Types and Quantitative Characterization of Microfractures in the Continental Shale of the Da’Anzhai Member of the Ziliujing Formation in Northeast Sichuan, China

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Article Types and Quantitative Characterization of Microfractures in the Continental Shale of the Da’anzhai Member of the Ziliujing Formation in Northeast Sichuan, China

Zhujiang Liu 1,2,3, Hengyuan Qiu 1,2, Zhenxue Jiang 1,2,*, Ruobing Liu 3, Xiangfeng Wei 3, Feiran Chen 3, Fubin Wei 3, Daojun Wang 3, Zhanfei Su 1,2 and Zhanwei Yang 1,2

1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China; [email protected] (Z.L.); [email protected] (H.Q.); [email protected] (Z.S.); [email protected] (Z.Y.) 2 Unconventional Natural Gas Research Institute, China University of Petroleum, Beijing 102249, China 3 Southern Company of Exploration SINOPEC, Chengdu 610041, China; [email protected] (R.L.); [email protected] (X.W.); [email protected] (F.C.); [email protected] (F.W.); [email protected] (D.W.) * Correspondence: [email protected]

Abstract: A number of wells in the Sichuan Basin of China have tested industrial gas flow pressure arising from the shale of the Da’anzhai section of the Ziliujing Formation, revealing good exploration potential. Microfractures in shales affect the enrichment and preservation of shale gas and are important storage spaces and seepage channels for gas. In order to increase productivity and to reduce the risks associated with shale gas exploration, the types, connectivity, and proportion Citation: Liu, Z.; Qiu, H.; Jiang, Z.; of microfractures in the Da’anzhai Member have been studied in this work by core and thin sec- Liu, R.; Wei, X.; Chen, F.; Wei, F.; tion observations, micro-CT, scanning electron microscopy, nitrogen adsorption, and high-pressure Wang, D.; Su, Z.; Yang, Z. Types and mercury intrusion. The results show that four types of fractures have developed in the shale of the Quantitative Characterization of Da’anzhai section: microfractures caused by tectonic stress, diagenetic shrinkage fractures of clay Microfractures in the Continental Shale of the Da’anzhai Member of the minerals, marginal shrinkage fractures of organic matter, and microfractures inside mineral particles. Ziliujing Formation in Northeast Among these, structural fractures and organic matter contraction fractures are the main types and are Sichuan, China. Minerals 2021, 11, 870. significant for shale reservoirs and seepage. The structural microfractures are mainly opened and are https://doi.org/10.3390/ well-developed in the shale, with a straight shape, mainly between bedding, with the fracture surface min11080870 being curved, fully opened, and mainly tensile. Organic matter fractures often develop on the edge of the contact between organic matter and minerals, presenting a slit-like appearance. The fractures Academic Editor: Luca Aldega related to bedding in the shale are particularly developed, with larger openings, wider extensions, intersecting and expanding, and forming a three-dimensional interconnected pore-fracture system. Received: 26 May 2021 Based on image recognition, generally speaking, microfractures account for about 20% of the total Accepted: 3 August 2021 pore volume. However, the degree of the microfractures’ development varies greatly, depending Published: 12 August 2021 upon the structural environment, with the proportion of microfractures in fault-wrinkle belts and high-steep zones reaching 40% to 90% of the total pore space. On the other hand, micro-fractures in Publisher’s Note: MDPI stays neutral areas with underdeveloped structures account for about 10% of the total pore space. with regard to jurisdictional claims in published maps and institutional affil- iations. Keywords: shale reservoir; microfracture; Da’anzhai Member; Sichuan Basin; quantitative characterization

Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. This article is an open access article Fractures in shale affect the enrichment and preservation of shale gas [1–5], where they distributed under the terms and form the seepage channels for gas. Fracture studies in the shale gas exploration stage can conditions of the Creative Commons therefore provide a reference for the optimal areas for shale gas exploration [6–9]. In the Attribution (CC BY) license (https:// development stage of shale gas reservoirs, the effectiveness and permeability anisotropy creativecommons.org/licenses/by/ of fractures directly affects the pattern of the deployment of the extraction wells and the 4.0/). reservoir fracturing reformation [10–13], which is an important reference for horizontal

Minerals 2021, 11, 870. https://doi.org/10.3390/min11080870 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 870 2 of 17

well deployment and geosteering [14]. In addition, the existence of shale cracks provides storage space for shale gas [15–17]. Fractures can be classified according to their cause and scale, and there are various classification schemes [18]. Due to their various scales and development characteristics, these fracture types have different effects on shale gas accumulation and enrichment [19–21]. For example, shrinkage fractures and abnormally high-pressure fractures account for a small proportion and are mostly filled by later miner- alization [22–24], so they have little impact on the enrichment and preservation of shale gas. However, interlayer shear fractures, bedding detachment fractures, interlayer ex- pansion fractures, and interlayer fractures have a greater impact on gas enrichment and preservation [25–28]. Microfractures are mainly micro-scale cracks smaller than 100 µm [29], which are difficult to identify with the naked eye, and their evaluation requires observation and statistical analysis with the help of microscopy instrumentation. In association with macroscopic fractures, microfractures also have an important influence on the physical and gas-bearing properties of shales [30,31]. In the process of shale gas development, natural gas is usually produced by gradual release, and the free gas in microfracture pores is discharged first [32]. Then, the adsorbed gas attached to the microfracture surface is released [33,34]. Finally, adsorbed gas escapes from the shale matrix. The desorption process of adsorbed gas is very slow, but when microfractures develop near the bottom of a well, the free gas in the microfractures quickly discharges, forming a low-pressure zone near the microfractures, in turn promoting the desorption of adsorbed gas in the area near the fractures and improving the gas production efficiency [35,36]. Microfractures in the shale can be used as penetration channels. Pores in a shale matrix are extremely undeveloped, being mostly microcapillary pores, and the permeability is much lower than that of tight sandstone; however, microfractures can significantly increase the permeability of this material [37–40]. Therefore, the development of effective microfractures is of great significance for increasing the permeability of shale. Previous descriptions of fractures have mainly focused on the core to outcrop scale, with few studies describing micro-scale fractures or the influence of microfracture development on shale reservoir space, with no quantitative studies to the authors’ knowledge of the influence of microfractures on the physical properties of shales. In this work, we take the Yuanba 21, Yuanba 102, Yuanlu 4, and another 10 wells in Northeast Sichuan and carry out a detailed description, classification, and quantitative characterization of microfractures based on a large number of observations of core and thin section samples, micro-CT, scanning electron microscopy, nitrogen adsorption, and high-pressure mercury injection tests. These results potentially play an important role in clarifying the effective reservoir space and gas-bearing properties of continental shale gas in the Sichuan Basin and are of great significance in clarifying the enrichment mechanism of continental shale gas and in supporting the exploration and deployment of continental shale gas resources.

2. Geological Setting The study area is located in the northeastern part of the Sichuan Basin, namely around the Longmen Mountain, Micang Mountain, and Daba Mountain front [41–43]. The structural division is located in the gentle fold belt of northern Sichuan and the highland fold belt of eastern Sichuan. North Sichuan is the main area for the exploration and development of shale gas reservoirs for the entire Sichuan Basin [44–47]. The study area is located on the northern margin of the Yangtze Plate and is adjacent to the Qinling fold belt. The Lower Jurassic Da’anzhai Member of the North Sichuan Basin is mainly distributed across the Micangshan and Dabashan arc structures [48–50]. The belt overlaps with the arc- shaped skirt belt in eastern Sichuan. During the Jurassic period, due to the disappearance of the Songpan Ganzi Sea and the Western Sea, all of the surrounding areas were surrounded and evolved into inland lakes [51–53]. The Early to Middle Jurassic basins were deep lakes, semi-deep lakes, and shallow lakes, where rivers, lakes, and deltas formed [54,55]. The facies are mainly composed of interbedded sand and mudstone. Among them, there Minerals 2021, 11, x FOR PEER REVIEW 3 of 18

disappearance of the Songpan Ganzi Sea and the Western Sea, all of the surrounding areas Minerals 2021, 11, 870 were surrounded and evolved into inland lakes [51–53]. The Early to Middle Jurassic3 of 17 basins were deep lakes, semi-deep lakes, and shallow lakes, where rivers, lakes, and deltas formed [54,55]. The facies are mainly composed of interbedded sand and mudstone. Among them, there are dark clastic rocks and carbonate strata from the Early to early areMiddle dark Jurassic clastic rocks and continental and carbonate red strata strata from from the the late Early Middle to early Jurassic Middle to the Jurassic Paleogene and continental[56,57]. The red Lower strata Jurassic from theDa’anzhai late Middle Member Jurassic is composed to the Paleogene of lakeside, [56 shallow,57]. The lake, Lower and Jurassicsemi-deep Da’anzhai lake deposits, Member with is composeda thickness of ranging lakeside, between shallow 100 lake, to 160 and m semi-deep in northeastern lake deposits,Sichuan with(Figure a thickness 1). These ranging deposits between are mainly 100 composed to 160 m in of northeastern shell limestone Sichuan and (Figuredark shale1). Theseinterbedded deposits with are mainlyeach other composed with unequal of shell thicknesses, limestone and and dark siltstone shale can interbedded be seen locally with each[58].other These with depos unequalits are thicknesses,rich in freshwater and siltstone bivalves can and be ostracod seen locally fossils, [58]. and These are depositscommon arein scale rich indeposits. freshwater Among bivalves these anddeposits, ostracod the second fossils, member and are commonof the Da’anzhai in scale Member deposits. is Amonga semi-deep these to deposits, deep lake the deposit, second and member is the ofmain the mudstone Da’anzhai development Member is a interval semi-deep in this to deepsection lake [59,60]. deposit, and is the main mudstone development interval in this section [59,60].

FigureFigure 1. 1.Structural Structural of of the the Sichuan Sichuan Basin Basin and and histogram histogram of of regional regional Lower Lower Jurassic Jurassic stratigraphy. stratigraphy.

3.3. SamplingSampling andand Laboratory Laboratory Methods Methods InIn this this study, study, samples samples were were collected collected from from 15 15 wells wells in in the the Yuanba Yuanba area area of of the the Sichuan Sichuan BasinBasin (YB (YB 21, 21, YB102, YB102, YBYB 104, 104,YB YB 122, 122, YB YB 221, 221, YB YB 273, 273, YL YL 4, 4, YL YL 30. 30. YL YL 17,17, YL YL 171,171, YL YL 175, 175, YLYL 176, 176, FY FY 1, 1, FY FY 4, 4, FYFY 5),5), withwith corescores fromfrom thethe Da’anzhaiDa’anzhai sectionsection ofof thethe ZiliujingZiliujing Formation.Formation. SomeSome ofof thethe well positions are are shown shown in in Figure Figure 11. .The The clay clay minerals minerals in inthe the Da’anzhai Da’anzhai lake lakefacies facies mud mud shale shale are very are very developed, developed, making making up from up from 21% 21%to 56%, to 56%,with withmost mostbeing being about about40% (illite40% (illite and and Imonite Imonite mixed mixed layer layer account account for for 50% 50% and and 25% 25% of of the clay clay minerals, minerals, respectively).respectively). TheThe secondsecond mostmostcommon commonare are brittle brittle minerals minerals such such as as quartz quartz and and feldspar feldspar withwith a a content content of of 30% 30% to to 45%, 45%, and and 5% 5% to to 25% 25% of of carbonate carbonate minerals. minerals. The The organic organic content content isis about about 0.8%, 0.8%, and and the the mineral mineral composition composition of of the the shale shale constitutes constitutes the the material material basis basis for for thethe development development ofof microscopic microscopic pores pores and and fractures. fractures. TheThe organicorganic matter matter content content of of the the lacustrinelacustrine mudstone mudstone in in the the Da’anzhai Da’anzhai section section of of the the YuanbaYuanba area area varies varies significantly, significantly, with with thethe TOC TOC total total organic organic carbon carbon (TOC) (TOC) values values of of the the analyzed analyzed samples samples ranging ranging from from0.04 0.04 to to 2.30%,2.30%, of of which which moremore thanthan halfhalf of of the the samples samples were were higher higher than than 0.5%, 0.5%, and and 27% 27% of of the the samples were higher than 1.0%, with some belonging to type III kerogen, and some to type samples were higher than 1.0%, with some belonging to type III kerogen, and some to II kerogen. The Ro of the samples shows little variation, ranging from 1.29% to 1.97%, with type II kerogen. The Ro of the samples shows little variation, ranging from 1.29% to 1.97%, an average of 1.63%. Readers should note that the analyses undertaken for this work were with an average of 1.63%. Readers should note that the analyses undertaken for this work carried out at the State Key Laboratory of Petroleum Resources and Prospecting, China were carried out at the State Key Laboratory of Petroleum Resources and Prospecting, University of Petroleum (Beijing), except where stated otherwise. China University of Petroleum (Beijing), except where stated otherwise. 3.1. Rock Slices and Identification Rock sections from the samples were polished and identified. To prevent man-made cracks from occurring, the samples were cut into sections of 3 to 5 cm in length and width with a wire cutting device. Then, sample which was processed was then glued onto the Minerals 2021, 11, 870 4 of 17

ground glass with epoxy resin. After gluing, the sample was placed on a ﬁxed platform and baked at a low temperature of 50 ◦C for about 6 to 8 h to consolidate and harden to avoid rock fracturing caused by later observation. After the glue hardened, the specimen was cut on a petro-thin machine and ground to a thickness of between 100 and 150 µm, then ground on a Lapping plate to a standard thickness of 30 µm. A Leica DM4P polarized light microscope was used to observe the prepared rock slices, to determine the mineral composition of the sample, to study its structure, to analyze the sequence of mineral generation, and to determine the rock type and its microcrack characteristics.

3.2. Micro-CT Scanning The micro-CT experiment was carried out at the Petroleum Geology Experiment Center, PetroChina Research Institute of Petroleum Exploration and Development. The instrument was a XRadia Ultra-XRM L200 stereoscopic microscope, with the voltage set to 8 kV, and the instrument’s limit resolution being 0.7 µm. The sample was made by vertical shale line cutting, and the samples’ shapes were cylinders about 1 cm in length and 4 mm in width. A sample would be ﬁxed in the device vertically and X-ray scanning was used to obtain the three-dimensional data volume of the core. In this paper, the software named Avizo 9.0 was used for the routine analysis of the three-dimensional data. According to the different X-ray absorption capacities of materials with different densities and thicknesses, the materials in the shale could be divided into three types: iron-bearing minerals, mineral matrix, and pores and microcracks.

3.3. Field Emission Scanning Electron Microscope (FE-SEM) Analysis FE-SEM analysis was performed using a Quanta200F cold-field scanning electron microscope. The Quanta 200 F is a high-performance multi-purpose high-resolution FE-SEM with a high comprehensive field emission electron microscope. Resolution and environmental scanning electron microscopes (ESEM) are suitable as they have the advantages of sample diversity and can perform static and dynamic observations and analyses of various samples in high vacuum mode, low vacuum mode, and environmental scanning mode. Each sample was cut along the direction perpendicular to the bedding and polished with an argon ion beam to produce a smooth and flat surface with little surface change. In a high vacuum with 30 kV, the highest resolution can reach 2 nm, while the low vacuum with 3 kV can reach 3 nm.

3.4. Nitrogen Adsorption Nitrogen adsorption was performed using a Kangta AbsorbIQ gas adsorption analyzer. The experiment required the use of high-purity nitrogen, with the sample needing to be degassed before the test, as the extent of the degassing directly affects the test results. Generally, a vacuum degassing system is adopted. High-resolution physical adsorption, chemical adsorption, and vapor adsorption isotherms can be measured to obtain the most accurate estimates of the pore sizes, the surface areas, and the specific gas/solid interactions. The sample was grinded to 60 to 80 mesh particles and degassing at 150 ◦C for more than 12 h.

3.5. High-Pressure Mercury Injection High-pressure mercury intrusion was performed using the Mike 9505 automatic mercury intrusion instrument. As mercury does not wet general solids, to make mercury enter the pores, an external pressure must be applied. The greater the external pressure, the smaller the radius of the pore that the mercury can enter. By measuring the amount of mercury entering the pores under different external pressures, the pore volume of the corresponding pore size can be found. The maximum pressure of the mercury porosimeter is about 228 MPa, and the diameters of the measurable holes range between 50 to 360 µm. These measurements are used to draw capillary pressure curves and can also be used to describe the characteristics of multiple reservoirs, especially the size distribution of pore bellows in porous media. Minerals 2021, 11, x FOR PEER REVIEW 5 of 18

Minerals 2021, 11, 870 50 to 360 μm. These measurements are used to draw capillary pressure curves and5 of can 17 also be used to describe the characteristics of multiple reservoirs, especially the size distribution of pore bellows in porous media.

4.4. ResultsResults 4.1.4.1. FeatureFeature ofof FractureFracture Morphology Morphology under under Rock Rock Slice Slice MudMud shaleshale isis the the main main component component of of unconventional unconventional shale shale reservoirs reservoirs in in the the second second sub-membersub-member of of the the Sophomore Sophomore stage. stage. Through Through a a microscope,microscope, we we can cansee see thethe clayclay structure,structure, silt-containingsilt-containing clayclay structure,structure, and silty silty-clay-clay structure, structure, among among which which shale shale is is often often seen. seen. A Asmall small amount amount of of scale scale fragments fragments (two (two-petal-petal species, mainly ostracods), ostracods), and and some some silt silt particlesparticles can can be be seen. seen.When Whenthe theorientation orientation of of those those detrital detrital particles particles is is more more obvious, obvious, the the directiondirection of of bedding bedding can can be be identified, identified, and and some some authigenic authigenic minerals minerals are are common common(Figure (Figure2) . For2). For example, example, in pyrite in pyrite particles, particles, a large a large amount amount of flaky of flaky organic organic matter matter orientated orientated in the in direction of the bedding can often be seen. The development forms of microcracks are the direction of the bedding can often be seen. The development forms of microcracks are mainly hairline, dendritic, and fractured networks. They are mainly distributed in the mainly hairline, dendritic, and fractured networks. They are mainly distributed in the bedding and develop in the layers. In the well-defined shale, some shell-bearing shale and bedding and develop in the layers. In the well-defined shale, some shell-bearing shale and silt-bearing shale also have a few microcracks. Among them, silt-bearing shale has the least silt-bearing shale also have a few microcracks. Among them, silt-bearing shale has the proportion of microcracks. When the silt content is high, it is difficult to find microcracks least proportion of microcracks. When the silt content is high, it is difficult to find under the microscope. microcracks under the microscope.

FigureFigure 2.2. PhotographPhotograph ofof typicaltypical micro-fracturesmicro-fractures underunder shaleshale thinthin section.section. ((AA)) YL30-2,YL30-2, 3597.363597.36 m,m, mudstonemudstone with with shell, shell, micro-fractures micro-fractures cutting cutting through through shell particles; shell particles; (B) YB21-2, (B) 4026.05 YB21- m,2, 4026.05mudstone, m, micro-fracturesmudstone, micro along-fractures the bedding along unfilled;the bedding (C) YL30-24, unfilled; 4007.5 (C) YL30 m, mudstone,-24, 4007.5 nearly m, mudstone, parallel micro- nearly fracturesparallel inmicro mudstone;-fractures (D in) YL4-6, mudstone; 3748.38 (D m,) YL4 mudstone,-6, 3748.38 micro-fractures m, mudstone, extending micro-fractures along the extending edge of along the edge of the shell into the mudstone, unfilled; (E) YL4-12, 3753.38 m, mudstone, along the the shell into the mudstone, unfilled; (E) YL4-12, 3753.38 m, mudstone, along the shell micro-fractures extending from the edge to mudstone, unfilled; (F) YL4-16, 3756.4 m, calcareous mudstone, interlayer fractures and nearly parallel fractures connected into a network; (G) YL17-2, 3864.22 m, calcareous mudstone, unfilled micro-fractures; (H) YB102-6, 3922.79 m, shell mudstone, micro-fractures, severe silicification of bioclastic edges; (I) FY1-1, 2571 m, calcareous mudstone, networked micro-fractures in mudstone.

4.2. Type, Occurrence, and Scale of Micro-CT Scan Cracks At the micrometer scale, the bedding phenomenon of the shale samples is not sig- niﬁcant, but we can see from the three-dimensional display images that the microcracks are almost parallel and spread out in the layers. Excluding other inﬂuencing factors, we conclude that the microcracks we observe are bedding-related microcracks, which either Minerals 2021, 11, x FOR PEER REVIEW 6 of 18 Minerals 2021, 11, x FOR PEER REVIEW 6 of 18

shell micro-fractures extending from the edge to mudstone, unfilled; (F) YL4-16, 3756.4 m, shell micro-fractures extending from the edge to mudstone, unfilled; (F) YL4-16, 3756.4 m, calcareous mudstone, interlayer fractures and nearly parallel fractures connected into a network; calcareous mudstone, interlayer fractures and nearly parallel fractures connected into a network; (G) YL17-2, 3864.22 m, calcareous mudstone, unfilled micro-fractures; (H) YB102-6, 3922.79 m, shell (G) YL17-2, 3864.22 m, calcareous mudstone, unfilled micro-fractures; (H) YB102-6, 3922.79 m, shell mudstone, micro-fractures, severe silicification of bioclastic edges; (I) FY1-1, 2571 m, calcareous mudstone, micro-fractures, severe silicification of bioclastic edges; (I) FY1-1, 2571 m, calcareous mudstone, networked micro-fractures in mudstone. mudstone, networked micro-fractures in mudstone. 4.2. Type, Occurrence, and Scale of Micro-CT Scan Cracks 4.2. Type, Occurrence, and Scale of Micro-CT Scan Cracks At the micrometer scale, the bedding phenomenon of the shale samples is not At the micrometer scale, the bedding phenomenon of the shale samples is not significant, but we can see from the three-dimensional display images that the Minerals 2021, 11, 870 significant, but we can see from the three-dimensional display images that6 of the 17 microcracks are almost parallel and spread out in the layers. Excluding other influencing microcracks are almost parallel and spread out in the layers. Excluding other influencing factors, we conclude that the microcracks we observe are bedding-related microcracks, factors, we conclude that the microcracks we observe are bedding-related microcracks, which either extend along the bedding or the main body extends along the shale bedding, which either extend along the bedding or the main body extends along the shale bedding, extendbut intersects along the with bedding it at a orlow the angle. main This body is extends fully illustrated along the in shale the bedding,sample FY1 but-13 intersects (Figure but intersects with it at a low angle. This is fully illustrated in the sample FY1-13 (Figure with3). it at a low angle. This is fully illustrated in the sample FY1-13 (Figure3). 3).

Figure 3. Three-dimensional distribution of bedding-related micro-fractures. (A) YB102-7 3923.19 m 3D gray scale model Figure 3. Three-dimensional distribution of bedding-related micro-fractures. (A) YB102-7 3923.19 m 3D gray scale model Figureof the core; 3. Three-dimensional (B) FY1-13 2636 m distribution 3D gray scale of bedding-related model of the core; micro-fractures. (C) YB102-7 3923.19 (A) YB102-7 m 3D 3923.19 pore model m 3D of gray the scalecore; model(D) FY1 ofof the core; (B) FY1-13 2636 m 3D gray scale model of the core; (C) YB102-7 3923.19 m 3D pore model of the core; (D) FY1- the13 2636 core; m (B 3D) FY1-13 pore model 2636 m of 3D the gray core. scale model of the core; (C) YB102-7 3923.19 m 3D pore model of the core; (D) FY1-13 263613 2636 m 3D m pore3D pore model model of the of core.the core. 4.3. Types of Microcracks Observed under a Scanning Electron Microscope 4.3.4.3. TypesTypes of of Microcracks Microcracks Observed Observed under under a a Scanning Scanning Electron Electron Microscope Microscope According to our observations, there are mainly the following types of microfractures According to our observations, there are mainly the following types of microfractures in theAccording shale of to the our Da’anzhai observations, section: there structural are mainly microfractures, the following types diagenetic of microfractures shrinkage in the shale of the Da’anzhai section: structural microfractures, diagenetic shrinkage infractures the shale of of clay the Da’anzhai minerals, section: marginal structural shrinkage microfractures, fractures diagenetic of organic shrinkage matter, frac- and fractures of clay minerals, marginal shrinkage fractures of organic matter, and turesmicrofractures of clay minerals, inside mineral marginal particles. shrinkage Among fractures them, of organicthe structural matter, microcracks and microfractures that are microfractures inside mineral particles. Among them, the structural microcracks that are insideof greatest mineral significance particles. are Among the extension them, the of structural microcracks microcracks at the thin that slice are scale of greatest to the of greatest significance are the extension of microcracks at the thin slice scale to the signiﬁcancenanoscale. We are have the extensionanalyzed ofand microcracks counted them, at the and thin other slice types scale toof themicrofractures nanoscale. Weare havenanoscale. analyzed We and have counted analyzed them, and and counted other types them, of and microfractures other types are of eithermicrofractures too underde- are either too underdeveloped, or not typical enough, or even unable to be called fractures. veloped,either too or underdeveloped, not typical enough, or ornot even typical unable enough, to be calledor even fractures. unable Whileto be called they have fractures. little While they have little impact on the reservoir, we nonetheless present here a brief impactWhile on they the have reservoir, little we impact nonetheless on the present reservoir, here a we brief nonetheless introduction present to them here (Figure a bri4).ef introduction to them (Figure 4). introduction to them (Figure 4).

A B C A Structural microfractures B C Structural microfractures 267nm 267nm

Organic matter 61nm Organic matter 61nm

D E F D E F 124nm 124nm

Microfractures 68nm Microfractures 156nm 68nm Shrinkage joints between 156nm Shrinkagelayers in clay joints minerals between layers in clay minerals

shrinkage seam; (D) YB21-2 Organic matter edge shrinkage seam; (E) FY1-13 Diagenetic shrinkage joints in clay minerals; (F) YL4-8 Internal micro-fractures of shell particles.

Figure 4. Three-dimensional distribution of bedding-related micro-fractures. (A) YL30-19 Structure micro-fractures, cracks are impregnated with organic matter; (B) YL4-20 Bedding structure micro-fractures; (C) YL171-5 Organic matter edge shrinkage seam; (D) YB21-2 Organic matter edge shrinkage seam; (E) FY1-13 Diagenetic shrinkage joints in clay minerals; (F) YL4-8 Internal micro-fractures of shell particles.

4.4. Joint Evaluation of Pore Structure by High-Pressure Mercury Intrusion and Nitrogen Adsorption According to the International Union of Pure and Applied Chemistry (IUPAC) Minerals 2021, 11, 870 classification, the measured nitrogen adsorption curve belongs to a type IV loop, with the7 of 17 hysteresis loop similar to H2 and H4. It has the characteristics of flaky pores and ink bottle- like throat, with small pore packing, point B, and multi-layer adsorption. The capillary agglomerates multiple processes, reflecting the existence of micropores and mesopores, agglomeratesand finally, no multiple platform processes, appears, reflectingindicating the the existence existence of of micropores macropores, and which mesopores, may be and finally,caused no by platform microcracks appears, (Figure indicating 5A). In the N2 existenceadsorption, of macropores, the NLDFT whichmodel may pays be more caused byattention microcracks to the (Figure characterization5A). In N 2 ofadsorption, micro-mesopores, the NLDFT and themodel BJH pays model more pays attention more to theattention characterization to the characterization of micro-mesopores, of meso-macropores. and the BJH In model this paper, pays N more2 adsorption attention and to the characterizationhigh-pressure mercury of meso-macropores. injection were used In this to characterize paper, N2 adsorption the pore size, and and high-pressure the range of mer- curypore injectionsize selected were by used high to-pressure characterize mercury the poreinjection size, was and greater the range than of 5 pore0 nm. size Therefore, selected by high-pressureNLDFT model, mercury which focuses injection more was on greater micro-mesopore than 50 nm. characterization, Therefore, NLDFT was selected model, for which focusescharacterization. more on micro-mesoporeWe used the NLDFT characterization, model to explain was selectedits pore size for characterization. distribution, and We usedfound the that NLDFT the pore model size to explainis mainly its distributed pore size distribution, between 1 and to 2 found nm (Figure that the 5B pore). The size is mainlycombination distributed of mercury between intrusion 1 to 2 and nm nitrogen (Figure5 B).adsorption The combination can measure of mercurythe distribution intrusion andof full nitrogen-aperture adsorption pores and can cracks measure from 0.8 the to distribution 1500 nm, with of full-aperture the high-pressure pores mercury and cracks fromintrusion 0.8 to being 1500 used nm, to with evaluate the high-pressure the development mercury of microcracks. intrusion beingThe flaky used clay to is evaluate plastic, the developmentand high pressure of microcracks. will produce The cracks. flaky There clay is are plastic, errors and in interpreta high pressuretion. Cracks will produce and cracks.microcracks There generally are errors have in interpretation.a width of more Cracksthan 100 and nm, microcracks and a macro generally-hole radius have of more a width ofthan more 50 thannm is 100 just nm, in the and macro a macro-hole-hole interval radius (Figure of more 6A,B than). Therefore, 50 nm is just the inmacro the macro-hole-hole is intervalused to (Figure evaluate6A,B). the Therefore, proportion the of macro-hole microcracks, is and used the to evaluate development the proportion degree of of microcracks,microcracks should and the not development exceed this value. degree of microcracks should not exceed this value.

(A) (B)

Figure 5. N2 adsorption and desorption curve (A) and pore size distribution of shale N2 adsorption Minerals 2021, 11, x FOR PEER REVIEWFigure 5. N2 adsorption and desorption curve (A) and pore size distribution of shale N2 adsorption8 of 18 NLDFT model (B). NLDFT model (B).

(A) (B)

FigureFigure 6. The curve curve of of mercury mercury intrusion intrusion volume volume with with pressure pressure in high in high pressure pressure mercury mercury intrusion intrusion (A) and pore size distribution explained by high pressure mercury injection (B). (A) and pore size distribution explained by high pressure mercury injection (B).

5.5. Discussion 5.1.5.1. Comprehensive Qualitative Qualitative Evaluation Evaluation of ofMulti Multi-Scale-Scale Microcracks Microcracks Through multi multi-scale-scale analyses analyses of of the the shale shale from from the the Da’anzhai Da’anzhai Member, Member, we we have have observedobserved numerous numerous micro-scale micro-scale fractures. fractures. Generally Generally speaking, speaking, the microfractures the microfractures identiﬁed atidentified the slice at scale the areslice similar scale are to those similar observed to those at observed the core scaleat the and core are scale mainly and microfracturesare mainly microfractures caused by tectonic stress, followed by smaller clay mineral diagenetic shrinkage joints, organic matter edge shrinkage joints, and microcracks inside mineral particles. Through the combined observation of single-biased and orthogonal light, we found that the structural microfractures are mainly open, developed in the bedding-developed shale, with straight shapes and mainly bedding. In some areas, interlayer fractures have developed and are connected into dendrites and nets in fine-grained mud shale deposits, since mud shale is composed of a large number of lamellar clay minerals which have cleavage. The development of most microcracks is bedding-compliant, and we believe this may be related to the sheet-like arrangement of clay minerals (Figure 2B–I). However, in other lithologies without obvious bedding, such as siltstone and fine sandstone, the microcracks are not developed. In addition, in the calcareous cement layers distributed along the beds in some mudstones and the calcareous cement near some organic fragments, microcracks (laminas) are particularly developed, which may have formed by stress. Under an electron microscope, the widths of the structural microfractures usually range between 0.1 and 3 μm, the fracture surfaces are curved, fully opened, mainly tensile, and often extend along the edges of the clay mineral flakes and brittle mineral particles (Figure 4A,B). They occasionally cut through particles, which can be simply divided into marginal fractures and interlaminar fractures according to their development positions. They are the nano-scale extension of microfractures observed at the slice scale, which play a major role in the quality of shale reservoirs. The communication effect is conducive to the improvement of its porosity and permeability, which may be caused by the stress generated by the source rock overpressure and the regional tectonic stress. Based on micro-CT imaging, we can clearly see that the seams related to the bedding are particularly developed, with larger openings, wider extensions, and some interlayer seams that cross and extend, constituting a three-dimensional interconnected hole-slit system (Figure 3C,D). We believe this is sufficient to show that the cracks we have identified are mainly bedding-related. The other three fracture types have different degrees of development in different samples. The clay mineral composition of the Da’anzhai section contains more than 50% of illite, and more than 25% of the Imonite mixed layer. In the process of converting illite to montmorillonite, the syneresis of clay minerals will occur. During this process, a large number of diagenetic shrinkage microfractures will form, mainly in the clay mineral

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caused by tectonic stress, followed by smaller clay mineral diagenetic shrinkage joints, organic matter edge shrinkage joints, and microcracks inside mineral particles. Through the combined observation of single-biased and orthogonal light, we found that the structural microfractures are mainly open, developed in the bedding-developed shale, with straight shapes and mainly bedding. In some areas, interlayer fractures have developed and are connected into dendrites and nets in fine-grained mud shale deposits, since mud shale is composed of a large number of lamellar clay minerals which have cleavage. The development of most microcracks is bedding-compliant, and we believe this may be related to the sheet-like arrangement of clay minerals (Figure2B–I). However, in other lithologies without obvious bedding, such as siltstone and fine sandstone, the microcracks are not developed. In addition, in the calcareous cement layers distributed along the beds in some mudstones and the calcareous cement near some organic fragments, microcracks (laminas) are particularly developed, which may have formed by stress. Under an electron microscope, the widths of the structural microfractures usually range between 0.1 and 3 µm, the fracture surfaces are curved, fully opened, mainly tensile, and often extend along the edges of the clay mineral flakes and brittle mineral particles (Figure4A,B) . They occasionally cut through particles, which can be simply divided into marginal fractures and interlaminar fractures according to their development positions. They are the nanoscale extension of microfractures observed at the slice scale, which play a major role in the quality of shale reservoirs. The communication effect is conducive to the improvement of its porosity and permeability, which may be caused by the stress generated by the source rock overpressure and the regional tectonic stress. Based on micro-CT imaging, we can clearly see that the seams related to the bedding are particularly developed, with larger openings, wider extensions, and some interlayer seams that cross and extend, constituting a three- dimensional interconnected hole-slit system (Figure3C,D). We believe this is sufficient to show that the cracks we have identified are mainly bedding-related. The other three fracture types have different degrees of development in different samples. The clay mineral composition of the Da’anzhai section contains more than 50% of illite, and more than 25% of the Imonite mixed layer. In the process of converting illite to montmorillonite, the syneresis of clay minerals will occur. During this process, a large number of diagenetic shrinkage microfractures will form, mainly in the clay mineral flakes. The edges without structural microfractures are rough, their extension length is short, and the fracture width is narrow. These are mainly less than 100 nm, concentrated in the range of 30 to 80 nm (Figure4E). These appear less frequently than structural microcracks, and only in large numbers in a few samples. They are also identified as clay mineral interlayer pores in other pore and fracture classifications, and their pore size distribution interval is much smaller than that of conventional microfractures. Organic matter edge shrinkage joints are formed by the volume shrinkage of organic matter during the thermal evolution associated with the burial and conversion of organic matter to hydrocarbons. They generally occur at the edges where organic matter and minerals are in contact, forming a slit-like shape. In the past, some researchers believed that these may be an important storage space for shale gas. In our observation of organic matter, their occurrence frequency is very high, and the fracture width is generally about 30 to 100 nm (Figure4C,D). The common minerals in the shale of the Da’anzhai Member are terrigenous quartz, feldspar debris, and some shell particles. In addition to some structural microcracks that are prone to occur on the edges of minerals, some microcracks also occur inside mineral particles. The widths of the fractures are very small and they are difficult to find, being more common in shells, quartz, and pyrite particles. Since shell limestone is also an important potential reservoir in the Da’anzhai Member, the pore structure inside shell particles will have a certain reference value, hence some examples are presented (Figure4F). Minerals 2021, 11, 870 9 of 17

5.2. Comprehensive Quantitative Characterization of Multi-Scale Microfractures 5.2.1. Quantitative Characteristics of Micro-CT Microcracks Micro-CT images were segmented and reconstructed using Avizo software. Firstly, the units of interest are selected and the image is segmented by using the threshold tool in Avizo software. The brightness of the high-resolution image is proportional to the atomic number of the sample components, which can clearly show the difference in the gray value of shale pores and cracks, organic matter and inorganic mineral matrix. According to the above principle, manual threshold method is adopted to extract the pores and cracks, organic matter and minerals respectively, and display their three-dimensional spatial structure. Then the Avizo Pore Network Model (PNM) module software was used to analyze the pore structure. The data type stored by the PNM module represents a grid composed of multiple linear lines in three-dimensional space. The branches or endpoints of the grid represent pores, and the straight lines connecting the pores are called throats. For each pore and throat, parameters such as radius, throat length, and coordination number can be calculated. Using the model that comes with the software, we extracted the relevant parameters of the pores. We calculated the average porosity of the sample to be 1.49%. It is almost the same as the hole and seam distribution observed in our images and the actual test data. The volume and area percentages of the holes and seams were calculated, and the statistical results are as follows. The distribution first shows a two-peak situation, indicating that the sample is a double-porosity medium with small micropores and large cracks. We believe that the large pores identified by the software are the cracks that can be seen, but the pores with an equivalent diameter of 50 to 100 nm only account for 16.9% of the entire pore volume (Figure7). This seems to be very low compared to the other peak, but the ratio is still high. Due to the problem of threshold segmentation, some of the fractures are divided into many disconnected points (as can be seen from the model), with the actual contribution of the fractures to the porosity being much greater than this value. We can see from the three-dimensional display of the pore model that the pore distribution in this sample is mainly related to pores of a size between 1 to 2 µm and 2 to 5 µm (Figure8) . They contribute more than 71% of the pore volume and 81% of the pore µ Minerals 2021, 11, x FOR PEER REVIEWsurface area, and show that the peak of the pore size distribution of 1 to 2 m is higher10 andof 18 that the software cannot identify pores less than 1 µm in size. This means that although we have identified a large number of pores ranging between 1 to 2 µm, it may mean that there are more pores less than 1 µm in size that have not been identified. This is also one of the limitationsranging between of this 1 method; to 2 μm, however, it may mean there that appears there toare be more no issue pores when less than analyzing 1 μm poresin size 1thatµm have or more notin been size. identified. This is also one of the limitations of this method; however, there appears to be no issue when analyzing pores 1 μm or more in size.

FigureFigure 7. 7.Percentage Percentage distribution distribution of of pore pore volume volume of of different different equivalent equivalent diameters diameters of of micron micron CT. CT.

Figure 8. Percentage distribution of specific surface area of pores with different equivalent diameters in micron CT.

5.2.2. Quantitative Identification of Microcracks by Scanning Electron Microscopy The microfractures observed under an electron microscope are all open, which is of great significance to the improvement of the porosity and permeability of shale reservoirs. Among them, a small number of fractures become channels for the migration and accumulation of hydrocarbons, which are contaminated by the migration of organic matter. Therefore, parameters such as the width and length of the fractures are critical to their connectivity. We have conducted statistical analyses on the distribution of the widths of the microcracks and found that they are mainly distributed between 200 to 400 nm, followed by between 400 to 1000 nm, and the number of microcracks can be ignored. To measure the degree of development of the microcracks, their face ratios in the electron microscope field of view is a parameter worth assessing (Figure 9A). We used a magnification of 500 to 2000 times to observe and photograph microcracks. After calculating their face ratios, we found that they are mainly distributed below 0.8%. Although this value is not high, it is indeed an objective microscopic channel for oil and gas migration and accumulation and is of great potential significance to improving shale storage and permeability (Figure 9B).

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ranging between 1 to 2 μm, it may mean that there are more pores less than 1 μm in size that have not been identified. This is also one of the limitations of this method; however, there appears to be no issue when analyzing pores 1 μm or more in size.

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Figure 7. Percentage distribution of pore volume of different equivalent diameters of micron CT.

FigureFigure 8. 8.Percentage Percentage distribution distribution of speciﬁc of specific surface surfac areae of area pores of with pores different with equivalentdifferent diametersequivalent diameters in micron CT. in micron CT.

5.2.2.5.2.2. Quantitative Quantitative Identification Identification of of Microcracks Microcracks by by Scanning Scanning Electron Electron Microscopy Microscopy TheThe microfractures microfractures observed observed under under an an electron electron microscope microscope are are all all open, open, which which is is of of greatgreat significance significance to to the the improvement improvement of of the the porosity porosity and and permeability permeability of of shale shale reservoirs.reservoirs. AmongAmong them, them, a smalla small number number of fracturesof fractures become become channels channels for the for migration the migration and accu- and mulationaccumulation of hydrocarbons, of hydrocarbons, which which are contaminated are contaminated by the migrationby the migration of organic of matter.organic Therefore,matter. Therefore, parameters parameters such as thesuch width as the and width length and oflength thefractures of the fractures are critical are critical to their to connectivity.their connectivity. We have We conducted have conducted statistical statistical analyses analyses on the distributionon the distribution of the widths of the widths of the microcracksof the microcracks and found and that found they that are mainlythey are distributed mainly distributed between 200between to 400 200 nm, to followed 400 nm, byfollowed between by 400 between to 1000 400 nm, toand 1000 the nm, number and the of number microcracks of microcracks can be ignored. can be To ignored. measure To themeasure degree the of developmentdegree of development of the microcracks, of the micr theirocracks, face ratios their in face the electronratios in microscopethe electron fieldmicroscope of view isfield a parameter of view worth is a assessingparameter (Figure worth9A). assessing We used (Figure a magnification 9A). We of 500used to a 2000magnification times to observe of 500 and to photograph2000 times microcracks.to observe Afterand photograph calculating theirmicrocracks. face ratios, After we foundcalculating that they their are face mainly ratios, distributed we found below that 0.8%.they Althoughare mainly this distributed value is not below high, 0.8%. it is Minerals 2021, 11, x FOR PEER REVIEW 11 of 18 indeedAlthough an objective this value microscopic is not high, channel it is indeed for oil an and objective gas migration microscopic and accumulation channel for andoil and is ofgas great migration potential and significance accumulation to improving and is of grea shalet potential storage and significance permeability to improving (Figure9B). shale storage and permeability (Figure 9B).

FigureFigure 9. 9. StatisticalStatistical graph graph of of we we have have observed observed numerous numerous micro-scale micro-scale fractures fractureswidth width under under scanning electron microscope (A), and statistics of micro-crack face rate under scanning electron scanning electron microscope (A), and statistics of micro-crack face rate under scanning electron microscopel (B). microscopel (B).

5.2.3.5.2.3. Comprehensive Comprehensive Quantitative Quantitative Characterization Characterization of of Multi-Scale Multi-Scale Microfractures Microfractures AlthoughAlthough we we have have obtained obtained hole-fracture hole-fracture distribution distribution information, information, we face we new face prob- new lemsproblems given given that wethat intend we intend to describe to describe and and characterize characterize the the microfractures microfractures in in detail. detail. A A methodmethod such such as as micro-CT micro-CT gives gives a a too-low too-low resolution resolution in in terms terms of of its its accuracy accuracy in in recognizing recognizing microcracks,microcracks, with with the the connected connected three-dimensionally three-dimensionally distributed distributed microcracks microcracks being iden- being tiﬁedidentified as many as many isolated isolated small holes.small holes.This leads This to leads a decrease to a decrease in the contribution in the contribution of cracks of tocracks porosity to porosity and an and increase an increase in the contributionin the contribution of porosity of porosity to porosity. to porosity. In addition, In addition, the the scale of various experiments is different, and it is difficult to determine the proportion of microfractures in the pore-fracture system. On this basis, a comprehensive quantitative characterization of multi-scale microfractures was carried out. First, the combined quantitative characterization of high- pressure mercury intrusion and nitrogen adsorption can measure the distribution of pores over a range of 0.8 to 1500 nm. The pore size data interpreted by nitrogen adsorption give values below 50 nm, and the data interpreted by high-pressure mercury injection provide values above 50 nm. These were sampled and statistics at uniform intervals were undertaken to obtain the total pore size distribution of the shale (Figure 10A). It is found that the shale pore size has two peaks, both occupying the mesopore interval (2 to 50 nm), meaning the pore volume is mainly provided by mesopores. These peaks were at 1 to 10 nm and 10 to 100 nm, indicating that there are multiple types of pores. There were also multiple peaks above 100 nm, between 100 and 1000 nm, and between 1000 to 10,000 nm, but the ratios were small. This shows that the shale pore structure is complex, and multi- scale pores and fractures develop (Figure 10B). Observed by light and electron microscopes, microfractures are basically macro-pores with a diameter greater than 50 μm. From the point of view of pore size distribution, the error associated with using macropores to estimate the proportion of microfractures will not be too large. However, due to the low overall diagenesis of the continental shale in the study area, there are still some primary pores and clay mineral pores in the shale that will be counted among them.

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Figure 9. Statistical graph of we have observed numerous micro-scale fractures width under scanning electron microscope (A), and statistics of micro-crack face rate under scanning electron microscopel (B).

5.2.3. Comprehensive Quantitative Characterization of Multi-Scale Microfractures Although we have obtained hole-fracture distribution information, we face new problems given that we intend to describe and characterize the microfractures in detail. A method such as micro-CT gives a too-low resolution in terms of its accuracy in recognizing Minerals 2021, 11, 870 11 of 17 microcracks, with the connected three-dimensionally distributed microcracks being identified as many isolated small holes. This leads to a decrease in the contribution of cracks to porosity and an increase in the contribution of porosity to porosity. In addition, scalethe scale of various of various experiments experiments is different, is different, and itand is difﬁcultit is difficult to determine to determine the proportionthe proportion of microfracturesof microfractures in the in pore-fracturethe pore-fracture system. system. OnOn this this basis, basis, a comprehensive a comprehensive quantitative quantitative characterization characterization of multi-scale of microfrac- multi-scale turesmicrofractures was carried was out. carried First, theout. combined First, the quantitativecombined quantitative characterization characterization of high-pressure of high- mercurypressure intrusion mercury andintrusion nitrogen and adsorption nitrogen adsorption can measure can themeasure distribution the distribution of pores overof pores a rangeover ofa range 0.8 to of 1500 0.8 nm.to 1500 The nm. pore The size pore data size interpreted data interpreted by nitrogen by nitrogen adsorption adsorption give values give belowvalues 50 below nm, and 50 nm, the dataand the interpreted data interpreted by high-pressure by high-pressure mercury mercury injection injection provide valuesprovide abovevalues 50 abovenm. These 50 nm. were These sampled were and sampled statistics and at uniform statistics intervals at uniform were undertaken intervals were to obtainundertaken the total to poreobtain size the distribution total pore size of the distr shaleibution (Figure of the 10 shaleA). It (Figure is found 10 thatA). theIt is shale found porethatsize the shale has two pore peaks, size has both two occupying peaks, both the occupying mesopore the interval mesopore (2 to interval 50 nm), (2 meaning to 50 nm), themeaning pore volume the pore is mainlyvolume providedis mainly byprovided mesopores. by mesopores. These peaks These were peaks at 1 towere 10 nmat 1 andto 10 10nm to 100and nm,10 to indicating 100 nm, indicating that there arethat multiple there are types multiple of pores. types There of pores. were There also multiplewere also peaksmultiple above peaks 100 above nm, between 100 nm, 100 between and 1000 100 nm,and and1000 between nm, and 1000between to 10,000 1000 nm,to 10,000 but the nm, ratiosbut the were ratios small. were This small. shows This that shows the that shale the pore shale structure pore structure is complex, is complex, and multi-scale and multi- poresscale and pores fractures and fracturesdevelop (Figure develop 10 B).(Figure Observed 10B). by Observed light and by electron light microscopes, and electron µ microfracturesmicroscopes, microfractures are basically macro-pores are basically with macro a diameter-pores with greater a diameter than 50 greaterm. From than the 50 pointμm. of From view the of point pore sizeof view distribution, of pore size the distribution,error associated the witherror using associated macropores with using to estimatemacropores the proportion to estimate of the microfractures proportion of will microfractures not be too large. will not However, be too duelarge. to However, the low overall diagenesis of the continental shale in the study area, there are still some primary due to the low overall diagenesis of the continental shale in the study area, there are still pores and clay mineral pores in the shale that will be counted among them. some primary pores and clay mineral pores in the shale that will be counted among them.

Figure 10. N2 adsorption and mercury porosity distribution diagram (YL4-10) (A) and shale N2 adsorption-high pressure mercury injection full pore size distribution (B).

Thin slices can allow macroscopic microcracks above 5 µm to be identified, and electron microscopy can identify microcracks below this size, with some overlap between the observable scales of these methods. According to the statistics of fracture width, the widths of microcracks are not arbitrary. The slit width of the sheet is mainly 10 to 100 µm, and the aperture ratio is about 1%. The slit widths under the electron microscope are mainly 100 to 1000 µm, and the aperture ratio is about 0.2%. The distribution of slit widths does not overlap, indicating that the observed objects are of different scales, so they can complement each other. Therefore, a full-scale quantitative evaluation of microcracks can be performed by combining thin slices and electron microscope crack identification methods. Due to different magnifications used under microscopes, differing minimum crack widths can be observed, i.e., the higher the magnification, the wider the scale of the microcrack that can be observed, and the greater the number of microcracks observed, the higher the calculated face rate. Therefore, the large image stitching function of the NIS-Elements software was used to continuously stitch multiple microscope images at high magnifications into a single large image. The image obtained in this way not only shows more microscopic microfractures, but also avoids the influence of heterogeneity of microfracture development in shale. For example, using a 5× objective lens and a 10× eyepiece, stitching slices of the full field of view for statistical analysis, the average aperture ratio of microfractures is Minerals 2021, 11, x FOR PEER REVIEW 12 of 18

Figure 10. N2 adsorption and mercury porosity distribution diagram (YL4-10) (A) and shale N2 adsorption-high pressure mercury injection full pore size distribution (B).

Thin slices can allow macroscopic microcracks above 5 μm to be identified, and electron microscopy can identify microcracks below this size, with some overlap between the observable scales of these methods. According to the statistics of fracture width, the widths of microcracks are not arbitrary. The slit width of the sheet is mainly 10 to 100 μm, and the aperture ratio is about 1%. The slit widths under the electron microscope are mainly 100 to 1000 μm, and the aperture ratio is about 0.2%. The distribution of slit widths does not overlap, indicating that the observed objects are of different scales, so they can complement each other. Therefore, a full-scale quantitative evaluation of microcracks can be performed by combining thin slices and electron microscope crack identification methods. Due to different magnifications used under microscopes, differing minimum crack widths can be observed, i.e., the higher the magnification, the wider the scale of the microcrack that can be observed, and the greater the number of microcracks observed, the higher the calculated face rate. Therefore, the large image stitching function of the NIS- Elements software was used to continuously stitch multiple microscope images at high magnifications into a single large image. The image obtained in this way not only shows more microscopic microfractures, but also avoids the influence of heterogeneity of Minerals 2021, 11, 870 12 of 17 microfracture development in shale. For example, using a 5× objective lens and a 10× eyepiece, stitching slices of the full field of view for statistical analysis, the average aperture ratio of microfractures is 1.08%, 1.08%,and the and aperture the aperture ratio of ratio most of shale most samples shale samples is mainly is distributed mainly distributed below 0.7%. below However, 0.7%. However,there are therestill some are still samples some sampleswith exceptionally with exceptionally developed developed microcracks, microcracks, with facewith ratios facebetween ratios 1.5% between and 1.5%3%, and and the 3%, standard and the deviation standard deviationreaches 1.163%, reaches which 1.163%, is higher which than is higherthe average than the value, average indicating value, that indicating the data that are thevery data discrete. are very It also discrete. shows Itthat also the shows degree thatof the differentiation degree of differentiation of microfracture of microfracture development development is very is high. very high. We We evaluated evaluated the thedevelopment development of of microcracks microcracks in in the the microstructure microstructure by randomly randomly taking taking more more than than ten ten photosphotos of of each each sample sample under under the electronthe electron microscope microscope at magniﬁcations at magnifications of 400 toof 2000 400 totimes. 2000 Thetimes. average The aperture average ratio aperture of the ratio nano-scale of the microcracks nano-scale microcracks under the electron under microscope the electron ismicroscope 0.53%, and is the 0.53%, difference and the is notdifference large, withis not all large, of them with concentrated all of them concentrated below 0.8%, below and mostly0.8%, below and mostly 0.2%. We below believe 0.2%. that We the believe development that the of development these nanoscale of microfractures these nanoscale willmicrofractures play a positive will role play in shalea positive reservoir role properties.in shale reservoir The microfracture properties. contributionThe microfracture rate (thecontribution ratio of fracture rate (the porosity ratio of to fracture total porosity), porosity to is calculatedtotal porosity), according is calculated to the porosityaccording ofto different the porosity samples, of different including samples, the total including porosity the of shale total inporosity the Yuanba of shale area in of the northern Yuanba Sichuanarea of andnorthern the Fuling Sichuan area and of eastern the Fuling Sichuan, area whichof eastern are 5%Sichuan, and 7%, which respectively. are 5% and Then, 7%, therespectively. microfractures’ Then, contribution the microfractures’ rates for contribution different regions rates andfor different wells are regions calculated. and Thewells dataare arecalculated. highly dispersed, The data andare highly in general, dispersed, microfractures and in general, account microfractures for about 20% of account the total for poreabout volume 20% of (Figure the total 11). pore volume (Figure 11).

Figure 11. Micro-slice photos and scanning electron microscope full-scale micro-fractures hole ratio evaluation. 5.2.4. Controlling Factors of Microfractures Development Through our analysis of the previously calculated microfracture contribution rate, we can see that the degree of the microfractures’ inﬂuence on the development of porosity is still very large. Generally speaking, the inﬂuencing factors of the development of microfractures in shale include the brittle mineral content, lithological combination, and regional structure. Therefore, we will discuss these three factors to determine which controls the development of microfractures. We use the brittleness index (BI) to evaluate the evaluation index of brittle minerals. There are many calculation methods, with generally the method of rock mineralogy being used. Quartz, carbonate minerals account for percentage of quartz, carbonate minerals, and clay minerals. The percentage of these is the brittleness index. During our analysis, we found that in addition to these minerals, pyrite is also a component of our samples. However, pyrite is also a brittle mineral and is not accounted for. Therefore, we used the content of other minerals, except for clay minerals, to describe the brittleness index of shale, and calculated the brittleness index of each sample separately, and hence determined the brittleness index, porosity, fracture porosity, and fracture contribution rate (Table1). These parameters were then, respectively, analyzed for correlations. Minerals 2021, 11, x FOR PEER REVIEW 13 of 18

Figure 11. Micro-slice photos and scanning electron microscope full-scale micro-fractures hole ratio evaluation.

5.2.4. Controlling Factors of Microfractures Development Through our analysis of the previously calculated microfracture contribution rate, we can see that the degree of the microfractures’ influence on the development of porosity is still very large. Generally speaking, the influencing factors of the development of microfractures in shale include the brittle mineral content, lithological combination, and regional structure. Therefore, we will discuss these three factors to determine which controls the development of microfractures. We use the brittleness index (BI) to evaluate the evaluation index of brittle minerals. There are many calculation methods, with generally the method of rock mineralogy being used. Quartz, carbonate minerals account for percentage of quartz, carbonate minerals, and clay minerals. The percentage of these is the brittleness index. During our analysis, we found that in addition to these minerals, pyrite is also a component of our samples. However, pyrite is also a brittle mineral and is not accounted for. Therefore, we used the content of other minerals, except for clay minerals, to describe the brittleness index of shale, and calculated the brittleness index of each sample separately, and hence Minerals 2021, 11, 870 13 of 17 determined the brittleness index, porosity, fracture porosity, and fracture contribution rate (Table 1). These parameters were then, respectively, analyzed for correlations.

TableTable 1.1. BriefBrief tabletable ofof brittlenessbrittleness indexindex calculationcalculation resultsresults ofof thethe sample.sample.

Sample PotashPotash BrittlenessBrittleness TotalTotal Sample ID CClaylay Quartz Quartz PlagioclasePlagioclase Calcite Calcite Dolomite Dolomite Siderite Pyrite Pyrite ID FeldsparFeldspar IndexIndex BIBI PorosityPorosity FY1-9FY1-9 56.2 31.2 31.2 0.8 0.8 2.5 8.2 1.1 43.8 43.8 1.49 FY1-13FY1-13 50.1 28.6 28.6 1.9 18.1 1.3 49.9 49.9 4.74 YB102-7YB102-7 40.6 45.8 45.8 0.7 0.7 1.8 8.4 2.7 59.4 59.4 2.46 YL171-5YL171-5 32.8 26.9 26.9 1 17.4 3030 0.70.7 1.2 67.2 67.2 1.26 YL176-7YL176-7 40.5 31.5 31.5 1.1 1.1 2.4 23.4 1.11.1 59.5 59.5 1.83 YL4-6YL4-6 48.1 34.1 34.1 1.2 1.2 4.6 9.1 1.51.5 1.4 51.9 51.9 1.70 YL4-10YL4-10 26.6 16.2 16.2 1.6 55.6 73.4 73.4 1.53

We analyzed analyzed the the correlation correlation between between the the three three factors factors of total of porosity, total porosity, fracture fracture porosity,porosity, and the and contribution the contribution rate of fracturesrate of fractures to porosity to presentedporosity presented in Table1 andin Table the brittleness 1 and the index.brittleness The index. analysis The results analysis are resultsshown inare Figure shown 12 in. From Figure the 12 results. From of the the results correlation of the analysis,correlation these analysis, three parametersthese three parameters are seen to beare very seen similar, to be very and similar, the resulting and the graphs resulting are closegraphs to are each close other. to each However, other. fromHowever, these from results, these we results, cannot we prove cannot that prove the total that porosity, the total fractureporosity, porosity, fracture and porosity, fracture-to-porosity and fracture of-to the-porosity rock have of athe signiﬁcant rock have correlation a significant with thecorrelation brittleness with index the brittleness of the rock. index of the rock.

Figure 12. The relationship between sample brittleness index and microfracture porosity, contribution of microfracture Figure 12. The relationship between sample brittleness index and microfracture porosity, contribution of microfracture and and total porosity. total porosity. InIn terms terms of of the the lithological lithological combination, combination, for forthe samplesthe samples examined, examined, there there is no muta- is no tionmutation from mudstone from mudstone to limestone to limestone or sandstone or sandstone in terms in of terms their lithology. of their lithology. In a previous In a article, it was found that a small amount of biological shells and sandstone bands exist in some samples of the fracture morphology. In shell-bearing shale and silt-bearing shale, a small amount of microcracking can be seen, but their proportions are less than that of shale and cannot play a signiﬁcant role in the development of microfractures. In terms of regional structural distribution, wells YB21 and YB102 are located in structurally stable areas, and large regional faults are not developed in their vicinity. The porosity in this area is well developed, but the contribution rate of microcracks to porosity is not high, with the microcracks accounting for about 10% of the total pores. Wells YL4, YL171, and YL176 are located in structurally developed areas, with fault-folded belts and high-steep belts. Their porosity is similar, but their microfractures lead to very high porosity, with the microfractures accounting for up to 40% to 90% (Figure 13). Through the above discussion of the contribution rate of microfractures, we gain the following basic understanding that the contribution rate of microfractures is less correlated with the content of brittle minerals, and the contribution of microfractures is higher in areas with developed structures than in areas with underdeveloped structures (Figure 14). The development of shale microfractures is mostly shale-related microfractures, and their degree of development is related to the development of the shale, and the regional tectonic environment. Minerals 2021, 11, x FOR PEER REVIEW 14 of 18 Minerals 2021, 11, x FOR PEER REVIEW 14 of 18

previous article, it was found that a small amount of biological shells and sandstone bands previous article, it was found that a small amount of biological shells and sandstone bands exist in some samples of the fracture morphology. In shell-bearing shale and silt-bearing exist in some samples of the fracture morphology. In shell-bearing shale and silt-bearing shale, a small amount of microcracking can be seen, but their proportions are less than shale, a small amount of microcracking can be seen, but their proportions are less than that of shale and cannot play a significant role in the development of microfractures. that of shale and cannot play a significant role in the development of microfractures. In terms of regional structural distribution, wells YB21 and YB102 are located in In terms of regional structural distribution, wells YB21 and YB102 are located in structurally stable areas, and large regional faults are not developed in their vicinity. The structurally stable areas, and large regional faults are not developed in their vicinity. The porosity in this area is well developed, but the contribution rate of microcracks to porosity porosity in this area is well developed, but the contribution rate of microcracks to porosity is not high, with the microcracks accounting for about 10% of the total pores. Wells YL4, is not high, with the microcracks accounting for about 10% of the total pores. Wells YL4, YL171, and YL176 are located in structurally developed areas, with fault-folded belts and YL171, and YL176 are located in structurally developed areas, with fault-folded belts and high-steep belts. Their porosity is similar, but their microfractures lead to very high high-steep belts. Their porosity is similar, but their microfractures lead to very high porosity, with the microfractures accounting for up to 40% to 90% (Figure 13). Through porosity, with the microfractures accounting for up to 40% to 90% (Figure 13). Through the above discussion of the contribution rate of microfractures, we gain the following basic the above discussion of the contribution rate of microfractures, we gain the following basic understanding that the contribution rate of microfractures is less correlated with the understanding that the contribution rate of microfractures is less correlated with the content of brittle minerals, and the contribution of microfractures is higher in areas with content of brittle minerals, and the contribution of microfractures is higher in areas with developed structures than in areas with underdeveloped structures (Figure 14). The developed structures than in areas with underdeveloped structures (Figure 14). The Minerals 2021, 11, 870 development of shale microfractures is mostly shale-related microfractures, and14 oftheir 17 development of shale microfractures is mostly shale-related microfractures, and their degree of development is related to the development of the shale, and the regional tectonic degree of development is related to the development of the shale, and the regional tectonic environment. environment.

Figure 13. Relationship between the contribution of microfracture porosity and the degree of Figure 13. Relationship between the contribution of microfracture porosity and the degree of regional Figureregional 13 .tectonic Relationship development. between the contribution of microfracture porosity and the degree of regionaltectonic development.tectonic development.

Figure 14. Distribution map of the top boundary and bottom boundary of the Da’anzhai section in Yuanba area. Figure 14. DistriDistributionbution map of the top boundary and bottom boundary of the Da’anzhai section in Yuanba area.area.

6. Conclusions Through thin slice section, micro-CT scanning, scanning electron microscopy analysis, and nitrogen adsorption and high-pressure mercury intrusion measurements, we analyzed the characteristics of microfractures in the Da’anzhai shale from Lower Jurassic artesian

wells in northeastern Sichuan. The conclusions are as follows: (1) Four types of fractures appear to have developed in the shale of the Da’anzhai section: mainly microfractures caused by tectonic stress, followed by smaller clay mineral diagenetic shrinkage fractures, organic marginal shrinkage fractures, and microfractures inside mineral particles. Among them, structural fractures and organic matter contraction fractures are the main development types and are of signiﬁcance for shale reservoir development and seepage. (2) The structural microfractures are mainly open and have developed in the bedding- developed shale, with straight shapes and bedding orientation. In some areas, interlayer seams are developed and formed into dendritic and net-like shapes, with curved seam surfaces, fully opened, and mainly tensile. Organic matter cracks often develop on the edge of the contacts between organic matter and minerals, showing a slit-like shape. Through micro-CT imaging, we clearly see that the seams related to the bedding are particularly developed, with larger openings, wider extensions, intersecting and expanding, forming a three-dimensional interconnected hole-slit system. Minerals 2021, 11, 870 15 of 17

(3) Multi-scale stitching allows the calculation of the contribution rate of microfractures for different regions and wells. Generally, microfractures account for about 20% of the total pore volume. However, the degree of development of microfractures in different structural regions varies greatly. Microfractures in fault-wrinkle belts and high-steep belts can account for up to 40% to 90%, and microfractures in areas with underdeveloped structures account for about 10% of the total pore volume.

Author Contributions: Conceptualization, Z.L. and H.Q.; methodology, F.W.; software, F.W.; valida- tion, Z.Y., Z.S.; formal analysis, H.Q.; investigation, R.L., X.W.; resources, Z.J.; data curation, Z.L.; writing—original draft preparation, H.Q.; writing—review and editing, Z.J.; visualization, D.W.; supervision, Z.S.; project administration, F.C.; funding acquisition, Z.J. All authors have read and agreed to the published version of the manuscript. Funding: This study is supported by the National Natural Science Foundation of China (no. 41872135 & no.42072151)and the National Major Science and Technology Projects of China (2016ZX05034001-005). Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to some basic research involves confidentiality. Acknowledgments: The Southern Company of Exploration SINOPEC is thanked for providing shale samples and geological background references. We are grateful to editors and reviewers for their constructive comments and suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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