The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shales are the only shale formations in China from which shale gas has been developed commercially. Shale gas fields such as Fuling, Weirong, Weiyuan, Changling and Zhaotong have been discovered in this formation in Sichuan Basin and surrounding areas^[1,2,3,4,5], and they have total proved geological reserves of 10 455×10⁸ m³, shale gas productivity of 150×10⁸ m³. Among them, the Fuling Shale Gas Field is the first commercially developed shale gas field, with the feature of overpressure^[6]. In this gas field, the Wufeng Formation-Longmaxi Formation shale is about 90 m thick, in which the high quality shale reservoir sections (TOC≥2%) are about 30-40 m thick, high in thermal evolution degree with R_o of 2.0%-3.5%, high in porosity (1.85%- 8.32%, 5.22% on average). The shale reservoirs have mainly organic pores, total gas contents of 3.52-8.90 m³/t (5.96 m³/t on average), good fracturing effect, and high production. The average production of 313 wells during test was 24.2×10⁴ m³/d. Some researchers have studied the depositional environment, reservoir features and main factors controlling gas accumulation and high production of the Wufeng Formation-Longmaxi Formation deep-water shelf shale and got some achievements^{[7,8,9,10,11,12,13,14]}, and proposed the finding that two factors controlling the shale gas enrichment in marine shale of southern China^[10]. Some researchers have also studied the hydrocarbon generation and storage mechanisms of deep-water shelf shale and got some understandings. But most of the studies haven’t examined the dynamic evolution and the relationship between hydrocarbon generation mechanism and storage mechanism^{[6, 10-11, 15-16]}. Therefore, in this study, starting from analysis of the features of shale gas reservoirs of Wufeng Formation-Longmaxi Formation, we investigated the hydrocarbon generation and accumulation mechanisms of the Wufeng Formation-Longmaxi Formation shale and the relationship between them, and main factors affecting hydrocarbon generation and accumulation in shale formations of different depths of deep-water shelf facies through physical modeling, in the hope to find out the inner mechanism of shale gas accumulation, stable and high production of deep high quality shale (at the depth of 4 000-5 000 m) with high thermal evolution degree.

The Sichuan Basin is located in south-western China, and can be divided into six structural belts, namely, low and steep structural belt in western Sichuan, low and gentle structural belt in northern Sichuan, gentle structural belt in central Sichuan, low and steep structural belt in south-western Sichuan, high and steep structural belt in eastern Sichuan, and low and steep structural belt in southern Sichuan. Shale gas fields such as Fuling, Weiyuan, Weirong, Changling and Zhaotong discovered in Sihuan Basin and its surrounding areas are mainly distributed in high and steep structural belt in eastern Sichuan, low and steep structural belt in southern Sichuan and low and steep structural belt in south-western Sichuan (Fig. 1). The Upper Ordovician-Lower Silurian organic-rich shale (TOC> 0.5%) mainly occurs in Wufeng Formation-1^st member of Longmaxi Formation (hereinafter shortened as Long 1 Member) shelf sediments, while high quality shale (TOC>2%) is in Wufeng Formation-Long1 Member, which is deep shelf sediment with a thickness of 20-40 m, and is mainly distributed in south-western part, southern part and eastern part of Sichuan Basin. The shales include two types, calcium-bearing siliceous shale and siliceous shale, of which, the former is mainly distributed in southwestern Sichuan Basin and Weiyuan-Changning-Renhuai-Lintanchang in southern Sichuan Basin, while the latter in the areas north of Jiaoshiba in south-eastern Sichuan Basin and north-eastern Sichuan Basin.

The Wufeng-Longmaxi Formation shale deposited in deep-water shelf has high TOC and silicon content, with TOC of 1.04%-5.89%, 3.50% on average, and silicon content of 31.00%-70.60%, 44.57% on average (Fig. 2). The TOC and silicon content show obviously positive correlation, and the high quality shale sections have coupling of high TOC and high silicon content (Fig. 3). There are two reasons: one is that the high quality shale has abundant biological fossils such as grapholites, silicon radiolarians and sponge bone needles etc., and high organic productivity, suggesting the quiet, anoxic and strong reducing deep-water shelf environment is favorable to enrichment and preservation of organic matter, and thus has obvious control on the enrichment and preservation of organic matter; the other side is that the large amount of radiolarians and sponge bone needles can form endogenous high silicon-content minerals^[1].

In shallow-water shelf, due to sufficient supply of terrestrial debris, the organic matter would be destructed or diluted, so the shale of this facies shows low TOC and silicon content on the whole (Fig. 2), and the silicon is dominated by terrestrial quartz debris, so the silicon content and TOC have poor correlation (Fig. 3).

Kerogens of different geneses have different hydrocarbon- generation ability and degradation products^[16]. The organic matter of Wufeng-Longmaxi Formation has mainly Ⅰ-Ⅱ₁ type kerogens, and the shale has high TOC and high thermal evolution degree with R_o of 2.2%-3.4%^[17,18,19]. According to the hydrocarbon-generation theory, simulation by using TOC, thickness and R_o of high quality shale, the Wufeng-Longmaxi Formation shale has high hydrocarbon-generating rate. Taking Well JY1 for example, the shale of shallow-water shelf facies is 51 m thick, and has a hydrocarbon-generation intensity of 25×10⁸ m³/km², while the high quality shale of deep-water shelf, 38 m thick, has a hydrocarbon-generating intensity of 35×10⁸ m³/km² and huge hydrocarbon-generation potential^[14].

Comparing δ¹³C of natural gas and kerogens of Wufeng Formation-Longmaxi Formation in Fuling Shale Gas Field with those of kerogens in other formations of Sichuan Basin shows that the natural gas in Wufeng Formation-Longmaxi Formation of Fuling Gas Field comes from in-situ source rock^[13]. The gas of Fuling Gas Field has entire reversal of C isotope values, namely, δ¹³C₁>δ¹³C₂>δ¹³C_3, suggesting that the natural gas is a mixture of gases generated by the same source rock in different geological times^{[20,21,22,23]}.

As Wufeng-Longmaxi Formation experienced three stages of evolution, early continuous burial and hydrocarbon-generation, cracking gas generation at maximum burial depth, and reservoir destruction and adjustment in the late stage (Fig. 4), Devonian shale samples from Luquan county, Yunnan province with TOC of 2.64%, Ⅱ₁ kerogen and R_o of 0.48% similar to the Wufeng Formation-Longmaxi Formation deep-water shelf shale was taken to model hydrocarbon-generation course in the key oil & gas national laboratory of Wuxi Petroleum Geological Research Institute of Petroleum Exploration & Development Institute of SINOPEC. The principles and apparatus of the modeling experiment are elaborated in reference [24]. The modeling results show that cracking of kerogen to gas has three peaks at the R_o of 1.0%, 2.1% and 3.2% respectively. R_o 1.0% is the peak of oil generation (Fig. 5a), while organic matter is pyrolyzed into oil, part of little molecules with weak active energy fall off to generate gas. When R_o is 2.1%, due to high energy by pyrolysis, the organic matter has the strongest gas-generating capacity, then because of polycondensation of organic matter, solid bitumen is produced, and gas-generating capacity decreases. When R_o is more than 3.2%, due to the underground water, some hydrocarbon gas is generated, but from the trend of total hydrocarbon-generating rate (Fig. 5b), the amount of gas generated in the late stage is small. The total generating capacity of kerogen is 2 m³/t rock, residual oil makes great contribution to gas generated in high and over mature stage, the retained oil in shale with oil expulsion efficiency of 68% (original TOC 2.64%) during the oil generation peak has the highest gas generating capacity of 6 m³/t.

Based on results from the experiments, the proportions of gas cracked by kerogen and gas cracked by residual oil in different evolution stages were calculated (Fig. 5b). After the stage of high- over-maturity, gas cracked by residual oil makes up 70%, and gas cracked by kerogens 30%.

The roof and floor beds of shale gas reservoir feature great thickness, stable distribution, tight lithology, high breakthrough pressure and good sealing capacity^[21]. The roof bed of shale gas reservoir is grey-dark grey medium-thick siltstone and muddy siltstone of 2^nd member of Longmaxi Formation about 50 m thick; the floor bed is dark grey muddy nodular limestone and limestone etc. of Upper Ordovician Linxiang Formation about 30-40 m thick in total, which is distributed stable regionally. The siltstone of 2^nd member of Longmaxi Formation in Jiaoshiba area has an average porosity of 2.4% and average permeability of 0.001 6×10^-3 μm². Testing results by special petrophysical laboratory of national key laboratory of Chengdu University of Technology with TS-100 Breakthrough Pressure Instrument under the temperature 80 °C show the breakthrough pressure of the roof bed is 69.8-71.2 MPa. The floor Linxiang Formation has an average porosity of 1.58%, average permeability of 0.001 7×10^-3 μm², and a breakthrough pressure of 64.5-70.4 MPa at the temperature of 80 °C. High breakthrough pressure of roof and floor beds is crucial for the accumulation of shale gas^[17]. The sealing of roof and floor beds is favorable for crude oil generated in early stage to be retained in great amount, and is also favorable for the retention and preservation of gas cracked in late stage. Based on many experiments on measuring isotopes of alkane gas such as methane and ethane etc. of mixed two kinds of end-element gases, the kerogen-cracking gas and oil-cracking gas, at various proportions in various evolution stages and theory calculation of C isotope of composition fractionation, a template to forecast values of C isotope of alkane gas of the mixed two kinds of end-element gases at any evolution stage^[23] has been made. Conversely, the thermal maturity and mixture proportion can be forecasted by measuring the carbon isotope of methane and ethane in the two kinds of gas to judge whether there is cracked gas from crude oil or not. By putting current isotope compositions of methane and ethane of shale gas in Jiaoshiba Area in the quantitative identification template of kerogen-cracking gas and oil-cracking gas, it is found that the oil-cracking gas (residual oil) accounts for 60%-80% (Fig. 6), which proves further the reliability of the modelling experiments.

Generally, plankton with rich silicon mainly includes silicon algae, dinoflagellate, radiolarian, and sponge etc., their siliceous shells are made of opal A, which is a kind of non- crystalline mineral with amorphous structure^[25]. During early diagenesis, the silicon filled inside the planktons would be dissolved ahead of siliceous shell to form multiple pore structure, while opal A of siliceous shell would transform into crystalline structure with high hardness. According to former studies, the biogenesis siliceous shale formations in current or ancient deposits in the oceans have pores of 5-10 nm in average diameter from sedimentation and diagenesis, and porosity of up to 35%-50%^[25].

High quality shale of Wufeng-Longmaxi Formation developing in deep-water shelf environment has a great number of biological fossils such as grapholites, foraminifera, radiolarians and spongy bone needles etc. and high silicon content. Moreover, its organic matter content and silicon content are in positive correlation, and on the triangle diagram of Al-Fe-Mn, most of the measured values fall in biogenesis area (Fig. 7a), so it is concluded that the siliceous minerals of high quality shale of Wufeng-Longmaxi Formation deposited in deep- water shelf are largely biogenesis silicons^[1]. Analysis show that the porosity and content of siliceous minerals of Wufeng-Longmaxi shale samples from Well DYS1 have a positive correlation (Fig. 7b), so, it is inferred that biogenesis silicon content is an important factor controlling the development of pores in high quality shale.

As the depth increases, temperature, pressure and thermal maturity increase quickly, before hydrocarbons are generated at great scale (R_o<1.3), the opal would be transformed into quartz micro-crystals, and quartz microcrystalline grains would constitute a large number of hard framework pores. Organic matter with low hardness of 1.5-3.0 in general is easy to be compacted, whereas quartz with a hardness of up to 7.0 is difficult to be compacted, so the framework pores of quartz grains can be preserved easily. On one hand, unstable biogenesis silicon unstable in structure in the early stage (opal) in the deep-water shelf shale would be transformed into crystalline quartz in plaque and lump through continuous dehydration (Fig. 8), at the same time, a great number of hard quartz intergranular pores with strong anti-compaction capacity would be formed (Fig. 9a); whereas shale of shallow-water shelf facies has no rigid quartz intergranular pores and has weak anti-compaction capacity (Fig. 9b). On the other hand, the silicon released during the course of erosion of biogenesis silicon and clay minerals would recrystallize into authigenic quartz in shapes of flake, ovum and ellipsoid to form clay mineral intergranular pores with relatively high hardness (Fig. 9c, 9d). Therefore, the shale with biogenetic silicon has abundant pores in net structure and high original porosity (Fig. 9e, 9f and Table 1), providing space for crude oil charge, and growth and preservation of organic pores. Pores in quartz sandstone in many petroliferous basins are preserved well soundly prove the view too^[26,27].

Hence, it is considered that during the course of diagenesis, the transformation of structure of minerals in the inner part of siliceous living organisms such as radiolarians and spongy bone needles is an important reason for the high porosity of high quality shale, and is also the key for the preservation of massive intergranular pores and organic pores.

When hard quartz intergranular pores are formed at the early stage of diagenesis, the thermal maturity of shale increases gradually, then when the shale enters into oil-generation stage, siliceous mineral framework has high resistance to compaction (Fig. 9a), the crude oil generated can be stored in the intergranular pores supported by siliceous framework; as the burial depth increases, thermal maturity of the shale increases further, the crude oil and kerogens remaining in the framework pores enter into the stage of cracking gas generation stage, nanometer scale organic pores begin to develop and be preserved. Clearly, the coupling relationship of hard quartz intergranular pore of deep-water shelf shale and the oil generating stage of organic matter results in high porosity and high specific area of the high quality shale of Wufeng-Longmaxi Formation (Fig. 10), which is favorable for shale gas accumulation and adsorption, thus the shale shows high gas content.

In contrast, the clay shale of shallow-water shelf facies is undeveloped in quartz intergranular pore and highly compacted later, all these are not conducive to crude oil charge and formation and preservation of organic pores (Fig. 9b). Consequently, the clay shale of shallow-water shelf facies has much lower porosity and specific surface area (Fig. 9), which aren’t favorable for shale gas accumulation and adsorption, so this kind of shales show much lower gas content.

Apparently, biogenesis silicon is the key for development and preservation of organic pores.

Observations of thin sections and SEM pictures show the high quality Wufeng-Longmaxi shale of deep-water shelf facies has good homogeneity, has no obvious laminar heterogeneity like that in Wufeng-Longmaxi Formation shale layers of shallow-water shelf facies, Permian Longtan Formation and Jurassic, and has organic matter and siliceous minerals in scattered distribution (Fig. 11a, 11b). The high silicon content in the siliceous shale increases the brittleness of the rock (Table 2), so during fracturing, this shale is more likely to form complicated and net-like pore-fissure system. Furthermore, the organic matter pores in intergranular pores are mostly nanometer scale and in nearly circular, oval, pit, meniscus and narrow fissure etc. shapes under electronic microscope, more commonly in cell-like irregular oval shape with good connectivity between each other (Fig. 11c, 11d).

In summary, siliceous mineral framework pores and organic pores coexist in Wufeng-Longmaxi shale of deep-water shelf facies.

Siliceous minerals in shale have high elastic modulus, low Poisson’s ratio, and good brittleness, which are conducive to the formation of reticular fracture network. In contrast, clay minerals and organic matter have low elastic modulus, high Poisson’s ratio, poor brittleness, and thus poor fracability (Table 2).

During fracturing, brittle siliceous mineral grains will be fractured open first to form net-like fissure connecting the scattered and isolated quartz intergranular pores, while the organic pores, clay mineral pores and siliceous mineral framework pores are paragenetic, and organic pores and clay mineral pores have good connectivity, thus the fractured net fissures can connect with organic pores and clay mineral pores that store shale gas, forming channels of organic pore-intergranular pore-micro-fissure (Fig. 12) favorable for high production of shale gas.

Hence, though the high quality shale of Wufeng-Longmaxi Formation with large depth (more than 4000 m) in China has higher thermal maturity than shale in medium-shallow depth (They have undergone similar maximum depth in geological history, but structural uplifting and erosion in the later stage lead to the current differences of them in depth), due to the hard biogenesis siliceous framework, the high quality shale with larger depth can maintain high porosity and high gas content, which are favorable for shale gas accumulation. The deep shale gas of Wufeng-Longmaxi Formation (at the depth of 4 000-5 000 m) has resources of over 7×10¹² m³ and great exploration potential.

High quality shale of Wufeng-Longmaxi Formation at high thermal maturity has high hydrocarbon generation intensity and good material base. Good roof and floor conditions kept a large amount of crude oil generated early retained in the shale, and the gas from cracking of crude oil provides sufficient gas source, laying solid material foundation for shale gas reservoir with high abundance.

Biogenesis silicon (opal A) of Wufeng-Longmaxi Formation in deep-water shelf facies transformed into crystalline quartz with high hardness at the early stage of diagenesis, at the same time a lot of micro intergranular pores were formed. Furthermore, the biogenetic siliceous framework has strong resistance to compaction, which not only provides space for crude oil charge at the early stage but also conditions for development and preservation of cell organic pores, so it is the key for the preservation of organic pores.

Wufeng-Longmaxi siliceous shale of deep-water shelf facies has high brittleness, strong homogeneity, paragenetic micron scale siliceous intergranular pores and nanometer scale organic pores, which are the keys to form complicated net fissures by fracturing to connect effectively micron scale siliceous intergranular pores and realize the connectivity of siliceous intergranular nanometer scale organic pores, and high and stable production of shale gas.