Microfractures in black shale are not only good for the accumulation of free gas and desorption of adsorbed gas^[1], but also the important paths of gas migration and seepage^[2]. In the reservoir fracturing, microfractures can reduce the fracture initiation pressure^[2], form artificial fracture net and enlarge the whole fracture volume, thereby increase well production and ultimate recovery^[3]. The Ordovician Wufeng Formation- Silurian Longmaxi Formation in south China are rich in shale gas resources, where the microfractures of “sweet spot areas” control the “artificial gas reservoir” effect and well production^[4,5]. The formation of microfractures in shale is controlled by regional tectonic activity^[6], composition^[7] and physical property of shale^[8] and sedimentary diagenesis^[9]. In the same regional stress setting, the contents of organic matter, quartz, feldspar and carbonate are the important factors affecting mi-crofracture development^[10,11]. At present, studies on microfractures of Wufeng-Longmaxi formations in south China are concentrated on the extension laws of hydraulic fracture^[12], stress sensitivity of fracture^[13], extension laws of microfracture^[14], microfracture distribution^[15] and so on, but there is no systematic knowledge of microfracture types, vertical distribution laws, main controlling factors and genetic mechanisms of microfractures. This study takes the black shale of Wufeng-Longmaxi Formations of Shuanghe profile in the Changning County as the example, analyses the microfracture types and vertical distribution laws, discusses the main controlling formative factors and genetic mechanisms, to provide theoretical foundation and practical evidence of marine shale gas enrichment mechanisms and “sweet spot areas” optimization of shale gas.

The outcrop profile of Shuanghe of Ordovician Wufeng Formation-Silurian Longmaxi Formation is located in south Sichuan Basin (Fig. 1, left), the Wufeng Formation-Longmaxi Formation expose completely, with clear top and bottom interfaces. The Wufeng Formation is in parallel unconformable contact with the underlying Baota Formation (Fig. 1, right), and the Longmaxi Formation is in conformable contact with the underlying Wufeng Formation. Wufeng-Longmaxi Formations of Shuanghe profile are big series of black carbonaceous shale, black shale and dark grey silt shale containing rich graptolite fossils and organic matter. The graptolite fossils and organic matter reduce gradually from Wufeng Formation bottom to Longmaxi Formation top. By means of comprehensive research of outcropping sequence stratigraphy and graptolite fossil stratigraphy, it is found that the WF1-WF4 graptolite fossil zones of Wufeng Formation and LM1 and LM2 graptolite fossil zones of lower Longmaxi Formation of the Shuanghe profile are complete, with one or more quasi-sequence group in each graptolite fossil zone.

The Wufeng-Longmaxi formations in south Sichuan Basin were formed in the period when Nanhua basin was extinct and Nanhua orogenic belt developed, and Yangtze platform entered the tectonic evolutionary stage of foreland basin and was in the semi-euxinic stagnant marine basin sedimentary environment; the basin base was high in southeast and low in the northwest, and the marine areas turned deeper gradually from southeast to northwest^[5].

The samples were taken from SWF4-WF8 members of Wufeng Formation and SLM1-SLM5 members of Longmaxi Formation on the Shuanghe profile. Continuous rock samples of 14 meters, holographic photographs statistics on tensile and microcracks of 203 large thin slices and microcrack of 203 small thin slices, TOC content of 110 samples, whole rock X-ray diffraction analysis of 110 samples, and major element and trace element test of 103 samples were done (Fig. 2). The outcropping rocks of SWF1-SWF2 members were too fragmented, so only the top member was sampled. SLM5 member on the top of Longmaxi Formation and the strata above with very low organic content are not shale gas production layers, so they were not sampled. In the process of sampling, two outcropping rock samples of 14 meters long 10 cm wide and 5-10 cm thick were cut from SWF2 to SLM2 member continuously with cutting machine, and labeled with sequence, direction and length in the same way as drilling cores, and then, one of them was polished into 7 cm wide by 5 cm thick big sample for direct observation. Two hundred and three (203) standard large thin sections of 5 cm by 7 cm and 203 standard small thin sections of 1cm by 1 cm were cut from the other sample at equal intervals and one-to-one correspondence. The large thin sections were observed with German Leica4500P high-precision microscope, and the small thin sections ordinary optical microscope. One hundred and ten (110) specimens for TOC content test and whole rock X-ray diffraction test each, and 103 specimens for major element and trace element test were then taken. The mass of specimens for TOC test, whole rock X-ray diffraction analysis and major element and trace element test were more than 100 g, 20 g, and 200 g respectively. The TOC content test, whole rock X-ray diffraction test and major element and trace element test were done on TOC analysis machine, X-ray diffraction machine and element detection machine respectively. All above tests were completed in the National Energy Shale Gas R&D (experiment) Center.

The microfracture research had two stages, namely, the precise measurement of the large rock sample and the collection and analysis of large thin sections. In the precise measurement of the large rock sample, the rough large sample was cut into a cuboid 7 cm wide by 5cm thick in perpendicular to the bedding plane, then the surface of the cuboid was sanded with emery of Grade 120, 320 and 400 to the particle diameter of 0.080 mm, 0.050 mm and 0.032 mm, and then sanded with micro-powder of 7 μm and 3 μm to grain sizes of 0.031 mm and 0.030 mm further, finally the surface of the large rock sample was polished by polishing agent on the machine. After the preparation, the large rock sample was continuously scanned by Itrax core XRF scan machine, and the scanning pictures were spliced and processed by Adobe Photoshop CS5 or advanced version in appropriate and consistent brightness and contrast. Finally, based on the sliced pictures, the types and number of microfractures of target layers from bottom to top were counted.

Whole section data collection and splicing and fracture research was conducted on the 203 large thin sections. The whole section data collection was done with 20 by 10 times lens of German Leica4 500 P microscope precise digital platform. Each 5c m by 7 cm large thin section was divided into 645 lines by 460 rows, S-shaped image collection was done by using orthogonal light, and 3200 images were captured from each thin section. The diagonal line (or cross line, or groined line, or gridding) sight was selected to focus and the focal length (Z) was recorded by digital platform automatically with the auto-focusing function in the process of image acquisition. Finally, the 3200 pictures were spliced fracturelessly on the advanced configuration work station by using Photoshop to put the images of whole section together. After the images of all the large thin sections were spliced, the types and characteristics of microfractures were observed and number of different types of microfractures were counted, and based on the TOC and whole rock X-ray diffraction data, the main factors controlling formation and genetic mechanisms of different microfractures were analyzed.

According to the location relationship between the microfracture and bedding surface, the microfractures of Wufeng-Longmaxi Formations can be divided into bedding fracture and non-bedding fracture. The bedding fractures are parallel to the bedding plane or have an angle of less than 15° with the bedding plane, are linear or en echelon distribution, and filled with silicic matter, including bedding-plane slip fractures (Fig. 3a), lamellation fractures (Fig. 3b, 3e) and structural en echelon fractures (Fig. 3c). The bedding-plane slip fractures appear in the fine shale in bundles, and are more than 2 mm long. Occurring at the lamella interfaces, the lamellation fractures are mostly straight and intermittent and more than 2 mm long. Appearing in fine shale, the en echelon fractures are generally less than 2 mm long and in en echelon arrangement. Non- bedding fractures intersect with (Fig. 3d) or are perpendicular to the bedding plane (Fig. 3b), and are filled with silicic matter, and they can be divided into shearing fractures (Fig. 3d) and extension fractures (Fig. 3b, 3f) according to their shapes. The shearing fractures are more than 2 mm long and 15°-20° in dip angle, while the extension fractures length are more than 2 mm and nearly perpendicular to the bedding plane. The bedding fractures of Wufeng Formation are mainly bedding-plane slip fractures and en echelon fractures, while those of Longmaxi Formation are largely lamellation fractures. The non-bedding fractures of Wufeng- Longmaxi Formations often cut through the bedding fractures (Fig. 3b, 3f) or change dip angle at the bedding fractures.

On the Shuanghe profile, Longmaxi Formation has higher density of microfracture, while Wufeng Formation has lower density of microfracture (Fig. 4). Vertically, Longmaxi Formation has 0-20 bedding fractures and 0-43 non-bedding fractures per millimeter, with an average density of 0-57 microfractures each millimeter. In comparison, Wufeng Formation has 0-10 bedding fractures and 0-13 non-bedding fractures each millimeter, and a total average density of 0-15 microfractures each millimeter. In the Longmaxi Formation, SLM1 member has the highest microfracture density, (Fig. 4), more specifically, it has an average density of bedding fractures, non-bedding fractures and total fractures of 10, 14 and 25 fractures per millimeter respectively. SLM2 member has more bedding fractures, with an average 6.6 bedding fractures each millimeter, but fewer non-bedding fractures, to be specific, 2.6 non-bedding fractures each millimeter, and total average density of 9 microfractures per millimeter. The microfracture density of SLM3-SLM5 members becomes lower gradually, with an average bedding fracture density of 1.6 per millimeter, an average non-bedding fracture density of 4.2 per millimeter, and total average fracture density of 5.8 per millimeter. In the Wufeng Formation, the microfracture density of SWF4 member is the highest, followed by SWF5-SWF6 members, SWF7-SWF8 members are the lowest. The SWF4 member has a density of bedding fractures, non-bedding fractures and total fractures of 2.7, 3.8 and 6.5 per millimeter respectively. For SWF5-SWF6 members, the values are 3.7, 2.2 and 6 per millimeter respectively.

SLM1-5 members differ widely in microfracture characteristics. SLM1 member has rich bedding fractures and non- bedding fractures, which form a microfracture net in space (Fig. 5a); SLM2 member has rich bedding fractures too but much fewer non-bedding fractures (Figs. 4 and 5b); SLM3-SLM5 members have even fewer bedding fractures and non-bedding fractures (Fig. 5c), and fewer fractures in total (Fig. 4).

The TOC test results of 110 shale samples from Wufeng- Longmaxi Formations on the Shuanghe profile show (Fig. 6): the samples have TOC values between 1.7% and 12.0%, and an average value of 5.6%. Longmaxi Formation has higher TOCs from 3.9% to 12.0%, and an average of 7.9%. Wufeng Formation is lower in TOC, from 1.7% to 8.3%, and 4.9% on average. In the Longmaxi Formation, SLM1 member is the highest in TOC, with a range of 5.6%-12.0% and an average of 9.6%; SLM2-5 members decrease gradually in TOC, with a range of 3.9%-7.6% and an average of 5.2%. In the Wufeng formation, from SWF4 member to SWF6 member, the TOC value increases from 1.7% to 8.3%, the TOC value of SWF4 member is the highest, after SWF7 member, the TOC value decreases gradually and falls to 2.0% at top of the Wufeng Formation.

There is a positive correlation between the TOC values and the density of fractures of Longmaxi Formation, but a negative correlation of those of Wufeng Formation (Fig. 7). In the Longmaxi Formation, when the TOC values are less than 2%, the fracture density is zero, when the TOC values are more than 2%, the higher the fracture density, the higher the TOC value, when the TOC value is up to 9%, the fracture density peaks at 60 fractures each millimeter. In the Wufeng Formation, the TOC value is 2%-8%, the fracture density is 0-12 fractures each millimeter, the density decreases obviously with the increase of TOC value.

The identification and holographic photograph results of 203 large thin sections show (Fig. 8): on the Shuanghe profile, the black shale beddings of Wufeng-Longmaxi Formations can be divided into unclear bedding, obscure bedding, relatively clear bedding and clear bedding according to clarity^[16]. The massive unclear beddings (Fig. 8a) occurring at the top of Wufeng Formation, are composed of medium granular mudstone and coarse mudstone, and blocky inside. The obscure beddings (Fig. 8b) appearing in the middle part and bottom of Wufeng Formation, are made up of medium granular mudstone, the inside lamella interfaces are obscure, and tabular, continuous and parallel, multiple lamellae vertically can constitute a normal grading sequence, reverse grading sequence or homogeneous sequence^[17]. The relatively clear beddings (Fig. 8c), distributed at the bottom of Longmaxi Formation, are comprised of fine-grained mudstone, there are interlayered dark lamellae and light lamellae, the lamella interfaces are clear relatively, the lamellae are broken, tabular or parallel, the dark lamellae are thicker, which can constitute normal grading sequence, reverse grading sequence or homogeneous sequence; the light lamellae are thinner. The clear beddings (Fig. 8d) are found at the middle and top of Longmaxi Formation, there are interlayered dark lamellae and light lamellae, the light lamellae are much thicker than those in relatively clear beddings, the interfaces are clear, the lamellae are tabular or wavy, continuous, parallel, the interfaces are usually in abrupt contact.

Beddings of different types are different in microfracture density (Fig. 9). The relatively clear beddings have the highest microfracture density of 12.4 microfractures each millimeter on average. Clear beddings come second with a microfracture density of 8.6 microfractures each millimeter. Massive unclear beddings decrease further in microfracture density, with 5.2 microfractures each millimeter. Obscure beddings are the lowest with 4.6 microfractures per millimeter. Because of the differences of mechanical properties in the layered media, shear strain is likely to occur at the bedding interfaces, which causes the extension and deflection of fractures, the larger the shear deformation at the interface, the bigger the deflection angle of microfractures will be^[18].

The results of whole rock X-ray diffraction test of 110 shale samples from the Wufeng-Longmaxi Formations of Shuanghe profile show (sheet 1): the samples have silicon contents from 14% to 75%, on average 42.5%; carbonate (calcite and dolomite) contents from 5.4% to 72.5%, on average 37.0%; clay mineral contents from 7% to 32%, on average 20.5%; and feldspar contents from 1% to 9%, on average 2.95%. The shale samples of Longmaxi Formation have silicon contents from 32% to 75%, on average 57%; carbonate contents from 5% to 49%, on average 22%; and clay mineral contents between 13% and 31%, on average 21%. The shale samples from Wufeng Formation have silicon contents from 14% to 71%, on average 37.5%; carbonate contents from 14% to 73%, on average 42.6%; and clay mineral contents from 7% to 32%, on average 19.9%. Compared with the Wufeng Formation, the Longmaxi Formation shale has higher silicon content, lower carbonate content and similar clay mineral content.

In the Jiaoshiba shale gas field in Sichuan Basin, Wufeng- Longmaxi Formations shale has a silicon content of 44.4%, carbonate content of 9.7% and clay mineral content of 34.6%^[19]. In the Weiyuan shale gas field, Wufeng-Longmaxi Formations shale has a silicon content of 49%, carbonate content of 2.5%, and clay mineral content of 24%_. The Barnett shale in Fort Worth Basin of North America has a silicon content of 45%, carbonate content of 8% and clay mineral content of 27%^[2]. Compared with Jiaoshiba, Weiyuan and Barnett shale, Wufeng-Longmaxi formation shale on Shuanghe profile has similar silicon content, but higher carbonate content and lower clay mineral content.

In the Longmaxi Formation, black shale of SLM1 member has the highest silicon content of (Fig. 10) 63% on average and the lowest carbonate content of 14% on average. With silicon content decreasing and carbonate content increasing gradually, the SLM2-SLM5 members have an average contents of silicon and carbonate of only 47% and 34%. In the Wufeng Formation, the lower part of SWF5 member has the highest silicon content of 71% and the lowest carbonate content of 14%, and the silicon content drops while the carbonate content rises towards upper and lower strata. According to the classification standard of Lazar (2015)^[20], the shale grain size was preliminarily identified by scratch method, the lithology of Wufeng Formation SWF4 member and Longmaxi Formation SLM1 member in the Shuanghe profile is mainly fine shale, other members are medium and coarse shale.

In the Longmaxi Formation and Wufeng Formation, the microfracture density of black shale is in positive correlation with silicon content (Figs. 11 and 12), and negative correlation with carbonate content. In the Longmaxi Formation, when the silicon content is lower than 30%, the microfracture density is zero, when the silicon content is higher than 30%, the higher the silicon content, the higher the microfracture density, and when the silicon content is higher than 70%, the microfracture density reaches 60 fractures each millimeter. When the carbonate content is 10%, the microfracture density is 60 fractures each millimeter, when the carbonate content is 50%, the microfracture density is almost zero. In the Wufeng Formation, when the silicon content is lower than 30%, the microfracture density is zero, when the silicon content is higher than 30%, the microfracture density is 1-13 fractures each millimeter; the carbonate content is between 30% and 50%, and the microfracture density is 0-14 fractures each millimeter.

For the black shale of Wufeng-Longmaxi Formations, the finer the shale grain size, the higher the microfracture density (Fig. 9). The fine shale has the highest microfracture density of 22.5 fractures each millimeter on average, followed by medium shale of 6.5 fractures each millimeter, and coarse shale of 5.5 fractures each millimeter respectively.

The silicic minerals of Wufeng-Longmaxi Formations in the Sichuan Basin are mainly biogenic, and a small amount is terrigenous detrital minerals. The major element and trace element tests of 103 samples show that there is a negative correlation between the content of Zr and SiO₂ of Longmaxi Formation (Fig. 13), the Wufeng Formation also has this negative correlation on the whole, only a few samples in the SWF7-SWF8 members show positive correlation between the content of Zr and SiO₂. The mineral Zr in the silicate sediments mainly comes from orogeny in continental margin, and there is often a positive correlation between the content of Zr and SiO₂ in the terrigenous sediments. In view of the negative correlation in Wufeng-Longmaxi formations, it is inferred that the silicic minerals formed in this period are mainly biogenic. Meanwhile, previous researches of the occurrence state of quartz, statistics on trace elements and excess silicon content in thin rock slices also support this view^[21,22], the silicic organisms like siliceous sponge and radiolarian which lived in the water in the sedimentary period died and stacked massively^[16], forming black shale with high silicon content and high TOC content.

The microfracture density is dependent upon the biogenic silicon content, the higher the biogenic silicon content, the higher the microfracture density. There is a positive correlation between microfracture density and silicon content of black shale in the Wufeng-Longmaxi Formations, the higher the silicon content, the higher the microfracture density. The silicic minerals of the Barnett shale of Mississippian in the Fort Worth Basin of America and the Woodford shale of Devonian in the Arkama passive continental margin basin are all biogenic origin^[7], and these shales are both well-developed in microfracture nets; while the silicic minerals of carbonaceous shale of Devonian Ohio Formation in the Appalachian Basin and black shale of Devonian Antrim Formation in the Michigan Craton Basin are terrigenous detrital minerals, so these shales only have high-angle natural fractures but few microfractures. Three factors are behind the situation. The first one is that the biogenic silicate has rich matrix pores and high organic matter content (Fig. 14), the organic matter would transform during the diagenetic evolution, which results in the increase of porosity^[23], and drop of the compressive strength and tensile strength of rock at the same time, that is good for the development of microfracture^[11,24]. The second factor is that the large number of organic matter in biogenic silicic matter would generate hydrocarbons in the diagenetic evolution, leading to increase of pore pressure, when the pressure exceeds the tensile strength of the rock, a great number of microfractures would occur. The third factor is that under the same stress conditions, silicic shale has lower Poisson’s ratio, higher Young’s modulus and lower tensile strength, so it is easier to generate the microfractures^[25]; moreover, the organic shale has low cohesive force and angle of internal friction, under the regional horizontal compression or tensile stress, so it is more likely to break along the bedding surface, giving rise to low angle slippage fractures^[26]. Meanwhile, the black shale rich in silicon is more brittle than the shale rich in calcite^[27], provided that the total content of minerals is constant, if the calcite content is high, the silicon content will be low, and the microfracture density will decrease.

The lamellation surface in the fine shale is the key position of microfracture development. The finer the grain size of black shale, the higher the clay mineral content, and the lower the contents of brittle minerals like quartz, feldspar and carbonate are, under the regional horizontal compression or tensile stress, shear fracturing is more likely to occur along the lamellation surface, giving rise to low angle slippage fractures. The shale strata can be divided into seven units, namely, lamella, lamina set, bed, bed set, quasi-sequence, systems tract and sequence^[28]. Because of the differences in lithology, structure and texture above or below the stratigraphic unit interfaces, there are mechanical weakness interfaces. Under the external forces or in the diagenetic evolution, stress concentration could occur above and below the interfaces of the stratigraphic units, so peeling off is likely to happen along the interfaces, forming lamellations or bedding fractures^[24]. Compared with the interfaces of lamella and lamina set, the mechanical properties of bedding interfaces are weaker, so bedding surfaces are more likely to become key interfaces of microfracture development. The interlayer lamellation fractures in Wufeng-Longmaxi Formations are generally small in aperture and mostly filled by silicic matter completely.

Exploration practice reveals that the SLM1 member of Longmaxi Formation in Changning area features abundant lamellations and nanometer pores, high content of TOC and brittle minerals, higher gas content and physical property, and well-developed microfracture net ^{[4, 29]}, so it is the “sweet spot segment” of shale gas exploration and development in this area.

The regional tectonic activities are the key factor controlling the formation of microfractures in the “sweet spots segments” of the shale reservoir of Longmaxi Formation. Previous studies show in the Changning area, the shale of Longmaxi Formation has experienced the Caledonian, Hercynian, Indo-Sinian, Yanshanian epoch and Himalayan tectonic movements, and thus the strata have suffered multiphase compression, uplifting, denudation and deformation^[30]. On the one hand, in the period of structural compression, the tectonic stress would generate a series of large vertical fractures and oblique fractures, as well as a large number of bedding fractures, non-bedding fractures, sliding fractures and en echelon fractures, forming a relatively developed fracture network. On the other hand, during the period of uplifting and denudation, due to the poor seepage of pore fluid in the shale, part of the original pressure was preserved. In the process of rock strata denudation and pressure unloading, rock fluid rebounded, causing pressure changes, which is directly related to the mechanical properties of rock and fluid^[31]. Compared with sandstone, shale has larger compressibility factor, especially in the middle and late diagenetic stage, compared with the early diagenetic stage, the shale has weaker rebounding ability. Large compressibility factor and weak rebounding ability result in the preservation of the original strata pressure after uplifting and denudation, so abnormal pressure is likely to occur in the space, which is conducive to the formation of microfractures.

Diagenetic shrinkage is an important force for the formation of lamellation fractures. In the early stage of diagenetic evolution, due to dehydration, shrinkage and phase transformation of minerals, the volume and structure of rock would change, thus giving birth to diagenetic shrinkage fractures. The diagenetic shrinkage fractures usually develop along the lamella surface and are filled or semi-filled with late silicic matter. In the late stage of diagenetic evolution, the regional tectonic extension caused the development of tension fractures in Wufeng-Longmaxi Formations nearly vertical to the bedding plane, the non-bedding fractures to cut through the bedding fractures and to be filled with silicic matters in late stage.

Pressurization caused by hydrocarbon generation and strong diagenetic shrinkage are the main reasons for the development of large number of microfractures in the “sweet spots members” of shale reservoir. There are evidences of four aspects: (1) The black shale of the “sweet spot member” has high TOC value of 1.9% on average, R_o of 2.5%-3.98%^[4] which means the shale has reached mature-over mature stages, so the shale has the material base and conditions to generate large amounts of hydrocarbon; (2) Black shale of “the sweet spot member” has abundant lamellations, and vertical permeability much lower than lateral permeability^[32], which can effectively seal the hydrocarbon substance; (3) In the peak period of hydrocarbon generation, the black shale in the “sweet spot section” was still in deep burial, and the unexposed strata nearby show significant overpressure, there is a positive correlation between the pressure and TOC content and gas content of shale^[33]; (4) Microfracture density is positively correlated with TOC and silicon content, and the TOC content is also positively correlated with the silicon content (Fig. 14) but negatively correlated with the carbonate content, from the SLM1 member to the SLM5 member, as the TOC and silicon content decrease gradually, the microfracture density also reduce accordingly.

During the burial process of organic matter, as the buried depth and strata temperature increase, the strata pressure increases due to the hydrocarbon generation of organic matter^[34]. At the same time, because of the poor physical properties of the upper and lower strata^[35], oil and gas can’t migrate out or fully migrate out of the formation, leading to abnormal high pore fluid pressure. On the one hand, the abnormal high pore fluid pressure reduces the friction coefficient between the shale particles and rock strength, on the other hand, changes the effective stress field when rock breaks, prompting the fracture. Previous studies show that when the pores of brittle-elastic rock are full of fluid, the rock is under the total stress and effective stress field, and when the pore fluid pressure increases to a certain level, effective stress field leads to the shear or tensile fractures of rock, in the strata with abnormal high pore fluid pressure, tension fractures are likely to be created, while in the hydrostatic pressure zone, only shear fractures would be produced under the tectonic stress^[36,37]. The “sweet spot section” is high in TOC, the abnormal high pressure caused by hydrocarbon generation would lead to tensile fracture of black shale, forming a high-density fracture network system. Meanwhile, high silicon content is likely to cause shrinkage during diagenesis, giving rise to bedding-parallel fractures^[24]. From SLM1 member to SLM5 member, as the TOC and silicon content decrease, the density of microfractures caused by hydrocarbon-generating pressurization and diagenetic shrinkage decrease correspondingly.

On the Shuanghe profile, there are two types of microfractures, bedding fractures and non-bedding fractures, and most of the microfractures are along the lamellation interfaces. The microfracture density of Longmaxi Formation is higher than that of Wufeng Formation. In the Longmaxi Formation, SLM1 member has the highest density of microfractures, and SLM2- SLM5 members see gradual drop of microfracture density. Fine shale has the highest density of microfracture, while medium and coarse shale have lower density of microfracture. The relatively clear beddings have the highest density of microfracture, followed by clear beddings, and obscure beddings and massive beddings are lowest in microfracture density. The microfracture density is in positive correlation with silicon content and negative correlation with carbonate content. The finer the black shale, the higher the microfracture density will be. The microfracture density is controlled by the biogenic silicon content, the higher the biogenic silicon content, the higher the microfracture density will be. The microfracture are formed firstly at the lamella interfaces of fine shale under ground stress. Regional tectonic activities are the key factor affecting the development of microfractures in the “sweet spot section” of Longmaxi Formation shale reservoir, diagenetic shrinkage is the major force driving the creation of lamellation fractures, and hydrocarbon-generating pressurization is the main cause of large-scale development of microfractures.