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Lithofacies and sedimentary sequence of the lower Cambrian Niutitang shale in the upper Yangtze platform, South China
Journal of Natural Gas Science and Engineering 43 (2017) 124e136
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Lithofacies and sedimentary sequence of the lower Cambrian Niutitang shale in the upper Yangtze platform, South China Yue Wu a, Tailiang Fan b, *, Shu Jiang c, **, Xiaoqun Yang b a
Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, People's Republic of China School of Energy Resources, China University of Geosciences, Beijing 100083, People's Republic of China c Energy and Geoscience Institute, University of Utah, Salt Lake City, UT 84108, United States b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2016 Received in revised form 27 March 2017 Accepted 3 April 2017 Available online 7 April 2017
The lower Cambrian Niutitang shale in the Upper Yangtze Platform is a hot target for shale gas exploration in China. In this paper, the lithofacies and sedimentary evolution characteristics about this shale formation are studied on the basis of outcrop investigation and experimental measurements. The variations on geochemistry, mineralogy, lithofacies and stratigraphic architecture of this shale are analyzed. This shale is rich in organic matter, with an average TOC content of 5.5%. Quartz and clay mineral dominate the mineral composition, with an average content of 57.3% and 29.8%, respectively. The TOC content correlates positively to quartz content, but negatively to clay mineral content. Trace element ratios of V/Cr, V/(V þ Ni) and Ni/Co indicate anoxic to dysoxic conditions prevailed during the Niutitang shale deposition period in Early Cambrian, and moderate to weak degree of restriction are inferred by the covariation of Mo and TOC content. Five lithofacies are identified based on TOC content, mineral composition, texture and fabric: organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Four lithofacies associations are interpreted to represent four different depositional facies from shelf margin to abyssal environments. The Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and can be divided into two large shale cycles. From bottom to top, the organic matter content and anoxic degree decreases largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies. © 2017 Elsevier B.V. All rights reserved.
Keywords: Niutitang shale Lithofacies Sedimentary sequence Upper Yangtze platform
1. Introduction Lithofacies and sedimentary environments analysis, as the fundamental work in sedimentology and reservoir geology, have long been of great scientific interest. Understanding the lithofacies characteristics and their stacking patterns are critical for predicting favorable reservoirs (Jarvie et al., 2007; Loucks and Ruppel, 2007; Abouelresh and Slatt, 2012; Slatt and Rodriguez, 2012; Wang et al., 2012, 2014; Bhattacharya et al., 2016). Generally, lithofacies is the overall reflection of mineral composition, texture, bedding,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Jiang). http://dx.doi.org/10.1016/j.jngse.2017.04.002 1875-5100/© 2017 Elsevier B.V. All rights reserved.
(T. Fan), [email protected]
structure, color, size distribution, sorting and roundness. At present, different opinions and methods have been suggested in terms of the shale lithofacies classification, which can be generally grouped into three types: (1) macroscopic sedimentary characteristics such as shale texture and structure characteristics; (2) mineral composition and organic matter; (3) paleontology such as graptolite shale lithofacies and radiolarian shale lithofacies (Hickey and Henk, 2007; Li and Quan, 1992; Chen et al., 2016). Among them, the second method was applied in this study to define and describe the shale lithofacies. Shale gas exploration and production in China has drawn much attention from the public in recent years. Technically recoverable shale gas reserves in China are estimated to range from 10 to 45 1012 m3 (MLR, www.mlr.gov.cn). The lower Cambrian Niutitang shale in the Upper Yangtze Platform in South China was
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investigated as one of the important Phanerozoic source rock for conventional oil and gas before. It is now considered as potential shale gas play on the basis of its rock properties and shale gas accumulation conditions (Jiang et al., 2016; Dong et al., 2010, 2012; Tan et al., 2011). Its properties of shale distribution, organic matter richness, thermal maturity, and mineral composition were commonly reported on the basis of drilling data, field study and experimental measurements (Tan et al., 2015, 2014; Han et al., 2013; Lin et al., 2014; Wang et al., 2013; Luo et al., 2014a, b). However, little work has been conducted for the detailed lithofacies analysis of this shale and only few published papers discuss the depositional settings and evolution history. Due to few wells targeting the lower Cambrian Niutitang shale have been drilled in the Upper Yangtze Platform, this paper selects two type outcrops to describe the lithofacies features and their stacking patterns of this shale. The detailed lithofacies analysis presented in this study will provide information on the lateral changes of the depositional environments. The study of the lithofacies stacking may help to correlate the outcrops across the platform and thus will shed new light on the sedimentary evolution of the Upper Yangtze Platform through Niutitang Formation times. 2. Geological setting 2.1. The yangtze platform The Yangtze Platform is located between the Qinling-Dabie and Longmenshan orogens to the north and the Cathaysia suture to the south (Wang and Mo, 1995; Wan, 2011). These major boundary structures resulted from the collision between the Yangtze Platform and the North China craton during the early Triassic and the collision between Yangtze Platform and the Cathaysia volcanic arc during the Silurian (Wang and Li., 2004; Ma et al., 2004). The
Fig. 2. Lithofacies and sequence stratigraphy framework of the lower Cambrian Niutitang Formation in the southeastern Upper Yangtze Platform (referring to Mei et al., 2006). TST ¼ transgressive system tract, HST ¼ highstand system tract. The data of d13C (‰, VPDB) as an indicator for the relative sea level change is from Zhu (2007).
regional stratigraphy of the Yangtze Platform shows the sedimentary successions spanning from Precambrian Sinian to
Fig. 1. The paleoenvironmental map of the South China Craton in the early Cambrian. This paleogeography is modified from Och et al. (2013).
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Cenozoic. These strata are preserved with a thickness on the order of several kilometers and are moderately folded with large-scale features (Chen et al., 2004). During early Cambrian times, the Yangtze Platform was a passive margin basin and consisted of inner shelf, shelf margin/slope to deep ocean basin from northwest to southeast (Fig. 1) (Vernhet, 2005; Jiang et al., 2011; Och et al., 2013). Volcano erupted and sea level rose rapidly with the crust regional extension in the Yangtze Platform at the end of Precambrian (Brasier, 1992; Ma et al., 2009). During this period, thick siliceous rock was developed in the southeast margin of the Yangtze Platform (Jiang et al., 2011; Wang and Carr, 2012). At the early Cambrian, the crust extension entered into the late stage, and the Yangtze Platform evolved into thermal subsidence stage of passive-margin setting (Vernhet, 2005; Feng et al., 2004). From this time, the Niutitang shale began to deposit under the conditions of sea level rise, hydrothermal activity and bio-radiation (Steiner et al., 2001). Previous studies reported the Niutitang Formation recorded two large marine transgressions in the early Cambrian, and represented a second-order sedimentary sequence (Mei et al., 2006a, b). From bottom to top, the Niutitang Formation can be divided into two members, and the lithology changes from siliceous shale to argillaceous shale and to silty shale in basinal environment (Fig. 2) (Lin et al., 2014; Yang et al., 2016).
prepared for Light Microscopy to analyze the shale's texture and fabric. 62 samples were measured for TOC content using a CS580-A carbon-sulfur analyzer. 44 samples were selected to perform the XRD (X-ray diffraction) experiment to quantify the mineral composition. 29 samples were used to measure the trace element concentrations by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Below describes the methods of the specific measurements. TOC: The TOC contents of 62 samples were measured through a LECO CS230 carbon/sulfur analyzer. Samples were firstly crushed to powder with particle less than 100-mesh, then 1e2 g samples were pyrolyzed up to 540 C. X-ray diffraction (XRD): Bulk mineralogical composition of shales was derived from the X-ray diffraction patterns. 44 samples were firstly ground into powder, and then XRD analysis was performed on the randomly oriented powder through a Rigaku D/max-2600 diffractometer with Cu Ka radiation, automatic divergent and anti-scatter silts, and a secondary graphite monochromator with a scintillation counter. The generator settings were 40 KV and 40 mA. The diffraction data were recorded from 2 to 76 2qwith a step width of 0.02 and a counting time of 4s per step. The mineral content was semi-quantitatively determined on the basis of the intensity of specific reflections, the density, and the mass adsorption coefficient (Cu Ka) of the identified mineral phases. Trace element (TE): Trace element concentrations of a total of 29
2.2. The two studied outcrops The Nangao (NG) outcrop lies in Jiumenchong Country of Guizhou province in the southeastern Upper Yangtze Platform, with the geographic coordinate of 26 22.8950 N, 108 52.8590 E. Previous studies demonstrated that the Niutitang shale at the NG outcrop was deposited in deep ocean basinal setting (Liang et al., 2009; Xu et al., 2015). This outcrop was selected as a type outcrop for the research of lower Cambrian geochemistry and paleo-biology before (Jiang et al., 2007; Steiner et al., 2005). The Niutitang Formation at NG outcrop has a thickness of 118 m, and is of conformable contact to the Precambrian upper Sinian Laobao siliceous rocks below, and to the lower Cambrian Jiumenchong limestone above. The Longbizui (LBZ) outcrop lies in Guzhang Country of Hunan province in the eastern Upper Yangtze Platform, with the geographic coordinate of 28 29.8890 N, 109 50.5670 E. Previous studies indicated that the Niutitang shale at the LBZ outcrop was deposited in slope setting during the early Cambrian (Liang et al., 2009; Xu et al., 2015). This outcrop was investigated to reconstruct the paleoenvironment in the Yangtze Platform at the transition time from Precambrian to Cambrian before (Wang et al., 2012; Cremonse et al., 2014). The Niutitang Formation at LBZ outcrop has a thickness of 156 m, and is conformably bounded by the Precambrian upper Sinian Liuchapo siliceous rocks below, and by the lower Cambrian Palang siltstone above. 3. Samples and methods 3.1. Samples In this study, a total of 88 shale samples were collected. 46 samples are from Nangao (NG) outcrop, and 42 samples are from Longbizui (LBZ) outcrop. The locations for the two sampled outcrops are shown in Fig. 1. 3.2. Methods A total of 53 thin-sections and ion polished-sections were
Table 1 The measured TOC contents for shale samples from NG and LBZ outcrop. Code
Depth (m)
TOC (%)
Code
Depth (m)
TOC (%)
NG07 NG08 NG09 NG10 NG11 NG12 NG13 NG14 NG15 NG16 NG17 NG18 NG19 NG20 NG21 NG22 NG23 NG24 NG25 NG26 NG27 NG28 NG29 NG30 NG31 NG32 NG33 NG34 NG35 NG36 NG37 NG38 NG39 NG40 NG41 NG42 NG43 NG44 NG45
0.51 0.63 0.78 1.22 1.46 2.36 2.5 3.23 4.9 6.43 6.87 7.6 8.03 9.38 10.56 12.56 14.78 15.9 16.23 23 28.2 32.5 33.1 36.06 38.6 42.2 63.26 64.8 65.15 71.7 75.7 78.18 81.6 90.33 92.8 96.7 104.65 114.9 118.2
11.2 8.42 6.08 15.4 14.1 10.3 9.3 8.43 8.24 4.08 4.24 3.98 4.66 4.98 4.7 7.38 4.11 4.79 4.39 7.02 2.55 6.44 7.43 8.22 7.33 12.9 3.38 1.56 7.08 6.02 11.2 8.04 3.08 2.23 3.06 2.64 3.01 3.4 2.89
LBZ01 LBZ02 LBZ04 LBZ05 LBZ06 LBZ08 LBZ09 LBZ10 LBZ12 LBZ14 LBZ16 LBZ18 LBZ19 LBZ21 LBZ23 LBZ25 LBZ27 LBZ29 LBZ32 LBZ34 LBZ36 LBZ38 LBZ40
0 1.71 2.49 3.35 4.83 9.92 14.25 18.97 21.79 25.61 30.54 37.07 85.29 89.86 94.35 101.29 107.5 114.47 122.65 130.33 140.63 148.69 156.17
9.23 7.25 6.86 9.19 8.76 6.2 6.61 6.64 6.69 4.06 6.55 5.83 1.22 1.26 1.36 1.5 1.07 1.49 1.7 1.68 0.88 3.02 1.29
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Table 2 The quantified mineral composition by XRD for shale samples from NG and LBZ outcrop. Code
Depth (m)
Quartz (%)
Clay mineral (%)
Calcite (%)
Dolomite (%)
K-feldspar (%)
Plagioclase (%)
Pyrite (%)
NG10 NG11 NG12 NG14 NG17 NG19 NG20 NG21 NG22 NG23 NG25 NG27 NG30 NG31 NG32 NG34 NG35 NG36 NG37 NG39 NG40 NG41 NG42 NG45
1.22 1.46 2.36 3.23 6.87 8.03 9.38 10.56 12.56 14.78 16.23 28.2 36.06 38.6 42.2 64.8 65.15 71.7 75.7 81.6 90.33 92.8 96.7 118.2
56.8 60.4 58 63.8 62.1 53.8 51.6 51.6 51 59.1 63.5 53 61.8 62 60 49 69 52 65.4 43 40.4 43 46 51
24 30.5 31.4 21.8 25.8 29.3 37.4 34.7 27.9 26.3 22.2 30 25.5 27 21 43 26.9 35 25.4 53 59.6 53 50 44
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
10.4 6.7 7.2 6.9 7.3 5.3 7.7 7.4 10.4 9.5 9.1 5 4.1 8 9 7 4.1 10 5.2 4 0 4 4 5
0 0 0 0 0 5.3 3.3 6.3 5.2 0 0 0 0 0 0 0 0 0 4 0 0 0 0 0
5.8 0 2.3 6.6 4.8 6.3 0 0 0 0 0 2 0 3 0 0 0 0 0 0 0 0 0 0
Code
Depth (m)
Quartz (%)
Clay mineral (%)
Calcite (%)
Dolomite (%)
K-feldspar (%)
Plagioclase (%)
Pyrite (%)
LBZ02 LBZ04 LBZ05 LBZ06 LBZ07 LBZ08 LBZ09 LBZ10 LBZ19 LBZ21 LBZ23 LBZ25 LBZ27 LBZ29 LBZ32 LBZ34 LBZ36 LBZ38 LBZ40
1.71 2.49 3.35 4.83 5.78 9.92 14.25 18.97 85.29 89.86 94.35 101.29 107.5 114.47 122.65 130.33 140.63 148.69 156.17
94.8 64.7 83.8 86.9 51.3 64.6 54.1 76.6 56.6 48.8 48.3 50.8 50.6 45.1 46.3 46.3 59.8 56.8 53.5
5.2 11.2 8.7 5.1 6 0 31.6 15.5 36.1 43.6 40 40.7 39.7 42.6 39.6 39 29.3 28.9 31
0 0 0 0 0 22.3 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 34.3 7 0 0 0 0 0 0 0 0 0 0 0 0 0
0 4.9 0 3.6 3 2.7 0 0 0 0 0 0 0 0 0 0 0 0 0
0 10.8 5.3 0 0 0 12.8 7.9 7.3 7.6 9.1 8.5 9.7 12.3 10.9 11.7 10.9 11.7 15.5
0 8.4 2.2 4.4 5.4 0 1.5 0 0 0 2.6 0 0 0 3.2 3 0 2.6 0
Fig. 3. Ternary diagrams of mineralogy for the lower Cambrian Niutitang shale. (A) Is for NG outcrop and (B) is for LBZ outcrop.
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samples were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) using sample powders dissolved in HF, HClO4 and HNO3. 4. Results 4.1. TOC and XRD The measured TOC contents are presented in Table 1. The samples from NG outcrop show an average TOC content of 6.29%, ranging from 1.08% to 15.4%. The TOC contents for the samples from LBZ outcrop range from 0.88% to 9.23%, averaging at 4.36%. The average TOC content for all samples is 5.5%. The analyzed XRD results are shown in Table 2 and Fig. 3. The data indicates that quartz and clay mineral dominate the mineral composition of the Niutitang shale. Other minerals including calcite, dolomite, K-feldspar, plagioclase, and pyrite have the content less than 5% averagely. Specifically, for the samples from NG outcrop, quartz content ranges from 36% to 69%, with 54.21% on average, and clay mineral content ranges from 21% to 59.6%, with 33.47% on average. For the samples from LBZ outcrop, quartz content varies from 44.4% to 94.8%, with 58.7% on average, and clay mineral content varies from 0% to 43.6%, with 25% on average. The mineral composition for all samples shows an average content of 57.3% quartz and 29.8% clay mineral. 4.2. Trace elements The trace element contents for samples from NG outcrop are summarized in Table 3. The concentration of Mo ranges from 14.4 mg/g to 149 mg/g. The ratios of V/Cr, V/(V þ Ni) and Ni/Co show variations from 1.17 to 21.59, from 0.58 to 0.99, and from 6.45 to 53.15, respectively.
Table 3 TOC, Mo, V/Cr, Ni/Co, and V/(V þ Ni) data of samples from NG outcrop. Code
Depth (m)
TOC (%)
Mo (ppm)
V/Cr
V/V þ Ni
Ni/Co
NG07 NG10 NG11 NG12 NG13 NG14 NG15 NG17 NG18 NG20 NG21 NG22 NG23 NG24 NG26 NG27 NG28 NG29 NG31 NG32 NG34 NG36 NG39 NG41 NG42 NG44 NG45
0.51 1.22 1.46 2.36 2.5 3.23 4.9 6.87 7.6 9.38 10.56 12.56 14.78 15.9 23 28.2 32.5 33.1 38.6 42.2 64.8 71.7 81.6 92.8 96.7 114.9 118.2
11.2 15.4 14.1 10.3 9.3 8.43 8.24 4.24 3.98 4.98 4.7 7.38 4.11 4.79 7.02 2.55 6.44 7.43 7.33 12.9 1.56 6.02 3.08 3.06 2.64 3.4 2.89
117 149 132 91.7 96.5 106 102 83.3 62.4 58.8 59.7 48.2 46.4 48.7 79.54 24.23 115.4 66.59 25.15 133.3 61.12 102.4 15.9 50.2 31.87 7.63 40.61
1.17 5.79 6.91 16.3 18.64 8.42 3.94 13.69 21.59 13.95 5.44 2.52 2.74 7.33 11.1 14.45 2.98 4.09 8.65 2.96 5.7 11.1 2.62 11.28 7.34 2.15 2.8
0.91 0.8 0.83 0.97 0.98 0.82 0.58 0.94 0.95 0.97 0.95 0.82 0.97 0.95 0.96 0.99 0.69 0.93 0.96 0.84 0.93 0.97 0.94 0.98 0.97 0.79 0.85
16.82 12.9 25.79 24 38.33 9.2 11.46 9.59 16.68 18.53 26.89 8.1 6.45 31.78 53.15 8.93 22.5 29.83 30.34 25.1 53.05 51.26 15.14 24.85 24.54 2.96 7.44
5. Discussion 5.1. The correlations between TOC and mineral composition Many authors reported that there were certain correlations existing between TOC and mineral composition (Luo et al., 2014b; Han et al., 2013; Wu et al., 2014). The cross-plots of TOC vs Quartz and TOC vs Clay mineral for the Niutitang shale are shown in Fig. 4. A moderately positive correlation between TOC and quartz content is shown for samples from Both NG and LBZ outcrop. Similar relationship can also be found in the Barnett shale in America. Several literature reported that part of quartz in Niutitang shale was biological silica from radiolarian and sponge spicule. Those organisms can also contribute to the organic matter accumulation (Wang et al., 2014; Luo et al., 2014b and Zhang et al., 2015). In addition, the TOC content shows a moderately negative correlation to clay mineral content in the Niutitang shale, and the same to the Gordondale shale in Canada. 5.2. Evolution patterns of redox and restrictive conditions 5.2.1. Redox condition In general, redox condition of water column can be classified into oxic, dysoxic and suboxic/anoxic (sulfidic and nonsulfidic). Trace elements ratios of V/Cr, V/(V þ Ni) and Ni/Co have been widely used to reconstruct paleo-redox conditions. They usually decrease with the increase of oxygenation level in water body. Jones and Manning (1994) suggested that V/Cr ratios <2 inferred oxic condition, 2e4.25 for dysoxic condition and >4.25 for suboxic to anoxic condition. They also used Ni/Co ratios of <5 to infer oxic condition, 5e7 for dysoxic condition, and >7 for suboxic to anoxic condition. Hatch and Leventhal (1992) suggested V/(V þ Ni) ratios greater than 0.84 for euxinic conditions, 0.46e0.60 for dysoxic conditions and 0.54e0.82 for anoxic waters. In this study, most of the samples fall into the anoxic domain indicated by the ratios of Ni/Co, and just one sample with the value of Ni/Co lower than 5, was deposited in oxic condition. Most of the V/Cr values are higher than 4.25, indicating that an anoxic condition prevailed when the Niutitang shale was deposited. Few samples show low V/Cr values, inferring dysoxic even oxic condition occasionally existed. As for the parameter of V/(V þ Ni), almost all samples fall into the area of anoxic and euxinic settings, indicating that a persistent oxygen rare condition, sometimes with free H2S occurred during the Niutitang shale deposition (Fig. 5). Based on the above analysis, it can demonstrate that the Niutitang shale was mainly deposited in anoxic-to -dysoxic conditions, with occasionally oxic condition. This is consistent with the results of studies from Zhang et al. (2016) and Och et al. (2013). The vertical variations of ratios of V/Cr, V/(V þ Ni) and Ni/Co indicates an obvious decreasing trend of anoxia degree in bottom water upwards within each member framework (Fig. 6). 5.2.2. Restrictive condition Covariation of TOC and Mo content has been proposed as a proxy for watermass restriction on the basis of the observation that the amount of Mo taken up by sediments in anoxic marine systems depends on both the aqueous concentration of Mo and the concentration of organic matter (Algeo and Lyons, 2006; Algeo and Rowe, 2012). The ratios of Mo/TOC for studied samples can be divided into two domains. Domain 1 points to a weak degree of deep water restriction, similar on average to that of the modern Saanich Inlet. Domain 2 is similar to that of the modern Framvaren Fjord, indicating a moderate degree of deep water restriction.
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Fig. 4. The correlation plots of TOC vs Quartz and TOC vs Clay for Niutitang shale of this study and Barnett shale in U.S. and Gordondale shale in Canada. Data of Barnett and Gordondale shales are from Bustin et al. (2008) and Ross, 2007, respectively.
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Fig. 5. The cross-plots of trace elements ratios for the lower and upper members of the Niutitang shale at NG outcrop. (A) Ni/Co vs. V/Cr. (B) Ni/Co vs. V/(V þ Ni). Boundaries for different redox environments from Jones and Manning (1994) for Ni/Co and V/Cr, and Hatch and Leventhal (1992) for V/(V þ Ni).
Almost all samples from upper member fall into domain 1, and samples from lower member fall into both domains (Fig. 7). The restrictive degree of the watermass shows an obviously decreasing trend from lower member to upper member during the Niutitang shale deposition period. 5.3. Lithofacies classification and characterization 5.3.1. Lithofacies types In this study, the shale lithofacies are classified and characterized on the basis of TOC content, mineral composition, texture, and fabric. Each lithofacies corresponds to specified sedimentary process and environment. Shown following are the identified five lithofacies and their characteristics. Organic-rich siliceous shale: The quartz content is more than 50%, compared to clay mineral and carbonate. The texture is characterized by more than 75% micro particles (diameter lower than 0.0039 mm) and less than 25% silty particles (diameter ranging between 0.0039 and 0.0625 mm). Organic material commonly appears in the form of aggregates and strips, with the TOC content more than 10% averagely. Burrows and fossil traces are extremely scarce, indicating rare-to -no bioturbation. Weak-to -well laminations are sometimes presented resulting from the interbedded quartz and organic matter laminae. This lithofacies is generally
interpreted to develop in anoxic deep water and low energy situation (Fig. 8AeC, Fig. 9AeD). Silty siliceous shale: Quartz is dominant in the mineral composition, with the content more than 50%. Silty grains content is above 25% revealed by micrographs. Organic material with the content lower than 5%, commonly appears in the form of aggregates or disseminated particles. All mineral grains mix homogeneously, exhibiting non-to -weak lamination structure. A relative low energy and shallow water environment is indicated by this lithofacies (Fig. 8DeE, Fig. 9EeH). Calcareous siliceous shale: The mineral composition in this lithofacies contains more than 20% carbonate and more than 50% quartz. The carbonate minerals occur as matrix and/or natural fracture fillings. Organic matter commonly appears in the form of aggregates and strips, with the TOC content ranging from 2% to 5%. Microphotographs reveal the diagenetic alteration of this lithofacies in the form of strong recrystallization and replacement by microcrystalline dolomite. A relative low energy and shallow water environment is suggested by this lithofacies (Fig. 10AeB). Argillaceous shale: This lithofacies is characterized by more than 50% clay mineral content and lower than 25% silty sized particles. Biological fossils, such as radiolarian and sponge spicule are abundant. Organic material with the content between 2% and 3%, mixes homogenously with clay mineral to form aggregates or strips. Micro-quartz is orderly ranged to form silica laminae. This lithofacies may be associated with low energy and deep water conditions (Fig. 8F, Fig. 10CeF). Silty mixed shale: Samples are classified into this lithofacies when quartz and clay mineral content are both less than 50%. The silty grain content is generally more than 25%. TOC content is less than 2%. Laminated structure can be observed by the alternation of black and bright layer. The black layer is composed of organic matter and clay mineral, and the bright layer is aggregated by quartz, feldspar and mica. This lithofacies is interpreted to develop in high energy and shallow water environment (Fig. 8GeH, Fig. 10GeH). 5.3.2. Lithofacies associations Four lithofacies associations (LA1~4) have been identified based on lithofacies stacking patterns that reflect correlative sedimentary environments (Fig. 11). Each association represents a lithofacies succession organized within one regressive cycle. LA1 includes two lithofacies, which are deposited in anoxic-to -suboxic, low-to -moderate energy, and hot water prevailed environment. Organic-rich siliceous shale was developed at the bottom of LA1 after an extensive marine transgression at the end of Meishucun stage. The succeeded silty siliceous shale was then deposited in a relative high energy environment as a result of sea level fall. These two lithofacies are organized in a shallowingupward regressive cycle with more terrigenous input upwards. LA1 indicates siliceous basinal facies. LA2 comprises of three lithofacies, which are deposited in anoxic-to -oxic, and low-to -high energy environment, recording a shallowing-upward cycle. Organic-rich siliceous shale was developed at the bottom after an extensive marine transgression at the early of Qiongzhusi stage. With the water shallowing upwards, argillaceous shale was then presented in the middle of the cycle, and argillaceous siltstone was finally developed on the top. LA2 suggests argillaceous basinal facies. LA3 is composed of three types of lithofacies, which are related to anoxic-to -suboxic, low-to -moderate energy, and hot water predominated environment. Organic-rich siliceous shale at the
Fig. 6. Vertical variations in the content of Mo and the ratios of V/Cr, V/(V þ Ni) and Ni/Co at NG outcrop.
Fig. 7. The cross-plots between TOC content and Mo content showing the domains of tested data for the Niutitang shale at NG outcrop. The dashed lines represent modern anoxic marine systems. This figure is modified from Algeo and Lyons (2006).
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Fig. 8. Outcrop photographs for different lithofacies. (A) Organic-rich siliceous shale; (B) Organic-rich siliceous shale containing phosphatic nodules; (C) Thin layer organicrich siliceous shale; (D) Thick layer silty siliceous shale; (E) Silty siliceous shale with dissolved pores and microfractures; (F) Argillaceous shale showing spheroidal weathering; (G) Silty mixed shale and argillaceous siltstone interbedding; (H) Silty mixed shale with silt laminae. Photographs of A, C, D, G are from NG outcrop, and B,E, F, H are from LBZ outcrop.
bottom recorded an extensive marine transgression at the end of Meishucun stage. With the sea level fall, subsequent deposition was the alternated silty siliceous shale and calcareous siliceous shale. The depositional environment for this association follows an upward shallowing trend. The total proportions of silty grains and carbonate minerals in the succession increase upwards. LA3 represents lower slope facies. LA4 includes three lithofacies, which are deposited in anoxic-to -oxic, and low-to -high energy environments. Organic-rich siliceous shale at the base was developed in the anoxic and low energy environment after a large marine transgression at the early of Qiongzhusi stage. The following deposited silty mixed shale and argillaceous siltstone indicate that the depositional setting turned into high energy and shallow water environment. The vertical
Fig. 9. Microphotographs for different lithofacies. A ~ D are for organic-rich siliceous shale. Micro silica grains are concentrated to form patchness in Pictures A and B, or well oriented to form thin laminae in Picture D. E ~ H are for silty siliceous shale. Silt sized grains distribute randomly, not forming obvious laminae.
stacking pattern of the lithofacies in this sequence collectively reflects a shallowing-upward trend. LA4 suggests upper slope or shelf margin facies. 5.4. Shale cyclicity and sedimentary sequence The lithofacies association analysis indicates the Niutitang Formation generally exhibits a shallowing-upward prograding depositional sequence. On the basis of the vertical variations of TOC content, mineral composition, redox conditions and lithofacies stacking pattern, the Niutitang shale can be divided into two large cycles (named cycle A and B), which are corresponded to the two large marine transgressions at the end of Meishucun stage and at the early of Qiongzhusi stage, respectively (Mei et al., 2006a, b; Lin et al., 2014; Yang et al., 2016). Additionally, cycle A can be further divided into two sub-cycles (named cycle A1 and A2) (Fig. 12, Table 4).
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high TOC and quartz contents. Cycle A2: This cycle lies above the cycle A1, and is totally or partly absent in the two studied outcrops. The depositional successions in this cycle may resemble those in cycle A1. Cycle B: This cycle is located at the upper member of the Niutitang Formation, and results from an extensive marine transgression at the early of Qiongzhusi stage in early Cambrian. For the NG outcrop in basinal facies, the depositional sequence in this cycle is composed of LA2, with a thickness of 53 m. The TOC content ranges from 2.23% to 11.2%, with an average of 6.3%. The average content of quartz is about 49.5% (varying from 36% to 65.4%), and the clay mineral content ranges between 25.4% and 59.6% (43.1% on average). For the LBZ outcrop in slope facies, this cycle is composed of LA4, with a thickness more than 65 m. The TOC content ranges from 0.88% to 3.02%, with an average of 1.5%. The average content of quartz is about 51.2% (varying from 45.1% to 59.8%), and the clay mineral content ranges between 28.9% and 43.6% (37.3% on average). The bottom section in this cycle may hold shale gas potential because of the high TOC and quartz contents. From Cycle A to Cycle B, the sedimentary facies evolved from siliceous basin to argillaceous basin at NG outcrop, and from lower slope to upper slope at LBZ outcrop. The organic matter content and anoxic degree decrease largely because of the changes of sea level and their corresponding depositional environments. For the mineral composition, the average content of quartz deceases, but the average content of clay mineral increases. Compared each shale cycle at NG outcrop and LBZ outcrop, the organic matter and clay mineral contents show an obviously increasing trend, but quartz content shows a decreasing trend from slope to basinal facies. More terrigenous input and stronger dilution in the slope environment but more suspension clay sediments in the basinal environment may account for this changing trend.
6. Conclusion
Fig. 10. Microphotographs for different lithofacies. A ~ B are for calcareous siliceous shale. It shows that dolomite replaces siliceous fossils in Picture A, and calcite fills in nature fractures in Picture B. C ~ F are for argillaceous shale, rich in bio-fossils. Silica laminae are obvious. G ~ H are for silty mixed shale. Quartz and mica concentrate in the bright laminae, and clay mineral and organic matter concentrate in the black laminae in Picture H.
Cycle A1: This cycle is located at the bottom of the Niutitang Formation, which was developed after a rapid sea level rise at the end of Meishucun stage in early Cambrian. For the NG outcrop in basinal facies, this cycle is made up of LA1, with a thickness of 32 m. The TOC content ranges from 1.08% to 15.4%, with an average of 7.3%. The quartz content ranges from 51% to 63.8% (56.2% on average), and from 21.8 to 37.4% (28.3% on average) for clay mineral content. For the LBZ outcrop in slope facies, this cycle is made up of LA3, with a thickness over 40 m. The TOC content ranges from 4.06% to 9.23%, with an average of 7%. The quartz content ranges from 51.3% to 94.8% (72.1% on average), and from 0% to 31.6% for clay mineral (10.4% on average). Cycle A1 is considered as the most shale gas potential interval because of the
(1) For the lower Cambrian Niutitang shale, the TOC content is positively correlated to quartz content, but negatively correlated to clay mineral content. Anoxic to dysoxic conditions, and moderate to weak degree of restriction are the major depositional conditions during the Niutitang shale deposition period in Early Cambrian. (2) On the basis of TOC content, mineral composition, texture and fabric, five lithofacies are identified: Organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Additionally, four lithofacies associations are interpreted to represent four different sedimentary facies, including siliceous basinal facies, argillaceous basinal facies, lower slope facies, and shelf margin facies. (3) The lower Cambrian Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and consists of two large shale cycles. From bottom to top, the organic matter content and anoxic degree decrease largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies.
Fig. 11. Four types of lithofacies associations. Each lithofacies association shows a shallowing-upward stacking pattern and indicates a certain depositional setting.
Fig. 12. Vertical evolution patterns of lithofacies, geochemistry and mineralogy, and stratigraphic correlation for the lower Cambrian Niutitang shale. (a) Is for NG outcrop in basinal facies, and (b) is for LBZ outcrop in slope facies. See Fig. 1 for the locations of the two outcrops.
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Table 4 Statistic average contents of TOC, quartz and clay mineral. Outcrop
Cycle
Thickness (m)
TOC (%)
Quartz (%)
Clay mineral (%)
Nangao (NG)
A1 A2 B
32 33 53
7.3 / 6.3
56.2 / 49.5
28.3 / 43.1
Longbizui (LBZ)
A1 A2 B
>40 <50 >65
7 / 1.5
72.1 / 51.2
10.4 / 37.3
5.5
57.3
29.8
Average
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Lithofacies and sedimentary sequence of the lower Cambrian Niutitang shale in the upper Yangtze platform, South China Yue Wu a, Tailiang Fan b, *, Shu Jiang c, **, Xiaoqun Yang b a
Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, People's Republic of China School of Energy Resources, China University of Geosciences, Beijing 100083, People's Republic of China c Energy and Geoscience Institute, University of Utah, Salt Lake City, UT 84108, United States b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2016 Received in revised form 27 March 2017 Accepted 3 April 2017 Available online 7 April 2017
The lower Cambrian Niutitang shale in the Upper Yangtze Platform is a hot target for shale gas exploration in China. In this paper, the lithofacies and sedimentary evolution characteristics about this shale formation are studied on the basis of outcrop investigation and experimental measurements. The variations on geochemistry, mineralogy, lithofacies and stratigraphic architecture of this shale are analyzed. This shale is rich in organic matter, with an average TOC content of 5.5%. Quartz and clay mineral dominate the mineral composition, with an average content of 57.3% and 29.8%, respectively. The TOC content correlates positively to quartz content, but negatively to clay mineral content. Trace element ratios of V/Cr, V/(V þ Ni) and Ni/Co indicate anoxic to dysoxic conditions prevailed during the Niutitang shale deposition period in Early Cambrian, and moderate to weak degree of restriction are inferred by the covariation of Mo and TOC content. Five lithofacies are identified based on TOC content, mineral composition, texture and fabric: organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Four lithofacies associations are interpreted to represent four different depositional facies from shelf margin to abyssal environments. The Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and can be divided into two large shale cycles. From bottom to top, the organic matter content and anoxic degree decreases largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies. © 2017 Elsevier B.V. All rights reserved.
Keywords: Niutitang shale Lithofacies Sedimentary sequence Upper Yangtze platform
1. Introduction Lithofacies and sedimentary environments analysis, as the fundamental work in sedimentology and reservoir geology, have long been of great scientific interest. Understanding the lithofacies characteristics and their stacking patterns are critical for predicting favorable reservoirs (Jarvie et al., 2007; Loucks and Ruppel, 2007; Abouelresh and Slatt, 2012; Slatt and Rodriguez, 2012; Wang et al., 2012, 2014; Bhattacharya et al., 2016). Generally, lithofacies is the overall reflection of mineral composition, texture, bedding,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Jiang). http://dx.doi.org/10.1016/j.jngse.2017.04.002 1875-5100/© 2017 Elsevier B.V. All rights reserved.
(T. Fan), [email protected]
structure, color, size distribution, sorting and roundness. At present, different opinions and methods have been suggested in terms of the shale lithofacies classification, which can be generally grouped into three types: (1) macroscopic sedimentary characteristics such as shale texture and structure characteristics; (2) mineral composition and organic matter; (3) paleontology such as graptolite shale lithofacies and radiolarian shale lithofacies (Hickey and Henk, 2007; Li and Quan, 1992; Chen et al., 2016). Among them, the second method was applied in this study to define and describe the shale lithofacies. Shale gas exploration and production in China has drawn much attention from the public in recent years. Technically recoverable shale gas reserves in China are estimated to range from 10 to 45 1012 m3 (MLR, www.mlr.gov.cn). The lower Cambrian Niutitang shale in the Upper Yangtze Platform in South China was
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investigated as one of the important Phanerozoic source rock for conventional oil and gas before. It is now considered as potential shale gas play on the basis of its rock properties and shale gas accumulation conditions (Jiang et al., 2016; Dong et al., 2010, 2012; Tan et al., 2011). Its properties of shale distribution, organic matter richness, thermal maturity, and mineral composition were commonly reported on the basis of drilling data, field study and experimental measurements (Tan et al., 2015, 2014; Han et al., 2013; Lin et al., 2014; Wang et al., 2013; Luo et al., 2014a, b). However, little work has been conducted for the detailed lithofacies analysis of this shale and only few published papers discuss the depositional settings and evolution history. Due to few wells targeting the lower Cambrian Niutitang shale have been drilled in the Upper Yangtze Platform, this paper selects two type outcrops to describe the lithofacies features and their stacking patterns of this shale. The detailed lithofacies analysis presented in this study will provide information on the lateral changes of the depositional environments. The study of the lithofacies stacking may help to correlate the outcrops across the platform and thus will shed new light on the sedimentary evolution of the Upper Yangtze Platform through Niutitang Formation times. 2. Geological setting 2.1. The yangtze platform The Yangtze Platform is located between the Qinling-Dabie and Longmenshan orogens to the north and the Cathaysia suture to the south (Wang and Mo, 1995; Wan, 2011). These major boundary structures resulted from the collision between the Yangtze Platform and the North China craton during the early Triassic and the collision between Yangtze Platform and the Cathaysia volcanic arc during the Silurian (Wang and Li., 2004; Ma et al., 2004). The
Fig. 2. Lithofacies and sequence stratigraphy framework of the lower Cambrian Niutitang Formation in the southeastern Upper Yangtze Platform (referring to Mei et al., 2006). TST ¼ transgressive system tract, HST ¼ highstand system tract. The data of d13C (‰, VPDB) as an indicator for the relative sea level change is from Zhu (2007).
regional stratigraphy of the Yangtze Platform shows the sedimentary successions spanning from Precambrian Sinian to
Fig. 1. The paleoenvironmental map of the South China Craton in the early Cambrian. This paleogeography is modified from Och et al. (2013).
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Cenozoic. These strata are preserved with a thickness on the order of several kilometers and are moderately folded with large-scale features (Chen et al., 2004). During early Cambrian times, the Yangtze Platform was a passive margin basin and consisted of inner shelf, shelf margin/slope to deep ocean basin from northwest to southeast (Fig. 1) (Vernhet, 2005; Jiang et al., 2011; Och et al., 2013). Volcano erupted and sea level rose rapidly with the crust regional extension in the Yangtze Platform at the end of Precambrian (Brasier, 1992; Ma et al., 2009). During this period, thick siliceous rock was developed in the southeast margin of the Yangtze Platform (Jiang et al., 2011; Wang and Carr, 2012). At the early Cambrian, the crust extension entered into the late stage, and the Yangtze Platform evolved into thermal subsidence stage of passive-margin setting (Vernhet, 2005; Feng et al., 2004). From this time, the Niutitang shale began to deposit under the conditions of sea level rise, hydrothermal activity and bio-radiation (Steiner et al., 2001). Previous studies reported the Niutitang Formation recorded two large marine transgressions in the early Cambrian, and represented a second-order sedimentary sequence (Mei et al., 2006a, b). From bottom to top, the Niutitang Formation can be divided into two members, and the lithology changes from siliceous shale to argillaceous shale and to silty shale in basinal environment (Fig. 2) (Lin et al., 2014; Yang et al., 2016).
prepared for Light Microscopy to analyze the shale's texture and fabric. 62 samples were measured for TOC content using a CS580-A carbon-sulfur analyzer. 44 samples were selected to perform the XRD (X-ray diffraction) experiment to quantify the mineral composition. 29 samples were used to measure the trace element concentrations by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Below describes the methods of the specific measurements. TOC: The TOC contents of 62 samples were measured through a LECO CS230 carbon/sulfur analyzer. Samples were firstly crushed to powder with particle less than 100-mesh, then 1e2 g samples were pyrolyzed up to 540 C. X-ray diffraction (XRD): Bulk mineralogical composition of shales was derived from the X-ray diffraction patterns. 44 samples were firstly ground into powder, and then XRD analysis was performed on the randomly oriented powder through a Rigaku D/max-2600 diffractometer with Cu Ka radiation, automatic divergent and anti-scatter silts, and a secondary graphite monochromator with a scintillation counter. The generator settings were 40 KV and 40 mA. The diffraction data were recorded from 2 to 76 2qwith a step width of 0.02 and a counting time of 4s per step. The mineral content was semi-quantitatively determined on the basis of the intensity of specific reflections, the density, and the mass adsorption coefficient (Cu Ka) of the identified mineral phases. Trace element (TE): Trace element concentrations of a total of 29
2.2. The two studied outcrops The Nangao (NG) outcrop lies in Jiumenchong Country of Guizhou province in the southeastern Upper Yangtze Platform, with the geographic coordinate of 26 22.8950 N, 108 52.8590 E. Previous studies demonstrated that the Niutitang shale at the NG outcrop was deposited in deep ocean basinal setting (Liang et al., 2009; Xu et al., 2015). This outcrop was selected as a type outcrop for the research of lower Cambrian geochemistry and paleo-biology before (Jiang et al., 2007; Steiner et al., 2005). The Niutitang Formation at NG outcrop has a thickness of 118 m, and is of conformable contact to the Precambrian upper Sinian Laobao siliceous rocks below, and to the lower Cambrian Jiumenchong limestone above. The Longbizui (LBZ) outcrop lies in Guzhang Country of Hunan province in the eastern Upper Yangtze Platform, with the geographic coordinate of 28 29.8890 N, 109 50.5670 E. Previous studies indicated that the Niutitang shale at the LBZ outcrop was deposited in slope setting during the early Cambrian (Liang et al., 2009; Xu et al., 2015). This outcrop was investigated to reconstruct the paleoenvironment in the Yangtze Platform at the transition time from Precambrian to Cambrian before (Wang et al., 2012; Cremonse et al., 2014). The Niutitang Formation at LBZ outcrop has a thickness of 156 m, and is conformably bounded by the Precambrian upper Sinian Liuchapo siliceous rocks below, and by the lower Cambrian Palang siltstone above. 3. Samples and methods 3.1. Samples In this study, a total of 88 shale samples were collected. 46 samples are from Nangao (NG) outcrop, and 42 samples are from Longbizui (LBZ) outcrop. The locations for the two sampled outcrops are shown in Fig. 1. 3.2. Methods A total of 53 thin-sections and ion polished-sections were
Table 1 The measured TOC contents for shale samples from NG and LBZ outcrop. Code
Depth (m)
TOC (%)
Code
Depth (m)
TOC (%)
NG07 NG08 NG09 NG10 NG11 NG12 NG13 NG14 NG15 NG16 NG17 NG18 NG19 NG20 NG21 NG22 NG23 NG24 NG25 NG26 NG27 NG28 NG29 NG30 NG31 NG32 NG33 NG34 NG35 NG36 NG37 NG38 NG39 NG40 NG41 NG42 NG43 NG44 NG45
0.51 0.63 0.78 1.22 1.46 2.36 2.5 3.23 4.9 6.43 6.87 7.6 8.03 9.38 10.56 12.56 14.78 15.9 16.23 23 28.2 32.5 33.1 36.06 38.6 42.2 63.26 64.8 65.15 71.7 75.7 78.18 81.6 90.33 92.8 96.7 104.65 114.9 118.2
11.2 8.42 6.08 15.4 14.1 10.3 9.3 8.43 8.24 4.08 4.24 3.98 4.66 4.98 4.7 7.38 4.11 4.79 4.39 7.02 2.55 6.44 7.43 8.22 7.33 12.9 3.38 1.56 7.08 6.02 11.2 8.04 3.08 2.23 3.06 2.64 3.01 3.4 2.89
LBZ01 LBZ02 LBZ04 LBZ05 LBZ06 LBZ08 LBZ09 LBZ10 LBZ12 LBZ14 LBZ16 LBZ18 LBZ19 LBZ21 LBZ23 LBZ25 LBZ27 LBZ29 LBZ32 LBZ34 LBZ36 LBZ38 LBZ40
0 1.71 2.49 3.35 4.83 9.92 14.25 18.97 21.79 25.61 30.54 37.07 85.29 89.86 94.35 101.29 107.5 114.47 122.65 130.33 140.63 148.69 156.17
9.23 7.25 6.86 9.19 8.76 6.2 6.61 6.64 6.69 4.06 6.55 5.83 1.22 1.26 1.36 1.5 1.07 1.49 1.7 1.68 0.88 3.02 1.29
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Table 2 The quantified mineral composition by XRD for shale samples from NG and LBZ outcrop. Code
Depth (m)
Quartz (%)
Clay mineral (%)
Calcite (%)
Dolomite (%)
K-feldspar (%)
Plagioclase (%)
Pyrite (%)
NG10 NG11 NG12 NG14 NG17 NG19 NG20 NG21 NG22 NG23 NG25 NG27 NG30 NG31 NG32 NG34 NG35 NG36 NG37 NG39 NG40 NG41 NG42 NG45
1.22 1.46 2.36 3.23 6.87 8.03 9.38 10.56 12.56 14.78 16.23 28.2 36.06 38.6 42.2 64.8 65.15 71.7 75.7 81.6 90.33 92.8 96.7 118.2
56.8 60.4 58 63.8 62.1 53.8 51.6 51.6 51 59.1 63.5 53 61.8 62 60 49 69 52 65.4 43 40.4 43 46 51
24 30.5 31.4 21.8 25.8 29.3 37.4 34.7 27.9 26.3 22.2 30 25.5 27 21 43 26.9 35 25.4 53 59.6 53 50 44
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
10.4 6.7 7.2 6.9 7.3 5.3 7.7 7.4 10.4 9.5 9.1 5 4.1 8 9 7 4.1 10 5.2 4 0 4 4 5
0 0 0 0 0 5.3 3.3 6.3 5.2 0 0 0 0 0 0 0 0 0 4 0 0 0 0 0
5.8 0 2.3 6.6 4.8 6.3 0 0 0 0 0 2 0 3 0 0 0 0 0 0 0 0 0 0
Code
Depth (m)
Quartz (%)
Clay mineral (%)
Calcite (%)
Dolomite (%)
K-feldspar (%)
Plagioclase (%)
Pyrite (%)
LBZ02 LBZ04 LBZ05 LBZ06 LBZ07 LBZ08 LBZ09 LBZ10 LBZ19 LBZ21 LBZ23 LBZ25 LBZ27 LBZ29 LBZ32 LBZ34 LBZ36 LBZ38 LBZ40
1.71 2.49 3.35 4.83 5.78 9.92 14.25 18.97 85.29 89.86 94.35 101.29 107.5 114.47 122.65 130.33 140.63 148.69 156.17
94.8 64.7 83.8 86.9 51.3 64.6 54.1 76.6 56.6 48.8 48.3 50.8 50.6 45.1 46.3 46.3 59.8 56.8 53.5
5.2 11.2 8.7 5.1 6 0 31.6 15.5 36.1 43.6 40 40.7 39.7 42.6 39.6 39 29.3 28.9 31
0 0 0 0 0 22.3 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 34.3 7 0 0 0 0 0 0 0 0 0 0 0 0 0
0 4.9 0 3.6 3 2.7 0 0 0 0 0 0 0 0 0 0 0 0 0
0 10.8 5.3 0 0 0 12.8 7.9 7.3 7.6 9.1 8.5 9.7 12.3 10.9 11.7 10.9 11.7 15.5
0 8.4 2.2 4.4 5.4 0 1.5 0 0 0 2.6 0 0 0 3.2 3 0 2.6 0
Fig. 3. Ternary diagrams of mineralogy for the lower Cambrian Niutitang shale. (A) Is for NG outcrop and (B) is for LBZ outcrop.
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samples were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) using sample powders dissolved in HF, HClO4 and HNO3. 4. Results 4.1. TOC and XRD The measured TOC contents are presented in Table 1. The samples from NG outcrop show an average TOC content of 6.29%, ranging from 1.08% to 15.4%. The TOC contents for the samples from LBZ outcrop range from 0.88% to 9.23%, averaging at 4.36%. The average TOC content for all samples is 5.5%. The analyzed XRD results are shown in Table 2 and Fig. 3. The data indicates that quartz and clay mineral dominate the mineral composition of the Niutitang shale. Other minerals including calcite, dolomite, K-feldspar, plagioclase, and pyrite have the content less than 5% averagely. Specifically, for the samples from NG outcrop, quartz content ranges from 36% to 69%, with 54.21% on average, and clay mineral content ranges from 21% to 59.6%, with 33.47% on average. For the samples from LBZ outcrop, quartz content varies from 44.4% to 94.8%, with 58.7% on average, and clay mineral content varies from 0% to 43.6%, with 25% on average. The mineral composition for all samples shows an average content of 57.3% quartz and 29.8% clay mineral. 4.2. Trace elements The trace element contents for samples from NG outcrop are summarized in Table 3. The concentration of Mo ranges from 14.4 mg/g to 149 mg/g. The ratios of V/Cr, V/(V þ Ni) and Ni/Co show variations from 1.17 to 21.59, from 0.58 to 0.99, and from 6.45 to 53.15, respectively.
Table 3 TOC, Mo, V/Cr, Ni/Co, and V/(V þ Ni) data of samples from NG outcrop. Code
Depth (m)
TOC (%)
Mo (ppm)
V/Cr
V/V þ Ni
Ni/Co
NG07 NG10 NG11 NG12 NG13 NG14 NG15 NG17 NG18 NG20 NG21 NG22 NG23 NG24 NG26 NG27 NG28 NG29 NG31 NG32 NG34 NG36 NG39 NG41 NG42 NG44 NG45
0.51 1.22 1.46 2.36 2.5 3.23 4.9 6.87 7.6 9.38 10.56 12.56 14.78 15.9 23 28.2 32.5 33.1 38.6 42.2 64.8 71.7 81.6 92.8 96.7 114.9 118.2
11.2 15.4 14.1 10.3 9.3 8.43 8.24 4.24 3.98 4.98 4.7 7.38 4.11 4.79 7.02 2.55 6.44 7.43 7.33 12.9 1.56 6.02 3.08 3.06 2.64 3.4 2.89
117 149 132 91.7 96.5 106 102 83.3 62.4 58.8 59.7 48.2 46.4 48.7 79.54 24.23 115.4 66.59 25.15 133.3 61.12 102.4 15.9 50.2 31.87 7.63 40.61
1.17 5.79 6.91 16.3 18.64 8.42 3.94 13.69 21.59 13.95 5.44 2.52 2.74 7.33 11.1 14.45 2.98 4.09 8.65 2.96 5.7 11.1 2.62 11.28 7.34 2.15 2.8
0.91 0.8 0.83 0.97 0.98 0.82 0.58 0.94 0.95 0.97 0.95 0.82 0.97 0.95 0.96 0.99 0.69 0.93 0.96 0.84 0.93 0.97 0.94 0.98 0.97 0.79 0.85
16.82 12.9 25.79 24 38.33 9.2 11.46 9.59 16.68 18.53 26.89 8.1 6.45 31.78 53.15 8.93 22.5 29.83 30.34 25.1 53.05 51.26 15.14 24.85 24.54 2.96 7.44
5. Discussion 5.1. The correlations between TOC and mineral composition Many authors reported that there were certain correlations existing between TOC and mineral composition (Luo et al., 2014b; Han et al., 2013; Wu et al., 2014). The cross-plots of TOC vs Quartz and TOC vs Clay mineral for the Niutitang shale are shown in Fig. 4. A moderately positive correlation between TOC and quartz content is shown for samples from Both NG and LBZ outcrop. Similar relationship can also be found in the Barnett shale in America. Several literature reported that part of quartz in Niutitang shale was biological silica from radiolarian and sponge spicule. Those organisms can also contribute to the organic matter accumulation (Wang et al., 2014; Luo et al., 2014b and Zhang et al., 2015). In addition, the TOC content shows a moderately negative correlation to clay mineral content in the Niutitang shale, and the same to the Gordondale shale in Canada. 5.2. Evolution patterns of redox and restrictive conditions 5.2.1. Redox condition In general, redox condition of water column can be classified into oxic, dysoxic and suboxic/anoxic (sulfidic and nonsulfidic). Trace elements ratios of V/Cr, V/(V þ Ni) and Ni/Co have been widely used to reconstruct paleo-redox conditions. They usually decrease with the increase of oxygenation level in water body. Jones and Manning (1994) suggested that V/Cr ratios <2 inferred oxic condition, 2e4.25 for dysoxic condition and >4.25 for suboxic to anoxic condition. They also used Ni/Co ratios of <5 to infer oxic condition, 5e7 for dysoxic condition, and >7 for suboxic to anoxic condition. Hatch and Leventhal (1992) suggested V/(V þ Ni) ratios greater than 0.84 for euxinic conditions, 0.46e0.60 for dysoxic conditions and 0.54e0.82 for anoxic waters. In this study, most of the samples fall into the anoxic domain indicated by the ratios of Ni/Co, and just one sample with the value of Ni/Co lower than 5, was deposited in oxic condition. Most of the V/Cr values are higher than 4.25, indicating that an anoxic condition prevailed when the Niutitang shale was deposited. Few samples show low V/Cr values, inferring dysoxic even oxic condition occasionally existed. As for the parameter of V/(V þ Ni), almost all samples fall into the area of anoxic and euxinic settings, indicating that a persistent oxygen rare condition, sometimes with free H2S occurred during the Niutitang shale deposition (Fig. 5). Based on the above analysis, it can demonstrate that the Niutitang shale was mainly deposited in anoxic-to -dysoxic conditions, with occasionally oxic condition. This is consistent with the results of studies from Zhang et al. (2016) and Och et al. (2013). The vertical variations of ratios of V/Cr, V/(V þ Ni) and Ni/Co indicates an obvious decreasing trend of anoxia degree in bottom water upwards within each member framework (Fig. 6). 5.2.2. Restrictive condition Covariation of TOC and Mo content has been proposed as a proxy for watermass restriction on the basis of the observation that the amount of Mo taken up by sediments in anoxic marine systems depends on both the aqueous concentration of Mo and the concentration of organic matter (Algeo and Lyons, 2006; Algeo and Rowe, 2012). The ratios of Mo/TOC for studied samples can be divided into two domains. Domain 1 points to a weak degree of deep water restriction, similar on average to that of the modern Saanich Inlet. Domain 2 is similar to that of the modern Framvaren Fjord, indicating a moderate degree of deep water restriction.
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Fig. 4. The correlation plots of TOC vs Quartz and TOC vs Clay for Niutitang shale of this study and Barnett shale in U.S. and Gordondale shale in Canada. Data of Barnett and Gordondale shales are from Bustin et al. (2008) and Ross, 2007, respectively.
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Fig. 5. The cross-plots of trace elements ratios for the lower and upper members of the Niutitang shale at NG outcrop. (A) Ni/Co vs. V/Cr. (B) Ni/Co vs. V/(V þ Ni). Boundaries for different redox environments from Jones and Manning (1994) for Ni/Co and V/Cr, and Hatch and Leventhal (1992) for V/(V þ Ni).
Almost all samples from upper member fall into domain 1, and samples from lower member fall into both domains (Fig. 7). The restrictive degree of the watermass shows an obviously decreasing trend from lower member to upper member during the Niutitang shale deposition period. 5.3. Lithofacies classification and characterization 5.3.1. Lithofacies types In this study, the shale lithofacies are classified and characterized on the basis of TOC content, mineral composition, texture, and fabric. Each lithofacies corresponds to specified sedimentary process and environment. Shown following are the identified five lithofacies and their characteristics. Organic-rich siliceous shale: The quartz content is more than 50%, compared to clay mineral and carbonate. The texture is characterized by more than 75% micro particles (diameter lower than 0.0039 mm) and less than 25% silty particles (diameter ranging between 0.0039 and 0.0625 mm). Organic material commonly appears in the form of aggregates and strips, with the TOC content more than 10% averagely. Burrows and fossil traces are extremely scarce, indicating rare-to -no bioturbation. Weak-to -well laminations are sometimes presented resulting from the interbedded quartz and organic matter laminae. This lithofacies is generally
interpreted to develop in anoxic deep water and low energy situation (Fig. 8AeC, Fig. 9AeD). Silty siliceous shale: Quartz is dominant in the mineral composition, with the content more than 50%. Silty grains content is above 25% revealed by micrographs. Organic material with the content lower than 5%, commonly appears in the form of aggregates or disseminated particles. All mineral grains mix homogeneously, exhibiting non-to -weak lamination structure. A relative low energy and shallow water environment is indicated by this lithofacies (Fig. 8DeE, Fig. 9EeH). Calcareous siliceous shale: The mineral composition in this lithofacies contains more than 20% carbonate and more than 50% quartz. The carbonate minerals occur as matrix and/or natural fracture fillings. Organic matter commonly appears in the form of aggregates and strips, with the TOC content ranging from 2% to 5%. Microphotographs reveal the diagenetic alteration of this lithofacies in the form of strong recrystallization and replacement by microcrystalline dolomite. A relative low energy and shallow water environment is suggested by this lithofacies (Fig. 10AeB). Argillaceous shale: This lithofacies is characterized by more than 50% clay mineral content and lower than 25% silty sized particles. Biological fossils, such as radiolarian and sponge spicule are abundant. Organic material with the content between 2% and 3%, mixes homogenously with clay mineral to form aggregates or strips. Micro-quartz is orderly ranged to form silica laminae. This lithofacies may be associated with low energy and deep water conditions (Fig. 8F, Fig. 10CeF). Silty mixed shale: Samples are classified into this lithofacies when quartz and clay mineral content are both less than 50%. The silty grain content is generally more than 25%. TOC content is less than 2%. Laminated structure can be observed by the alternation of black and bright layer. The black layer is composed of organic matter and clay mineral, and the bright layer is aggregated by quartz, feldspar and mica. This lithofacies is interpreted to develop in high energy and shallow water environment (Fig. 8GeH, Fig. 10GeH). 5.3.2. Lithofacies associations Four lithofacies associations (LA1~4) have been identified based on lithofacies stacking patterns that reflect correlative sedimentary environments (Fig. 11). Each association represents a lithofacies succession organized within one regressive cycle. LA1 includes two lithofacies, which are deposited in anoxic-to -suboxic, low-to -moderate energy, and hot water prevailed environment. Organic-rich siliceous shale was developed at the bottom of LA1 after an extensive marine transgression at the end of Meishucun stage. The succeeded silty siliceous shale was then deposited in a relative high energy environment as a result of sea level fall. These two lithofacies are organized in a shallowingupward regressive cycle with more terrigenous input upwards. LA1 indicates siliceous basinal facies. LA2 comprises of three lithofacies, which are deposited in anoxic-to -oxic, and low-to -high energy environment, recording a shallowing-upward cycle. Organic-rich siliceous shale was developed at the bottom after an extensive marine transgression at the early of Qiongzhusi stage. With the water shallowing upwards, argillaceous shale was then presented in the middle of the cycle, and argillaceous siltstone was finally developed on the top. LA2 suggests argillaceous basinal facies. LA3 is composed of three types of lithofacies, which are related to anoxic-to -suboxic, low-to -moderate energy, and hot water predominated environment. Organic-rich siliceous shale at the
Fig. 6. Vertical variations in the content of Mo and the ratios of V/Cr, V/(V þ Ni) and Ni/Co at NG outcrop.
Fig. 7. The cross-plots between TOC content and Mo content showing the domains of tested data for the Niutitang shale at NG outcrop. The dashed lines represent modern anoxic marine systems. This figure is modified from Algeo and Lyons (2006).
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Fig. 8. Outcrop photographs for different lithofacies. (A) Organic-rich siliceous shale; (B) Organic-rich siliceous shale containing phosphatic nodules; (C) Thin layer organicrich siliceous shale; (D) Thick layer silty siliceous shale; (E) Silty siliceous shale with dissolved pores and microfractures; (F) Argillaceous shale showing spheroidal weathering; (G) Silty mixed shale and argillaceous siltstone interbedding; (H) Silty mixed shale with silt laminae. Photographs of A, C, D, G are from NG outcrop, and B,E, F, H are from LBZ outcrop.
bottom recorded an extensive marine transgression at the end of Meishucun stage. With the sea level fall, subsequent deposition was the alternated silty siliceous shale and calcareous siliceous shale. The depositional environment for this association follows an upward shallowing trend. The total proportions of silty grains and carbonate minerals in the succession increase upwards. LA3 represents lower slope facies. LA4 includes three lithofacies, which are deposited in anoxic-to -oxic, and low-to -high energy environments. Organic-rich siliceous shale at the base was developed in the anoxic and low energy environment after a large marine transgression at the early of Qiongzhusi stage. The following deposited silty mixed shale and argillaceous siltstone indicate that the depositional setting turned into high energy and shallow water environment. The vertical
Fig. 9. Microphotographs for different lithofacies. A ~ D are for organic-rich siliceous shale. Micro silica grains are concentrated to form patchness in Pictures A and B, or well oriented to form thin laminae in Picture D. E ~ H are for silty siliceous shale. Silt sized grains distribute randomly, not forming obvious laminae.
stacking pattern of the lithofacies in this sequence collectively reflects a shallowing-upward trend. LA4 suggests upper slope or shelf margin facies. 5.4. Shale cyclicity and sedimentary sequence The lithofacies association analysis indicates the Niutitang Formation generally exhibits a shallowing-upward prograding depositional sequence. On the basis of the vertical variations of TOC content, mineral composition, redox conditions and lithofacies stacking pattern, the Niutitang shale can be divided into two large cycles (named cycle A and B), which are corresponded to the two large marine transgressions at the end of Meishucun stage and at the early of Qiongzhusi stage, respectively (Mei et al., 2006a, b; Lin et al., 2014; Yang et al., 2016). Additionally, cycle A can be further divided into two sub-cycles (named cycle A1 and A2) (Fig. 12, Table 4).
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high TOC and quartz contents. Cycle A2: This cycle lies above the cycle A1, and is totally or partly absent in the two studied outcrops. The depositional successions in this cycle may resemble those in cycle A1. Cycle B: This cycle is located at the upper member of the Niutitang Formation, and results from an extensive marine transgression at the early of Qiongzhusi stage in early Cambrian. For the NG outcrop in basinal facies, the depositional sequence in this cycle is composed of LA2, with a thickness of 53 m. The TOC content ranges from 2.23% to 11.2%, with an average of 6.3%. The average content of quartz is about 49.5% (varying from 36% to 65.4%), and the clay mineral content ranges between 25.4% and 59.6% (43.1% on average). For the LBZ outcrop in slope facies, this cycle is composed of LA4, with a thickness more than 65 m. The TOC content ranges from 0.88% to 3.02%, with an average of 1.5%. The average content of quartz is about 51.2% (varying from 45.1% to 59.8%), and the clay mineral content ranges between 28.9% and 43.6% (37.3% on average). The bottom section in this cycle may hold shale gas potential because of the high TOC and quartz contents. From Cycle A to Cycle B, the sedimentary facies evolved from siliceous basin to argillaceous basin at NG outcrop, and from lower slope to upper slope at LBZ outcrop. The organic matter content and anoxic degree decrease largely because of the changes of sea level and their corresponding depositional environments. For the mineral composition, the average content of quartz deceases, but the average content of clay mineral increases. Compared each shale cycle at NG outcrop and LBZ outcrop, the organic matter and clay mineral contents show an obviously increasing trend, but quartz content shows a decreasing trend from slope to basinal facies. More terrigenous input and stronger dilution in the slope environment but more suspension clay sediments in the basinal environment may account for this changing trend.
6. Conclusion
Fig. 10. Microphotographs for different lithofacies. A ~ B are for calcareous siliceous shale. It shows that dolomite replaces siliceous fossils in Picture A, and calcite fills in nature fractures in Picture B. C ~ F are for argillaceous shale, rich in bio-fossils. Silica laminae are obvious. G ~ H are for silty mixed shale. Quartz and mica concentrate in the bright laminae, and clay mineral and organic matter concentrate in the black laminae in Picture H.
Cycle A1: This cycle is located at the bottom of the Niutitang Formation, which was developed after a rapid sea level rise at the end of Meishucun stage in early Cambrian. For the NG outcrop in basinal facies, this cycle is made up of LA1, with a thickness of 32 m. The TOC content ranges from 1.08% to 15.4%, with an average of 7.3%. The quartz content ranges from 51% to 63.8% (56.2% on average), and from 21.8 to 37.4% (28.3% on average) for clay mineral content. For the LBZ outcrop in slope facies, this cycle is made up of LA3, with a thickness over 40 m. The TOC content ranges from 4.06% to 9.23%, with an average of 7%. The quartz content ranges from 51.3% to 94.8% (72.1% on average), and from 0% to 31.6% for clay mineral (10.4% on average). Cycle A1 is considered as the most shale gas potential interval because of the
(1) For the lower Cambrian Niutitang shale, the TOC content is positively correlated to quartz content, but negatively correlated to clay mineral content. Anoxic to dysoxic conditions, and moderate to weak degree of restriction are the major depositional conditions during the Niutitang shale deposition period in Early Cambrian. (2) On the basis of TOC content, mineral composition, texture and fabric, five lithofacies are identified: Organic-rich siliceous shale, silty siliceous shale, calcareous siliceous shale, argillaceous shale, and silty mixed shale. Additionally, four lithofacies associations are interpreted to represent four different sedimentary facies, including siliceous basinal facies, argillaceous basinal facies, lower slope facies, and shelf margin facies. (3) The lower Cambrian Niutitang Formation generally shows a shallowing-upward prograding depositional sequence and consists of two large shale cycles. From bottom to top, the organic matter content and anoxic degree decrease largely with the sea level fall. The average content of quartz deceases, but the average content of clay mineral increases. Stratigraphic correlation for this shale reveals an obviously decreasing trend of quartz content, but an increasing trend of organic matter and clay mineral contents from slope to basinal facies.
Fig. 11. Four types of lithofacies associations. Each lithofacies association shows a shallowing-upward stacking pattern and indicates a certain depositional setting.
Fig. 12. Vertical evolution patterns of lithofacies, geochemistry and mineralogy, and stratigraphic correlation for the lower Cambrian Niutitang shale. (a) Is for NG outcrop in basinal facies, and (b) is for LBZ outcrop in slope facies. See Fig. 1 for the locations of the two outcrops.
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Table 4 Statistic average contents of TOC, quartz and clay mineral. Outcrop
Cycle
Thickness (m)
TOC (%)
Quartz (%)
Clay mineral (%)
Nangao (NG)
A1 A2 B
32 33 53
7.3 / 6.3
56.2 / 49.5
28.3 / 43.1
Longbizui (LBZ)
A1 A2 B
>40 <50 >65
7 / 1.5
72.1 / 51.2
10.4 / 37.3
5.5
57.3
29.8
Average
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