Organic nanopore structure and fractal characteristics of Wufeng and lower member of Longmaxi shales in southeastern Sichuan, China

Marine and Petroleum Geology 103 (2019) 456–472

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Research paper

Organic nanopore structure and fractal characteristics of Wufeng and lower member of Longmaxi shales in southeastern Sichuan, China

T

Nyujia Penga, Sheng Hea,∗∗, Qinhong Hub,∗, Boqiao Zhangc, Xipeng Hed, Gangyi Zhaie, Chencheng Hea, Rui Yanga a

Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, 430074, China Department of Earth and Environmental Sciences, The University of Texas at Arlington, 500 Yates Street, Arlington, TX, 76019, USA c Jianghan Oilfield Company, SINOPEC, Qianjiang, Hubei, 433124, China d Research Institute of Exploration and Development, Sinopec East China Oil & Gas Company, Nanjing, 210011, China e Oil & Gas Survey, China Geological Survey, Beijing, 100083, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Wufeng formation Longmaxi formation Southeastern Sichuan Organic nanopore structure Fractal dimension of organic pore Shale gas reservoir

Organic nanopores in shale gas reservoirs are the main storage space for free gas and adsorbed gas and directly affect the occurrence mode and movement of shale gas. In this study, field emission-scanning electron microscopy (FE-SEM), low-pressure gas (N2 and CO2) adsorption and mercury injection capillary pressure (MICP) analyses were performed and interpreted with statistical and fractal analyses to investigate the organic pore structure in the Upper Ordovician Wufeng shale (O3w) and the Lower Silurian lower member of Longmaxi shale (S1l). It was found that organic pores in 15 samples from 8 layers of the Wufeng-Longmaxi shale section are usually developed in discrete organic matter particles, organic matter associated with clay minerals and organic matter associated with pyrite framboids. The size, shape and quantity of organic pores vary greatly. The organic pores in Wufeng Formation (layer 1) are densely developed in organic matter particles with relatively small sizes and irregular shapes. The organic pores in the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5) are less developed and have relatively larger pore sizes with an elliptical shape. In contrast, the organic pores in the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) are the least developed, with elliptical shapes and pore sizes in between the two cases above. A higher value of fractal dimension refers a more complex form and this value of whole organic pores with full range of sizes (Dwop) is the largest and followed by organic macropores (Dop > 50), organic pores with pore sizes between 4 nm and 50 nm (Dop4-50), and organic pores with pore sizes less than 4 nm (Dop < 4). The fractal dimension of the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) is lower than that of both the Wufeng Formation (layer 1) and the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5). Possible causes leading to pore complexity and heterogeneity include the TOC content, pore size, mineral composition and tectonic effect. Wufeng shale is located at the bottom of Wufeng-Longmaxi shale weakness zone and had experienced more tectonic compression, nappe-slip and reconstruction, which may be the main reason that organic nanopores in Wufeng shale (layer 1) are more complex than other layers.

1. Introduction Shale gas has become the new focus of energy exploration in United States, Canada, China, Australia and other countries as a result of technical progress in horizontal drilling and hydraulic fracturing (Bernard et al., 2012; Dong et al., 2017). Shales act as both the source and reservoir in unconventional gas systems (Curtis, 2002; Hill et al., 2007). Organic-rich shales, deposited in marine, transitional marine or

∗

lacustrine settings, are widely distributed from Sinian to Jurassic strata in China (Hao et al., 2013; Yang et al., 2016a). The marine shales of the Upper Ordovician Wufeng Formation (O3w) to the Lower Silurian Longmaxi Formation (S1l) are the most extensive and organic-rich shales in southeastern Sichuan, and are the current major target for shale gas development (Guo and Zhang, 2014; Liang et al., 2017). Shale is laminated and fine-grained argillaceous rock, with low porosity and extremely low permeability (Curtis, 2002; Jarvie et al.,

Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. He), [email protected] (Q. Hu).

∗∗

https://doi.org/10.1016/j.marpetgeo.2019.03.017 Received 10 February 2019; Received in revised form 9 March 2019; Accepted 12 March 2019 Available online 13 March 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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minor uplift. Affected by the Yanshanian and Himalayan movements, most of the sedimentary rocks of the Jurassic – Early Cretaceous have been eroded by tectonic deformation and uplift. During the period from late Ordovician Wufeng Formation to early Silurian Longmaxi Formation, due to the low-energy, anoxic sedimentary environment caused by two global transgressions, a set of organic rich marine shale was formed (Guo and Zhang, 2014; Guo et al., 2014). The present depth of the Wufeng-Longmaxi shale in the study area for economic shale gas exploration is about 2000–3000 m. Longmaxi Formation is divided into 3 intervals: the lower member of Longmaxi, the middle member of Longmaxi and the upper member of Longmaxi. According to biological fossils, rock mineralogy, authigenic minerals and electrical properties, high-quality shales (Wufeng and the lower member of Longmaxi) are divided into 9 layers by Jianghan Oilfield Company, the major operator of the filed. Those high-quality shales can also be divided into two sections: the highest-quality shale and secondary high-quality shale. The highest-quality shale section is composed of the Wufeng Formation and the lower part of the lower member of Longmaxi Formation: Wufeng Formation is further divided into layer 1 and layer 2 (layer 2 is mainly the argillaceous limestone interlayer of Guanyinqiao section with about 0.4 m), and the lower part of the lower member of Longmaxi Formation is divided into layers 3 to 5. The secondary high-quality shale section is the upper part of lower member of Longmaxi Formation, which is divided into layers 6, 7, 8, and 9. The present-day thickness of high-quality shale (Wufeng shale and the lower member of Longmaxi shale) is about 60–160 m and its maturity ranges around 2.5–3.0% Ro (Li et al., 2012; Han et al., 2013; Guo, 2015). The Wufeng shale is conformably underlain by Longmaxi shale, and its thickness is about 5–8 m (Yang et al., 2016b). The Wufeng shale is characterized by black fine grain carbonaceous and siliceous shale, and abundant graptolites, calthrop and radiolarian can be observed in the shales. While for the Longmaxi shale, from the bottom to the top, the fossils turn fewer and the lithology changes from black fine carbonaceous-rich and siliceous-rich shale, argillaceous siltstone to dark-gray mudstone, gray shale and gray siltstone.

2007; Wood and Hazra, 2017). The abundance and distribution of shale gas is closely related to its pore structure, as different pore sizes create different storage patterns and therefore capacities (Cao et al., 2015). Several different pore classification schemes have been proposed based on pore size, pore morphology, pore origin, or positional relationship between pores and rock particles (Rouquerol et al., 1994; Desbois et al., 2009; Loucks et al., 2009, 2012; Slatt and O'Brien, 2011; Kuila et al., 2014; Ma et al., 2015). The pore size classifications published by the International Union of Pure and Applied Chemistry (IUPAC) are commonly used for pores in shales. IUPAC classify pores as micropores (diameters less than 2 nm), mesopores (2–50 nm in diameter) and macropores (diameters more than 50 nm) (Rouquerol et al., 1994). Shales typically have a variety of pore types such as organic pores, interP (inter-particle) pores (pores between grains), intraP (intra-particle) pores (pores within particles) and micro-fractures. Among them, nanoscale-organic pores present in kerogen and pyrobitumen have been identified as a significant pore type in gas shales (Curtis et al., 2012; Modica and Lapierre, 2012; Milliken et al., 2013; Loucks and Reed, 2014; Löhr et al., 2015). Organic pores are the main reservoir space for free gas and have a certain local connectivity. The inner surface of organic pores has a strong adsorption capacity for shale gas (Yang et al., 2016b). The organic pores of shale directly affect the storage and movement of shale gas (Yang et al., 2017). The pore structure in shale is very complicated due to the location, size, shape, and connectivity of pores. To elucidate the pore structure of shales, researchers have utilized a range of fluid invasion methods, radiation methods and direct-imaging methods to characterize the pore morphology, porosity, pore size distribution (PSD), specific surface area and organic matter surface pore percentage (the percent of organic pores surface area versus total organic matter area) (Mendhe et al., 2017; Gao and Hu, 2018; Zhao et al., 2018; Yang et al., 2019). To date, many studies have used a series of methods to characterize the pore structure of Wufeng-Longmaxi shales in southeastern Sichuan (Tang et al., 2015; Yang et al., 2016a; Ji et al., 2017; Zhao et al., 2017a; Wu et al., 2018). This study focuses on the characteristics of organic nanopores in this formation, which have been less studied. The detailed characterization of the organic pore structure and its developmental characteristics, have practical value for geological evaluation and modeling of shale gas, it may have an important impact on the gas bearing capacity of shale and the occurrence state of shale gas (i.e., the content of free gas and adsorbed gas). Using a combination of FE-SEM, low-pressure gas (CO2 and N2) adsorption, mercury injection capillary pressure methods, assisted with systematic statistical and fractal analyses for Wufeng-Longmaxi shale, the major objectives of this study were to: (1) quantify the organic nanopore structure such as morphology, PSD and surface pore percentage of organic matter; (2) assess the complexity and heterogeneity of the organic nanopore structure; and (3) discuss the possible causes of organic nanopore complexity and heterogeneity.

3. Samples and methods 3.1. Samples A total of 15 shale core samples were obtained from Wufeng and the lower member of Longmaxi shales in wells PY1 (7 samples) and JY1452 (8 samples), which were drilled in the Pengshui area (external area of the basin) and Wulong area (internal area of the basin), respectively. Shale thicknesses of the Wufeng Formation and the lower member of Longmaxi Formation in these two wells are 132.8 m and 151.3 m, respectively (Fig. 2). The locations of wells PY1 and JY145-2 are shown in Fig. 1b, and the sampling location and other relevant information of the shales in Wufeng Formation and the lower member of Longmaxi Formation are presented in Fig. 2. Due to the uniform distribution of Wufeng-Longmaxi shales in the study area, every layer is sampled to have a representative set of samples. Each sample was cut into several parts for different analyses.

2. Geological setting The study area is located in southeastern Sichuan, and structurally belongs to the Chuandong Fold Belt (Tian et al., 2013). It is divided into the internal (west of the fault) and external (east of the fault) areas of the basin by the Qiyueshan fault (Fig. 1). The area to the west of the fault belongs to the eastern part of the Sichuan Basin, and contains a wide and gentle anticline confined by some high angle reverse faults with outcrops of Jurassic-Triassic strata. The area to the east of the fault is dominated by a gentle syncline and exposed Triassic-Silurian strata (Nie et al., 2009; Guo and Liu, 2013; Guo, 2014). The tectonic and sedimentary evolution of the study area includes five stages: Caledonian (late Sinian-Silurian), Hercynian (Devonian-Permian), Indosinian (Triassic), Yanshanian (Jurassic-Cretaceous), and Himalayan (TertiaryQuaternary) (Yang et al., 2016a; Wang et al., 2017). Prior to the late Cretaceous, the study area was mainly characterized by subsidence and

3.2. TOC content, helium porosity and mineralogy In order to characterize the organic matter richness, all the samples were crushed and processed according to the Chinese Oil and Gas Industry Standard (GB/T) 19145-2003. TOC contents were determined using an Elementar rapid CS cube (precision of 1%), and calculated as the difference between total carbon and inorganic carbon. The total porosity of shale samples was determined based on Boyle's Law, by measuring the grain volume under ambient conditions and bulk volume by Archimedes' principle with helium pycnometry. Samples were oven dried at 115 °C to a constant weight ( ± 1 mg). Porosity was calculated from the difference between bulk volume and grain volume. 457

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Fig. 1. (a) Location map of the study area in the eastern margin of Sichuan Basin. (b) Tectonic map of study area and wells (modified from Peng et al., 2017).

Bustin, 2008).

All samples were tested for mineralogical compositions by X-ray diffractometry (XRD) (X'Pert PRO MPD from the Panalytical Company). Powder samples were smear mounted on glass slides for XRD analyses. The working voltage, current and measuring angle range are 40 kV, 40 mA and 5°–90°, respectively. The relative mineral percentages were calculated by the area under the curves of the major peaks of each mineral with a correction for Lorentz Polarization (Chalmers and

3.3. Field emission-scanning electron microscopy (FE-SEM) for organic nanopore statistical analysis An Ion Beam Milling System Leica EM TIC 3X was used to create a very smooth surface to fifteen samples which the polished surface are

Fig. 2. Stratigraphic column of the Wufeng Formation and the lower member of Longmaxi Formation of wells PY1 and JY145-2. 458

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Table 1 Total organic carbon, helium porosity and mineral contents of the shale samples from Wufeng and Longmaxi Formations in the two wells. Well name

Sample ID

Depth (m)

Formation

layer

TOC (wt.%)

Porosity (%)

Quartz (%)

Feldspar (%)

Calcite (%)

Dolomite (%)

Pyrite (%)

Clay (%)

PY1 PY1 PY1 PY1 PY1 PY1 PY1 JY145-2 JY145-2 JY145-2 JY145-2 JY145-2 JY145-2 JY145-2 JY145-2

P8 P7 P6 P5* P4 P3 P1* J9 J8 J7 J6 J5* J4 J3 J1*

2076.09 2101.86 2113.65 2134.03 2143.03 2148.84 2157.23 2128.89 2180.69 2202.29 2220.95 2233.91 2250.89 2262.80 2275.70

Longmaxi Longmaxi Longmaxi Longmaxi Longmaxi Longmaxi Wufeng Longmaxi Longmaxi Longmaxi Longmaxi Longmaxi Longmaxi Longmaxi Wufeng

8 7 6 5 4 3 1 9 8 7 6 5 4 3 1

0.93 0.11 1.01 1.17 2.19 2.83 3.17 0.94 1.48 1.54 2.27 2.93 3.34 4.14 5.48

2.96 1.35 1.44 1.93 3.44 2.25 4.06 2.20 1.30 2.39 2.10 2.82 1.83 1.74 2.89

39.02 39.40 29.80 33.12 40.10 48.05 39.38 37.95 40.84 43.57 46.99 45.13 53.59 64.53 45.79

5.02 16.55 16.80 13.97 5.00 7.05 11.58 10.31 8.85 11.23 14.44 11.32 9.09 6.12 12.54

1.99 7.80 5.20 3.02 5.00 5.15 1.84 1.60 2.15 1.53 2.68 1.42 5.22 2.98 nd

2.01 5.65 6.20 1.99 2.00 2.95 4.27 nd 9.92 3.18 2.64 2.61 2.17 3.13 4.75

1.99 2.65 3.20 3.01 3.00 3.00 2.56 1.92 2.36 1.88 2.53 2.53 3.58 2.26 1.79

49.97 27.95 38.80 44.89 44.90 33.80 40.37 48.22 35.88 38.61 30.72 36.99 26.35 20.98 35.13

Note:* means this sample has two argon ion polished sub-samples of which the polished surfaces are vertical and parallel to the bedding, respectively. While the others only have polished samples that were vertically to the bedding; nd = no data.

Fig. 3. Classification of shale lithofacies from different layers in study area (ternary chart model from Wu et al., 2018). Larger solid circles are the 15 samples used in this article and smaller solid circles are the samples only do the XRD to analyses shale lithofacies of each layer. The ternary chart divides the shale lithofacies into 4 categories and 16 subcategories: (1) Siliceous shale lithofacies association: S is siliceous shale lithofacies, S-1 is carbonate-rich S, S-2 is argillaceous/carbonate mixed S, S-3 is argillaceous-rich S; (2) calcareous shale lithofacies association: C is calcareous shale lithofacies, C-1 is siliceous-rich C, C-2 is argillaceous/siliceous mixed C, C-3 is argillaceous-rich C; (3) argillaceous shale lithofacies association: CM is argillaceous shale lithofacies, CM-1 is siliceous-rich CM, CM-2 is carbonate/siliceous mixed CM, CM-3 is carbonate-rich CM; and (4) mixed shale lithofacies association: M is mixed shale lithofacies, M-1 is carbonate/siliceous M, M-2 is argillaceous/ siliceous M, M-3 is argillaceous/carbonate M.

circular sectional area, which is equal to the cross-sectional area of each organic pore in the FE-SEM image calculated by Image-Pro Plus software. The minimum and maximum pore sizes are the minimum and maximum values of the actual pore size obtained for each organic pore. Circularity value is a parameter used to semi-quantitatively describe the pore shape, which ranges between 0 and 1. The closer the circularity value is to 1, the more round the pore shape is; a smaller circularity value tends to be a more irregular pore shape. The equation of circularity value is defined as

vertical to the bedding and four samples which the polished surface are parallel to the bedding (P5, P1, J5 and J1, see Table 1). Then samples were imaged with a Zeiss Merlin Field Emission Scanning Electron Microscope without coating, because samples coated with carbon or gold may cover up some of the actual pore characteristics. Using a low accelerating voltage (2.00 kV) can reduce charging and help obtain clear high-resolution FE-SEM images. When the magnification is 10-50k times, the organic pores with sizes > 10 nm can be clearly observed. When the magnification was increased to 50-120k times, organic pores with diameters of 5–10 nm can be observed and some pores with sizes between 2 and 5 nm can also be observed. To quantify the pore characteristics, all high-resolution FE-SEM images were processed by Image-Pro Plus software. At present, the commonly used image processing software employs gray values to automatically identify pores. Due to factors such as image resolution, the irregularity of organic pores, brightness and contrast, this approach may miss pores, especially those less than 5 nm. Therefore, this work combines Image-Pro Plus software and manual identification to obtain the parameters of organic nanopores such as the number, cross-sectional area, circumference, equivalent circular diameter (ECD), minimum size, maximum size and circularity value. The ECD of pore is the diameter of organic pores calculated according to the equivalent

k = 4 × π × S/C2

(1)

where k is circularity value, S is cross-sectional area, and C is circumference.

3.4. Low-pressure gas adsorption Low-pressure nitrogen (N2) and carbon dioxide (CO2) gas adsorption were conducted using a Quantachrome ASiQwin system, which can generate adsorption-desorption isotherms for calculating nanopore structure parameters such as PSD, specific surface area and pore volumes based on multiple adsorption theories. CO2 was used to identify 459

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Fig. 4. FE-SEM images of organic nanopores. (a) elliptical organic pores with different pore sizes, sample ID J8, 2180.69 m; (b) high resolution organic pores, with a minimum diameter of 3.35 nm as observed in the figure, sample ID P3, 2148.84 m; (c) some small organic pores can be observed in bigger organic pores, sample ID J5, 2233.91 m; (d) organic pores with subcircular shapes, sample ID P4, 2143.03 m; (e) organic pores with irregular shapes, sample ID P1, 2157.23 m; (f) heterogeneously distributed organic pores, sample ID J1, 2275.70 m; (g) organic pores in the OM-clay aggregates, sample ID P7, 2101.86 m; (h) organic pores in organic matter associated with pyrite, sample ID J6, 2220.95 m; (i) organic pores in organic matter associated with pyrite, sample ID J9, 2128.89 m.

et al., 2017).

micropores that are more accessible by CO2 than by N2 (Mastalerz et al., 2013), and N2 was used to identify mesopores and macropores. In preparation for analyses, samples were crushed to 60–80 mesh grains (250-180 μm), then degassed at about 110 °C under vacuum for at least 8 h to remove adsorbed moisture and volatile matter that might be present in the samples. For low-pressure N2 adsorption, the adsorption-desorption isotherms were obtained at a pressure of up to 97.3 kPa at 77.35 K (the temperature of liquid nitrogen). Low-pressure CO2 adsorption was measured at a pressure of up to 104.5 kPa and 273 K (ice-water mixture). The relative pressures (P/P0) for N2 and CO2 adsorption range from 0.0001 to 0.995 and 0.0006 to 0.029, respectively. For N2 adsorption, the Barrett Joyner Halenda (BJH) method was applied to calculate PSD (Gregg and Sing, 1982). Micropore size distributions were interpreted from CO2 adsorption data using the nonlocal density functional theory (NLDFT) (Fan and Ziegler, 1992). Equivalent surface areas (m2/g) were calculated using the Brunauer Emmette Teller (BET) method in a relative pressure range of 0.05–0.3.

3.6. Fractal theory It is well known that the PSD of shale is not uniform and cannot be represented by the traditional Euclidean geometry (Wang et al., 2012). Therefore, a view of fractal geometry can describe the essential characteristics of complex forms (Mandelbrot, 1967). The method of fractal geometry is used to analyze the heterogeneity and complexity of pore structure. Fractal dimension (D) is the key parameter in the fractal geometry that can quantify irregular patterns. Key methods to calculate the fractal dimension include fractal BET model (Brunauer et al., 1938), Frenkel-Halsey-Hill (FHH) model (Avnir and Jaroniec, 1989), small angle X-ray scattering(Wong et al., 1986), electronic energy transfer method (Even et al., 1984), image analysis (Krohn, 1988; Dathe et al., 2001) and thermodynamic method (Pfeifer et al., 1989). In this study, a fractal analysis of organic pores was achieved by box counting and dilation methods based on FE-SEM images and FHH model from gas adsorption isotherms. The box counting can be simplified as (Dathe et al., 2001):

3.5. Mercury injection capillary pressure (MICP)

log N(ε) = -DL log ε +c

Mercury intrusion data were collected on a mercury injection porosimeter (Micromeritics AutoPore IV 9510). Prior to the MICP test, each sample was cut into a 1 cm3 cube and oven-dried at 60 °C for 48 h to remove moisture, then cooled to room temperature in a desiccator. During the MICP test, the pressure of Hg was increased continuously from 0.034 to 413 MPa (5–60,000 psia), and this can detect a lowestlimit of pore-throat diameter of approximately 3 nm. The contact angle and surface tension of mercury were adopted as 130° and 485 dyne/cm, respectively (Gan et al., 1972; Gregg and Sing, 1982). The pore throat diameter was determined using the Washburn equation (Washburn, 1921a; b), and is commonly smaller than the actual pore width (Hu

(2)

where ε is box size; N(ε) is the number of boxes with size of ε that containing any part of the focused structure; DL stands for length fractal dimension; and c is a constant that describes the ordinate intercept. As the fractal dimension of a line (DL) within a plain has been measured, the fractal dimension of a surface (DS) in 3D space is obtained by DS = DL + 1 assuming an isotropic structure(Mandelbrot, 1983). The FHH model can be simplified as (Liu et al., 2017): ln(V/V0) = constant +(D-3)ln[ln(P0/P)] 460

(3)

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Fig. 5. Organic pore size distribution from different layers in the study area.

2) as shown in Fig. 3.

where V is the volume of gas adsorption at equilibrium pressure P; V0 is the monolayer coverage volume; P0 is the saturation pressure; P is the equilibrium pressure; D is fractal dimension.

4.2. FE-SEM observation and statistics

4. Results

Based on observation of the FE-SEM images, organic pores are common in all 15 samples. Organic pores are generally believed to be formed in the high thermal evolution stage of organic-rich shale. They form the main gas storage space of gas-bearing shale and play an important role in the enrichment and migration of shale gas (Slatt and O'Brien, 2011; Chalmers et al., 2012; Curtis et al., 2012). Variable numbers of organic pores can be observed in a single particle of organic matter. The size of organic pores varies greatly (Fig. 4a), with the smallest observed pores ranging from 2 to 3 nm (Fig. 4b). Some small organic pores can be observed in larger ones (Fig. 4c). Morphologically, most organic pores in Longmaxi Formation are elliptical or nearly circular (Fig. 4d), while those in Wufeng Formation are irregular and polygonal (Fig. 4e). In the same organic matter particle, the distribution of organic pores can be non-uniform, and the distributions are highly heterogeneous (Fig. 4f). In addition to being distributed in separate organic matter particles, organic pores are often developed in organic matter associated with clay minerals and pyrite framboids (Fig. 4g, h, i). These phenomena are observed in all 8 layers. Due to the strong heterogeneity of development degree of organic matter pores, a total of 216 FE-SEM images containing organic matter were randomly selected for statistical analyses, with a total of 88,249 organic pores being counted. Comparing the organic pores from different layers based on statistical analyses, the organic pore size

4.1. Organic geochemistry, porosity and shale composition TOC and helium porosity values of the studied samples are exhibited in Table 1. In general, TOC values of shales from 1 to 5 layers (except 2 layer) are higher than that from 6 to 9 layers. One hundred and thirty samples were selected for analysis of XRD, of which fifteen samples were used for the study, and the remaining samples were only used to analyze the lithofacies of each layer. XRD analysis suggests that the shale samples from the study area are composed of quartz, feldspar and clay minerals, with less than 10 wt% dolomite, calcite and pyrite for each (Table 1). According to the classification methods of Wu et al. (2018), the shale lithofacies of each layer are shown in Fig. 3. In general, the shale of layers 1, 3, 4, 5, 6 and 7 in wells PY1 and JY145-2 from the study area are argillaceous-rich siliceous shale lithofacies (S-3) with some argillaceous/siliceous mixed shale lithofacies (M-2) and argillaceous/carbonate mixed siliceous shale lithofacies (S-2), layer 8 and 9 are mainly argillaceous/siliceous mixed shale lithofacies (M-2) and siliceous-rich argillaceous shale lithofacies (CM-1). Each layer has similar shale lithofacies except layer 9. Samples chosen for this study belong to argillaceous-rich siliceous shale lithofacies (S-3) and argillaceous/siliceous mixed shale lithofacies (M461

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Fig. 6. Organic pore circularity value distribution from different layers in the study area. Table 2 Quantitative organic nanopore parameters from FE-SEM images for shale samples of wells PY1 and JY145-2. Sample ID

Layer

TOC (wt. %)

No. of FE-SEM images

No. of organic pores

Organic matter surface pore percentage (%): range (avg.)

Avg. of organic pore circularity value

Organic pore equivalent circular diameter (nm): Main range (avg.)

P8 P7 P6 P5 P4 P3 P1 J9 J8 J7 J6 J5 J4 J3 J1

8 7 6 5 4 3 1 9 8 7 6 5 4 3 1

0.93 0.11 1.01 1.17 2.19 2.83 3.17 0.94 1.48 1.54 2.27 2.93 3.34 4.14 5.48

9 15 21 15 15 19 20 12 14 16 12 10 13 17 8

3127 4011 8412 7554 5266 7134 8902 4068 4875 6098 4975 4065 5131 8387 6244

2.19–16.85 (6.47) 1.12–9.76 (4.32) 1.97–9.62 (6.88) 3.04–21.08 (8.17) 1.37–17.14 (10.64) 3.97–33.24 (14.79) 6.55–32.11 (19.57) 0.2–21.48 (5.43) 0.93–17.02 (7.78) 1.31–13.17 (9.70) 5.12–19.12 (10.63) 4.83–23.03 (13.16) 2.07–27.17 (17.32) 2.40–33.76 (18.01) 4.63–30.75 (20.88)

0.692 0.711 0.677 0.671 0.676 0.657 0.643 0.701 0.689 0.682 0.672 0.669 0.671 0.658 0.629

5-30 (22.8) 5-35 (24.5) 5-30 (27.0) 5-35 (27.8) 5-35 (41.5) 10-35 (44.0) 5-25 (14.9) 5-30 (21.1) 5-25 (33.1) 10-30 (25.6) 10-30 (25.5) 10-25 (45.3) 10-35 (44.9) 10-35 (44.2) 5-25 (27.4)

mesopores in shale samples accounts for the highest proportion of total pores, and the number of organic macropores from 50 nm to 900 nm accounts for a relatively small proportion of total pores. The organic pores in layer 1 are well developed and relatively small in size, especially in well PY1. The organic pores in layers 3, 4 and 5 are less

distributions and circularity value distributions are shown in Figs. 5 and 6. The organic nanopore parameters, such as organic matter surface pore percentage and average circularity value, are presented in Table 2. Due to the limitation of FE-SEM resolution, the number of organic pores less than 10 nm is underestimated. In general, the number of organic 462

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Fig. 7. Low pressure N2 adsorption-desorption isotherms collected for the shale samples in wells PY1 and JY145-2. The solid-point connected line is the adsorption curve and the unfilled point line is the desorption curve.

Fig. 8. Pore volume distributions with pore width obtained from low pressure CO2 adsorption collected for the shale samples in wells PY1 and JY145-2.

463

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Fig. 9. Pore volume distributions with pore width obtained from low pressure N2 adsorption collected for the shale samples in wells PY1 and JY145-2.

shale samples are disordered porous materials with non-uniform pore network structures. In general, the pores of shale samples in the study area are mainly composed of nanopores with complex structure, including open cylindrical-like pores, layered slit-like pores, and inkbottle pores. Both hysteresis loops of type H2(a) and H2(b) are composed of more complicated pore structures, in which network effects are very important. The main difference in shape between type H2(a) and type H2(b) is that the desorption branch of H2(a) loop is very steep. Comparing the adsorption-desorption isotherms between different layers (Fig. 7), the desorption curves of layer 1 (Wufeng shale) and layers 6, 7, 8, 9 (the upper part of lower member of Longmaxi Formation) are steeper, and the hysteresis loop is more inclined to type H2(a), while the hysteresis loops of layers 3, 4, 5 (the lower part of lower member of Longmaxi shale) are more inclined to type H2(b). According to Thommes et al. (2015), pores of Wufeng shale and the upper part of the lower member of Longmaxi shale tend to have a narrow range of pore necks whereas the size distribution of neck widths is much larger in pores of the lower part of the lower member of Longmaxi shale. The pore volume distributions obtained from low pressure CO2 adsorption, N2 adsorption and MICP are shown in Figs. 8–10. Generally, micropore characteristics were quantified by CO2 adsorption, mesopores by N2 adsorption, and macropores by MICP. The quantitative pore parameters of the samples are presented in Table 3. Since the micropore volume is highly correlated with TOC, it can be inferred that the pore

developed than in layer 1, and have relatively larger pore sizes. Finally, layers 6, 7, 8 and 9 contain the least amount of organic pores and their pore sizes are intermediate compared to layers 1, 3, 4 and 5. Fig. 6 shows that the organic pore circularity value (k) is the lowest on average in layer 1, and then gradually increases through to layer 9, which means the shape of organic pores in layer 1 tends to be irregular and polygonal, while the shape of organic pores in the lower member of Longmaxi tends to be subcircular.

4.3. Pore volume, size distribution and surface area examined by gas adsorptions and MICP Fig. 7 shows the low-pressure N2 adsorption-desorption isotherms of all samples. According to the IUPAC classification (Thommes et al., 2015), the isotherms have both type II and type IV(a) characteristics, suggesting that samples are dominated by micropores (pore size < 2 nm) and mesopores (pore size between 2 and 50 nm) with a heterogeneous planar surface. The adsorption volume increases rapidly when the relative pressure is greater than 0.95 and no plateau at higher P/P0 values, indicating that there may be some large pores or nano-fractures in these samples. All samples show adsorption-desorption isotherms with clear hysteresis loops, implying that capillary condensation occurs within the mesopores (Sing et al., 1985). Based on the IUPAC classification of hysteresis (Thommes et al., 2015), the shapes of hysteresis loops are characterized by H2(a), H2(b), H3 and H4 types, namely the superposition of multiple standard hysteresis loops, reflecting that these 464

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Fig. 10. Pore volume distributions with pore throat diameter obtained from MICP collected for the shale samples in wells PY1 and JY145-2.

Table 3 Quantitative pore parameters of the shale samples in wells PY1 and JY145-2 from gas adsorption and MICP analyses. Sample ID

P8 P7 P6 P5 P4 P3 P1 J9 J8 J7 J6 J5 J4 J3 J1

layer

8 7 6 5 4 3 1 9 8 7 6 5 4 3 1

TOC (wt.%)

0.93 0.11 1.01 1.17 2.19 2.83 3.17 0.94 1.48 1.54 2.27 2.93 3.34 4.14 5.48

CO2 adsorption

N2 adsorption

MICP

VCO2

SCO2

SBET

VBJH

10−3 cm3/g

m2/g

m2/g

10−3 cm3/g

4.87 3.06 4.37 4.17 5.43 6.05 6.56 5.12 5.79 5.86 6.13 6.96 7.51 7.33 9.36

13.96 8.36 12.74 12.31 16.66 19.48 19.41 15.92 17.87 17.61 18.65 20.85 23.44 23.65 29.29

11.24 4.80 9.94 11.19 14.83 18.55 14.31 11.50 14.39 12.45 14.08 17.48 19.97 22.51 29.07

23.34 11.56 17.62 19.61 25.54 29.27 22.70 24.93 30.74 23.10 26.29 29.55 37.56 34.02 45.02

Vmic

Vmes

Vmac

V < 10

VHg

V3-50

V > 50

10−3 cm3/g 7.88 3.22 6.89 7.49 9.71 11.94 9.25 6.55 8.69 7.99 9.13 9.41 13.43 11.32 20.03

8.36 4.29 7.00 7.44 9.84 11.57 9.76 12.08 12.00 9.89 10.39 15.38 15.41 17.02 16.68

7.10 4.06 3.73 4.68 5.99 5.76 3.70 6.29 10.05 5.22 6.77 4.76 8.72 5.68 8.31

12.20 5.24 10.54 11.37 15.00 18.81 15.38 13.54 15.67 13.49 15.09 19.33 22.77 23.15 31.77

7.92 2.61 5.86 6.31 8.77 4.90 3.55 4.17 2.60 3.16 3.41 3.96 2.94 2.87 6.35

6.79 0.80 2.29 2.57 7.64 3.65 2.18 2.16 1.60 1.72 1.20 1.55 1.56 1.44 4.52

1.13 1.80 3.56 3.74 1.13 1.25 1.37 2.01 0.99 1.44 2.21 2.42 1.37 1.43 1.83

VCO2 and SCO2 are micropore volume and surface area obtained from CO2 adsorption; SBET is surface area using BET method; VBJH, Vmic, Vmeo, Vmac and V < 10 are total pore volume, micropore volume, mesopore volume, macropore volume and pore volume with a diameter < 10 nm calculated from N2 adsorption by BJH, respectively; VHg, V3–50 and V > 50 are total pore volume, mesopore volume with a diameter between 3 and 50 nm and macropore volume by MICP, respectively.

Layer 1 has more micropores than other layers. Fig. 9 shows the PSD of mesopores that not only contain organic pores but also inorganic pores and nano-fractures. It shows that layer 3 has the largest volume of mesopores and the upper part of lower member of Longmaxi (layers 6 to 9) has the smallest volume of mesopores. Fig. 10 reflects pore throat distribution from 3 nm to 36 μm. Distributions in well PY1 cover the full pore size range, whilst distributions in well JY145-2 are more

type of micropores is dominated by organic pores. This means that Fig. 8 roughly reflects the PSD of organic micropores, making up for the lack of statistics of organic micropores caused by the resolution of FESEM. Fig. 8 shows that the incremental pore volume of shale samples is similar in pore width, but the variation of incremental pore volume is obviously different. The peak of incremental pore volume is mainly distributed in pore width of 0.35 nm, 0.45–0.6 nm and 0.8–0.9 nm. 465

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Fig. 11. Fractal analyses of organic pores for one of the FE-SEM images measured by box counting. (a) For whole organic pores. (b) For organic pores with pore sizes > 50 nm. (c) For organic pores with pore size between 4 nm and 50 nm. (d) For organic pores with pore sizes < 4 nm.

Table 4 Fractal analyses of organic pores measured by box counting and dilation methods. Sample ID

P8 P7 P6 P5 P4 P3 P1 J9 J8 J7 J6 J5 J4 J3 J1

No. of images

9 15 21 15 15 19 20 12 14 16 12 10 13 17 8

Dwop (whole)

Dop > 50

Dop4-50

Dop < 4

Range

Average

Range

Average

Range

Average

Range

Average

2.464–2.594 2.476–2.610 2.506–2.631 2.542–2.655 2.445–2.660 2.532–2.675 2.653–2.712 2.478–2.585 2.483–2.600 2.512–2.621 2.438–2.599 2.552–2.667 2.565–2.678 2.557–2.682 2.594–2.709

2.551 2.559 2.569 2.582 2.576 2.629 2.675 2.550 2.558 2.571 2.570 2.573 2.581 2.598 2.653

2.365–2.477 2.371–2.479 2.385–2.511 2.388–2.504 2.376–2.510 2.400–2.595 2.487–2.665 2.362–2.480 2.369–2.487 2.382–2.494 2.384–2.483 2.393–2.497 2.381–2.505 2.400–2.536 2.466–2.590

2.425 2.435 2.467 2.458 2.441 2.515 2.593 2.421 2.426 2.447 2.435 2.449 2.448 2.479 2.530

2.250–2.341 2.257–2.352 2.275–2.381 2.284–2.390 2.265–2.382 2.299–2.412 2.337–2.463 2.247–2.339 2.252–2.366 2.263–2.385 2.256–2.383 2.259–2.391 2.266–2.387 2.289–2.398 2.301–2.420

2.308 2.310 2.324 2.316 2.339 2.352 2.355 2.308 2.313 2.329 2.327 2.333 2.340 2.355 2.358

2.069–2.203 2.042–2.292 2.061–2.326 2.159–2.276 2.172–2.298 2.211–2.305 2.231–2.331 2.181–2.297 2.166–2.290 2.199–2.278 2.200–2.281 2.188–2.283 2.191–2.301 2.207–2.313 2.213–2.399

2.199 2.173 2.200 2.226 2.244 2.265 2.270 2.229 2.228 2.238 2.245 2.252 2.260 2.269 2.350

dominantly below 40 nm and above 2 μm. Sample P6 shows a higher volume ratio of pore throat below 20 nm (Fig. 10), and the pore width distribution is not high in this range (Figs. 8–9). The reason for this phenomenon is that pore sizes measured by MICP represents the pore throats. Therefore, pores with larger bodies but smaller necks are counted as the pore size of corresponding neck sizes to skew the pore size distribution.

sub-division of organic pore groups based on the pore area exhibits a fractal behavior and the fractal dimension value of those samples are presented in Table 4. The fractal dimension of whole organic pores with full range of pore sizes (Dwop) is the largest, followed by organic macropores (Dop > 50), organic pores with pore size between 4 nm and 50 nm (Dop4-50), and then organic pores with pore size less than 4 nm (Dop < 4). The reason for Dwop > Dop > 50 > Dop4-50 > Dop < 4 may be that the morphology of small pores is more circular while larger pores more complicated. Full range of pore sizes contain pores all above and are the most complex pore network. Comparing the fractal dimensions of the different layers, layer 1 has the largest fractal dimension value (Dwop, Dop > 50, Dop4-50, Dop < 4) and the upper part of the lower member of Longmaxi Formation (layers 6 to 9) the smallest with

4.4. Fractal analysis Fractal analysis was employed to quantify the complexity of the pore structure. Fig. 11 shows the fractal analysis results for one of the FE-SEM images with organic pores in organic matter particles. Each 466

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Fig. 12. Plots of lnV vs. ln(ln(P0/P)) from N2 adsorption isotherms using the FHH model. Table 5 Fractal dimensions obtained from FHH model. Sample ID

Slope > 50

R2> 50

D > 50

Slope4-50

R24-50

D4-50

Slope < 4

R2< 4

D<4

P8 P7 P6 P5 P4 P3 P1 J9 J8 J7 J6 J5 J4 J3 J1

−0.2347 −0.2627 −0.1477 −0.1698 −0.1726 −0.1461 −0.1194 −0.1770 −0.2484 −0.1603 −0.1965 −0.1045 −0.1763 −0.1203 −0.1505

0.9958 0.9998 0.9951 0.9996 0.9992 0.9956 0.9953 0.9994 0.9883 0.9942 0.9950 0.9959 0.9982 0.9961 0.9810

2.7653 2.7373 2.8523 2.8302 2.8274 2.8539 2.8806 2.8230 2.7516 2.8397 2.8035 2.8955 2.8237 2.8797 2.8495

−0.2194 −0.2505 −0.2100 −0.2028 −0.2024 −0.1901 −0.2035 −0.2480 −0.2211 −0.2169 −0.2061 −0.2078 −0.2024 −0.1829 −0.1550

0.9993 0.9996 0.9991 0.9992 0.9981 0.9894 0.9757 0.9902 0.9917 0.9950 0.9934 0.9765 0.9887 0.9638 0.9623

2.7806 2.7495 2.7900 2.7972 2.7976 2.8099 2.7965 2.7520 2.7789 2.7831 2.7939 2.7922 2.7976 2.8171 2.8450

−0.5971 −0.6042 −0.6011 −0.5735 −0.5556 −0.5302 −0.5607 −0.5834 −0.5736 −0.5639 −0.5288 −0.5361 −0.5598 −0.5315 −0.4951

0.8518 0.8521 0.8635 0.8786 0.8913 0.8962 0.9009 0.9682 0.9183 0.9055 0.9020 0.9760 0.9144 0.9533 0.9313

2.4029 2.3958 2.3989 2.4265 2.4444 2.4698 2.4393 2.4166 2.4264 2.4361 2.4712 2.4639 2.4402 2.4685 2.5049

decreases, some data in right region start to deviate from the fitted line, which may be caused by the increase of van der Waals force in the micropores. The fractal analysis results from N2 adsorption for all samples are presented in Table 5. Although the pore fractal analyses based on N2 adsorption contains pores other than organic pores, it still has a certain significance in reflecting the fractal behavior of organic pores, especially for pores less than 4 nm. The results show that the fractal dimension of the sample basically conforms to D > 50 > D4-50 > D < 4 which is the same as the fractal analysis based on FE-SEM images. Comparing the fractal dimension values among the two formations, the average of D > 50, D4-50 and D < 4 is 2.8651, 2.8208 and 2.4721 in Wufeng shales, while the average of D > 50, D4-50 and D < 4 in Longmaxi shales is 2.8218, 2.7877 and 2.4355, respectively. The higher average D > 50, D4-50 and D < 4 of Wufeng shale suggests its more complex pore

correspondingly simple pore complexity. In addition, layers 1 and 3 in well PY1 have higher Dwop values than those of the corresponding layers in well JY145-2, indicating those organic pores in these two layers are more complex in the PY1 well. Fig. 12 illustrates FHH plots for samples J8, J3, P5 and P1. Previous studies (Liu et al., 2017; Hazra et al., 2018) have commonly divided FHH plots into two regions due to two phases of the N2 adsorption process, i.e. i) monolayer-multilayer adsorption based mainly on van der Waals forces and ii) capillary condensation based mainly on surface tension. In this study, the dividing line of the two phases of N2 adsorption process corresponds to pore radius of about 1.7–2 nm from the Kelvin equation. In order to better analyze the fractal differences of micro-, meso- and macro-pores, the Kelvin equation is applied to calculate the relative pressure when pore diameter is 50 nm, and then divide the third region. Fig. 12 shows that when the relative pressure 467

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For instance, Fig. 13 shows that compared with adjacent data points, the D > 50 of sample P7 is significantly smaller while the value of Dop > 50 is between samples P8 and P6, indicating sample P7 probably has some inorganic pores which are less complex than organic pores. 5. Discussion 5.1. Impact of organic geochemistry on organic nanopore structure The Wufeng-Longmaxi marine shale samples from wells PY1 and JY145-2 primarily contain type I organic matter (Mao and Guo, 2018), and equivalent vitrinite reflectance converted from the reflectance of bitumen range from 2.51 to 2.81% (Yang et al., 2016b). The shale samples are rich in organic matter with an average TOC content > 2%. The development of an organic pore structure is a complicated process with many variables and controls. As the main pore type of shale, organic pores are formed mainly because volatiles were generated and expelled from the kerogen and secondary cracking occurred (Curtis et al., 2012; Modica and Lapierre, 2012; Milliken et al., 2013). Previous studies (Klaver et al., 2016; Hu et al., 2017; Zhao et al., 2017b) reported the relationship between thermal maturity and organic pore evolution. Both the Wufeng and Longmaxi shales have experienced high thermal evolution and are currently considered to be overmature for petroleum generation, and the maturity difference between these two formations of shale is not expected to be significant and considered in this study. Table 2 shows the average organic matter surface pore percentage (%) increases with TOC, indicating the quantity of organic pores increases with an increases of TOC. To investigate the relationship

Fig. 13. Comparison of fractal dimension between FE-SEM using box counting and dilation methods (closed symbols) and N2 adsorption isotherms using FHH model (open symbols).

structure. The fractal dimension values calculated by the two methods are quite different, mainly because of two different models being used to interpret data from two different methods of pore characterization. Although these two methods of fractal dimensions alone are not comparable, it is highly likely that the complexity of the inorganic pores will cause the corresponding trend of two methods to change greatly.

Fig. 14. Relationships between TOC content and pore structure parameter. (a) Vmic-CO2: micropore volume obtained from CO2 adsorption, V < 10 nm-N2: pore volume with a diameter < 10 nm by N2 adsorption, Vmes-N2: mesopore volume by N2 adsorption, Vtotal-N2BJH: total pore volume calculated from N2 adsorption by BJH, and Vmac-MICP: macropore volume obtained from MICP; (b) micropore, mesopore and macropore surface areas that all obtained from N2 adsorption by BJH method; (c) fractal dimension of the organic macropores (Dop > 50), pore size in 4–50 nm (Dop4-50) and pore size less than 4 nm (Dop < 4); (d) fractal dimension obtained from N2 adsorption in macropores (D > 50), pore size in 4–50 nm (D4-50) and pore size less than 4 nm (D < 4). 468

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Fig. 15. Organic pore shapes comparison between parallel bedding samples and vertical bedding samples in wells PY1 and JY145-2. (a) organic pores in a single organic matter grain, parallel bedding sample of J5, Mag = 86.05k × ; (b) organic pores in a single organic matter grain, vertical bedding sample of J5, Mag = 86.05k × ; (c) organic pores in a single organic matter grain, parallel bedding sample of J1, Mag = 66.79 k × ; (d) organic pores in a single organic matter grain, vertical bedding sample of J1, Mag = 66.79 k × ; (e) organic pores within the pyrite framboids, parallel bedding sample of P5, Mag = 17.35 k × ; (f) magnification of the rectangle shown in Fig. 15e, Mag = 88.78 k × ; (g) organic pores within the pyrite framboids, vertical bedding sample of P5, Mag = 17.17 k × ; (h) magnification of the rectangle shown in Fig. 15g, Mag = 65.67 k × ; (i) organic pores in a single organic matter grain, parallel bedding sample of P1, Mag = 120 k × ; (j) organic pores in organic matter grain, vertical bedding sample of P1, Mag = 120 k × .

micropore volume, mesopore volume and total pore volume (Fig. 14a), yielding correlation coefficients of 0.90, 0.69 and 0.75, respectively. It indicates that shale samples with a higher TOC content tend to have larger volumes of micropores, mesopores and total pores. However, the relationship between macropore volume and TOC content is not apparent, suggesting that the organic pores accounts for a small

between TOC content and organic pore structure, several plots are presented in Fig. 14. TOC values of 15 samples suggest the pore volume with pore sizes less than 10 nm increases with the increase of TOC (Fig. 14a), with a correlation coefficient of 0.90, suggesting that a meaningful proportion of pores with lengths < 10 nm are derived from organic matter. Similar correlations occur between TOC content and

469

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Fig. 16. Organic pore circularity value distribution between parallel bedding samples and vertical bedding samples.

Fig. 17. Representative core photographs of Wufeng Formation (layer 1). (a) high-angle tension-shear fracture, well PY1, 2154.55–2054.65 m; (b) rumpled structure, well PY1, 2156.75–2156.87 m; (c) compressive-torsional high-angle fractures are penetrated by low-angle fractures, well JY143-5 (same block as well JY145-2), 2893.31–2893.42 m; (d) low-angle slip fracture, see obvious scratches, step and smooth mirror features, well JY143-5 (same block as well JY145-2), 2893.46 m.

parallel and vertical bedding were analysed by FE-SEM; typical organic pore images are shown in Fig. 15. For the same samples, the size of organic pores, organic pore circularity value and average of organic matter surface pore percentage do not change much from the observation between vertical and parallel bedding samples. Fig. 16 shows the organic pore circularity value distribution between vertical and parallel bedding samples in layer 1 and layer 5 from wells PY1 and JY145-2. Both samples from vertical and parallel bedding have the same distribution trends with circularity value, as shown by previous results in Fig. 6. In conclusion, based on observations of argon ions polished surfaces between vertical bedding and parallel bedding for the same sample, the obvious difference in pore morphology does not appears. The effect of vertical load compaction and lateral compression during the processes of uplift and tectonic deformation of Yanshanian and Himalayan movements on pore deformation seems to be the same.

proportion of the macropores. Specific surface area also shows a similar trend in that micropore and mesopore surface areas have a positive correlation to TOC, while macropore surface area has an insignificant relationship with TOC (Fig. 14b). Values of fractal dimension obtained from FE-SEM images and N2 adsorption show an increasing trend with the TOC contents and have high correlation coefficients (0.79, 0.90, 0.77 and 0.79) in Dop4-50, Dop < 4, D4-50 and D < 4 (Fig. 14c and d). Meanwhile, Dop > 50 and D > 50 has lower correlation coefficients with TOC contents. It can be inferred that the organic pores of shale samples with a higher TOC content tend to be more complicated. Among them, the complexity of micropores is mainly affected by TOC, followed by mesopores, while the complexity of macropores is controlled not only by TOC contents but also other factors. 5.2. Impact of vertical and lateral compaction on organic pore structure

5.3. Impact of slippage effect on organic pore structure Loucks et al. (2012) documented that mechanical compaction may be the most significant process in shale pore development. High resolution FE-SEM images make it possible to study the formation and development of shale pores. It is reported that some macropores may be deformed by strong compaction or asphaltene filling (Yang et al., 2016a). To investigate how compaction affects the organic pore structure, four samples with the argon ions polished surface from both

Zhu et al. (2018) noted that the evolution and development of pore structure and network is mainly affected by mechanical deformation during the process of tectonism. Based on the observation of shale cores in the study area, more high-angle tectonic fractures are developed in Wufeng Formation than in Longmaxi Formation, and the fractures are filled with calcite and quartz veins (Fig. 17a, c). Meanwhile, it can be 470

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Science Foundation of China (No. 41830431, 41690134), China National Science and Technology Major Projects (No. 2016ZX05034002-003), China Geological Survey Project Grant (No. DD20160185), the Programme of Introducing Talents of Discipline to Universities (No. B14031) and the Project Funded by China Postdoctoral Science Foundation (No. 2018M640740).

observed that a compression-crushed layer is developed in the middle and lower part of the Wufeng Formation. Rumpled structure can be observed in well PY1 in the Wufeng Formation (Fig. 17b), and lowangle slip fracture also can be observed with obvious scratches, step and smooth mirror features (Fig. 17d). This may indicate that under the tectonic stress of the regional compressive nappe, Wufeng Formation is above the hard rock strata of the underlying carbonate rocks, which is at the bottom of the Wufeng-Longmaxi shale weakness zone. The bottom of Wufeng shale is the main slippage surface, therefore, Wufeng shale experienced more tectonic compression, slippage and reconstruction, compared with that of Longmaxi shale. As a result, the organic pores of the Wufeng shale are subject to lateral stress-induced deformation and compression, which may be the main reason that organic pores in Wufeng shale (layer 1) are more complex than other overlying layers (Fig. 6, Tables 2 and 4).

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6. Conclusions Based on the results of this study, the following conclusions can be derived: (1) Organic pores in the study wells are usually developed in separate organic matter particles and organic matter associated with clay minerals or pyrite framboids. They account for the highest proportion of micropores and mesopores and a relatively small proportion in macropores. The number, structure and development of organic pores vary greatly in each layer of Wufeng and the lower member of Longmaxi shales, and the occurrence of organic pores is highly heterogeneous. (2) Morphologically, most organic pores in Wufeng Formation are irregular and polygonal, while those in Longmaxi Formation are elliptical or nearly circular. The organic pores in layer 1 of Wufeng Formation are well developed, with the highest organic matter surface pore percentage and relatively small sizes. The organic pores in layers 3, 4 and 5 of lower part of Longmaxi Formation are less developed than layer 1 and have relatively larger pore sizes. While layers 6, 7, 8 and 9 of the upper part of the Longmaxi Formation are least developed, and their pore size are intermediate between layer 1 and layers 3–5. (3) Higher fractal dimension means more complicated forms. The fractals of pores based on FE-SEM image analysis and N2 adsorption follow the trend of Dwop > Dop > 50 > Dop4-50 > Dop < 4 and D > 50 > D4-50 > D < 4. Comparing the fractal dimension of different layers, layer 1 is the highest in fractal dimension value and upper part of Longmaxi Formation (layers 6 to 9) is the lowest. This indicates that the organic pore complexity of the upper part of the lower member of Longmaxi Formation is relatively simpler than that of the Wufeng Formation and the lower part of the lower member of Longmaxi Formation. (4) Possible causes that lead to differences in organic pore structure and pore complexity, reflected in fractal dimension, include TOC content, pore size, tectonic effect and shale lithofacies. Higher TOC content tends to make organic pores more complicated, whereby TOC has the largest impact on micropores, followed by mesopores, while the complexity of macropores is controlled not only by TOC contents but also by other factors such as tectonic effect. The main slippage surface is at the base of the Wufeng Formation, so that the shale and organic pores in this formation experienced more obvious tectonic compression, and layered decollement reconstruction than Longmaxi shale, which may be the main reason that organic pore structures in Wufeng shale (layer 1) are more complex than other layers. Acknowledgements This study was financially supported by the National Natural 471

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