geochemistry, paleoenvironment, and mechanism …...lateral variation of sedimentary environments in...

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Running title: Mechanism of Organic-Matter Enrichment of shale in Sichuan Basin

Geochemistry, Paleoenvironment, and Mechanism of Organic-Matter

Enrichment in the Lower Silurian Longmaxi Formation Shale in the

Sichuan Basin, China

Zhipeng CHEN1,2,3,5

, Junping CUI1,2, *

, Zhanli REN1,2

, Shu JIANG3,4

, Xing LIANG5, Gaocheng WANG

5, and

Chen ZOU 5

1 State Key Laboratory of Continental Dynamics of Ministry of Geology, Northwest University, Xi'an 710069, Shaanxi,

China 2 Department of Geology, Northwest University, Xi'an 710069, Shaanxi, China 3 State Key Laboratory of Sedimentary Basin and Energy Resources & Faculty of Earth Resources, China University of

Geosciences, Wuhan 430074, Hubei, China 4 Energy and Geoscience Institute, University of Utah, Salt Lake City, UT 84108, USA 5 PetroChina Zhejiang Oilfield Company, Hangzhou, Zhejiang 310023, China

Abstract: To investigate the mechanism of the organic-matter enrichment in the Lower Longmaxi Formation shale, the

geochemistry and total organic carbon (TOC) of the Longmaxi Formation black shales in the Jiaoshiba, Zhaotong, and

Weiyuan areas of the Sichuan Basin were analyzed. Paleoproductivity proxy parameters (Babio, Siex, and Ni/Al), clastic

influx proxies (TiO2 and Ti/Al), redox indices (V/Cr, Ni/Co, V/[V+Ni], and U/Th), and hydrothermal indicators (Fe, Mn, and

Y concentrations; Fe/Ti ratio and a Ni-Zn-Co diagram) were employed to decipher the paleoenvironment of the Lower

Longmaxi Formation shales. TiO2 and Ti/Al indicated low terrigenous detrital influx in all three areas. However, Babio, Siex,

and Ni/Al indicated high productivity in the Jiaoshiba area. V/Cr, Ni/Co, and U/Th indicated higher oxygen content with

larger fluctuations in the Zhaotong and Weiyuan areas. Fe, Mn, and Y concentrations and the Fe/Ti ratio implied greater

active hydrothermal activity in the Weiyuan area. These heterogeneities were considered to be closely related to the

paleoenvironment and paleogeography, and the large basement faults that developed during the Chuanzhong paleo-uplift

could have provided vents for deep-hydrothermal-fluid upwelling. The redox indices (V/Cr, Ni/Co, and U/Th) and a

paleoproductivity proxy (Ni/Al) displayed a significant correlation with the TOC, suggesting that both excellent preservation

conditions and high paleoproductivity were the controlling factors for the enrichment of organic matter in the Longmaxi

Formation shale. There was no obvious correlation between the clastic influx proxy (Ti/Al) and the TOC due to the

extremely low supply of terrigenous debris. The hydrothermal indicator (Fe/Ti) was negatively correlated with the TOC in

the Weiyuan area, indicating that hydrothermal activity may have played a negative role in the accumulation of organic

matter. This study suggests that the enrichment of organic matter in the Longmaxi Formation marine shale varied according

to the paleogeography and sedimentary environment.

Key words: Geochemical index; Paleogeography; Organic-matter enrichment; Hydrothermal fluid; Marine shale; Sichuan

Basin

E-mail: [email protected]

1 Introduction

Marine black shale is mainly deposited in deep water environments with rapid deposition rates and closed geological conditions (Arthur et al., 1994). Because the enrichment of organic matter in marine shale is an important material basis for hydrocarbon generation, it has received much attention. Recent studies have shown that the enrichment of organic matter is the result of multiple factors (Rimmer et al., 2004; Wu et al., 2017). The accumulation of organic matter can be significantly impacted by not only primary productivity and preservation under low oxygen levels (Arthur et at., 1994; Murphy et al., 2000) but also clastic influx, oceanic circulation and hydrothermal activity (Shock and Schulte, 1998; Sageman et al., 2003; Riquier et al., 2006; Gomez-Saez, 2016). Therefore, before we predicted the distribution of shale organic matter, it was necessary to distinguish the main factor controlling organic matter enrichment according to the different sedimentary environments. Additionally, a depositional environment not only controls the formation of the original organic matter but also greatly affects inorganic geochemical variations. Because inorganic geochemistry records paleoenvironmental properties and their evolution (Fernex et al., 1992; Morford and Emerson, 1999; Brumsack, 2006), it plays an important role in the analyses of sedimentary conditions. Extensive research has been conducted on the inorganic geochemistry of shale, and considerable progress has been made in the understanding of the variations in the concentrations of elements in different depositional environments (Morford and Emerson, 1999; Brumsack, 2006; Tribovillard et

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10.1111/1755-6724.13868.

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https://doi.org/10.1111/1755-6724.13868

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al., 2006). A series of geochemical indicators for deciphering redox conditions, primary productivity, detrital influx and hydrothermal activity has been established (Toth, 1980; Boström, 1983; Hatch and Leventhal, 1992; Jones and Manning, 1994; Bertrand et al., 1996; Murphy et al., 2000). These indicators allow for greater precision when discussing the depositional factors that affect organic-matter accumulation and the prediction of favorable areas for organic-rich shale deposition.

The thick Early Silurian Longmaxi Formation organic-rich shale, widely distributed throughout the Sichuan Basin, is not only the significant source rock for hydrocarbon generation but also the most favored reservoir for China’s shale gas exploration (Dai et al., 2014; Guo, 2013; Guo and Zhang, 2014; Li et al., 2016; Ran et al., 2016). The commercial development of shale gas has been successful in the Jiaoshiba, Changning, Fushun-Yongchuan, Zhaotong and Weiyuan areas of the Sichuan Basin (Zhai et al., 2018; Zou et al., 2018). Although the Longmaxi Formation shale is generally considered to have been deposited in an oxygen-deprived deep-sea environment, the abundance of organic matter in the Longmaxi Formation shale shows strong vertical and lateral heterogeneity (Zhang et al., 2012; Guo, 2013; Guo and Zhang, 2014; Li et al., 2016). Therefore, numerous studies have been conducted on the mechanism of the organic-matter enrichment in the Longmaxi Formation shale in the Sichuan Basin and its peripheral areas (Li et al., 2015; Wang et al., 2015, 2016; Yan et al., 2015; Zhao et al., 2016; Wu et al., 2017; Zhang et al., 2018). These studies have generally suggested that the lower part of the shale, characterized by high organic-matter abundance, was deposited in an anoxic or dysoxic environment, while the upper shale, characterized by low organic-matter content, was deposited in an oxic environment. The paleoenvironment’s change from anoxic or dysoxic conditions in the lower part to oxic conditions in the middle and upper parts has been shown to be the main reason for the vertical heterogeneity of organic-matter abundance in the Longmaxi Formation shale. However, these studies mainly concentrated on a single block or well, so they lack an analysis of contrasting blocks and wells. Therefore, knowledge of the lateral variation of sedimentary environments in the Longmaxi Formation shale and a distinction of the factors that controlled the organic-matter enrichment mechanism in different areas is still limited.

In this paper, a systematic comparison of the geochemistry of the Jiaoshiba, Zhaotong, and Weiyuan areas, where high-yielding shale gas was developed, was conducted. Primary productivity, detrital influx, redox conditions, and hydrothermal activity were chosen to indicate the paleoconditions of the organic-rich sediment. Multiple geochemical parameters (U/Th, V/Cr, Ni/Co, and V/[V +Ni] ratios) were used as proxies for the redox state of the lower water. Excess Si and the Ni/Al ratio were used to indicate primary productivity. The role of detrital input was evaluated using the Ti/Al ratio. Hydrothermal activity was determined using a Ni-Co-Zn diagram; Fe, Mn, and Y concentrations and the Fe/Ti ratio. These geochemical indicators were combined to discuss the differences in the paleoenvironmental conditions of the Longmaxi Formation and their effect on the organic-matter accumulation mechanism. 2 Geological Settings

The Sichuan Basin, located on the Upper Yangtze Plate in southwestern China (Mei and Liu, 2017), is the

most important petroliferous basin in South China and is currently the largest shale gas producing region outside North American (Guo, 2013; Dai et al., 2014; Guo and Zhang, 2014). Although the Jiaoshiba, Zhaotong, and Weiyuan areas are all important shale gas producing fields within the Sichuan Basin, they are remote from each other (Fig. 1). The Jiaoshiba area is located in the southeastern Sichuan Basin; the Zhaotong area is located in the southern Sichuan Basin, adjacent to the Qianzhong uplift, and Weiyuan area is located in the southwestern Sichuan Basin, close to the Chuanzhong uplift (Fan et al., 2018).


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Fig. 1. Early Silurian paleogeographic map of the Sichuan Basin, China, showing the sedimentary facies of the Lower

Silurian Longmaxi Formation (modified after Dai et al., 2014; 2016, Zou et al., 2015; Guo and Zhang, 2014).

During the Late Ordovician–Early Silurian period, Chuanzhong uplift in the northwest of the Upper Yangtze

Platform, Qianzhong uplift in the south of the Upper Yangtze Platform and Jiangnan-Xuefeng uplift in the southeast of the Upper Yangtze Platform extended under the intensive compression of the Caledonian orogeny (Chen et al., 2004; Liang et al., 2009; Xu et al., 2012; Li et al., 2017). Coincident climate warming and the global melting of ice sheets resulted in rapid sea level rises that lead to global transgressions (Yan et al., 2010; Zhang et al., 2012). Under these circumstances, a broad, semi-enclosed deep-water bay oriented to the north was formed in the Upper Yangtze Plate, resulting in the deposition of the Wufeng and Longmaxi Formations black shales (Guo and Zhang, 2014; Zhao et al., 2017).

Both the Wufeng Formation and the Lower Longmaxi Formation (LLF) are composed of graptolite-bearing organic-rich shales that reflect a deep-shelf depositional environment. The Upper Longmaxi Formation is mainly composed of gray siltstone, silty mudstone, and argillaceous limestone, which suggests a semi-deep-shelf depositional environment following a regression. The Wufeng Formation and the LLF shales display similar characteristics and are the main shale gas producing reservoirs in the Sichuan Basin (Guo, 2013, Ran et al., 2016). However, the Wufeng Formation and the LLF were deposited during two completely different global transgressions during the Late Ordovician–Early Silurian (Su et al., 2007; Yan et al., 2010; Wang et al., 2016). The top of the Wufeng Formation (the GuanyinQiao Member) contains shell-rich marl and limey mudstone and is considered to have been deposited during a rapid sea-level fall, a result of abrupt Gondwana glaciation between two transgressions (Chen et al., 2004; Yan et al., 2010; Wang et al., 2016). In the Yangtze Platform, the widely distributed Guanyinjiao Member is regarded as the boundary between the two sets of shales. In addition, the LLF shale predominate the development of shale gas, having a greater thickness than the Wufeng Formation shale. Therefore, the current study mainly focused on the LLF shale.

Previous studies have indicated that the lower and upper LLF shales exhibit differing geochemistry characteristics and abundance of organic matter (Zhang et al., 2012; Guo, 2013; Guo and Zhang, 2014; Li et al., 2016). To accurately distinguish the shales in the three study areas, the author divided the LLF into layer A


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(LLA), the lower section, and layer B (LLB), the upper section, according to lithological and logging characteristics (Fig. 2). LLA shale is mainly lithologically characterized by siliceous shale and high levels of logged natural gamma rays (GR > 200 API). LLB shale is principally lithologically characterized by silty shale and argillaceous shale and low levels of logged natural gamma rays (GR < 200 API).

Fig. 2. Lithology, total organic carbon (TOC), and gamma-ray (GR) characteristics of the studied profiles. The well depth flatten according to the top of the Upper Ordovician Baota Formation. (The stratigraphic columns of wells JY2 and W201 are

modified after Guo, 2013; Wang et al., 2015, 2016; Zhao et al., 2016). LLA = layer A: the lower section of Lower Longmaxi Formation (LLF) and

LLB = layer B: the upper section of the LLF.

3 Samples and Methods

This study was conducted on data from the LLF shales collected from Well JY2 in the Jiaoshiba area, Well YS12 in the Zhaotong area, and Well W201 in the Weiyuan area. JY2, YS12, and W201 are all key exploration wells in the Sichuan Basin and could respectively reflect the geological characteristics of the Longmaxi Formation shale in these areas. The data for JS2 and W201 were provided by Wang et al. (2015, 2016) and Zhao et al. (2016), respectively. The data for YS12 were provided by the current study’s experimental analyses and well logs. A total of 19 LLF shale core samples from the YS12 Well were analyzed for total organic carbon (TOC) and major and trace elements.

The TOC content was determined using infrared radiation spectroscopy using a carbon-sulfur analyzer (LECO CS-400) at the Langfang Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation (CNPC), after pretreating the samples with hydrochloric acid and water (1:9) to remove any carbonate minerals. The precision of this analysis is regarded to be <0.5%. The major elements were measured using an X-ray fluorescence spectrometer (PANalytical Axios) at the Hangzhou Research Institute of Petroleum Exploration and Development, CNPC. The analytical fusion glass was made from a 1: 8 mixture of powdered sample and flux (Li2B4O7), and the analytical uncertainty is considered to be < 3%. Trace and rare element analyses were undertaken with an inductively coupled plasma mass spectrometer (ICP-MS) (Thermo X Series II) at the Hangzhou Mineral Resources Supervision and Inspection Center, Ministry of Land and Resources, China. Before the ICP-MS analysis, sample splits (50 mg) were digested in a screw-cap PTFE-lined stainless-steel bomb using a 1 ml ultrapure HF and 0.5 ml HNO3 solution and dried in an oven at 200°C for 48 h. The experimental analysis was calibrated to standard reference materials GSR5 and GSD12. The relative standard deviation was estimated to be < 5%. Additionally, the elemental capture spectrum log (ECS), including Si, Al and Ti; the natural gamma spectrometry log (NGS), including U and Th; and calculated TOC were provided by Schlumberger Ltd., Houston, TX, USA.


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4 Results 4.1 The abundance of organic matter

The TOC of the LLF shale from JY2, YS12, and W201 is shown in Fig. 2 and Table 1. The mean TOC in LLA in the JY2 samples was 3.82% (3.35%–4.23%), 2.34% (1.43%–3.60%) in the YS12 samples and 2.74% (1.78%–4.28%) in the W201 samples. The mean TOC in LLB in the JY2 samples was 1.53% (0.52%–2.46%), 0.67% (0.31%–1.2%) in the YS12 samples and 0.48% (0.05%–1.13%) in the W201 samples. The TOC in LLA from the three areas was more than 2%, which was significantly larger than the TOC in LLB. This result was also found by calculating the TOC from the YS12 ECS and NGS logs (Fig. 3). It should be noted that the TOC of both LLA and LLB was noticeably larger in JY2 than in YS12 or W201, and there was no significant difference between the latter two. 4.2 Main and trace elements

The stratigraphic variations of Si, Al, Ti, U and Th and Ti/ Al and U/Th ratios from YS12 are shown in Fig. 3. The ECS logging data were consistent with the results of the core analysis. The Si contents of the LLF shale varied between 24.2% and 35.52% (avg. 28.61%) and are very close to post-Archean Australian shale (PAAS; Taylor and McLennan, 1985). The Ti content, which ranged from 0.16% to 0.45% (avg. 0.35%), was much lower than PAAS (0.60%). The Al content, which ranged from 2.64% to 10.62% (avg. 7.39%), was slightly lower than PAAS (10%). The Ti/Al ratio was relatively consistent, with values ranging from 0.034% to 0.061% (avg. 0.049%), throughout the LLF shale. The NGS logging data indicated that the U content was highly variable, ranging from 0.78 to 40.33 ppm (avg. 6.10 ppm), whereas the Th content showed relatively consistent values ranging from 12.07 to 35.63 ppm (avg. 20.76 ppm). The U/Th ratio significantly increased downward, from 0.04 to 2.08, reaching a maximum at the base of the LLF shale. The stratigraphic variations of U content and U/Th ratio showed a very similar trend to the variation in TOC, indicating that the U concentration was closely connected with the accumulation of organic matter.

Fig. 3. Stratigraphic distribution of Si, Al, Ti, U, and Th elements, ratios of Ti/Al and U/Th, excess Si and TOC of the Lower

Longmaxi Formation (LLF) in YS12, Zhaotong area, Sichuan Basin. LLA = layer A: the lower section of the LLF and LLB = layer B: the upper section of the LLF.

The ternary plot of the major elements (SiO2, Al2O3, and CaO) showed obvious differences among the LLF shales in JY2, YS12, and W201, as well as between LLA and LLB (Fig. 4). In a ternary plot, SiO2 content generally indicates clastic quartz and biogenic Si, Al2O3 content indicates clay and CaO content indicates


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carbonate. The SiO2 content of the LLF shale in JY2 (avg. 66.08%) was the largest, followed by YS12 (avg. 61.26%) and W201 (avg. 50.11%), which indicated higher silicate mineral contents. In contrast, the CaO contents of the LLF shale in W201 (avg. 8.62%) and YS12 (avg. 7.20%) were greater than in JY2 (avg. 1.97%), which indicated higher carbonate contents. The Al2O3 contents of the LLF shale in JY2, YS12, and W201 displayed low and stable values (avg. 12.85%, 12.91%, and 13.98%, respectively), which indicated a poor supply of terrigenous debris. In addition, LLA was characterized by a rich SiO2 content relative to LLB. It should be noted that the TFe2O3 content of the LLF shale in W201 (avg. 8.95%) was noticeably greater than that of JY2 (avg. 4.76%) and YS12 (avg. 4.71%), and also greater than PAAS (7.22%).

Fig. 4. Ternary plot of SiO2, Al2O3, and CaO for the Lower Longmaxi Formation (LLF) shale in the Jiaoshiba, Zhaotong, and

Weiyuan areas, Sichuan Basin.

The trace elements (V, Cr, Co, Ni, Zn, Ba, Rb, La, Ce, and Y) of the LLF shale in JY2, YS12, and W201 were

normalized against PAAS (Fig. 5). The ratios of most of the elements varied between 0.1 and 10 times PAAS, except for Y in W201, which was greater than 10 times. The PAAS-normalized V, Cr, Co, and Ni element values for YS12 were relatively lower than those of JY2 and W201. However, PAAS-normalized Ba element values of JY2 were noticeably greater than those of YS12 and W201, while PAAS-normalized Y element values for W201 were noticeably greater than those for JY2 and YS12. There were small differences among the three wells of the large ionic lithophile element Rb and the rare earth elements, La and Ce.

The TOC and indexes of primary productivity (Babio, Siex and Ni/Al), detrital influx (TiO2 and Ti/Al), redox conditions (V/Cr, Ni/Co, V/[V+Ni] and U/Th) and hydrothermal activity (Fe/Ti and Co/Zn) of LLA and LLB shales in JY2, YS12 and W201 are listed in Table 1.

Fig. 5. Comparison of trace elements in the Lower Longmaxi Formation (LLF) from the Jiaoshiba, Zhaotong, and Weiyuan

areas, Sichuan Basin. The trace elements (V, Cr, Co, Ni, Zn, Ba, Rb, La, Ce, and Y) were normalized against PAAS (Taylor and McLennan, 1985).


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Table 1. Total organic carbon (TOC) and indexes of primary productivity (Babio, Siex and Ni/Al), detrital influx (TiO2 and Ti/Al), redox conditions (V/Cr, Ni/Co, V/[V+Ni] and U/Th)

and hydrothermal activity (Fe/Ti and Co/Zn) of the Lower Longmaxi Formation (LLF) shale in the Weiyuan, Zhaotong and Jiaoshiba areas, Sichuan Basin, China.

Block/Layer TOC (%) Primary productivity

Detrital influx

Redox conditions

Hydrothermal activity

Babio(ppm) Siex(%) Ni/Al TiO2 (%) Ti/Al V/Cr Ni/Co V/(V+Ni) U/Th Fe/Ti Co/Zn

JY2 in Jiaoshiba area

LLA 3.35-4.23

(3.82)

1504-2072

(1784)

14.74-19.85

(16.87)

15.36-22.38

(18.56)

0.44-0.53

(0.49)

0.053-0.062

(0.058)

2.90-3.55

(3.28)

4.44-7.65

(5.49)

0.68-0.73

(0.71)

0.93-1.89

(1.26)

6.28-10.00

(7.74)

0.08-0.16

(0.14)

LLB 0.52-2.46

(1.53)

1559-11323

(3065)

0.17-13.28

(7.72)

4.86-13.60

(8.42)

0.45-0.79

(0.62)

0.042-0.061

(0.049)

1.63-2.85

(2.04)

2.19-5.66

(3.54)

0.62-0.83

(0.73)

0.19-0.56

(0.41)

8.76-12.89

(10.31)

0.06-0.24

(0.17)

YS12 in Zhaotong area

LLA 1.43-3.60

(2.34)

429-555

(498)

1.35-21.59

(10.95)

6.13-20.99

(11.16)

0.37-0.75

(0.56)

0.045-0.056

(0.048)

1.42-3.59

(2.26)

3.50-8.08

(5.20)

0.66-0.71

(0.69)

0.23-2.12

(0.77)

8.60-10.35

(9.28)

0.09-0.24

(0.17)

LLB 0.31-1.2

(0.68)

200-955

(591)

0.84-16.00

(5.78)

2.67-5.33

(4.28)

0.27-0.73

(0.6)

0.034-0.061

(0.049)

0.64-1.35

(1.14)

1.89-3.49

(2.69)

0.65-0.73

(0.70)

0.04-0.33

(0.17)

7.16-21.68

(10.39)

0.05-0.31

(0.18)

W201 in Weiyuan area

LLA 1.78-4.28

(2.74) (157)

0.29-24.97

(4.73)

6.45-38.73

(12.3)

0.21-0.65

(0.49)

0.041-0.069

(0.052)

1.49-4.27

(3.03)

2.51-9.54

(4.74)

0.19-0.82

(0.72)

0.26-3.30

(0.78)

11.64-49.44

(18.32) (0.24)

LLB 0.05-1.13

(0.48) (89) -

5.16-5.96

(5.59)

0.57-0.72

(0.62)

0.036-0.045

(0.039)

1.30-1.61

(1.44)

2.42-2.86

(2.55)

0.70-0.76

(0.73)

0.17-0.25

(0.21)

21.45-23.14

(22.55) (0.33)

Note: Values separated by a dash indicate the range. Values in parentheses are averages. No values represent no data. The data for W201, located in the Weiyuan area, and JY2 in the Jiaoshiba area were provided by Wang et al. (2015, 2016) and Zhao et al. (2016), respectively. LLA= layer A of the Lower Longmaxi Formation (LLF) at the lower section. LLB = layer B of the LLF at the upper section. LLA= the lower layer of the LLF and LLB = the upper layer of the LLF.


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5 Discussions 5.1 Implications for the paleoenvironment 5.1.1 Primary productivity

Ba is widely used to reconstruct primary productivity because extensive studies have indicated that Ba enrichment is closely associated with biological processes (Dymond et al., 1992; Pedersen and Calvert, 1990; Francois et al., 1995; McManus et al., 1998; Eagle et al., 2003). In addition, barite, the main form of Ba in deep oceans, is considered to be the consequence of organic-matter decomposition (Dymond et al., 1992; Francois et al., 1995). In addition to biogenic Ba, some of the Ba found in marine sediments is of terrigenous origin (Eagle et al., 2003). Therefore, the crucial step when reconstructing primary productivity from Ba content is eliminating the impact of terrigenous material. Dymond (1992) proposed an equation based on Ba and Al elements to calculate a biogenic Ba content (Babio):

Babio = Batotal − Alsed × (Ba/Al)terr (1) In this equation, all the aluminum in the sediments is assumed to have originated from terrigenous material.

Because of the relatively stable Ba/Al ratio in the crust (ranging from 0.005 to 0.01), the terrigenous Ba content can be calculated. Thus, the biogenic Ba content in sediments equals the total Ba concentration minus the terrigenous Ba content. Considering the relatively large clay content of the Longmaxi Formation shale, we took the standardized value of the Ba/Al ratio as 0.005.

The mean Babio content in LLA was highest in the Jiaoshiba area (1,784 ppm), medium in the Zhaotong area (498 ppm) and lowest in the Weiyuan area (157 ppm). Additionally, the Babio content of LLA in both the Jiaoshiba and Zhaotong areas was significantly smaller than that of LLB (Table 1). This result is consistent with recent studies on the Longmaxi Formation in the Sichuan Basin (Yan et al., 2015; Li et al., 2015; Zhao et al., 2016). Since it is possible for barite to dissolve in sulfate-reducing conditions, leading to Ba loss (van Os et al., 1991; Torres et al., 1996; van Santvoort et al., 1996), the Babio content might not completely reflect the paleoproductivity of the Longmaxi Formation shale. However, the Ba and Babio contents of the LLF were far greater in the Jiaoshiba area compared with the Zhaotong and Weiyuan areas, corresponding to the high abundance of organic matter in the Jiaoshiba area. This implies that Ba and Babio are reliable paleoproductivity indicators, and the primary productivity might have been better in the Jiaoshiba area than in the Zhaotong and Weiyuan areas.

Many microfossils, such as foraminiferas, sponge spicules, and radiolarians, are observed in the Longmaxi Formation shale (Zhang et al., 2012; Guo and Zhang, 2014; Li et al., 2016) and the Barnett Shale (Loucks and Ruppel, 2007; Pan et al., 2015), especially in organic-rich layers. Previous studies have suggested that biogenic silica is an important component of the siliceous component of marine shale (Ross and Bustin, 2009; Zhao et al., 2016). Therefore, excess Si in shale determined from the total Si concentration minus the Si associated with terrigenous source material is considered to be a good proxy for paleoproductivity. Excess Si (Siex) is calculated from the total Si and terrigenous Si normalized against a fixed Si/Al ratio by the following equation (Ross and Bustin, 2009):

Siex = Sisample − [(Si/Al)background × Alsample] (2) The commonly used value for (Si/Al)background is the value of Average Shale (AS), 3.11 (Wedepohl, 1971). The

calculated results for Siex for the Jiaoshiba, Zhaotong, and Weiyuan areas are shown in Fig. 6a and Table 1. The average Siex of LLA and LLB in the Jiaoshiba area were 16.87% and 7.72% and accounted for 52% and 22% of the total Si, respectively. The average Siex of LLA and LLB in the Zhaotong area were 10.95% and 6.8% and accounted for 28% and 19% of the total Si, respectively. The average Siex of LLA in the Weiyuan area was 4.52% and accounted for 15% of the total Si, while all the Si of LLB was indicated to be the result of inorganic sediment, according to Eq. (2). The Siex of LLA being far larger than that of LLB verified the biogenic origin of the excess silica. Meanwhile, the stratigraphic variations of the Siex in the LLF corresponded to the TOC in YS12 (Fig. 4). Among the three areas, the Siex was largest in the Jiaoshiba area, lowest in the Weiyuan area and medium in the Zhaotong area. This might also indicate that the paleoenvironment of the Jiaoshiba area had greater primary productivity compared with the Zhaotong and Weiyuan areas during the LLF deposition period.


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Fig. 6. Comparison of primary productivity indexes (Siex and Ni/Al) of the Lower Longmaxi Formation (LLF) in the

Jiaoshiba, Zhaotong and Weiyuan areas, Sichuan Basin. The vertical boxes represent the variation of the values (min and max) and the horizontal boxes represent the average values. LLA = layer A: the

lower section of the LLF and LLB = layer B: the upper section of the LLF.

Ni as a trace nutrient noticeably displays a correlation with organic matter in marine formations (Lewan and

Maynard, 1982; Piper and Perkins, 2004). When organic matter decays, released Ni can be incorporated into pyrite under sulfate-reducing conditions (Fernex et al., 1992; Piper and Perkins, 2004). Therefore, the concentration of Ni can point to the original state. Trace element concentrations normalized against Al can estimate the effects of dilution in sedimentary rocks (Pedersen and Calvert, 1990; Morford and Emerson, 1999). Therefore, the Ni/Al ratio is a reliable indicator for paleoproductivity (Zhao et al., 2016). The Ni/Al ratio in the Jiaoshiba, Zhaotong, and Weiyuan areas are shown in Fig. 6b and Table 1. The average Ni/Al ratios of LLA and LLB were 18.56 and 8.42 in the Jiaoshiba area, 11.16 and 4.23 in the Zhaotong area and 12.30 and 5.59 in the Weiyuan area, respectively. The Ni/Al ratio of the Longmaxi Formation shale is far larger in the Jiaoshiba area than in the Zhaotong and Weiyuan areas. Meanwhile, the Ni/Al ratio in LLA was far greater than that of LLB, which indicated a correlation between organic matter and the Ni/Al ratio.

The Babio, Siex, and Ni/Al ratio all indicated that the primary productivity of the organic-rich LLA shale was higher than the organic-poor LLB shale, and the primary productivity was far higher in the Jiaoshiba area than in the Zhaotong and Weiyuan areas. This could have been caused by the strong closure of sea water that resulted in seawater stratification, which led to an eutrophication pump in base waters that would have promoted primary productivity in the water column (Murphy et al., 2000; Sageman et al., 2003; Zhang et al., 2018). Additionally, seawater stratification is more likely to form in thicker water bodies. The Jiaoshiba area is situated in the center of a deep shelf with a thick water body, which may be the main reason for the higher primary productivity in the Jiaoshiba area. 5.1.2 Detrital influx

Detrital input can influence the organic-matter concentration of marine sediments by serving as a variable dilution, providing a source of organic matter or influencing the burial rate and decomposition of organic matter (Sageman et al., 2003; Rimmer et al., 2004). SiO2 and the Si/Al ratio have been widely used as indicators of detrital input in many previous studies (Bertrand et al., 1996; Murphy et al., 2000; Sageman et al., 2003; Rimmer et al., 2004). However, silica in the LLF has been shown to originate from both detrital input and biogenic fossils, as mentioned above. Therefore, SiO2 and the Si/Al ratio are no longer reliable proxies for terrestrial clastic input in these study areas. Alternatively, Ti is a conservative detrital component present in both clays and heavy minerals, such as ilmenite, sphene, augite and rutile (Rimmer et al., 2004; Brumsack, 2006; Li et al., 2015). Consequently, TiO2 and the Ti/Al ratio are generally used as reliable indicators of clastic influx. The average TiO2 and Ti/Al ratio of the LLF are 0.58% and 0.052 in the Jiaoshiba area, 0.59% and 0.049 in the Zhaotong area and 0.53% and 0.049 in the Weiyuan area. There was no significant difference among the TiO2 values and Ti/Al ratios in three study areas (Fig. 7). In addition, the TiO2 and Ti/Al ratio of the LLF are mostly less than PAAS (TiO2 = 1 and Ti/Al = 0.06) and AS (TiO2 = 0.78 and Ti/Al = 0.053), which indicates a very small supply of detrital matter during the whole period of the LLF deposition. Meanwhile, a similar distribution of TiO2 and Ti/Al ratios combined with a similar distribution of Rb, La, and Ce elements in the Jiaoshiba, Zhaotong, and Weiyuan areas reflected the rather homogenous terrigenous clastic input in the LLF shale.


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Fig. 7. Comparison of detrital influx indexes (TiO2 and Ti/Al ration) in the Lower Longmaxi Formation (LLF) among the Jiaoshiba, Zhaotong and Weiyuan areas, Sichuan Basin.

The vertical boxes represent the variation of the values (min and max) and the horizontal boxes represent the average values. LLA = layer A: the

lower section of the LLF and LLB = layer B: the upper section of the LLF.

5.1.3 Redox conditions

Some trace elements (Th, U and Ni) are sensitive to changes in redox conditions, while other trace elements (Cr and Co) are relatively conservative, and appropriate trace element ratios can provide information about redox conditions (Pedersen and Calvert, 1990; Morford et al., 2001). The trace element ratios V/Cr, Ni/Co, V/(V+Ni) and U/Th are widely accepted as indicators of paleoredox conditions and discrimination criteria based on these proxies have been established (Hatch and Leventhal, 1992; Jones and Manning, 1994; Wignall and Twitchett, 1996; Rimmer et al., 2004; Ross and Bustin, 2009). Ratios of V/Cr > 4.25, Ni/Co > 7.0, V/(V+Ni) > 0.54, and U/Th > 1.25 are interpreted as anoxic conditions, while ratios of 4.25 > V/Cr > 2.0, 7.0 > Ni/Co > 5.0, 0.6 > V/(V+Ni) > 0.46, and 1.25 > U/Th > 0.75 are interpreted as dysoxic condition, and V/Cr < 2.0, Ni/Co < 5.0, V/(V+Ni) < 0.46, and U/Th < 0.75 are interpreted as oxic condition (Hatch and Leventhal, 1992; Jones and Manning, 1994; Wignall and Twitchett, 1996). In the current study, we utilized these redox indicators to analyze the differences among the paleoredox conditions of the LLF shales in the Jiaoshiba, Zhaotong, and Weiyuan areas.

As shown in Fig. 8, all four redox indicators in LLA produced consistent results of either dysoxic or anoxic conditions. Additionally, V/Cr, Ni/Co, and U/Th ratios from LLB indicated a primarily oxic condition. However, the V/(V+Ni) ratio from LLB indicated an anoxic condition and showed little difference from the V/(V+Ni) ratio from LLA. Therefore, we inferred that the V/(V+Ni) ratio could not accurately distinguish similar depositional environments, while previous studies have shown that LLB shale was generally deposited during a marine regression period, leading to lower sea levels (Zhang et al., 2012; Li et al., 2016). Meanwhile, distinctions in the V/Cr, Ni/Co, and U/Th ratios were observed among the LLF shales sampled from the three study areas. The values of the V/Cr, Ni/Co, and U/Th ratios of LLA were noticeably higher and more consistent in the Jiaoshiba area than in the Zhaotong and Weiyuan areas. This indicated that the sedimentary environment during the LLA depositional period contained less oxygen and experienced smaller sea-level fluctuations in the Jiaoshiba area compared with the Zhaotong and Weiyuan areas.


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Fig. 8. Comparison of redox indexes (V/Cr, Ni/Co, V/[V+Ni], and U/Th) from the Lower Longmaxi Formation (LLF) in the

Jiaoshiba, Zhaotong, and Weiyuan areas, Sichuan Basin. The vertical boxes represent the variation of the values (min and max) and the horizontal boxes represent the average values. The redox proxy ranges

(V/Cr, Ni/Co, V/[V+Ni], and U/Th) are from Hatch and Leventhal (1992), Jones and Manning (1994) and Wignall and Twitchett (1996). LLA = layer

A: the lower section of the LLF and LLB = layer B: the upper section of the LLF.

These distinct redox conditions may be closely related to the paleogeography of the Longmaxi Formation

deposition in the Sichuan Basin. During the Early Rhuddanian-Aeronian stage of the Early Silurian, when LLF was deposited, global climate warming and ice sheet melting led to a rapid rise in sea level (Liang et al., 2009; Zhang et al., 2012). On the Upper Yangtze Platform, a large-scale transgression formed a semi-enclosed, restricted deep-water bay constrained by adjacent paleo-uplifts (Su et al., 2007; Zhang et al., 2012). As Fig. 1 shows, the Jiaoshiba area was located near the center of a deep shelf during the Longmaxi Formation deposition period, while the Weiyuan and Zhaotong areas were located on the edge of the deep shelf. Therefore, the water column was relatively deeper in the Jiaoshiba area than in the Weiyuan and Zhaotong areas, and the variation in redox conditions caused by sea-level fluctuations was reflected more in the Weiyuan and Zhaotong areas compared with the Jiaoshiba area. This might have been the primary reason for the higher oxygen content and dramatic oxygen fluctuations in the Weiyuan and Zhaotong areas. 5.1.4 Hydrothermal activity

Elemental anomalies caused by hydrothermal activity are significant features in hydrothermal sediments (Toth, 1980; Boström, 1983; Stoffers, 1990; Choi and Hariya, 1992; Rona et al., 1993; Shock and Schulte, 1998). Numerous studies have demonstrated that hydrothermal activity generally leads to the enrichment of Fe and Mn concentrations (Stoffers, 1990; Rona et al., 1993). The TFe2O3 (Fe2O3+FeO) and MnO contents of the shale samples from the Jiaoshiba area (average 4.76% and 0.03%) and the Zhaotong area (average 4.71% and 0.07%) were relatively low compared with PAAS (7.22% and 0.11%), respectively. However, the TFe2O3 and MnO contents of the shale samples from the Weiyuan area (average 8.95% and 0.18%) were significantly greater compared with those from the other two areas and PAAS. The Y concentration of hydrothermal fluid has been observed to be considerably higher than that of ambient seawater in many deep-sea hydrothermal systems located on the Mid-Atlantic Ridge and the East Pacific Rise and in the back-arc basins on the southwest Pacific (Douville et al., 1999; Bau and Dulski, 1999). Therefore, the extremely high Y content in the Weiyuan area is probably the consequence of the influx of hydrothermal fluid (Fig. 5).

Boström (1983) proposed a Fe/Ti ratio greater than 20 as a typical characteristic of marine hydrothermal sediment. The Fe/Ti ratios of the shale samples from the Weiyuan area ranged from 11.64 to 49.44, and 8 of the 22 samples had ratios greater than 20. The Fe/Ti ratios of the shale samples from the Zhaotong area ranged from 7.16 to 21.68, and 2 of the 19 samples had ratios greater than 20. The Fe/Ti ratios of the shale samples from the Jiaoshiba area ranged from 6.28 to 12.89, and all the 12 samples had ratios lower than 20. Toth (1980) found that marine hydrothermal sediment is generally characterized by a low Co/Zn ratio of approximately 0.15, while


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normal marine sediment has a relatively high Co/Zn ratio of around 2.5. The Co/Zn ratios of the shale samples ranged from 0.04 to 1.25 in the Weiyuan area, 0.02 to 0.31 in the Zhaotong area and 0.06 to 0.24 in the Jiaoshiba area.

The elements Ni, Co, and Zn are regarded as indicators of the background and source of marine sediment (Choi and Hariya, 1992). Co is considered to be associated with a common seawater source, while Zn and Ni are primarily considered to be from hydrothermal origins. Therefore, a Ni-Zn-Co ternary diagram was established and is used to distinguish hydrogenic and hydrothermal sediments (Choi and Hariya, 1992; Wu et al., 2017). Fig. 9 shows that most of the LLF samples fall in the hydrothermal sediments region of the Ni-Zn-Co ternary diagram, with some samples falling outside this region.

Fig. 9. Ni-Zn-Co ternary diagram for the Lower Longmaxi Formation in the Jiaoshiba, Zhaotong and Weiyuan areas,

Sichuan Basin, distinguishing hydrothermal and hydrogenic sediments (modified after Choi and Hariya, 1992; Wu et al.,

2017).

All the evidence discussed here, combined with reported hydrothermal sedimentary phenomenon in the

Longmaxi Formation shale (Wu et al., 2017; Zhang et al., 2018), indicates that hydrothermal intrusion might have occurred during the deposition of the LLF in the Sichuan Basin, and the impact of these hydrothermal fluids varied in different regions at different periods. The geochemical indexes of hydrothermal activity (Fe, Mn, and Y concentrations and Fe/Ti ratio) indicated that the LLF in the Weiyuan area was more affected by hydrothermal activity than the LLF in the Jiaoshiba and Zhaotong areas. This might have been because of the paleogeography of the Weiyuan area, which placed it near the Chuanzhong uplift, where enhanced tectonic activity took place (Xu et al., 2012; Li et al., 2017). The Caledonian orogeny movement not only extended the Chuanzhong uplift but also activated basement faults around the uplift, providing vents for deep-hydrothermal-fluid upwelling. 5.2 Controls on the accumulation of organic matter

The deposition of marine-sourced rocks, as organic-rich layers, is controlled by the sedimentary environment. To clarify whether the factors affecting the accumulation of organic matter in the LLF shale in the three study areas were consistent, the current study analyzed the relationships between TOC and the geochemical indicators of the sedimentary environment, including redox proxies (V/Cr, Ni/Co, U/Th, and V/[V+Ni]), paleoproductivity indices (Ni/Al and Siex), the detrital influx index (Ti/Al) and the hydrothermal activity parameter (Fe/Ti).

The results indicated medium-strong correlations between TOC and the redox proxies for all the samples from the three study areas, while the correlation coefficients (R

2) for V/Cr, Ni/Co and U/Th V/(V+Ni) were 0.63,

0.41 and 0.62, respectively (Fig. 10a, b, c). This indicated that the organic matter in the LLF shale increases with lower oxygen content, which is consistent with previous studies that have shown that a hypoxic reducing environment is a key factor for organic matter accumulation in marine shale (Demaison and Moore, 1980; Arthur et al., 1998; Mort et al., 2007). However, TOC was not related to V/[V+Ni) (Fig. 10d). As mentioned above, this may be ascribed to the fact that the V/[V+Ni) ratio cannot reflect subtle variations in oxygen content.


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Fig. 10. Cross-plots of TOC vs. (a) V/Cr, (b) Ni/Co, (c) U/Th, (d) V/[V+Ni], (e) Ni/Al, (f) Siex, (g) Fe/Ti, and (h) Ti/Al of the

Lower Longmaxi Formation in the Jiaoshiba, Zhaotong, and Weiyuan areas, Sichuan Basin. V/Cr, Ni/Co, V/(V+Ni), and U/Th ratios indicate redox conditions. Ni/Al and the Siex indicate paleoproductivity. Fe/Ti indicates hydrothermal activity

and Ti/Al indicates clastic influx.

Fig. 10e shows the relationship between TOC and the detrital influx index (Ti/Al). The TOC showed an

extremely weak correlation with Ti/Al (R2 = 0.18) for all the samples from the three study areas, which

indicated that the detrital influx index did not reflect any obvious enrichment of organic matter. We assumed that the detrital influx was too low to affect the enrichment of organic matter. Additionally, the widespread lack of terrestrial detrital supply that resulted in the strong closure of the water body led to seawater stratification and reducibility, which provided a fundamental condition for the accumulation of organic matter in the deep-marine shale.


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As shown in Fig. 10f, the TOC displayed a strong positive correlation with the paleoproductivity index, Ni/Al (R

2 = 0.62), for all the samples from the three study areas. This indicated that the primary productivity

significantly controlled the accumulation of the organic matter, which suggested the characteristics of a ―productivity‖ mode (Pedersen and Calvert, 1990). However, the TOC exhibited different relationships with the Siex in the samples from the three study areas. There was a medium correlation between the TOC and the paleoproductivity index, Siex (R

2=0.46), in the samples from the Jiaoshiba and Zhaotong areas, whereas there

was no obvious correlation between the TOC and the Siex in the samples from the Weiyuan area (Fig. 10g). This odd phenomenon might have been due to the multiple origins of Si, generally derived from terrestrial clastic sources, biogenesis, and hydrothermal origins. We speculated that hydrothermal activity was very strong during the deposition of the LLF in the Weiyuan area and might have severely affected biological processes. Additionally, partial excess Si can be produced by hydrothermal fluids. Conversely, the hydrothermal activity was relatively week during the deposition of the LLF in the Jiaoshiba and Zhaotong areas, and most of the excess Si may have resulted from a biogenic origin. This assumption was also supported by the relationship between the TOC and the hydrothermal activity index (Fe/Ti). There was a noticeably negative correlation between the TOC and the Fe/Ti index (R

2 = 0.41) in the samples from the Weiyuan area, while there was no

correlation between the TOC and the Fe/Ti index in the samples from the Jiaoshiba and Zhaotong areas (Fig. 10h).

The current study suggests that the accumulation of organic matter in the LLF shale was mainly controlled by high primary productivity and the enhanced preservation of organic matter under reducing conditions, which resulted from the combination of both the ―preservation‖ and ―productivity‖ models. The comparison of organic abundances in the LLF shale from the three study areas indicated that the area closest to the paleoceanographic center tended to have a higher concentration of organic matter. One reason for this is that the large water depth in the paleoceanographic center led to an oxygen-deficient environment, which promoted the preservation of organic matter (Mort et al., 2007). Another reason might be that the efficient recycling of nutrients in the base waters enhanced the primary productivity, which is more likely to take place in relatively deep water, and clastic dilution and water column mixing in shallow water could terminate this recycling (Murphy et al., 2000; Sageman et al., 2003).

Hydrothermal activity has been shown to play a negative role in the enrichment of organic matter. Some studies have shown that hydrothermal fluids bring numerous nutrient elements (P and Mn) and energy for the growth of microbiology (Dando et al., 1995) or reduced inorganic sulfur compounds for sulfur-oxidizing bacteria (Sievert et al., 1999). Both inputs would contribute to primary productivity and promote the accumulation of organic matter. However, recent research has indicated that hydrothermal activity can negatively influence the accumulation of organic matter. Dissolved organic matter in deep-ocean seawater can be removed by hydrothermal circulation within the Earth’s crust or consumed by microbial communities and interaction with minerals (Lang et al., 2006; Hawkes et al., 2015; Gomez-Saez, 2016). In addition, dissolved organic matter can be thermally degraded at temperatures above 68°C (Hawkes et al., 2016), and hydrothermal fluids are generally much hotter than this. It was assumed that the effect of hydrothermal activity on the accumulation of organic matter depends on the physicochemical properties of the hydrothermal fluid and the intensity of the hydrothermal fluid influx. 6 Conclusions

Based on the detailed geochemical characterization and comparison of the LLF shale in the Jiaoshiba, Zhaotong and Weiyuan areas in the Sichuan Basin, China, we concluded the following:

(1) The TOC in the Longmaxi Formation shales is higher in the Jiaoshiba area than in the Weiyuan and Zhaotong areas. V/Cr, Ni/Co, and U/Th ratios show more oxidic and greater sea-level fluctuation in the Zhaotong and Weiyuan areas than in the Jiaoshiba area. The Siex and the Ni/Al proxy demonstrate that the Jiaoshiba area was more productive than the Zhaotong and Weiyuan areas. The Ni-Zn-Co ternary diagram suggests that most of the Longmaxi Formation shales in all three areas was influenced by hydrothermal fluid, while Fe, Mn, and Y concentrations and Fe/Ti ratios imply that hydrothermal activity was more active in the Weiyuan area than the Jiaoshiba and Zhaotong areas. TiO2 level and the Ti/Al proxy indicated very low terrigenous clastic input during the deposition period of the LLF in all three study areas.

(2) The relationship between the TOC and the geochemical indicators in the LLF implies that high organic-carbon accumulations were mainly related to both an anoxic water environment and high primary productivity, the detrital influx had no effect on the enrichment of organic matter and hydrothermal activity could have played a negative role in the accumulation of organic matter in specific circumstances.

(3) The current study indicates that Longmaxi Formation shale deposited under different tectonic conditions exhibits different paleoenvironmental conditions and organic-matter enrichment mechanisms. Geochemical data can effectively decipher these differences and distinguish the factors that controlled the organic-matter accumulation mechanism. Acknowledgements

This work was supported by U.S. National Science Foundation (No. 1661733), the National Science and Technology Major Project of China (No. 2017ZX05005-002-008), the National Natural Science Foundation of


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About the first author CHEN Zhipeng, male, born in 1987 in Jian City, Jiangxi province; master; graduated from China University of Petrolem(Beijing); PhD student in Department of Geology in Northwest University. His current research is focus on basin sedimentation, tectonic evolution, and evaluation of shale gas resource. Email: [email protected].

About the corresponding author CUI Junping, male, born in 1978 in Xian City, Shaanxi Province; Ph.D.; graduated from Department of Geology in Northwest University; associate professor at the Northwest University. He is interested in basin geothermal evolution history and oil-gas generation and reservoir. Email: [email protected].


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