Fracture development characteristics and controlling factors for reservoirs in the Lower Silurian Longmaxi Formation marine shale of the Sangzhi block, Hunan Province, China

Journal of Petroleum Science and Engineering 184 (2020) 106470

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Fracture development characteristics and controlling factors for reservoirs in the Lower Silurian Longmaxi Formation marine shale of the Sangzhi block, Hunan Province, China

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Gang Zhao (Ph.D)a,b,c,d, Wenlong Ding (Ph.D)a,b,c,d,∗, Yaxiong Suna,b,c,d, Xinghua Wanga,b, Li Tiane, Jingshou Liua,d, Siyu Shia,c, Baocheng Jiaoa,d, Long Cuif a

School of Energy Resources, China University of Geosciences, Beijing 100083, China Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China Beijing Key Laboratory of Unconventional Natural Gas Geology Evaluation and Development Engineering, China University of Geosciences, Beijing 100083, China d Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, China University of Geosciences, Beijing 100083, China e China National Administration of Coal Geology, Beijing 100036, China f Beijing Dadi Special Exploration Branch, China Coal Geological Engineering Corporation, Beijing 100161, China b c

A R T I C LE I N FO

A B S T R A C T

Keywords: The Longmaxi Formation shale Fracture development characteristics Fracture distribution controlling factors Sangzhi block

The Lower Paleozoic Silurian Longmaxi Formation shale in the Sichuan Basin of southern China has achieved commercial exploitation in recent years, and has become a rapid growth point in China's oil and gas reserves. However, the Longmaxi Formation on the peripheral margin of the Sichuan Basin has not made substantial progress. Fractures, that as fluid reservoirs and migration channels, play an important role in shale gas exploration and development. Based on data from field outcrop observations, drilling cores, well logging, geochemical tests, microscopic thin section observations, and focused-ion-beam scanning electron microscope (FIBSEM) combined with energy dispersive spectrometer (EDS) analysis and cathodoluminescence microscope (CLM) technology, the development characteristics of fractures in the Longmaxi Formation SY-5 well in the Sangzhi block of Sangzhi-Shimen synclinorium, NW Hunan Province, are described systematically, and their important roles, primary dominant factors and effectiveness in the shale gas reservoir are discussed. The results showed that tectonic factors are the dominant factors of shale fracture development, followed by nonstructural factors. The fractures were mainly ductile shear fractures, with short extension distances and small apertures, and they had at least two openings in the burial history. The statistical fracture density of drilling cores was no obvious correlated with the contents of total organic carbon (TOC) (0.18%–2.49%) and clay minerals (13.0%–34.0%), and the same with brittle quartz (32%–72%), which indicates that they have controlled the development of fractures together. These results could be used for estimating effective permeability, fracture distribution, numerical simulation, well deployment, hydraulic fracturing and reserves calculation in the processes of exploration and development.

1. Introduction With the successful of the “shale gas revolution” in North America, China also ramped up exploration research related to unconventional petroleum (Curtis, 2002; Zou et al., 2010; Zou, 2017; Zeng et al., 2013a; Jiu et al., 2013; Ding et al., 2013; He et al., 2015; Wang et al., 2016b; Wang et al., 2018). The marine shale in southern China is mainly concentrated in the Sichuan Basin and adjoining regions (Huang et al., 2011; Nie et al., 2012; Guo and Zhang, 2014; Zhou et al., 2014b). The two most favorable marine shale formations, the Lower Cambrian

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Niutitang Formation shale and the Lower Silurian Longmaxi Formation shale are widely developed in the Yangtze region, which primarily includes the southeastern Sichuan, northeastern Guizhou, northwestern Hunan, and southwestern Hubei areas (Zhou et al., 2014b; Jing et al., 2016; Zou, 2017). Although industrial production has been preliminarily realized in the Longmaxi Formation marine shale gas in the Sichuan Basin, with an evaluated and ascertained favorable exploration and development area of 4.5 × 104 km2, and proven in-place shale gas exceeding 5 440 × 108 m3, showing the rudiments of a 1 × 1012 m3 large gas province (Zou et al., 2015, 2016), the adjoining regions have

Corresponding author. School of Energy Resources, China University of Geosciences, Beijing 100083, China. E-mail addresses: [email protected] (G. Zhao), [email protected] (W. Ding).

https://doi.org/10.1016/j.petrol.2019.106470 Received 28 September 2018; Received in revised form 26 August 2019; Accepted 4 September 2019 Available online 09 September 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) The location of study area (red box below A) and the Sangzhi-Shimen synclinorium belt; (B) Main folds, faults and outcrops in the Sangzhi block and the key well of location of SY-1, SY-2, SY-5; L1, L2, L2, L3, L4, L5 and L6 are the location of fractures observation in the field corresponding to Fig. 11; (C) Cross sections a-a’ and b-b’ through the Sangzhi block shown in B. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

not achieved the industrial oil flows and further breakthroughs. The reason is that although marine shale strata in North America were formed in cratonic and foreland basins, with a large-scale stable base and relatively simpler history of tectonic movement (Ma et al., 2018), the Yangtze region has not. The Yangtze region has suffered at least four periods of large movements from the Low Paleozoic Silurian until now (Guo, 2013). As a result of those tectonic movements, faults and fractures are well developed, including in the Sangzhi block, NW Hunan Province. Shale gas can be commercialized by hydraulic fracturing, and the development of natural fractures directly affects pressure, direction, and angle, and then impacts the overall fracturing effectiveness (Gale et al., 2007; Bustin et al., 2008). Thus, the development degree of a natural fracture system not only directly affects the exploitation efficiency, but also determines its quality and yield of shale gas reservoirs (Bowker, 2007; Gale et al., 2007; Zeng et al., 2013a; Wang et al., 2018). In addition, the rock particles and organic matter on the shale surface,

are in an adsorption state, and some of the shale gas is a free state in the fractures; therefore, characterizing natural fractures in shale from macro to micro scales is critical to and cannot be neglected (Zhang et al., 2004; Wang et al., 2018). Previous scholars have undertaken considerable efforts in this study area. In terms of structure, some studies have analyzed the tectonic control of the Silurian reef distribution and development on the Upper Yangtze Platform (Zhang et al., 1996) and studied the fold style and its formation mechanism (Yan et al., 2000, 2008). Regarding the Niutitang Formation shale, some work undertook comparative characterization (Wan et al., 2018), examined the stable isotopic geochemistry of organic carbon and pyrite sulfur (Cao et al., 2004), and studied the characteristics and significance of the fractures in well SY-2 employing drilling data (Wang et al., 2018). Regarding the Lower Silurian Longmaxi Formation shale, some papers analyzed shale gas reservoir conditions and favorable areas (Ye and Li, 2014), evaluated the shale gas

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2.3. Stratigraphic burial history

potential (Wan et al., 2017) and investigated pore structure and fractal characteristics (Hu et al., 2016). Although these studies have made great contributions to the area, for evaluating mining, the development degree of natural fractures is controlled by many external factors and internal characteristics, which control the occurrence of fractures, combined characteristics, mineral filling degree, formation stage, and area and linear densities. These characteristics have an important impact on the evaluation of resources, and thus, it is essential and urgent to characterize the natural fractures of the Longmaxi Formation shale. The development characteristics of natural fractures and controlling factors in the Lower Silurian Longmaxi Formation shale, and the relationships between the TOC, vitrinite reflectance (Ro) and shale mineral content are studied in this work, which further analyses the main factors controlling of the natural fractures based on detailed testing data from drilling cores, high-precision observations of casting thin sections and FIB-SEM combined with EDS, CLM, and careful fracture observations from well SY-5. The results provide practical significance for the exploration and development of Lower Paleozoic marine shale gas around the Sichuan Basin.

The Chinese landmass, as a composite continent, consists of multiple small continental blocks, such as Sino-Korea, Yangtze, and Tarim, and orogenic belts, and is thus characterized by an unstable base and strong tectonic movements (Guo et al., 2004; Ma et al., 2018). Over its long geological history, the southern China marine facies have undergone multicycle tectonic movements and intense post reconstruction. For example, the Sichuan Basin is a complex basin, that is composed of ancient cratonic and Mesozoic-Cenozoic foreland basins, and two sets of Cambrian and Silurian shale strata experienced Caledonian, Hercynian, Indosinian, Yanshanian and Himalayanian tectonic movements. During the late Yanshanian movement, the two sets of shale strata experienced 6000–9000 m of deep burial, which was followed by sharp uplifts during the Yanshanian and Himalayan compressions (Ma et al., 2018). The buried history types of southern marine facies are divided into two categories and six types (Wo et al., 2007). The situation for the Sichuan Basin is similar to that of the West Hunan and Hubei Province, belonging to early subsidence and late uplift types. It has undergone early settlement at the Early Yanshanian and then overall uplift at the Late Yanshanian-Himalayanian stage (Fig. 3), which consisted of single subsidence during a macro lift cycle. From the point of view of the hydrocarbon source rock, the basic hydrocarbon generation process continued to completion after being briefly slowed by the Indosinian movement, only experiencing a process of hydrocarbon generation. There was no significant subsidence after the late uplift. The difference between the Sichuan Basin and West Hunan and Hubei Province lies in the subsidence period of Sichuan type is longer than the uplift period, while the situation was opposite for the West Hunan and Hubei Province. Fig. 3 shows the single well burial history curve for well SY-5 in the study area. The figure shows the burial history of the study area from the Caledonian to the Late Yanshanian-Himalayanian. In the Caledonian stage, Cambrian, Ordovician, and Silurian strata were deposited in a marine shelf sedimentary environment. The strata were uplifted, and at the end of the Caledonian stage, sedimentation ended, and the Upper Silurian strata suffered denudation. In the Hercynian-Indo-China stage, the Early Hercynian was in a state of erosion, and the Upper Silurian strata were experiencing denudation, subsequently entering the overall shock stage, with a lack of Carboniferous strata. Overall rapid subsidence occurred in the Early Permian and into the hydrocarbon generation threshold stage (Fig. 3), subsequently, Permian to Early Triassic strata were deposited. During the Early Yanshanian period, the burial continued, and at the end of the Triassic period into the Middle Jurassic period, the wet gas and the dry gas stages began (Fig. 3). In the Late Yanshanian-Himalayanian period, the study area entered the orogenic period, the northwest to southeast experienced strong compressional stresses, that formed the northeast, linear fold, which basically led to the present tectonic framework in the work area.

2. Geological setting 2.1. Structure The Sangzhi block (Fig. 1A) is located in the Sangzhi-Shimen synclinorium belt, in which there is a secondary structure unit in the most southern part of the NW HunanHubei Fold Belt (Guo et al., 2004; Guo et al., 2005). It lies to the north of the Baojing-Cili Great Fault (the F2 fault is the southern boundary), and to the south of the Jianshi-Pengshui Fault (the F1 fault is the northern boundary) Fig. 1A. The Sangzhi-Shimen synclinorium is under the influence of Xuefeng movement (Zhou et al., 2014a), with the emergence of the Baojing-Cili Fault, and gradually developed a series of NE to EW anticlines and synclines. Its the basic structural form is concave in the north and south but uplifts in its centre. The tectonic line is in a generally NNE-NE-EW direction, and has an arc-shaped distribution of NW-trending protrusions (Fig. 1A). The folds are mainly characterized by the wide and narrow south-east and steep and narrow north-west wings of the anticlines, whereas the synclines are reversed. The NW flanks are narrow and steep (from 40° to 60°), whereas SE flanks of anticlines are usually wide and flat (from 20° to 30°). However, the synclines, on the contrary, indicate an extrusion deformation in the direction from SE to NW or from south to north (Fig. 1B and C). According to the analysis of the unconformity or pseudo conformity of the Silurian in the Shimen-Sangzhi area over the Middle Triassic, the structure was formed in the Indosinian and was superimposed the structural traces of the Yanshanian (Wang et al., 2018). 2.2. Stratigraphic sedimentation

3. Experiments and methods Examinations of outcrops and subsurface investigations of the Sangzhi block by researchers have revealed the presence of Sinian, Cambrian, Ordovician, Middle-Lower Silurian, Devonian, Permian and Triassic deposits (Fig. 2) (Hu et al., 2016; Wang et al., 2018). Owing to the Hercynian-Indo-China movement, there is lack of the Upper Silurian-Lower Devonian and Carboniferous strata. The Jurassic, Cretaceous, Paleogene and Neogene strata are completely eroded because of Yanshanian and Himalayanian orogeny. Wells SY-1 and SY-5 in the study area targeted the Lower Silurian Longmaxi Formation, whereas well SY-2 had the Lower Cambrian Niutitang Formation as the goal. Well SY-5 reached a maximum depth of 1186.52 m, penetrated Silurian units, including Lower Silurian Longmaxi and Middle Silurian Luoreping Formations and terminated at Ordovician Guniutang Formation. The Longmaxi Formation shale is gray or black shale and 70.90 m thick, with a depth range from 1024.82 m to 1095.72 m depth in well SY-5.

The experimental analysis of the SY-5 well mainly included TOC, whole rock X-ray diffraction (XRD), quantitative mineral clay analysis, vitrinite reflectance (Ro) and rock pyrolysis. The Geochemistry Laboratory of the Jiangsu Geological Mineral Design and Research Institute conducted the organic geochemistry tests of the Longmaxi Formation samples. The protocol referred in the “Determination of Total Organic Carbon in Sedimentary Rock” in Chinese standard GB/T19145–2003 was followed for the TOC samples. Rock pyrolysis measurements followed Chinese standard GB/T 186022001 (Wu et al., 2001). The XRD analysis followed analytical protocol outlined by the SY/T6201-1996 (Wang et al., 1996) by oil and gas industry standard. Determination of vitrinite reflectance (Ro) referred to the method of vitrinite reflectance determination in sedimentary rocks in the SY/T5124-2012 (Tu et al., 2012) petroleum industry standard of 3

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Fig. 2. Stratigraphic column of the Sangzhi block (the target formation is shown light-gray).

4. Results

China. The experimental conditions were conducted at 60% relative humidity and 25 °C. The Electron Probe Microanalysis laboratory of the Beijing Research Institute of Uranium Geology of China National Nuclear Corporation conducted microscope, cathodoluminescence microscope (CLM) and Focused Ion Beam Scanning Electron Microscope (FIB-SEM) combined with Energy Dispersive Spectrometer for a large number of observations of shale thin sections prepared in this study for the development of microfractures. The microstructure of the natural fractures in the shale were directly observed in each shale sample using the Secondary Electron mode and Back Scattering mode of a TESCAN GAIA3 Field Emission environment scanning electron microscope. The tests referred to the oil and gas standards of GB/T 15074-2008 (Fan et al., 2008) national standard and the general principles for electron probe quantitative analyses. SEM inspection was conducted at 25 °C and 35% relative humidity.

4.1. Outcrop fractures characteristics It was found that fractures (Fig. 4) are well-developed in the Longmaxi Formation outcrop observed in the study area, showing that it has experienced a strong tectonic evolutionary process. Outcrop fractures in the field are mainly weathering fractures (Fig. 4A and C). Two groups of regional fractures on the same layer are generally developed simultaneously, and they cut the rock layers into a checkerboard pattern on the layer (Fig. 4B). Graptolite is seen in the Longmaxi Formation black carbonaceous shale (Fig. 4D). Fractures are not only good reservoir spaces, but also channels for fluid migrations, and can improve reservoirs the permeability. Due to the high content of shale and its foliated or thin laminar bedding, its resistance to weathering is relatively weak. If subjected to long-term weathering, leaching and 4

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Fig. 3. The single well burial history curve for well SY-5 in the study area.

Fig. 4. Outcrop fractures of the Longmaxi Formation shale in the study area (modified from (Wan et al., 2017)). (A) Lower Longmaxi outcrop, rocks are highly weathered and fragmented; (B) Crossed relationship fractures; (C) Upper Longmaxi outcrop and (D) Longmaxi Formation black carbonaceous shale with graptolite.

by rock bursts under tectonic stress (Fig. 5). According to the stress state during the formation of structural fractures, fractures can be divided into three types: tension, shear and tension-shear fracture. Base on those three fractures classifications, we selected the core photographs of the target interval, taking into account the change of fracture occurrence and the depth distribution range (Fig. 5). (1) Tension fractures are formed by tensile stresses perpendicular to the fracture plane and extension direction. They present high dip angles, large apertures and rough fracture surfaces. Extensions do not disappear over large distances: in the fractures, two walls open, the fractures are sometimes

denudation, physical and chemical breaks along the bedding surfaces easily occur, forming large numbers of weathering fractures.

4.2. Core fractures Core fracture observation is an important means of studying fractured reservoirs. It is the most direct means of fracture research to visually describe the fractures in the formation mainly based on the development of core fractures. Core observations for well SY-5 show that the fractures related to structure are well developed and may be caused 5

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Fig. 5. Fractures in cores of the Longmaxi shale in the SY-5 well. (A) 1033.4 m, three parallel slickensides with no fillings; (B) 1039.2 m, horizontal fracture filled with calcite and oriented parallel to bedding; (C) 1041.8 m, low dip fracture sealed with calcite; (D) 1048.1 m, two high dip angle fractures sealed with calcite; (E) 1055.5 m, vertical compression-shear fracture sealed with calcite; (F) 1062.9 m, anti-Y shaped fracture sealed with calcite and the surface of shale and siltstone; (G) 1064.1 m, the complex fractures systems between three high dip angle fractures sealed with calcite; (H) 1068.4 m, high dip angle fractures interaction cut by horizontal fractures filled with calcite; (I) 1072.3 m, slickenside and high dip angle fractures; (H) 1073.6 m, complex fractures system and a slickenside along the fracture; (K) 1035.6 m, two high dip angle fractures observed from the core interface; (L) 1038.2 m, horizontal slip fracture with a smooth mirror.

filled with minerals and mineral growth line direction and two walls directly intersect (Fig. 5I–H). (2) Shear fractures are formed by shear stresses parallel to the fracture plane and extension direction. Shear fractures observed occur mainly at high dip angles, with stable occurrences, have long extensions, small widths and are even closed and filled with small minerals (Fig. 5C–F and K). There are also two kinds of low dip angle shear fractures: the first being slippage fracture and decollement fractures, which are mainly developed in the argillaceous rocks, and are mainly distributed in the top, and bottoms of the shale formations, especially in the area near the sandstone reservoir and lithological change development. Most common fracture surfaces have obvious scratch steps, are ordered and have smooth mirror characteristics (Fig. 5A, F and L), and are completely open, but they tend to vary considerably. The dip angle between slippage fractures and the direction of maximum principal pressure stress (σ1) is not subject to the distribution rule of shear angle in the shear fracture criterion of rock. The other type comprises the near-horizontal shear fracture. Nearhorizontal shear fractures (Fig. 5B) show small angle, oblique intersections between the fracture surfaces and micro level. (3) Tensionshear fractures are caused by shear stresses parallel to the fractures planes, are perpendicular to the propagation direction and are of the tearing type. Two kinds of stresses are associated with tensile-shear fractures, and thus the fractures have the characteristics of tensile and shear fractures. Some fractures are filled with calcite, and display slickensides, and chatter marks suggestive of shear displacement (Fig. 5G–H). Observations and measurements of fracture attitude and statistical filling in shale cores (Fig. 6) are the basic methods used to study shale fracture development degree and distribution law. The structural fractures of the Longmaxi shale for the SY-5 cores are steeply dipping (> 75°), and comprise approximately 9% of all fractures (Fig. 6A). Approximately 45% of the fractures have dip angles in the range of 50–75°, while the subhorizontal fractures (15–50°) and horizontal

fractures (< 15°) account for 24% and 22% of the total fractures, respectively. The aperture sizes of filling fracture are mainly 0.1–0.5 mm and 0.5–1 mm, and the ratios are 70% and 16% (Fig. 6B). Fracture lengths ranged from 0-5 cm to 5–10 cm in the ratio of 54%–40%, and from 10-20 cm to 6% of the total fracture length (Fig. 6C). Calcite is the primary fracture filling mineral, composing 80% of the total, and quartz and pyrite each constitute 8% of the total (Fig. 6D). In summary, the fractures in the Longmaxi Formation shale are dominated by high dip angle (50–75°), the fracture lengths are predominantly 0–5 cm, and the fracture widths are predominantly by 0.1–0.5 mm. Most fractures are filled with calcite. The results show that the cracks in the Longmaxi formation are mainly ductile shear fractures that are affected by regional structures. From the comprehensive column chart of fracture characteristics for well SY-5 (Fig. 7), it can be seen that all of the fractures in the Longmaxi Formation are well developed. From the calculated line and area densities data, the upper and middle fractures are well developed. The dip angles, lengths and apertures of the fractures are all larger than those for the bottom, and the linear and area density can reach 189 fracs/m and 102 m/m2, respectively. Although the development of fractures in the bottom of the Longmaxi Formation is poor, the reason will be discussed later, in the contact area between the Longmaxi Formation and Baota Formation, the linear and area densities of the fractures are more developed, similar to the upper and middle sections. This phenomenon may be related to the lithological heterogeneity under regional tectonic stresses. 4.3. Micro fractures Macroscopic fractures such as outcrops and cores can be observed with the naked eye, but microscopic fractures that provide important reservoir space and migration channels for fluids need to be observed in the laboratory. (Ding et al., 2011; Dai et al., 2016; Wang et al., 2016a; 6

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Fig. 6. Pie charts of (A) Dip angles, (B) Apertures, (C) Lengths and (D) Filled mineral of fractures observed in the Longmaxi shale of well SY-5 cores.

in Fig. 3. Fig. 8G–H show us the results observed in the same thin sections under OM, CLM and EDS, In Fig. 8G, the fractures are sealed with calcite, and there are two dissociation groups and many secondary fractures that are not filled with calcite. Corresponding to Fig. 8H, the calcite is found only in the middle of the fracture calcite mineral phenomenon of orange light, and the fracture's edges and small branch fractures do not present this kind of phenomenon, which may be related to crystal growth processes, due to different ions that exist in the fluid, causing fractures in crystals containing different ionic compositions, parts of which contain quenching agents, such as Fe2+. It can also provide evidence of CLM for different times when minerals filled the fractures. Fig. 8I–L show a series of photos. Fig. 8I is an OM photo, and Fig. 8J is a FIB-SEM photo, in which two test point located in the light and dark areas were analyzed by EDS, as shown in Fig. 8K–L. A blue color was observed by OM, but there is a rhombic orange color filling in the middle (Fig. 8I). There are only light and dark colors under EP (Fig. 8J), and in the figure, two test points are marked by five-pointed stars. Fig. 8K shows the spectral analysis diagram, K points showing Ba, S and O, and the mineral was speculated to be BaSO4, while Fig. 8L shows Ca, C and O elements, for which the mineral was speculated to be CaCO3. As seen in Fig. 8I, barium sulfate has many unfilled fractures, which may be formed in the late stage. From the perspective of the whole, the main space comprises calcite fractures, with BaSO4 filling in calcite. According to the analysis above, quartz minerals precipitate and crystallize along the edges of fractures after they are opened, and calcite precipitates along the middles of fractures as they opened wider. Considering the metasomatic sequence, fractures are opened at least two times, and calcite formation occurred subsequent to quartz. There was an anhydride filling period suffering from the influence of seafloor hydrothermal activity, which can constitute mineralogical evidence of different times of fracturing. Resolution for Secondary Electron of FIB-SEM-EDS analysis indicates the varied nature of micro natural fractures hosted by the Longmaxi Formation (Fig. 9). Some minerals suffered tension to form

Zeng et al., 2016; Wang et al., 2018). The microscopic fractures were examined in this paper polished thin sections of Longmaxi Formation samples by using of Optical Microscope (OM), Electron Probe & Energy Dispersive Spectrometer (EP-EDS), CLM, and FIB-SEM-EDS. Using CLM, OM, and SEM-EDS observations, natural microfractures were found to be very developed in the Longmaxi Formation shale. As observed in core fractures, these microfractures are usually filled with calcite. Calcite-filled microfractures exhibit wider openings and extend farther than unfilled ones. The time sequence of microfractures can be determined by analyzing the cross-sectional relationship between the interference colors of microfractures and the filling material. For instance, different fracture geometries were observed, including Y-shaped fractures sealed with calcite (Fig. 8A). Two parallel fractures and secondary unsealed fractures in calcite cement with an obvious edge (Fig. 8B). A complex natural fracture network was filled with calcite with different optical colors, showing that fracture F2 cuts across F1 and ends at F3 (Fig. 8C), and these network fractures are more easily induced by hydraulic fracturing to form a complex fracture network. These cross-cutting relations could constitute a geometric evidence to determine the order of fracture formation. Three parallel fractures were filled with calcite in the middle, and the observed opaque square black particles could be concluded to be pyrite particles (Fig. 8D). Unlike the fractures in Fig. 8B, the secondary fractures were filled with calcite later. They are presumed to have opened again after stress-induced fracturing. At the edges of the fractures, relatively fine grains of calcite and quartz minerals are clearly visible, although some of the filling fractures can be seen in both types of calcite. These unfilled secondary fractures can be divided into two groups, one parallel to the main fracture direction and the other perpendicular to the main fracture direction (Fig. 8E). Many unfilled fractures can be seen in the observational range, and there are many asphalts filling around those fractures (Fig. 8F), indicating that a large amount of hydrocarbon generation has occurred in the history of this area, which corresponds to the past hydrocarbon generation stage in the SY5 well burial history 7

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Fig. 7. A comprehensive column chart of fracture characteristics in the Longmaxi shale of the SY-5 well.

5. Discussion

unfilled fractures in the middle (Fig. 9A). Numerous pyrite framboids grow in the shale, and micro compression fractures occur between the pyrites (Fig. 9B). These fractures have small apertures and short extension distances. Some fractures are in a closed or open state, indicating that they were caused by strike-slip compression stress (Fig. 9C). Organic matter can be found in clay minerals (Fig. 9D). Some irregular fractures can be found on the rugged surfaces of shale (Fig. 9E). Moreover, many microscale fractures are abundant inside the detached quartz (Fig. 9F), and these fractures' apertures are extremely small and even closed, with extensions different in directions. Fig. 9G–I show a series of photographs at different resolutions. There is an obvious performance difference in nano space between barytite and calcite in color (Fig. 9G). Fig. 9H shows that there are many secondary unfilled fractures and some dissolution holes in the barytite fracture, providing a strong space for shale gas reservoirs. The secondary unfilled fractures are irregular, and there are two orthogonal groups. The main secondary fractures tend to follow the fracture in Fig. 9G, and there is a group of vertical fractures with short extension distances, smooth edges and irregular shapes (Fig. 9I). These filling fractures and secondary unfilled fractures, on one hand, increase the storage space for shale gas; on the other hand, they become channels connecting pores and macroscopic fractures, which is conducive to the rapid migration of shale gas and reconstruction of artificial hydraulic fracturing in the later period, and is of great significance for the development of shale gas.

5.1. Paleo-stress investigation Stress condition has an important impact on the degree of fractures (Yin et al., 2019, Yin and Ding, 2019). The influence of Caledonian, Indosinian and early Yanshanian movements on the southeastern Chongqing-western and Hunan-Hubei region was mainly uplift, but there was no strong orogeny (Li et al., 2013). In the late YanshanianHimalayan period, the strong orogeny characterized by folds and faults made the crust continuously uplift, tilt, depression, and suffered strong erosion. Therefore, we mainly considered the Late Yanshanian and Himalayan stress fields orientations and the magnitudes in the study area. Some researchers (Ju et al., 2017; Ju et al., 2019;He, 2018; Zeng et al., 2013b; Wu et al., 2017) have studied the paleostress fields and stress orientations in Sichuan basins and Hunan-Hubei fold belts, which shows that orientation of the maximum principal stress (σ1) during the Late Yanshanian and Late Himalayan period was NW-SE and NE-SW trending, respectively. The in-situ stress magnitudes can be measured by the acoustic emission test (AET), which can be completed by MTS286 rock testing system. The main principle of AET is that rock has memory for in-situ stress and gradually pressurizes rock. When it is subjected to stress again in geological history, strain energy of the internal reservoir will be released rapidly, resulting in a phenomenon of transient elastic wave, also known as the Kaiser effect. The AET cumulative curve shows a sudden increase of the turning point, which becomes Kaiser point, and the corresponding stress is the corresponding paleostress of the stage. The AET of core design sample plot and cumulative curve are shown in 8

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Fig. 8. Characteristics of micro-fractures of the Longmaxi Formation in well SY-5. (A) 1062.2 m, Y shaped fractures filled by calcite; (B) 1085.5 m, two parallel fractures and secondary unsealed fractures are well developed in calcite cement; (C) 1054.1 m, complex natural fracture network, fracture F2 cuts across F1 and ends at F3; (D) 1085.8 m, three parallel factures are filled by calcites and pyrites, the left fracture was dissolved and filled by calcite again; (E) 1030.2, the edge of the fracture is filled with some fine minerals such as quartz and calcite; (F) 1064.2 m, around unsealed fractures filled with Organic matter; (G) 1085.2 m, fractures filled with calcite and some secondary unsealed fractures; (H) 1085.2 m, the G section observed using a fluorescence microscope (note the edge of fracture); (I) 1083.5, fracture is filled with calcite and barium sulfate; (J) 1083.5, the I section observed using an electronic probe analysis; (K) Spectral map of the yellow star located in the G section, main elements are barium, sulfur and oxygen, full scale is 189 cts and cursor is 6.532 cts; and (L) Spectral map of the red star located in the G section, main elements are calcium, carbon and oxygen, full scale is 277 cts and cursor is 6.929 cts. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

are NW and NE. From the point of view of the fold axis in this area shown in Fig. 1, the direction of the axis is mainly NW-SE, so are the main thrust faults shown in Fig. 12. Based on the previous research results and the actual investigation in the region, we can see that these fractures formed corresponding to the Late Yanshanian and Himalayan stress fields, whose orientations are approximately NW-SE and NE-SW.

Fig. 10, which indicates that there are six times Kaiser effects at least. The maximum stress is 146.1Mpa and followed is 97.7Mpa. Maybe these phase stresses formed in Late Yanshanian and Himalayan. Previous studies (Zeng et al., 2013b; Wu et al., 2017) and the acoustic emission results in regions of South China shows that the Late Yanshanian movement was relatively intense with the maximum principal stress (σ1) magnitude of approximately 145–165 MPa and the Late Himalayan movement with the magnitude of approximately 110–120 MPa during the Cenozoic era. At the same time, we also made field outcrop observations and statistical work of outcrop fractures, which could help us to understand well the direction of the paleostress field. We counted the strike of fractures in six outcrop points to make rose maps in the block. The outcrops location is shown in Fig. 1 and strike of fracture of different outcrops are shown in Fig. 1, which indicates that the main orientations

5.2. Structural factors It is clear that structural factors are the most important factors affecting the development characteristics of shale fractures in the study area. As mentioned above, the Lower Silurian Longmaxi Formation in the study area has mainly experienced tectonic movements such as the Early Paleozoic Caledonian, Late Paleozoic-Mesozoic HercynianIndosinian, Mesozoic Yanshanian and Cenozoic Himalayanian. The 9

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Fig. 9. Secondary Electron of Focused Ion Beam Scanning Electron Microscope images of the Longmaxi shale; (A) 1071.1 m, open tension fractures sealed with no minerals; (B) 1054.7 m, fracture sealed with pyrites and micro-compression-fractures between pyrites; (C) 1060.1 m, nonlinear curve shape of compression fracture; (D) 1064.2 m, organic matter in clay mineral; (E) 1081.8 m, open tension fractures at the edge of the feldspar; (F) 1069.5 m, open tension fractures and compression fractures; (G) 1083.5 m, micro fractures sealed with barytite at low resolution; (H) and (I) 1083.5 m, secondary open tension fractures and dissolution pores in barytite.

observation in Fig. 5 and Longmaxi Formation observation under the microscope in Fig. 8. In addition, the shale has good plastic characteristics, easy flow and bending deformation, which provides a basis for the formation of bedding fractures and small interlayer folds; slip joints are often formed on the lithological change interface or on the shale bedding surface, accompanied by a series of smooth mirror and scratch features. In the vicinity of the fault, the fracture is often the most developed, and with increasing distance from the fault, the degree of fracture development gradually decreases. These fractures, whether filled or not, are effective and may be reopened even if they are currently filled with minerals.

manifestations of tectonic movements in different stages exhibit regional differences, especially since the Indosinian movement. The uplift and subsidence, denudation and sedimentation in different areas are obviously different. The burial history is also full of ups and downs. Even if the reservoir is continuous, it is not a simple one-time reservoir formation. It is often a small uplift in the middle of the continuous buried background. Under such a background, the uplift or subsidence amplitudes of sedimentary strata differ greatly or slightly. This tectonic subsidence of different sizes produced a series of folds and faults, so that the fault fold-related fractures are well developed in the study area. For anticline folds, the curvature is the largest at the core of the anticline, and the rock is most likely to rupture and form tensile joints under the actions of forces. Shear joints and tensile shear joints are often formed under the stress neutral plane; for syncline folds, the opposite is true. As shown in Fig. 12, the fault system in the bottom of the Longmaxi Formation is distributed in the NE direction, and in the SY-5 well on the left flank of the syncline, shear and tension shear fractures are easily formed. As it is close to the F4 fault, it is easily to be affected, resulting in fractures forming. This is compared with the core

5.3. TOC content TOC content test data for the Lower Silurian Longmaxi Formation shale samples (Fig. 13A) in Sangzhi block show that the TOC contents of Longmaxi Formation shale is 0.18%–2.49%, the average value is 1.34%, and the main distribution is 1.0%–2.0%. Among them, the TOC content exceeds 1.0%, accounting for 34.61% of the total. The vitrinite 10

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Fig. 10. The AET design point location of core sample and typical curve between AE cumulative times and applied stress.

reflectance Ro uncorrelated with fracture density.

reflectance data from 14 samples (Fig. 13B) show that the organic matter maturity of the Longmaxi Formation is from 2.46 to 2.76, indicating that all kerogens are in the overmatured stage, which is consistent with the burial history of well SY5 in Fig. 3. In addition, we use two red dotted lines to circle most of the data points in Fig. 13, where this method is used by Dong et al. (2018). It is speculated that the data points may fall within the range of two red dotted lines and follow the relationships when the sample size is larger. This is consistent with the observation of a fracture development interval in the core. The TOC content is higher in the lower part of the Longmaxi Formation, and the fracture density value is relatively small. It is found that vitrinite

5.4. Mineral composition and content According to the analysis of shale mineral composition, the results show that the mineral composition of Longmaxi Formation shale (Fig. 14D) is mainly quartz and clay minerals, followed by feldspar, carbonate minerals (such as dolomite and calcite) and sulfide minerals (such as pyrite and siderite). The content of quartz in Longmaxi shale samples are from 32.0% to 72.0%, with an average of 56.85%. The clay mineral content ranges from 13.0% to 34.0%, with an average of

Fig. 11. The outcrop fracture observation and rose diagram of fracture strike; the outcrops point location of L1 to L6 are shown in Fig. 1. 11

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Fig. 12. Distribution map of fault system at the bottom of the Longmaxi Formation.

mineral filled fractures effective fractures? From OM, FIB-SEM, it can be seen that there are many secondary unfilled fractures, and there are many dissolution pores in the filling minerals of the fractures. All of these can become shale gas reservoirs and migration channels. From the perspective of view of stresses, these fractures will become the preferred fracture sites in the later hydraulic fracturing processes and play an important role in the formation of fracture networks. The meaning of the existence fracture is two-fold. On one hand, they provide spaces for shale gas reservoirs and migration channels. On the other hand, given the existence of fractures, which form a low-value area of fluid potential energy, early shale gas formed and served in the role of providing force into this area, causing shale gas moved the fracture into another large open fractures, thus migrating and leaving the shale reservoir, into the surrounding rock, other reservoir space or even in the air. Fractures can also become free water, hydrothermal and other flow channels, so that shale gas generated in early hydrocarbon generation and expulsion entered those fluids, under the gravity effect, with the flow of those, and other fluids leading to the fracture precipitation, thus, resulting in smaller effective reservoir spaces.

21.0%. The clay minerals are mainly composed of illite, chlorite and immiscible minerals. According to the statistics of fracture linear density and quartz content at the same fracture depth, it is found that there is no obvious correlation between quartz content and fracture linear density (Fig. 14A), so as the clay content (Fig. 14B) and feldspar and carbonate mineral content (Fig. 14C). It indicates that there may be no such relationship between the fracture and one of these parameters in the study area and they control the fracture development together. In addition, we use two red dotted lines to circle most of the data points in Fig. 14, the method followed as Fig. 13. The stronger the heterogeneity and the higher the dispersion of the mineral components in the vertical direction are, the easier the fracture will occur in the same lithology. By calculating the extreme deviations, mean deviations, standard deviations and dispersion coefficients of quartz, clay minerals, feldspar, carbonate minerals and pyrite in Longmaxi Formation shale (Table 1), it can be concluded that feldspar, calcite and pyrite have large dispersion coefficients, high dispersion degrees, and strong heterogeneities of the mineral components in the vertical direction in the shale, which is favorable for fracture generation.

6. Conclusions 5.5. Fracture effectiveness On the basis of previous studies, fractures parameters are characterized and its controlling factors are discussed in the paper. The research scales are from macro-regional geological structure evolution, structural characteristics analysis of study area to micro-fracture

From the above, it can be seen that most of the fractures observed under microscope are filled with minerals, and a few of the unfilled fractures are filled with asphalt and other organic matter. Are these

Fig. 13. (A) Relationship of TOC content and fracture density; (B) Average Ro to fracture density plot. 12

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Fig. 14. Relationships between fracture density and the mineral composition of the Longmaxi Formation. (A) the relationship between quartz and fracture density; (B) the relationship between clay and fracture density; (C) the relationship between feldspar, carbonate minerals and pyrite content and fracture density and (D) Stacking bar diagrams of mineral content at different sampling depths, the depth code S1 to S14 corresponding to the sampling point is shown in Fig. 7.

pyrite, and 4% are filled with clay minerals. (3) Through thin-section OM, CLM and FIB-SEM-EDS observations, there are abundant microfractures in the Longmaxi Formation shale, that form complex fracture networks, and asphalt fillings around the unfilled fracture, indicating that these fracture networks are important channels for shale gas storage and migration. Based on the analysis of the intersection geometry of micro fractures, the CLM of minerals and the different types of fillings, it is considered that the fractures have been opened at least twice, and the hydrothermal activity of the seabed has affected the study area. (4) The structural factor is among the most important factors affecting the development characteristics of shale fractures in the study area, which was uplifted rapidly after the middle Yanshanian period, and suffered long-term weathering and denudation. The upper Silurian strata exposed to the surface, and some large faults directly reached the surface, which was unfavorable to the early formation of oil and gas preservation. Second, organic matter and mineral composition are the main nonstructural factors controlling the development of shale fractures, which may have comprehensive effects on the development of shale fractures not only single factor controlling. The dispersion coefficients of feldspar, calcite and pyrite are large in the vertical direction, indicating that the shale is heterogeneous. (5) The development of fractures is controlled by paleostress field. Its effectiveness is difficult to evaluate. Fractures provides assistance in fluid migration on the one hand, but also provides a channel for fluid preservation and destruction on the other hand in geological history.

Table 1 Mineral dispersion of Longmaxi formation in the Sangzhi block. Formation

Mineral

Range

Average deviation

Standard deviation

Dispersion coefficient

Longmaxi

Quartz Clay Feldspar Calcite Pyrite

40 21 21 29 6

10.43 3.71 4.30 5.18 2.48

11.61 5.24 5.62 7.64 2.87

0.20 0.24 0.41 1.41 1.49

analysis using various analysis methods. We also used advanced geochemical analysis and experimental testing technique, trying to carry out multi-faceted and multi-level fracture analysis and hoping to give some hints to the latter researcher. So, the main conclusions of this paper are as follows. (1) The study area has undergone a burial history of early subsidence and late uplift. It has experienced Caledonian, HercynianIndosinian and Yanshanian-Himalayanian tectonic movements. Faults and fractures are well developed. (2) The core fractures of the Longmaxi Formation in well SY-5 are mainly structural fractures, which can be divided into three types according to mechanical origin: tension, shear and tension-shear fractures. According to the statistics of the fracture parameters, it is found that the main horizontal, low angle, high angle and vertical fractures account for 22%, 24%, 45%, 9%, respectively. The fractures apertures less than 0.1 mm account for 8%, fractures with width 0.1–0.5 mm account for 70%, fractures with widths of 0.5–1 mm account for 16%, and fractures with lengths exceeding 1 mm width account for 6%. Approximately 54% of the fractures are 5 cm, 40% are 5–10 cm, and 6% are 10–20 cm; and 80% are filled with calcite, 8% are filled with quartz, 8% are filled with

Acknowledgments This research was supported by the National Natural Science Foundation of China (Project Nos. 41072098 and 41372139) and the National Major Science and Technology Projects of China (Nos. 13

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2016ZX05034-004-003, 2016ZX05046-003-001 and 2016ZX05033002-005-001). We are grateful to the Beijing Dadi Special Exploration Branch, China Coal Geological Engineering Corporation and the CNNC Beijing Research Institute of Uranium Geology, for their assistance in testing and analyzing the samples. The authors would like to thank the staffs of all of the laboratories that cooperated in performing the tests and analyses.

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