black carbonaceous shale, and the GR curve is a stable box shape between 140 and 180 (API). Bed Long1-₁³ is composed of black carbonaceous graptolite shale, and the GR value changes dramatically at the boundary between Bed Long-1₁² and Bed Long-1₁³, which increases upward in funnel shape, and the shape is similar to a gyroscope, with a value between 160 and 270 (API). Bed Long-1₁⁴ is dominated by gray and black shale, and the GR value changes obviously at the boundary between Bed Long-1₁⁴ and Bed Long-1₁³, and the upper bell shape decreases, while Bed Long-1₁⁴ is distributed in a stable box shape between 120 and 150 (API). The total bed thickness in the research area is 30–60 m, among which Submember Long-1₁ is the best reservoir layer [17].

3. Sample and Method

The samples in this study were collected from the Upper Ordovician Wufeng Formation and Submember Long-1₁ of the Lower Silurian Longmaxi Formation in the Changning area, including 70 shale samples from seven core holes, including N203 (6 pieces), N208 (11 pieces), N209 (6 pieces), N210 (7 pieces), N211 (10 pieces), N215 (12 pieces) and N217 (18 pieces). The sampling location is shown in Figure 1. The analysis and test of major and trace elements of the samples were completed in Guangzhou ALS Minerals Laboratory.

The test method of the major element was to add the lithium borate–lithium nitrate melting flux containing lithium nitrate into the sample and mix, and then melt at high temperature. After the molten material was poured into the platinum mold to form a flat glass sheet, it was analyzed with an X-ray fluorescence spectrometer. The test method of trace elements involved taking two test samples: one was decomposed with perchloric acid, nitric acid and hydrofluoric acid, which is then dissolved by dilute hydrochloric acid to constant volume after nearly drying, and then analyzed via inductively coupled plasma emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometer (ICP-MS); the other test sample was added to lithium metaborate/lithium tetraborate flux, mixed evenly, and melted in a furnace above 1025 °C. After cooling, the molten liquid was metered to constant volume with nitric acid, hydrochloric acid, and hydrofluoric acid, and then analyzed via inductively coupled plasma mass spectrometer (ICP-MS). TOC data was derived from well logging interpretation.

Enrichment coefficient (EF) is used to evaluate the enrichment degree of an element in this shale sample. The formula for calculating EF is as follows [8]:

$${EF}_X=\frac{{\left(X/Al\right)}_{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}{{\left(X/Al\right)}_{PAAS}}$$

(1)

where ${\left(X/Al\right)}_{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}$ refers to the percentage ratio of $X$ element to aluminum element in the sample, ${\left(X/Al\right)}_{PAAS}$ refers to the percentage ratio of $X$ element to aluminum element in the post-Archean Australian shale (PAAS) and ${EF}_X$ is the enrichment coefficient of $X$ element. When ${EF}_X$> 1, it means that $X$ element is more enriched than post-Archean Australia shale (PAAS); otherwise, it is deficient.

Degree of mineralization of pyrite (DOP_T) is used to judge the REDOX environment of water body during deposition. It refers to the ratio of Fe element in pyrite to total active Fe (Fe element dissolved by hydrochloric acid and Fe element in pyrite), and the content of Fe element in pyrite is calculated through the content of S element [10,18,19]. Assuming that all S elements exist in rocks in the form of pyrite (FeS₂), the degree of mineralization of pyrite (DOP_T) can be calculated using the following formula:

$${\mathrm{D}\mathrm{O}\mathrm{P}}_T=\frac{\omega \left(\mathrm{S}\right)\times 55.85/64.13}{\omega \left(\mathrm{F}\right)}$$

(2)

Wilde believes that $\mathrm{C}\mathrm{e}$ anomaly degree can indicate the relative change in sea level, and the calculation formula of $\mathsf{\delta }\mathrm{C}\mathrm{e}$ is as follows:

$$\mathsf{\delta }\mathrm{C}\mathrm{e}=\lg \frac{{5Ce}_N}{\left({4La}_N+{Sm}_N\right)}$$

(3)

${Ce}_N$, La_N and Sm_N refer to the values of $\mathrm{C}\mathrm{e}$, La and Sm normalized by the Post-Archaean Australian shale (PAAS). The δCe value of the sample decreases with the rise in sea level, and it increases otherwise.

Wignall [20] proposed that the authigenic uranium (δU) could reflect the REDOX condition of sedimentary environment of the sample, and its calculation formula is as follows:

$$\mathsf{\delta }\mathrm{U}=\frac{2\mathrm{U}}{\mathrm{U}+\mathrm{T}\mathrm{h}/3}$$

(4)

When δU > 1, it reflects the anaerobic environment when the sample was deposited; otherwise, it is the normal sedimentary environment.

Biogenic barium (Ba_XS) represents the biogenic barium content, which can be obtained by subtracting the contribution of terrigenous barium from the total content of this element in shale [4,21,22,23,24,25,26,27,28,29,30,31,32]. The calculation formula is as follows:

$${\mathrm{B}\mathrm{a}}_{XS}={\mathrm{B}\mathrm{a}}_{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}-{Al}_{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\times {\left(\frac{Ba}{Al}\right)}_{PAAS}$$

(5)

Ba_Sample and Al _Sample refer to the content of barium and aluminum in the sample in the unit of μg/g, and ${\left(\frac{Ba}{Al}\right)}_{PAAS}$ refers to the ratio of barium and aluminum in the post-Archean Australian shale.

4. Laboratory Result

4.1. TOC

According to the content of organic matter in the shale of the Wufeng Formation–Longmaxi Formation in Sichuan Basin, Wang Yuman et al. divided the shale into the organic-rich shale (TOC > 2%), organic-containing shale (2% > TOC > 1%) and organic-poor shale (1% > TOC). Based on TOC data, the TOC range of Wufeng Formation–Submember Long-1₁ in the Changning area of southern Sichuan is 1.12%~6.59%. The TOC of N203, N208, N209, N210, N211, N215 and N217 varies greatly, but it shows the same regularity. The maximum shale TOC of Bed Long-1₁¹ is 4.62~6.59%, and the average TOC is 5.5%. The second TOC is Bed Long-1₁³, which is 1.38–5.16%, with an average of 4.06%. The TOC of Bed Long-1₁² is 2.52~5.44%, with an average of 3.94%. The TOC of the Wufeng Formation is 1.37~5.44%, with an average of 5.36%. The minimum TOC is Bed Long-1₁⁴, which is 1.12~3.42%, with an average of 2.14%. The TOC data is based on core calibration logging data, and an accurate logging interpretation model is established to calculate the TOC value of the rock. The error between the calculated TOC after the sample is compared to the TOC analyzed in the experiment is less than 1%. Except for organic-containing shale in a small part of Bed Long-1₁⁴, Bed Long-1₁³ and the Wufeng Formation, the other sample points are the organic-rich shale, indicating that the shale of Wufeng Formation–Submember Long-1₁ in the Changning area is generally the organic-rich shale.

4.2. Chemical Characteristics of Elements

There is little difference in total rare earth content among the test wells in the Changning area, including ΣREE of N203 is 91.93~272.86 μg/g, ΣREE of N208 is 91.68~241.41 μg/g, ΣREE of N209 is 124.83~212.32 μg/g, ΣREE of N210 is 81.80~160.99 μg/g, ΣREE of N211 is 84.40~201.63 μg/g, ΣREE of N215 is 115.47~198.97 μg/g and ΣREE of N217 is 99.00~227.93 μg/g. The distribution of rare earth elements in Wufeng Formation–Submember Long-1₁ is quite flat (Figure 2), indicating that shale deposits in the research area are weakly influenced by terrigenous materials and have a relatively stable structure [33].

In general, the trace elements such as Mo, U, Ba and Ni are relatively enriched in each well, while the Co element is deficient. The enrichment coefficients of Zn, V and Sr vary greatly, and the content is 21.00–645.00 μg/g, 30.47–619 μg/g and 52.00–852.00 μg/g, respectively. The enrichment coefficients of Co and Th are 4.50–22.50 μg/g and 2.81–33.00 μg/g, respectively. The average values of enrichment coefficients of Mo, V and U elements in Bed Long-1₁¹ are the highest, and the maximum contents are 143.50 μg/g, 619.00 μg/g and 75.50 μg/g, respectively.

5. Discussion

The enrichment of organic matter in marine shale is influenced by the rise and fall in sea level, tectonic movement of the basement, primary productivity, terrigenous material supply and REDOX conditions during deposition. Li Yanfang [9] has studied the Wufeng Formation–Longmaxi Formation and found that the enrichment of organic matter was controlled by REDOX conditions and influenced by the background of high primary productivity. Li Jin [22] believed that the relative height of sea level affected the enrichment of organic matter by influencing the supply of terrigenous material and the REDOX environment of water.

5.1. Relationship between REDOX Conditions and Total Organic Carbon Content

In the oxidation environment, U element is generally dissolved in marine water with +6 valence, and in the reduction environment, U6+ is reduced to U4+, which is absorbed by asphalt in the form of oxide or forms a complex and enriched in sediments [23,24,25,26,27]. In the oxidation environment, V element is dissolved in water with +5 valence in the states of HVO₄²⁻ and H₂VO₄₋. In the reduction environment, it will be reduced to +4 or +3 valence, and it is often enriched in sediments in the form of insoluble hydroxides, organometallic extraction coordination, or adsorption by groups [28]. Mo is mainly dissolved in marine water in the form of MoO₄²⁻ [29], and it can be enriched in sediments in the form of sulfide under the reduction condition of sulfate-reducing bacteria. The REDOX environment controls the enrichment of REDOX sensitive elements such as U, V and Mo, and the content of REDOX sensitive elements in sediments can reflect the REDOX conditions during deposition [9]. However, due to their own chemical characteristics, the elements tend to attract each other and combine into compounds. Therefore, in this study, the element ratio and multiple indexes are used to judge the REDOX characteristics during deposition.

Some scholars have proposed to indicate the REDOX environment during deposition by U/Th, Ni/Co, V/Sc, δU and DOP (

Table 1. Discrimination index of REDOX environment.

Discrimination Index	Anaerobic	Oxygen-Poor	Oxygen-Rich	Proposed by
U/Th	>1.25	0.75~1.25	<0.75	Hatch and Leventhal [30]
Ni/Co	>7.00	5.00~7.00	<5.00	Jones and Manning [31]
V/Sc	>9.10	>9.10	<9.1	Kimura 2001 [34]
δU	>1.00	>1.00	<1.00	Wignall [20]
DOPT	>0.75 with H2S, 0.42~0.75 without H2S		<0.42	Raiswell 1988 [35]

). The REDOX index of the Wufeng Formation of the seven test wells in the research area is shown as (Figure 3). The U/Th value ranges from 0.21 to 3.82, with an average of 1.29. The Ni/Co value ranges from 5.20 to 15.98, with an average of 9.37. The V/Sc value ranges from 8.06 to 33.54, with an average of 21.13. The δU value ranges from 0.76 to 1.84, with an average of 1.40. The DOP_T value ranges from 0.19 to 0.77, with an average of 0.46.

In Bed Long-1₁¹, the U/Th value ranges from 1.79 to 4.63, with an average of 3.1. The Ni/Co value ranges from 10.85 to 15.69, with an average of 13.27. The V/Sc value ranges from 30.28 to 49.78, with an average of 40.53. The δU value ranges from 1.69 to 1.87, with an average of 1.79. The DOP_T value ranges from 0.55 to 0.62, with an average of 0.59. Combining all the indexes, it reflects that the whole Bed Long-1₁¹ is an anaerobic environment.

In Bed Long-1₁², the U/Th value ranges from 1.06 to 3.78, with an average of 1.87. The Ni/Co value ranges from 8.15 to 18.57, with an average of 11.95. The V/Sc value ranges from 18.53 to 77.08, with an average of 34.44. The δU value ranges from 4.52 to 1.84, with an average of 1.67. The DOP_T value ranges from 0.43 to 0.71, with an average of 0.62. Combining all the indexes, it reflects that the whole Bed Long-1₁² is an anaerobic environment.

In Bed Long-1₁³, the U/Th value ranges from 0.24 to 1.79, with an average of 1.15. The Ni/Co value ranges from 4.00 to 7.85, with an average value of 6.29. The V/Sc value ranges from 6.67 to 21.28, with an average of 16.37. The δU value ranges from 0.84 to 1.69, with an average of 1.50. The DOP_T value ranges from 0.22 to 0.70, with an average of 0.53. Combining all the indexes, it reflects that the whole Bed Long-1₁³ is an oxygen-poor environment.

In Bed Long-1₁⁴, the U/Th value ranges from 0.30 to 1.47, with an average of 0.56. The Ni/Co value ranges from 3.61 to 8.26, with an average value of 5.29. The V/Sc value ranges from 8.90 to 19.53, with an average of 12.80. The δU value ranges from 0.95 to 1.63, with an average of 1.2. The DOP_T value ranges from 0.24 to 0.52, with an average of 0.35. Combined with all the indexes, it reflects that Bed Long-1₁⁴ is changed from the oxygen-poor environment at the bottom to the oxygen-rich environment at the top.

Combined with various indexes, it can be seen that the Wufeng Formation is dominated by an oxygen-poor and anaerobic environment. The index of REDOX increases first from Wufeng Formation Bed Long-1₁⁴. From the profile of REDOX index of N215 and N217, the index is high in Bed Long-1₁¹ and the Wufeng Formation. The REDOX index at the boundary between Bed Long-1₁¹ and the Wufeng Formation is generally small, and it is in the oxidizing environment, and the REDOX index of Bed Long-1₁¹-Long-1₁⁴ gradually decreases. The anaerobic environment of Bed Long-1₁¹ and Bed Long-1₁² gradually changes to an oxygen-poor environment of Bed Long-1₁³. The lowest REDOX index of Bed Long-1₁⁴ is generally oxygen-rich environment.

In Wufeng Formation–Submember Long-1₁ of the Changning area, there are obvious linear positive correlations bet[ween the U/Th and TOC content (Figure 3a), between the Ni/Co and TOC content (Figure 3b), between the V/Sc and TOC content (Figure 3c), between the δU and TOC content (Figure 3d), and between the DOPT and TOC content (Figure 3e). The REDOX index and TOC value of Bed Long-1₁¹ and Bed Long-1₁² are the highest, indicating the typical anaerobic sedimentary environment during deposition, and reflecting that REDOX conditions control TOC content during the shale deposition.

5.2. Primary Productivity

In this study, the productivity index biogenic barium (Ba_XS) was used to evaluate the primary productivity of the ocean during deposition [36]. When Ba_XS > 1000 μg/g, it is considered that the paleoproductivity was high during deposition. The Ba_XS value of the seven test wells in the Wufeng Formation in the research area ranged from 723.97 to 2469.94 μg/g, with an average of 1420.76 μg/g. The Ba_XS value of Bed Long-1₁¹ ranged from 667.98 to 952.98 μg/g, with an average of 810.48 μg/g. The Ba_XS value of Bed Long-1₁² ranged from 543.98 to 1119.98 μg/g, with an average of 831.68 μg/g. The Ba_XS value of Bed Long-1₁³ ranged from 575.96 to 1999.97 μg/g, with an average of 1231.47 μg/g. The Ba_XS value of Bed Long-1₁⁴ ranged from 685.97 to 1354.97 μg/g, with an average of 1134.17 μg/g.

Vertically, the content of barium in Wufeng Formation was higher, while those in Bed Long-1₁¹ and Bed Long-1₁² were lower. The TOC of samples with Ba_XS > 1000 μg/g was 1.38–5.16%, with an average of 3.72%, and that of samples with Ba_XS < 1000 μg/g was 1.12–6.59%, with an average of 3.5%. From Figure 3f, the barium of Wufeng Formation–Submember Long-1₁ is basically unrelated to TOC. This article discusses that biological barium is basically unrelated to TOC, indicating that the primary productivity of the ocean does not control TOC. According to the analysis in the article, high TOC is mainly due to the fact that organic matter is not decomposed and is mainly preserved in anaerobic environments.

5.3. Terrigenous Material Content

Rare earth elements are almost insoluble in sea water and come from terrigenous detritus, which can reflect the injection amount of terrigenous materials [9]. For the seven test wells in the research area, the total rare earth content ΣREE in the Wufeng Formation is 112.47~472.86 μg/g, with an average value of 190.39 μg/g. The total rare earth content ΣREE in Bed Long-1₁¹ is 81.80~182.60 μg/g, with an average value of 118.80 μg/g. The total rare earth content ΣREE in Bed Long-1₁² is 91.56~179.62 μg/g, with an average value of 122.58 μg/g. The total rare earth content ΣREE in Bed Long-1₁³ is 125.33~227.93 μg/g, with an average value of 162.38 μg/g. The total rare earth content ΣREE in Bed Long-1₁⁴ is 138.89~223.79 μg/g, with an average value of 181.53 μg/g.

Continuous injection of terrigenous detrital materials would destroy the anoxic and anaerobic sedimentary environment of the bottom seawater, and it would dilute the organic matter, resulting in a decrease in TOC content. Based on the ΣREE content data of Wufeng Formation–Submember Long-1₁ in the Changning area, it is indicated that the amount of terrigenous injection in the Wufeng Formation is higher than that in Submember Long-1₁, and the terrigenous injection gradually increased from Bed Long-1₁¹ upward to Bed Long-1₁⁴. It can be seen from Figure 3g that there is an obvious negative linear correlation between the terrigenous detrital injection amount and TOC content.

5.4. Relative Height of Sea Level

The δCe value reflects the rise and fall in sea level, and the higher the sea level is, the smaller δCe is. For the seven test wells in the research area, the δCe value of the Wufeng Formation ranges from −0.086 to 0.004, with an average of −0.045. The δCe value of Bed Long-1₁¹ ranges from −0.117 to −0.056, with an average of −0.075. The δCe value of Bed Long-1₁² ranges from −0.086 to −0.051 with an average of −0.068. The δCe value of Bed Long-1₁³ ranges from −0.086 to −0.033, with an average of −0.057. The δCe value of Bed Long-1₁⁴ ranges from −0.065 to −0.026, with an average of −0.046.

It shows that the sea level of the Wufeng Formation is lower than that of Submember Long-1₁, and the sea level of Bed Long-1₁¹ to Bed Long-1₁⁴ gradually decreases. There is a linear negative correlation between δCe and TOC (Figure 3h). The higher the sea level is, the higher TOC content will be. This is due to the formation of a reduction environment on the sea bottom as the sea level rises, and the death of benthic organisms as the sea level rises, leading to the enrichment of organic matter.

6. Conclusions

(1)

The trace elements such as Mo, U, Ba and Ni are relatively enriched in the shale of Wufeng Formation–Submember Long-1₁ in the Changning area, while the Co element is deficient. The distribution pattern of rare earth is flat, which indicates that the shale deposits in the research area are weakly influenced by terrigenous materials.

(2)

During the shale deposition period of Wufeng Formation–Submember Long-1₁ in the Changning area, the REDOX conditions of the water body changed many times, and they were generally dominated by an anaerobic environment. The shale of Bed Long-1₁¹ and Bed Long-1₁² was generally an anaerobic environment, the shale of Bed Long-1₁³ was generally an oxygen-poor environment, and the shale of Bed Long-1₁⁴ changed from an oxygen-poor environment to an oxygen-rich environment. The water REDOX conditions of the Wufeng Formation varied greatly, mainly in terms of the oxygen-poor and anaerobic environment.

(3)

Organic matter enrichment in Wufeng Formation–Submember Long-1₁ in the Changning area was mainly controlled by REDOX conditions; the amount of terrigenous injection in Wufeng Formation was higher than that in Bed Long-1₁¹, and the amount of terrigenous injection gradually increased from Bed Long-1₁¹ to Bed Long-1₁⁴, which resulted in the reduction and dilution of organic matter, and this was an important reason for the decrease in organic matter content. A relatively high sea level has a positive effect on organic matter enrichment. There is no obvious relationship between the paleoproductivity and organic matter enrichment.