Shale gas is a self-generation, self-storage, and self- sealing unconventional oil and gas resource, which requires a comprehensive evaluation method by combining "geological sweet spots" and "engineering sweet spots" to optimize "sweet spots" for shale gas economic development^[1]. Large-scale commercial development of shale gas has been achieved in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shales in the Sichuan Basin and its periphery, China, which has provided good support for natural gas supply in China^[2]. As the only set of industrially developed marine shale, a large number of researches on the Wufeng-Longmaxi Formation shale have been done before, covering the sedimentary model of the Wufeng- Longmaxi Formation organic-rich shale^[3], reservoir qualitative and quantitative characterization^[4], main factors controlling shale gas enrichment and high production^[5,6], and resource evaluation^[7], etc. The understandings of "two types of enrichment models" and "four main control factors" have been achieved from these researches^[3]. These studies mainly focused on the formation mechanism and enrichment characteristics of highquality shale intervals (with TOC≥2%), but did not analyze the storage mechanism of high-quality shale reservoirs.

Shale has complex compositions. However, quartz is one of the common mineral components in shale, and it is also an important indicator for the fracturability of shale reservoirs^[8], which is critical for shale to become reservoir. Besides quartz from terrestrial input, organic-rich shale also contains biogenic quartz and quartz transformed from clay minerals etc.^[9]. Due to the limited mobility in shale, quartz transformed from clay minerals exists within the clay matrix. The formation of biogenic quartz is closely related to organic matter enrichment, runs throughout the entire shale "hydrocarbon generation-accumulation" process, and controls the enrichment and high production of shale gas. High-quality shale layers in North America are rich in biogenic silica. The Woodford shale has a biogenic silica content range of 7.7%-31.0%, accounting for 30.8%-84.7% of the total silica^[10]. The Haynesville shale has a biogenic silica content range of about 5%-15%^[11]. Although many researches on quartz in shale have been done before, they mainly focused on microscopic identification, semi-quantitative characterization, and modification of quartz to reservoir. The types, mechanisms, and functions of biogenic quartz haven’t been delved into^{[8,9,10,11,12,13,14]}. This study takes the biogenic quartz in the Wufeng-Longmaxi Formation shale in the Sichuan Basin as the research object, and analyzes its occurrence characteristics and formation mechanism through a large number of experiments, and based on the experiments to discuss the storage mechanism of the Wufeng Formation-Longmaxi Formation high-quality shale.

The Sichuan Basin located in southwestern China is a large gas-rich superimposed basin. The Wufeng-Longmaxi Formation shale gas play, the only industrially developed shale gas layer in China, mainly occurs in the interior Sichuan Basin, in which Weiyuan and Weirong shale gas fields in the central part of southwestern Sichuan, Changning and Zhaotong shale gas fields in the southern edge of Sichuan, and Fuling shale gas field in eastern Sichuan have been discovered, with cumulative gas production of 791×10⁸ m^3[12,13]. Currently, the main interval produced is concentrated in the deep-water shelf deposits of Wufeng Formation-first member of Longmaxi Formation (Long 1 for short), with a thickness of 20-40 m, organic matter content (TOC) generally greater than 2%, and content of brittle minerals such as quartz greater than 40%. During the depositional stage of Wufeng Formation to the first member of the Longmaxi Formation, due to the weakening of the compression between the Cathaysian massif and the Yangtze Plate, the paleogeographic pattern was relatively stable. As a result, a semi-enclosed bay opening to the north and surrounded by Center Sichuan uplift, Qianzhong ancient land and Xiangexi submarine-uplift in the other three sides was formed. The depo-centers were mainly located in the present Luzhou-Zigong and Jiaoshiba-Shizhu areas (Fig. 1). With different provenances and relative structural positions, the two depo-centers differ somewhat in shale lithology. The high-quality shale layers in the Changning, Weiyuan, and Zhaotong shale gas fields are mainly composed of calcium-bearing siliceous shale or calcareous-siliceous hybrid shale. In contrast, the Fuling shale gas field is dominated by siliceous shale^[1].

The core samples used in the experiments were taken from typical wells in the Changning and Weiyuan shale gas fields. Experimental tests included TOC content analysis, organic matter extraction, physical property analysis, low-temperature nitrogen adsorption experiment, whole-rock X-ray diffraction analysis (XRD), major and trace element analysis, rare earth element analysis (REE), X-ray fluorescence spectroscopy (XRF), optical microscope observation, large thin section splicing, and a variety of scanning electron microscope combined analysis (argon ion polishing-secondary electron imaging, field emission scanning electron microscope and helium ion scanning electron microscope). All analysis and test projects were carried out following relevant test standards. The tests were mainly completed by the National Energy Shale Gas Research and Development (Experiment) Center, the Institute of Geology and Geophysics, the Chinese Academy of Sciences, the Beijing Institute of Geology of the Nuclear Industry, and Nanjing University.

Quartz is one of the main brittle minerals in shale. The quartz content accounts for over 40% in high-quality shale of the Wufeng-Longmaxi Formation, and includes two types: extrabasinal detrital quartz supplied by terrestrial sources outside the basin and intrabasinal authigenic quartz (from clay transformation, and biogenic or hydrothermal origins). In the organic-rich shale, the detrital quartz is low in content, and the authigenic quartz transformed from clay mainly exists in the clay matrix and hardly directly involves in the high-quality shale "hydrocarbon generation-accumulation" process. Therefore, this work only focused on biogenic quartz.

Through observation with ordinary optical microscope, a variety of scanning electron microscope imaging, and energy spectrum analysis, the biogenic quartz in the Wufeng-Longmaxi Formation in the Sichuan Basin can be classified into two types based on the growth morphology.

2.1.1. Type I quartz

Type I quartz mainly occurs in siliceous shale and calcareous-siliceous hybrid shale, in quartz aggregates composed of many sub-micron-level quartz microcrystals, mostly filling the shells and edges of biological body cavities such as sponge spicules and radiolarian (Figs. 2a and 3a). Shape of single crystal in this type of quartz cannot be observed under ordinary optical microscope. The quartz aggregates of this type are round or elliptical depending on body cavities they fill, and some of them are associated with pyrite (Fig. 2b). Backscattering and secondary electron imaging show that the quartz aggregates are mainly composed of subhedral-euhedral quartz crystals of less than 3 μm, with blurred boundaries in between (Figs. 2c and 3a).

2.1.2. Type II quartz

Type II quartz mainly appears in siliceous shale or calcareous shale and is "powder-like" grain aggregates under unpolished scanning electron microscope (Figs. 2d and 3d). Observed under both argon ion polishing scanning electron microscope and helium ion scanning electron microscope, this type of quartz is mainly composed of cryptocrystalline nano-scale quartz grains less than 100 nm in ellipsoidal or spherical shape floating on organic matter on the whole (Figs. 2e and 3d) and in point contact or no contact (Fig. 2f).

These two types of quartz discussed above are authigenic. Authigenic quartz has a variety of silica sources^[14], mainly including biogenic silica dissolution, seafloor hydrothermal silica input, silica released by clay mineral conversion, silica released by volcanism, pressure solution of detrital quartz, and alteration of feldspar. Therefore, it is necessary to conduct careful analysis by comprehensive methods. In this study, the major and trace elements and rare earth elements analysis and petrographical observation were used to find out the sources of the silica comprehensively.

The trace element Zr exists stably in terrestrial silicate sediments. Positively correlated with the SiO₂ content, it is an important parameter indicating the source of terrestrial silica and biologic silica. Besides, in marine sediments, the major elements Al, Fe, and Mn are hardly affected by later diagenesis and weathering, thus are effective geochemical indicators for discriminating silica of biogenic origin and hydrothermal origin^[14]. By analyzing major elements in the high-quality shale intervals of several typical wells in Weiyuan gas field, combined with the research data of Changning and Fuling areas^[15,16,17], we found that the quartz in Wufeng-Longmaxi Formation of the Sichuan Basin is mainly of biological origin (Fig. 4). But quartz in some shale intervals (Fig. 4b) differs in genesis, and may be affected by hydrothermal activity.

This is consistent with the results of previous studies on shale in Fuling area^[14].

2.2.1. Formation of type I quartz

Type I quartz mainly appears as submicron quartz crystals at the edge of the biological body cavity or organic matter, is associated with pyrite in local parts, and comes entirely from biologic silica. In the early depositional stage of the Wufeng-Longmaxi Formation, volcanic activities were frequent. The nutrients carried by volcanic ash caused flourishment of plankton, including silica-rich radiolarians, sponge spicules, and foraminifera^[18]. A large number of studies have shown that the organic matter in the lowermost Longmaxi Formation (especially from Persulptograptus persulptus to Parakidograptus acuminatus graptolite zone) is mainly composed of planktonic algae, acritatch, bacteria, solid asphalt, and other non-bioclasts, accounting for 70%-80% of the total microscopic components; the bioclast is mainly graptolite, and some sponge spicules, radiolarians and chitins, etc.^[19], which shows that the paleo-water body at that time had abundant radiolarians, sponges, and other silica-rich planktons. In addition, the organic-rich shale was deposited in an anaerobic-anoxic deep-water shelf environment (some of the interval in partially sulfurized environment), which was conducive to enrichment and preservation of biological silica. The original mineral composition of these silica-rich organisms was generally opal-A, a disordered amorphous, unstable mineral with the molecular formula of SiO₂·nH₂O. In the process of burial diagenesis, due to the effect of temperature and pressure, the opal-A transformed to opal-CT, gradually forming aggregates of cryptocrystalline and microcrystalline quartz with a high-hardness structure^{[3, 20-21]} (Fig. 5a). The particles of type I quartz in the Wufeng-Longmaxi Formation shale are consistent in size with the opal phosphor balls and their recrystallized products discussed in previous studies^[22], which further demonstrates that this type of quartz is biogenic silica coming mainly from the transformation of silicon-rich organisms. Type I quartz was seen associated with pyrite in the organic matter of some shale sections. This is mainly because the silica conversion process was right in the sulfate reduction zone, and Fe²⁺ in the pores formed by the conversion contacted with the released S^2- to form pyrite precipitate. The reaction equation is as follows formulas (1) and (2):

(2)H₂O + HS^- + Fe²⁺ → FeS + H₃O⁺

The paragenesis of pyrite and type I quartz reflects the formation of many open pores during the diagenesis process, and the conversion of biological silica and pyrite mineralization took place simultaneously in silica-rich organisms.

2.2.2. Formation of type II quartz

Although the major element Al-Fe-Mn chart reveals that the quartz in the Wufeng Formation-Long 1 Member is mainly of biologic origin, some samples from Wufeng Formation and bottom of Longmaxi Formation in the Weiyuan area show deviation and are close to the hydrothermal silica origin area (Fig. 4b). Previous studies have revealed that the modern seafloor hydrothermal silica deposition is the result of precipitation of supersaturated amorphous silica from the upwelling high-temperature thermal fluid under the cooling effect of the upper cold seawater^[23]. This kind of hydrothermal origin silica is significantly different from the siliceous rocks in other environments in morphology and structure. It is mainly in sponge or honeycomb form, and appears mainly as nano-scale cryptocrystalline siliceous grains under scanning electron microscope.

The type II quartz in the Wufeng-Longmaxi Formation shale is similar to hydrothermal fluid origin silica to some extent. Through observation under microscope and geochemical analysis, it is concluded that type II quartz is mainly biogenic origin and affected by hydrothermal effect limitedly, and the layer with this kind of quartz is thin. Standard analysis of shale samples from Wufeng Formation-Long1 Member of Well W201 in the Weiyuan area shows that some shale intervals do not have apparent characteristics of low ∑REE content, negative Ce anomaly, and positive Eu anomaly (Fig. 6), indicating hydrothermal activity has little effect on the deposition of the shale. The dissolved liquid of silica-rich organisms might mix with a small amount of siliceous hydrothermal fluid on the seafloor and fill the biological cavities (algae or radiolarians, etc.) or attach on organic matter surface. The process of hydrocarbon generation and expulsion of organic matter released organic acid, and silicon-rich organisms were dissolved, resulting in the supersaturation and precipitation of silicic acid. Through bacterial action (such as sulfate-reducing bacteria), nano-scale siliceous pellets were formed, which finally deposited on organic matter surface due to chemical adsorption (Fig. 5b)^{[20,21,22,23,24]}. Besides, as algae's outer wall has complex lipid components^[25] which can effectively prevent biodegradation and chemical dissolution, nano-scale siliceous spheroids could exist in algal body cavities too.

The high-quality shale has a large thickness and continuous distribution. Thus the horizontal wells have a higher chance to encounter the shale, and the stimulated volume of staged fracturing in the high quality shale section in the later stage is large, so the wells will have higher initial production and ultimate recoverable shale gas reserves^[26]. Biogenic quartz content can directly reflect the fracturability of the interval and indirectly reveal the degree of organic matter enrichment in the interval, so it is one of the important indicators of high-quality shale distribution. Therefore, it is essential to calculate the content of biogenic silica. This content can be calculated by the average shale Si/Al ratio of 3.11^[27]. The excess silica is biogenic silica^[15,16,17].

By examining typical cross-sections and drilling information in the Changning, Weiyuan, and Fuling areas, it is found that the biogenic silica content of the Wufeng- Longmaxi Formation in the Sichuan Basin gradually decreases upwards, and is below 30%, but the biogenic silica accounts for a high proportion of the total silica (up to 91.3%) (Table 1). The siliceous shale is mainly concentrated in the bottom 10-40 m of the Wufeng-Longmaxi Formation, and is relatively stable in distribution vertically, with local differences (Fig. 7). The shale section in Well JY1^[16] and Well JY2^[17] of Fuling area have a biogenic silica content range of 0.17%-32.6%, 13.3% on average. That is about 4%-12% higher than that in Changning^[15] and Weiyuan blocks (0-28.9%, 5.1% on average). Analysis shows that this is caused by the change of organic-rich shale depositional environment. The main body of the Wufeng-Long 1 Member in southern Sichuan deposited in a deep-water shelf setting, and has calcareous shale with low biogenic silica content developing in the central part. In southeastern Sichuan, the main body of the shale deposited in deep-water shelf setting; rising ocean currents on the seafloor caused by volcanic activity made the seawater rich in silica and various nutrients, so the whole shale interval here is more rich in siliceous shale. Therefore, the biogenic silica-rich shale in the Fuling area (30-40 m) is thicker than that in the Changning and Weiyuan areas (10-20 m). The high production sections with a testing production rate greater than 20×10⁴ m³/d^[26] are in good consistence with the biogenic silica-rich shale sections (Fig. 7), accounting for about 62%-80% of the total shale thickness. Therefore, tracking the distribution of biogenic silica-rich shale has important guiding significance for exploration and development of shale gas in the Sichuan Basin.

During formation of type I and type II quartz, the cysts of siliceous planktons dissolved first to form residual internal cavities. These cavities were large, generally tens of microns to hundreds of microns in size (Figs. 2e, 3a and 5a) and often filled with co-deposited in-situ organic matter. Therefore, the siliceous planktons are parent material for hydrocarbon generation and provide good space for organic matter preservation. The content of biogenic silica enriched in shale is positively correlated with the organic matter content (Fig. 8), and can indirectly indicate the amount of organic matter.

Organic matter and type I quartz enrichment in the body cavities laid foundation for the later development of organic matter pores. Previous studies show that compared with calcareous shale and argillaceous shale, siliceous shale has higher hydrocarbon expulsion efficiency and higher solid bitumen yield, mainly related to low adsorption to soluble organic matter, and easy formation of micro cracks due to high brittleness of quartz^[28]. The hydrocarbon generation and expulsion of organic matter would create a large number of honeycomb nano-scale pores. The secondary cracking of solid bitumen would generate organic matter pores, and its flow network would be good migration channel^[29]. Organic matter was extracted from different lithofacies of shale samples from the Wufeng-Longmaxi Formation in two wells of the Weiyuan area. The low-temperature nitrogen adsorption experiments of whole rock and organic matter extract were conducted under the same conditions. The normalized results show that siliceous shale has the largest specific surface area of organic matter pore and highest proportion of organic matter pore volume of 12.53%- 43.81% and 18.46%-23.32% respectively; calcareous shale takes the second place, with organic matter pore contributing 7.71%-12.85% and 5.48%-7.76% to pore surface and volume, argillaceous shale has the lowest proportion of organic matter pore (Table 2). Apparently, biogenic silica has a significant promoting effect on pore formation.

Because the two types of biogenic quartz help pore preservation and have high-quality fracture-forming ability, high-quality shale intervals have relatively high porosity and permeability. Type I quartz was mainly formed in the early stage of diagenesis. Before the large-scale hydrocarbon generation of organic matter, the siliceous plankton shells solidified to form high-hardness crystalline structures (Fig. 3a). This kind of rigid grid composed of quartz minerals with a Mohs hardness of 7.0 has strong compression resistance. At the beginning stage of hydrocarbon generation, the pores formed by organic matter existing in organism cavities in the early stage can be well preserved. The rigid framework can also protect the large number of organic pores formed by secondary cracking of retained crude oil in the late stage. The organic acids and bacteria released by hydrocarbon generation and expulsion would promote the formation of massive type II authigenic quartz. Observation under microscope shows that some of the type II quartz exists in the honeycomb-like organic pores, enhancing the organic pores' anti-compaction ability (Fig. 9a). The discharged crude oil would migrate into the intergranular pores in the low potential zone. It would later crack to form good migration channels of "cavities-solid asphalt-residual intergranular pores."

Quartz is a mineral with high brittleness and is likely to break and form cracks^[30]. The fracture formation mechanisms mainly include hydrocarbon generation pressurization, tectonic activity, and diagenesis^[31]. In the Wufeng-Longmaxi Formation shale's organic matter, type I quartz formed early experienced hydrocarbon generation pressurization and multi-phase tectonic activities; type II quartz formed in the large-scale hydrocarbon generation stage experienced multi-phase tectonic activities. The Wufeng-Longmaxi Formation shale section has well-developed structural fractures. Characterization of filled micro-fractures in the entire thin-section by splicing large thin sections shows that the shale intervals with rich biogenic silica have more micro-fractures developed under the same stress field environment. Observation under microscope shows that the shale sections rich in biogenic silica in the Weiyuan area have a micro-crack density of 6-12 cracks/cm, up to 20 cracks/cm. Semi-quantitative analysis of silica in shale thin sections using XRF shows that thin sections with high content and uniform distribution of biogenic silica have more filled microcracks. The thin-sections from the bottom to the top of Wufeng Formation-Long1 Member decrease in TOC and biogenic silica content. The top of the Wufeng Formation-Long1 Member has lower biogenic silica content, and fewer micro-cracks (dominantly bedding fractures). The middle and bottom sections have higher content of biogenic silica in uniform distribution, very well-developed micro-cracks (some filled with calcareous minerals), forming an “artifact” that the overall silica content in the middle and bottom is relatively low. In the large thin-section, the middle and bottom have alternate low-angle oblique fractures and bedding fractures. These fractures are wide and penetrate multiple laminae, significantly increasing the shale reservoir's storage space (Fig. 9b, 9c).

Different from conventional reservoirs, shale reservoirs have nano-scale pores as main storage space, and fractures under-developed and unable to connect matrix pores in a large area. Horizontal well-staged fracturing must be employed to form complex fracture systems in shale reservoirs to improve the reservoir's seepage capacity, and make them economically recoverable "artificial gas reservoirs"^[32]. Shale reservoir fracturing is a prerequisite for large-scale shale gas exploration.

The Wufeng-Longmaxi Formation shale has both type I and type II quartz occurring in organic matter, and symbiosis of biogenic silica pores, organic matter pores, and micro-cracks, so the seepage system of "organic matter pores-net like cracks" is likely to come up and the fracture-making capacity of the reservoir can be effectively improved. The tightly bound type I microcrystalline quartz has a clear boundary with organic matter. Under external force, slip joints are likely to occur along the weak contact between this type of silica grain and organic matter^[33], to connect biologic silica lattice pores and organic matter pores in a large area (Figs. 3a and 5a). The type II nano-scale quartz grains are evenly embedded in organic matter, and in point-point or surface contact with each other. According to Hertz's contact theory, under the same load, the smaller the mineral particle size, the more significant the stress at the mineral contact point^[34], and therefore the rigid particles are likely to move against each other to form cracks under the same external force (Figs. 2f and 5b). The type II quartz has intergranular pores well-developed, and these pores can couple with organic matter pores and artificial fractures to form more efficient seepage channels.

There are two types of biogenic quartz in the Wufeng- Longmaxi Formation shale in the Sichuan Basin. The type I is submicron quartz aggregates less than 3 µm in size in semi-automorphic-automorphic structure. The quartz aggregates mostly occur at the edge of the organic matter and have clear boundaries from organic matter. This kind of biogenic quartz is formed by the dissolution and transformation of silica-rich plankton. The type II is nano-scale quartz grain less than 100 nm in size. Spherical or ellipsoidal, the quartz grains of this kind "float" on organic matter, and are in point or surface contact and uniform distribution with blurred boundaries with organic matter. This kind of quartz is mainly of biological origin with no obvious hydrothermal silicon input characteristics.

The biogenic silica in the Wufeng-Longmaxi Formation shale is continuously distributed vertically, concentrated in the bottom 20-40 m interval, and less than 30% in content generally. Due to the variations in the depositional environment of organic-rich black shale in the Sichuan Basin, the black shale in southern and eastern Sichuan differ somewhat in biogenic silica content and thickness. The black shale in eastern Sichuan has higher biogenic silica content and larger thickness.

The type I quartz aggregates formed early have strong compression resistance, so the intergranular pores and organic pores in them have been preserved well, and slip joints are likely to occur between the quartz aggregates and organic matter under external force. The type II quartz has rich intergranular pores. In point or surface contact, the grains of this kind of quartz are likely to move against each other to form micro-cracks. These two types of quartz are coupled with organic pores to create high-speed migration channels of "biologic silica framework pores-organic pores-microcracks", and effective seepage channels can be formed during fracturing to facilitate high and stable shale gas production.