The laminae and their combinations in gas-bearing shale control the material composition, pore characteristics and micro-fracture distribution in the shale^[1], thus controlling the porosity and permeability of the shale. In the past few decades, a lot of studies have been done in terms of shale lamina terminology^[2], description method^[3], lamina type^[4], formation environment^[5,6], hydrodynamic conditions for forming different laminae^[7] and shale gas exploration and development significance^[8]. According to previous studies, the laminae can be described from three aspects, including the composition, the texture and the structure^[3]. According to the lamina composition, black shale can be divided into organic-rich lamina, organic-bearing lamina and clay lamina^[4]. According to the lamina texture, black shale can be classified as clayey lamina and silty lamina^[9]. According to the lamina structure, laminae can be subdivided into 12 types based on the continuity (continuous, discontinuous), shape (planar, wavy, curved) and geometry (parallel, nonparallel).

The black shale is mainly composed of particles with diameter less than 62.5 μm^[3]. According to particle diameter, it can be divided into coarse silt (31.2-62.5 μm), fine silt (3.9-31.2 μm) and fine mudstone (less than 3.9 μm)^[10]. The minimum diameter of mineral particles which can be recognized by optical microscope is 31.2 μm. In view of this, the particles with diameter less than 31.2 μm are collectively called as muddy particles, and the particles with diameter of 31.2-62.5 μm are collectively called as silty ones. According to the contents of muddy and silty materials, the shale laminae can be divided into two types, namely clayey laminae and silty laminae. The clayey laminae have muddy content over 50%, and the silty laminae have silty content over 50%. The clayey and silty laminae are mutually overlapped to constitute various lamina combinations^[6]. There are also great differences in pore composition, pore texture and micro-fracture distribution between the clayey and silty laminae in the black shale. The clayey laminae are rich in clay-size mineral particles^[11], and the silty laminae often have better reservoir space and permeability at the low-middle thermal maturity evolution stage^[8]. During the process of diagenetic evolution, due to the composition difference, the clayey and silty laminae had different diagenetic paths and reservoir characteristics^[12].

In the first member of the Lower Silurian Longmaxi Formation (Long-1 Member for short) in the Sichuan Basin, the laminae have become the best target layer for shale gas exploration and development due to the high TOC, high gas content, high brittle mineral content and high porosity^[1]. In view of the mineral composition and classification of laminae^[4], the overall characteristics of shale reservoirs with different lamina texture^[13], a series of previous studies have been conducted. However, there are still three problems in the current researches: (1) whether there are differences in reservoirs and micro-fracture characteristics between clayey and silty laminae; (2) the genetic mechanisms of different laminae; (3) the influences for the different laminae and combinations on the physical properties of shale. In this study, with the gas-bearing shale in the Long-1 Member in the Sichuan Basin as an example, through the integrated study methods of conventional thin section observation, large thin section, argon-ion polished large-area imaging and nano-CT scanning, the differences in reservoir characteristics and geneses of the clayey and silty laminae have been discussed, and the controlling roles of different lamina combinations on the porosity and permeability of shale have been analyzed.

A large set of black shale is well developed in the Longmaxi Formation in the Sichuan Basin (Fig. 1). The Longmaxi Formation is in conformity contact with the underlying Ordovician Wufeng Formation, and in angular unconformity contact with the overlying Shiniulan Formation, Xiaoheba Formation or Liangshan Formation. From lower to upper, the Longmaxi Formation can be divided into the Long-1 Member and the Long-2 Member. The Long-1 Member is further divided into Long-1₁ and Long-1₂ sub-members. The Long-1₁ sub-member is subdivided into four sub-layers (Long-1₁¹ to Long-1₁⁴). The Long-1 Member is black and gray-black thin-layer shale or massive shale interbedded with thin-layer siltstone. The Long-2 Member is muddy siltstone, interbedded with siltstone occasionally.

The studied samples of the Long-1 Member are from cores of wells Wei-201, Wei-202, Wei-204H10-5, Zi-201, Zu-202, Yanjin-1, and from Shuanghe outcrop in the Sichuan Basin. Five large thin sections have been prepared from the cores of wells Wei-201, Wei-202, Zi-201, and Zu-202 each. 42 thin sections have been prepared from the core of Well Yanjin-1. One CT scanning sample has been prepared from the cores of Well Wei-204H10-5 and Zu-202 each. Continuous sampling method has been used for collection of outcrop samples. 203 large thin sections, 203 small thin sections, 13 pieces/times for physical property analysis, and 1 piece/time of argon-ion polished section have been made. The size of the large thin sections is 5 cm×7 cm×30 μm, and the size of the samples for analyzing physical properties is 25 mm×10 mm. All the analysis and tests have been completed in National Energy Shale Gas R&D (Experiment) Center.

The description of laminae was based on the full-scale photography and polarizing microscope observation of large thin section. The Leica 4500P micro high-precision digital platform made in Germany has been used in full-scale thin section photography, and 3200 images have been acquired for each large thin section. After the image acquisition, the 3200 images acquired were seamlessly spliced by graphics processing software (Adobe Photoshop CS5 and higher version) on the high-configured workstation to complete the full-scale photography. After the full-scale photography, lamina characteristics have been described, and Leica DMIP polarizing microscope equipped with Leica DFC450 photographic system has been used to study the petrological characteristics of standard thin sections.

In order to obtain high-precision and large-view nano-pore images, the following research steps and methods have been used, such as making argon-ion polished slice, image acquisition and splicing, analysis of pore composition and pore diameter distribution, etc. The size of the argon-ion polished slice is 10 mm×10 mm×5 mm. Hitachi field emission scanning electron microscope with cold emission has been used for image acquisition, equipped with secondary electron probe and X-ray energy disperse spectrometer (EDS). The magnification of the scanning electron microscope is 30×10³ times (the maximum resolution of single photo is 9 nm). In order to confirm the nano-pore types and pore diameter distribution of different laminae, the image acquisition area is perpendicular to the lamina plane. Seven images have been acquired in horizontal direction, and 80 images have been acquired in vertical direction. The size of a single image is 8.172 μm× 11.829 μm, and the total acquired area is 82.80 μm×653.76 μm. After the image acquisition, Adobe Photoshop graphics processing software has been used to splice the images.

After image splicing, according to the particle composition of SEM images, the clayey and silty laminae have been distinguished, and the thicknesses of the laminae have been measured. The clayey laminae are dominated by clay-size particles, with relatively dark SEM images. The silty laminae are mainly silt-size particles, with relatively bright SEM images. After laminae identification, the software tracking and manual correction have been combined to conduct the statistical analysis of pore quantity, pore diameter size, pore diameter distribution, pore area, plane porosity and so on. Firstly, the high-resolution argon-ion polished images of shale have been processed by binary processing module in Adobe Photoshop, the pore boundaries have been identified automatically, and the pore types have been identified manually. Then, for different types of pores in each lamina, statistical analysis have been conducted on the amount of pores, proportion, area, plane porosity and area proportion. The statistical analysis has also been conducted on the amount of pores, proportion, area and area proportion for different types of pores in different particle diameter intervals. Finally, Excel software has been used to compile the composition percentage graphs (quantity and area) and pore diameter distribution graphs (quantity and area), and pore composition distribution graphs (quantity and area) of different types of pores with the same pore diameter. Relevant graphs have also been compiled. During the process of image recognition, the pore diameter has been calculated from the cross-sectional area equivalent to the same circle in the FE-SEM image, which can also be called the equivalent circle pore diameter or equivalent pore diameter.

In the gas-bearing shale in the Long-1 Member, clayey and silty laminae are well developed (Fig. 2). The integrated analysis of polarizing microscope and SEM images shows that the thickness of the single clayey lamina is 64.80-92.80 μm (76.54 μm in average ), and the thickness of the single silty lamina is 23.20-87.30 μm (54.14 μm in average).

The organic matters in the clayey laminae are connected from each other, while the organic matters in the silty laminae are disconnected from each other. In the clayey laminae, organic matters are mostly distributed in disperse, banded or lumpy shape (Fig. 3a), and different organic matters communicate with each other to form a network in space. In the silty laminae, most of the silty particles are in point contact or line contact (Fig. 3b), a few are in disperse; the organic matters are distributed between the silty particles in banded, dispersive or lumpy form (Fig. 3b), and most of them are disconnected from each other. At the interface between the clayey and silty laminae, due to the sudden change of mineral composition and particle size, the vertical extension of organic matter particles have been hindered.

In the clayey laminae, the quartz content is more than 70%, and organic matter content is more than 15%. In the silty laminae, the carbonate content is more than 50%, quartz content is more than 20%, and organic matter content is 5%-15%. SEM study shows that the main muddy materials in the clayey laminae are quartz (70%-90%), organic matter (15%-25%) and a few other minerals (5%-15%); while in the silty laminae, the main silty materials are calcite (25%-35%), dolomite (25%-35%) and quartz (10%-20%), with locally enriched pyrite; the main muddy materials are quartz (20%-30%) and organic matter (5%-10%). In the clayey laminae, the quartz particles are 1-3 μm in diameter, and are distributed in isolated style or aggregated form. In the silty laminae, the diameters of calcite and dolomite particles are mostly 20-40 μm. Under the polarizing microscope, the color of the clayey laminae is darker, often called as dark laminae (Fig. 2); and the color of the silty laminae is brighter, often called as bright laminae^[4].

In the black shale, organic pores, inorganic pores and micro-fractures are well developed. The organic pores are distributed in organic matters, in elliptical, nearly spherical, irregular honeycomb, stomatal or slit shapes (Fig. 4a, 4b). The plane porosity of organic pores in a single organic matter is 13.6%-33.8%. The inorganic pores are distributed in or between mineral particles, in triangular, angular or rectangular shapes. The inorganic pores can be divided into inter-granular pores (Fig. 4c, 4d) and dissolution pores (Fig. 4e, 4f). The dissolution pores are mainly formed by dissolution of carbonate minerals and a small amount of feldspar. The micro-fractures are mainly distributed between mineral particles, inside organic matter, or between mineral particles and organic matter (Fig. 4a). They are in banded shape, and can often communicate with various pores.

In the clayey laminae, organic pores are dominated. In the silty laminae, and inorganic pores are dominated. The proportions of different types of pores in the clayey and silty laminae have been calculated according to the area of 82.800 μm in length and 8.172 μm in width of a single line in the SEM image (Fig. 5). In five clayey laminae, there are 3799, 14 775, 9737, 4540 and 6679 organic pores (with an average of 7906); 0, 0, 0, 1 and 0 inter-granular pores; 7, 25, 5, 1 and 18 dissolution pores (with an average of 11.2); 0, 0, 1, 5 and 2 micro-fractures (with an average of 1.6), respectively. In five silty laminae, there are 2644, 4915, 3031, 2642, and 1227 organic pores (with an average of 2891.8); 0, 4, 3, 0 and 0 inter-granular pores; 36, 21, 24, 26 and 17 dissolution pores (with an average of 24.8); 1, 0, 0, 5 and 1 micro-fractures (with an average of 1.4), respectively. The proportion of organic pores in clayey laminae is 2.73 times that in silty laminae. The proportion of dissolution pores in silty laminae is 2.2 times that in clayey laminae.

In clayey laminae, the organic pores communicate with each other to form network. In silty laminae, organic and inorganic pores are all in disperse, and are disconnected from each other. The organic pores in clayey laminae are widely distributed along organic matters, and communicate with each other in organic matters to form mutually connected network in 3D space. The inorganic pores in silty laminae are mostly dispersed (Fig. 4h), and organic pores are also discontinuously distributed, leading to mutually disconnected pores in the silty laminae. Between the clayey laminae and the silty laminae, due to the discontinuity of mineral composition and organic matter distribution, the pore connectivity between various laminae is poor.

In the clayey laminae, the inorganic pore is small in diameter, while in the silty laminae, the inorganic pore is large in diameter. In clayey laminae, most of the inorganic pores are dissolution pores formed by dissolution of tiny particles. In silty laminae, most of the inorganic pores are inter-granular or intragranular dissolution pores formed by dissolution of larger particles, and some calcite is even dissolved to form network dissolution pores (Fig. 4e, 4f and 4h).

The plane porosity value of a lamina can reflect the porosity value. The study results show that the plane porosity of the clayey laminae is basically the same as that of the silty laminae. Statistical analysis has been conducted on plane porosity values of the clayey and silty laminae according to the area of 82.800 μm in length and 8.172 μm in width of a single line in the SEM image (Fig. 6). In the five clayey laminae, the plane porosity is 0.81%, 2.80%, 2.26%, 1.08% and 1.73% respectively (with an average of 2.09%) (Fig. 6a). In the five silty laminae, the plane porosity is 3.02%, 4.35%, 2.20%, 1.80% and 1.73%, respectively (with an average of 2.62%). The average plane porosity value of the silty laminae is 0.5% higher than that of the clayey laminae. The previous study results indicate that in the gas-bearing shale in Long-1 Member, the micro pores account for about 25%-35% of the total organic pores^[14]. In view of the fact that SEM images can only identify meso-pores and macro-pores with diameter over 10 nm, the reduced total plane porosity of clayey and silty laminae is 2.65% and 2.93%, respectively. Therefore, there is little plane porosity difference between the clayey laminae and the silty laminae.

In the clayey laminae, the plane porosity of organic pores is higher, while in silty laminae, the plane porosity of inorganic pores is higher (Fig. 6b). In the five clayey laminae, the proportion of the plane porosity of organic pores is 52.9%, 58.7%, 60.6%, 53.4% and 26.6% (50.4% in average), higher than that of inorganic pores. In the five silty laminae, the proportion of the plane porosity of inorganic pores is 73.2%, 78.3%, 81.7%, 83.5% and 87.9% (80.9% in average), much higher than that of organic pores.

The gas-bearing shale in the Long-1 Member is dominated by nano-pores (Fig. 7), with pore diameter ranging from 0 to 1000 nm (Fig. 8a), and the distribution frequency of the pore diameter ranging from 0 to 100 nm is the largest.

In the clayey laminae, the distribution frequency of the pore diameter ranging from 10 to 40 nm is the largest. In the silty laminae, the distribution frequency of the pore diameter ranging from 100 to 1000 nm is the largest. The diameters of organic pores are mainly between 0-100 nm, in which, the distribution frequency of the pore diameter ranging from 10 to 40 nm is the largest (Fig. 8b). The diameters of inorganic pores are mainly between 200-1000 nm, in which, the distribution frequency of the pore diameter ranging from 500 to 1000 nm is the largest (Fig. 8c). The diameters of dissolution pores are between 40-1000 nm, in which, the distribution frequency of the pore diameter ranging from 100 to 1000 nm is the largest (Fig. 8d). The lengths of micro-fractures are between 10-200 nm, in which, the distribution frequency of the micro- fracture length ranging from 40 to 200 nm is larger (Fig. 8e).

In clayey laminae, the contents of organic pores in different pore diameter intervals are all higher than those of silty laminae (Fig. 8b), and in silty laminae, the contents of inorganic pores in different diameter intervals are higher than those of clayey laminae (Fig. 8c, 8d). Statistical analysis has been conducted on the pore contents of different pore diameter intervals in the clayey and silty laminae according to the area of 82.800 μm in length and 8.172 μm in width of a single line in the SEM image (Fig. 8). The contents of organic pores in pore diameter ranging from 0 to 100 nm in the clayey laminae are 2-3 times those in the silty laminae. The contents of inter-granular pores in pore diameter ranging from 200 to 1000 nm in the silty laminae are 2-3 times those in the clayey laminae. The distribution frequencies of dissolution pores in pore diameter ranging from 100 to 1000 nm in the silty laminae are 1-2 times those in the clayey laminae.

The diameters of organic pores in the clayey laminae are smaller, while those in the silty laminae are larger. The statistical results show that the plane porosity proportion of organic pores less than 100 nm of clayey laminae is higher than that of the silty laminae, while the plane porosity proportion of organic pores greater than 100 nm in the silty laminae is higher than that of the clayey laminae (Fig. 9). The average plane porosity of organic pores ranging from of 20 to 40 nm in the clayey laminae is 25.9%, and that in the silty laminae is 20.3%; the average plane porosity of organic pores ranging from 40 to 100 nm in the clayey laminae is 31.8%, and that in the silty laminae is 24.1%; the average plane porosity of organic pores ranging from 100 to 200 nm in the clayey laminae is 18.1%, and that in the silty laminae is 18.9%; the average plane porosity of organic pores ranging from 200 to 500 nm in the clayey laminae is 17.9%, and that in the silty laminae is 23.6%; the average plane porosity of organic pores ranging from 500 to 1000 nm in the clayey laminae is 6.3%, and that in the silty laminae is 13.1%.

In the gas-bearing shale in the Long-1 Member, a large amount of micro-fractures are well developed. According to the relationship with the lamina plane, the micro-fractures can be divided into bed-parallel fractures and non-parallel fractures^[11]. Under the polarizing microscope, the bed-parallel fractures are parallel to or slightly inclined to lamina plane (Fig. 10a), and the non-parallel fractures are oblique and perpendicular to lamina plane (Fig. 10b). The bed-parallel fractures and non-parallel fractures intersect mutually to form network (Fig. 10c). Most of the gas-bearing bed-parallel fractures and non-parallel fractures in the Long-1 Member are filled with calcite (Fig. 10c, 10d), organic matter (Fig. 10e) or siliceous matter (Fig. 10f), and a few are half or completely filled with argillaceous matter, pyrite and other fillings^[15].

In the clayey laminae, bed-parallel fractures are well developed, while in the silty laminae, there are fewer bed-parallel fractures. The abundance of the bed-parallel fractures in the gas-bearing shale in the Long-1 Members is 3 times that of the non-parallel fractures, and the length of a single fracture is 5-6 times that of a single non-parallel fracture. The length of a bed-parallel fracture is controlled by the continuity and thickness of a clayey lamina. The more continuous the lamina is, the greater the length is, the larger the thickness of a single lamina is, and the more developed the bedding parallel fracture is. The bed-parallel fractures are mainly distributed in the clayey laminae, along the middle of clayey laminae or the contact surface between clayey and silty laminae (Fig. 10a). There are fewer bed-parallel fractures in the silty laminae. In SEM photos, the starting points of bed-parallel fractures and non-parallel fractures are located within the interior of organic matter or the contact surface between organic matter and clastic particles^[16]. Their length and abundance are controlled by the abundance of organic matter distributed along the bedding.

According to the shape, contact relationship and thickness distribution of clayey and silty laminae, three kinds of combinations can be divided, namely siltstone bearing laminae, graded (siltstone to claystone) laminae and siltstone and claystone interlaminated laminae. In the Long-1 Member of the Sichuan Basin, the siltstone bearing laminae are distributed mainly in the Long 1₁¹ sub-layer, the graded (siltstone to claystone) laminae are mainly distributed in the Long 1₁² sub-layer, and the siltstone and claystone interlaminated laminae are distributed mainly in the Long 1₁³ to Long 1₁⁴ sub-layers.

The siltstone bearing laminae combinations exist mainly in clayey laminae. The thickness ratio between clayey and silty laminae is generally more than 10. The silty laminae are mostly in lenticular shape (Fig. 11a), in dispersive or banded shape (Fig. 11b). The top and bottom interfaces between the clayey and silty laminae are mostly abrupt contact, and the interfaces are generally intermittent, planar and parallel (Fig. 11a), occasionally continuous, planar and parallel (Fig. 11b).

The graded (siltstone to claystone) laminae combination is composed of interbedded clayey and silty laminae, of which, the thickness ratio between clayey and silty laminae is generally 2-3. The top or bottom interfaces of the silty laminae are usually graded contact to form reversed grading (Fig. 11c) or normal grading (Fig. 11d). The lamina interfaces are generally continuous, planar and parallel, or intermittent, planar and parallel.

The porosity of siltstone bearing laminae is the largest, the graded (siltstone to claystone) laminae is the second, and the siltstone and claystone interlaminated laminae is the smallest (Table 1). On the outcrops of Shuanghe outcrop, the porosity of three samples from siltstone bearing laminae is 9.04%, 4.13% and 6.31%, respectively (with an average of 6.49%); the porosity of five samples from gradating sand-mud combination is 5.76%, 6.17%, 5.98%, 2.17% and 4.17%, respectively (with an average of 6.49%); the porosity of five samples from siltstone and claystone interlaminated laminae is 2.63%, 2.73%, 2.29%, 2.48% and 1.83%, respectively (with an average of 2.39%).

The siltstone bearing laminae combination has the largest ratio of horizontal to vertical permeability, followed by the gradating sand-mud combination, and the siltstone and claystone interlaminated laminae has the lowest value. On the Shuanghe outcrop, the ratio of horizontal to vertical permeability of three samples from siltstone bearing laminae is 8.62, 6.53 and 7.64, respectively; that of five samples from gradating sand-mud combination is 3.72, 4.56, 4.69, 3.15 and 3.47, respectively; that of five samples from siltstone and claystone interlaminated laminae is 2.13, 1.47, 2.66, 1.98 and 2.54, respectively.

The siltstone and claystone interlaminated laminae combination is composed of interbedded silty laminae and clayey laminae. In which, the thickness ratio between clayey and silty laminae is generally 1-20. The silty laminae are mostly in long strip shape, and the top and bottom interfaces of the laminae are in abrupt contact, most of them are continuous, planar, parallel (Fig. 11e) or intermittent, planar, parallel (Fig. 11f), a few are continuous, planar, nonparallel (Fig. 11e).

The genetic mechanisms of laminae in black shale usually include pulse flowing^[17], accumulation of multiple deposition events with different water energy^[18], algal blooming^[19], deposition differentiation^[20], or water transport differentiation^[9], etc.

The blooming of silicon-rich organisms may be the main genetic mechanism for laminae in gas-bearing shale in the Long-1 Member, Sichuan Basin. The main evidences are as follows:

(1) The muddy matters in silty laminae and clayey laminae are all biogenic silicon, indicating that the siliceous organisms were flourishing during the depositional period. In the silty laminae and clayey laminae, there were a large number of biological skeletons (such as radiolaria and siliceous sponge spicules) (Fig. 12). Most of the biological skeletons are filled with siliceous and organic matters, and a few are filled with pyrite. In addition, the muddy matters in silty laminae and clayey laminae are generally crypto-crystal, micritic or quartz aggregate. Under the illumination of cathodoluminescence, they emit weak light or are nonluminous, indicating that they are autogenetic or biogenetic. Moreover, through the study of quartz occurrence state, trace element statistics and excessive silicon content, the predecessors also think that these siliceous components are mainly biogenetic^[21,22,23]. According to the integrated analysis, the content of biogenic silicon in clayey laminae is more than 70%, and that in silty laminae is over 20%.

(2) Dong et al.^[11] have found that the Zr content is negatively related to the SiO₂ content in the Longmaxi Formation through the analysis of the main and trace elements of 103 gas-bearing shale samples from the Shuanghe outcrop, and speculated that the siliceous minerals in this period were mostly biogenic.

(3) The interfaces between silty laminae and clayey laminae are mostly planar parallel texture, without any cross bedding and erosion (Fig. 13). Schieber et al.^[24] found that in the current genetic laminae, cross bedding or erosion phenomenon occurred, while in the biological blooming genetic laminae, planar parallel textures are developed.

The biological blooming may be related to the seasonal change of paleo-climate. In the relatively warm and humid season, terrigenous fresh water brought many nutrients, which led to the blooming growth of siliceous organisms. The clayey laminae might be formed during biological blooming period, and the silty laminae might be formed during intermittent period. During the blooming period of silicon-rich organisms, a large number of biogenic silicon and organic matters were formed due to the large-scale growth of siliceous organisms. At the same time, biological blooming caused serious consumption of carbon dioxide in water body, resulting in precipitation of a large amount of calcium carbonate^{[19, 25]}, forming considerable calcite, dolomite and biological skeleton. Calcite, dolomite and biological skeleton have large particle diameter and density, and their sedimentation rates are relatively larger, forming silty laminae during the biological blooming period. Due to the small density and particle size, siliceous organisms and organic matter precipitated slowly, and organic-rich clayey laminae were formed.

Due to the intermittent blooming of siliceous organisms and the differentiation of diagenesis and evolution of different shale laminae, there are great differences in thickness, material composition, pore texture and plane porosity between the clayey laminae and the silty laminae. The clayey laminae were formed during the intermittent of the blooming period, when a large number of siliceous biological wreckages accumulated slowly. Therefore, the thickness of the clayey laminae formed by siliceous biological wreckages is large, and the content of organic matter is high. The silty laminae were formed during the blooming period. Due to the short formation time, the thickness is smaller and organic matter content is lower. During the syn-depositional period, in both the clayey and silty laminae, inorganic pores were dominated, and the organic pores were not developed or under-developed^[26]. During the sedimentary diagenetic stage, with the increasing in the thermal evolution degree of organic matter, inorganic pores decreased, and organic pores gradually formed and increased^[27,28,29]. Due to the higher content of organic matter, many organic pores were developed in the clayey laminae; due to the lower content of organic matter, more inorganic pores were developed in the silty laminae. In addition, due to the lower content of brittle minerals, in the clayey laminae, the compaction degree is higher, and the proportion of the plane porosity of organic pores with diameter less than 100 nm is higher; due to the higher content of brittle minerals and lower compaction degree, the proportion of the plane porosity of organic pores with diameter greater than 100 nm is higher^[30,31,32].

The difference in material composition resulted in the difference of micro-fractures between the clayey and silty laminae. In the clayey laminae, due to the higher content of organic matter and siliceous matter, it is easier to form micro-fractures^[33]. In addition, due to the higher content of organic matter, it was more likely to form fractures by increasing pressure during hydrocarbon generation^{[7, 16, 31, 34]}. In silty laminae, the contents of organic matter and siliceous matter were relatively low, so it is less likely to form micro-fractures under the same stress. At the same time, due to the development of inorganic pores, the silty laminae during the early stage of diagenesis had better permeability and are not easy to form fractures by increasing pressure during hydrocarbon generation. In addition, the interface between the clayey and silty laminae also belongs to the weak surface of rock mechanical strength, and micro-fractures are often easily formed along the interface^[35].

The ratio of horizontal to vertical permeability of the clayey laminae is large due to larger proportion of organic pores and micro-fractures. In the clayey laminae, the organic pores are high in proportion, and are connected mutually in space, thus with stronger permeability. In the silty laminae, although the inorganic pores are high in proportion, most of them are isolated, and difficult to form effective connected network, thus they have poor permeability. In the direction parallel to lamina surface, the bed-parallel fractures in the clayey laminae are connected mutually, thus with stronger horizontal permeability^[8]. In the direction vertical to lamina surface, the densities of the non-parallel fractures in the clayey laminae and the silty laminae are both lower, and most fractures are terminated at the lamina surface, thus they have worse vertical permeability. Wang et al.^[36] concluded that the permeability of shale samples can be greatly improved by micro-fractures, and the average permeability of samples with micro-fractures is 62.9 times of that of samples without micro-fractures.

The measurement methods might result in the difference in porosity of different lamina combinations. In this study, the porosity values have been measured by helium method, being effective porosity. In black shale, most organic pores are effective porosity, while most inorganic pores are inactive porosity. In the clayey laminae, the organic pore proportion is higher, thus the effective porosity is higher. In the silty laminae, the inorganic pore proportion is higher, thus the inactive porosity is higher. In the siltstone bearing laminae, the proportion of clayey laminae is the highest, thus it has the largest effective porosity. In the siltstone and claystone interlaminated laminae, the proportion of clayey laminae is the lowest, thus it has the lowest effective porosity.

The difference of the content ratio between the clayey and silty laminae resulted in the difference of the ratio of horizontal to vertical permeability of different lamina combinations. In the strip-shaped silty lamina combination, the ratio between the clayey and silty laminae is the highest, thus the organic pore proportion is the highest, the abundance of bed-parallel fractures is the largest, and ratio of horizontal to vertical permeability is the biggest. In the gradating sand-mud lamina combination, the ratio between the clayey and silty laminae is smaller, thus the organic pore proportion is lower, the abundance of bed-parallel fractures is lower, and the ratio of horizontal to vertical permeability is also lower. In the sand-mud thin interlayer lamina combination, the ratio between the clayey and silty laminae is the lowest, thus organic pore proportion and bedding parallel fracture abundance decrease further, and the ratio of horizontal to vertical permeability is the smallest.

In the gas-bearing shale in the Long-1 Member of the Sichuan Basin, clayey and silty laminae are well developed, with great difference in single layer thickness, the material composition, pore type and structure, plane porosity and pore diameter distribution.

The clayey lamina is about 100 μm in single layer thickness, with organic matter content more than 15%, quartz content more than 70%, high organic pore proportion, high plane porosity, and well developed bed-parallel fractures. The organic matters and organic pores are communicated with each other to form network. The silty lamina is about 50 μm in single layer thickness, with organic matter content of 5%-15%, carbonate content over 50%, high inorganic pore proportion, and bed-parallel fractures are poorly developed.

In silty laminae, the organic matters are relatively dispersive, organic pores and inorganic pores are disconnected from each other. The formation of the clayey and silty laminae may be related to the blooming of silicon-rich organisms. The clayey laminae were formed during intermittent period, and the silty laminae were formed during blooming period. Different geneses and late diagenesis of the laminae cause different reservoir characteristics of the clayey and silty laminae.

The clayey and silty laminae can constitute three types of combinations, including strip-shaped silt, gradating sand-mud and siltstone and claystone interlaminated laminaes. The strip-shaped silty lamina combination has the largest porosity and the highest ratio of horizontal to vertical permeability, followed by the gradating sand-mud lamina combination, and the sand-mud thin interlayer lamina combination has the lowest ratio of horizontal to vertical permeability. The measurement methods can cause the difference of porosity for different lamina combinations. The content ratio between the clayey and silty laminae can lead to different ratio of horizontal to vertical permeability.