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Volume 31 Issue 2
Apr.  2020
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Bao Zhang, Detian Yan, Hassan Jasmine Drawarh, Xiangrong Yang, Jin He, Liwei Zhang. Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China. Journal of Earth Science, 2020, 31(2): 354-367. doi: 10.1007/s12583-019-1247-4
Citation: Bao Zhang, Detian Yan, Hassan Jasmine Drawarh, Xiangrong Yang, Jin He, Liwei Zhang. Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China. Journal of Earth Science, 2020, 31(2): 354-367. doi: 10.1007/s12583-019-1247-4

Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China

doi: 10.1007/s12583-019-1247-4
Funds:

the Natural Science Foundation of Hubei Province 2019CFA028

the National Natural Science Foundation of China 41572327

the National Natural Science Foundation of China 41690131

the National Natural Science Foundation of China 4127300

the Program of Introducing Talents of Discipline to Universities of China B14031

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  • The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments. We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section,South China,based on thin-section studies using SEM (scanning electron microscope),SEM-CL (cathodoluminescence),XRD (X-ray diffraction) and geochemical analyses. There are two types of quartz in the shales:detrital quartz and authigenic quartz. Detrital quartz is mostly silt-size,typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape,and brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape:grain overgrowths and crystallite grains. Overgrowth surfaces are subhedral. Crystallite grains are typically less than 10 μm in size,euhedral or subhedral monocrystal in shape. Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon. In the numerical model,the content of detrital quartz is relatively consistent (20%); the content of biogenic quartz ranges from 40% to 70%,with a sharp fall (0-30%) in the Guanyinqiao mudstone. During the Katian,a lower anoxic and dense water column make the dissolution of biogenic silica well preserved. Biogenic quartz is the major contributor to the sediment. During the Early Hirnantian interval,due to the drop of sea level and the oxygenation of seafloor,the sediment is mainly composed of clay transformed quartz and detrital quartz. During the Latest Hirnatian and Rhuddanian,rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica,thereby biogenic quartz re-emerges as the major contributors to the sediment. Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity,however,as a rigid framework,they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz,which fortifies the brittleness of rocks and is beneficial to the development of shale gas.
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Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China

doi: 10.1007/s12583-019-1247-4
Funds:

the Natural Science Foundation of Hubei Province 2019CFA028

the National Natural Science Foundation of China 41572327

the National Natural Science Foundation of China 41690131

the National Natural Science Foundation of China 4127300

the Program of Introducing Talents of Discipline to Universities of China B14031

    Corresponding author: Detian Yan

Abstract: The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments. We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section,South China,based on thin-section studies using SEM (scanning electron microscope),SEM-CL (cathodoluminescence),XRD (X-ray diffraction) and geochemical analyses. There are two types of quartz in the shales:detrital quartz and authigenic quartz. Detrital quartz is mostly silt-size,typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape,and brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape:grain overgrowths and crystallite grains. Overgrowth surfaces are subhedral. Crystallite grains are typically less than 10 μm in size,euhedral or subhedral monocrystal in shape. Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon. In the numerical model,the content of detrital quartz is relatively consistent (20%); the content of biogenic quartz ranges from 40% to 70%,with a sharp fall (0-30%) in the Guanyinqiao mudstone. During the Katian,a lower anoxic and dense water column make the dissolution of biogenic silica well preserved. Biogenic quartz is the major contributor to the sediment. During the Early Hirnantian interval,due to the drop of sea level and the oxygenation of seafloor,the sediment is mainly composed of clay transformed quartz and detrital quartz. During the Latest Hirnatian and Rhuddanian,rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica,thereby biogenic quartz re-emerges as the major contributors to the sediment. Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity,however,as a rigid framework,they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz,which fortifies the brittleness of rocks and is beneficial to the development of shale gas.

Bao Zhang, Detian Yan, Hassan Jasmine Drawarh, Xiangrong Yang, Jin He, Liwei Zhang. Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China. Journal of Earth Science, 2020, 31(2): 354-367. doi: 10.1007/s12583-019-1247-4
Citation: Bao Zhang, Detian Yan, Hassan Jasmine Drawarh, Xiangrong Yang, Jin He, Liwei Zhang. Formation Mechanism and Numerical Model of Quartz in Fine-Grained Organic-Rich Shales: A Case Study of Wufeng and Longmaxi Formations in Western Hubei Province, South China. Journal of Earth Science, 2020, 31(2): 354-367. doi: 10.1007/s12583-019-1247-4
  • Quartz, one of the most widely distributed minerals on the earthʼs surface, usually presents in shales as detrital component and as an authigenic cement (Milliken et al., 2016, 2012; Day-Stirrat et al., 2010; Blatt and Schultz, 1976). Detrital quartz, including α-quartz mechanically transported in the basin (Milliken et al., 2016) and silicic acid flux dissolution of terrigenous silicates deposited in sediments (Tréguer and De La Rocha, 2013), general forms quartz overgrowth (Dowey and Taylor, 2017; Zhao et al., 2017). Authigenic quartz cement consists of biotic and abiotic quartz. Biotic quartz refers to quartz precipitated by biological opal or amorphous silicon. The amorphous silicon is mainly derived from diatoms, radiolaria, siliceous dinoflagellates and sponge bone needles, and siliceous organisms differ in silica productivity in different ecosystems (Tréguer and De La Rocha, 2013). Abiotic quartz includes the dissolution of potassium feldspar, the pressure dissolution of detrital quartz and clay minerals transformation during the diagenetic stage (Liang et al., 2017; Milliken et al., 2016, 2012; Milliken, 2014; Milliken and Day-Stirrat, 2013; Isaacs, 1982, 1981). In contrast to other diagenetic forms (such as grain replacement), the grain-binding authigenic quartz cement with significant impact on mechanical properties of rock can cause lithification and stiffening of sediments (Thyberg et al., 2010), and it can also reduce reservoir porosity and permeability, fortify the brittleness of rock in nature and induce deformation eventually (Zhao et al., 2017).

    Over the past several decades, many experimental investigations have been performed on quartz in shales to explore the sources. Previous studies indicate that adequate potassium feldspar dissolution might be the source of authigenic quartz in the Wilcox Group mudstone, the nucleation of authigenic quartz in the periphery of detrital quartz was less to be limited by clay coating inhibition and smaller porosity (Day-Stirrat et al., 2010). In addition, some researchers believed that the source of quartz in the Haynesville-Bossier shale is from the pressure dissolution of detrital quartz and clay minerals transformation; authigenic quartz cement is present in two phases, grain replacements and grain overgrowth, and can fortify the brittleness and reduce reservoir porosity (Milliken et al., 2016). Until now, most researchers believed that quartz in shale is dominated by authigenic quartz (up to 60%); biogenic silica (radiolarian skeletons) dissolution and precipitation provide materials for authigenic quartz crystals. Quartz is present in infilling primary intragranular pores and replacing allochem grains in two phases in the Eagle Ford Formation (Dowey and Taylor, 2017; Jiang et al., 2017; Zhao et al., 2017). Although the predecessors have done a lot of research on the morphology and origin of different types of quartz in shales, the formation mechanism of quartz and the content and vertical variations of different types of quartz are less discussed.

    Organic-rich shales of the Wufeng and Longmaxi formations are potential targets in the exploration and development of shale gas in South China, but the types and development mechanism of quartz as a brittle mineral are not clear. The purpose of this paper is to clarify: (1) what are the types of quartz in the study area; (2) what are the potential sources of authigenic quartz; (3) what is the development mechanism of quartz; (4) how to establish a mathematical model of quartz development; and (5) how does quartz cement impact on reservoir properties. This study presents summary and conclusions of the change of quartz in diagenesis of shale reservoirs, and also provides new evidence for seawater chemistry and the catastrophic biotic events during the Ordo-Silurian transition.

  • During the Mid-Paleozoic, the South China Block existed as a distinctly separate plate, but was connected to the Gondwana edge during the Late Ordovician to Early Silurian (Metcalfe, 1994) (Fig. 1a). The Yangtze Plate was covered by a broad epeiric sea, and to its southeast there was the deep Pearl River, which may be more or less connected with the open ocean. Regional paleogeographic reorganization during the O-S transition coincides with sea level changes (Chen et al., 2004; Zhang et al., 2000). Jiujiang Strait divided the Yangtze Platform into the western Upper Yangtze Platform and the eastern Lower Yangtze Platform. The Upper Yangtze Platform was to a great extent closed as sea level dropped (Chen et al., 2000; Wang et al., 1993). The Wangjiawan Section, situated in Yichang, Hubei Province, is the Global Stratotype Section and Point (GSSP) for the Hirnantian (Late Ordovician) (Chen et al., 2006). In ascending order, it includes Wufeng, Guanyinqiao and Longmaxi formations (Fig. 1). The Wufeng Formation is generally composed of graptolite-rich black shale and Guanyinqiao limestone (argillaceous shelly limestone containing Hirnantian fauna less than 50 cm). The overlying Longmaxi Formation makes up of black shale with abundant graptolite fossils (Figs. 2b-2e). Extensive studies indicate that two pulses of mass extinction of the Latest Ordovician occurred at the beginning of Guanyinqiao interval for the first time and at the end of Guanyinqiao interval for the second time (Yan et al., 2010).

    Figure 1.  Late Ordovician paleogeography of the South China Block. (a) Details of the Yangtze Platform (after Wang et al., 1993), the proposed semi-enclosed nature of the Yangtze Sea, opening eastwards into the Pearl River Sea. (b) Stratigraphic column of measured sections from Wangjiawan Section.

  • Thirty-five samples of the Wufeng and Longmaxi formations shales were collected from Wangjiawan Section. The sample spacing was generally between 0.2-0.5 m, and the boundary line encrypted up to 5 cm. Samples of different layers were chosen and made into probe chip (thickness 60-80 μm). Thin sections were examined under 5×, 10×, 20× and 50× lenses with Nikon Eclipse microscope. The photos were taken by the Nikon DXM digital camera. The SEM (scanning electron microscope) data set includes secondary-electron, back scattered electron imaging, and energy dispersive spectrum of elementary data. A Gatan Chroma CL detector coupled with a field emission scanning electron microscope were used for the CL (cathodoluminescence) imaging. During SEM/EDS/CL operation, thin sections were covered with a thin carbon layer to prevent any charge build-up.

    For each sample, about 100 g of fresh rock was pulverized into ground granules using a steel jaw crusher, and then pulverized to a size of 200 mesh or more in an agate mortar. After grinding, each powder sample was structured in several sections for separate analysis.

    The XRD (X-ray diffraction) analyses of the samples were carried out using a Rigaku automated powder diffractometer with Cu Kα (wave length=0.154 16 nm) at 40 kV and 40 mA, a step interval 0.02° from 4° to 70° 2θ at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan.

    The XRF (X-ray fluorescence) spectrometer was used to measure the concentrations of the major element at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The analysis accuracy is generally better than 5% and uncertainty < 5%. Fine-grained shale samples were crushed (~5 g) into 0.075 mm diameter pellets for analysis with fusion glass (a 1 : 5 mixture of sample powder and flux (Li2B4O7)). Inductively-coupled plasma mass spectrometer (ICP-MS) was used to measure the concentrations of trace elements. About 50 mg powder sample digested in 0.5 mL HNO3 and 1 mL HF solution. The relative standard uncertainties of results are < 3%.

    Total organic carbon (TOC) was carried out using a vario EL III at the Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan. About 2 g sample (M1) was taken and added with excessive 0.5 mL HCl solution. After removal of all carbonate, it was washed with plenty of deionized water until pH reached neutrality. Solid residual sample (M2) was obtained by centrifugation and drying, and C% was measured. TOC=M2/M1×C%. The relative standard uncertainties of results are < 3%.

  • Clay strips are distributed intermittently in parallel while quartz grains are dispersed (Fig. 2c). Siliceous sponge spicules, are mainly needle columnar spine spicules (Figs. 2d, 2e). Injective spicules can also be observed (Fig. 2f).

    Figure 2.  Outcrops and optical microscope images of the shale sample in Wufeng and Longmaxi formations. (a) Limestone of Guanyinqiao Section; (b) polarized light image of detrital quartz grains; (c) polarized light image of quartz grains (distributed dispersedly); (d), (e) polarized light image of needle columnar sponge spicules; (f) polarized light image of injective sponge spicules.

    The quartz is initially divided into 2 categories by SEM. The first type of quartz is mostly silt-size and subangular to angular monocrystal (Fig. 3a). The second type quartz occurs mainly as crystallite, euhedral or subhedral monocrystal in shape (Figs. 3b, 3c).

    Figure 3.  SEM images of the shale sample in Wufeng and Longmaxi formations. (a) SEM image of detrital quartz; (b) SEM image of authigenic quartz near clay; (c) SEM image of authigenic quartz near sponge spicules.

    The first type of quartz defined as macrocrystalline or silt-size and subangular to angular monocrystal shows CL intensity and color that were distinctly bright and variable compared to the second type quartz (Figs. 4a, 4b, 4d, 4e). The first type of quartz has a dark circle of irregular edges outside the bright luminescent core (Figs. 4b, 4e). The CL intensity and color of the dark rim are similar to the second type of quartz.

    Figure 4.  SEM and CL images of the shale samples in Wufeng and Longmaxi formations. (a) SEM image of sponge spicules; (b) CL image of sponge spicules; (c) content of detrital quartz in sponge spicules; (d) SEM image of clay; (e) CL image of clay; (f) content of detrital quartz in clay. GO. Grain overgrowths.

    Figure 5.  (a) SEM image of authigenic crystallite quartz grain near sponge spicules; (b) SEM image of authigenic crystallite quartz grain near clay; (c) SEM images of numerous quartz silt grain contacts in coarse mudstone microfacies; (d) CL images of numerous quartz silt grain contacts in coarse mudstone microfacies; (e) SEM-CL image of the inside of the sponge spicules; (f) SEM-CL image of the inside of the clay. CG. Crystallite grain; Q. quartz; F. feldspar.

    In the Wangjiawan Section, the Si abundance varies of 32.38%-88.32% (avg. 68.93%); the Al abundance varies of 2.98%-13.24% (avg. 8.59%); the Mn abundance varies of 0-0.57% (avg. 0.05%); the K abundance varies of 0.70%- 5.51% (avg. 2.01%); the Rb abundance varies of 31.15 ppm- 149.7 ppm (avg. 82.83 ppm) (Table S1) (Fig. 6a). The ratio of Rb/K2O in the studied samples is equal to the Post-Archean Average Shale (PAAS). The Si/Al ratios of the samples in the study area are very high (avg. 11.08, > 3.11, Si/Al background values are 3.11 (Wedepohl, 1971)).

    Figure 6.  Geochemical characterization of the shale samples in Wufeng and Longmaxi formations. (a) Relationship between K2O and Rb for shale samples from Wufeng and Longmaxi. The Rb/K2O ratios are similar to the average ratio of the PAAS. The K2O and Rb concentrations of bentonites are from Su et al. (2009). (b) Al+Fe (%) and Al (%) concentration in studied samples. (c) Ti (%) and Fe (%) concentration. (d) Ternary diagram of Al-Fe-Mn showing the low Mn concentrations and within the non-hydrothermal field. (e) Si+Al+Fe (%) and Si (%) concentration. (f) Fe+Al+Mn (%) and Al (%) concentration. (g) Zirconium (ppm) and SiO2(non-clay) (%) concentration. (h) Aluminium (%) and SiO2(non-clay) (%) concentration. (i) TOC (%) and SiO2(non-clay) (%) concentration in the studied samples.

    Based on the whole rock analysis of outcrop samples by XRD, it is found that the shales are characterized by a high content of quartz and clay, and low content of plagioclase and pyrite. The samples are composed of 27%-86% quartz, 12%-45% clay, 1%-9% plagioclase. Quartz content varies generally between 8% and 86% (avg. 60.15%), with the lowest abundance in the lower Hirnantian (avg. 38.57%). The clay content is variable between 13% and 79% (avg. 27.53%), clay mineral assemblages include 26%-75% illite, 12%-72% illite- smectite mixed-layer minerals, 5%-15% smectite, and 1%-16% chlorite (Table S2).

  • Cathodoluminescence (CL) is often used to identify quartz provenance even on very small grain. The authigenic quartz is commonly observed with dull luminescent in SEM-CL while high luminescent grain is detrital quartz (Boggs, 2006; Sprunt, 1981). In other words, there are two types of quartz in the shales from Wufeng and Longmaxi formations: (1) detrital quartz, and (2) authigenic quartz.

  • Detrital quartz is mostly silt-size (Figs. 4a, 4d, 5e, 5f), typically ranging from 10 to 60 μm in size and from subangular to angular monocrystal in shape, brighter than authigenic quartz by CL intensity. Detrital quartz is present throughout Wufeng and Longmaxi formations shale samples. We estimate the content of detrital quartz of high luminescent in SEM-CL using point calculation method by software ImageJ (Figs. 4c, 4f), it is found that the content of detrital quartz accounts for about 20% of the sediment, whether rich in siliceous organism (Fig. 4c) or clay minerals (Fig. 4f).

  • The SEM-CL imaging shows that authigenic quartz cement with dull luminescent in SEM-CL is present in two phases in shape: (1) grain overgrowths, and (2) crystallite grains. Grain overgrowths are found alongside the detrital quartz grains. Overgrowth surfaces are subhedral (Figs. 4a, 4d). Crystallite grains are extant on illite or siliceous organism (sponge spicules), and their sizes are small, typically less than 10 μm in size, euhedral or subhedral monocrystal in shape (Figs. 5e, 5f). Aggregates of crystalline grains may develop into grain overgrowths.

    Authigenic quartz can be divided into biogenic quartz (Fig. 4a) and clay mineral transformed quartz (Fig. 4d) according to the source of silicon.

    It is worth noting that in the shales from Wufeng and Longmaxi formations we can easily observe siliceous organisms (Figs. 2d, 2e, 2f, 4a), while in the Guanyinqiao mudstone we can easily observe detrital quartz (Fig. 2b) and clay minerals (Fig. 4d). Unlike detrital quartz, the point calculation method is not applied to calculate the contents of biogenic quartz and clay mineral transformed quartz, because siliceous organism or clay minerals can't be observed in every photo, and the randomness of the thin sections may also affect the results.

  • Changes in lithological components in marine sediments often result in the enrichment of different trace elements (Zhu et al., 2018), such as alkaline elements (K, Rb and Cs) which are usually enriched in detrital components but are diluted by biological components in marine sediment. The Rb/K2O ratio provides information on the source of the deposit (Ran et al., 2015; Plank and Langmuir, 1998). Sediment is rich in volcaniclastic components or undergoes K absorption leading to low Rb/K2O ratio; in contrast, sediment from ancient and highly weathered sources has high Rb/K2O ratio (McLennan et al., 1990). The Rb/K2O ratio in the shales from Wufeng and Longmaxi formations are similar to the Post-Archean Average Shale (PAAS) (Fig. 6a), which suggests that the sediments in the study area are mainly derived from the upper continental crust, but are dissolved by the smaller potassium feldspar. Meanwhile, the Si/Al ratios are very high (ave. 11.08, > 3.11, Si/Albackground values are 3.11 (Wedepohl, 1971)), indicates that quartz is mainly authigenic quartz in the Wangjiawan Section.

    The SEM-CL imagings (Figs. 4a-4d) show the presence of detrital quartz has a dim circle of irregular edges outside the bright luminescent core nearby the illite or sponge spicules. Generally, quartz overgrowths tend to develop euhedral crystal. Quartz overgrowths of the study area are mainly anhedral or subhedral, with only a small part of euhedral ones, which is due to the low porosity and permeability of the shales, compaction or pressure melting will produce anhedral or subhedral quartz from euhedral quartz.

    The enrichment of Fe and Mn in shales is usually associated with the hydrothermal activity, while the enrichment of Al and Ti indicates that the sediment is from terrigenous debris. Boström et al. (1973) thought that siliceous rocks with the ratio of Al/(Al+Fe) < 0.4 and Fe/Ti > 20 were hydrothermal origins. Yamamoto (1987) considered that the Al/(Fe+Al+Mn) ratio is likewise a crucial indicator of the quartz. The hydrothermal cause ratio is 0.01, and the pure biogenic rate is higher than 0.60. Also, the Fe-Al-Mn ternary diagram can be used to estimate the sources of quartz. Harris et al. (2011) considered that the ratio of Si/(Si+A1+Fe) is an essential parameter in determining the source of silica, and the biogenic ratio is higher, generally greater than 0.9. In addition to the above-mentioned method, the relationship between silica and zirconium can also be utilized to determine the source of quartz (Blood et al., 2013; Wright et al., 2010). Zirconium in sediment generally comes from detritus material input. If there is a positive correlation between Si and Cr, it indicates that the quartz in the study area mainly comes from detrital material input; on the contrary, the quartz is mainly derived from siliceous organisms in the ocean, which was also confirmed by Muskwa Formation where the quartz was mainly a biogenic source (Wright et al., 2010).

    Through petrographic evidence, it is found that quartz in the study area is mainly composed of slowly crystallized subhedral authigenic quartz, but it is not clear that authigenic quartz is the product of diagenetic crystallization or as a result of hydrothermal fluid activity. The average ratio of Al/(Al+Fe) is 0.69 (> 0.4) (Fig. 6b), Fe/Ti is 6.2 (< 20) (Fig. 6c), the Mn concentration is low, and Ternary diagram of Al-Fe-Mn is located in the non-hydrothermal field (Fig. 6d), suggesting that the study area did not undergo significant hydrothermal sedimentary transformation, and the studied quartz crystals were formed during diagenesis.

    The source of authigenic quartz cementation in shales falls into the following four categories: (1) detrital potassium feldspar, (2) pressure dissolution, (3) dissolution of amorphous silica organisms, and (4) transformation of clay minerals (illiitisation/ chloritisation of smectite) (Worden and Morad, 2000).

    (1) The dissolution of potassium feldspar can provide raw materials for the crystallization of authigenic quartz. However, it is not the case with the source of authigenic quartz in the study area because K-feldspar concentrations are low in the samples (Table S2), and a large amount of potassium feldspar dissolution has not been observed in SEM (Figs. 5g, 5h). Meanwhile, the Rb/K2O ratio is similar to the PAAS, indicating that sediment has not experienced potassium uptake, and K- feldspar dissolution may not be the principal sources of quartz cementation.

    (2) The shale is a system of low permeability and porosity, pressure solution in contact with quartz grain may be the source of authigenic quartz cementation (Figs. 5g, 5h), quartz overgrowths can either engulf or replace (Day-Stirrat et al., 2010). It can be observed from the SEM images that quartz overgrowths could replace primary detrital framework and dissolution of K-feldspars (Figs. 5g, 5h) could occur, thereby pressure solution is unlikely to be a main source because fewer particles are observed to be in contact with each other. Meanwhile, the low permeability and diffusion coefficient of shale will greatly reduce the mobility of silica, which is also a factor limiting the dissolution of pressure into crystallization of authigenic quartz.

    (3) Siliceous organisms in marine sediments are part of the essential raw materials for the growth of authigenic quartz during diagenesis (Peltonen et al., 2009). A large number of siliceous sponge spicules were found in the study area, mainly needle columnar spine spicules (Figs. 2g, 2h). Injective spicules can also be observed (Fig. 2i). Previous studies have revealed that opal-A from biogenic amorphous silica gradually transform into opal-CT in diagenetic evolution, and finally crystallise into quartz in shales (Dowey et al., 2017; Milliken et al., 2016). Both quartz and clay minerals contain Si, so in order to lower the influence of Si in clay minerals, here, we use SiO2(non-clay) instead of quartz, and the content of SiO2(non-clay) is similar to quartz in shales. SiO2(non-clay) was determined as SiO2(total) minus SiO2(clay) through the major elements and XRD data. Among them, SiO2(clay) includes TOT clay minerals (mainly illite and smectite) and TOTO clay minerals (mainly chlorite). This calculation assumes that Al is all derived from clay minerals and TOT clay minerals have twice the Si : Al molar ratio than TOTO clay minerals (Liu et al., 2017).

    where the molar mass of Si is 28.1 g, the molar mass of Al is 27 g, and the molar mass of SiO2 is 60.1 g. The average ratio of Si/(Si+A1+Fe) is 0.85 (Fig. 6e), the average ratio of Al/(Fe+ Al+Mn) is 0.68 (Fig. 6f), indicating that the silica within the shales from Wufeng and Longmaxi formations is a biogenic mineral phase, authigenic quartz may come from siliceous organisms dissolution and re-crystallization. The zirconium abundance in the Katian samples shows a weak negative correlation with SiO2(non-clay) content (R2 value of 0.067 5; Fig. 6g); the zirconium abundance in the Hirnantian samples is positively correlated with SiO2(non-clay) content (R2 value of 0.835 1; Fig. 6g); the zirconium abundance in the Rhuddanian samples shows a weak negative correlation with SiO2(non-clay) content (R2 value of 0.048 9; Fig. 6g). The negative correlated relationship of Katian and Rhuddanian samples indicates that quartz is mainly a biotic source, but R2 value is small, shows abiotic source provides partly materials for authigenic quartz crystals; the positive correlated relationship of Hirnantian samples indicates that quartz is mainly a detrital origin. The TOC abundance in the Katian samples shows a weak negative correlation with SiO2(non-clay) content (R2 value of 0.114 3; Fig. 6i); the TOC abundance in the Hirnantian samples shows a weak positive correlation with SiO2(non-clay) content (R2 value of 0.13; Fig. 6i); the TOC abundance in the Rhuddanian samples shows a weak negative correlation with SiO2(non-clay) content (R2 value of 0.093 9; Fig. 6i). The negative correlated relationship of Katian and Rhuddanian samples indicates that quartz is mainly a biological source; the positively correlated relationship of Hirnantian samples indicates that quartz is mainly a detrital origin.

    (4) In the process of illiitisation/chloritisation of smectite, silica was released and finally crystallized into quartz, that is to say, the smectite-illite reaction is a dissolution precipitation reaction (Nadeau et al., 2002; Stixrude and Peacor, 2002). The smectite particles will gradually dissolve while the newly formed illite particles release free silicon (Środoń, 1999). Free silicon will increase the silicon content in the pore water (Abercrombie et al., 1994), and excess silica (aq) will slowly crystallize in the shale particles (newly formed illite-smectite), implying that the authigenic microcrystalline quartz formed by the transformation of clay minerals don't inherit CL-properties from the smectite. The XRD data indicate that the clay minerals in the study area were mainly illite/smectite mixed layer and illite, indicating that most of the smectite was converted into illite, so the high concentration of illite indicates that the conversion of clay minerals may be one of the sources of authigenic quartz crystals. The Al abundance in the samples is negatively correlated with SiO2(non-clay) content (R2=0.52; Fig. 6h) indicates that silica released by the conversion of clay minerals is not the main route for the growth of authigenic quartz (Fig. 6h), excess silica produced by clay reactions may partially precipitate quartz (Peltonen et al., 2009).

    Petrographic and geochemical data suggest that dissolution of biogenic amorphous silica and clay mineral transformation is probably the dominant process supplying silica for authigenic quartz crystals. Although the dissolution of K-feldspar and pressure solution can provide abundant raw materials for the growth of authigenic quartz, the crystallization of authigenic quartz in the study area does not depend mainly on these two pathways.

  • We use a numerical box model to simulate the change of quartz content in the Wangjiawan Section. Firstly, the model conforms to the basic mass balance Eq. (1)

    where $M_{\rm{Q}}^{{\rm{SW}}}, t, {F_{{\rm{in}}}^{\rm{Q}}} $ and ${F_{{\rm{out}}}^{\rm{Q}}} $ represent the mass of quartz in the ocean, time, total input and output fluxes of quartz respectively. Time data 0-4.8 Ma is Katian, 4.8-6.2 Ma is Early Hirnantian, 6.2-11.4 Ma is Late Hirnantian and Rhuddanian.

    The rate of change of the seawater quartz is indicated by Eq. (2)

    where $ {R_{\rm{Q}}^{{\rm{SW}}}}$ represents the ration of quartz in the ocean, ${R_{\rm{Q}}^{{\rm{in}}}} $ represents input quartz.

    In our simulation, we focus on the three inputs of quartz, siliceous organism transformation, detrital input and clay mineral transformation. Utilize Pexp, Al, illite and chlorite content to replace siliceous organisms, detrital input and clay mineral transformation, respectively.

    Detrital input quartz, including α-quartz mechanically transported in the basin and the low-temperature dissolution of terrigenous silicates deposited in sediments. In order to estimate the content of the detrital input, the content of Al is used in the simulation calculation. The Al ($ R_{{\rm{in}}, {\rm{Ti}}}^{\rm{Q}}$) in the sediment is mainly found in terrigenous silicate such as smectite, which represents the detrital input. The Si/Al of the average crust is the coefficient of detrital input ($ f_{{\rm{in}}, {\rm{Ti}}}^{\rm{Q}}$), and Si/Al background values are 3.11 (Wedepohl, 1971).

    Petrographic and XRD data indicate that the transformation of clay minerals can be converted to quartz. In the smectite-illite and smectite-chlorite variations, the silica content is reduced relative to alumina. Using end-member chemical formulae and molecular weights for smectite, illite, and muscovite, with balanced reactions assuming Al2O3 to be constant, estimated silica yields are as follows (van de Kamp, 2008).

    Smectite-illite

    Total weight of the reaction product=1 101.1 g; total weight of clay of product=787.2 g; silica released during conversion= (197.7/1 101.1)×100%=18.0%.

    Smectite-chlorite

    Total weight of the reaction product=1 146.2 g; total weight of clay of product=395.43 g; silica released during conversion=(395.43/1 146.2)×100%=34.5%.

    The shales from Wufeng and Longmaxi formations in the Wangjiawan Section experienced similar tectonic activities, so the shale samples were subjected to similar diagenetic transformations. Using the content of illite and illite/smectite in XRD data as ($R_{{\rm{in}},{\rm{I}}}^{\rm{Q}} $)(for the brief calculation, take half of illite/ smectite as illite), chlorite as ($ R_{{\rm{in}},{\rm{C}}}^{\rm{Q}}$) respectively, the coefficient of conversion as ($ f_{{\rm{in}},{\rm{I}}}^{\rm{Q}}$) and ($f_{{\rm{in}},{\rm{C}}}^{\rm{Q}} $).

    As a discriminating indicator of paleoproductivity, Ba is the most extensive trace element. Here, content of the biological Ba is calculated mainly by the following Eq. (5)

    where Babio indicates the content of biological Ba; Bat indicates the total amount of Ba in the sediment; (Ba/Ti)PAAS is the ratio of Ba to Ti, which is derived from the average shale of the Post-Archean Average Shale (PAAS), the ratio is 0.003 8. The paleoproductivity is estimated by the Francois model.

    where Pexp indicates paleoproductivity input; FBa indicates the theoretical Ba flux; FBam indicates the measured Ba flux; RMA indicates the sediment accumulation rate; RS is the deposition rate; DBD is the sediment dry density (RS=0.78 m/Ma; DBD of the Guanyinqiao Section is 397 mg/cm3; DBD of the Wufeng and Longmaxi formations is 501 mg/cm3). Using Pexp as $ R_{{\rm{in}}, {P_{\rm{exp}}}}^{\rm{Q}}$ they are shown in Table S3.

    The modern diatoms account for about 40% of marine primary productivity (P), a part of the gross production is exported toward the ocean shales while the other part is recycled in the surface layer via the dissolution of the biogenic silica (D) (Tréguer et al., 2017). Factors such as surface layer temperature, water depth and diatom characteristics seriously affect the preservation of silicon (the ratio of D : P is different in different ecosystems using isotopic tracer methods), the global burial/ production ratio is approximately 3% (Nelson et al., 1995). Deposition rate and redox may affect the preservation of biogenic silica (Jurkowska et al., 2019).

    The predecessors have done a lot of work on the redox and sea level changes in Wangjiawan Section. During the O-S transition, sea level (Fig. 7f), redox conditions (Fig. 7d), surface layer temperature (Fig. 7e) and deposition rate have changed, the water depth of the Wangjiawan Section has also changed. There is a causal link between the rapid climatic changes and the extinction. It is difficult to have a suitable modern parameter to simulate changes in biogenic silica. In our simulation, we focus on the Wangjiawan samples which are divided into seven parts (Table S1) by Graptolite zone. Give different $ f_{{\rm{in}}, {P_{\rm{exp}}}}^{\rm{Q}}$ values according to graptolite zone and correct it by the actual content of quartz (XRD data), higher in the D. complexus Zone (0.025), gradually decreasing upwards, in the N. extraordinarius-N. ojsuensis carbonaceous mudstone $f_{{\rm{in}}, {{\rm{BA}}_{{\rm{bio}}}}}^{\rm{Q}} $ values of 0, then gradually rise, in the C. vesiculosus Zone $ f_{{\rm{in}}, {P_{\rm{exp}}}}^{\rm{Q}}$ values of 0.01.

    Figure 7.  (a) Stratigraphic column of measured sections from Wangjiawan Section; (b) changes in the number of graptolite species at Wangjiawan Section from Chen et al. (2005); (c) chemical index of alteration (CIA) variations across Ordovician-Silurian boundary at Wangjiawan Section from Yan et al. (2010); (d) V/(V+Ni) ratios of Ordovician-Silurian sedimentary rocks from the Wangjiawan Section from Yan et al. (2009); (e) Δ47 derived near-surface ocean temperature trend for the Early Katian to Rhuddanian interval from Finnegan et al. (2011); (f) sea-level variations for the Late Ordovician to Late Silurian in South China from Liu et al. (2017).

    Different amounts of coefficient ($f_{{\rm{in}}, {P_{\rm{exp}}}}^{\rm{Q}} $) combined with detrital quartz, clay quartz are used to simulate the change of total quartz content. The change in total quartz content conforms to the following Eq. (6). $R_{\rm{Q}}^{{\rm{SW}}}\left({{t_0}} \right) $ is considered to be the content of Si in the submarine siliceous precipitates, greater than 50%, the residence time of silicon ($ \tau _{\rm{Q}}^{{\rm{SW}}}$) in the modern ocean is 15 000 years (Tréguer and De La Rocha, 2013).

    In the simulation results, the variation trend of simulated quartz content is similar to the change trend of actual quartz content (Fig. 8b).

    Figure 8.  Numerical simulation of different quartz vertical changes at Wangjiawan Section. (a) Stratigraphic column of measured sections from Wangjiawan Section; (b) quartz variations across Ordovician-Silurian boundary by XRD data; (c) numerical simulation of different quartz vertical changes; (d) numerical simulation of quartz vertical changes; (e) numerical simulation of biological quartz vertical changes; (f) numerical simulation of clay transformed quartz vertical changes; (g) numerical simulation of detrital quartz vertical changes. Boundary conditions in numerical simulation are presented in Table S4.

    The clay transformed quartz do not change much in the vertical direction (Fig. 8g), which is consistent with the similar diagenesis of the shales from Wufeng and Longmaxi formations in the Wangjiawan Section. This is similar to the results observed with the thin sections that in the Guanyinqiao mudstone we can easily observe clay minerals (Section 4.1.2). It is noteworthy that the N. extraordinarius-N. ojsuensis carbonaceous mudstone has a higher clay content, especially WJW-25 sample, clay content is 79% while quartz content is 8%. We think that the quartz of the WJW-25 sample is entirely clay transformed quartz, the contribution of detrital input and biotransformation is almost 0. It is unlikely that they slumped to 0, in the simulation calculation they gradually reduced to 0 and then rose.

    The content of detrital quartz is relatively consistent from Wufeng Formation (fluctuates around 20%) (Fig. 8f). This is basically consistent with the SEM-CL observation (Figs. 4c, 4f), which further verifies the correctness of the simulation. They fall to 0 in the N. extraordinarius-N. ojsuensis carbonaceous mudstone, further upward the content of detrital quartz quickly returns to relatively high values, and upward falls sharply. Then returns to relatively high values (fluctuates around 20%) in the Longmaxi Formation.

    The content of biogenic quartz is higher in the Wufeng Formation, and slowly decreases from 65% to about 30% (Fig. 8e). They fall to 0 sharply in the N. extraordinarius-N. ojsuensis carbonaceous mudstone, further upward the content of biogenic quartz quickly returns to relatively high values, and upward falls sharply. Then returns to relatively high values (fluctuates around 40%) in the Longmaxi Formation. This is consistent with the results observed with the thin sections that in the shales from Wufeng and Longamaxi formations we can easily observe siliceous organisms (Section 4.1.2).

    In summary, during the Katian, a lower anoxic and dense water column (Fig. 7d) makes the dissolution of biogenic silica under the photic layer well preserved, biogenic quartz is the major contributors to the sediment. During the Early Hirnantian interval, as the drop of sea level (Fig. 7f) and the oxygenation of seafloor (Yan et al., 2012), leads to the increase of biological productivity, however, it also increases the difficulty of the biogenic silica preservation; decreases weathering intensity (Fig. 7c) (Yan et al., 2010) may reduce detrital source input. In particular, there is almost no preserved biogenic silica in the N. extraordinarius-N. ojsuensis carbonaceous mudstone, and the sediment is mainly composed of clay transformed quartz and detrital quartz. During the Latest Hirnatian and Rhuddanian, rapid sea level rise (Fig. 7f) and anoxic ocean (Fig. 7d) enhance the preservation of the biogenic silica, thereby biogenic quartz re-emerges as the major contributors to the sediment.

  • The formation mechanism of quartz in the shales from Wufeng and Longmaxi can be roughly divided into the following four main stages (Fig. 9).

    Figure 9.  Schematic diagram outlining the development mechanism of quartz.

    During weathering, transport and deposition, detrital quartz, clay mineral and the remains of siliceous organism get to the ocean.

    During early diagenesis, dissolution of metastable opal- A/CT and smectite produces silica super-saturation estimated 5-10 times higher than quartz saturation in the pore water (Thyberg et al., 2010), the high silicon concentration of pore water will crystallize into micro-sized-quartz (Figs. 5e, 5f). The stage of transformation of clay minerals into quartz may be later than dissolution of metastable opal-A/CT. The illitization reaction of clay minerals in shales requires that the solubility of silicon in pore water approaching saturation (Bjørlykke and Egeberg, 1993; Egeberg and Aagaard, 1989). The first occurrence of illitization reaction, after the dissolution of biogenic amorphous silica does not meet the conditions of illitization reaction. The most suitable condition is that opal-A is converted into better-ordered opal-CT in diagenesis to produce free silicon, which increases the saturation of silicon in the pore water, then smectite becomes unstable and gradually transforms into the mixed layer minerals (illite-smectite or chlorite-smectite) (Table S2). This process also releases free silicon to increase the silicon saturation of the pore water. Excess silica (aq) will be slowly crystallized into micro-quartz in the shale particles (newly formed illite-smectite).

    High silica saturation makes microcrystalline quartz grow slowly and detrital quartz crystals nucleate (Figs. 4b, 4e, 5d, 5e). Parts of these micro-quartz crystals have been cemented together, probably resulting in connections between smaller individual micro-quartz networks forming larger aggregates (Figs. 5d, 5e). The formed microcrystalline quartz may integrate into a larger quartz aggregate (Thyberg et al., 2010). The aggregates are linked to each other and may form a quartz network. The larger quartz network formed may be transformed into the detrital quartz overgrowths. The crystallite crystals nucleate on suitable particles, such as detrital quartz and early-formed micro-quartz crystals. Detrital quartz and micro-quartz crystals will accelerate quartz crystals nucleate (crystals accelerate crystallization of saturated solutions).

    Shale is a low permeability and diffusion coefficient closed system. During burial diagenesis, the most important process is compaction, and detrital quartz and marine kerogen are the most primitive skeletons of shale, which are destroyed by pressure and cause a large amount of primary pore damage (Dowey et al., 2017; Pommer and Milliken, 2015; Mondol et al., 2007). Then authigenic micro-quartz networks and quartz overgrowths serve as a rigid framework for shale, which can restrain compaction and increase the brittleness of the rock. Pressure solution occurs at contact associated quartz overgrowths may enhance the concentration of silicon in pore water while the excess silica will increase the size of quartz overgrowths (Day-Stirrat et al., 2010). The quartz overgrowths between adjacent quartz particles (rigid minerals) may be engulfed while quartz particles may replace adjacent feldspar particles (plastic minerals) due to compression (Figs. 5c, 5d). Chemical compaction joins mechanical compaction to reduce porosity and permeability further.

  • The effects of authigenic quartz on rock structures in sandstone reservoirs have been extensively studied (Worden and Morad, 2000), quartz cementation mainly reduces porosity and affects permeability by increasing rock brittleness and occupying pore space. Due to the unique properties of shales with low permeability and low porosity and the significance of rock brittleness during hydraulic fracturing, authigenic quartz will have a greater impact on shale structure (Dowey et al., 2017). The brittleness of shale reservoirs is an important rock parameter in hydraulic fracturing because it affects the formation and propagation of shale fractures. The shale, which is largely developed from authigenic quartz, is more susceptible to cracking when it is subject to spontaneous or induced fracturing than shale without cementation.

    Detrital quartz and marine kerogen (large and rigid grains shelter) are the most primitive shale skeletons that are destroyed by pressure and cause large amounts of primordial pore damage (Dowey et al., 2017; Mendhe et al., 2017; Wood and Hazra, 2017; Pommer and Milliken, 2015). Clay mineral transformation or dissolution of biogenic amorphous silica amorphous silicon microcrystalline quartz (Fig. 5a) can crystallize in appropriate pore space, a large number of microcrystalline quartz cementation, and then form a self-generated micro-quartz network, accompanied by an increase in pore water concentration, the nucleation of detrital quartz forms quartz overgrowths (Figs. 4b, 4e), authigenic microcrystalline quartz aggregates and quartz overgrowths filled in primary pore space and then, the authigenic micro-quartz network and quartz overgrowth as a rigid frame of shale can suppress compaction, especially in the interior of siliceous (Fig. 4e) and clay minerals (Fig. 4f), only a small amount of minerals are compacted. But authigenic quartz reduce the porosity of the rock, may have an enormous influence on porosity and permeability of shale, fortify the brittleness of rock. The formation of authigenic quartz results in the increase of total quartz, which fortifies the brittleness of rock and is beneficial to the development of shale gas.

  • According to the SEM/CL characteristics and geochemical data, the types of quartz in the shales from Wufeng and Longmaxi formations in South China have been distinguished and their development patterns are summed up in this study, coming to the following conclusions.

    (Ⅰ) There are two types of quartz in the fine grained shales Wufeng and Longmaxi formations: (1) detrital quartz and (2) authigenic quartz. Detrital quartz is mostly silt-size, typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape, which is brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape: (1) grain overgrowths and (2) crystallite grain. Overgrowths surfaces are subhedral. Crystallite grain is typically less than 10 μm in size, euhedral or subhedral monocrystal in shape.

    (Ⅱ) Petrographic and geochemical data indicate that dissolution of biogenic amorphous silica and clay mineral transformation is probably the dominant sources of authigenic quartz.

    (Ⅲ) During early diagenesis, in the pore water, dissolution of metastable opal-A/CT and smectite produce silica super- saturation. Smectite is gradually replaced by mixed layer minerals and releases free silicon. High silica saturation makes microcrystalline quartz grow slowly and detrital quartz crystals nucleate.

    (Ⅳ) In the numerical model, during the Katian, a lower anoxic and dense water column makes the dissolution of biogenic silica well preserved, and biogenic quartz is the major contributors to the sediment. During the Early Hirnantian interval, as the drop of sea level and the oxygenation of seafloor, which turns the clay transformed quartz and detrital quartz the major contributors. During the Latest Hirnatian and Rhuddanian, rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica, thereby biogenic quartz re-emerges as the major contributors to the sediment.

    (Ⅴ) Authigenic crystallite grain and grain overgrowths have filled in primary pore space and have decreased the porosity, however, as a rigid framework, they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz, which fortifies the brittleness of rock and is beneficial to the development of shale gas.

  • This study was supported by the National Natural Science Foundation of China (Nos. 41690131, 41572327, 4127300), the Natural Science Foundation of Hubei Province (No. 2019CFA028), and the Program of Introducing Talents of Discipline to Universities of China (No. B14031). We appreciate constructive reviews from the anonymous reviewers and the editors. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1247-4.

    Electronic Supplementary Materials: Supplementary materials (Table S1-S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1247-4.

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