Multiple geochemical proxies controlling the organic matter accumulation of the marine-continental transitional shale: A case study of the Upper Permian Longtan Formation, western Guizhou, China

Journal of Natural Gas Science and Engineering 56 (2018) 152–165

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Multiple geochemical proxies controlling the organic matter accumulation of the marine-continental transitional shale: A case study of the Upper Permian Longtan Formation, western Guizhou, China

T

Shunxi Liua,b, Caifang Wua,∗, Teng Lic, Haichao Wangd a Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu Province, 221008, China b School of Resources and Environment, Henan Polytechnic University, Jiaozuo, Henan Province, 454000, China c College of Petroleum Engineering, Xi'an Shiyou University, Xi'an, Shaanxi Province, 710065, China d Institute of Geology and Mining Engineering, Xinjiang University, Urumqi, Xinjiang, 830047, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Marine-continental transitional organic-rich shale Longtan formation Geochemical proxies Organic matter accumulation

The organic-rich shale of the Longtan Formation of the Upper Permian in western Guizhou formed during the marine-continental transitional facies depositional environment. With a high total organic carbon (TOC) content and a large cumulative thickness, it is thought to be the superior source rock for shale gas development. The depositional environment of marine-continental transitional shale is significantly different from marine shale, which leads to the various accumulation characteristics of the organic matter. In this paper, shale samples were collected from the Longtan Formation of the Upper Permian, which is typical marine-continental transitional shale. The TOC, major elements and trace elements were measured, and the formation and preservation conditions were investigated using multiple geochemical proxies, including paleoclimate, detrital influx, redox parameters, paleoproductivity and sedimentation rate. The TOC decreases first and then increases from the bottom to the top of the Longtan Formation shale, and the TOC for the lower Longtan Formation is higher than the upper Longtan Formation. For the lower Longtan Formation, the positive correlations between TOC and redox indicators (V, U and V/Cr) demonstrate that the dysoxic bottom water environment was the key factor that controlled the accumulation of organic matter. For the upper Longtan Formation, there are positive correlations between the TOC and the paleoclimate and sedimentation rate, which suggests that the enrichment of the organic matter was influenced by both a warm and humid paleoclimate and the high sedimentation rate of an oxic environment. However, the high detrital influx (aluminosilicate) occurred as the diluent decreased the concentration of organic matter. The paleoproductivity has a poor correlation with TOC for the Longtan Formation, suggesting that it was inferior to the gathering of organic matter. The sedimentary models built for the upper and lower Longtan Formation shale can reproduce the enrichment of organic matter.

1. Introduction The marine-continental transitional organic-rich shale is mainly dark coal-measured shale, with a high total organic carbon (TOC) content and large cumulative thickness, which are the best features and qualities for exploiting shale gas (Zou et al., 2010). The CarboniferousPermian is the key period for the change of sedimentary environment from marine facies to continental facies in China, and the marinecontinental transitional organic-rich shale is widely deposited, including Northern China, the Tarim Basin and the Junggar Basin of the Carboniferous-Permian, the Upper Carboniferous Benxi Formation, the

Lower Permian Taiyuan Formation- Shanxi Formation in the Ordos Basin (Zou et al., 2011; Song et al., 2016). The Upper Permian Longtan Formation shale is deposited in the environment of a lagoon-tidal flat and delta at the Yangtze Platform in Southern China, which consists of carbon mudstone, silty mudstone and pelitic siltstone. Due to the abundance of organic matter, this area has prime geological conditions for the development of shale gas (Luo et al., 2017; Zhang et al., 2017). The accumulation of organic matter is a complex physical and chemical process. According to detailed research studies of dark organic-rich shale in the marine strata, the influence of organic matter accumulation is multi-faceted. However, there are still several

∗ Corresponding author. Address: Room 338, Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, South Jiefang Road, Xuzhou, Jiangsu Province, 221008, China. E-mail address: [email protected] (C. Wu).

https://doi.org/10.1016/j.jngse.2018.06.007 Received 11 January 2018; Received in revised form 28 April 2018; Accepted 3 June 2018 Available online 07 June 2018 1875-5100/ © 2018 Elsevier B.V. All rights reserved.

Journal of Natural Gas Science and Engineering 56 (2018) 152–165

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Fig. 1. (A) Tectonic location of study area and division of regional tectonic units (modified from Luo et al., 2017); (B) Paleogeographic map of late Permian sediment in the study area (modified from Xu and He, 2003).

a positive correlation between the nutrient element P and paleoproductivity. This is similar to the Ba accumulation rate therefore, they are considered indicators of marine paleoproductivity (Dymond et al., 1992; Filippelli and Delaney, 1994; Zeng et al., 2015). To avoid the dilution effects of other components, the ratios of Ba/Al and P/Ti are replaced (Algeo et al., 2011). Recently, the geochemical analysis methods have been used to investigate the organic enrichment law of marine-continental transitional organic-rich shale. Adegoke et al. (2015) found that the suboxic to relatively anoxic bottom water conditions contribute to the enrichment of organic matter for Gongila shales from northeastern Nigeria, these shales are mainly influenced by the marine environment. Wang et al. (2017) studied the enrichment of organic matter for the Tumengela Formation shale from the north Qiangtang Depression under delta environment, they presented that the high paleoproductivity and fast sedimentation rates favor the enrichment of organic matter under the oxidizing environment. Previous studies on the enrichment of organic matter with geochemical analysis methods mainly focus on the marine shale. Although there are a spot of relevant research on that for the marine-continental transitional organic-rich shale, they paid attention to the part of the marine-continental transitional deposition environment (Adegoke et al., 2015; Wang et al., 2017). Then, it is lack of the scientific study on the enrichment of organic matter in the marine-continental transitional organic-rich shale completely. In this paper, the major elements, trace elements and TOC of the Upper Permian Longtan Formation organicrich shale in western Guizhou, China are tested. The dynamic response relationships between the enrichment of organic matter and paleoclimate, redox conditions, paleoproductivity, and detrital influx are built. Finally, the key factors that influence the enrichment of marine-continental transitional organic-rich shale are discussed.

controversies, including the factors controlling the accumulation of organic matter during shale formation (Murphy et al., 2000); the two main schools of thought are the preservation model and productivity model (Talbot, 1988; Carroll and Bohacs, 1999). The preservation model emphasizes that redox conditions, sedimentation rate, and water depth variation are the key factors that influence organic matter accumulation, especially the bottom water dysoxia/anoxia plays a decisive role (Demaison and Moore, 1980; Arthur and Sageman, 1994; Arthur et al., 1998). For example, the diminution of aerobic decomposition of the Black Sea contributes to organic matter preservation (Yan et al., 2015). The productivity model favors the organic carbon flux influenced by paleoclimate, paleoproductivity and detrital influx (Wignall and Newton, 2001). This model claims that marine surficial primary productivity is the key factor (Pedersen and Calvert, 1990; Sageman et al., 2003; Gallego-Torres et al., 2007; Lash and Blood, 2014). The deposition of shale is influenced by the paleoenvironment and contributes to the differences in the controlling factors at various depositional environments (Arthur and Sageman, 1994; Ganeshram et al., 1999; Lash and Blood, 2014), and no single factor can clarify the mechanisms of organic matter accumulation (Rimmer, 2004). Marine shale is usually deposited in deep water with large quantity of microplankton, the anoxic preservation condition and high paleoproductivity contribute to the enrichment of organic matter, such as the Barnett shale and Antrim shale in North America and the Longmaxi Formation shale in South China (Martini et al., 2003; Loucks and Ruppel, 2007; Ma et al., 2016). The marine-continental transitional organic-rich shale is usually deposited in the shallow water environment (dysoxic-oxic depositional environment and mixed organic source), which is influenced by both the ocean and river, and that is really different from that for marine shale (Adegoke et al., 2014; Hou et al., 2015). Then, the key factors that influence the enrichment of organic in the marine-continental transitional shale would be the primary coverage in this paper. Geochemistry is used extensively to elucidate the evolution and reconstruction of the paleoenvironment (Böning et al., 2004; Tribovillard et al., 2006). Nesbitt and Young (1982) first presented the chemical index of alteration (CIA) to investigate the weathering degree of feldspar form the Paleoproterozoic period in Canada. The CIA, which can reflect the degree of chemical weathering of the source rock and rebuild the paleoclimate, has been utilized widely (McLennan, 1993; Ahmad et al., 1998; Vital and Stattegger, 2000; Roddaz et al., 2006; Li and Yang, 2010). The elements Al and Ti are considered detrital influx indicators with stable chemical properties. Combined with the Si, the source and component of the clastics can be acquired (Yan et al., 2015; Zeng et al., 2015; Chen et al., 2016; Zhao et al., 2016). The trace elements U, V, Cr and Ni collected from sediment are insoluble under reduced conditions. These elements and their ratios can be used to identify the redox environment of the water column (Riboulleau et al., 2003; Algeo and Maynard, 2004; Rimmer, 2004). For plankton, there is

2. Geological setting Guizhou Province is in the south of the Upper Yangtze plate with 7 secondary tectonic units, including the northern Yunnan-Guizhou depression, the eastern Yunnan uplift, the central Guizhou uplift, Wuling depression, the southwestern Guizhou depression, the southern Guizhou depression and the Xuefeng uplift (Luo et al., 2017) (Fig. 1A). From the Middle Permian to the Late Permian period, the Dongwu Movement elevated the West Sichuan Central Yunnan oldland. The fierce rifting led to the extensive eruption of basalt, which created a gentle slope with a trend from northwest to southeast, and the seawater retreated eastward. During the Late Permian period, seawater intruded from the southeast with frequent transgression and regression. Finally, the paleogeographic pattern of the continental facies, transitional facies and marine facies from northwest to southeast were formed (Fig. 1B). The study area is located in the western part of Guizhou (Fig. 1). The Upper Permian Longtan Formation is influenced by both the river from 153

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Fig. 2. Stratigraphic column and vertical distribution of TOC content of the Longtan Formation. Samples are numbered according to depth in the drill.

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Table 1 Major oxides, TOC contents and the chemical index of alteration (CIA) of the Longtan Formation samples. Sample

WS-1 WS-2 WS-3 WS-4 WS-5 WS-6 WS-7 WS-8 WS-9 WS-10 WX-1 WX-2 WX-3 WX-4 WX-5 WX-6 WX-7 WX-8

TOC(wt.%)

9.10 5.79 4.61 3.70 0.50 2.69 1.39 2.10 4.85 3.17 3.48 4.01 4.49 2.50 2.74 4.66 11.42 1.46

Major oxides(%)

CIA

SiO2

Al2O3

Fe2O3

FeO

CaO

MgO

Na2O

K2O

TiO2

MnO

P2O5

20.55 43.77 50.04 42.74 50.95 44.34 50.86 49.85 34.49 49.40 41.75 34.48 62.97 53.31 52.13 47.43 41.47 45.19

12.59 29.87 12.41 17.37 22.05 21.42 20.17 19.50 16.59 16.70 15.69 15.07 17.75 22.75 14.50 25.72 29.36 22.65

2.88 0.07 2.03 1.37 3.51 4.52 1.54 3.00 3.14 2.72 1.54 0.21 0.48 0.05 1.14 2.76 0.85 1.80

29.35 1.30 15.18 14.82 4.75 8.32 4.98 6.10 18.36 10.98 16.54 19.60 1.44 2.80 10.42 1.84 1.79 10.66

1.24 0.24 0.74 0.70 0.37 0.97 2.23 1.68 1.11 0.92 1.64 2.27 0.20 1.39 1.44 0.36 0.34 1.96

1.40 0.58 2.79 2.00 1.66 2.63 1.97 1.84 1.50 2.27 2.18 2.95 0.87 1.27 1.84 1.48 0.71 2.16

0.37 1.00 0.54 1.11 1.38 1.16 2.90 2.50 1.36 1.38 1.34 2.09 0.76 2.37 0.52 1.03 1.02 0.85

0.41 1.15 0.44 1.38 2.67 1.74 2.02 1.74 1.60 0.85 1.34 1.84 3.45 3.33 2.73 3.41 1.92 1.86

2.11 7.30 2.52 3.46 4.56 3.97 4.30 4.17 3.59 3.43 3.56 2.33 2.70 2.96 2.57 3.78 4.41 2.59

0.49 0.01 0.27 0.36 0.01 0.04 0.06 0.05 0.29 0.08 0.55 0.35 0.01 0.08 0.63 0.01 0.01 0.07

0.38 0.08 0.21 0.24 0.12 0.44 0.39 0.46 0.20 0.50 0.25 0.33 0.08 0.29 0.08 0.09 0.09 0.15

88 90 85 81 80 83 67 71 75 82 73 63 77 71 76 82 88 82

The insoluble residues were dissolved in a 130 °C 5 ml 30% (v/v) HNO3 for 3 h and diluted to 25 ml. Then, the X Series II type ICP-MS instrument was used. The analytical uncertainty of the major and trace elements was usually < 5%. The element enrichment factor (EF) can be used to describe the enrichment degree of the element in shale, and it can reflect the deviation of element concentrations between the sample and average shale (Wedepohl, 1971); it can be calculated as follows (Ross and Bustin, 2009):

the west and the sea from the southeast, and it is a typical transitional facies deposit. The delta system and lagoon-tidal flat system are distributed from west to east (Shao et al., 1998). The thickness of the Longtan Formation ranges from 190 to 320 m. The organic-rich shale is mainly composed of light-dark gray pelitic siltstone, silty mudstone, mudstone, dark carbon mudstone and coal seams. These organic-rich sediments develop flasher bedding, wavy bedding, horizontal bedding and thin sand-mud interbred bedding, containing fossils of plants, brachiopods and bivalves.

EF=(element/Al)sample/(element/Al)average 3. Samples and analytical methods

shale

(1)

where the EF is the element enrichment factor; (element/Al)sample is the value of Al-normalization in the sample; and the (element/Al)average shale is the value of Al-normalization in the average shale. The chemical index of alteration (CIA) can be used to evaluate the chemical weathering intensity of source rock (Fedo et al., 1997; Feng et al., 2004; Zeng et al., 2015; Chen et al., 2016). The high CIA means the strong chemical weathering, and reflects hot, humid climates. The CIA is expressed as:

According to the lithostratigraphic unit, the Longtan Formation in western Guizhou can be divided into Upper Longtan Formation and Lower Longtan Formation. The depositional environment of the former is a delta, and that for the latter is lagoon-tidal flat. Eighteen samples from different horizons were collected from the BD1 well (Fig. 1), including 8 samples from the Lower Longtan Formation and 10 samples from the Upper Longtan Formation. The thickness of the Longtan Formation coal measure strata from this well is 258 m. The WX-8 sample is fine sandstone with pelitic strip, while other samples are pelitic siltstone, silty mudstone and mudstone. All of the samples were packed in plastic bags to avoid contamination. TOC contents were determined at Jiangsu Geology and Mineral Resources Research Institute, using a Multi-EA 4000 carbon and sulfur analyzer (GB/T19145-2003, Chinese national standard). First, the samples were crushed and ground to < 80 mesh, and the inorganic carbon was removed along with any excess dilute hydrochloric acid. Then, the samples were burned in the high temperature oxygen stream, where the C would transfer to CO2. Finally, the TOC was measured by the infrared detector. Every sample was tested two times to ensure that the reproducibility of test results was better than 0.1%. Geochemical analyses of samples were completed at the Hebei Regional Institute of Geology and Mineral Resources, and the samples were crushed and ground to < 200 mesh. Major elements were tested with the X-ray fluorescence spectrometer (XRF) (GB/T14506.28–2010, Chinese national standard). The powdered samples were prepared with the fusion glasses method, and then the AxiosMAX XRF instrument was used. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to determine trace element contents (DZ/T 0223-2001, Chinese national standard). The powdered samples were dried at 105 °C. Then, 25 mg samples were put into a screw-top PTFE-lined stainless-steel bomb and digested with 1 ml HF and 0.5 ml HNO3 at 190 °C for 24 h.

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

(2)

where the CaO* is the content of CaO in silicate minerals. It can be adjusted with the content of P2O5, CaOadjust = CaO-P2O5 × 10/3. When the CaOadjust > Na2O, CaO* = Na2O; when the CaOadjust ≤ Na2O, CaO* = CaOadjust. The molar percent of the element is used in Eq (2) (McLennan, 1993). 4. Results 4.1. TOC content characteristics The TOC of the Longtan Formation has a considerable change in the vertical direction, while the TOC first decreases and then increases from the bottom to the top (Fig. 2). The TOC ranges from 1.46% to 11.42% for the lower Longtan Formation with an average of 4.35% (Table 1). The highest value appears at the bottom (WX-7 sample). The TOC varies from 0.5% to 9.1% for the upper Longtan Formation, with an average of 3.79% (Table 1), which is lower than that of the lower Longtan Formation, and the highest TOC is shown in the top (WS-1 sample). 4.2. Major element geochemistry Results of the major element analyses are presented in Table 1. 155

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Fig. 3. Ternary diagram showing relative proportions of major Longtan shale elements SiO2, Al2O3 and CaO. Average shale also shown as checked box (after Wedepohl, 1971).

Marine shale can be regarded as an admixture of three end-member oxides: SiO2 (detrital quartz and/or biogenic silica), Al2O3 (clay fraction), and CaO (carbonate content) (Ross and Bustin, 2009). The ternary plot of the three major oxides indicates that the Longtan shales are enriched in SiO2 relative to CaO and Al2O3 (Fig. 3). The contents of SiO2 for the lower and upper Longtan Formation range from 34.48% to 62.97% and 20.55%–50.95%, with an average of 47.34% and 43.70%, respectively, which are lower than that of average shale. Thus, the content of Al2O3 ranges from 14.50% to 29.36% for the lower Longtan Formation and 12.41%–29.87% for the upper Longtan Formation, with an average value of 20.44% and 18.87%, respectively, which are higher than that of average shale. The CaO content for the lower and upper Longtan Formation is approximately 0.2%–2.27% and 0.24%–2.23%, with an average of 1.20% and 1.02%, respectively; they are only half that of average shale. It can be concluded that the average contents of SiO2, Al2O3 and CaO for the lower Longtan Formation are higher than that of the upper Longtan Formation. The element K has a relationship with muscovite and illite (Chen et al., 2016), and the content of Na has a correlation with the content of plagioclase (Wedepohl, 1971; Ross and Bustin, 2009), while high Fe2+ content is related to abundant pyrite and siderite (Zeng et al., 2015). The enrichment of K and Na in both the lower and upper Longtan Formation is not apparent, EFk = 0.56, EFNa = 0.64 and EFk = 0.34, EFNa = 0.76, respectively (Table 3, Fig. 4), and the enrichment of Fe2+ is high, EFFe2+ = 1.80 and EFFe2+ = 2.73 for the lower and upper Longtan Formation (Table 3, Fig. 4), which indicate the lower content of muscovite, illite, plagioclase and high content of iron mineral. The Ti, as the diagenetically stable constituent of marine sediments, has a high EFTi in both the lower and upper Longtan Formation (3.26 and 4.47). The correlation between the content of TiO2 and Al2O3 in the upper Longtan Formation is clear (R2 = 0.95, Fig. 5) when compared with that of the lower Longtan Formation. It indicates that the Ti comes from the clay lattices or constant detrital material (Ross and Bustin, 2009).

Fig. 4. Enrichment factors of TOC and major elements in Longtan shale. Horizontal line drawn at EFaverage shale = 1 to highlight enrichment or depletion of elements.

4.3. Trace element geochemistry The content of various trace elements in the Longtan Formation are presented in Table 2. Table 3 and Fig. 6 also show the value and variation tendency of the EF of the elements. Compared with the EF of the average shale, the Be, V, Cr, Co, Cu, Zn, Ga, Sr, Zr, Nb, Hf and Ta show various enrichment characteristics. Cu, Zr, Nb, Hf and Ta are highly enriched in the upper and lower Longtan Formation, EFCu = 3.12, EFZr = 2.43, EFNb = 4.01, EFHf = 3.15, EFTa = 2.39 and EFCu = 2.85, EFZr = 3.38, EFNb = 5.43, EFHf = 4.27, EFTa = 3.20 respectively. The Cr, Co and Zn mainly gather in the upper Longtan Formation (EFCr = 2.68, EFCo = 5.56, EFZn = 2.07), while the Be is enriched in the lower Longtan Formation (EFBe = 2.30).

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the detrital influx in provenance terrains, which finally contributed to the mineralogical and chemical compositions of the sediments. The CIA can be used to evaluate the chemical weathering intensity and was applied to describe the climate changes (Fedo et al., 1997; Young and Nesbitt, 1999; Feng et al., 2004; Zeng et al., 2015; Chen et al., 2016). When the climate is cold and arid, the CIA ranges from 50 to 70, indicating a low degree of chemical weathering, while the CIA of the warm and humid climate varies from 70 to 85, indicating a moderate degree of chemical weathering. When the CIA exceeds to 85, the value indicates a high degree of chemical weathering associated with a hot and humid climate (Nesbitt and Young, 1982). The CIA of the Longtan Formation ranges from 63 to 90 and appears to be W-shaped from the bottom to the top (Table 1, Fig. 7), which indicates the study area mainly experiences from hot and humid to warm and humid to hot and humid with the transient cold and arid climate (Sample WS-7 and WX2). The CIA of the upper Longtan Formation is higher (average 80), and it has a positive correlation with the TOC (R2 = 0.42, Fig. 8). This indicates that the climate has a great influence on TOC. The warm and humid climate favors the growing of the advanced plants. With the strong chemical weathering, terrigenous clastic mixed with plant debris were brought into sedimentary water by the river, which provided abundant organic matter to the delta depositional system that was close to the source rock. The average CIA of the lower Longtan Formation is slightly lower than that of the upper Longtan Formation (77 and 80, respectively). Although the climate for the lower Longtan Formation is

Fig. 5. The correlation between TiO2 and Al2O3.

5. Discussion 5.1. Paleoclimate proxies The paleoclimate influenced the chemical weathering intensity and Table 2 Trace elements content and ratios of the Longtan Formation samples. Sample

WS-1 WS-2 WS-3 WS-4 WS-5 WS-6 WS-7 WS-8 WS-9 WS-10 WX-1 WX-2 WX-3 WX-4 WX-5 WX-6 WX-7 WX-8 Sample

WS-1 WS-2 WS-3 WS-4 WS-5 WS-6 WS-7 WS-8 WS-9 WS-10 WX-1 WX-2 WX-3 WX-4 WX-5 WX-6 WX-7 WX-8

Trace elements (ppm) Li

Be

Sc

V

Cr

Co

Ni

Cu

Zn

Ga

Rb

Sr

Zr

Nb

54.17 168.48 49.05 41.46 26.94 58.85 20.58 19.10 16.98 42.96 27.06 17.18 7.82 14.32 11.94 43.62 129.28 47.90

3.53 7.09 2.46 3.17 4.64 4.47 3.87 3.19 4.79 3.44 3.98 4.46 5.21 7.15 8.29 12.87 17.67 8.03

22.34 24.66 13.23 26.22 24.96 25.70 22.18 17.62 24.22 17.54 18.75 15.17 11.08 8.07 14.90 15.84 29.86 9.06

192.70 349.70 166.70 264.80 271.90 310.30 307.50 254.80 285.90 251.50 267.70 173.40 165.80 187.90 188.30 297.20 504.80 143.10

501.06 244.30 333.90 327.06 290.43 125.91 218.25 142.29 375.84 169.74 311.67 114.75 70.28 72.16 181.62 116.73 82.39 149.49

80.35 15.23 703.71 60.36 28.92 40.23 66.17 47.59 89.91 60.31 54.27 30.53 15.51 23.89 27.65 30.66 21.15 29.90

58.03 101.43 544.80 110.34 69.07 68.30 97.56 72.24 99.18 104.49 83.10 47.42 25.83 27.87 20.65 45.90 32.18 44.32

92.07 147.60 65.46 171.63 184.50 177.93 179.46 176.22 187.47 203.76 158.31 112.23 202.23 140.31 109.35 247.14 224.46 62.15

148.30 96.97 415.20 113.70 221.30 199.10 213.60 204.00 390.20 218.70 171.30 178.30 96.06 287.40 132.10 263.90 162.90 337.10

21.94 56.88 25.93 34.02 41.91 37.24 37.10 29.91 32.39 33.07 28.83 31.59 35.42 43.20 30.06 51.55 56.70 39.36

17.08 45.28 16.94 50.89 83.00 60.31 61.02 47.31 48.50 22.18 39.56 47.33 94.43 70.54 74.47 100.43 62.95 32.96

206.03 424.83 213.03 379.43 402.73 458.03 482.43 431.23 530.93 438.43 449.63 471.33 455.43 734.13 458.73 717.33 696.13 463.43

285.80 664.76 270.45 416.09 495.44 438.42 500.30 532.77 371.95 416.74 388.00 540.86 512.72 773.03 457.39 1026.42 945.98 655.20

43.86 133.61 46.35 81.32 95.63 72.05 90.95 86.08 78.48 87.62 74.69 114.44 91.94 126.32 83.26 152.42 175.01 138.38

Trace elements (ppm)

Element ratios

Cd

Cs

Ba

Hf

Ta

Tl

Th

U

Th/U

V/Cr

P/Ti

Ba/Al(10−4)

LaN/YbN

0.20 0.01 0.55 0.15 0.27 0.29 0.41 0.42 0.95 0.39 0.26 0.28 0.18 0.74 0.27 0.96 0.89 0.71

1.51 3.18 1.13 2.72 3.87 2.93 2.38 1.85 2.26 1.49 1.37 1.74 3.89 2.39 2.78 4.78 3.05 1.74

306.29 866.70 288.68 762.80 1049.54 897.91 778.90 676.40 800.93 532.33 630.40 618.73 754.60 879.66 565.92 849.74 553.39 335.12

7.40 15.32 7.00 9.70 11.13 10.42 10.77 11.31 7.69 8.92 8.77 10.99 11.77 16.07 9.55 22.33 23.16 14.48

3.46 10.24 3.07 5.33 6.23 4.76 5.53 5.26 4.75 5.46 4.62 6.64 5.73 9.41 5.06 11.95 11.58 7.65

0.07 0.11 0.23 0.15 0.25 0.19 0.17 0.16 0.14 0.09 0.11 0.14 0.23 0.31 0.20 0.39 0.20 0.14

6.93 16.38 5.57 8.31 7.42 10.79 10.24 10.40 5.51 9.18 5.83 9.35 10.11 14.83 9.25 17.94 22.26 9.33

1.44 4.36 1.22 2.40 3.04 2.62 2.34 2.09 2.01 2.26 1.85 2.24 2.81 3.93 2.59 5.77 7.75 1.88

4.81 3.76 4.55 3.47 2.44 4.11 4.38 4.98 2.74 4.06 3.15 4.17 3.59 3.77 3.58 3.11 2.87 4.95

0.38 1.43 0.50 0.81 0.94 2.46 1.41 1.79 0.76 1.48 0.86 1.51 2.36 2.60 1.04 2.55 6.13 0.96

0.13 0.01 0.06 0.05 0.02 0.08 0.07 0.08 0.04 0.11 0.05 0.10 0.02 0.07 0.02 0.02 0.01 0.04

46 55 44 83 90 79 73 66 91 60 76 78 80 73 74 62 36 28

1.39 1.72 3.06 1.78 4.37 2.02 2.37 2.01 2.58 2.25 1.69 2.02 2.35 2.45 1.96 1.41 1.89 2.25

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Table 3 Average concentrations of Al-normalization all elements of the Longtan Formation samples. Oxide/element

TOC (wt.%) SiO2(%) Al2O3(%) Fe2O3(%) FeO(%) CaO(%) MgO(%) Na2O(%) K2O(%) TiO2(%) MnO(%) P2O5(%) Li(ppm) Be(ppm) Sc(ppm) V(ppm) Cr(ppm) Co(ppm) Ni(ppm) Cu(ppm) Zn(ppm) Ga(ppm) Rb(ppm) Sr(ppm) Zr(ppm) Nb(ppm) Mo(ppm) Cd(ppm) Cs(ppm) Ba(ppm) Hf(ppm) Ta(ppm) Th(ppm) U(ppm) La(ppm) Ce(ppm) Y(ppm)

Average shale

Upper Longtan FM.(n = 10)

Lower Longtan FM.(n = 8)

Abundance

/Al

Abundance

/Al

EF

Abundance

/Al

EF

0.20 58.90 16.70 2.80 3.70 2.20 2.60 1.60 3.60 0.78 0.09 0.16 66.00 3.00 13.00 130.00 90.00 19.00 68.00 45.00 95.00 19.00 140.00 300.00 160.00 18.00 2.60 0.80 5.50 580.00 2.80 2.00 12.00 3.70 40.00 95.00 41.00

0.01 3.53 1.00 0.17 0.22 0.13 0.16 0.10 0.22 0.05 0.01 0.01 7.47 0.34 1.47 14.70 10.18 2.15 7.69 5.09 10.75 2.15 15.84 33.93 18.10 2.04 0.29 0.09 0.62 65.60 0.32 0.23 1.36 0.42 4.52 10.75 4.64

3.79 43.70 18.87 2.48 11.41 1.02 1.86 1.37 1.40 3.94 0.17 0.30 49.86 4.06 21.87 265.58 272.88 119.28 132.54 158.61 222.11 35.04 45.25 396.71 439.27 81.60 5.78 0.36 2.33 696.05 9.97 5.41 9.07 2.38 76.4874 171.87 31.0208

0.20 2.32 1.00 0.13 0.60 0.05 0.10 0.07 0.07 0.21 0.01 0.02 4.99 0.41 2.19 26.59 27.32 11.94 13.27 15.88 22.24 3.51 4.53 39.72 43.98 8.17 0.58 0.04 0.23 69.69 1.00 0.54 0.91 0.24 7.66 17.21 3.11

16.77 0.66 1.00 0.78 2.73 0.41 0.63 0.76 0.34 4.47 1.63 1.67 0.67 1.20 1.49 1.81 2.68 5.56 1.73 3.12 2.07 1.63 0.29 1.17 2.43 4.01 1.97 0.40 0.38 1.06 3.15 2.39 0.67 0.57 1.69 1.60 0.67

4.35 47.34 20.44 1.10 8.14 1.20 1.68 1.25 2.48 3.11 0.21 0.17 37.39 8.46 15.34 241.03 137.39 29.19 40.91 157.02 203.63 39.59 65.33 555.77 662.45 119.56 1.77 0.53 2.72 648.45 14.64 7.83 12.36 3.60 110.8395 237.29875 52.645

0.21 2.32 1.00 0.05 0.40 0.06 0.08 0.06 0.12 0.15 0.01 0.01 3.46 0.78 1.42 22.28 12.70 2.70 3.78 14.51 18.82 3.66 6.04 51.37 61.23 11.05 0.16 0.05 0.25 59.93 1.35 0.72 1.14 0.33 10.24 21.93 4.87

17.75 0.66 1.00 0.32 1.80 0.45 0.53 0.64 0.56 3.26 1.94 0.87 0.46 2.30 0.96 1.51 1.25 1.26 0.49 2.85 1.75 1.70 0.38 1.51 3.38 5.43 0.55 0.55 0.40 0.91 4.27 3.20 0.84 0.80 2.26 2.04 1.05

Average shale composition is from Wedepohl (1971).

Fig. 6. Enrichment factors of trace elements in Longtan shale. Horizontal line drawn at EFaverage

similar to the upper, it is weakly correlated with TOC (R2 = 0.25, Fig. 8). This indicates that the lagoon-tidal flat depositional environment for the lower Longtan Formation is far away from the source area, and it may be influenced more largely by the ocean. Therefore, the control of the climate on the concentration of organic matter is weakened.

shale

= 1 to highlight enrichment or depletion of elements.

5.2. Detrital influx proxies For the transitional sediments, the detrital influx can not only directly provide the abundant organic matter but also serve as a variable diluent (Yan et al., 2015). However, the clay minerals in the clastic rocks can also influence the concentration of the organic matter by providing the adsorption space (Rimmer et al., 2004; Ross and Bustin, 158

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Fig. 7. The stratigraphic distribution of TOC, detrital influx and CIA of the Longtan Formation vertical section.

Fig. 8. Crossplots of TOC content versus CIA and Si of the samples in the Longtan Formation.

do not exhibit similar variation patterns, especially at the bottom of the lower Longtan Formation. There is no correlation between the TOC and Si (R2 = 0.05, Fig. 8), and because silicon can come from different sources, it creates a complex relationship with the TOC.

2009). The chemical properties of Al, Ti, Zr and Th are stable, and can be widely used as detrital influx indicators (Murphy et al., 2000; Tribovillard et al., 2006; Zhao et al., 2016). Al commonly occurs in the aluminosilicate minerals as fine-grain sediment (Calvert and Pedersen, 2007). High field strength elements Zr and Th are also the essential components of the aluminosilicate (Zhao et al., 2016). Ti mainly occurs in the clay minerals, ilmenite and rutile. Si mainly comes from both the siliciclastic (quartz) and the biogenic fractions (Kidder and Erwin, 2001; Zeng et al., 2015). For the upper Longtan Formation, the variation pattern of Si is similar to Al, Ti, Zr and Th (Fig. 7), indicating that the source of the Si is mainly the aluminosilicate minerals. However, the negative correlation between the TOC and Si means that high detrital influx from aluminosilicate decreases the concentration of organic matter as the diluent (R2 = 0.73, Fig. 8). The distribution of elements for the lower Longtan Formation is different from the upper, and they

5.3. Redox proxies Due to the enrichment of V and U, which is controlled by the redox conditions of the water column, these two elements are considered redox-sensitive indicators that determine the paleoredox conditions of the ocean (Calvert and Pedersen, 1993; Jones and Manning, 1994; Crusius et al., 1996; Wignall and Twitchett, 1996; Kimura and Watanabe, 2001; Algeo and Maynard, 2004), and high concentrations of V and U commonly suggest an oxygen-deficient depositional environment (Lézin et al., 2013). Under an oxidizing environment, V can 159

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Fig. 9. The stratigraphic distribution of TOC and redox proxies (U, V, Th/U and V/Cr) of the Longtan Formation vertical section.

Twitchett, 1996; Kimura and Watanabe, 2001). The ratio of Th/U for the lower Longtan Formation ranges from 2.87 to 4.95 (average 3.65) and that for the upper Longtan Formation varies from 2.44 to 4.98 (average 3.93) (Table 2, Fig. 9), which suggests a dysoxic-oxic depositional environment and the oxygen content of the water in the lower Longtan Formation is lower than that of the upper Longtan Formation. The V/Cr ratio increases with the decreasing content of oxygen, high V/Cr reflects anoxic conditions. The oxic environment has the low ratio of V/Cr; it is less than 2. When it ranges from 2 to 4.25, it indicates a dysoxic environment. For the suboxic to anoxic environment, the ratio of V/Cr exceeds 4.25 (Jones and Manning, 1994). The V/Cr ratio for the lower Longtan Formation ranges from 0.86 to 6.13 (average 2.25), which indicates predominantly dysoxic conditions. Regarding the upper Longtan Formation, the ratio of V/Cr varies from 0.38 to 2.46 (average 1.2), and indicates predominantly oxic conditions (Table 2, Fig. 9). Combined with the ratios of Th/U and V/Cr, the shale of the lower Longtan Formation was deposited in a dysoxic environment, while the shale for the upper Longtan Formation was deposited in a relatively oxic environment. There is a negative correlation between the TOC and Th/U for the lower Longtan Formation (R2 = 0.42, Fig. 10), while correlations for TOC and U, V, V/Cr are positive (R2 = 0.7, 0.85 and 0.85, respectively, Fig. 10), it indicates that the dysoxic environment of the bottom water with drawdown oxygen content favors the accumulation of TOC. For the upper Longtan Formation, there are weak negative correlations between the TOC and V, V/Cr (R2 = 0.12 and 0.25, respectively, Fig. 10), with no relationship between the TOC and U, Th/U (R2 = 0.05 and 0.08, respectively, Fig. 10); this means the redox condition has little influence on the enrichment of TOC under the oxic environment

be adsorbed by Fe and Mn hydroxide (Calvert and Piper, 1984). However, in a reducing environment, the V precipitates as V(OH)3 (Breit and Wanty, 1991; Wanty and Goldhaber, 1992). For U, the molecule can be dissolved in water as U6+ under oxidizing conditions, but the insoluble U4+ precipitate gathers in the sediments under reducing conditions (Wignall and Twitchett, 1996; Kimura and Watanabe, 2001). For the Longtan Formation, V displays enrichment with a high content of 143.1 ppm–504.8 ppm (average 254.7 ppm), and the EFv is higher than 1, which is higher than that of average shale. The U is deficient with a content of 1.22 ppm–7.75 ppm (average 2.92 ppm). The EFu is less than 1, which is lower than that of the average shale (Table 3, Fig. 6). Although the abundance of V and U are different, the variation trend of V and U concentrations at the vertical direction is similar, especially for the relationships between the TOC and V and U in the lower Longtan Formation (Fig. 9). The high content of V and U corresponds to the high TOC of shale, which indicates the decrease of oxygen content in water. The enhanced reducing environment favors the preservation of the organic matter. The TOC of sample WX-7 (11.42) is the highest, and it also has the highest content of elemental V and U: 504.8 ppm and 7.75 ppm, respectively. The ratios of Th/U and V/Cr can also estimate the degree of bottom water oxygenation during sedimentation (Myers and Wignall, 1987; Wignall and Twitchett, 1996; Kimura and Watanabe, 2001; Rimmer et al., 2004). Th mainly exists as insoluble Th4+ in the depositional environment. For U, it appears as soluble U6+ in the oxidizing environment, leading to the loss of U in the sediments. Therefore, the high ratio of Th/U commonly indicates intense oxidizing conditions. When the ratio of Th/U is less than 2, this indicates an anoxic environment. With the increase of the oxygen content, it can reach to 8 (Wignall and 160

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Fig. 10. Crossplots of TOC content versus redox proxies (U, V, Th/U and V/Cr) of the samples in the Longtan Formation.

lower Longtan Formation ranges from 28 to 80 (average 63) and that for the upper Longtan Formation varies from 44 to 91 (average 69). These values are lower than those from laminated sediment in the continental margins of Central California (100–200) (Dean et al., 1997), which indicates the paleoproductivity of the Longtan Formation is lower. For the lower Longtan Formation, the Ba/Al ratio rises gradually, indicating moderate paleoproductivity (Fig. 11). For the upper Longtan Formation, the Ba/Al ratio reaches the highest at the sample WS-9 (91), and then decreases rapidly. Following that, it increases, and the high value of Ba/Al of 90 occurred at sample WS-5, and then decreases again. This indicates that there is complex and multivariate paleoproductivity for the upper Longtan Formation. However, the vertical variation trend between the TOC and of Ba/Al ratio is the same for that of P/Ti (Fig. 11). For the lower Longtan Formation, the ratio of Ba/Al increases slowly with the rapid increase of TOC (with the maximum TOC of 11.42). For upper Longtan Formation, the high ratio of Ba/Al in the sample of WS-5 and WS-9, the TOC is 0.5 and 4.85 respectively. The concentration of Ba can also be affected by redox conditions, especially the reducing environment (Chen et al., 2016). Therefore, the redox environment of water is essential to determine the paleoproductivity with Ba. There is a very weak negative correlation between the TOC and P/ Ti, Ba/Al ratio for lower and upper Longtan Formation (R2 = 0.14, 0.11 and 0.35 respectively, Fig. 12), and a poor positive correlation between the TOC and of P/Ti ratio for upper Longtan Formation (R2 = 0.11, Fig. 12). This indicates that the paleoproductivity does not contribute to the high TOC. The enrichment of organic matter is multi-controlled by the paleoclimate and the redox condition of bottom water and detrital influx.

with the increasing content of oxygen.

5.4. Paleoproductivity proxies P, as the essential nutritional element for plankton, is the ultimately limited element for the paleoproductivity in the marine environment (Tyrrell, 1999). P is used to evaluate marine paleoproductivity (Latimer and Filippelli, 2002; Pujol et al., 2006; Algeo et al., 2011). To avoid the dilution effect of the sedimentary organic matter and authigenic minerals on the content of P in terrigenous clastics, the P/Ti ratio can be used to present the nutriture of the ocean (Algeo et al., 2011). The P/Ti ratio for the lower Longtan Formation ranges from 0.01 to 0.1 (average 0.04), and that for upper Longtan Formation varies from 0.01 to 0.13 (average 0.06) (Table 2), which is lower than that for the average shale (0.15) and average pelagic clay (0.33). It is far below the elevated productivity of the modern equatorial Pacific regions (2–8) (Murray et al., 1993). This indicates a poor paleoproductivity of the Longtan Formation. Even though the low P/Ti ratio reflects low productivity, it is found that the vertical variation trend between TOC and P/Ti ratio was not very consistent (Fig. 11). Sample WX-7 has the highest content of TOC (11.42), while the P/Ti ratio is lower (0.01). For the samples WS-5 and WX-3, the ratios of P/Ti are same, but the TOC values are 0.5 and 4.49, respectively. The content of P in the sediments can be affected by both the redox condition and adsorption capacity of Fe compounds, and the P can be released from the organic matter in a reducing environment (Algeo and Ingall, 2007; Westermann et al., 2013), and this could have led to the high TOC and the lower ratio of P/Ti in the WX-7 sample. The element Ba can also indicate paleoproductivity. Combined with Ba2+ in the marine environment and the high concentration of SO42− on the surface of the cankered organic matter, barite (BaSO4) is deposited. This creates a positive correlation between the accumulation rate of Ba and primary productivity (Dymond and Collier, 1996; Paytan and Griffith, 2007). The ratio of Ba/Al can be utilized to qualitatively assess paleoproductivity (Dean et al., 1997), while Al is used as the denominator to eliminate the dilution effects of other components (Algeo and Maynard, 2004; Zeng et al., 2015). The ratio of Ba/Al for the

5.5. Controls on the accumulation of organic matter The transitional deposition is affected by both the river and marine environment; the accumulation of organic matter is a result of multiple factors, which is a complex physical-chemical process. For the productivity factors, the paleoclimate contributed the abundant plants and the weathering degree of rocks in the source area. The detrital influx 161

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Fig. 11. The stratigraphic distribution of TOC and paleoproductivity proxies (P, Ba, P/Ti and Ba/Al) of the Longtan Formation vertical section.

paleoclimate transformed from hot and humid, to warm and humid; the sedimentary environment was a dysoxic lagoon-tidal flat system. The vertical variation trends of TOC and redox indicators are very consistent, which indicates that the decreasing oxygen content responded to the increasing TOC. There are positive correlations between the TOC and U, V, V/Cr (R2 = 0.7, 0.85 and 0.85, respectively). However, the paleoproductivity indicators, such as the P/Ti and Ba/Al, tend to show less influence; they present poor negative correlations with the TOC

provided abundant organic matter (the phytoclasts) on one hand, and acted as the diluents for the organic matter on the other hand. The paleoproductivity controlled the abundance of organic matter directly when the shale was deposited. For the preservation factors, the decrease in the oxygen content in the bottom water provided good preservation conditions for the organic matter, and the deposition rate also contributed to the accumulation of organic matter. During the deposition of the lower Longtan Formation shale, the

Fig. 12. Crossplots of TOC content versus paleoproductivity proxies (P/Ti and Ba/Al) of the samples in the Longtan Formation. 162

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(R2 = 0.14 and 0.11, respectively). These results indicate that the dysoxic conditions of the bottom water preservation environment was the key factor that contributed to the accumulation of organic matter for the lower Longtan Formation shale. Additionally, there is a weak positive correlation between the CIA and TOC (R2 = 0.25), meaning that the warm and humid paleoclimate also had a certain contribution to the enrichment of the organic matter. For the upper Longtan Formation shale, the paleoclimate was opposite of that for the lower Longtan Formation (from warm, humid to hot, humid); the sedimentary environment was a relatively oxic delta system. The indicators of redox and paleoproductivity have no relationship with the TOC (Figs. 10 and 12), indicating that the increasing oxygen content that leads to the redox conditions did not influence the organic matter preservation. Meanwhile, there is a positive correlation between the CIA and TOC (R2 = 0.42) and a negative correlation between the content of Si and TOC (R2 = 0.73). This indicates that the paleoclimate and detrital influx of the productivity factors contributed to the accumulation of organic matter in the upper Longtan Formation shale. Because the delta depositional zone is closer to the provenance terrains, the warm and humid climate promoted land-based plantgrowth, which provided enough organic matter. However, the high detrital influx (aluminosilicate) should not be ignored because it also diluted the concentration of the organic matter. The rare earth elements (REE) are mainly combined with detritus or suspended substances and transported into the sea. Then, their retention time in marine would be the key factor to the REE fractionation. With the fast sedimentation rate, the REE are less exchanged with the sea water, and the REE fractionation is weak. Contrarily, the REE have enough time to be adsorbed on clay and combine with organic matter leading to the intensive REE fractionation (Tenger et al., 2006). The normalized ratio of w(La)N/w(Yb)N with the North America shale can reflect the degree of REE fractionation. When the value of w(La)N/w (Yb)N is close to 1, it reflects the fast sedimentation rate (weak REE fractionation). Contrary, if the value of w(La)N/w(Yb)N is significant less or exceed 1 (intensive REE fractionation), which means the lower sedimentation rate (Tenger et al., 2006; Zhang et al., 2013; Zeng et al., 2015). The w(La)N/w(Yb)N ratios for the lower and upper Longtan Formation range from 1.41 to 2.45 (average 2) and 1.39 to 4.37 (average 2.36), respectively, suggesting a fast sedimentation rate during the Longtan Formation shale deposition. For the upper Longtan Formation, there is a negative correlation between the TOC and the w (La)N/w(Yb)N ratio (R2 = 0.35, Fig. 13), indicating a faster sedimentation rate responds to the high TOC, in which w(La)N/w(Yb)N is closer to 1. The depositional environment of the upper Longtan

Formation shale is delta with a high oxygen content; with the high speed of sedimentation, it can reduce the resolving time of the organic matter, which favors the preservation and enrichment of the organic matter (Stow et al., 2001). 5.6. Sedimentary model of the marine-continental transitional shale The evolution of the paleoenvironment for the Longtan Formation shale of the Late Permian in western Guizhou can be divided into two stages. For the Lower Permian, the shale deposited in the lagoon-tidal flat sedimentary environment under dysoxic conditions (Fig. 14A). The semi-restricted, low hydrodynamic shallow basin (lagoon) and gently sloping coastal plain (tidal flat) are the ideal environment for the accumulation of the organic matter. At this time, study area located in the low latitudes (Wang and Li, 1998; Rees et al., 2002) with a warm and humid climate. This favors the plant and microbes growth, which can provide abundant and diverse sources of organic matter. According to the redox indicators, good preservation conditions associated with a decrease in oxygen content mainly contributed to the accumulation of organic matter for the lower Longtan Formation shale. Regarding the upper Longtan Formation, the fluviation was enhanced and the sedimentary environment transformed to the delta with high oxygen content (Fig. 14B). The paleoenvironment was warm and humid; the river brought terrigenous clastics together with land plant debris and poured into the basin with a high sedimentation rate. Then, the delta plain (swamp) had a favorable environment for the enrichment of organic matter. Because of the enhanced oxygen content, the climate and detrital influx were the major controlling factors that influenced the concentration of organic matter for the upper Longtan Formation shale. 6. Conclusions The depositional environment of the marine-continental transitional shale is significantly different from that of typical marine shale, which leads to the variable accumulation characteristics of the organic matter. Geochemical data provides a good constraint on the accumulation of organic matter. Based on the analyses of geochemical parameters of the Upper Permian Longtan Formation shale in western Guizhou, these followings are concluded: (1) Organic-rich shale of the lower Longtan Formation is deposited in the environment of the lagoon-tidal flat. There are significant positive correlations between TOC and redox indicators (V, U and V/ Cr) (R2 = 0.7, 0.85 and 0.85, respectively). However, there is a weak positive relationship between TOC and CIA (R2 = 0.25). This indicates that the dysoxic bottom water conditions played a dominant role in the enrichment of organic matter. (2) Organic-rich shale of the upper Longtan Formation is deposited in the environment of the delta. There is a positive correlation between TOC and CIA (R2 = 0.42). Combined with the negative correlation between the TOC and w(La)N/w(Yb)N (R2 = 0.35), it indicates that the warm and humid climate under oxidizing environment and high sedimentation rate jointly controlled the enrichment of organic matter. However, the significant negative relationship between TOC and Si (R2 = 0.73) implies the detrital influx (aluminosilicate) can serve as the variable diluent, which decreased the concentration of TOC. (3) According to the paleoproductivity proxies, poor relationships between TOC and ratios of P/Ti, Ba/Al indicate that the paleoproductivity is not the critical factor that controls the enrichment of organic during deposition of Longtan Formation. Acknowledgments

Fig. 13. Crossplot of TOC content versus sedimentation rate of the samples in the Longtan Formation.

This work was supported by the Natural Science Foundation of China (41572140), the National Major Special Project of Science and 163

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Fig. 14. Schematic illustration to show factors controlling the accumulation of organic matter in the Longtan sediments in western Guizhou.

Technology of China (2016ZX05044001), the Fundamental Research Funds for the Central Universities (2015XKZD07), and the Qing Lan Project.

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