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Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China
Accepted Manuscript Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China Shufang Wang, Dazhong Dong, Yuman Wang, Xinjing Li, Jinliang Huang, Quanzhong Guan PII:
S1875-5100(15)30285-7
DOI:
10.1016/j.jngse.2015.11.045
Reference:
JNGSE 1141
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 11 February 2015 Revised Date:
13 October 2015
Accepted Date: 26 November 2015
Please cite this article as: Wang, S., Dong, D., Wang, Y., Li, X., Huang, J., Guan, Q., Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China, Journal of Natural Gas Science & Engineering (2016), doi: 10.1016/j.jngse.2015.11.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: a case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China Shufang Wang*, Dazhong Dong, Yuman Wang, Xinjing Li, Jinliang Huang, Quanzhong Guan
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Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083,China. *
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Corresponding author,E-mail address:[email protected]
Abstract
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Based on element geochemical and total organic carbon (TOC) analyses of core samples from the Silurian Longmaxi Formation black shales in the Sichuan Basin, this study tried to figure out the temporal changes of palaoenvironmental settings for the shale deposition. The redox-sensitive element ratios, such as U/Th, V/Sr, V/Cr, Ni/Co and V/(V+Ni) are useful indicators to define the redox condition of marine
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black shale deposition. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in an
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oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich
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black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales. Furthermore, geochemical indicators should be combined with sedimentary and paleontological features in the study of palaeoenvironment conditions for shale deposition. Key words: Sichuan Basin; Longmaxi Formation; paleoenvironment; trace-element redox proxies; TOC
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1. Introduction With the deepening of shale gas exploration in China, the Lower Silurian Longmaxi Formation black shales in the Sichuan Basin have been chosen as a key
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target for shale gas generation. Among 301 wells drilled from this formation, 124 wells have got gas shows. The knowledge of Longmaxi Shale is very important for resource assessment and exploration deployment, therefore many studies have been
al., 2010; Liang et al, 2012; Huang et al., 2012).
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carried out on the Longmaxi Formation in the Sichun Basin (Zou et al., 2010; Dong et
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The shale gas production is related to shale porosity, permeability, organic carbon content, petrophysical properties, sedimentary environment, lithologic association and burial history. Among these factors, organic carbon content is strongly affected by paleoenvironment when the shale deposited. The redox environment of water during shale deposition is a main factor controlling the development of
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organic-rich shale, and has strong effects on the preservation and enrichment of organic matter and shale gas accumulation (Murphy et al., 2000; Rimmer, 2004; Rue et al., 2007; Loucks and Ruppel, 2007). Therefore, the determining of sedimentary
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environment during shale deposition is of great significance for the prospective shale gas exploration.
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In the last few decades, geochemical indicators, especially redox indices, were wildly used in the study of sedimentary environment for organic-rich gas shale (Rimmer, 2004; Sageman et al., 2003). For example, Rowe et al. (2008) defined the water body redox conditions for Barnett shale deposition based on the study of elements;
and
Murphy
et
al.
(2000)
carried
a
geochemical
study
on
Devonian-Mississippian New Albany shale in North America to identify its paleoenvironment. For the Longmaxi Formation, Zhou et al. (2011) and Yan et al. (2009) carried out isotopes and trace elements studies on outcrop shale samples in the Hubei Province and identified the sedimentary environments based on geochemical 2
ACCEPTED MANUSCRIPT indicators. Up to now, the sedimentary environment of the Longmaxi Shale in the Sichuan Basin has not been studied. Meanwhile, previous studies of the Longmaxi Shale were made mainly on outcrop samples, without study on core samples (Yan et al., 2009; Zhou et al., 2011). As the breakthrough of shale gas exploration on the
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Longmaxi Formation in the Sichuan Basin, several wells were drilled and cores were collected, which provided a good opportunity for us to study the paleoenvironment of the Longmaxi Formation with core samples.
In this paper, core samples from two
wells in central (W201) and southern (Z106) Sichuan Basin were collected for the
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study of the redox environment during shale deposition based on core observation, microscope observation, and inorganic and organic geochemical analyses.
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2. Geologic setting and well description
The Sichuan Basin, located in the southwestern China, is a superimposed basin developing on the western part of the Yangtze Craton. Due to several orogenic events, especially the collision between India and Eurasia, the Sichuan Basin is surrounded
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by series of mountains, forming the landform nowadays (Wang, 1989). From Late Neoproterozoic to Mesozoic, more than 10 km-thick sediments were deposited in the Sichuan Basin, with several black shale sequences developing in the periods of Ediacaran, Early Cambrian, Late Ordovician-Early Silurian, Permian and Late
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Triassic-Early Jurassic (Zou et al., 2010; 2014). Shale gas has been found from several series of these strata (Zou et al., 2010). However, industry shale gas
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exploration was only from the Lower Silurian Longmaxi Formation. The Longmaxi Formation organic-rich shales are widely distributed in the
Sichuan Basin and outside of Southern Sichuan in the Yangtze Craton (Fig. 1; Chen et al., 2004; Zhou et al., 2015). The Longmaxi Shale was deposited in the deep shelf and was in conformable contact with the underlying Ordovician strata (Chen et al., 2004; Zou et al., 2010). Due to the impact of the Caledonian orogeny, a large-scale tectonic uplifting occurred in western and eastern parts of the Sichuan Basin at the end of the Ordovician Period (Chen et al., 2004).
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ACCEPTED MANUSCRIPT The well W201 is located in the central part of the Sichuan Basin (Fig. 1) The Longmaxi Formation of well W201 is in the depth of 1380.8 - 1543.3 m, with the thickness of 162.5 m (Fig. 2). It can be divided into two sections: the lower Section A is composed of organic-rich shale, which is rich in organic carbon and graptolite; and
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the upper Section B is composed of lamellation grapholith without graptolite. The well Z106 is located to the southern Sichuan Basin (Fig. 1). The Longmaxi Formation of well Z106 is in the depth of 1197.54-1447.69 m, with the thickness of 250.15 m thick. It can be divided into three sections from bottom to top: the lower Section A is
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composed of organic-rich shale; the middle Section B is composed of black shale sandwiched with marl; and the upper Section C consists of calcareous shale
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alternating with marl (Fig. 2). 3. Samples and analytical methods
Twenty-six core samples from well W201 and 43 samples from well Z106 were collected for TOC and major and trace element analysis. A consensus has been
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achieved by previous studies that the high-TOC black shales were mainly deposited in the bottom part of the Longmaxi Formation (Zou et al., 2010; Dong et al., 2010; Liang et al, 2012; Huang et al., 2012). So, for the new wells, Oil Company just focused on the high-TOC (bottom) part of the Longmaxi Formation, and cores were
upper part.
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mainly sampled from the bottom part, with relative fewer cores from middle and
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Major and trace element analysis was conducted in the Analytical Laboratory of
Beijing Research Institute of Uranium Geology, China National Nuclear Corporation. Major elements were determined using X-ray fluorescence (XRF) with detection limit 0.01%. Trace and rare earth element concentrations were analyzed by coupled plasma-mass spectrometry (ICP-MS) with a Finnigan MAT Element I mass spectrometer at the Beijing Research Institute of Uranium Geology, China. The analytical uncertainties are estimated to be 5 %. The standard rock reference materials GSR4 and GSD12 were used to monitor the analytical accuracy and precision. XRD mineral analysis and TOC measurement were done in the Petroleum Geology 4
ACCEPTED MANUSCRIPT Research and Laboratory Center, Research Institute of Petroleum Exploration and Development, PetroChina. 4. Results The analytical results for well W201 and Z106 are presented in Supplementary
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Tables 1 and 2, respectively. XRD mineral analytical result is listed in Supplementary Table 3. 4.1 TOC abundance
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For Well W201, samples from the bottom organic-rich black shales (section A) show relatively high TOC contents ranging from 1.13 % to 4.28 %, with an average of
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2.64 %. For samples from the upper organic-poor shales (section B) have low TOC abundance ranging from 0.05 % to 0.24 %, with an average of 0.14 % (Supplementary Table 1).
For Well Z106, samples from the bottom organic-rich black shales (section A)
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show relatively high TOC contents ranging from 2.23 % to 5.25 %, with an average of 3.18 %. For samples from the upper organic-poor shales (sections B and C) have low TOC abundance ranging from 0.1 % to 1.96 %, with an average of 0.56 %
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(Supplementary Table 2).
4.2 Major and trace element
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For both two wells, the abundance of major elements shows unobvious differences between organic-rich and organic-poor shales (Supplementary Tables 1 and 2). However, the ternary plot of major elements (SiO2, Al2O3 and CaO) shows that the lower shale section contains more SiO2 than Al2O3 and CaO (Fig. 3). In general, trace element concentration are different for organic-rich and organic poor shales (Supplementary Tables 1 and 2). For example, the organic-rich shales show higher concentration of V, Ni, U and Mo, whereas lower concentration of Cr and Th than that of the organic-poor shales. Therefore, big differences of the V/Cr, Ni/Cr, U/Th, U/Mo and Re/Mo values can be identified between organic-rich and 5
ACCEPTED MANUSCRIPT organic-poor layers for each well (Figs. 4 and 5). Meanwhile, V/Cr and Ni/Co values show positive correlation with TOC content (Fig. 6), indicating these elemental ratios are strongly related with TOC. Laterally, wells W201 and Z106 have big differences in ΣREE abundance. For well W201, ΣREE ranges from 47.3 to 803.9 ppm with an
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average of 244.6 ppm. For well Z106, ΣREE ranges from 94.5 to 307.2 ppm with an average of 186.9 ppm. Both wells exhibit a flat pattern of REE distribution (Figures not shown).
Ce and Eu anomalies can be calculated with the following formulas:
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δCe=Cen/(Lan×Prn)1/2 and δEu=Eun/(Smn×Gdn)1/2 (Taylor and McClennan, 1985). Ce would exhibit positive anomaly in a reducing environment and negative anomaly
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in an oxidizing environment. REE analysis of shales from Well W201 shows no big differences of δCe and δEu between organic-rich and organic-poor shales (Fig. 4). However, for well Z106, an increasing trend of δCe from bottom to top can be identified (Fig. 5).
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4.3 Main mineral composition
For both two wells, organic-rich shales show higher quartz and pyrite contents than that of organic-poor shales. The clay mineral content is high for both wells and
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there is no obvious difference between organic-rich and organic-poor shales (Supplementary Tables 3).
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5. Discussion
5.1 Paleoenvironment constrained from major element Marine shale minerals include quartz, carbonate and clay minerals, and are often
represented with the ternary phase diagram of three oxides, i.e. SiO2 (clastic quartz and/or biogenic silicon), Al2O3 (clay) and CaO (carbonate). The ternary plot of major elements show the lower organic-rich shale section contains more SiO2 than Al2O3 (Fig. 3). It is necessary to calculate the excess silica of these shale samples as the excess silica can be a good indicator for the good preservation of organic matters (Ross and Bustin, 2009). Excess SiO2 content refers to the SiO2 content exceeding the 6
ACCEPTED MANUSCRIPT limit for normal clastic rocks (Ross et al.,2009) and is calculated with the following expression (Ross and Bustin, 2009): Siexcess=Sisample-[(Si/Al)background*Alsample], The value of (Si/Al)background is 3.11 (Wedepohl,1971), the average value for shales.
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The calculated result of excess Silica was presented in Supplementary tables 1 and 2, which shows high concentration of excess silica in the bottom organic-rich part of each well. Meanwhile, our microscopic observation identified many microfossils from
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the organic-rich layers, such as sponge spiculae, radiolarian, and foraminifera (Fig. 8). This kind of microfossils-rich organic shale is similar to the organic-rich siliceous
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shale in Barnett (Loucks and Ruppel, 2007), which is also riched in microfossils. No doubt that some siliceous components in Longmaxi Shales come from clastic rocks, but excess SiO2 must be of biogenic origin because (1) high TOC content about 5 % and (2) high correlation between Al2O3 content and TiO2 content. These two facts indicate small contribution of terrigenous substances to siliceous components and
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more likely biogenic origin of siliceous components (Ross and Bustin, 2009). 5.2 Paleoenvironment constrained from trace elemental redox indices Different valence states of an element would be rearranged in a redox reaction,
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so the concentration of a trace element in shale would be dominated by the redox condition of water during deposition. Some trace elements are sensitive to the redox
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condition and can be used as redox indicators, such as U/Th, V/Sc, V/Cr, Ni/Co, V/(V+Ni), V/Mo, U/Mo, Re/Mo, etc.(e.g., Hatch and Leventhal, 1992; Jones and Manning, 1994; Crusius et al., 1996; Rimmer, 2004; Abanda and Hannigan, 2006; Ross and Bustin, 2009; Xu et al., 2012; Wang et al., 2015). In a strong reducing environment, the sediments would be rich in U in the form of insoluble U4+ and in an oxidizing environment, U would occur in the form of soluble U6+. The occurrence of Th will not be affected by the redox conditions of water. Therefore the U/Th ratio can reflect the redox condition during deposition. In general, the ratio of U/Th > 1.25 indicates an anoxic environment; in the range of 7
ACCEPTED MANUSCRIPT 0.75-1.25 indicates a dysoxic environment and U/Th < 0.75 indicates an oxidizing environment (Jones and Manning, 1994). Section A of well W201 has a U/Th ratio of 0.42-1.15, with an average of 0.7, indicating an oxic to dysoxic environment. Section B composed of grapholith has a U/Th ratio of 0.13-0.25, with an average of
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0.18suggesting an oxic environment (Fig. 7). The organic-rich section A of well Z106 has a U/Th value of 0.46-2.21, on average 1.19 and mostly higher than 0.75, indicating an anoxic to dysoxic environment (Fig. 7). Section B in the middle has a U/Th of 0.15-0.44, on average of 0.23, indicating an oxic environment (Fig. 7). Upper
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Section C has a U/Th of 0.13-0.14, on average of 0.135, also indicating an oxic environment (Fig. 7).
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Like the U/Th values, previous studies has established the standard for elemental ratios of V/(V+Ni), Ni/Co and V/Cr when they were used as redox indices (Hatch et al., 1992; Jones and Manning, 1994; Arthur and Sageman, 1994; Tribovillard et al., 2006). A comprehensive plot using these redox indices was presented in Figure 7. For well W201, samples from the organic-rich section A were plotted in the oxic to
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dysoxic field, whereas samples from the organic-poor section B were plotted in the oxic field. Although samples from organic-rich layers display relative less oxygen depositional environment than those from the organic-poor layers, the difference is
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not significantly (Fig. 7). For well Z106, a clear transition from anoxic to oxic environment can be observed from bottom organic-rich section A to organic-poor
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section B and C (Fig. 7). The four redox indices of U/Th, V/(V+Ni), Ni/Co and V/Cr give consistent result that the organic-rich shales were deposited in anoxic environment, whereas the organic-poor shales were deposited in relative oxic environment (Fig. 7).
Mo has been used as an index of anoxic environment (Wilkin et al., 1997) and a high Mo content indicates a constantly anoxic environment (Dean et al., 1997; 2006). In the Mo-TOC diagram, a clear positive correlation can be observed (Fig. 6), indicating that an oxygen free environment for the high-TOC shales. Meanwhile, the ratio of Re/Mo can be used to differentiate between an anoxic environment and a 8
ACCEPTED MANUSCRIPT suboxic environment (Crusius et al., 2006). The oxic environment generally corresponds to high Re content, low Mo content and Re/Mo>9×10-3, whereas the anoxic environment has low Re/Mo and high Mo content. Bottom water in an anoxic or sulfurized environment usually has low Re/Mo value (<9×10-3), close to the value
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(0.8×10-3) of present sea water (Crusius et al., 2006). For well W201, Section A composed of organic-rich shale at the bottom of Core W201 has a Re/Mo of 0.0007-0.007 with an average of 0.002, indicating an anoxic
0.02, indicating an oxic environment (Fig. 4).
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environment. Section B of grapholith has a Re/Mo of 0.002-0.02 with an average of
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For well Z106, organic-rich Section A at the bottom has a Re/Mo ratio of 0.0003-0.0007, on average of 0.0004. Section B has a Re/Mo of 0.0005-0.004, with an average of 0.002. Section C at the top has a Re/Mo of 0.009-0.014, on average of 0.01. The increase of Re/Mo ratios from bottom to top indicates a transition from bottom anoxic into oxic water condition (Fig. 5).
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Ce and Eu anomalies (δCe and δEu) were considered as effective redox indicators for the sedimentary environment of shale (Elderfield et al., 1982; Wright et al., 1987; Murray et al., 1992). δCe would exhibit negative anomaly in a reducing
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environment. Well W201 shows no obvious differences of δCe and δEu values between organic-rich and organic-poor layers (Fig. 4). However, for well Z106, an increasing trend of δCe from bottom to top can be identified (Fig. 5), indicating a
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transition from bottom anoxic to upper oxic environment, which is consistent with the aforementioned conclusion by trace element redox indices (Fig. 7). In addition, Ce anomaly can also be used to indicate relative change of sea level.
A sea-level rise may lead to less oxygen in bottom water and reduction of δCe in sediments. On the contrary, a sea-level drop would lead to more oxygen in bottom water and increase of δCe (Wilder et al., 1996). Ce anomalies in Wells W201 and Z106 suggest a sea level decline from bottom to upper part. 5.3 Relationship between TOC and trace element redox indices 9
ACCEPTED MANUSCRIPT Organic matter enrichment would be largely dependent on anoxic condition and a high TOC content usually indicates an anoxic environment (Loucks and Ruppel, 2007). Organic matter tends to be preserved in bottom water with little or no oxygen due to inhibited organic matter degradation from anaerobic bacteria and less
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meiobenthos and macrobenthos in anoxic conditions. TOC contents of both wells show positive correlation with V/Cr and Ni/Co ratios and Mo contents (Fig. 6), indicating that the anoxic environment is beneficial to organic matter preservation. The organic-rich shale at the lower part of the Longmaxi Formation is relatively rich
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in rare element V and those elements (e.g. Cr, Cu, Ni and Zn) related to biological cycle, demonstrating the relation between organic carbon and productivity. The
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increase in the residence time of organic matter in water and adsorption of metallic ions illustrate that the organic-rich shales of the Longmaxi Formation deposited in water with high biological productivity.
Meanwhile, a lightly higher TOC content of organic-rich shales of well Z106 can be identified than that of organic-rich shales of well W201 (Figs. 6 and 7). This might
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be ascribed to the different positions of these two wells. It is notable that U/Th, Ni/Co, and V/Cr ratios of section A in well Z106 exhibit wide variation than the section A of well W201 after precluding one unusual sample (Fig. 7). The wide variation indicates
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oscillating redox conditions between dyoxic and anoxic. This kind of oscillating conditions may play a key in promoting the nutrient recycling in the water column
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and enhance high primary production. This is also supported by higher TOC conents of samples in well Z106.
5.4 Sedimentary and paleontological characteristics Pyrite occurrences may also be used to identify redox conditions of sea water (Loucks and Ruppel, 2007). Small sized pyrites generally occur in an anoxic environment with a narrow distribution of particle diameter from 1-18 µm and an average of 5 µm; pyrites larger than 20 µm usually appear in an oxidizing 10
ACCEPTED MANUSCRIPT environment (Wilkin et al., 1997). For both wells W201 and Z106, framboids pyrites are only found in the bottom organic-rich layers (Fig. 2), which is confirmed by XRD mineral analysis (Supplementary Table 3). Itcan also be revealed by the differences of FeO/Fe2O3 ratios between organic-rich and organic-poor shales (Supplementary tables 1 and 2). Small coalball-shaped pyrites in bottom organic-rich shale are 5.4-11.6 µm
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in diameter and may reach 15.2 µm at most for well W201 and are 4.6-9.3 µm for well Z106, indicating an anoxic environment during organic-rich shale deposition (Fig. 9). Praptolite can only be preserved in anxoic environment. For the Longmaxi
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Formation, abundant of praptolites were only preserved in the bottom part, with cephalopod instead of praptolite in the upper part, indicating an anxoic environment at
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the bottom and a shallowing upward sequence.
Conclusions
From the geochemical analysis on TOC, major and trace elements, and petrological observation of Wells W201 and Z106 in central and southern Sichuan
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Basin, we can get the following conclusion: the shale at the bottom of the Longmaxi Formation has high-TOC abundance. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows
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that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in
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an oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales.
Furthermore, it is suggested to combine geochemical indexes
with sedimentary and paleontological features in the study of the palaeo-redox conditions for shale deposition.
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ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (Grant No. 2013CB228000) National Science and Technology Special (Grant No. 2011ZX05018-001). We appreciate very much the constructive and
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detailed comments and suggestions by two anonymous reviewers that significantly improved our manuscript. We are also grateful to Boling Pu for her help with core
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collection of samples.
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ACCEPTED MANUSCRIPT Biogenic silica of organic-rich shale in Sichuan Basin and its significance for shale gas. Acta Scientiarum Naturalium Universitatis Pekinensis 50(3), 476-486. Wang, S., Zou, C., Dong, D., Wang, Y., Li, X., Huang, J., Guan, Q., 2015. Multiple controls on the paleoenvironment of the Early Cambrian marine black shales in
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the Sichuan Basin, SW China: Geochemical and organic carbon isotopic evidence. Marine and Petroleum Geology, doi: 10.1016/j.marpetgeo.2015.07.009.
Wedepohl, K.H., 1971. Environmental influences on the chemical composition of shales and clays, Physics and Chemistry of the Earth 8, 307-331.
The whole-rock cerium anomaly,
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Wilder, P., Quinby, M.S., Erdtmann, B.D., 1996.
a potential indicator of eustatic sea-level changes in shales of anoxic
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facies.Sedimentary Geology 101, 43-53.
Wilkin, R.T., Arthur, M.A., Dean, W.E., 1997. History of watercolumn anoxia in the Black Sea indicated by pyrite framboids size distributions.Earth and Planetary Science Letters 148, 517- 525.
Wright, J., Schrader, H., Holser, W.T., 1987. Paleoredox variations in ancient oceans
Acta 51, 637-644.
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recorded by rare earth elements in fossil apatite. Geochimica et Cosmochimica
Xu, L., Lehmann, B., Mao, J., Nagler, T.F., Neubert, N., Bottcher, M.E., Escher, P.,
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2012. Mo isotope and trace element patterns of Lower Cambrian black shales in South China: multi-proxy constraints on the paleoenvironment. Chem. Geol.
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318-319, 45-59.
Yan, D.T., Chen, D.Z., Wang, Q.C., Wang, J.G., 2009. Geochemical changes across the Ordovician -Silurian transition on the Yangtze Platform, South China. Science China, Earth Sciences 52(1), 38-54.
Zhou, L., Su, J., Huang, J.H., Yan, J.X., Xie, X.N., Gao, S., 2011. A new paleoenvironmental index for anoxic events-Mo isotopes in black shales from Upper Yangtze marine sediments. Science China, Earth Sciences 41(3), 309-319. Zou, C.N., Dong, D.Z., Wang, S.J., Li, J.Z., Li, X.J., Wang, Y.M., Li, D.H., Cheng, K.M., 2010. Geological characteristics, formation mechanism and resource 15
ACCEPTED MANUSCRIPT potential of shale gas in China. Petroleum Exploration and Development. 37(6), 641-653. Zou, C.N., Du, J.H., Xu, C.C., Wang, Z.C., Zhang, B.M., Wei, G.Q., Wang, T.S., Yao, G.S., Deng, S.H., Liu, J.J., Zhou, H., Xu, A.N., Yang, Z., Jiang, H., Gu, Z.D.,
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2014. Formation, distribution, resource potential and discovery of the Sinian−Cambrian giant gas field, Sichuan Basin, SW China. Petroleum
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Exploration and Development 41(3), 278-293.
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Figure captions
Figure 1. Latest Ordovician-Early Silurian palaeogeographic maps of the Yangtze Block areas (modified from Chen et al. (2004); Zhou et al. (2015)). The studied wells W201 and Z106 are located in the central and southern Sichuan Basin, western part of
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Yangtze Block.
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Figure 2. Stratigraphic column of wells W201 and Z106.
Figure 3. Al2O3-SiO2-CaO plot for Longmaxi Formation shale of well W201 and
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Z106 in the Sichuan Basin.
Figure 4. Stratigraphic profiles of Well W201 for TOC, trace element redox indices, Ce and Eu anomalies.
Figure 5. Stratigraphic profiles of Well Z106 for TOC, trace element redox indices, Ce and Eu anomalies.
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Figure 7. Crossplots of trace-element ratios used as redox indices of well W201 and
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Z106. Ranges for ratios from Jones and Manning (1994) and Hatch and Leventhal (1992).
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Figure 8. Microfossiles from the organic-rich black shales of the Longmaxi Formation
Figure 9. SEM photomicrographs of pyrite framboids in Longmaxi Formation shale.
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(a) well W201, 1537m; (b) well Z106, 1422.8m
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1. Organic-rich shales are located at the lower part of Longmaxi Formation. 2. Organic-rich shales were deposited in anoxic environment, while the organic-poor shales in oxic condition. 3. Redox-sensitive elements proxies should be combined with sedimentary and biotic characteristics in study of paleo-redox condition.
S1875-5100(15)30285-7
DOI:
10.1016/j.jngse.2015.11.045
Reference:
JNGSE 1141
To appear in:
Journal of Natural Gas Science and Engineering
Received Date: 11 February 2015 Revised Date:
13 October 2015
Accepted Date: 26 November 2015
Please cite this article as: Wang, S., Dong, D., Wang, Y., Li, X., Huang, J., Guan, Q., Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China, Journal of Natural Gas Science & Engineering (2016), doi: 10.1016/j.jngse.2015.11.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: a case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China Shufang Wang*, Dazhong Dong, Yuman Wang, Xinjing Li, Jinliang Huang, Quanzhong Guan
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Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083,China. *
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Corresponding author,E-mail address:[email protected]
Abstract
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Based on element geochemical and total organic carbon (TOC) analyses of core samples from the Silurian Longmaxi Formation black shales in the Sichuan Basin, this study tried to figure out the temporal changes of palaoenvironmental settings for the shale deposition. The redox-sensitive element ratios, such as U/Th, V/Sr, V/Cr, Ni/Co and V/(V+Ni) are useful indicators to define the redox condition of marine
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black shale deposition. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in an
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oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich
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black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales. Furthermore, geochemical indicators should be combined with sedimentary and paleontological features in the study of palaeoenvironment conditions for shale deposition. Key words: Sichuan Basin; Longmaxi Formation; paleoenvironment; trace-element redox proxies; TOC
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1. Introduction With the deepening of shale gas exploration in China, the Lower Silurian Longmaxi Formation black shales in the Sichuan Basin have been chosen as a key
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target for shale gas generation. Among 301 wells drilled from this formation, 124 wells have got gas shows. The knowledge of Longmaxi Shale is very important for resource assessment and exploration deployment, therefore many studies have been
al., 2010; Liang et al, 2012; Huang et al., 2012).
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carried out on the Longmaxi Formation in the Sichun Basin (Zou et al., 2010; Dong et
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The shale gas production is related to shale porosity, permeability, organic carbon content, petrophysical properties, sedimentary environment, lithologic association and burial history. Among these factors, organic carbon content is strongly affected by paleoenvironment when the shale deposited. The redox environment of water during shale deposition is a main factor controlling the development of
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organic-rich shale, and has strong effects on the preservation and enrichment of organic matter and shale gas accumulation (Murphy et al., 2000; Rimmer, 2004; Rue et al., 2007; Loucks and Ruppel, 2007). Therefore, the determining of sedimentary
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environment during shale deposition is of great significance for the prospective shale gas exploration.
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In the last few decades, geochemical indicators, especially redox indices, were wildly used in the study of sedimentary environment for organic-rich gas shale (Rimmer, 2004; Sageman et al., 2003). For example, Rowe et al. (2008) defined the water body redox conditions for Barnett shale deposition based on the study of elements;
and
Murphy
et
al.
(2000)
carried
a
geochemical
study
on
Devonian-Mississippian New Albany shale in North America to identify its paleoenvironment. For the Longmaxi Formation, Zhou et al. (2011) and Yan et al. (2009) carried out isotopes and trace elements studies on outcrop shale samples in the Hubei Province and identified the sedimentary environments based on geochemical 2
ACCEPTED MANUSCRIPT indicators. Up to now, the sedimentary environment of the Longmaxi Shale in the Sichuan Basin has not been studied. Meanwhile, previous studies of the Longmaxi Shale were made mainly on outcrop samples, without study on core samples (Yan et al., 2009; Zhou et al., 2011). As the breakthrough of shale gas exploration on the
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Longmaxi Formation in the Sichuan Basin, several wells were drilled and cores were collected, which provided a good opportunity for us to study the paleoenvironment of the Longmaxi Formation with core samples.
In this paper, core samples from two
wells in central (W201) and southern (Z106) Sichuan Basin were collected for the
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study of the redox environment during shale deposition based on core observation, microscope observation, and inorganic and organic geochemical analyses.
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2. Geologic setting and well description
The Sichuan Basin, located in the southwestern China, is a superimposed basin developing on the western part of the Yangtze Craton. Due to several orogenic events, especially the collision between India and Eurasia, the Sichuan Basin is surrounded
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by series of mountains, forming the landform nowadays (Wang, 1989). From Late Neoproterozoic to Mesozoic, more than 10 km-thick sediments were deposited in the Sichuan Basin, with several black shale sequences developing in the periods of Ediacaran, Early Cambrian, Late Ordovician-Early Silurian, Permian and Late
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Triassic-Early Jurassic (Zou et al., 2010; 2014). Shale gas has been found from several series of these strata (Zou et al., 2010). However, industry shale gas
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exploration was only from the Lower Silurian Longmaxi Formation. The Longmaxi Formation organic-rich shales are widely distributed in the
Sichuan Basin and outside of Southern Sichuan in the Yangtze Craton (Fig. 1; Chen et al., 2004; Zhou et al., 2015). The Longmaxi Shale was deposited in the deep shelf and was in conformable contact with the underlying Ordovician strata (Chen et al., 2004; Zou et al., 2010). Due to the impact of the Caledonian orogeny, a large-scale tectonic uplifting occurred in western and eastern parts of the Sichuan Basin at the end of the Ordovician Period (Chen et al., 2004).
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ACCEPTED MANUSCRIPT The well W201 is located in the central part of the Sichuan Basin (Fig. 1) The Longmaxi Formation of well W201 is in the depth of 1380.8 - 1543.3 m, with the thickness of 162.5 m (Fig. 2). It can be divided into two sections: the lower Section A is composed of organic-rich shale, which is rich in organic carbon and graptolite; and
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the upper Section B is composed of lamellation grapholith without graptolite. The well Z106 is located to the southern Sichuan Basin (Fig. 1). The Longmaxi Formation of well Z106 is in the depth of 1197.54-1447.69 m, with the thickness of 250.15 m thick. It can be divided into three sections from bottom to top: the lower Section A is
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composed of organic-rich shale; the middle Section B is composed of black shale sandwiched with marl; and the upper Section C consists of calcareous shale
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alternating with marl (Fig. 2). 3. Samples and analytical methods
Twenty-six core samples from well W201 and 43 samples from well Z106 were collected for TOC and major and trace element analysis. A consensus has been
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achieved by previous studies that the high-TOC black shales were mainly deposited in the bottom part of the Longmaxi Formation (Zou et al., 2010; Dong et al., 2010; Liang et al, 2012; Huang et al., 2012). So, for the new wells, Oil Company just focused on the high-TOC (bottom) part of the Longmaxi Formation, and cores were
upper part.
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mainly sampled from the bottom part, with relative fewer cores from middle and
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Major and trace element analysis was conducted in the Analytical Laboratory of
Beijing Research Institute of Uranium Geology, China National Nuclear Corporation. Major elements were determined using X-ray fluorescence (XRF) with detection limit 0.01%. Trace and rare earth element concentrations were analyzed by coupled plasma-mass spectrometry (ICP-MS) with a Finnigan MAT Element I mass spectrometer at the Beijing Research Institute of Uranium Geology, China. The analytical uncertainties are estimated to be 5 %. The standard rock reference materials GSR4 and GSD12 were used to monitor the analytical accuracy and precision. XRD mineral analysis and TOC measurement were done in the Petroleum Geology 4
ACCEPTED MANUSCRIPT Research and Laboratory Center, Research Institute of Petroleum Exploration and Development, PetroChina. 4. Results The analytical results for well W201 and Z106 are presented in Supplementary
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Tables 1 and 2, respectively. XRD mineral analytical result is listed in Supplementary Table 3. 4.1 TOC abundance
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For Well W201, samples from the bottom organic-rich black shales (section A) show relatively high TOC contents ranging from 1.13 % to 4.28 %, with an average of
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2.64 %. For samples from the upper organic-poor shales (section B) have low TOC abundance ranging from 0.05 % to 0.24 %, with an average of 0.14 % (Supplementary Table 1).
For Well Z106, samples from the bottom organic-rich black shales (section A)
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show relatively high TOC contents ranging from 2.23 % to 5.25 %, with an average of 3.18 %. For samples from the upper organic-poor shales (sections B and C) have low TOC abundance ranging from 0.1 % to 1.96 %, with an average of 0.56 %
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(Supplementary Table 2).
4.2 Major and trace element
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For both two wells, the abundance of major elements shows unobvious differences between organic-rich and organic-poor shales (Supplementary Tables 1 and 2). However, the ternary plot of major elements (SiO2, Al2O3 and CaO) shows that the lower shale section contains more SiO2 than Al2O3 and CaO (Fig. 3). In general, trace element concentration are different for organic-rich and organic poor shales (Supplementary Tables 1 and 2). For example, the organic-rich shales show higher concentration of V, Ni, U and Mo, whereas lower concentration of Cr and Th than that of the organic-poor shales. Therefore, big differences of the V/Cr, Ni/Cr, U/Th, U/Mo and Re/Mo values can be identified between organic-rich and 5
ACCEPTED MANUSCRIPT organic-poor layers for each well (Figs. 4 and 5). Meanwhile, V/Cr and Ni/Co values show positive correlation with TOC content (Fig. 6), indicating these elemental ratios are strongly related with TOC. Laterally, wells W201 and Z106 have big differences in ΣREE abundance. For well W201, ΣREE ranges from 47.3 to 803.9 ppm with an
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average of 244.6 ppm. For well Z106, ΣREE ranges from 94.5 to 307.2 ppm with an average of 186.9 ppm. Both wells exhibit a flat pattern of REE distribution (Figures not shown).
Ce and Eu anomalies can be calculated with the following formulas:
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δCe=Cen/(Lan×Prn)1/2 and δEu=Eun/(Smn×Gdn)1/2 (Taylor and McClennan, 1985). Ce would exhibit positive anomaly in a reducing environment and negative anomaly
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in an oxidizing environment. REE analysis of shales from Well W201 shows no big differences of δCe and δEu between organic-rich and organic-poor shales (Fig. 4). However, for well Z106, an increasing trend of δCe from bottom to top can be identified (Fig. 5).
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4.3 Main mineral composition
For both two wells, organic-rich shales show higher quartz and pyrite contents than that of organic-poor shales. The clay mineral content is high for both wells and
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there is no obvious difference between organic-rich and organic-poor shales (Supplementary Tables 3).
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5. Discussion
5.1 Paleoenvironment constrained from major element Marine shale minerals include quartz, carbonate and clay minerals, and are often
represented with the ternary phase diagram of three oxides, i.e. SiO2 (clastic quartz and/or biogenic silicon), Al2O3 (clay) and CaO (carbonate). The ternary plot of major elements show the lower organic-rich shale section contains more SiO2 than Al2O3 (Fig. 3). It is necessary to calculate the excess silica of these shale samples as the excess silica can be a good indicator for the good preservation of organic matters (Ross and Bustin, 2009). Excess SiO2 content refers to the SiO2 content exceeding the 6
ACCEPTED MANUSCRIPT limit for normal clastic rocks (Ross et al.,2009) and is calculated with the following expression (Ross and Bustin, 2009): Siexcess=Sisample-[(Si/Al)background*Alsample], The value of (Si/Al)background is 3.11 (Wedepohl,1971), the average value for shales.
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The calculated result of excess Silica was presented in Supplementary tables 1 and 2, which shows high concentration of excess silica in the bottom organic-rich part of each well. Meanwhile, our microscopic observation identified many microfossils from
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the organic-rich layers, such as sponge spiculae, radiolarian, and foraminifera (Fig. 8). This kind of microfossils-rich organic shale is similar to the organic-rich siliceous
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shale in Barnett (Loucks and Ruppel, 2007), which is also riched in microfossils. No doubt that some siliceous components in Longmaxi Shales come from clastic rocks, but excess SiO2 must be of biogenic origin because (1) high TOC content about 5 % and (2) high correlation between Al2O3 content and TiO2 content. These two facts indicate small contribution of terrigenous substances to siliceous components and
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more likely biogenic origin of siliceous components (Ross and Bustin, 2009). 5.2 Paleoenvironment constrained from trace elemental redox indices Different valence states of an element would be rearranged in a redox reaction,
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so the concentration of a trace element in shale would be dominated by the redox condition of water during deposition. Some trace elements are sensitive to the redox
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condition and can be used as redox indicators, such as U/Th, V/Sc, V/Cr, Ni/Co, V/(V+Ni), V/Mo, U/Mo, Re/Mo, etc.(e.g., Hatch and Leventhal, 1992; Jones and Manning, 1994; Crusius et al., 1996; Rimmer, 2004; Abanda and Hannigan, 2006; Ross and Bustin, 2009; Xu et al., 2012; Wang et al., 2015). In a strong reducing environment, the sediments would be rich in U in the form of insoluble U4+ and in an oxidizing environment, U would occur in the form of soluble U6+. The occurrence of Th will not be affected by the redox conditions of water. Therefore the U/Th ratio can reflect the redox condition during deposition. In general, the ratio of U/Th > 1.25 indicates an anoxic environment; in the range of 7
ACCEPTED MANUSCRIPT 0.75-1.25 indicates a dysoxic environment and U/Th < 0.75 indicates an oxidizing environment (Jones and Manning, 1994). Section A of well W201 has a U/Th ratio of 0.42-1.15, with an average of 0.7, indicating an oxic to dysoxic environment. Section B composed of grapholith has a U/Th ratio of 0.13-0.25, with an average of
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0.18suggesting an oxic environment (Fig. 7). The organic-rich section A of well Z106 has a U/Th value of 0.46-2.21, on average 1.19 and mostly higher than 0.75, indicating an anoxic to dysoxic environment (Fig. 7). Section B in the middle has a U/Th of 0.15-0.44, on average of 0.23, indicating an oxic environment (Fig. 7). Upper
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Section C has a U/Th of 0.13-0.14, on average of 0.135, also indicating an oxic environment (Fig. 7).
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Like the U/Th values, previous studies has established the standard for elemental ratios of V/(V+Ni), Ni/Co and V/Cr when they were used as redox indices (Hatch et al., 1992; Jones and Manning, 1994; Arthur and Sageman, 1994; Tribovillard et al., 2006). A comprehensive plot using these redox indices was presented in Figure 7. For well W201, samples from the organic-rich section A were plotted in the oxic to
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dysoxic field, whereas samples from the organic-poor section B were plotted in the oxic field. Although samples from organic-rich layers display relative less oxygen depositional environment than those from the organic-poor layers, the difference is
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not significantly (Fig. 7). For well Z106, a clear transition from anoxic to oxic environment can be observed from bottom organic-rich section A to organic-poor
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section B and C (Fig. 7). The four redox indices of U/Th, V/(V+Ni), Ni/Co and V/Cr give consistent result that the organic-rich shales were deposited in anoxic environment, whereas the organic-poor shales were deposited in relative oxic environment (Fig. 7).
Mo has been used as an index of anoxic environment (Wilkin et al., 1997) and a high Mo content indicates a constantly anoxic environment (Dean et al., 1997; 2006). In the Mo-TOC diagram, a clear positive correlation can be observed (Fig. 6), indicating that an oxygen free environment for the high-TOC shales. Meanwhile, the ratio of Re/Mo can be used to differentiate between an anoxic environment and a 8
ACCEPTED MANUSCRIPT suboxic environment (Crusius et al., 2006). The oxic environment generally corresponds to high Re content, low Mo content and Re/Mo>9×10-3, whereas the anoxic environment has low Re/Mo and high Mo content. Bottom water in an anoxic or sulfurized environment usually has low Re/Mo value (<9×10-3), close to the value
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(0.8×10-3) of present sea water (Crusius et al., 2006). For well W201, Section A composed of organic-rich shale at the bottom of Core W201 has a Re/Mo of 0.0007-0.007 with an average of 0.002, indicating an anoxic
0.02, indicating an oxic environment (Fig. 4).
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environment. Section B of grapholith has a Re/Mo of 0.002-0.02 with an average of
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For well Z106, organic-rich Section A at the bottom has a Re/Mo ratio of 0.0003-0.0007, on average of 0.0004. Section B has a Re/Mo of 0.0005-0.004, with an average of 0.002. Section C at the top has a Re/Mo of 0.009-0.014, on average of 0.01. The increase of Re/Mo ratios from bottom to top indicates a transition from bottom anoxic into oxic water condition (Fig. 5).
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Ce and Eu anomalies (δCe and δEu) were considered as effective redox indicators for the sedimentary environment of shale (Elderfield et al., 1982; Wright et al., 1987; Murray et al., 1992). δCe would exhibit negative anomaly in a reducing
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environment. Well W201 shows no obvious differences of δCe and δEu values between organic-rich and organic-poor layers (Fig. 4). However, for well Z106, an increasing trend of δCe from bottom to top can be identified (Fig. 5), indicating a
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transition from bottom anoxic to upper oxic environment, which is consistent with the aforementioned conclusion by trace element redox indices (Fig. 7). In addition, Ce anomaly can also be used to indicate relative change of sea level.
A sea-level rise may lead to less oxygen in bottom water and reduction of δCe in sediments. On the contrary, a sea-level drop would lead to more oxygen in bottom water and increase of δCe (Wilder et al., 1996). Ce anomalies in Wells W201 and Z106 suggest a sea level decline from bottom to upper part. 5.3 Relationship between TOC and trace element redox indices 9
ACCEPTED MANUSCRIPT Organic matter enrichment would be largely dependent on anoxic condition and a high TOC content usually indicates an anoxic environment (Loucks and Ruppel, 2007). Organic matter tends to be preserved in bottom water with little or no oxygen due to inhibited organic matter degradation from anaerobic bacteria and less
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meiobenthos and macrobenthos in anoxic conditions. TOC contents of both wells show positive correlation with V/Cr and Ni/Co ratios and Mo contents (Fig. 6), indicating that the anoxic environment is beneficial to organic matter preservation. The organic-rich shale at the lower part of the Longmaxi Formation is relatively rich
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in rare element V and those elements (e.g. Cr, Cu, Ni and Zn) related to biological cycle, demonstrating the relation between organic carbon and productivity. The
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increase in the residence time of organic matter in water and adsorption of metallic ions illustrate that the organic-rich shales of the Longmaxi Formation deposited in water with high biological productivity.
Meanwhile, a lightly higher TOC content of organic-rich shales of well Z106 can be identified than that of organic-rich shales of well W201 (Figs. 6 and 7). This might
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be ascribed to the different positions of these two wells. It is notable that U/Th, Ni/Co, and V/Cr ratios of section A in well Z106 exhibit wide variation than the section A of well W201 after precluding one unusual sample (Fig. 7). The wide variation indicates
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oscillating redox conditions between dyoxic and anoxic. This kind of oscillating conditions may play a key in promoting the nutrient recycling in the water column
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and enhance high primary production. This is also supported by higher TOC conents of samples in well Z106.
5.4 Sedimentary and paleontological characteristics Pyrite occurrences may also be used to identify redox conditions of sea water (Loucks and Ruppel, 2007). Small sized pyrites generally occur in an anoxic environment with a narrow distribution of particle diameter from 1-18 µm and an average of 5 µm; pyrites larger than 20 µm usually appear in an oxidizing 10
ACCEPTED MANUSCRIPT environment (Wilkin et al., 1997). For both wells W201 and Z106, framboids pyrites are only found in the bottom organic-rich layers (Fig. 2), which is confirmed by XRD mineral analysis (Supplementary Table 3). Itcan also be revealed by the differences of FeO/Fe2O3 ratios between organic-rich and organic-poor shales (Supplementary tables 1 and 2). Small coalball-shaped pyrites in bottom organic-rich shale are 5.4-11.6 µm
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in diameter and may reach 15.2 µm at most for well W201 and are 4.6-9.3 µm for well Z106, indicating an anoxic environment during organic-rich shale deposition (Fig. 9). Praptolite can only be preserved in anxoic environment. For the Longmaxi
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Formation, abundant of praptolites were only preserved in the bottom part, with cephalopod instead of praptolite in the upper part, indicating an anxoic environment at
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the bottom and a shallowing upward sequence.
Conclusions
From the geochemical analysis on TOC, major and trace elements, and petrological observation of Wells W201 and Z106 in central and southern Sichuan
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Basin, we can get the following conclusion: the shale at the bottom of the Longmaxi Formation has high-TOC abundance. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows
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that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in
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an oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales.
Furthermore, it is suggested to combine geochemical indexes
with sedimentary and paleontological features in the study of the palaeo-redox conditions for shale deposition.
11
ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (Grant No. 2013CB228000) National Science and Technology Special (Grant No. 2011ZX05018-001). We appreciate very much the constructive and
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detailed comments and suggestions by two anonymous reviewers that significantly improved our manuscript. We are also grateful to Boling Pu for her help with core
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collection of samples.
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Figure captions
Figure 1. Latest Ordovician-Early Silurian palaeogeographic maps of the Yangtze Block areas (modified from Chen et al. (2004); Zhou et al. (2015)). The studied wells W201 and Z106 are located in the central and southern Sichuan Basin, western part of
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Yangtze Block.
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Figure 2. Stratigraphic column of wells W201 and Z106.
Figure 3. Al2O3-SiO2-CaO plot for Longmaxi Formation shale of well W201 and
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Z106 in the Sichuan Basin.
Figure 4. Stratigraphic profiles of Well W201 for TOC, trace element redox indices, Ce and Eu anomalies.
Figure 5. Stratigraphic profiles of Well Z106 for TOC, trace element redox indices, Ce and Eu anomalies.
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Figure 7. Crossplots of trace-element ratios used as redox indices of well W201 and
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Z106. Ranges for ratios from Jones and Manning (1994) and Hatch and Leventhal (1992).
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Figure 8. Microfossiles from the organic-rich black shales of the Longmaxi Formation
Figure 9. SEM photomicrographs of pyrite framboids in Longmaxi Formation shale.
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(a) well W201, 1537m; (b) well Z106, 1422.8m
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1. Organic-rich shales are located at the lower part of Longmaxi Formation. 2. Organic-rich shales were deposited in anoxic environment, while the organic-poor shales in oxic condition. 3. Redox-sensitive elements proxies should be combined with sedimentary and biotic characteristics in study of paleo-redox condition.