A comparative discussion of the evidence for biogenic silica in Wufeng-Longmaxi siliceous shale reservoir in the Sichuan basin, China

Marine and Petroleum Geology 109 (2019) 70–87

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Research paper

A comparative discussion of the evidence for biogenic silica in WufengLongmaxi siliceous shale reservoir in the Sichuan basin, China

T

Guoheng Liua,b,1, Gangyi Zhaia,∗∗,1, Caineng Zouc,1, Lijuan Chengd,∗,1, Xiaobo Guoe, Xianghua Xiaa, Dishi Shia, Yuru Yanga,b, Cong Zhanga,b, Zhi Zhoua a

Oil and Gas Survey, China Geological Survey, Beijing 100083, China Key Laboratory of Unconventional Oil and Gas Geology, China Geological Survey, Beijing 100029, China c Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China d Accumulation and Development of Unconventional Oil and Gas, State Key Laboratory Cultivation Base Jointly-constructed by Heilongjiang Province and Ministry of Science and Technology, Northeast Petroleum University, Daqing 163318, China e School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an 710065, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Biogenic silica Airborne volcanic ash-derived silica Lucaogou formation Wufeng-Longmaxi siliceous shale

The siliceous Wufeng-Longmaxi Formation is the main shale gas play in China. It contains a large amount of silica, which has been reported to be of biogenic origin. The previous evidences for a biogenic origin include the presence of siliceous skeletal debris resembling organisms, such as sponge spicules, various parameters, such as the ratios of Al/(Al + Fe + Mn) and Si/(Si + Fe + Al + Ca), and a number of different correlations, such as a positive relationship of silica content with total organic matter (TOC). However, these evidences, especially various parameters and correlations, become unpersuasive when comparing Wufeng-Longmaxi Formation siliceous shale with other shale in these parameters and correlations. A series of petrographic thin section and scanning electron microscope (SEM) observations, elemental analysis, TOC analysis and mineral composition analysis were conducted to verify the unpersuasiveness of the existing evidences, and then propose new evidences to demonstrate the abundance of biogenic silica in Wufeng-Longmaxi siliceous shale. It is shown that siliceous ellipsoids are a better mark of biogenic silica in Wufeng-Longmaxi siliceous shale than sponge spicule, radiolarian and foraminifera. Except for the Barium (Ba) concentration, most elemental composition data cannot be used to distinguish biogenic silica from silica with an airborne volcanic ash origin. Furthermore, the poor crystal morphology of the silica in Wufeng-Longmaxi siliceous shale was a clear difference from the airborne volcanic ash-derived silica in Lucaogou Formation and hydrothermal silica in Nutitang and Doushantuo formations. Hence, the crystal morphology can be considered as a distinguishing feature for biogenic silica.

1. Introduction Industrial exploitation of shale gas in China has started following the commercial development of shale gas in the USA. In China, the commercial developed shale gas plays, including Fuling-Jiaoshiba, Weiyuan, Changning and Zhaotong, are mainly located in Sichuan basin, which is estimated to hold approximately 17.73 × 1012 m3 of technically exploitable shale gas resources (Jiang et al., 2017). Shale gas production in China increased rapidly from 25 × 106 m3 in 2012 to greater than 90 × 108 m3 in 2017, and will reach 300 × 108 m3 by the end of 2020 (Ma et al., 2018). It was frequently reported that the Wufeng-Longmaxi Formation (WL Fm.) is the main production interval

of shale gas in China (Wu et al., 2016; Wu et al., 2017a,b,c; Wu et al., 2017a,b,c; Zhang et al., 2018a,b,c). For instance in the Fuling-Jiaoshiba area, approximately 50 × 108 m3 gas was produced from the WL Fm. in 2016 (Guo, 2016). The Jiaoye 1 well alone has already produced 1.0 × 108 m3 shale gas from the WL Fm. (He et al., 2017). In fact, gas are especially concentrated in certain parts of the WL Fm., and not the entire shale, because of the heterogeneity of the finegrained sediments (Zhang et al., 2018a,b,c). High shale gas production mainly appears in the organic-rich siliceous shale layers, which are mainly developed in the upper part of the Ordovician Wufeng Formation and the lower part of the Silurian Longmaxi Formation shale (Wang et al., 2017; Zhao et al., 2017a,b,; Hu et al., 2018a,b; Zhang

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Zhai), [email protected] (L. Cheng). 1 means equal contributions. ∗∗

https://doi.org/10.1016/j.marpetgeo.2019.06.016 Received 9 February 2019; Received in revised form 6 June 2019; Accepted 8 June 2019 Available online 10 June 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. A) Location of the Sichuan Basin and Malang Sag in Santanghu basin, B) the study area and C, D) tectonic geological map with well location.

quartz, silica or excess silica (Siexcess), was inferred to indicate biogenic origin, since there was no obvious correlation between TOC and detrital quartz (Rowe et al., 2008; Wang et al., 2014a,b; Zhao et al., 2016a,b; He et al., 2017; Liu et al., 2017; Yan et al., 2018). Compared with shale deposited in a normal detrital sedimentary environment, such as the Triassic Yanchang Formation shale in the Ordos basin, the WL Fm. siliceous shale displays much higher silica content and larger Si/Al ratios, which have been regarded as additional evidence of biogenic silica (Rowe et al., 2008; Wang et al., 2014a,b; Huang et al., 2017; Liu et al., 2017; Zhao et al., 2017a,b; Yang et al., 2018; Zhang et al., 2018a,b,c). The Si/Al ratios of siliceous shale layers in WL Fm. range from 5.16 to 12.37, while those of argillaceous shale vary from 3.39 to 3.99 (Yang et al., 2018). Furthermore, Si/(Al + Fe + Si) or Si/ (Al + Fe + Si + Ca) ratios, and Al/(Al + Fe + Mn) ratio or an Al-FeMn triangle plot, were also used to prove the biogenic origin of silica in WL Fm. siliceous shale (Wang et al., 2014a,b; Yang et al., 2018; Zhang et al., 2018a,b,c). The Al/(Al + Fe + Mn) ratio for hydrothermal silica are lower than 0.35, while that for biogenic silica is larger than 0.6 and the Si/(Al + Fe + Si) ratio for biogenic silica is commonly larger than 0.9 (Boström, 1973; Adachi et al., 1986; Yamamoto, 1987; Liu et al., 2017). Based on the cutoff values for these parameters, silica in WL Fm. siliceous shale is biogenic in origin (Wang et al., 2014a,b; Yang et al., 2018; Zhang et al., 2018a,b,c). The presence of biogenic barite (Babio), connected with phytoplankton decay, is further evidence, which no relationship between Babio and quartz, or very low Babio values,

et al., 2019). A lot of researches has been conducted on siliceous shale layers enriched in organic matter, especially on the origin and source of silica. It was reported that the authigenic quartz in siliceous shale of the WL Fm. reached up to 60% of the whole minerals, and a large percentage of this authigenic quartz was biogenic in origin (Wang et al., 2014a,b; Zhao et al., 2016a,b; Liu et al., 2017; Zhao et al., 2017a,b; Botting et al., 2018; Lu et al., 2018; Yang et al., 2018; Zhang et al., 2018a,b,c; Zhang et al., 2019). Yang et al. (2018) conducted a quantitative research and proposed that the percentage of biogenic silica could be up to 56% of the whole minerals. The evidences for biogenic silica above mentioned in the literature falls into two categories. The first category has been used in all of the previous work mentioned above and involves the use of micropalaeontology, which identifies siliceous debris and material with a biological structure observed through petrographic thin section and scanning electron microscope (SEM) with energy dispersive X-ray spectrometry (EDS). The debris and biological structures were proven to be composed of silica by EDS, and Zhao et al. (2017a,b) proved that the silica was authigenic origin rather than detrital through cathode luminescence (CL) response characteristics. Their biological structures suggest that these are the remains of siliceous organisms, such as radiolarians, siliceous sponges or foraminifera. The second category includes various parameters and a number of correlations involving organic matter, minerals, and elements. In many studies, the positive correlation of total organic carbon (TOC) with 71

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in the upper part of Longmaxi formation have low TOC content, and the silica in LM5-LM9 is derived from the terrigenous clastic input (Yan et al., 2018). A sharp fall in sea level happened in the latest Ordovician (late Katian to the Hirnantian) and continued into the earliest Silurian (Miller et al., 2005; Haq and Schutter, 2008). During the sharp sea level fall, a mass extinction occurred. However, sponges flourished in the near-shore benthic zone across South China after the biological extinction (Botting et al., 2018).

indicates that the majority of the silica is non-biogenic, and vice versa (Dymond et al., 1992; Zhao et al., 2016a,b; Yang et al., 2018). The positive correlation of Babio with quartz means most of the silica is biogenic in WL Fm. siliceous shale (Yang et al., 2018). The correlation between zirconium (Zr) and silica has also been used to differentiate biogenic silica from detrital silica (Wright et al., 2010; Dong et al., 2017). The negative correlation between Zr and silica indicates a biogenic origin for silica in WL Fm. siliceous shale (Yang et al., 2018). Finally, there are other parameters and correlations that play an instrumental or supplementary role in proving the biogenic origin of silica in WL Fm. siliceous shale, mainly from the perspective of demonstrating that the silica is not terrestrial. For example, when the content of Al or Al2O3 shows no correlation with that of Si or SiO2, as well as a negative correlation with Siexcess, this suggests that most of the silica is not detrital (Wang et al., 2014a,b; Zhao et al., 2016a,b; Zhang et al., 2018a,b,c). All these evidences are unconvincing in supporting the presence of biogenic silica in WL Fm. When the data are compared with the same data for shale affected by volcanism, such as the Permian Lucaogou Formation tuffaceous shale in Santanghu basin. This is because the data and derived parameters used previously do not accomplish discrimination between biogenic silica in WL Fm. and volcanically derived silica in the Lucaogou Formation shale. Using these two case studies, this work proposes means to distinguish between biogenic silica and volcanogenic silica, and presents new evidence to more strongly support the predominance of biogenic silica in the WL Fm.

3. Samples, experiments and data sources 3.1. Samples The samples used in this study were taken from 12 Wells and 2 outcrops (Fig. 1C). The weathered surface of outcrops samples were removed by grinding, and the core rock samples were cleaned by washing away the soil and dust contaminated during sample collection and storage. Samples of the WL Fm. shale come from wells ZN-1, SD-1, SY-1, JSB-1 and outcrops of YDH-1 and MQH-1. Samples of Lucaogou formation shale are from wells Gh45, Ghl6, Lgh3, Lh6, Ma720 and Ml2. Samples from the lower Permian Qixia formation of TD-1 well are also used to figure out the difference between biogenic silica in the WL Fm. and that in the Qixia formation, which has been commonly recognised as being biogenic (Yang et al., 2014; Cheng et al., 2015; Jianatiguli et al., 2017). Samples of Niutitang formation and Doushantuo formation shale from wells of YY-1 and outcrops of YDH-1 and MQH-1 are also applied to demonstrate the difference between biogenic silica and hydrothermal silica, which has been proved to be the main components to the silica in Niutitang formation and Doushantuo formation shale (Yin et al., 2017; Gao et al., 2018; Zhang et al., 2019).

2. Geological setting The Santanghu basin is situated in the northwest part of China (Fig. 1A), and is classified into three tectonic blocks: the northeast uplift zone, the central depression zone, and the southern thrust belt (Fig. 1B). The Malang sag is located in the southeast part of the central depression zone (Fig. 1D). The sediments in Santanghu Basin range in age from the Carboniferous to Quaternary. Lucaogou and Tiaohu Formations are Permian strata developed in the Santanghu basin, with great hydrocarbon generation and accumulation potential (Wang et al., 2014a,b). A set of lacustrine sediments enriched in volcanic materials released from volcanic activities induced by the Indosinian movement was developed in the Lucaogou Formation (Meng et al., 2014; Wu et al., 2017a,b,c; Zhang et al., 2018a,b,c). The Lucaogou Formation can be subdivided into three members, and the second member, comprising the principal source rock units, mainly consists of two rock types (Fig. 2). The first type is chiefly composed of shale, mudstones and tuffaceous shale, which has high TOC and silica content, high porosity and oil saturation, and better kerogen Type for oil generation. The second type is chiefly composed of limestone, dolomite and argillaceous dolomite, with low TOC and silica content, low porosity and oil saturation (Hu et al., 2018a,b; Liu et al., 2018). The Sichuan basin is one of the most stable sedimentary basins in the upper Yangtze Craton. It extends from the northeastern Qinling orogenic belt and northwestern Longmenshan orogenic belt (Fig. 1C) to the southwestern Emei-Liang Mountain and southeastern Hubei-Hunane-Guizhou fold belt (Dai et al., 2014; Jin et al., 2018). The Sichuan basin and surrounding areas have experienced a series of complicated tectonic movements and most of the Carboniferous strata were eroded. The well-known WL Fm. is subdivided into the WF1–WF4 and LM1–LM9 members according to graptolite identification (Fig. 2) (Yan et al., 2018; Han et al., 2019). The high quality source rock in the WL Fm. shale in the Sichuan Basin and surrounding areas can be classified into two intervals, which are the graptolite shale interval of WF2-WF3 at the upper part of the Wufeng formation and the graptolite shale interval of LM1-LM4 at the bottom of the Longmaxi formation (Jin et al., 2018). Both the two intervals exhibit high contents of TOC and biogenic silica. However, the calcareous shale interval of WF4 in the top part of the Wufeng formation and the argillaceous shale interval of LM5-LM9

3.2. Experiments Rock slice identification and SEM observation was conducted using a Leica microscope with a CRAIC Microscope photometer and FEI Quanta-200F apparatus with an energy-dispersive spectrometer (EDS) to ascertain the optical property and crystalline morphology of silica in samples. Both samples with fresh planes of fracture and samples with Ar-ion polished planes are used in SEM observation. The crystalline morphology of silica can be more integrally observed in samples with fresh planes of fracture and can be used to illustrate these silica is authigenic minerals. Samples with Ar-ion polished planes were more suitable in observing the contact features between silica and organic matter. Both X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis were conducted to obtain the mineral and elemental composition, respectively. The mineral analysis was conducted using a Rigaku automated powder diffractometer (D/MAX-RA, Japan Rigaku Corporation) with a Cu X-ray source (40 kV, 35 mA). The elemental compositions were measured by a Thermo Scientific Niton XL portable energy-dispersive X-ray fluorescence (ED-XRF) analyser, the parameters and functions of which have been described in detailed in Zhao et al. (2017a,b) and Yang et al. (2018). A total of 68 samples from Lucaogou formation shale were submitted to XRD analysis to obtain mineral compositions. In the context of the overall lacustrine mixed dolomiticclastic sediment system of the Lucaogou Formation, 57 samples were classified as tuffaceous shale, and 11 samples as argillaceous dolomite. 16 samples of the Lucaogou formation tuffaceous shale were also submitted to XRF analysis for elemental composition. A total of 18 samples from the Lucaogou formation were analysed for Zirconium (Zr) at the Geological Experimental department of the China National Nuclear Corporation (CNNC). Nine samples were from the tuffaceous shale, and nine samples from the argillaceous dolomite. The Zr element contents were measured using a laser-ablation microprobe associated with an inductively coupled plasma mass spectrometer (LA-ICP-MS) following the criteria of JY/T 015-1996 (Zhang 72

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Fig. 2. The comprehensive stratigraphic column of WL Fm., Lucaogou formation, Niutitang formation and Doushantuo formation shale of Well M720, SY-1 and YY-1.

et al., 2018a,b,c). A total of 50 samples from the tuffaceous shale analysed by XRD were also submitted for TOC measurements using a Leco TOC (CS230HC) instrument. Before the experiments, the sample mass was measured and then the carbonate minerals in these samples were eliminated by immersing in 10% hydrogen chloride (HCl) solution and totally dried in a drying oven at 65 °C for 1.5 days.

collected data was presented in Table 1. All these data points corresponding to WL Fm. shale are from the high quality siliceous shale source rock interval of WF2-WF3 and LM1-LM4.

3.3. Published data sources

The Lucaogou formation tuffaceous shale is chiefly composed of quartz in the range of 16%∼73% (average: 34%) by weight (Table 2). The contents of clay minerals, plagioclase and dolomite are very similar to each other, namely 16%, 16% and 17% on average, respectively

4. Results 4.1. Mineral composition and silica content

Apart from the data points acquired from our experiments, a lot of data points were collected from references. All the information of the Table 1 Summary sheet of data points collected from references. Comparative item

Data points

Samples origin

References

Si-TOC relationship

15 22 52 19 16 32 13 19 43

Shuanghe outcrops Barnett shale Jiaoye 1 well JY2, DY1 and YY1 Wells Changning outcrops Barnett shale Well A JY2, DY1 and YY1 Wells Shuanghe outcrops Gesala outcrops Jiaoye 1 Well Wangjiawan outcrop JY2, DY1 and YY1 Wells Well Imperial Komie D-069-K/094-O-02 Wangjiawan outcrop Yangjiawan outcrop Hehuazhen outcrop Lu 1 well

Wang et al. (2014a),b

Si-Al relationship

Al, Fe and Mn elemental data

biogenic Ba (Babio)-quartz relationship Zirconium (Zr) - silica relationship

7 19 80 18

25

73

He et al. (2017) Yan et al. (2018) Wang et al. (2014a),b Liu et al. (2018) Zhao et al. (2016a),b Yan et al. (2018) Wang et al. (2014a),b Zhang et al. (2018a),b,c Zhang et al. (2018a),b,c Yang et al. (2018) Yan et al. (2018) Dong et al. (2017) Yang et al. (2018)

Zhang et al. (2018a),b,c

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Table 2 The mineral composition, TOC content and Zr element concentration of tuffaceous shale and argillaceous dolomite in Lucaogou formation. Well name

Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Gh45 Ghl6 Lgh3 Lgh3 Lgh3 Lgh3 Lgh3 Lgh3 Lh6 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ma720 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ml2 Ghl6 Lgh3 Lgh3 Lgh3 Lgh3 Ma720 Ma720 Ma720 Ml2 Lgh3 Ma720 Ma720 Ma720 Ml2 Ml2 Ml2 Ml2 Ml2 Ghl6* Ghl6* Ghl6* Ghl6*

Lithology

TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS AD AD AD AD AD AD AD AD AD TS TS TS TS

Depth (m)

2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2260 2262 3070.41 2591.61 2592.21 2598.81 2600.21 2601.81 2602.81 3057.65 2794.71 2816.91 2817.81 2868.01 2869.01 2870.01 2871.01 2873.01 2874.01 2875.21 2878.01 3323 3343 3344 3345 3346 3347 3348 3349.75 3354.89 3355.13 3376.49 3376.87 3380.02 3397 3405 3461 3477 3070.45 2598.83 2601.84 2601.89 2602.85 2794.76 2817.88 2869.05 3397.1 2602.85 2794.75 2817.85 2869.05 3346.05 3346.09 3354.85 3354.81 3397.15 3112.16–3112.34 3216.91–3217.13 3194.83–3195.01 3070.47–3070.57

Mineral composition (wt. %) Clay minerals

Quartz

K-feldspar

Plagioclase

Calcite

Dolomite

Pyrite

Analcime

Siderite

Glauberite

28 38 20 15 18 16 17 19 18 28 17 15 18 18 11 22 26 26 34 16 11 42 18 15 18 42 12 19 18 18 23 34 19 23 22 22 12 22 33 34 22 2 32 4 2 16 3 2 30 26 10 11 10 5 5 11 4 12 4 2 5 2 1 2 2 3 3 4 0 4 0 0

16 16 22 24 28 29 33 28 30 26 26 27 28 25 40 33 36 41 27 46 47 33 44 25 54 31 42 37 39 36 38 29 30 28 23 21 25 24 22 23 21 23 21 22 20 34 24 24 21 20 55 50 65 45 40 60 50 42 45 36 22 30 38 41 25 39 28 35 35 38 42 50

12 14 14 13 25 0 0 0 0 0 0 15 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 11 0 5 0 5 0 5 0 4 0 0 12 5 11 13 11 6 9 15 4 3 0 11 5 5 19 1 1 2 4 3 3 3 14 4 2 7 7 6 11

20 21 19 17 0 20 21 23 22 25 22 16 23 21 22 19 26 15 26 10 13 15 21 22 17 18 15 25 23 26 26 18 29 17 22 12 17 32 10 18 13 30 11 19 18 34 15 19 16 14 4 4 3 0 16 6 22 7 1 2 4 4 2 2 3 8 2 3 2 0 3 19

5 0 5 5 6 7 5 8 7 5 4 4 5 6 12 9 0 5 0 21 11 3 12 3 4 0 11 5 3 6 4 6 3 3 0 7 0 0 10 5 20 0 0 15 14 0 5 10 0 0 4 4 3 13 10 2 4 4 1 20 12 14 7 5 19 2 3 3 3 2 2 6

9 6 11 11 13 14 11 12 14 4 20 16 20 21 0 6 9 7 4 3 14 2 4 27 4 0 14 11 4 11 5 5 3 27 25 35 38 20 12 17 13 41 33 22 36 0 35 29 25 28 12 22 14 37 16 11 11 13 45 38 51 45 46 44 47 33 55 48 43 42 39 8

0 0 0 0 0 0 0 4 3 6 5 3 3 4 5 11 3 6 6 4 3 5 1 3 3 4 6 3 4 3 4 5 5 2 3 3 3 2 3 3 5 4 3 6 5 5 5 5 2 3 0 4 2 0 0 2 3 1 1 1 4 1 1 2 1 1 1 2 2 3 2 6

10 5 9 15 10 14 13 6 6 6 6 4 3 5 0 0 0 0 1 0 0 0 0 5 0 5 0 0 0 0 0 1 0 0 0 0 0 0 1 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 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 2 0 2 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 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 2 3 1 2 2 0 0 0 2 1 0 0 4 3 8 4 6 0

TOC %

Zr (ppm)

2.02 1.11 3.03 3.32 2.38 6.06 8.09 4.78 4.1 6.58 7.9 5.94 4.91 5.75 10.9 5.53 4.46 10.25 8.61 5.93 8.28 5.41 9.07 0.81 8.7 3.03 6.31 5.59 10.26 9.67 8.79 5.22 9.82 6.11 5.03 6.29 6.88 4.83 5.13 4.15 3.61 2.51 3.38 3.9 4.27 6.16 5.41 7.36 2.84 3.19 – – – – – – – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – 100 100 88 125 180 55 133 150 160 137 100 140 120 125 142 140 130 110 182 226 108 81.5

(continued on next page) 74

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Table 2 (continued) Well name

Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6* Ghl6*

Lithology

TS TS TS TS AD AD AD AD AD AD AD AD AD AD AD AD AD AD AD AD AD

Depth (m)

3126.05–3126.24 3040.19–3040.31 3129.70–3129.86 3088.13–3088.25 3054.96–3055.08 3151.53–3151.64 3076.48–3076.59 3046.07–3646.22 3190.75–3190.92 3045.46–3045.57 3201.55–3201.75 3136.31–3136.47 3173.74–3173.90 3141.79–3141.99 3102.86–3102.96 3144.38–3144.57 3122.60–3122.79 3183.15–3183.35 3091.48–3091.59 3123.89–3124.04 3063.62–3063.70

Mineral composition (wt. %) Clay minerals

Quartz

K-feldspar

Plagioclase

Calcite

Dolomite

Pyrite

Analcime

Siderite

Glauberite

0 0 0 0 5 3 3 4 0 2 0 5 0 0 3 0 0 0 0 2 0

51 58 73 31 30 31 32 33 34 35 4 9 11 12 16 17 19 21 25 27 28

14 0 4 23 13 13 0 0 8 3 10 0 3 12 0 4 2 9 9 11 4

12 13 6 13 19 13 23 20 4 17 3 15 12 20 24 11 2 11 19 19 2

0 19 0 0 0 4 0 0 3 0 3 1 0 3 12 1 6 3 23 3 0

7 10 14 19 29 19 38 43 41 43 70 62 69 41 36 60 65 48 13 27 66

3 0 0 14 4 4 4 0 3 0 4 4 2 0 2 2 3 2 1 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

13 0 3 0 0 13 0 0 7 0 6 4 3 12 7 5 3 6 10 11 0

TOC %

Zr (ppm)

– – – – – – – – – – – – – – – – – – – – –

165 146 26.8 194 125 153 83.7 147 172 91 111 79.2 83.1 122 39.8 52.1 130 103 80.8 119 52.2

“TS” means tuffaceous shale. “AD” means argillaceous dolomite. “–” means no test. “*” means data acquired from Zhang et al. (2018a,b,c).

Yangjiawan and Hehuazhen outcrops (Dong et al., 2017; Yang et al., 2018). From XRF analysis (Table 3), the Ba concentration in the Lucaogou formation tuffaceous shale ranges from 0.021% (210 ppm) to 0.047% (470 ppm) with an average of 0.035% (355 ppm), which is much less than Ba concentration in WL Fm. shale mentioned in Yan et al. (2018). The Ti concentration in the Lucaogou formation tuffaceous shale ranges from 0.128% (1280 ppm) to 0.371% (3710 ppm) with an average of 0.275% (275 ppm), which is also much lower than Ti concentration in WL Fm. siliceous shale and argillaceous shale of Wangjiawan, Yangjiawan and Hehuazhen outcrops mentioned in Yang et al. (2018).

(Fig. 3 A). Compared with the WL Fm. shale (Yan et al., 2018), the most surprising feature of the Lucaogou formation tuffaceous shale is its high feldspar content (22%), including K-feldspar (5%) and plagioclase (17%). Dolomite, as the major mineral in the argillaceous dolomite, ranges from 13% to 70% with an average of 45%, followed by quartz (average: 26%) and plagioclase (average: 10%) (Fig. 3 B). Compared with WL Fm. shale (Yan et al., 2018), the content of feldspar is still relatively high in the argillaceous dolomite, namely 15%, including Kfeldspar (5%) and plagioclase (10%). According to XRF analysis (Table 3), the elemental composition of the Lucaogou formation tuffaceous shale is dominated by SiO2 with a range of 64% ∼ 79% (average: 68%), which is much larger than the content of quartz. In fact, the content of SiO2 acquired in XRF analysis was calculated from the Si content, which was directly measured by XRF analysis. Besides quartz, feldspar, the formula of which can be described as (Ca, K or Na)AlSi3O8 in a brief way, also contain a large percentage of Si element.

5. Discussion 5.1. Evidence involving micropalaeontology The first evidence proposed in previous studies for proving the presence of biogenic silica is siliceous debris and lumps with organism features, such as sponge spicule, radiolarian and foraminifera presented by Wang et al. (2014a,b), Han et al. (2015), Zhao et al. (2016a,b), Han et al. (2017), Zhang et al. (2018a,b,c) and Lu et al. (2018). These debris and lumps were composed of silica assemblage identified by EDS. In other words, support for the presence of biogenic silica in previous work is the appearance of silica assemblages with organism-like features. However, these siliceous debris and lumps cannot be regarded as compelling evidence of biogenic silica. Firstly, it was not accurately demonstrated in these studies whether the silica with organism features was formed from the recrystallization of the siliceous skeletons of organisms or the recrystallization of silica released during diagenetic processes. In other words, pores may be formed from the dissolution of calcareous skeletons which are then filled by siliceous materials released during diagenetic processes, such as tuffaceous material alteration or clay mineral transformation. Secondly, Schieber et al. (2000) has demonstrated the in situ precipitation of silica released from the dissolution of opaline skeletons of planktonic organisms, such as diatoms, and showed the several generations of diagenetic quartz growth in algal cysts. From a biological perspective, sponges, radiolaria and foraminifera were not at the bottom of food chain. In fact, planktons such as diatoms in the ocean formed the bottom of the food chain. Generally speaking, the higher the

4.2. Total organic carbon The TOC values of tuffaceous shale samples vary from 0.81 wt% to 10.9 wt% with an average of 5.67%, and the TOC of a majority of samples is encompassed within the scope of 3 wt% ∼7 wt% (Fig. 4). Moreover, samples with TOC content higher than 7 wt% account for 26 percent of the whole samples. The TOC values of the Lucaogou formation tuffaceous shale are greater than those from the Ohio, Barnett, Lewis and WL Fm. shale (Curtis, 2002; Jarvie et al., 2007; Zhao et al., 2017a,b; Yan et al., 2018). 4.3. Element composition According to elemental measurement by LA-ICP-MS (Table 2), including the data in Zhang et al. (2018a,b,c), Zr elemental concentration in tuffaceous shale ranges from 26.8 ppm to 226 ppm (average: 131 ppm), while that in argillaceous dolomite varies from 39.8 ppm to 172 ppm with an average of 111 ppm. According to XRF analysis of the 16 samples (Table 3), Zr element concentration in Lucaogou tuffaceous shale ranges from 0.007% to 0.017% with an average of 0.013%. The average concentration of elemental Zr in the Lucaogou formation tuffaceous shale is much larger than that of Zr in the mudstone from well Imperial Komie D-069-K/094-O-02, but much lower than that of Zr in WL Fm. siliceous shale and argillaceous shale of Wangjiawan, 75

Fig. 3. Mineral composition of Lucaogou formation tuffaceous shale (A) and argillaceous dolomite (B).

position of an organism is in the food chain, the lower its abundance is. Hence, since these silica with organism features formed from the recrystallization of siliceous skeleton of sponges, radiolaria and foraminifera has been found, more silicified diatom or algal cysts should be observed because diatoms in the ocean formed the bottom of the food chain. However, in fact, none of silica assemblage in the form of diatom or algal cysts like that mentioned in Schieber et al. (2000) were found in WL Fm. siliceous shale. Thirdly, another aspect that conflicts with siliceous sponge spicule, radiolarian and foraminifera as evidence for biogenic silica in WL Fm.

76

78.984 70.227 67.922 67.102 65.925 65.881 65.786 65.298 65.129 69.121 65.033 72.804 64.778 64.39 64.217 69.199

SiO2

12.453 4.3 8.239 8.073 9.905 5.539 8.534 9.717 8.878 9.358 6.231 10.75 7.786 8.672 6.451 8.257

Al2O3

1.04 0.687 2.262 2.091 2.373 1.635 2.139 1.787 3.075 1.619 1.574 2.411 1.902 2.929 1.804 1.213

K2O

0.613 0.405 0.485 1.599 0.515 2.873 1.986 0.511 1.231 0.435 4.527 2.88 1.222 0.767 0.97 0.567

CaO

2.575 0.666 0.997 1.567 4.996 1.406 2.114 3.568 2.899 3.628 3.261 3.598 1.742 3.123 0.473 3.077

Fe2O3

Element concentration (%)

36.908 32.816 31.739 31.356 30.806 30.785 30.741 30.513 30.434 30.43 30.389 30.282 30.27 30.089 30.008 35.999

Si

Three decimals were kept in the ratios, such as Si/Al.

3424.6 3413.34 3412.99 3407.96 3418.06 3398.74 3407.98 3425.22 3419.12 3425.15 3398.94 3403.38 3404.17 3419.15 3402.43 3424.65

Depth (m)

6.589 2.275 4.359 4.272 5.241 2.93 4.515 5.142 4.697 4.951 3.297 5.688 4.119 4.588 3.413 4.369

Al 1.801 0.466 0.697 1.096 3.494 0.983 1.479 2.495 2.027 2.537 2.28 2.516 1.218 2.184 0.331 2.152

Fe 0.438 0.29 0.347 1.142 0.368 2.052 1.418 0.365 0.879 0.311 3.233 2.057 0.873 0.548 0.693 0.405

Ca 0.111 0.106 0.108 0.114 0.111 0.117 0.129 0.11 0.113 0.111 0.116 0.128 0.107 0.115 0.105 0.145

Mn 0.193 0.128 0.271 0.241 0.321 0.339 0.262 0.334 0.276 0.312 0.371 0.366 0.297 0.239 0.242 0.217

Ti

Table 3 Concentrations of elements and values of some parameters in the Lucaogou formation shale.

0.035 0.021 0.032 0.032 0.04 0.041 0.032 0.04 0.032 0.042 0.045 0.047 0.034 0.031 0.031 0.034

Ba 0.007 0.009 0.013 0.014 0.016 0.011 0.015 0.016 0.011 0.013 0.017 0.013 0.014 0.015 0.017 0.012

Zr 16.416 25.741 18.183 18.070 14.506 21.673 16.699 14.521 15.826 15.032 20.135 12.592 17.460 15.820 19.394 16.411

Siexcess

5.601 14.425 7.281 7.340 5.878 10.507 6.809 5.934 6.479 6.146 9.217 5.324 7.349 6.558 8.792 8.240

Si/Al

0.775 0.799 0.844 0.779 0.592 0.727 0.737 0.664 0.687 0.652 0.579 0.683 0.757 0.666 0.887 0.655

Al/(Al + Fe + Mn)

0.815 0.923 0.863 0.854 0.779 0.887 0.837 0.800 0.819 0.803 0.845 0.787 0.850 0.816 0.889 0.847

Si/(Al + Fe + Si)

0.807 0.915 0.855 0.828 0.772 0.838 0.806 0.792 0.800 0.796 0.775 0.747 0.830 0.804 0.871 0.839

Si/(Al + Fe + Si + Ca)

0.029 0.056 0.062 0.056 0.061 0.116 0.058 0.065 0.059 0.063 0.113 0.064 0.072 0.052 0.071 0.050

Ti/Al

0.014 0.007 0.002 0.005 0.005 0.004 0.003 0.003 0.002 0.008 0.004 0.007 0.001 0.005 0.004 0.010

Biogenic Ba

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ranging from tens to hundreds micrometres (Fig. 5 A, B, C). Each ellipsoid consists of a few to several tens of quartz particles with diameters ranging from several micrometres to tens micrometres (Fig. 5 D). The direction of extinction is different among these ellipsoids (Fig. 5 C). Different quartz particles in one ellipsoid also display different extinction direction (Fig. 5 C). The spaces among these ellipsoids are filled with argillaceous and calcareous matrix (Fig. 5 D). Silica particles can be more clearly observed in biogenic siliceous rock of Qixia formation because they are coarse-grained. In fact, a large content of siliceous ellipsoids in the WL Fm. siliceous shale can also be observed through polarizing light microscope (Fig. 6). Most of these siliceous ellipsoids are smaller than, and only a few ellipsoids are as large as, those in biogenic siliceous rock of Qixia formation (Fig. 6A and B). These siliceous ellipsoids fill nearly the entire field of view (Fig. 6B). Each of these ellipsoids in WL Fm. siliceous shale also consists of a few to several tens of quartz particles. The direction of extinction is also different among these ellipsoids and also different for quartz particles within one ellipsoid (Fig. 6C ∼ G). Foraminifera-like pores were also observed, and two periods of recrystallization could be clearly seen (Fig. 6F and G). But it is hard to be sure whether there are real remains of foraminifera or pores formed from dissolution and filled by siliceous materials released during the diagenetic process. Even if they were remains of foraminifera, they were not distributed over a large area. Moreover, they display obvious differences from detrital silica in appearance, which can be observed in shale from LM8 member of Longmaxi formation (Fig. 6H and I). The detrital silica particles are dispersed, and without the phenomenon of multiple particles grouped together (Fig. 6J and K). To sum up, siliceous ellipsoids may be better marks or signs of biogenic silica than sponge spicule, radiolarian and foraminifera in WL Fm. siliceous shale. Hydrothermal silica in Niutitang and Doushantuo formation shale and airborne volcanic ash origin silica in Lucaogou formation tuffaceous shale displays clear differences in morphological and distribution characteristics compared with biogenic silica in the WL Fm. shale. The first conspicuous difference is that silica particles in the Niutitang, Doushantuo and Lucaogou formations show no organism-like features, such as the siliceous ellipsoids in Qixia formation and WL Fm. (Fig. 7).

Fig. 4. Histogram of TOC for Lucaogou formation tuffaceous shale.

siliceous shale is the inconsistency between the high content of quartz or silica and the small total areas of these organisms. The content of quartz or silica, described in previous studies, such as Wang et al. (2014a,b) and He et al. (2017), is generally larger than 50%, and some samples even higher than 70%. But the areas of sponge spicule, radiolarian and foraminifera shown in the figures used in their studies are less than 25 percentage of the area of view. These inconsistencies mean a large percentage of quartz or silica was not from these siliceous sponge spicule, radiolarian and foraminifera. To establish the evidence of biogenic silica from the perspective of micropalaeontology, thin slices of typical biogenic siliceous rock from the Qixia formation and WL Fm. siliceous shale were observed for comparison. In addition, thin slices of hydrothermal siliceous shale from the Niutitang and Doushantuo formations and the Lucaogou formation tuffaceous shale were also observed. Biogenic siliceous rock of Qixia formation contains millions of siliceous ellipsoids with diameters

Fig. 5. Images of petrographic thin section for bio siliceous rock of Qixia formation from TD-1 Well. “-” means plane polarized light. “+” means perpendicular polarized light. The red arrows point to siliceous ellipsoids and quartz particles. The yellow arrows point to argillaceous and calcareous matrix. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 6. Images of petrographic thin section of siliceous shale from the WL Fm. in JSB-1, ZN-1, SY-1 Well. “-” means plane polarized light. “+” means perpendicular polarized light. The red arrows point to siliceous ellipsoids in WL Fm. siliceous shale. The blue arrows point to the foraminifera (A∼G) and detrital quartz (H ∼ I). “P1” means recrystallization of period one, and “P2” means recrystallization of period two. Image F is the higher magnification image of location marked by a blue box in Image E. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Lucaogou formation are non-biogenic in origin.

In other words, no sponge spicule, radiolarian, foraminifera or siliceous ellipsoids can be observed. For the Niutitang formation shale, silica particles distribute like scattered families (Fig. 7A and B). Silica particles in Doushantuo formation shale are also widely dispersed, but sometimes a large area of silica assemblages can be observed (Fig. 7C, D, E and F). However, silica particles in Lucaogou formation tuffaceous shale mainly display in zonal distribution features. Some silica particles are found dispersed among carbonate minerals (Fig. 7G and H), but most silica particles appear with argillaceous belts or bands (Fig. 7G ∼ L). The clear differences in morphological and distribution characteristics indicates that silica in Niutitang, Doushantuo and

5.2. Evidence involving geochemistry and mineralogy Previous studies have suggested that silica is biogenic origin when the content of TOC exhibits a positive correlation with silica (Rowe et al., 2008; Wang et al., 2014a,b; Zhao et al., 2016a,b; He et al., 2017; Liu et al., 2017; Yan et al., 2018). Some researchers used quartz to display the positive correlation with TOC, some based it on the content of SiO2 calculated from XRF, and some used Siexcess calculated through a specific formula in these studies mentioned above. However measured, 78

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Fig. 7. Images of petrographic thin section for Niutitang formation shale, Doushantuo formation shale and Lucaogou formation tuffaceous shale samples from SD-1, YY-1, Lgh3, Lh6, Ml2 wells and YDH-1 outcrops. “-” means plane polarized light. “+” means perpendicular polarized light. The yellow arrows point to the silica particles. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

samples was also from phytoplankton rather than higher plant. Phytoplankton flourished because of natural iron fertilization from material from volcanic activities. However, siliceous organisms, such as diatoms, did not flourish, which was indicated by no sign of biological activity like sponge spicule, radiolarian, foraminifera or siliceous ellipsoids. In other words, for Lucaogou formation tuffaceous shale, siliceous organisms did not play a role in assisting the fixation and sedimentation of Si element. The Lucaogou formation tuffaceous shale samples also display the positive correlation between TOC and silica, and hence the positive correlation is not a sufficient condition but merely a necessary condition for biogenic silica. The Si/Al ratios of WL Fm. siliceous shale are in the range of 5.16–12.37, while those of argillaceous shale range from 3.39 to 3.99 (Yang et al., 2018). However, Table 3 shows that the Si/Al ratios of Lucaogou formation tuffaceous shale vary from 5.324 to 14.425, which are also much higher than those of argillaceous shale and close to those of WL Fm. siliceous shale. Furthermore, in Si-Al chart, the Lucaogou formation tuffaceous shale samples plot in the same area as the WL Fm. siliceous shale samples and Barnett siliceous shale samples (Fig. 9). In conclusion, the Si/Al ratios or Si-Al chart are also not a sufficient condition for biogenic silica and cannot prove that the silica in WL Fm. siliceous shale is biogenic in origin. It is reported that the Al/(Al + Fe + Mn) ratios for biogenic silica are larger than 0.6 and the Si/(Al + Fe + Si) ratios for biogenic silica are commonly larger than 0.9 (Boström, 1973; Adachi et al., 1986; Yamamoto, 1987; Liu et al., 2017). A total of 4 WL Fm. siliceous shale samples from Changning outcrop exhibit Al/(Al + Fe + Mn) values ranging from 0.67 to 0.71, and display Si/(Si + Fe + Al) values varying from 0.89 to 0.93 (Wang et al., 2014a,b). The Al/(Al + Fe + Mn) values of WL Fm. siliceous shale samples from “Well A” in the article range from 0.58 to 0.76 (Zhao et al., 2016a,b). A total of 12 WL Fm. siliceous shale samples from Well JY2 and YZ1 contain Si/ (Si + Fe + Al + Ca) values within the range of 0.73–0.90 (average:

it is the Si element that exhibits a positive correlation with organic matter. For the WL Fm. siliceous shale, liptodetrinite, lamalginite and graptolite are the major components of organic matter, which indicates phytoplankton-derived origin (Luo et al., 2016). Therefore, for marine shale, higher TOC content mean more abundant phytoplankton during geological periods. Phytoplankton, such as algae, would flourish in the following intermittent periods of volcanic activities because material released from volcanic eruptions such as ash can act as a source of natural iron nutrients (Duggen et al., 2007; Jones and Gislason, 2008; Langmann et al., 2010; Lin et al., 2011; Tao et al., 2013,; Luo et al., 2017). For WL Fm. siliceous shale, siliceous phytoplankton, such as diatom and sponges (Dong et al., 2017), flourished, as suggested by the widely distributed siliceous ellipsoids. Minerals with abundant Si were also released by volcanic activities. As a result, it would be reasonable to expect that abundant diatom growth played a role in fixation and subsequent sedimentation of Si into biogenic silica. This research does not set out to prove that the Si in WL Fm. siliceous shale is from volcanic activities. In fact, the aim is to suggest that the positive correlation of TOC with silica cannot be used to prove that the silica in WL Fm. siliceous shale is biogenic in origin, because the same positive correlation also appear in Lucaogou formation tuffaceous shale samples (Fig. 8), which show no sign of biological silica (discussed above and see Fig. 7). The only difference among these positive correlations for Wufeng-Longmaxi siliceous shale, Barnett shale and Lucaogou formation tuffaceous shale lies in slope coefficients, even if the correlation of quartz based on the same way of measurement (XRD) with TOC (Fig. 8A) or the correlation of silica based on the same way of measurement (XRF) with TOC (Fig. 8B). More data about TOC and quartz of Barnett shale can be obtained from Rowe et al. (2008). The kerogen in the Lucaogou formation tuffaceous shale samples contains mainly lamalginite, a maceral which is part of the sapropelinite family (Liu et al., 2018). This suggests that organic matter in these shale 79

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Fig. 8. Plot of TOC versus content of quartz (A), SiO2 and Siexcess (B).

conclusion, the parameters of Al/(Al + Fe + Mn), Si/ (Si + Fe + Al + Ca), as well as Si/(Si + Fe + Al) ratios or Al-Fe-Mn triangle chart are also not a sufficient condition for biogenic silica and cannot accurately prove that the silica in WL Fm. siliceous shale is biogenic origin. The biogenic Ba (Babio) content is another parameter that can be used to demonstrate the biogenic origin of silica in WL Fm. siliceous shale. The biogenic Ba (Babio) content is calculated using a specific formula, which is (Yang et al., 2018):

0.82) (Zhao et al., 2017a,b). But the Al/(Al + Fe + Mn) values of Lucaogou formation tuffaceous shale are also in the range of 0.579–0.887, the Si/(Si + Fe + Al) values in the range of 0.779–0.923, and the Si/(Si + Fe + Al + Ca) values vary from 0.747 to 0.915 (Table 3). The numerical ranges of the three parameters for Lucaogou formation tuffaceous shale and WL Fm. siliceous shale are so close to each other that these parameters cannot be regarded as strong evidence of biogenic silica in WL Fm. siliceous shale, because silica in Lucaogou formation tuffaceous shale came from airborne volcanic ash alteration (Liu et al., 2016). Besides, in Al-Fe-Mn triangle chart, the Lucaogou formation tuffaceous shale samples also distribute in the same region (bio-origin zone) as WL Fm. siliceous shale samples (Fig. 10). In

Babio = Batot − [Titot × (Ba/ TiPAAS )]

Batot , Titot and TiPAAS represent the total Ba content, total Ti content

Fig. 9. Plot of Si versus Al. 80

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Fig. 10. The Al-Fe-Mn triangle diagram showing the distribution of Lucaogou formation tuffaceous shale samples and WL Fm. siliceous shale samples.

Fig. 11. Plot of Babio content versus content of silica or quartz.

shale samples also display a positive correlation between Babio content and silica (Fig. 11). The correlation coefficient (R2 = 0.7068) of the relationship for tuffaceous shale is much larger than that for siliceous shale. But, the Babio content ranges from 0.001% to 0.014% (Table 3), which is much lower than that of siliceous shale (Fig. 11). All in all, the Babio content is a better parameter to clearly distinguish siliceous shale from tuffaceous shale than the positive correlation between Babio and silica.

and the Ti content of Post-Archean average Australian Shale, respectively (Yang et al., 2018). In argillaceous shale, the Babio content accounts for 412ppm-813 ppm, which is much lower than that in siliceous shale (985ppm-1706 ppm). A good positive relationship between Babio and silica means a great contribution of biogenic silica to the total silica in shale. On the contrary, no correlation, as well as much lower Babio content, indicates a non-biogenic origin of silica (Yang et al., 2018). Compared with WL Fm. siliceous shale, Lucaogou formation tuffaceous 81

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Fig. 12. Plot of Zr element versus content of silica or quartz.

correlations of Siexcess with Ti/Al and Al2O3 demonstrate that it is nonterrestrial silica.

The correlation of silica or quartz with Zr can be used to distinguish biogenic silica from detrital silica (Wright et al., 2010), because Zr can represent terrestrial source input correlated with the heavy mineral zircon (Dong et al., 2017). Detrital silica positively correlates with Zr, while biogenic silica negatively correlates with Zr. For Lucaogou formation argillaceous dolomite samples, the quartz content is below about 40% and shows a positive correlation with Zr (Fig. 12). But the correlation turns out to be negative for tuffaceous shale samples, where the quartz content is larger than 40%. For Horn River group shale samples, the turning point appears when the silica content is approximately 57% (Fig. 12) (Dong et al., 2017). For WL Fm. argillaceous shale, the silica content shows a positive correlation with Zr, while the silica content displays a negative correlation with Zr for WL Fm. siliceous shale. It seems that the correlation of silica or quartz with Zr also cannot be used to recognize biogenic silica because it fails to differentiate siliceous shale from tuffaceous shale. Essentially speaking, Zr is mainly from terrestrial source input, but both biogenic silica and silica from airborne volcanic ash alteration do not belong to terrestrial clastic sediments.

5.4. Evidence involving crystallography In order to establish the differences in crystal morphology between biogenic silica, hydrothermal silica and volcanic origin silica, representative SEM images were compared. The quartz crystal morphology is affected by the intensity of diagenesis to some extent. During diagenesis, opal transfers into quartz. Samples at greater depth undergo severer diagenesis and contain quartz with better crystal morphology (Weibel et al., 2010). The intensity of diagenesis can be correlated to organic matter maturity, which can be further reflected by vitrinite reflectance (Ro). The vitrinite reflectance of tuffaceous Lucaogou shale is mainly in the range of 0.7%–0.9%Ro, while that of WL Fm. siliceous shale is known to be much higher (Luo et al., 2016; Liu et al., 2018). It can be observed that silica in tuffaceous Lucaogou shale display distinct and flawless crystal morphology. The crystal planes are smooth and straight, and consist of triangular plane, two kinds of quadrangular planes (parallelogram and regular quadrilateral), hexagonal planes, two kinds of heptagonal planes and octagonal planes (Fig. 15). However, silica in WL shale shows no distinct crystal morphology. The crystal faces are pockmarked with little holes (Fig. 16), which can also be clearly observed in Ar-ion polished samples. The edges of silica particles in samples with Ar-ion polished planes are not straight, which also illustrate the poor crystal morphology. Furthermore, hydrothermal silica displays different crystal morphology from biogenic silica and volcanic silica (Fig. 16). The extent of diagenesis of Lucaogou shale is much lower than that of WL shale. Hence, the poor crystallization for the silica in WL Fm. shale cannot be caused by insufficient diagenesis. The notable distinction in silica crystal morphology is another way to recognize biogenic silica. A∼L: the Lucaogou samples with fresh planes of fracture. “3” means triangular plane. “4” means quadrangular plane. “6” represents hexagonal plane. “7” means heptagonal plane. “8” represents octagonal

5.3. Evidence in instrumental or supplementary role Al and Ti also represent terrestrial source input, like Zr. When the content of Al2O3 shows no correlation with silica content, as well as a negative correlation with Siexcess, it means detrital silica is not the major contribution to silica in shale. Furthermore, detrital silica will also not account for the main component of total silica in shale when Siexcess has little or no correlation with Ti/Al ratio (Wang et al., 2014a,b; Zhao et al., 2016a,b; Zhang et al., 2018a,b,c). Both the tuffaceous Lucaogou shale and the siliceous WL shale display an ambiguous correlation of Siexcess with Ti/Al ratios and a clearly negative correlation of Siexcess with Al2O3 (Figs. 13 and 14). For the same reasons as discussed above, Al and Ti can neither be used to distinguish biogenic silica from silica from airborne volcanic ash alteration. Hence, it is difficult to prove that the silica in WL Fm. siliceous shale is biogenic origin, even if the 82

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Fig. 13. Plot of Al2O3 versus content of Siexcess.

field of thin section, which is keeping with the large content of quartz or silica in WL Fm. (2) Most parameters and correlations involving elemental analysis, such as Al/(Al + Fe + Mn), Si/(Si + Fe + Al + Ca), Zr element and the positive correlation of silica with TOC, can be merely applied to distinguish biogenic silica from detrital silica, but cannot tell biogenic silica from airborne volcanic ash derived silica. Hence, they cannot support the viewpoint that the silica in WL Fm. shale is biogenic in origin. However, the biogenic Ba concentration rather than the positive correlation of Ba with silica can be used to identify biogenic silica and regarded as evidence for biogenic origin. (3) The crystal morphology of biogenic silica in WL Fm. siliceous shale was obviously distinct from that of hydrothermal silica in Niutitang

plane. 6. Conclusion The previous conclusion that the biogenic origin of silica in the WL Fm. siliceous shale was not refuted in our study, but the evidences to support it need to be re-established. (1) Siliceous ellipsoids are a better mark of biogenic silica than sponge spicule, radiolarian and foraminifera in WL Fm. siliceous shale. Firstly, siliceous ellipsoids in the WL Fm. shale are much similar to those in typical biogenic siliceous rock of Qixia formation. Secondly, these siliceous ellipsoids occupy nearly the entire view

Fig. 14. Plot of Ti/Al ratios versus content of Siexcess. 83

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Fig. 15. Images of SEM observation for Lucaogou shale.

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Fig. 16. Images of SEM observation for WL shale, Niutitang and Doushantuo shale. A∼F: the WL samples with fresh planes of fracture, G ∼ I: the WL shale Ar-ion polished samples, J ∼ K: the Niutitang samples with fresh planes of fracture, L: the Doushantuo samples with fresh planes of fracture.

between China and the USA) (Grant No. 2017YFE0106300). We thank Dr. Chang Liu in the China Geological Survey (CGS) for his assistance in X-ray fluorescence (XRF) experiments.

and Doushantuo formations and airborne volcanic ash derived silica in Lucaogou formation. The crystal planes of biogenic silica in WL Fm. shale are defective with little holes, and can be regarded as an identifier of biogenic silica.

References Acknowledgement Adachi, M., Yamamoto, K., Sugisaki, R., 1986. Hydrothermal chert and associated siliceous rocks from the northern Pacific their geological significance as indication of ocean ridge activity. Sediment. Geol. 47 (1–2), 125–148. Boström, K., 1973. Provenance and accumulation rates of opaline silica, Al, Fe, Ti, Mn, Cu, Ni and Co in Pacific pelagic sediments. Chem. Geol. 11, 123–148. Botting, J.P., Muir, L.A., Wang, W., Qie, W., Tan, J., Zhang, L., Zhang, Y., 2018. Spongedominated offshore benthic ecosystems across South China in the aftermath of the end-Ordovician mass extinction. Gondwana Res. 61, 150–171.

This study was supported by the China Postdoctoral Science Foundation (Grant No. 2018M641431), the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2016ZX05034) and the Innovative Special Project of Sino-US Intergovernmental Cooperation in Science and Technology (Carboniferous-Permian shale reservoir evaluation and technology 85

Marine and Petroleum Geology 109 (2019) 70–87

G. Liu, et al.

Wufeng–Longmaxi formations (upper ordovician–lower Silurian) of south eastern chongqing, China: implications for gas shale evaluation. Int. J. Coal Geol. 153, 87–98. Luo, X., Gong, S., Sun, F.J., Wang, Z.H., Qi, J.S., 2017. Effect of volcanic activity on hydrocarbon generation: examples in Songliao, Qinshui, and Bohai Bay basins in China. J. Nat. Gas Sci. Eng. 38, 218–234. Ma, Y., Cai, X., Zhao, P., 2018. China's shale gas exploration and development: understanding and practice. Petrol. Explor. Dev. 45 (4), 561–574 (in Chinese with English abstract). Meng, Y., Zhu, H., Li, X., Wu, C., Hu, A., Zhao, Z., Zhang, L., Xu, C., 2014. Thermodynamic analyses of dolomite dissolution and prediction of the zones of secondary porosity: a case study of the tight tuffaceous dolomite reservoir of the second member, Permian Lucaogou Formation, Santanghu Basin, NW China. Petrol. Explor. Dev. 41 (6), 754–760 (in Chinese with English abstract). Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E., Sugarman, P.J., Cramer, B.S., Christie-Blick, N., Pekar, S.F., 2005. The phanerozoic record of Global sea-level change. Science 310 (5752), 1293–1298. Rowe, H.D., Loucks, R.G., Ruppel, S.C., Rimmer, S.M., 2008. Mississippian Barnett Formation, FortWorth Basin, Texas: bulk geochemical inferences and Mo-TOC constraints on the severity of hydrographic restriction. Chem. Geol. 257, 16–25. Schieber, J., Krinsley, D., Riciputi, L., 2000. Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature 406 (6799), 981–985. Tao, C.Z., Bai, G.P., Liu, J.L., Deng, C., Lu, X.X., Liu, H.W., Wang, D.P., 2013. Mesozoic lithofacies palaeogeography and petroleum prospectivity in North Carnarvon Basin, Australia. J. Palaeogeogr. 2 (1), 81–92. Wang, C., Kuang, L., Gao, G., Cui, W., Kong, Y., Xiang, B., Liu, G., 2014a. Difference in hydrocarbon generation potential of the shaly source rocks in Jimusar Sag, Permian Lucaogou Formation. Acta Sedimentalogica Sinica 32 (2), 385–390 (In Chinese with English abstract). Wang, G., Ju, Y., Huang, C., Long, S., Peng, Y., 2017. Longmaxi-wufeng shale lithofacies identification and 3-D modelling in the northern fuling gas field, Sichuan basin. J. Nat. Gas Sci. Eng. 47, 59–72. Wang, S., Zou, C., Dong, D., Wang, Y., Huang, J., Guo, Z., 2014b. Biogenic silica of organic-rich shale in Sichuan basin and its significance for shale gas. Acta Sci. Nauralium Univ. Pekin. 50 (3), 476–486 (in Chinese with English abstract). Weibel, R., Friis, H., Kazerouni, A.M., Svendsen, J.B., Stokkendal, J., Poulsen, M.L.K., 2010. Development of early diagenetic silica and quartz morphologies - examples from the Siri canyon, Danish north sea. Sediment. Geol. 228, 151–170. Wright, A.M., Spain, D., Ratcliffe, K.T., 2010. In: Application of Inorganic Whole Rock Geochemistry to Shale Resource Plays: Canadian Unconventional Resources and International Petroleum Conference, Calgary, Alberta, Canada. October 19-21, https://doi.org/10.2118/137946-MS. SPE-137946-MS. Wu, H., Hu, W., Tang, Y., Cao, J., Wang, X., Wang, Y., Kang, X., 2017a. The impact of organic fluids on the carbon isotopic compositions of carbonate-rich reservoirs: case study of the Lucaogou Formation in the Jimusaer Sag, Junggar Basin, NW China. Mar. Pet. Geol. 85, 136–150. Wu, J., Liang, F., Lin, W., Wang, H., Bai, W., Ma, C., Sun, S., Zhao, Q., Song, X., Yu, R., 2017b. Characteristics of shale gas reservoirs in Wufeng formation and Longmaxi formation in well WX2, northeast chongqing. Pet. Res. 2, 324–335. Wu, L., Hu, D., Lu, Y., Liu, R., Liu, X., 2016. Advantageous shale lithofacies of Wufeng formation-Longmaxi formation in fuling gas field of Sichuan basin, SW China. Pet. Explor. Dev. 43 (2), 208–217 (in Chinese with English abstract). Wu, W., Shi, X., Liu, J., Li, D., Xie, J., Zhao, S., Ji, C., Hu, Y., Guo, Y., 2017c. Accumulation conditions and exploration potential of Wufeng-Longmaxi formations shale gas in Wuxi area, northeastern Sichuan basin, China. J. Nat. Gas Geosci. 2, 263–271. Yamamoto, K., 1987. Geochemical characteristics and depositional environments of cherts and associated rocks in the Franciscan and Shimanto terrenes. Sediment. Geol. 52, 65–108. Yan, C., Jin, Z., Zhao, J., Du, W., Liu, Q., 2018. Influence of sedimentary environment on organic matter enrichment in shale: a case study of the Wufeng and Longmaxi Formations of the Sichuan Basin, China. Mar. Pet. Geol. 92, 880–894. Yang, R., Li, H., Liu, Y., Lei, C., Lei, Y., Feng, S., 2014. Origin of nodular cherts in limestones in middle Permian Qixia formation, Chaohu, Anhui province. Geoscience 28 (3), 501–511 (in Chinese with English abstract). Yang, X., Yan, D., Wei, X., Zhang, L., Zhang, B., Xu, H., Gong, Y., He, J., 2018. Different formation mechanism of quartz in siliceous and argillaceous shales: a case study of Longmaxi Formation in South China. Mar. Pet. Geol. 94, 89–94. Yin, L., Borjigin, T., Knoll, A.H., Bian, L., Xie, X., Bao, F., Ou, Z., 2017. Sheet-like microfossils from hydrothermally influenced basinal cherts of the lower cambrian (Terreneuvian) Niutitang formation, Guizhou, south China. Palaeoworld 26 (1), 1–11. Zhang, K., Song, Y., Jiang, S., Jiang, Z., Jia, C., Huang, Y., Wen, M., Liu, W., Xie, X., Liu, T., Wang, P., Shan, C., Wu, Y., 2019. Mechanism analysis of organic matter enrichment in different sedimentary backgrounds: a case study of the Lower Cambrian and the Upper Ordovician-Lower Silurian, in Yangtze region. Mar. Pet. Geol. 99, 488–497. Zhang, L., Li, B., Jiang, S., Xiao, D., Lu, S., Zhang, Y., Gong, C., Chen, L., 2018a. Heterogeneity characterization of the lower Silurian Longmaxi marine shale in the Pengshui area, South China. Int. J. Coal Geol. 195, 250–266. Zhang, Q., Wang, J., Yu, Q., Xiao, Y., Zhang, B., Wang, X., Zhao, A., 2018b. The silicon source and sedimentary environment of the lower Silurian Longmaxi formation in Yanyuan basin, western edge of the Yangtze platform. Geol. Rev. 64 (3), 610–622 (in Chinese with English abstract). Zhang, S.H., Liu, C.Y., Liang, H., Wang, J.Q., Bai, J.K., Yang, M.H., Liu, G.H., Huang, H.X., Guan, Y.Z., 2018c. Paleoenvironmental conditions, organic matter accumulation, and unconventional hydrocarbon potential for the Permian Lucaogou Formation organicrich rocks in Santanghu Basin, NW China. Int. J. Coal Geol. 185, 44–60.

Cheng, C., Li, S., Zhao, D., Yan, L., 2015. Geochemical characteristics of the Middle-Upper Permian bedded cherts in the northern margin of the Yangtze block and its response to the evolution of paleogeography and paleo-ocean. Bull. Miner. Petrol. Geochem. 34 (1), 155–166 (In Chinese with English Abstract). Curtis, J.B., 2002. Fractured shale-gas systems. AAPG Bull. 86 (11), 1921–1938. Dai, J., Zou, C., Liao, S., Dong, D., Ni, Y., Huang, J., Wu, W., Gong, D., Huang, S., Hu, G., 2014. Geochemistry of the extremely high thermal maturity Longmaxi shale gas, southern Sichuan Basin. Org. Geochem. 74, 3–12. Dong, T., Harris, N.B., Ayranci, K., Yang, S., 2017. The impact of rock composition on geomechanical properties of a shale formation: middle and Upper Devonian Horn River Group shale, Northeast British Columbia, Canada. AAPG Bull. 101 (2), 177–204. Duggen, S., Croot, P., Schacht, U., Hoffmann, L., 2007. Subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth: evidence from biogeochemical experiments and satellite data. Geophys. Res. Lett. 34 (1), 95–119. Dymond, J., Suess, E., Lyle, M., 1992. Barium in deep-sea sediments: a geochemical proxy for paleoproductivity. Paleoceanography 7, 163–181. Gao, P., He, Z., Li, S., Lash, G.G., Li, B., Huang, B., Yan, D., 2018. Volcanic and hydrothermal activities recorded in phosphate nodules from the Lower Cambrian Niutitang Formation black shales in South China. Palaeogeogr. Palaeocl. 505, 381–397. Guo, T., 2016. Discovery and characteristics of the Fuling shale gas field and its enlightenment and thinking. Front. Earth Sci. (China Univ. Geosci. (Beijing); Peking Univ.) 23 (1), 29–43 (in Chinese with English abstract). Han, C., Han, M., Jiang, Z.X., Han, Z.Z., Li, H., Song, Z.G., Zhong, W.J., Liu, K.X., Wang, C.H., 2019. Source analysis of quartz from the Upper Ordovician and Lower Silurian black shale and its effects on shale gas reservoir in the Southern Sichuan Basin and its periphery, China. Geol. J. 54 (1), 439–449. Han, Y.J., Horsfield, B., Wirth, R., Mahlstedt, N., Bernard, S., 2017. Oil retention and porosity evolution in organic-rich shales. AAPG Bull. 101 (6), 807–827. Han, Y.J., Mahlstedt, N., Horsfield, B., 2015. The Barnett Shale: compositional fractionation associated with intraformational petroleum migration, retention, and expulsion. AAPG Bull. 99 (12), 2173–2202. Haq, B.U., Schutter, S.R., 2008. A chronology of paleozoic sea-level changes. Science 322 (5898), 64–68. He, Z., Hu, Z., Nie, H., Li, S., Xu, J., 2017. Characterization of shale gas enrichment in the Wufeng Formation-Longmaxi Formation in the Sichuan Basin of China and evaluation of its geological construction-transformation evolution sequence. J. Nat. Gas Geosci. 2, 1–10. Hu, H., Hao, F., Guo, X., Dai, F., Lu, Y., Ma, Y., 2018a. Investigation of methane sorption of overmature Wufeng-Longmaxi shale in the Jiaoshiba area, eastern Sichuan basin, China. Mar. Pet. Geol. 91, 251–261. Hu, T., Pang, X.Q., Jiang, S., Wang, Q.F., Zheng, X.W., Ding, X.G., Zhao, Y., Zhu, C.X., Li, H., 2018b. Oil content evaluation of lacustrine organic-rich shale with strong heterogeneity: a case study of the Middle Permian Lucaogou Formation in Jimusaer Sag, Junggar Basin, NW China. Fuel 221, 196–205. Huang, Z., Liu, G., Li, T., Li, Y., Yin, Y., Wang, L., 2017. Characterization and control of mesopore structural heterogeneity for low thermal maturity shale: a case study of Yanchang Formation shale, Ordos basin. Energy Fuels 31, 11569–11586. Jarvie, D.M., Hill, R.J., Ruble, T.E., 2007. Unconventional shale-gas systems: the Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 91 (4), 475–499. Jianatiguli, W., Zhou, Y., Yao, X., Xu, H., Fang, X., 2017. Geochemical characteristics comparison and tectonic background analysis of siliceous rocks from Qixia formation and Gufeng formation of permian in chaohu area, Anhui province. Geoscience 31 (4), 734–745 (in Chinese with English abstract). Jiang, S., Tang, X., Cai, D., Xue, G., He, Z., Long, S., Peng, Y., Gao, B., Xu, Z., Nick, D., 2017. Comparison of marine, transitional, and lacustrine shales: a case study from the Sichuan Basin in China. J. Pet. Sci. Eng. 150, 334–347. Jin, Z., Nie, H., Liu, Q., Zhao, J., Jiang, T., 2018. Source and seal coupling mechanism for shale gas enrichment in upper ordovician Wufeng formation - lower Silurian Longmaxi formation in Sichuan basin and its periphery. Mar. Pet. Geol. 97, 78–93. Jones, M.T., Gislason, S.R., 2008. Rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments. Geochem. Cosmochim. Acta 72, 3661–3680. Langmann, B., Zakšek, K., Hort, M., Duggen, S., 2010. Volcanic ash as fertiliser for the surface ocean. Atmos. Chem. Phys. 10, 3891–3899. Lin, I.I., Hu, C.M., Li, Y.H., Ho, T.Y., Fischer, T.P., Wong, G.T.F., Wu, J.F., Huang, C.W., Chu, D.A., Ko, D.S., Chen, J.P., 2011. Fertilization potential of volcanic dust in the low-nutrient low-chlorophyll western North Pacific subtropical gyre: satellite evidence and laboratory study. Glob. Biogeochem. Cycles 25 (1), 1–12. Liu, G., Huang, Z., Guo, X., Liu, Z., Gao, X., Chen, C., Zhang, C., Wang, X., 2016. The SiO2 occurrence and origin in the shale system of middle permian series Lucaogou Formation in Malang sag, Santanghu basin, Xinjiang. Acta Geol. Sin. 90 (6), 1220–1235 (In Chinese with English abstract). Liu, G., Liu, B., Huang, Z., Chen, Z., Jiang, Z., Guo, X., Li, T., Chen, L., 2018. Hydrocarbon distribution pattern and logging identification in lacustrine fine-grained sedimentary rocks of the Permian Lucaogou formation from the Santanghu basin. Fuel 222, 207–231. Liu, J., Li, Y., Zhang, Y., Liu, S., Cai, Y., 2017. Evidences of biogenic silica of WufengLongmaxi Formation shale in Jiaoshiba area and its geological significance. J. China Univ. Pet. (Ed. Nat. Sci.) 41 (1), 34–41 (in Chinese with English abstract). Lu, L., Qin, J., Shen, B., Tenger, Liu, W., Zhang, Q., 2018. The origin of biogenic silica in siliceous shale from Wufeng-Longmaxi Formation in the Middle and Upper Yangtze region and its relationship with shale gas enrichment. Front. Earth Sci. (China Univ. Geosci. (Beijing); Peking Univ.) 25 (4), 226–236 (in Chinese with English abstract). Luo, Q., Zhong, N., Dai, N., Zhang, W., 2016. Graptolite-derived organic matter in the

86

Marine and Petroleum Geology 109 (2019) 70–87

G. Liu, et al.

and organic matters of the Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin, China: implications for pore systems, diagenetic pathways, and reservoir quality in fine-grained sedimentary rocks. Mar. Pet. Geol. 86, 655–674. Zhao, J., Jin, Z., Jin, Z., Wen, X., Geng, Y., Yan, C., 2016b. The genesis of quartz in Wufeng-Longmaxi gas shales, Sichuan Basin. J. Nat. Gas Geosci. 27 (2), 377–386 (in Chinese with English abstract).

Zhao, J., Jin, Z., Jin, Z., Wen, X., Geng, Y., 2017a. Origin of authigenic quartz in organicrich shales of the Wufeng and Longmaxi formations in the Sichuan basin, south China: implications for pore evolution. J. Nat. Gas Sci. Eng. 38, 21–38. Zhao, J., Jin, Z., Jin, Z., Geng, Y., Wen, X., Yan, C., 2016a. Applying sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale deposition in the Sichuan Basin, China. Int. J. Coal Geol. 163, 52–71. Zhao, J., Jin, Z., Jin, Z., Hu, Q., Hu, Z., Du, W., Yan, C., Geng, Y., 2017b. Mineral types

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