Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China

Journal Pre-proof Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China

Yong Ma, Omid H. Ardakani, Ningning Zhong, Honglin Liu, Haiping Huang, Steve Larter, Cong Zhang PII:

S0166-5162(19)30776-1

DOI:

https://doi.org/10.1016/j.coal.2019.103349

Reference:

COGEL 103349

To appear in:

International Journal of Coal Geology

Received date:

1 August 2019

Revised date:

15 November 2019

Accepted date:

19 November 2019

Please cite this article as: Y. Ma, O.H. Ardakani, N. Zhong, et al., Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China, International Journal of Coal Geology(2019), https://doi.org/10.1016/j.coal.2019.103349

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© 2019 Published by Elsevier.

Journal Pre-proof

Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China Yong Ma1,3* , Omid H. Ardakani2,3 , Ningning Zhong1 , Honglin Liu4 , Haiping Huang3 , Steve Larter1,3 , Cong Zhang5

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State Key Laboratory of Petroleum Resources & Prospecting, China University of Petroleum, Beijing, China. Natural Resource Canada, Geological Survey of Canada, Calgary, AB, Canada. 3 Department of Geoscience, University of Calgary, Calgary, AB, Canada. 4 PetroChina Research Institute of Petroleum Exploration & Development -Langfang, Langfang, China. 5 The Key Laboratory of Unconventional Oil & Gas geology, China Geological Survey, Beijing, China

Corresponding author: Yong Ma ([email protected])

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Abstract

Shale gas reservoir performance and canister desorption experiments of the Lower Cambrian

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organic-rich shales in the eastern Upper Yangtze Platform reveal a significant difference in shale gas content between the Dabashan arc-like fold-thrust belt in northeastern Chongqing (Deformed

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Zone) and the slightly folded area in southeastern Chong (Non-deformed Zone). Integrated pore characterization methods including scanning electron microscopy (SEM), low-temperature N 2 and mercury injection capillary pressure (MICP) analyses

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and CO 2 adsorption,

were

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comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment. The Lower Cambrian shales in both Deformed Zone (DZ) and Non-deformed Zone (NDZ) were deposited in the deep-water shelf and show similar organic matter richness and thermal maturity. The majority of organic matter (OM)-hosted pores in DZ samples are in nanoscale size range with the dominance of micro-fractures within the OM or at the interface of OM and minerals. In contrast, OM-hosted meso-(2-50 nm) to macropores (>50 nm) are the dominant pore types in the NDZ samples. OM-hosted micropores (<2nm) are abundant in both zones. Helium ion microscopy observations further confirm the presence of OM-hosted micropores in the studied samples. Mineral-hosted pores in carbonate minerals are abundant in both zones, while dissolution rims around carbonate minerals are more abundant in NDZ samples.

Journal Pre-proof The Dabashan arc-like fold-thrust belt took place by the end of the Late Triassic, while the Lower Cambrian shales have reached thermal maturity peak. OM-hosted meso-(2-50 nm) to macropores (>50nm) in DZ samples are most probably collapsed during structural deformation related to tectonic compression, while micropores due to their smaller size survived the tectonic stress. The OM-hosted micropores are the main storage space for adsorbed gas in the DZ area. The dominance of micro-pores in DZ and lack of connection between those pores and matrix pores led to higher gas content in DZ samples. On the contrary, the well-connected OM-hosted pore network in NDZ allows easier gas flow in the rock matrix that eventually led to significant

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gas leakage during uplift and exhumation and lower gas content in this zone. The results of this

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study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in

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south China where had experienced complex tectonic movements.

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Keywords: Pore structure deformation, Structural deformation, pore size distribution, shale gas,

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gas desorption

1. Introduction

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Shales have long been studied for their qualities as either seals or source rocks in conventional hydrocarbon systems, while in unconventional hydrocarbon plays, they act both as source and

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reservoir (Jarvie et al., 2007; Löhr et al., 2015). The potential of fine-grained organic-rich

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sediments to be economical reservoirs largely depends on their storage capacity, permeability, and reservoir mechanical properties suitable for hydraulic fracturing (Löhr et al., 2015). The nanometer- to micrometer-sized pores, along with the natural fractures, form the major storage space and flow path network for gas flow in shale reservoirs (Slatt and O’Neal, 2011; Loucks et al; 2012). Determining pore type, size distribution, and arrangement of pores is thus of central importance to evaluate the storage and flow capacity of hydrocarbons in shale reservoirs (Ambrose et al., 2010; Loucks et al., 2012; Milliken et al., 2013). In addition, understanding geological controls on shale pore structure is important to predict porosity trends and assess the potential of shale oil or shale gas resources. The dominant pores in shale reservoirs largely consist of three major matrix-related pore types as defined by Loucks et al (2012): (1) inter-particle, (2) intra-particle, and (3) OM-hosted pores. 2

Journal Pre-proof The first two pore types are associated with the mineral matrix that is usually are micrometersized, providing potential free gas storage spaces and permeability pathways through the shale matrix (Slatt and O’Neal, 2011; Loucks et al., 2012). Argon-ion-beam milling combined with scanning electron microscopy (SEM) or focused ion beam milling-SEM show that OM-hosted pores in comparison are mainly in nanometer scale and display connectivity in three dimensions (Loucks et al., 2009; Sondergeld et al., 2010a; Curtis et al., 2012b; Ma et al., 2016). The general positive correlation between total organic carbon (TOC) content and total porosity indicates the potential dominant contribution of organic porosity in total porosity (Milliken et al., 2013; Ma et

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al., 2016). The SEM-visible pores are far larger than nano-pores, as such pores smaller than 5 nm

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cannot readily be observed by regular SEM. The good linear relationship between micropore volume and TOC suggests the importance of OM-hosted micropores in total porosity, which also

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contributes to the significant methane sorption capacity of organic-rich shales (Ross and Bustin, 2009; Ma et al., 2015). In addition, hydrocarbon-wet organic fragments with connected pore

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networks are considered to be important flow pathways in shale reservoirs (Wang et al., 2009;

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Ma et al., 2016).

OM-hosted pores are formed due to hydrocarbon expulsion from kerogen structure during thermal maturity and leave behind pores in the more mechanically resilient mature kerogen

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(Brown, 2000; Loucks et al., 2009; Mastalerz et al., 2013; Milliken et al., 2013). Thermal maturity has been considered as an important control on shale porosity evolution, while other

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factors such as OM type, OM abundance, clay minerals, and compaction may also affect the

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evolution of the OM porosity during thermal maturation (Sandvik et al., 1992; Ritter, 2003; Kelemen et al., 2006; Curtis et al., 2012a; Milliken et al., 2013; Lewan et al., 2014; Wu et al., 2017; Katz and Arango, 2018). Previous studies on OM porosity and geological controls are commonly focused on shales from the same formation or different depositional environments but usually in the same tectonic setting. In contrast to marine gas shale resources in North America that generally formed in relatively stable tectonic settings, the marine organic-rich shales in south China have experienced several episodes of intensive tectonic movements after hydrocarbon generation (Ma et al., 2004; Hao et al., 2013). However, the gas preservation of the marine shale gas resources in South China has been overlooked. Though fractures, the lithology of underlying and overlying formations of shale reservoir and other macro-geological factors affect the gas leakage, the shale reservoir pore characteristics are of crucial importance (Sondergeld et al., 3

Journal Pre-proof 2010b; Bruner and Smosna, 2011). Characterizing the pore structure of the marine shales is of great significance for the understanding of the shale gas enrichment in south China. The Lower Cambrian and Upper Ordovician-Lower Silurian organic-rich shales in the Upper Yangtze Platform of South China are regarded as regional major target units for shale gas resource exploration and development (Zou et al., 2015; Zhao et al., 2016). Remarkable breakthroughs have been achieved in the Upper Ordovician-Lower Silurian shales in the Sichuan Basin, with the shale gas yields of around 4.5 billion m3 in 2015. However, gas content detected at the well site of the Lower Cambrian shales in northeastern and southeastern Chongqing

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revealed a significant difference. Tectonically, the northeastern Chongqing area was located at

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the Dabashan arc-like fold-thrust belt (Deformed Zone) while the southeastern Chongqing was in a slightly folded area (Non-deformed Zone). Does the structural deformation play an important

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role in the pore structure and thus affect the shale gas content in the two areas with the different tectonic conditions?

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In this study, canister desorption experiments of the Lower Cambrian shales from both

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southeastern and northeastern Chongqing in the eastern part of the upper Yangtze Platform, which are tectonically Non-deformed Zone (NDZ) and deformed zone (DZ), respectively, have been studied to evaluate their gas content. The organic petrography, low-temperature N 2 , and

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CO2 adsorption and mercury injection capillary pressure (MICP) analyses carried out in this study and published data in these two zones were comprehensively studied to investigate the

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content.

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possible structural deformation effect on pore size characteristics of shale reservoirs and their gas

2. Geological setting

The Upper Yangtze Platform tectonically comprises the western part of the Yangtze Platform in South China and is surrounded by the Longmenshan orogen in the west and the Qinling-Dabie orogen in the north (Meng et al., 2005; Plesch et al., 2007) (Fig. 1A). It regionally includes the eastern Sichuan, Chongqing area, most of Guizhou, western Hubei and Hunan, and northern Yunnan province. The Tongwan tectonic movements occurring at the end of the Sinian (Ediacaran) resulted in the disconformity between the Upper Sinian and the Lower Cambrian strata (Ma et al., 2007). The Lower Cambrian succession consists of carbonaceous and calcareous shale, carbonate, siltstone that were deposited during a major marine transgression from southeast to northwest in that period (Tan et al., 2013). The widely distributed Lower 4

Journal Pre-proof Cambrian black shales were deposited in the deep-water shelf, with thicknesses of 40 to 65 m and TOC contents of 2.7 - 8.0 wt. %, are over-mature with equivalent Ro range from 2.2 to 4.4%, were recognized as one of the major source rocks with promising shale gas potential in the Upper Yangtze Platform (Liang et al., 2009; Zhao et al., 2016). After the end of hydrocarbon generation from the Lower Cambrian shales in Triassic, several episodes of intensive tectonic movements such as the Indosinian (257 - 205 Ma), Yanshanian (199.6 - 133.9 Ma), and Himalayan (70-3 Ma) orogenies have taken place in the Upper Yangtze Platform (Ma et al., 2004; Hao, 2013). One important movement is the early Indosinian orogeny that happened at the end of the middle

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Triassic, when the rhomboid shape of the Sichuan Basin began to form as a result of

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compression from the Tethys Ocean plate to the southwest and the Pacific Ocean plate to the southeast (Ma et al., 2007). By the end of the Late Triassic, the collision of the South Qinling

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and the Yangtze Platform took place, and the Qinling orogen was affected by the formation of thrust fold systems and extensive granite intrusions (Zhang et al., 2001). The Dabashan arc-like

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fold-thrust belt took its shape during this collision orogenesis, forming a thrust-nappe

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deformation zone at the front edge of the orogenic belt (Fig. 1B). During the Yanshanian orogeny between the Jurassic and Late Cretaceous, most of the Sichuan Basin became folded, and open symmetrical folds in NW-SE direction were formed in southeastern Chongqing (Fig.

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1C). Rapid uplift occurred in the upper Yangtze Platform as a result of renewed compression from the Pacific Ocean plate during the Himalayan orogeny from the Late Cretaceous and

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Cenozoic (Ma et al., 2007). Tectonically, northeastern Chongqing, located in the Dabasha arc-

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like thrust fold belt, is in the transitional zone of the northern edge of the upper Yangtze block and the Qinling orogenic belt. Strata exhibited strong structural deformation in the thrust-nappe domain resulting in faults and penetrative cleavages (Ma et al., 2015), as folds and faults are common in the outcrop (Fig. 2A) and the stratigraphic dip of the Lower Cambrian shales are large (Fig. 2B). In this study, northeastern Chongqing in the Qinling orogenic belt with complex tectonic history is referred to as the deformed zone (DZ) (Zhang et al., 2001). Southeastern Chongqing, outside of the Sichuan Basin, tectonically appears as trough-like folds in an NW-SE direction and strata younger than Triassic have not been exposed (Fig. 1C). The layers in this area are almost horizontal in the outcrop and rock samples (Fig. 2C and D), the southeastern Chongqing area can reasonably consider as a Non-deformed Zone (NDZ). 3. Sampling and methodology 5

Journal Pre-proof In this study, core samples from DZ in northeastern Chongqing and NDZ in southeastern Chongqing were collected from the eastern part of the Upper Yangtze Platform, South China. It is noteworthy, the outcrop sample in southeastern Chongqing is characterized by fracture-filled solid bitumen in the Lower Cambrian shales (Fig. 2E), in which a network of honeycomb-like calcite-filled fracture assemblage cross-cuts the bitumen layer in some spots (Fig. 2F), which represents the pure migrated solid bitumen within the Lower Cambrian source rocks. The core samples drilled with fresh water were initially used for canister desorption tests to determine the gas content at the well site. Saturated saltwater was filled inside the canister to

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reduce solution of methane. Freshwater was used to flushing the salt on the shale surface prior to

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the organic petrography and pore structure analyses. Then the samples were cut perpendicular to bedding for optical and scanning electron microscopy (SEM) observations. Sample offcuts were

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prepared as small cubic blocks with length around 1 cm for MICP. The rest of the offcuts were crushed in a ring mill to smaller than 60 mesh (250 µm). These crushed offcuts were used for

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TOC content, X-ray diffraction, methane adsorption, and low-temperature N 2 adsorption and

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CO2 adsorption analyses. All analyses were performed at China University of Petroleum (Beijing), except for the optical microscope observation and MICP analyses, which were performed at Geological Survey of Canada-Calgary and at the China University of Mining and

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Technology at Xuzhou, respectively. 3.1. Total Organic Carbon

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TOC content (wt. %) was measured with a Leco-CS230 carbon and sulfur analyzer after the

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samples were treated first, with 10% hydrochloric acid to remove carbonate. Measurements have an analytical precision of ± 0.5%. 3.2. Organic Petrography

Organic petrography was carried out on selected samples using polished blocks made with a cold-setting epoxy–resin mixture. The resulting sample pellets were ground and polished, in final preparation for microscopy, using an incident light Zeiss Axioimager II microscope system equipped with ultraviolet (UV) light source and the Diskus-Fossil system for reflectance measurements.

Random reflectance measurements

were conducted

under oil immersion

(objective ×50) following ASTM standard methodology. The standard reference for reflectance measurement was yttrium-aluminum-garnet with a standard reflectance of 0.906% under oil

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Journal Pre-proof immersion. Then the random bitumen reflectance (Rb) was converted to equivalent vitrinite reflectance (EqVRo ) using Jacob (1989)’s equation: (1) 3.3. Scanning electron microscopy (SEM) and Quantitative Evaluation of Minerals by Scanning electron microscopy (QEMSCAN) The shale surfaces were initially mechanically polished using the Leica EM TXP target surfacing system to create a level surface and they were then milled by a Leica EM RES102 ion beam milling system with an accelerating voltage of 3 kV and a milling time of 6-10 h. Samples

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were mounted to SEM stubs using carbon paste and coated with carbon to provide a conductive

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surface layer. Each sample was inserted into a Zeiss Merlin field emission scanning electron microscopy (FE-SEM) for imaging with accelerating voltages (1‒2 kV) and working distances of

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about 3.5‒5.5 mm. In addition, dual Bruker Quantax energy-dispersive X-ray spectroscopy (EDS) detectors were installed into the Merlin FE-SEM for chemical analysis.

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Quantitative Evaluation of Minerals by Scanning electron microscopy (QEMSCAN), which

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comprises SEM, EDS detectors and software controlling automated data acquisition were used to provide a quantitative analysis of minerals. The elemental composition in combination with

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back-scattered electron (BSE) and X-ray count rate information is converted into mineral phases (Gottlieb et al., 2000). By mapping the sample surface, bulk mineralogy, textural properties, particle, and mineral grain sizes can be calculated from the QEMSCAN data using the Amics

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software. The pixel resolution of the images in this study is 1 µm.

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3.4. Pore size distribution (N 2 and CO2 adsorption) According to the International Union of Pure and Applied Chemistry (IUPAC), pores are generally classified as micropores (< 2 nm diameter), meso- to macropores (2－50 nm diameter), and macropores (> 50 nm diameter) (Sing, 1985). The N 2 and CO 2 adsorptions analyses were carried out on Micromeritics ASAP 2020M apparatus. Prior to the gas adsorption analyses, the crushed shale samples were dried and degassed in a high vacuum oven (< 10 mmHg) for 12 h at 110 °C to remove adsorbed moisture and volatile matter. The N 2 adsorption was measured at low temperature and pressure (77.35 K at 101.3 kPa) between the relative pressures (P/P0 ) of 0.01 and 0.997, the five-point Brunauer, Emmett, and Teller (BET) method and the Barrett, Johner, and Halenda (BJH) method were used to then calculate the surface area and pore volume of pores (2‒200 nm), respectively. For the CO 2 adsorption studies, samples were measured at 273 K 7

Journal Pre-proof (0 °C) with relative pressures (P/P0 ) ranges from 0.0003 to 0.023, the density functional theory (DFT) method (Lastoskie et al., 1993) was used for surface area and pore volume measurement of micropores (0‒2 nm). The repeatability of the measurements was ± 6%. 3.5. Mercury injection capillary pressure (MICP) Cubic block samples (about 1 cm in length) were oven-dried at 110 °C overnight, evacuated to 1×10-3 psi, and intruded with mercury from 1 to 60,000 psi using a Micromeritics T M Autopore IV 9500 porosimetry. The measured pressure range equates to an equivalent pore diameter range of 3 to 1.8×105 nm. Bulk and skeletal densities of the sample can be assessed from the mercury

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intrusion measurements at 1 and 60,000 psi, respectively. Together with the sample weight,

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porosity can be calculated from the mercury porosimetry information. 3.6. Desorbed gas content

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Fresh shale core segments with a diameter of 6.2‒10 cm and length of about 20 cm were quickly sealed in a desorption canister that was filled with saturated salt water after recovery at

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the well site. Desorption took place in the water bath immediately at reservoir temperature, the

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volumes of gas released inside the canister were periodically measured using a graduated cylinder. Desorbed gas volume gradually decreased with time, when the amount dropped below 5 ml in 2 h, a high temperature of 90 °C was used to expedite the gas releasing process. Gas

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content determination finished when the desorbed gas volume similarly fell below 5 ml in 2 h at 90 °C. Desorbed gas content equals the sum of the gas released at reservoir temperature and at 90

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°C. A detailed introduction of this procedure can be found in Ma et al. (2015).

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3.7. X-ray diffraction (XRD)

Approximately 10 g of powder from selected samples were used for mineral phase identification using a powder diffractometer (Bruker D8 Discover). The crushed samples were mixed with ethanol, hand-ground and then smear mounted on glass slides for X-ray diffraction analysis. A normal-focus Cu X-ray tube was used on the diffractometer at 40 kV and 40 mA. Subsequently, the semi-quantitative mineralogy was obtained by analyzing the spectra using Rockquan software. 3.8. Methane adsorption test Powder sample up to 150g were used for methane adsorption test that was performed on a volumetric sorption apparatus, according to the Chinese Standard GB/T 19560-2004. Pressure points were collected up to 11MPa using high-precision pressure transducers (± 0.689 kPa). All 8

Journal Pre-proof methane adsorption experiments were conducted at 30 ±0.1°C. Based on the isotherm adsorption curve and the Langmuir adsorption model (Langmuir, 1918), VL is the Langmuir volume or the maximum adsorption capacity of the adsorbent, PL is the Langmuir pressure or the pressure at which half the Langmuir volume of gas is adsorbed. 4. Results 4.1. Desorbed gas content Free gas in natural fractures and matrix pore structure and adsorbed gas on the surface of the shale matrix and in OM-hosted pores are the major components of gas in the shale gas reservoir.

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Gas desorption is a major controlling mechanism on gas production and can be in turn a key factor controlling ultimate gas recovery. The measured gas content of 158 samples from three

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wells in the DZ area ranges from 0.02 to 3.97 (m3 /t) with a mean value of 0.87 (m3 /t), exhibits

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relatively good depth profile gas content of the Lower Cambrian shales (Fig. 3A). Methane accounts for more than 90% of desorbed gas in Well C1 and D1, gas composition and isotopic

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compositions suggest the Lower Cambrian shale gas in the DZ samples are the mixture of two

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gases generated from cracking of liquid hydrocarbons at high maturity, and kerogen catagenesis at the early hydrocarbon-generating stage (Han et al., 2013; Ma et al., 2015; Ma et al., 2020). For the Lower Cambrian shales in the NDZ area, the desorbed gas content for shale samples

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from Well CY1 in NDZ range from 0.06 to 0.64 (m3 /t) with an average value of 0.29 (m3 /t). However, subsequent gas chromatography (GC) analyses show that most of the desorbed gas

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from Well CY1 is air, as methane accounts no more than 1.8% of total gas composition, while

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the other components are nitrogen, oxygen, and carbon dioxide. Desorbed gas content for the NDZ core samples from both Well Z608 and Z1206 is less than 0.02 (m3 /t) (Table 1 and Fig. 3A), while no hydrocarbons were detected by subsequent GC analyses. The burial depth of the Lower Cambrian shales in Well CY1 is deeper than 1950 m, which may better help the preservation of shale gas than the shale samples in Z608 and Z1206 with burial depths shallower than 850 m (Fig. 3B). This is consistent with the poor shale gas production of the Lower Cambrian shales in the southeastern part of the Upper Yangtze Platform (Zhao et al., 2016), which support the poor resource potential of the Lower Cambrian shale gas in the NDZ evaluated by desorption experiments from three wells in this study. 4.2. Geochemical characteristics

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Journal Pre-proof The TOC content of 221 DZ samples is highly variable and ranges from 0.39 to 41.3 wt. % with a mean value of 4.06 wt. % (Table 1 and Fig. 3). TOC contents are generally less than 10 wt. % in DZ except for shales in well D1, which are black organic-rich shales with TOC content as high as 41.3 wt. % (Fig. 3A). The mean TOC value for 73 NDZ samples is equal to 3.97 wt. %, slightly lower than that of DZ samples with a narrow range of variation from 0.04 to 12.6 wt. % (Table 1). The mean EqVRo value derived from the Rb is 3.3% for DZ samples, which indicates an over-mature source rock (Table 1). The mean EqVRo for NDZ samples is similar to the DZ samples, with a mean value of 3.1% (Table 1). The OM in both sample sets has Ro >

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1.5% (cf. Mastalerz et al., 2018), is considered as pyrobitumen.

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The OM (pyrobitumen), in the studied samples occurs in three forms: (1) fine laminations of pyrobitumen that resemble the structure of precursor lamellar algae (Fig. 3A) or algal

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accumulations (Fig. 4B); (2) pore-filling pyrobitumen (Fig. 4C-G); and (3) big fragments (25 – 100 µm) of bitumen engulfed within fracture-filled calcite and quartz (Fig. 4B, H-I). Pore-filling

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pyrobitumen pervasively or locally filled inter-crystalline pores between diagenetic minerals

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such as dolomite (Fig. 4C-D), inter-granular pores in the rock matrix (Fig. 4F-G), or microfractures (Fig. 4E). The sample D1-44 with TOC content of 24.1 wt. % has a pervasive porefilling pyrobitumen texture (Fig. 4C-D).

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The δ13 C values of the kerogen in both areas are in the range between -30‰ and -35.3‰ (Liang et al., 2009; Wang et al.,2000; Ma et al., 2015). SEM observations and kerogen maceral

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identifications suggest that the organic matter inputs in the Lower Cambrian source rock of the

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upper Yangtze area are algae and fungi (Liang et al., 2009). Both of these studies suggest that shale samples from DZ and NDZ areas in the Upper Yangtze Platform are of Type II kerogen (Liang et al., 2009; Ma et al., 2015). 4.3. Mineralogy, X-Ray diffraction (XRD) Quartz, carbonate (calcite and dolomite), clay minerals, and feldspars (plagioclase and Kfeldspar) are major mineral constituents of the lower Cambrian shales in Eastern Upper Yangtze Platform (Table 2; Fig. 3 and 5). Plagioclase, K-feldspar, and pyrite are the other mineral components in the studied samples (Table 2; Fig. 5). Illite and chlorite are the major clay minerals that formed the total clay content, with the dominance of illite in both sample sets (Table 2). Kaolinite is the other clay mineral that is detected only in the NDZ samples (Table 2). The mean quartz and total clay content in NDZ samples are higher than those in DZ samples 10

Journal Pre-proof (Fig. 3 and 5). Dolomite and calcite contents in DZ samples are significantly higher than those of the NDZ samples (Fig. 5). This led to significantly higher total carbonate content for the DZ samples (Fig. 3 and 5). 4.4. Scanning electron microscopy (SEM) 4.4.1. Quantitative Evaluation of Minerals by Scanning electron microscopy (QEMSCAN) The results of QEMSCAN mineralogical abundance mapping and estimated mineral composition of one DZ area sample (D1-44) and two NDZ area samples (CY1-2 and Z608-802) are presented in Figure 6. Almost all three maps show a subtle fine laminated framework in the

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studied rocks, while DZ sample is significantly coarser-grained than the NDZ samples. The

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major components of the mapped area on the DZ sample are the dolomite and organic matter with significantly lower amounts of quartz and pyrite (Fig. 6A). The high relative abundance of

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dolomite and TOC is evident from XRD (Table 2; Fig. 3 and 5) and TOC measurements (Table 1). The major mineralogical constituents of two NDZ samples are quartz, carbonate, feldspars

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and clay minerals (Figs. 1 and 6B-C). Almost no OM content was detected in two NDZ samples

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(Fig. 6B-C), which might be the size of the organic matter is too fine and lower than the pixel resolution of QEMSCAN images (1 µm in this study). The QEMSCAN mineral abundance results are generally consistent with the XRD mineralogy data and help provide a microscopic

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view of the distribution of minerals.

4.4.2. Pore type characterization

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Based on SEM observations, four major pore types were identified in the studied samples,

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those are (1) OM-hosted pores, (2) mineral-hosted or intra-crystalline pores, (3) micro-fractures within OM and minerals, and (4) dissolution rims around minerals (Fig. 7). The dominant pore types in the DZ samples are intra-crystalline and micro-fractures within OM and minerals with sporadic OM-hosted pores (Fig. 7A-D). The OM in DZ sample shows a pervasive solid bitumen or pyrobitumen filling inter-particle pore space that is evident from engulfed matrix minerals with OM (Figs. 4D, 7A-C; e.g., Wood et al., 2018a). The majority of carbonate minerals in the rock matrix (Figs. 4C-D, 6A, 7A) contain secondary pores (Fig. 7A, D) that is the likely result of dissolution from organic acids during oil migration (Ehrenberg et al., 2012) or degradation (Seewald, 2003). The observed intra-crystalline pores in calcite and dolomite, are sub-micron in size (Fig. 7D), and likely contribute to pore volume in the studied sample. OM-hosted pores distribution in the DZ sample are sporadic (Fig. 7A-C) while shrunk micro-fractures (e.g., Ko et 11

Journal Pre-proof al., 2016) within OM (Fig. 7A, C) and at the OM or mineral interface (Fig. 7B-C) are the abundant type of OM-hosted porosity. This is consistent with the SEM observation from Ma et al. (2015) and Wang et al. (2016) that OM-hosted pores in the DZ area are rare. Micro-fractures are also abundant in minerals in the DZ sample (Fig. 7B-C). Some of those fractures could be the result of the gas generation and the inability of the gas to escape or possibly formed after sample retrieval (Katz and Arango, 2018) or result of deformation through exhumation of strata (e.g., Akker et al., 2018). OM-hosted pores in the NDZ samples, with pore diameters ranging from 5 to 100 nm (pores

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smaller than 5 nm are invisible under SEM), are the dominant pore types (Fig. 7E-P). The

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mineral-hosted pores (Fig. 7G) and micro-fractures (Fig. 7E-G) are also abundant in the NDZ samples (Fig. 7K-O). Dissolution rims around diagenetic minerals (i.e., calcite, dolomite), or

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between them (Fig. 7H-I, M-N), is another major type of pore in the NDZ samples. Some of those micro-fractures and/or spaces between crystals are filled with pyrobitumen (Fig. 7K). This

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suggests that those fractures are formed before the emplacement of solid bitumen in paleo-pores.

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Solid bitumen filled some dissolution rims (Fig. 7H) further indicate corrosive fluids created secondary pores that subsequently filled with solid bitumen, while crystal edges are dissolved (e.g., Zhu et al., 2019).

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Although OM-hosted pores are more abundant in the NDZ samples (Fig. 7E-P), their size is in sub-micron to nanometer size range, specifically in the outcrop sample (STMD) OM-hosted

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pores are in nanometer scale (Fig. 7O-P). Pyrobitumen pervasively filled inter-granular (Fig. 7E-

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N) and inter-crystalline pores in framboidal pyrite (Fig. 7G). In larger pores, OM pervasively encloses mineral grains and clay mineral platelets (Fig. 7E-F, J-N), this suggests the pore-filling nature of post-oil pyrobitumen (e.g., Mastalerz et al., 2018; Wood et al., 2018a) in the studied samples. Pores formed within pyrobitumen fill inter-crystalline spaces in framboidal pyrite which indicates that during the processes of thermal maturation, pyrobitumen developed pore spaces (Fig. 6G). Porosity development in inter-crystalline pyrobitumen postdates bitumen migration. The bubbly nature of OM-hosted pores in the NDZ samples suggests those pores likely formed through the devolatilization of solid bitumen (Katz and Arango, 2018; Wood et al., 2015, 2018a, b). Overall, SEM images suggest that mineral-hosted pores are the dominant type in the DZ samples, while the OM- and mineral-hosted pores are the dominant types in the NDZ samples 12

Journal Pre-proof (Fig.7). Dissolution pores around the diagenetic minerals (Fig. 7H, I, M), are the second important pore location that likely contributes to the total porosity of the samples in NDZ samples. It is clear that the pore types are diverse and complex and there is great variability in the distribution of pore types. 4.5. Reservoir pore structure The porosity of the samples in Well Yuke1 and Youke1 were measured by Ultrapore-200A helium porosimeter (Cao et al., 2014; Sun et al., 2015), the others were carried out by MICP analyses. Porosity measured by helium is slightly higher than that measured by mercury because

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very fine pores are not necessarily accessible by mercury because the molecular diameter of Hg

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is larger than that of helium (Bustin et al., 2008). Anyhow these measured porosity data can provide some quantitative information about the reservoir property. The porosity of DZ samples

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ranges from 0.35 to 2.55% with a mean value of 0.81% (n=18), with slightly lower mean porosity than the NDZ samples that vary from 0.4 to 2.7% (mean 1.24%; n=30) (Table 1). The

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pore size distribution (PSD) by MICP is plotted as incremental pore volume (ml/g; Fig. 8A-B)

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and incremental pore surface area (m2 /g, Fig. 8C-D). Samples from both areas show a significant volume of pores with a diameter between 50 and 100 µm (Fig. 8A-B); however, these pores do not contribute to the total surface area of the reservoir as significant as the finer pores (Fig. 8C-

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D). The dominant pore volume for both the DZ and NDZ samples comprised of pores smaller than 100 nm in diameter (Fig. 8A-B). Pores smaller than 10nm diameter has a significant

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contribution to the surface area measured by MICP (Fig. 8C-D).

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CO2 adsorption at 0 °C can be used to investigate micropores (< 2 nm) and N 2 adsorption at 196 °C can be used to evaluate pore attributes (pore volume, surface area, pore size distribution) at larger pore scales within the meso- and lower macropore range (2–200 nm) (Clarkson et al., 2013). The results of CO 2 and N 2 adsorption experiments are presented in Table 1 and Figure 9. All studied samples display hysteresis between the adsorption and desorption branches (Fig. 9AB), however, the hysteresis for the DZ samples is not as significant as NDZ samples (Fig. 9B). The isotherm for the NDZ sample suite is Type IVa, suggesting the presence of mesoporosity (2 nm < pore width < 50 nm). The hysteresis loop-shapes are variable, suggesting mainly Type H3 for NDZ samples and Type H4 for DZ (Fig. 9A, B), according to the International Union of Pure and Applied Chemistry (IUPAC) classification (Thommes et al., 2015), which may indicate the presence of slot-shaped pores in NDZ samples. 13

Journal Pre-proof The combination of CO 2 and N 2 adsorption derived pore size distribution (PSD) curves using pore volume (Fig. 9C-D) and pore area (Fig. 9E-F) reveal that micropores are dominant in both areas, while fine meso- to macropores in NDZ samples are more developed than those of DZ samples. The outcrop sample (STMD), has the highest micropore volumes and surface area while it has the lowest mesopore volumes and surface area compared with the other samples (Fig. 9C-E). The PSD results further confirm the dominance of micropores in the solid bitumen that is not SEM-visible (Fig. 7O), but can be detected with helium ion microscopy (Fig. 7P). The gas adsorption analyses illustrate the importance of micropores with respect to containing the

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greatest amount of surface area.

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The BET specific surface area and BJH pore volume are significantly lower for the DZ samples in comparison to the NDZ samples (Fig. 10A, B). The BET specific area from the 66

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DZ samples, ranges from 0.71 to 26.71 (m2 /g) with a mean value of 7 (m2 /g), while it ranges from 0.35 to 29.3 (m2 /g) and a mean value of 11.4 (m2 /g) for the 45 NDZ samples (Table 1; Fig.

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10A). The BJH pore volume for the DZ samples ranges from 0.0002 to 0.0392 (cm3 /g), with a

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mean value of 0.0073 (cm3 /g), while it ranges from 0.0003 to 0.0279 (cm3 /g), with mean values of 0.011 (cm3 /g) in the NDZ samples (Table 1; Fig. 10B). The pore diameters of DZ samples are relatively similar to NDZ samples (Fig. 10E). The average pore diameter for DZ samples ranges

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from 2.1 to 14.2 (nm) with a mean value of 4.9 (nm), while it ranges for NDZ samples from 1.6 to 8 (nm), with a mean value of 4.8 (nm) (Table 1; Fig. 10C).

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The DFT derived CO 2 pore surface areas and volumes of DZ samples are lower than those of

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NDZ samples (Table 1; Fig. 10D-E). The DFT specific area from the 32 DZ samples, ranges from 3.03 to 39.26 (m2 /g) with a mean value of 13.58 (m2 /g), while it ranges from 1.86 to 36.98 (m2 /g) and a mean value of 19.77 (m2 /g) for the 18 NDZ samples (Table 1; Fig. 10D). The DFT pore volume for the DZ samples ranges from 0.001 to 0.0121 (cm3 /g), with a mean value of 0.0038 (cm3 /g), while it ranges from 0.0005 to 0.012 (cm3 /g), with mean values of 0.0056 (cm3 /g) for the NDZ samples (Table 1; Fig. 10E). Mesopore surface area and volume show positive relationships with TOC content for the NDZ samples (Fig. 11A and B), suggesting that OM-hosted pores are well developed in the NDZ shales, which is confirmed by the SEM observation (Fig. 7E-P). For the DZ samples, there is no correlation between TOC content and mesopore surface area and volume (Fig. 11A and B), this is consistent with the SEM results, as larger SEM-visible pores within the OM are rare. The 14

Journal Pre-proof developed mineral-hosted pores may contribute to the meso- to macropores of the DZ samples (Fig. 7A-D). The micropore surface area and volume show a good positive correlation with TOC contents for shale samples from both areas, with a better correlation for NDZ samples (Fig. 11C and D), which illustrates that the contribution of OM-hosted micropores. Helium ion microscopy images further confirm the abundance of micropores in solid bitumen that is not SEM-visible (Fig. 7O, P). This is consistent with the modeling imbibition data that micropores in unconventional rocks are dominantly OM-hosted pores (Shi et al., 2019). The porosity of DZ samples have a good correlation with TOC content and total carbonates

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(Fig. 12A-B), which suggests the OM-hosted micropores (Fig. 11C-D), micro-fractures

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associated with OM (Fig. 7A, C), together with intra-crystalline dissolved pores within carbonates (Fig. 7D) contribute to total porosity. For the NDZ samples, the shale porosity

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doesn’t show any correlation with TOC content, total carbonates, clay minerals, quartz or feldspars, which suggests both the OM-hosted pores (Fig. 7E-G, J, K) and mineral-hosted pores

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(Fig. 7 F, H, I, M) contribute to the total porosity, while either of them is dominant.

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5. Discussion

5.1. The potential effect of structural deformation on shale pore characteristics Understanding the complex pore structure of shale/mudrock reservoirs is one of the biggest

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challenges in estimating the transport and storage properties of these rocks. The microstructure can significantly vary between shale plays and often within the same play (Curtis et al., 2012b;

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Jarvie, 2012; Loucks et al., 2012; Katz and Arango, 2018). Pore size variation in shale can range

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over six orders of magnitude (Ko et al., 2017) and is generally dominated by meso- to macropores (King et al., 2015) with varying amounts of micro- and macropores (Katz and Arango, 2018). The micro-, meso-, and macro-porosity contribution to the total porosity in shale reservoirs depends on variation in mineralogy, texture, fabric, kerogen content and type, and maturity (Chalmers et al., 2012; Kuila et al., 2014; Klaver et al., 2016; Ko et al., 2016; Ma et al., 2016; Yang et al., 2016; Chalmers and Bustin, 2017; Teng et al., 2017). SEM observations combined with MICP porosity and gas adsorption analyses have shown a significant difference in OM-hosted porosity between DZ and NDZ samples. There are rare SEM-visible OM-hosted pores in the DZ samples, while micro-fractures associated with OM are widely observed (Fig. 7A-C). In contrast to the DZ samples, the dominant SEM-visible pore space in the NDZ samples is OM-hosted pores that formed within pore-filling pyrobitumen (Fig. 15

Journal Pre-proof 7E-P), which is further confirmed by the significant correlation between TOC content and mesopore volume and surface area (Fig. 11A, B). In addition, both mean mesopore specific surface area (11.4 m2 /g) and pore volume (0.011 cm3 /g) of NDZ samples are higher than that (7 m2 /g and 0.0073 cm3 /g, respectively) in DZ samples. Micropores are abundant in both DZ and NDZ samples, which can be inferred from the good correlation between TOC content and micropore volume and surface area (Fig. 11C, D), and helium ion microscopy observation (Fig. 7O, P). All shale samples from both DZ and NDZ areas were formed during a major marine

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transgression from southeast to northwest of the Upper Yangtze Platform in the Early Cambrian

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period (Tan et al., 2013). Similar depositional environment and stratigraphic unit make geochemical characteristics of the shale samples from both areas similar, too. The mean TOC

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content of DZ and NDZ samples are 4.06 wt. % and 3.97 wt. %, respectively (Table 1). Shale samples from both areas have a mean equivalent vitrinite reflectance (EqVRo ) of 3.1 % (Table

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1). Studies from kerogen isotope and macerals suggest that shales from both DZ and NDZ areas

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are of Type II kerogen (Liang et al., 2009; Wang et al., 2000; Ma et al., 2015). Organic petrography suggests that the OM in the studied samples in both areas (Fig. 4), is dominantly pore-filling pyrobitumen. It has been suggested that the majority of SEM-visible OM-hosted

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porosity developed within solid bitumen/pyrobitumen, rather than in kerogen (i.e., solid or more structured OM) (İnan et al., 2018) and the majority of the OM-hosted porosity likely develops at

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higher levels of thermal maturity (i.e., gas window; Mastalerz et al., 2013; Katz and Arango,

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2018). The organic matter richness, type and thermal maturity of shales from both DZ and NDZ areas are similar, which are the major controlling factors on organic porosity development in organic-rich shales (Ardakani et al., 2018; Loucks et al., 2012; Mastalerz et al., 2013; Ko et al., 2017; Löhr et al., 2015; Katz and Arango, 2018). So what causes the significant difference of OM-hosted porosity between DZ and NDZ shale samples? One possible reason might be due to structural deformation. The strong extrusion stress during the formation of the Dabashan arc-like thrust fold belt might have significant effect on the shale texture then led to changes in the pore structure of the DZ shales. In comparison to the NDZ areas, slight trough-like folds may not cause significant changes to the pore structure. Large pores tend to collapse earlier in response to increasing effective stress, therefore, generally in mudstone sedimentary rocks, much of the overall porosity loss occurs at shallow 16

Journal Pre-proof depths, during the collapse of pores larger than 15 nm (Yang and Aplin, 1998), while smaller pores (< 10 nm) are generally resistant to mechanical compaction (Loucks et al., 2009; Milliken and Curtis, 2016; Katz and Arango, 2018). As such, mechanical stress due to structural deformation has little effect on micropores due to the nanometer-scale of OM-hosted pores and they are less likely to be affected by structural deformation (e.g., Liang et al., 2017; Zhu et al., 2019). This is why micropores in DZ shales are preserved and have a similar slope with NDZ shales in correlation with TOC contents (Fig. 11C-D). Pore collapse and deformation are also largely controlled by mineralogy and in turn grain

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mechanical properties (Schieber et al., 2016), this is supported by data on the proportion of

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micropores, meso- to macropores, and macropores found in shales of the Upper Ordovician Wufeng- Lower Silurian Longmaxi formations in which strong size control follows structural

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deformation (Liang et al., 2017). The majority of meso- to macropores were found to be destroyed due to tectonic compression, and more macropores were formed due to the

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development of fractures. This is inconsistent with the results that OM-hosted meso- to

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macropores are rare in DZ shales (Fig. 7A-C; Fig. 11A-B). In addition, the formation of microfractures may occur during the exhumation and folding of the rocks (Akker et al., 2018). The lower porosity of the DZ samples in comparison to the NDZ samples is likely, therefore, due to

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structural deformation in DZ samples and destroyed the OM-hosted meso- to macropores (Table 1; Fig. 116A-B; Liang et al., 2017; Zhu et al., 2019).

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OM-hosted porosity heterogeneity, in terms of pore size and distribution, may also result

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from a combination of OM chemo-physical properties and the structural fabric of the rock (Mathia et al., 2016; Katz and Arango, 2018). Therefore, less rigid rock fabrics may or may not protect the OM-hosted pores from structural deformation stress and result in the loss of the small organic pores (i.e., meso- to macropores). It appears that this is the case for the DZ samples with lower quantities of OM-hosted pores, than micro-fractures within the OM and mineral matrix (Fig. 7A-C). In addition, increased OM plasticity as a result of compaction and/or structural deformation may result in the destruction of pores within OM during burial or deformation (e.g., Löhr et al., 2015; Katz and Arango, 2018). Therefore, structural deformation and OM plasticity can potentially be a controlling factor for the destruction of OM-hosted pores in the DZ samples and the destruction of meso- to macropores that are more susceptible to collapse. 5.2 Potential effect of pore deformation on shale gas preservation 17

Journal Pre-proof As mentioned above (section 4.1), both desorption experiments and reservoir production performance, show the substantial difference in the Lower Cambrian shale gas contents between DZ and NDZ areas, with high shale gas content in the DZ, in contrast to low or none gas contents in the NDZ (Fig. 3; Table 1; Ma et al., 2015; Tang et al., 2017; Zhao et al., 2016). According to the geochemical analyses, the Lower Cambrian shales in the DZ and the NDZ areas are organic-rich and over-mature, should have promising hydrocarbon generation potential for shale gas (Table 1; Zou et al., 2015; Zhao et al., 2016). The critical issue, therefore, is how those

structural deformation (Hao et al., 2013; Ma et al., 2015).

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generated gases can be preserved after subsequent uplifts, intensive tectonic movements and

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For DZ samples, OM-hosted meso- to macropores have most probably collapsed during the formation of the Dabasha arc-like thrust fold belt, caused by the collision of the South Qinling

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and the Yangtze Platform by the end of late Triassic time (Zhang et al., 2001), after the end hydrocarbon generation from the shales (Ma et al., 2004; Ma at al., 2015). As a result, there is no

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significant relationship between mesopore volume and surface area and desorbed gas content

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(Fig. 13A-B). The remaining OM-hosted micropores in the DZ shales, provides a significant amount of surface area (Fig. 11C-D) and storage space for adsorbed gas (Ross and Bustin, 2009; Moore, 2012), making significant accumulation of adsorbed gas in dominant OM-hosted

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micropores, which is strongly suggested by the good correlation between micropore volume and surface area and desorbed gas content (Fig. 13C-D). The significant correlation between

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Langmuir volume, TOC content and desorbed gas contents (Fig. 13E-F) further confirms that

(Fig. 13C-D).

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adsorbed gas is dominant in the DZ samples, especially, stored in the OM-hosted micropores The desorbed gas contents are far below the Langmuir volume (Fig. 13E),

indicating that the adsorbed gas is undersaturated, and no free gas exists in the DZ shales. Shale gas is mainly stored as free compressed gas in the meso- to macro-pores and cracks, as adsorbed gas in micropores associated with kerogen and clay minerals (Javadpour et al., 2007; Ross and Bustin, 2009; Sondergeld et al., 2010b). During shale gas production or loss, adsorbed gas desorbs from the surface of kerogen/clays when reservoir pressure drops. The desorbed gas then flows to mesopore, macropores, microfractures, and fractures following slip flow or Darcy flow equation according to the pore size (Javadpour et al., 2007; Sondergeld et al., 2010b). OMhosted porosity is considered as the most important storage space for shale gas (Sondergeld et al., 2010b; Curtis et al., 2012b). 18

Journal Pre-proof Fig. 14 schematically illustrates shale gas storage and flow capacity. The lower abundance of OM-hosted meso- to macropores in the DZ shales, makes free gas stored in OM-hosted porosity limited and difficult to connect with the mineral-hosted pores and micro-fractures (Fig. 14 B). The poor connectivity of OM-hosted porosity makes it difficult for pressure decline in the mineral-hosted pores and micro-fractures transmitted to the OM-hosted micropores during reservoir depletion, thus, adsorbed gas in these micropores is comparatively well preserved and is dominant in DZ samples (Fig. 11C-D; Fig. 13C-F; Fig. 14 A-B). In contrast,

the well-developed

OM-hosted

pore

network

(including micropore to

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macropores) in the NDZ samples will make the gas flow easier in the OM-hosted pores as well

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as to mineral pores and micro-fractures, as recent studies using focused ion beam scanning electron microscope (FIB-SEM), imbibition and wettability analyses of gas shales have

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illustrated that OM-hosted pores commonly develop a 3D interconnected pore network, which could form important pathways for gas to flow in shales (Ambrose et al., 2010; Curtis et al.,

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2012b; Lan et al., 2015a; Lan et al., 2015b; Yassin et al., 2016). Once exhumed during basin

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uplift, the well-connected OM-hosted pore network along with the mineral-hosted pores likely, easily allowed gas leakage through fractures or faults. The well-connected OM-hosted pore network also makes the transition of pressure to the micropores easily when reservoir pressure

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drops, thus adsorbed gas in OM-hosted micropores start to desorb and migrate to the pore network and escape (Fig. 14 C-D). Thus, shale gas in NDZ samples are more pron to escape

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during subsequent basin uplift that led to the preservation of negligible. Besides, burial depth

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may also be a potential control on shale gas preservation in NDZ samples, as shales in CY1 (with burial depth 1970–2019m) have average desorbed gas content of 0.29 m3 /t, while no hydrocarbons detected in the desorbed samples in well Z608 (burial depth between 802.2–838 m). This is because shales with deeper burial depth have larger reservoir pressure from overlying strata, which diminishes the desorption of adsorbed gas in OM-hosted micropores and helps to preserve the shale gas to some extent. 6. Conclusion Samples of Lower Cambrian shales from the deformed and undeformed zones in the Eastern Upper Yangtze Platform have been characterized using canister desorption, mineralogy, petrophysical properties, and various high-resolution microscopy techniques. The main conclusions are listed as follows: 19

Journal Pre-proof 1. Pore size characteristics of OM-hosted porosity in the DZ and NDZ shale samples that deposited in the same geological age with similar thermal maturity show noticeable differences, with a dominance of OM-hosted micropores in the DZ samples, while the NDZ samples show a wider range of OM-hosted pores from micropores, to pores at the lower end of the macropore range. The presence of HIM-visible nano-pores, combined with the good correlation between CO 2 micropore surface area and TOC content suggests that micropores within OM are abundant in both the DZ and NDZ sample sets. Sub-micron to micrometersized mineral-hosted pores are abundant in both sample sets. Besides, micro-fractures

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associated with OM are more abundant in DZ samples, while dissolved-rims around carbonate

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minerals contribute another major pore type in the NDZ samples.

2. Both shale gas desorption experiments and drilled wells reveal the significant difference of

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shale gas content in DZ and NDZ samples. Shale gas is rich in DZ, with a mean desorbed gas content of 0.87 (m3 /t). The significant correlation of desorbed gas contents to Langmuir

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volume, TOC content, micropore volume, and surface area confirm that adsorbed gas in OM-

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hosted micropores is dominant in the DZ samples, and no free gas exists. There are no commercial shale gas wells found for the Lower Cambrian shales in NDZ, the maximum desorbed gas content is 0.64 (m3 /t) while methane accounts no more than 1.81% in volume.

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3. The substantial difference in SEM- and HIM-visible pore types, meso- and micropore size distribution between the DZ and NDZ shale samples, suggests structural deformation can

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potentially have significant effects on the pore size characteristics and gas content of the

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Lower Cambrian shales in the Upper Yangtze Platform. Structural deformations from tectonic stress most probably come from the formation Dabashan arc-like fold-thrust belt, which may have destroyed the meso- to macropores within the OM in the DZ samples, while OM-hosted micropores are retained due to their greater mechanical resilience. The abundant microfractures in the DZ samples are likely formed due to the exhumation and folding of the rocks during deformation. The dominance of OM-hosted micropores in the DZ samples resulted in higher adsorbed gas content, however, the lack of meso- to macropores within the OM makes gas flow difficult in the DZ samples shale matrix. All these factors resulted in better preservation of gas in the structural deformation zone. In contrast, the well-developed OM network in the NDZ samples may make gas flow easier in the shale matrix pore system and fractures resulting in easier gas leakage during basin uplift. 20

Journal Pre-proof Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 41802149, 41573035), Science Foundation of China University of Petroleum-Beijing (No. 2462017YJRC019), PetroChina Innovation Foundation (2018D-5007-0102) and China Geological Survey Project (DD20160181-YQ17W06JJ03). The authors also acknowledge the support from MITACS, the University of Calgary Beijing Research Site, a research initiative associated with the University of Calgary Global Research Initiative in Sustainable Low Carbon Unconventional Resources and the Kerui Group. The authors would like to thank the IJCG editor

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for handling of the manuscript and constructive comments from two anonymous reviewers and

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comments on the initial version of the manuscript provided by Dr. D. Lavoie of Geological

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Survey of Canada.

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26

Journal Pre-proof Table captions: Table 1. List of studied samples with analytical results of TOC content, equivalent vitrinite reflectance (EqVRo ), N2 and CO2 adsorption and mercury injection analysis. Note: Porosity of samples from Yuke1 and Youke1 well were measured by helium porosimeter (Cao et al., 2014; Sun et al., 2015), while the porosity of other samples was measured by MICP analyses.

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Table 2. Mineralogical composition of studied samples based on X-ray diffraction (XRD) analysis.

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Journal Pre-proof Figure captions: Figure 1. Location of the study area with respect to the main structural elements of the upper Yangtze Platform. (A) Regional tectonic setting of the eastern upper Yangtze Platform (Modified from Ma et al., 2007). See the lower-left corner for its location in China. (B) Simplified geological cross-section AA’ (Modified from Ma et al., 2015). (C) Simplified geological cross-section BB’. See A for the location of the cross-section.

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Figure 2. Pictures showing the outcrops and rocks of the Lower Cambrian shale in the study area. (A) Fold and fault in an outcrop close to Well D1 in the northeastern Chongqing, deformed zone. (B) Lamination of pyrite showing the large dip angle in a core sample in the deformed zone, the right part shows the cross-section of the core sample, C1-22. Both view from (C) Outcrop and (D) rock showing the horizontal formation in the Non-deformed Zone (NDZ) area, Youyang, Chongqing. (E) Outcrop fracturefilled pyrobitumen in Non-deformed Zone (NDZ) area, Songtao, Guizhou. (F) Honeycomb structure developed in pyrobitumen layer, Songtao, Guizhou.

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Figure 3. Integrated columns showing the lithology, TOC content, desorbed gas content (DGC) and mineral compositions in the DZ area (C1, D1, and YC2) and NDZ area (CY1, Youke1, and Z608). See Fig.1 for the location of the well.

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Figure 4. Organic petrography photomicrographs (oil immersion, white incident light) showing habits of OM in the studied samples dominantly as pervasive solid bitumen or pyrobitumen filling original interparticle pore space between framework grains. For all photomicrographs, pyrobitumen is the grey material. (A) Finely laminated shale with OM (pyrobitumen) occurrence as fine laminae parallel to the bedding suggests an in-situ transformation of lamellar algae to pyrobitumen, YC2-52. (B) Pyrobitumen accumulation with abundant pyrite, some fragments of OM enclosed by fracture-filling calcite, D1-26. (C and D) pervasive pore-filling nature of pyrobitumen filled inter-particle pores and enclosed carbonate minerals, D1-44. (E) Pyrobitumen filling micro-fracture, D1-26. (F) Enclosed pyrobitumen particles within fracture-filling calcite. The high temperature of mineralizing fluids further thermally matured OM and developed anisotropy in OM, YC2-52. (G) pore-filling OM surrounded big digenetic dolomite crystal, CY1-2. (H) Big fragments of pyrobitumen enclosed by fracture-filling calcite, STMD. (I) Porefilling pyrobitumen filled inter-particle pore spaces, Z608-802.

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Figure 5. The average mineralogical composition variation of the deformed zone and Non-deformed Zone samples. Figure 6. QEMSCAN images of the deformed zone (A) and Non-deformed Zone (B and C) samples with their mineral composition. Figure 7. Scanning electron microscopy (SEM) images of studied samples. A-D images for samples from the deformed zone, E-P images from non-deformed zone samples. (A) Elongated fracture (white arrows) within OM along with intra-crystalline micropores within carbonate mineral crystals (white circles), D144. (B) No obvious pores within OM and micro-fractures between mineral particles interface, YC2-37. (C) Micro-fractures within OM and mineral. Micro-fractures within OM do not extend to adjacent mineral, D1-44. (D) Abundant micropores developed within carbonate minerals due to dissolution from organic acids, D1-44. (E) OM (pyrobitumen) filled inter-particle pores and enclosed matrix and clay minerals with developed pores within OM. Micro-fractures within the mineral and at mineral OM interface (white arrows) and intra-crystalline micropores are present (white circles), CY1-2. (F) Porous and non-porous pore-filling OM in close proximity, with large dissolution pores developed within carbonate minerals (black arrows), CY1-2. (G) Pores developed within pyrite intra-crystalline filled OM. 28

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OM filled all inter-particle pores between minerals, CY1-2. (H) Dissolution rim around carbonate minerals (black arrows). The remaining inter-particle pores filled with OM, CY1-2. (I) Pore-filling OM with micro-porosity fills all the inter-particle pores. Dissolution rim formed around carbonate mineral (black arrows), CY1-2. (J) Large inter-particle pore filled with pyrobitumen, enclosed clay and other matrix minerals with abundant pore spaces developed within OM, CY1-2. (K) OM filled inter-particle and micro-fractures in the rock matrix. Dissolution pores within mineral (white circles) and between minerals (black arrows) are present, Z608-802. (L) Porous and non-porous pore-filling OM in close proximity with micropores – fractures within minerals and at the mineral OM interface (white arrows), Z608-802. (M) Abundant micropores within minerals (white circles) and dissolution rims around minerals (black arrows). Pore-filling OM developed micro-porosity, Z608-802. (N) Porous and non-porous porefilling OM in close proximity. In addition OM in the center shows heterogeneity in development of OMhosted pores within single OM particle. White circles show micropores within carbonate minerals, black arrows show the dissolution pore between dolomite crystals, and white arrows show micro-fractures at the mineral OM interface, Z608-802. (O) Pore-filling OM in outcrop sample (STMD) in Non-deformed Zone with no regular SEM visible pores and sporadic dissolution pores within minerals (white circles), STMD. (P) The same sample under Helium ion microscope with OM-hosed nanometer-scale pores, STMD.

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Figure 8. Pore-size distribution defined by the pore volume (A and B) and pore (surface) area (C and D) obtained from mercury injection capillary pressure (MICP) analyses for Non-deformed Zone (A and C) and Deformed zone (B and D) samples.

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Figure 9. Nitrogen adsorption isotherms for Non-deformed Zone (A) and Deformed zone (B) samples. Pore size distribution defined by differential pore volume (C and D) and pore (surface) area (E and F) using low-pressure gas (LPG) adsorption analyses (N 2 and CO2 ) of Non-deformed Zone samples (C and E) and deformed zone samples (D and F). The boundaries between micropore, mesopore and macropores are highlighted by dashed lines.

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Figure 10. Nitrogen BET mesopore surface area (A), pore volume (B), average pore diameter, and CO2 DFT micropore surface area (D), pore volume (E) variations of Deformed and Non-deformed Zone samples.

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Figure 11. Relationship between N2 adsorption mesopore and macropore surface area and total pore area from MICP analyses (A), N2 adsorption mesopore and macropore and MICP porosity, for deformed and Non-deformed Zone samples. Relationship between total pore area from MICP analyses and CO 2 adsorption micropore surface area (C), porosity from MICP analyses and CO2 adsorption micropore volume (D) for the studied samples. Figure 12. Relationship between porosity and TOC content (A), total carbonates (calcite and dolomite) (B). Figure 13. Relationship between desorbed gas content (DGC) and, N2 adsorption mesopore volume (A) and surface area (B), CO2 adsorption micropore pore volume (C) and surface area (D), Langmuir volume (VL ) (E) and TOC content (F) for DZ samples. Figure 14. Schematic diagrams showing pore structure and gas storage in DZ and NDZ shales.

29

Journal Pre-proof Table 1.

D1

1434.7

6.76

0.71

0.0043

12.7

17.76

0.0045

YC2

759.0

3.02

6.41

0.0040

2.1

18.24

0.003

0.006

0.17

1.34

0.94

YC2-52

YC2

909.9

2.43

2.74

0.0040

3.4

3.85

0.001

0.002

0.02

0.39

0.79

0.0029

0.021

0.64

C1

244.06

4.06

0.0038

3.7

C1

463.85

3.43

18.44

0.0295

3.7

C1-20

C1

622.76

2.66

5.69

0.0060

4.2

C1-21

C1

645.96

C1-23

C1

667.52

4.48

16.06

0.0136

C1-25

C1

705.8

6.69

16.29

C1-27

C1

737.8

5.10

2.94

C1-31

C1

790.58

2.42

C1-33

C1

797.03

2.99

C1-36

C1

817.85

2.28

3.17

a n

1.99

0.0135

-p

re

P l 3.4

7.51

4.423 17.12

1.65

0.002 0.001

0.12

0.006

0.81

2.30

1.19

3.18

1.23 0.0025

0.008

0.55

1.28

3.3

2.91

5.6

0.7

0.0079

5.1

1.15

0.0052

2.7

1.35

0.0030

3.8

1.1

0.0041

3.81

C1-40

C1

842.34

1.78

0.0022

4.4

D1-31

D1

1257.8

14.78

2.83

r u 0.89

0.0031

13.9

32.25

0.0067

0.011

3.21

1.97

2.23

9.56

D1-44

D1

1376.1

24.07

2.65

0.79

0.0028

14.2

39.26

0.0087

0.015

4.80

2.55

2.73

9.88

0.0026

0.003

0.56

0.70

1.61

6.16 7.87

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This study

Deformed Zone

D1-56

C1-3

Source

0.28

YC2-37

C1-13

VL (m3 /t)

ro

Desorbed Gas content (m3 /t)

f o

Porosity(%)

Total pore area (m2/g)

Total intrusion volume(ml/g)

Microporous pore volume-CO 2 (cm3/g)

Microporous surface area-CO 2 (m2/g)

Average pore Diameter (nm)

1.32

Meso- to Macroporo volume-N2 (cm3 /g)

1152.3

Meso- to Macroporo surface area-N2 (m2 /g)

D1

EqVRo (%)

TOC (wt%)

Depth(m)

Well

Sample ID

Tectonic zone

D1-26

3.68

0.72 Ma et al. (2020)

CQ1

415.0

2.86

3.14

19.14

0.0289

20.65

0.0072

CQ2

1987.0

7.23

3.72

26.71

0.0374

37.39

0.0121

#11

CQ3

787.0

2.64

3.13

21.06

0.0354

14.74

0.0048

#12

CQ4

1411.0

1.71

3.37

6.97

0.0157

10.24

0.0035

#13

CQ1

805.0

4.60

3.21

20.84

0.0284

26.57

0.0092

#14

CQ2

1969.0

2.11

3.78

12.12

0.0276

11.09

0.0038

#15

CQ3

447.0

2.74

3.36

17.54

0.0324

13.66

0.0046

#16

CQ4

1480.0

6.23

3.59

7.80

0.0133

4.21

0.0017

C1-1

C1

217.08

2.53

5.38

0.0068

9.22

0.003

5

Wang et al.(2016)

#9 #10

Ma et al.(2015)

30

Journal Pre-proof 260.47

2.77

5.28

0.0063

4.8

8.80

0.002

0.0025

0.005

0.55

0.05

2.56

C1

440.63

2.58

1.46

0.0007

3.5

7.64

0.002

0.0023

0.004

0.51

0.09

2.50

C1-15

C1

518.07

1.97

3.89

0.0037

3.8

3.62

0.001

0.0021

0.001

0.48

0.63

1.72

C1-17

C1

576.18

4.68

8.23

0.0099

3.1

16.41

0.004

0.0027

0.005

0.57

1.70

3.82

C1-18

C1

589.41

1.41

17.53

0.0392

3.7

5.73

0.002

0.0032

0.012

0.71

1.18

2.97

C1-19

C1

613.45

1.89

1.12

0.0006

2.8

3.03

0.001

0.0015

0

0.35

0.85

1.98

C1-22

C1

663

4.01

1.76

0.0002

2.2

6.59

0.002

0.0033

0.004

0.72

1.41

3.25

C1-24

C1

687.61

6.06

6.24

0.0018

3.2

19.34

0.006

0.0034

0.286

0.72

2.43

5.63

C1-29

C1

762

6.91

18.32

0.0110

2. 9

30.88

0.007

0.0032

0.014

0.67

3.18

5.87

C1-30

C1

721.85

2.97

8.90

0.0080

3.6

5.99

0.002

0.0023

0.007

0.53

1.27

2.98

C1-38

C1

831.26

1.85

7.24

0.0063

3.5

4.59

0.001

0.0031

0.013

0.68

0.75

2.64

NT 1

C1

1.2

2.15

0.0038

4.11

0.0012

0.003

0.01

1.19

NT 2

C1

2

2.40

0.0040

6.49

0.0018

0.006

0.01

1.72

NT 3

C1

3.4

2.92

0.0049

8.21

0.0026

0.004

0.01

2.51

NT 5

C1

5.3

3.36

0.0074

0.0044

0.002

0.01

3.78

CQ2-1

YC2

409.7

3.21

8.16

0.0015

5.4

0.87

2.41

CQ2-2

YC2

457.4

3.38

2.59

0.0009

5.6

0.92

1.8

CQ2-3

YC2

469.2

2.84

7

0.0011

4.6

1.07

3.1

CQ2-4

YC2

519.4

2.14

6.91

0.0020

5.4

0.56

2.4

CQ2-5

YC2

567.2

3.10

8.12

0.0018

5.1

0.79

4

CQ2-6

YC2

619.8

1.32

1.48

0.0004

4.6

0.89

1.78

CQ2-7

YC2

719.8

2.37

5.3

0.0012

5.1

0.96

2

CQ2-8

YC2

769.4

3

5.15

0.0012

5.6

1.02

2.25

CQ2-9

YC2

778.1

2.67

7.84

0.0025

5.1

0.99

3.71

CQ2-10

YC2

840.6

2.05

3.73

0.0019

5.2

0.79

3.17

CQ2-11

YC2

850

2.89

1.74

0.0008

5.8

0.93

1.96

CQ2-12

YC2

860.3

3.3

2.47

0.0009

5.7

0.79

1.7

CQ2-13

YC2

869.5

2.47

3.88

0.0008

5.4

1.00

2.26

CQ2-14

YC2

900.8

2.89

5.55

0.0012

4.8

0.81

2.68

CQ2-15

YC2

919.3

2.48

5.93

0.0020

4.2

0.35

4.67

CQ2-16

YC2

958.4

2.4

4.42

0.0014

4.5

0.59

2.86

CQ2-17

YC2

971.2

2.11

3.57

0.0013

5.7

0.89

1.52

CQ2-18

YC2

979.8

2.82

5.46

0.0015

5.1

0.84

2.49

CQ2-19

YC2

990.1

2.49

8.77

0.0027

5.1

0.71

2.48

CQ2-20

YC2

1010.5

2.66

4.58

0.0018

5

0.9

3.34

l a

rn

J

u o

ro

-p

e r P 14.85

f o

Tang et al.(2017)

C1

Tang et al.(2016)

C1-4 C1-12

31

CQ2-21

YC2

1020.2

3.03

4.66

0.0010

5.7

0.97

3.38

CQ2-22

YC2

1032

3.08

7.16

0.0017

4.6

0.8

2.05

CQ2-23

YC2

1071.6

3.29

6.91

0.0020

4.9

0.91

2.48

CQ2-24

YC2

1089.7

2.62

2.82

0.0014

4.6

0.64

2.74

CQ2-25

YC2

1101.3

2.33

6.13

0.0025

5.4

0.66

2.55

CQ2-26

YC2

1112.5

2.21

4.99

0.0015

5

0.71

2.05

CQ2-27

YC2

1139.3

2.71

1.14

4.16

CY1-2

CY1

1986.5

6.98

Z608-802

Z608

802.2

Z608-1

Z608

807.4

3.88

Z608-2

Z608

814.0

Z608-3

Z608

828.0

3.93

0.0013

4.6

2.41

17.82

0.0144

3.7

13.58

0.0019

2.59

16.16

0.0182

4.9

16.46

0.0031

15.69

0.0128

3.3

12.65

0.004

0.005

0.05

1.05

3.55

9.55

0.0071

3.8

18.31

0.006

0.004

0.08

0.88

9.52

20.31

0.0216

4.2

29.5

0.009

0.006

2.45

1.16

7.02

0.012

0.006

1.62

1.17

7.37

ro

-p

Z608-8

Z608

838.0

10.88

27.86

0.0110

1.6

36.98

Z1206-38

Z1206

295.5

9.28

29.32

0.0212

2.9

35.32

Z1402-12

Z1402

82.3

4.02

16.53

0.0141

3.4

17.29

MK64-1

MK64

38.8

4.49

ST MD

Outcrop

4.8

259.27

0.055

2

Yuke1

4

Yuke1

7

23.71

e r P

3.36

0.009

7.68

0.004

3.74 3.75

5

2.7

3.34

0.0140

4.1

1.2

2.43

24.94

0.0230

3.6

1.1

4.36

27.77

0.0240

3.4

0.9

5.04

1.73

0.0021

4.8

2

2.01

2.8

0.0028

4

2.6

1.83

4.3

0.0051

4.8

1.1

2.62

22.14

0.0183

3.3

0.8

5.23

0.35

0.0003

25.9

3.66

2.98

16.4

0.0187

4.6

11.4

0.0033

28.1

4.67

3.08

17.5

0.0196

4.5

17.1

0.0051

Yuke1

36.98

5.69

3.12

22.3

0.0260

4.7

23.7

0.0069

8

Yuke1

37.63

5.91

3.13

23.4

0.0242

4.1

24.9

0.0073

10

Yuke1

44.92

7.96

3.22

26

4.3

29.9

0.0086

Yuke1

49.8

8.76

3.25

10.3

0.0138

5.4

28.9

0.0086

13

Yuke1

50.5

6.23

3.26

10.5

0.0104

4

18.2

0.0052

15

Yuke1

58.6

8.31

3.37

10.8

0.0145

5.4

16.9

0.0052

17

Yuke1

61.45

1.2

u o

0.0279

12

3.51

0.5

0.0010

7.8

1.86

0.0005

19

Yuke1

68.56

1.17

3.35

4.32

0.0050

4.6

2.92

0.0008

18

Yuke1

25.63

3.76

2.67

12.6

0.0160

65

Yuke1

47.82

3.66

3.04

14.06

20

Yuke1

36.97

6.23

3.07

41

Yuke1

49.69

9.83

2.9

38

Yuke1

70.90

1.15

36

Yuke1

68.74

0.36

66

Yuke1

79.50

2.81

3.44

60

Yuke1

58.18

9.74

3.09

l a

rn

0.0081

2.582

Cao et al.(2014) and Sun et al.(2015)

2.81

Sun et al. (2016)

1.7

2.53

J

f o

This study

Non-deformed Zone

Journal Pre-proof

32

Journal Pre-proof 15

Yuke1

24.79

0.09

2.63

0.0028

4.2

0.8

28

Yuke1

35.19

0.21

2.99

0.0036

4.9

0.6

6

Yuke1

22.95

0.13

3.23

0.0040

4.9

1.8

1.44

17

Yuke1

40.13

0.05

2.62

0.0036

5.6

0.5

1.97

56

Yuke1

77.93

0.04

2.06

0.0032

6.3

0.7

1.62

3

Yuke1

0.12

2.05

0.0037

7.2

0.8

11

Yuke1

24.13

0.13

2.61

0.0034

5.1

0.9

1.46

48

Yuke1

71.91

0.07

2.16

0.0033

6.1

0.4

2.1

46

Yuke1

0.17

2.77

0.0037

5.2

45

Yuke1

65.39

0.18

2.84

2.41

0.0046

7.6

3.05

4.62

0.0071

6.1

1.97

0.0035

7.1

3.44

0.0053

6.2

26

Yuke1

33.79

1.19

YK1-1

Youke1

1330.27

0.31

YK1-5

Youke1

1352.23

0.3

2.79

YK1-9

Youke1

1382.36

0.27

2.82

2.35

0.0042

7.2

YK1-10

Youke1

1334.62

5.69

3.03

22.39

0.0198

3.5

YK1-14

Youke1

1364.24

9.93

3.53

20.33

0.0160

3.1

YK1-16

Youke1

1393.86

1.03

2.87

0.0058

8

0.5 0.9

1.75

2

1.99

0.8

1.13

1.3

1.24

1.4 1.8

4.69

1.1

7.36

2.5

2.2

a n

r u

Jo

P l

re

-p

ro

f o

1.44

33

Journal Pre-proof Table 2. Sample ID

Te ctonic z one

Q uartz

kFeldspar

Plagioclase

Total Feldspar

D1-26

30.7

6.5

13.0

D1-56

55.6

1.1

YC2-37

33.3

0.5

YC2-52

34.0

1.1

Calcite

Dolomite

Total Carbonates

Pyrite

Total Clay

19.5

2.4

17.3

19.7

14.5

7.0

1.1

20.6

13.5

34.1

9.7

10.2

15.8

13.5

29.3

5.2

17.7

13.8

14.9

15.2

14.5

29.7

2.7

12.3

47.9

C1-20

42

1

10.9

10.9

17.3

2.9

20.2

3.4

17.6

10

11

15

12

27

Deformed Zone

C1-21

o r p

15

e

16

0

C1-23

50

1

5

6

13

10

C1-25

47

1

5

6

13

7

C1-27

75

0

10

5

C1-31

37

8

8

31

C1-33

38

2

5

7

35

C1-36

34

2

5

7

rn

C1-40

28

1

5

6

D1-31

41.0

1.0

3.4

D1-44

8.7

1.0

3.0

#9

49.6

#10

44.6

#11

35

u o

#12

14.8

#13

49.9

#14

J

4.4 4

24 29

l a 10 10 7

r P 23

4

Chlorite

Illite

I/S

3.8

2.8

2.4

3.0

0.4

6.2

11.0

0.2

0.1

3.2

8.9

0.1

C/S

Source

17.6 1.28

11.2

3.52 T his study

6

15

0.15

11.4

3.45

4

23

0.23

18.4

4.37

2

8

6.8

1.2

41

5

9

8

1

45

5

5

3.8

1.2

31

4

24

20.4

3.6

7

5.4

20

10

39

20

14.6

22.5

28.3

50.8

1.3

1.3

32.4

53.5

85.9

0.5

0.5

Ma et al.(2019)

14.7

15.9

2.2

17.6

9.5

4.8

5.9

35.2

9.5

28.5

2.8

55.1

7.6

23.3

6.1

13.1

41.7

10.1

9.1

4.0

35.1

#15

36.6

14.1

25.9

#16

51.6

7.7

10.3

8.0

22.4

C1-1

38

C1-4

48

C1-12

55

1

f o

5.4

C1-3 C1-13

Kaolinite

27.0 27.3

Wang et al.(2016)

23.4

20

20

4

7

11

4

25

20.3

4.8

12

12

12

9

21

3

15

13.2

1.8

6

7

17

5

22

2

14

9.52

4.5

Ma et al.(2015)

34

Journal Pre-proof C1-17

41.4

2.2

2.2

32.3

2

34.3

3.3

18.8

18.8

C1-18

33.5

10.9

10.9

9.7

15.8

25.5

8

20.6

20.6

C1-19

30

1

3

4

13

36

49

5

12

9.6

2.4

C1-22

39

1

5

6

13

14

27

6

22

14.7

7.3

C1-24

51

2

5

7

14

9

23

4

15

10.5

4.5

C1-29

58

7

7

9

3

12

5

18

15.5

2.52

C1-30

27.6

1.1

7.9

9

28.3

15.4

43.7

4.7

13

13

0

C1-38

22.5

1.2

5.7

6.9

40.2

8.6

48.8

3.6

17.1

17.1

0

NT 1

42.2

9.9

14.8

0.7

32.4

NT 2

40.5

14.3

14.1

0.7

30.4

NT 3

40.0

12.1

10.7

3.6

33.6

NT 5

43.1

7.3

13.2

1.7

34.7

CQ2-1

38.9

6.2

27.0

2.2

17.2

CQ2-2

34.3

7.9

29.2

2.1

20.6

CQ2-3

36.1

11.3

28.6

CQ2-4

37.8

16.4

CQ2-5

36.1

19.9

CQ2-6

39.6

3.0

CQ2-7

35.1

9.8

CQ2-8

34.0

9.9

CQ2-9

35.7

8.8

CQ2-10

31.8

11.6

CQ2-11

34.1

CQ2-12

36.9

CQ2-13

34.7

CQ2-14

39.3

CQ2-15

34.1

CQ2-16

e

r P

3.9

9.8

4.2

19.5

3.1

25.3

24.9

4.2

19.7

29.2

5.0

18.0

28.1

3.1

19.2

34.6

3.1

16.9

20.5

3.8

23.2

22.7

4.9

19.8

13.3

25.4

3.8

17.5

18.4 12.4

10.5

l a

rn

u o 13.9

J

o r p

f o

36.2

T ang et al.(2016)

11.0

8.3

25.4

3.7

18.1

14.5

29.2

2.7

11.9

37.6

15.1

21.6

3.0

17.6

CQ2-17

30.8

14.0

34.6

2.4

15.3

CQ2-18

33

11.7

29.7

3.4

17.0

CQ2-19

31.3

17.4

20.0

3.3

21.6

CQ2-20

31.4

14.6

34.6

4.0

11.9

CQ2-21

37.6

12.3

20.5

3.7

25.1

CQ2-22

35.4

14.0

26.5

3.4

16.5

CQ2-23

30.3

12.4

29.2

4.0

19.4

T ang et al.(2017)

35

Non-deformed Zone

Journal Pre-proof CQ2-24

30.3

11.8

34.6

2.4

17.7

CQ2-25

26.9

13.5

33.0

5.3

16.6

CQ2-26

35.4

18.0

17.3

4.3

22.9

CQ2-27

38.3

CY1-2 Z608802 Z608-1

56.4

Z608-2

48.0

Z608-3

65.0

Z608-8 Z120638 Z140212 MK64-1 ST MD

4.4

2

37

4

40

7

45

8

48

10

40

1

13

14

12

50

2

15

17

13

50

1

7

8

15

63

1

8

17

60

1

19

71

3

18

42

5

65

51

10

10

20

54

5

5

41

58

9

12

38

71

2

36

78

2

66

76

1

1

60

53

5

5

15

52

9

13

19.5

3.8

24.0

11.5

10.7 15.5

10.3

10.3

7.9

9.2

0.1

10.7

10.7

4.1

4.1

5.1

34.5

2.2

1

6

7

4

9

32.0

1

7

8

3

3

8

16.0

42.0

3

9

12

6

3

9

11

37.1

2.2

14.7

16.9

9.9

9.9

11.4

43.8

9.6

9.6

3.7

35.4

9.5

9.5

14.8

4

45.6

10

10

1

10

11

1

10

11

9

9

3

4

4

e r P 14.8

89.3

4.7

15

10

l a 9 6 6

94

4.8 7

4.1

2.1

16.4

24.7

1.482

1.235

22.0

38.1

3.429

4.191

30.5

33.3

0

8.658

24.6

26.0

2.7

5.6

f o

ro

-p

3.7

0.2

2.7

0.8

8.5

6.7

2.1

9.9

4.0 7.5

25

6

22

0.4

19.1

2.4

9

10

32

0.3

28.5

3.2

6

8

30

0.6

26.1

3.3

6

7

31

0.9

26.4

3.7

10

10

27

0.8

24.3

1.9

11

24

0.5

19.4

4.1

21

5

16

0.3

12.6

3.0

0.2

5

5

3

21

18.1

2.7

1

33

33

1

5

4.4

0.6

3

10

10

1

15

14.3

0.8

12

26

4

23

0.2

21.2

1.6

3

3

10

26

0.3

21.6

4.2

10

10

5

26

0.3

22.4

3.4

5

8

6

16

0.2

14.1

1.8

2

3

3

5

19

0.2

15.4

3.4

2

9

9

1

10

0.1

9.1

0.8

2

21

0.2

18.1

2.7

17

17

7

18

0.4

14.0

3.6

35

9.8

23.8

1.4

Jo

9

5

18

14

3

5 3

T his study

1.6

n r u 5

0.5

Sun et al. (2016)

Cao et al.(2014) and Sun et al.(2015)

36

Journal Pre-proof 28

41

7

8

16

16

6

46

1

8

8

9

9

17

42

16

16

56

42

9

9

3

38

15

15

11

36

18

18

48

40

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Figure 5.

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Journal Pre-proof Figure 6.

D1-44

A

Mineral Quartz Pyrite Dolomite Calcite Albite Orthoclase Oligoclase Rutile Apatite Barytocalcite Organic Unknown Pores

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CY1-2

Mineral Quartz Pyrite Albite Oligoclase Biotite Orthoclase Calcite Apatite Rutile Illite Chlorite Clinochlore Monazite Organic Unknown Pores

wt. % 61.67 5.76 7.02 5.65 0.9 6.26 1.16 0.04 0.03 3.47 2.12 2.02 0.02 0.05 3.85 0

Area% 63.09 3.08 7.18 5.71 0.78 6.28 1.14 0.04 0.02 3.38 1.99 2.04 0.01 0.09 5.16 0.01

Mineral Quartz Pyrite Albite Oligoclase Orthoclase Dolomite Calcite Apatite Rutile Zircon Biotite Illite Chlorite Clinochlore Organic Unknown Pores

wt. % 37.2 0.62 7.74 5.09 1.32 0.2 32.56 0.13 0.05 0.01 0.57 4 1.3 2.61 0.03 6.57 0

Area% 37.16 0.33 7.73 5.02 1.29 0.19 31.45 0.11 0.03 0 0.49 3.8 1.19 2.58 0.05 8.59 0

1 mm

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Area% 1.58 0.65 30.33 7.52 1.98 1.02 0.07 0.01 0.55 0.13 47.98 8.18 0

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1 mm

wt. % 1.47 1.13 30.55 7.23 1.84 0.93 0.07 0.01 0.61 0.17 50.21 5.8 0

1 mm

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Journal Pre-proof Figure 7.

A

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M icro-fracture

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10 µm

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porous OM

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J D D D

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K

10 µm

L D D D

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2 µm

2 µm

N D D D

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2 µm

M

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20 µm

H D D D

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5 µm

I

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O

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5 µm

5 µm

200 nm

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Figure 10.

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Journal Pre-proof Figure 11.

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Figure 14.

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CRediT author statement Yong Ma: Investigation, Methodology, Formal analysis, Writing – Original Draft Omid H. Ardakani: Methodology, Writing - Review & Editing Ningning Zhong: Supervision Honglin Liu: Funding acquisition Haiping Huang: Review & Editing

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Steve Larter: Review & Editing

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Cong Zhang: Data Curation

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Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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None

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Journal Pre-proof Highlights Shale gas content in Deformed and Non-deformed Zone show a significant difference.



Meso- to macroporous OM-hosted pores are collapsed in the Deformed Zone samples.

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OM-hosted meso- to macropores are dominant in the Non-deformed Zone shales.



Transmission of structural stress to shale may cause pore structure deformation.



Deformation of pore structure likely lead to different shale gas content.

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