Characteristics and evolution of pyrobitumen-hosted pores of the overmature Lower Cambrian Shuijingtuo Shale in the south of Huangling anticline, Yichang area, China: Evidence from FE-SEM petrography

Journal Pre-proof Characteristics and evolution of pyrobitumen-hosted pores of the overmature Lower Cambrian Shuijingtuo Shale in the south of Huangling anticline, Yichang area, China: Evidence from FE-SEM petrography Sile Wei, Sheng He, Zhejun Pan, Gangyi Zhai, Tian Dong, Xiaowen Guo, Rui Yang, Yuanjia Han, Wei Yang PII:

S0264-8172(20)30086-6

DOI:

https://doi.org/10.1016/j.marpetgeo.2020.104303

Reference:

JMPG 104303

To appear in:

Marine and Petroleum Geology

Received Date: 28 January 2019 Revised Date:

9 February 2020

Accepted Date: 16 February 2020

Please cite this article as: Wei, S., He, S., Pan, Z., Zhai, G., Dong, T., Guo, X., Yang, R., Han, Y., Yang, W., Characteristics and evolution of pyrobitumen-hosted pores of the overmature Lower Cambrian Shuijingtuo Shale in the south of Huangling anticline, Yichang area, China: Evidence from FE-SEM petrography, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2020.104303. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Sile Wei: Conceptualization, Methodology, Formal Analysis, Data Curation, Writing-Original Draft Preparation, Visualization. Sheng He: Conceptualization, Methodology, Writing - Review & Editing, Supervision, Project administration, Funding Acquisition. Zhejun Pan: Conceptualization, Methodology, Writing - Review & Editing, Supervision. Gangyi Zhai: Resources, Funding Acquisition. Tian Dong: Methodology, Formal Analysis, Data Curation. Xiaowen Guo: Software, Resources. Rui Yang: Software, Writing - Review & Editing. Yuanjia Han: Writing - Review & Editing. Wei Yang: Data Curation.

4

Characteristics and evolution of pyrobitumen-hosted pores of the overmature Lower Cambrian Shuijingtuo Shale in the south of Huangling anticline, Yichang area, China: Evidence from FE-SEM petrography

5 6

Sile Wei 1, 2, Sheng He 1, *, Zhejun Pan 2, **, Gangyi Zhai 3, Tian Dong 1, Xiaowen Guo 1, Rui Yang 1, Yuanjia Han 1, Wei, Yang 1

1 2 3

7 8 9 10 11 12 13 14 15

1

Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University

of Geosciences, Wuhan 430074, China 2

CSIRO Energy, Private Bag 10, Clayton South VIC 3169, Australia

3

Oil & Gas Survey Center, China Geological Survey, Beijing 100029, China

*Corresponding author. **Corresponding author. E-mail addresses: [email protected] (S. He), [email protected] (Z. Pan)

Abstract:

16

Organic pores, one of the significant pore types in shale reservoir, can be formed

17

in both primary organic matter (kerogen) and secondary organic matter such as solid

18

bitumen and pyrobitumen. Compared to the primary organic pores that are mainly

19

observed in immature kerogen, secondary organic pores in migrated organic matter

20

(solid bitumen and pyrobitumen) are more abundant and well connected to the matrix.

21

In this study, the petrographic characteristics between the organic matter and matrix as

22

observed in field emission-scanning electron microscopy (FE-SEM) images were

23

used to characterize the pore system in the overmature Lower Cambrian Shuijingtuo

24

(Niutitang-equivalent) Shale in the south of Huangling anticline, Yichang area, China.

25

Pyrobitumen-hosted pores were observed to be the predominant pore type in the

26

organic-rich Shuijingtuo Shales. The porous pyrobitumen occurs primarily in the

27

original interparticle pores filled with microcrystalline quartz, which is the primary

28

petrographic evidence to identify the migrated organic matter. Pore-filling organic

29

matter and precipitation of authigenic quartz, rather than mechanical compaction,

30

resulted in further loss of a large number of interparticle pores. The porous organic

31

matter that filled in the intraparticle space within the early-formed framboidal pyrite

32

and the pre-existing dissolution pores within the quartz and pyrite grains is

33

pyrobitumen. This pyrobitumen had migrated as a mobile phase into the

34

aforementioned pores spaces in the initial phase of hydrocarbon emplacement during

35

the petroleum expulsion and migration process. With increasing thermal maturity, this

36

migrated organic matter thermally transformed into pyrobitumen, and nanoscale pores

37

were developed with thermal cracking into gas. The results show that the pore volume

38

and surface area are positively correlated with the total organic carbon (TOC) content,

39

indicating that organic matter primarily controls shale porosity for the Shuijingtuo

40

Shales.

41

Keywords: pyrobitumen-hosted pores, pyrobitumen, pore type, petrographic

42

characteristic, FE-SEM, Shuijingtuo (Niutitang-equivalent) Shale

43 44

1. Introduction

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Organic matter is an important component in shale reservoirs, not only because it

46

is the source of petroleum, but also for its numerous nanoscale pores that contribute

47

significantly to shale porosity (Chalmers et al., 2012; Curtis, 2002; Jarvie et al., 2007;

48

Loucks et al., 2009; Milliken et al., 2013; Slatt and O'Brien, 2011). Organic pores

49

occur in both kerogen and secondary organic matter such as solid bitumen and

50

pyrobitumen (Bernard et al., 2012a; Bernard et al., 2012b; Loucks et al., 2012;

51

Loucks et al., 2009; Pommer and Milliken, 2015). The organic pores in kerogen are

52

mainly primary pores, which are commonly observed in immature shales (Fishman et

53

al., 2015; Han et al., 2017; Katz and Arango, 2018; Ko et al., 2017). Primary organic

54

pores are considered to be inherited from organic macerals and would diminish with

55

increasing burial depth (Katz and Arango, 2018; Pommer and Milliken, 2015). Pores

56

in secondary organic matter are secondary organic pores, and their development and

57

evolution are stimulated by the generation and expulsion of petroleum during thermal

58

maturation (Bernard et al., 2012a; Bernard et al., 2012b; Curtis et al., 2012; Jarvie et

59

al., 2007; Ko et al., 2016). Compared to the organic pores in in-situ organic matter

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(kerogen and its alteration), secondary pores in migrated organic matter (solid

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bitumen and pyrobitumen) are more abundant and well connected, forming an

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effective pore network that is advantageous for the storage and transport of shale gas

63

(Loucks et al., 2017; Pommer and Milliken, 2015). In addition to scanning

64

transmission X-ray microscopy (STXM) providing information on the bonding of

65

carbon and integrated correlative light and electron microscopy (iCLEM) combing the

66

traditional fluorescence microscopy and SEM to identify the types of organic matter

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(kerogen, solid bitumen, and pyrobitumen) (Bernard et al., 2012a; Bernard et al.,

68

2012b; Hackley et al., 2017), the petrographic texture between the organic matter and

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mineral matrix provides another method for distinguishing the migrated organic

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matter from the in-situ organic matter (İnan et al., 2018; Loucks and Reed, 2014;

71

Wood et al., 2018).

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The Shuijingtuo Shale has become the most important potential formation for

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shale gas exploration in South China after the Upper Ordovician-Lower Silurian

74

Wufeng-Longmaxi Shale (Chen et al., 2018; Luo et al., 2019). Shuijingtuo Shales are

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characterized by high total organic carbon (TOC) content (average of 1.0–5.5%) and

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thick depositional sequences with average thickness of approximately 100 m and have

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a large distribution area, mainly in the Eastern Hubei Province, Hunan-Guizhou

78

Provinces, and Eastern and Southern Sichuan Basin (Zou et al., 2010). In this study, a

79

suite of techniques including quantitative and qualitative methods was applied to

80

characterize the pore structure of the Shuijingtuo Shales. Quantitative techniques

81

include the helium (He) expansion and low-pressure nitrogen (N2) and carbon dioxide

82

(CO2) adsorption, which can provide information on bulk porosity, surface area, pore

83

volume, pore size distribution, and other pore structure parameters (He et al., 2018;

84

Mastalerz et al., 2013; Peng et al., 2017; Wu et al., 2019b; Zhang et al., 2018). Field

85

emission-scanning electron microscopy (FE-SEM) combined with ion-beam milling is

86

utilized to observe the pore types and morphology (Li et al., 2019; Loucks et al., 2009;

87

Sun et al., 2017; Yang et al., 2019; Zhou et al., 2019).

88

The aim of this study is to characterize the organic pores in the overmature Lower

89

Cambrian Shuijingtuo (Niutitang-equivalent) Shales based on the petrographic

90

characteristics shown in the FE-SEM images. Specific objectives are to (1) analyze

91

the predominant pore type, (2) interpret the pores in kerogen and pyrobitumen, and (3)

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summarize the evolution and preservation of organic pores.

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2. Geological setting

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Huangling anticline is located in the Yichang area, middle Yangtze Craton,

95

bordered by the Qinling-Dabie orogenic belt to the north and Xuefeng orogenic belt to

96

the south (Shen et al., 2009). Morphologically, the Huangling anticline is a vast dome

97

structure characterized by the Mesozoic and Paleozoic strata distributed around the

98

pre-Sinian basement (Chen et al., 2012; Liu et al., 2019) (Fig. 1A). In the late Sinian,

99

the orogenic movement (Tongwan movement) resulted in the formation of structural

100

units with uplift and depression (Chen et al., 2018). In the early Cambrian, a

101

transgression from south to north occurred and resulted in the deposition of the

102

Shuijingtuo Formation (Chen et al., 2018). The uplift area mainly developed

103

shallow-water platform facies, where the shale also deposited with a thickness of less

104

than 3 m. In the depression area, the rock of deep shelf facies has a thickness of

105

approximately 140 m in the EYY 1 Well, located in the south of Huangling anticline

106

(Fig. 1A). According to the lithology, the Shuijingtuo Formation was divided into

107

three parts. The bottom part is mainly dominated by black siliceous shale with high

108

TOC content, and it was deposited in the transgressive systems tract (TST). The

109

middle part is composed of dark gray clay-rich siliceous shale interbedded with

110

calcareous shale, which was deposited in the early highstand systems tract (EHST).

111

The upper part mainly consists of gray limestone, deposited in the late highstand

112

systems tract (LHST).

113

3. Samples and methods

114

3.1 Samples

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A total of 20 Shuijingtuo Shale samples were collected from the EYY 1 Well.

116

Published data shows that kerogen in the Lower Cambrian Shuijingtuo Shale is type I

117

(Liang et al., 2009; Peng et al., 2019; Yang et al., 2017a). The EYY 1 Well was drilled

118

in the south of the Huangling anticline in 2017, and the Shuijingtuo Shale Formation

119

is the main target layer for the well. It was reported that the gas production from the

120

Shuijingtuo Shales in the EYY 1 Well is of approximately 7.84 × 104 m3 per day

121

(Wang et al., 2018). The location of the EYY 1 Well is shown in Fig. 1A. All the

122

collected samples are approximately evenly distributed at the bottom and middle of

123

the Shuijingtuo Formation, taking into account the variability of mineral composition

124

and TOC content (Fig. 1B).

125

3.2 Organic geochemistry and mineralogy

126

The TOC content was determined with the Rapid CS cube. Powdered shale

127

samples with a grain size of < 75 um (200 mesh) were placed in silver paper and

128

treated with hydrochloric acid (HCl, 7% mass concentration) to remove the

129

carbonates. After drying at 80 °C for 10 h, the pre-processed samples were completely

130

combusted at 930 °C with 99.99% oxygen (O2) as the carrier gas. The TOC content

131

was calculated based on the amounts of CO2 released during sample oxidation.

132

Due to the absence of vitrinite in Lower Cambrian formations, the thermal

133

maturity was determined by the reflectance of the solid bitumen (BRo). Five shale

134

samples were used to measure the BRo using an MPV-III microphotometer 806

135

apparatus in the Huabei Oilfield laboratory. The kerogen experiment, such as

136

elemental analysis and solid bitumen reflectance were conducted in the Huabei

137

Oilfield laboratory. The kerogen separation was carried out according to national

138

standard GB/T 19144-2010, mainly including the removal of the clay mineral,

139

inorganic carbon minerals, siliceous minerals, pyrite, and soluble organic matter.

140

The mineral composition was determined by X-ray diffraction (XRD) with an

141

X'Pert PRO diffractometer (Panalytical, Netherlands) following the Chinese Oil and

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Gas Industry Standard (SY/T) 5163-2010. The working voltage, current, radiation,

143

and scanning speed are 40 kV, 40 mA, Cu Kα (λ = 0.15416 nm), and 0.417782° (2θ)/s,

144

respectively, in a continuous mode with a step size of 0.017° in the range of 3°–65°.

145

The calculation limit of the mineral composition is approximately 3%.

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3.3 Pore structure

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Cylindrical plugs with a diameter of 1 inch (approximately 2.5 cm) and length of

148

3–5 cm were prepared to measure the bulk porosity. To reduce the anisotropy of shale

149

reservoir and considering the horizontal drilling technique for shale development, the

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cylindrical plugs parallel to the bedding plane were cut from the shale cores. Bulk

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porosity is a function of bulk volume and grain (skeletal) volume. The bulk volume

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was obtained by measuring the diameter and height of the cylindrical plugs. The grain

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volume was determined with helium following the Boyle’s Law using a PoroPDP 200

154

apparatus (Core Lab, America) at approximately 200 psi (1 MPa = 145 psi) without

155

confining stress. The pressure is considered to reach a balance when the pressure

156

change is less than 0.001 psi within 20 s.

157

Both the CO2 and N2 adsorption analyses were carried out on Autosob iQ3

158

(Quantachrome, America). Before the experiment, shale samples with a grain size of

159

0.18–0.25 mm (60–80 mesh) were degassed at 110 °C for 12 h under vacuum. The

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CO2 adsorption isotherms were interpreted using the Dubinin-Astakhov (DA) models

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for micropore (< 2 nm) volume. The N2 adsorption was analyzed with the

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Brunauer-Emmett-Teller (BET) method to calculate the surface area, and with the

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Barrett-Joyner-Halenda (BJH) method to derive the total pore volume. The pore size

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distribution for pore widths from less than 2 nm up to 300 nm was determined using a

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combination of the density-functional-theory and BJH method. All of the above

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results can be available within the ASiQwin instrument software package (version

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4.0). The pores are classified according to the International Union of Pure and

168

Applied Chemistry (IUPAC) (Sing et al., 1985). A detailed description of these

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methods and theories can be found elsewhere (Barrett et al., 1951; Brunauer et al.,

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1938).

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3.4 FE-SEM

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To observe the pore type and morphology, FE-SEM analysis was conducted on

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Shuijingtuo Shale samples. The sample was first mechanically cut and then the side

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perpendicular to the bedding plane was polished by argon ion-beam milling using a

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Leica EM TIC 3X. The shale samples were inspected using a ZEISS Merlin and

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GeminiSEM instrument coupled with energy-dispersive X-ray spectroscopy (EDS)

177

and cathodoluminescence (CL) detector. Secondary electron 2 (SE2), InLens, and

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backscattered electron (BSE) images were used to observe the nanometer-scale pores

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and characterize the minerals. EDS mapping was used for mineral identification.

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SEM-CL images were applied to distinguish the authigenic quartz from the detrital

181

quartz based on the luminescence intensity. The sizes of the organic pores and

182

dissolution pores were determined by outlining and measuring all visible pores in

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high-resolution FE-SEM images with Image J software.

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4. Results and discussion

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4.1 Organic geochemistry and mineral composition

186

The TOC content ranges from 1.58 wt.% to 8.29 wt.% with an average of 4.40

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wt.% (Table 1), indicating that the Shuijingtuo samples are rich in organic matter. The

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Shuijingtuo samples have a wide range of mineralogical composition, dominated by

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quartz (20–77 wt.%) and clay minerals (9–34 wt.%) (Table 1). With increasing TOC

190

content, the clay minerals content decreases while the quartz content increases (Fig.

191

2).

192

The results of the BRo analysis are listed in Table 2. Only two shale samples have

193

solid bitumen particles with BRo of 2.91% and 2.48%. There is a good linear

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relationship between the BRo and vitrinite reflectance (VRo): VRo = 0.618 × BRo +

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0.40 (Jacob, 1989). Using this equation, the measured BRo can be converted into the

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vitrinite reflectance equivalent (EVRo). The converted EVRo is 2.20% and 1.93%,

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respectively, which are in the range of the reported value (2.18–2.73%, EVRo) (Chen

198

et al., 2018), indicating that the Shuijingtuo Shales in the Huangling anticline are at

199

dry gas window maturity.

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4.2 Factors controlling shale porosity

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4.2.1 Quantitative results of pore structure

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The results of the bulk porosity, micropore volume, total pore volume, and

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surface area are listed in Table 3. The bulk porosity measured by helium expansion

204

ranges from 1.29% to 4.01% with an average of 2.73%. Shale samples with no

205

porosities occur because the collected shale sample does not exceed 1 inch in

206

thickness for cutting the cylindrical plugs, or the cylindrical plug was ruptured during

207

the cut process. The Micropore volume calculated by the DA method varies from 0.38

208

× 10-3 cm3/g to 1.19 × 10-3 cm3/g. The total pore volume varies from 1.03 × 10-2 cm3/g

209

to 3.19 × 10-2 cm3/g. The surface areas obtained by the BET method from N2

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adsorption range from 9.25 m2/g to 44.67 m2/g with an average of 23.31 m2/g.

211

The contributions of pores with different pore diameter ranges (< 10 nm, 10–50

212

nm, and > 50 nm) to the pore volume and surface area are listed in Table 3. Pores with

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pore diameter less than 10 nm and macropores (> 50 nm) are the major contributors to

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the pore volume, accounting for 42.20–76.74% and 14.06–42.46%, respectively.

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Pores with pore diameter less than 10 nm make a significant contribution to the

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surface area, accounting for 93.48–98.78%, while the contribution of macropores is

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negligible (0.34–1.82%).

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4.2.2 Effects of organic matter and clay minerals on pore structure

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Fig. 3 shows the positive relationships between the TOC content and bulk

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porosity, micropore volume, total pore volume, and surface area. The positive

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relationships agree with the observation that abundant organic pores were developed

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in the organic matter particles shown in the figures in Section 4.3.1. These positive

223

relationships are also consistent with the previous studies on the overmature

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organic-rich shales, such as the Niutitang Shales in the south of Sichuan Basin (3.0–

225

3.5%, EVRo; 0.24–7.23/3.5 wt.%, TOC) (Wang et al., 2017), Wufeng-Longmaxi

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Shales in the Fuling shale gas field (2.39–3.60%, EVRo; 0.75–4.03/2.59 wt.%, TOC)

227

(Yang et al., 2016b), and Devonian Shales in the Horn River Basin (1.6–2.5%, Ro;

228

0.04–8.25/3.09 wt.%, TOC) (Dong et al., 2015). The observed positive relationships

229

indicate that the organic matter is the prominent factor controlling the pore volume

230

and surface area in the overmature organic-rich shales.

231

As a nanoscale porous material, a higher clay mineral content theoretically means

232

a larger surface area for the inorganic pores (Ross and Bustin, 2009). However,

233

negative relationships between the clay mineral content and the pore structure

234

parameters are obtained for the Shuijingtuo Shales (Fig. 4). The clay mineral content

235

in the Shuijingtuo Shales is low and the clay minerals mainly are the illite and

236

mixed-layer illite/smectite (I/S). The illite is the dominated clay mineral, accounting

237

for 55.56–90.32% (average of 77.74%) of the total clay mineral content (Table 1). It

238

was reported that the pure illite has the lowest surface area (7.1 m2/g) as compared to

239

other clay minerals such as the montmorillonite (76.4 m2/g), mixed-layer I/S (30.8

240

m2/g), kaolinite (15.3 m2/g), and chlorite (11.7 m2/g) (Ji et al., 2012). Compared with

241

the illite, porous organic matter has a much higher surface area: 161 m2/g for the

242

kerogen isolated from the Niutitang Shales (Cao et al., 2015). Although the surface

243

area of the pure illite and isolated kerogen could not represent their surface areas in

244

shale rock, yet it can indicate that they have a huge difference in the surface area. Due

245

to the linear positive correlation between the surface area and total pore volume both

246

obtained from the N2 adsorption isotherms, we can infer that the illite also has a much

247

lower total pore volume as compared to the porous organic matter. Furthermore,

248

owing to the overall negative relationship between the clay mineral and TOC content

249

and the positive influence of the TOC content on the pore parameters, we inferred that

250

the organic matter is most likely to have affected the contribution of clay minerals to

251

the pore volume and surface area. To examine this hypothesis, the pore structure

252

parameters were normalized to TOC content to eliminate the influence of organic

253

matter. As expected, positive relationships were revealed, although some correlation

254

coefficients were relatively low (Fig. 5).

255

Although organic matter and clay minerals are the two important porous materials

256

providing sites and space for shale gas adsorption, they have different effects on the

257

methane adsorption capacity, as determined from high-pressure methane adsorption

258

experiments (Sander et al., 2018; Tan et al., 2014; Wei et al., 2019). In dry conditions,

259

the adsorption capacity increases with the contents of both organic matter and clay

260

minerals, whereas the adsorption capacity of organic matter is much higher than that

261

of pure clay minerals, except for montmorillonite (Ji et al., 2012; Rexer et al., 2014).

262

In the moisture-equilibrium condition, which is similar to the subsurface condition,

263

there is still a positive relationship between the TOC content and adsorption capacity

264

(Yang et al., 2017b). However, the TOC normalized adsorption capacity is

265

independent of the clay mineral content (Chalmers and Bustin, 2008). This is related

266

to the difference in hydrophilicity between the organic matter and clay minerals.

267

Different from the organic matter (hydrophobic), clay mineral is hydrophilic, resulting

268

in the adsorption sites being occupied by the water molecules. Moreover, the swelling

269

of clay minerals after water sorption leads to a reduction in adsorption space and a

270

further decrease in the adsorption capacity. Therefore, the organic matter is the main

271

controlling factor for the pore structure (surface area and pore volume) and methane

272

adsorption capacity, especially in organic-rich shales.

273

4.3 Qualitative analysis of pore types

274

4.3.1 Organic pores

275

The organic pore is the predominant pore type in the Shuijingtuo Shale reservoir,

276

especially in organic-rich shale samples (Figs. 6–8). Organic pores generally have a

277

sponge-like appearance, rounded and sub-rounded in shape with diameters ranging

278

from 4 nm to 84 nm (average of 12 nm) (Table 4 and Fig. 9). Both the intraparticle

279

space within the early-formed framboidal pyrite (Fig. 6A–D) and the dissolution

280

space within pyrite (Fig. 7A, B) and quartz grains (Fig. 7C–E) provide spaces for the

281

porous organic matter. However, the lower pyrite content (average of 2.75 wt.% in

282

Table 1) and the uncommon dissolution pores imply that these corresponding spaces

283

are not the primary locations of porous organic matter. The large-scale FE-SEM

284

images show that the porous organic matter predominantly occurred in the original

285

interparticle space (Figs. 6A, C and 8A, C, E).

286

The pore-filling interparticle porous organic matter distributes in all shale

287

samples but is remarkably abundant in the samples with high TOC content that

288

contain high quartz content. The SEM-CL and SEM-EDS images show two types of

289

quartz in the Shuijingtuo Shales: detrital quartz and aggregates of microcrystalline

290

quartz (Fig. 10). The detrital quartz has a sub-rounded shape and has a uniform and

291

bright luminescence (Dong et al., 2017; Dong et al., 2019) (Fig. 10B). The

292

microcrystalline quartz has a euhedral shape and is usually less than 10 µm in length.

293

The microcrystalline quartz belongs to the authigenic quartz and is characterized by a

294

dark to low luminescence (Dong et al., 2017; Dong et al., 2019) (Fig. 10B, E). The

295

petrographic features shown in Fig. 10 indicate that most of the organic matter occurs

296

with the aggregates of microcrystalline quartz and the crystallized quartz is occluded

297

by the organic matter.

298

It should be noted that most of the aforementioned positions, including the

299

intraparticle space within the early-formed framboidal pyrite, dissolution pores, and

300

interparticle space developed between crystallized quartz grains, can protect the

301

organic pores from compaction. In addition, the small pore size is another reason for

302

the preservation, because pores with a pore size less than 100 nm are especially

303

resistant to compaction, and thus, it is difficult that they collapse (Loucks et al., 2009;

304

Milliken and Curtis, 2016). In fact, these pore-filling porous organic matters are

305

pyrobitumen, which will be discussed in the following section. It should be

306

recognized that not all organic matter contains organic pores. For instance, no visible

307

pores were detected in the elongated organic matter even at magnifications as high as

308

19000–27000× (Fig. 8G, H). The organic matters in Fig. 8G, H are relatively

309

featureless and have no signs of ordered structure or morphology, so they are

310

interpreted as pyrobitumen. However, the reasons for the absence of organic pores in

311

these materials are not clear.

312

4.3.2 Mineral pores

313

The interparticle pores in the investigated Shuijingtuo Shale samples are least

314

developed. The reduction in interparticle pores can be related to mechanical

315

compaction and material filling (Zhao et al., 2017). The fact that the clay minerals

316

bend toward the framboidal pyrite and the absence of intraparticle space between

317

peripheral particles in the framboidal pyrite (Fig. 11A) imply that the Shuijingtuo

318

Shales

319

post-compaction indicated by the deformed organic pores (Fig. 11B) indicates that

320

mechanical compaction is a long-term process. The Shuijingtuo Shales have a high

321

content of brittle minerals (average of 80 wt.%), especially in quartz content,

322

implying that substantial amounts of primary interparticle pores could be developed.

323

However, in shale reservoirs dominated by brittle minerals, mechanical compaction of

have

undergone

intense

mechanical

compaction.

Furthermore,

the

324

these minerals makes them become closely arranged, thus reducing the interparticle

325

space, which does not cause a large reduction in the primary interparticle pores

326

(Loucks et al., 2012; Pommer and Milliken, 2015). FE-SEM images show that most

327

of the original interparticle space is filled by both porous organic matter and

328

aggregates of microcrystalline quartz (Figs. 6, 8 and 10). Therefore, the limited

329

abundance of original interparticle pores within the organic-rich Shuijingtuo Shales is

330

mainly caused by the precipitation of microcrystalline quartz and filling of organic

331

matter.

332

The pores developed in pyrite, quartz, and carbonate grains are intraparticle pores

333

but they are not common in Shuijingtuo Shales (Figs. 7 and 12). Most of the

334

pyrite-hosted pores are filled with porous organic matter and authigenic quartz (Fig.

335

7A, B), and the quartz-hosted pores are filled with porous organic matter (Fig. 7C–E).

336

The filled intraparticle pores indicate that the grain dissolution occurred prior to the

337

petroleum migration and cementation, and these pores were connected to the

338

migration path. The pores that formed within anhedral and euhedral pyrite (Fig. 7A, B)

339

and quartz grains (Fig. 7C–E) are clearly dissolution pores and belong to secondary

340

pores. However, it remains uncertain whether the pores observed in the unfilled quartz

341

(Fig. 12A, B) and calcite grains (Fig. 12C, D) are primary pores, secondary pores, or

342

artifact-related pores. The dissolution of quartz is complex, and it is mainly formed in

343

an alkaline environment (Worden and Morad, 2000). The dissolution of carbonate is

344

related to the corrosion of acidic fluids, the source of which can be the organic acids

345

and CO2 released by the maturation of kerogen (Tissot and Welte, 1984).

346

Understanding the dissolution mechanism and relative timing of those dissolved

347

grains is important for the further research of diagenesis, but it is beyond the scope of

348

this study. Although the unfilled dissolution pores within quartz and carbonate grains

349

have larger pore diameter (9–305 nm, with an average of 43 nm) (Table 5 and Fig. 9),

350

they are uncommon and do not contribute significantly to the shale porosity.

351

Microfractures in shale or mudstone commonly form as opening fractures up to a

352

few millimeters in length (generally < 10 mm) and a few micrometers wide (generally

353

< 15 µm) (Anders et al., 2014; Gale et al., 2014; Loucks and Reed, 2016). Although

354

microfracture is an important pathway for shale gas transport (Fink et al., 2017;

355

Javadpour, 2009; Zhang et al., 2018), natural microfractures are relatively uncommon

356

at the nano- and micron-scale in FE-SEM images (Gale et al., 2014; Loucks and Reed,

357

2016; Reed, 2013). The shrinkage fractures between the organic matter and mineral

358

observed in Fig. 8G, H are not real or else they would not be observed. They are

359

artifacts of sampling and preparation and not present in the subsurface. They are

360

formed because the shrinkage of organic matter is larger than that of rigid minerals. In

361

addition, the cracks developed within rigid minerals (Fig. 8G, H) and the

362

microfractures separating the organic matter from the rigid mineral (Fig. 13A, B) are

363

artifacts. If they are natural in the subsurface, the oil/bitumen generated from the

364

nearby kerogen will be filled in the fractures. The artifact-related microfractures could

365

be induced during the coring process, post-coring dehydration, SEM sample

366

preparation (e.g., mechanical cutting), and other procedures. In contrast, the

367

microcracks developed within minerals and totally occluded with migrated organic

368

matter (now pyrobitumen) are natural microfractures (Fig. 13C, D).

369

4.4 Organic pores in kerogen and pyrobitumen

370

4.4.1 Primary organic pores in kerogen

371

Primary organic pores generally occur in kerogen in immature shales such as

372

Toarcian Posidonia Shales (Fig. 11A in Han et al, 2017), Cretaceous Eagle Ford

373

Shales (Fig. 12A in Ko et al., 2017; Fig. 3G in Pommer and Milliken, 2015), and

374

Permian Clear Fork Shales (Fig. 3A in Loucks and Reed, 2014). These primary pores

375

were suggested to be inherited from the organic maceral because they have regular

376

pore walls and are regularly arranged, and these features are similar to those observed

377

in some plant materials (Han et al., 2017; Xi et al., 2017). In the natural maturity suite

378

of the Eagle Ford Shales, organic pores in “low maturity” samples (VRo = 0.5%,

379

~100 m) are larger than those in “high maturity” samples (VRo = 1.2–1.3%, ~2500–

380

2800 m) (Pommer and Milliken, 2015), indicating that mechanical compaction has a

381

strong negative effect on the pores. The investigated Shuijingtuo Shale samples have

382

high rigid minerals content, which is beneficial for the preservation of the organic

383

pores formed in original interparticle space because of pressure shadow (Fishman et

384

al., 2012). However, it has no protection for the organic pores formed in kerogen

385

parallel to the bedding direction due to the absence of vertical support. The

386

low-temperature thermochronology revealed that the Huangling anticline has been

387

stripped by almost 6900 m since the late Jurassic, meaning that the Shuijingtuo Shales

388

have experienced strong mechanical compaction (Ge et al., 2016). Therefore, most of

389

the primary organic pores in kerogen have disappeared.

390

4.4.2 Identification of pyrobitumen-hosted pores based on FE-SEM petrography

391

Organic pores can be formed in both primary and secondary organic matter

392

(Bernard et al., 2012a; Bernard et al., 2012b; Loucks and Reed, 2014). Utilizing the

393

SEM to identify the organic matter types (kerogen, solid bitumen, and pyrobitumen)

394

is challenging because these organic matters have similar colors in FE-SEM images

395

(Zargari et al., 2015). Furthermore, the generated secondary organic matter that does

396

not migrate out from kerogen could form a kerogen and solid bitumen complex in situ,

397

which increases the difficulty of identification (Loucks and Reed, 2014). Through

398

providing the information on the bonding of carbon in organic compounds, the

399

scanning transmission X-ray microscopy (STXM) was successfully distinguished the

400

solid bitumen from kerogen. The integrated correlative light and electron microscopy

401

(iCLEM) was firstly applied to the shale petrography for imaging organic matter

402

(Hackley et al., 2019; Hackley et al., 2017). This technique combines the traditional

403

fluorescence microscopy and SEM, enabling identification of orgainnc matter types

404

via fluorescence microscopy at low resolution followed by high-resolution SEM

405

evaluation of nanoscale structures. To solve the issue of identifying organic matter

406

types in FE-SEM images, Loucks and Reed (2014) proposed the concept of migrated

407

organic matter to distinguish the in-situ organic matter. Then, organic pores formed in

408

the migrated organic matter are identified as secondary organic pores.

409

The key to determining whether the organic matter has migrated is to confirm that

410

the corresponding space is unfilled before the petroleum migration. Commonly,

411

organic matter embayed by euhedral terminations of authigenic mineral is solid

412

bitumen (Hackley and Cardott, 2016). However, there are exceptions, the sedimentary

413

organic matter such as bituminite, could also show that embayment textures (Hackley

414

et al., 2018; Hackley et al., 2017). In Figs. 6A, C and 8A–F, organic matter embayed

415

by the euhedral terminations of microcrystalline quartz grains is mostly solid bitumen,

416

which implies a situation where the petroleum migration was coeval with or occurred

417

in the later stages of the microcrystalline quartz precipitation (Hackley et al., 2020). If

418

it is not the aforementioned situation, i.e., the petroleum migration initiated before the

419

precipitation, there would be no void space available for the later cementation as the

420

plastic petroleum would fully fill the space. The framboidal pyrite with several

421

micrometers (< 5 µm) commonly formed earlier than shale deposition (Fishman et al.,

422

2015; Zhao et al., 2017). Similar to the migrated organic matter in a fossil body-cavity

423

(Fig. 5B in Loucks and Reed, 2014), the organic matter that filled the intraparticle

424

pores between pyrite crystallites within the early-formed framboidal pyrite is migrated

425

organic matter instead of in-situ organic matter (Fig. 6B–D). It is worth noting that we

426

found another irrefutable petrographic evidence to identify the migrated organic

427

matter: organic matter filled in the early-formed dissolution pores (Fig. 7). This newly

428

discovered petrographic evidence has not been observed or reported previously. More

429

importantly, the filling of the migrated organic matter in the dissolution pores

430

demonstrates that the latter is well-connected to the matrix-related pore network,

431

which can increase the shale gas transport capacity. It should be noted that although

432

the aforementioned voiding-filling and embayment textures frequently are the

433

petrography evidence to identify the migrated organic matter in the SEM images.

434

However, it is not enough using the SEM alone to distinguish the migrated organic

435

matter from the sedimentary and correlative microscopy approaches are needed, such

436

as iCLEM (Hackley et al., 2018; Hackley et al., 2017).

437

The types of migrated organic matter that host pores in shale reservoirs can be

438

solid bitumen or pyrobitumen. The solid bitumen defined by organic petrologists is

439

partially insoluble, emphasizing its pore and fracture filling (Jacob, 1989), which can

440

be an effective indicator of petroleum migration. The pyrobitumen used by both

441

organic geochemists and petrologists is a hard and insoluble secondary organic matter

442

that is generated from the thermal cracking of oil or solid bitumen in the gas window

443

(Tissot and Welte, 1984). The boundary between solid bitumen and pyrobitumen was

444

set at a BRo of 1.5% (Mastalerz et al., 2018), corresponding to an EVRo of

445

approximately1.33% based on the empirical formula by (Jacob, 1989). According to

446

this boundary of thermal maturity, pyrobitumen is the predominant migrated organic

447

matter in the overmature Shuijingtuo Shales (2.18–2.70%, EVRo) in this study.

448

Combined with the discussion in Section 4.3, we further infer that the majority of

449

the porous organic matter that predominantly occurs in the original interparticle pores

450

is the porous pyrobitumen, i.e., pyrobitumen-hosted pores are the predominant pore

451

type in the investigated Shuijingtuo Shales in this work.

452

4.5 Development and evolution of organic pores

453

The development and evolution of secondary organic pores are believed to be

454

largely related to the maturation of organic material (Bernard et al., 2012a; Jarvie et

455

al., 2007; Loucks et al., 2009; Wu et al., 2019a). Fig. 14 illustrates the development of

456

organic pores with increasing thermal maturity. The thermal evolution of organic

457

matter is divided into three stages: diagenesis, catagenesis, and metagenesis stages,

458

corresponding to the immature stage (< 0.5%, Ro), mature stage (0.5–2.0%, Ro), and

459

overmature stage (> 2.0%, Ro), respectively (Fig. 14). This classification is widely

460

adopted in the field of petroleum geology (Tissot and Welte, 1984) and is comparable

461

to the main stages of coalification (Teichmüller and Teichmüller, 1979).

462

Based on the published data illustrating the occurrence of secondary organic

463

pores, a thermal maturity of 0.80% VRo is regarded as the thermal level of the first

464

appearance of the secondary organic pores (Han et al., 2017). This thermal level is

465

reasonable because 0.80% VRo coincides with the peak stage of petroleum expulsion

466

(Han et al., 2017). Secondary organic pores in the form of gas bubbles are originally

467

occupied by oil and are difficult to migrate out because of the high viscosity of

468

organic matter in the oil-generation stage (Curtis et al., 2012; Löhr et al., 2015;

469

Pommer and Milliken, 2015). In other words, it is the petroleum expulsion that leads

470

to the appearance of organic pores. Many previous studies also support this view

471

highlighting, for example, the increase in pore volume and surface area of the

472

oil-window shales after the extraction of soluble organic matter (Wei et al., 2014;

473

Xiong et al., 2016) and the relatively well-developed organic pores in the gas-mature

474

Eagle Ford Shales compared with those in the oil-mature stage (Fishman et al., 2013).

475

Besides, the emergence of abundant pores after bitumen-extraction in the oil-mature

476

Woodford Shales is another favorable evidence to support this view (Löhr et al., 2015).

477

According to this view, it is reasonable to infer that the thermal maturity of organic

478

pores formation is less than that of pores appearance.

479

The thermal cracking of residual oil and solid bitumen is a significant process that

480

not only generates a large amount of gas but also forms nanoscale organic pores

481

(Bernard et al., 2012a; Bernard et al., 2012b; Jarvie et al., 2007; Ko et al., 2017). The

482

absence of organic pores in the oil window and their appearance in gas windows in

483

the Woodford Shale (Curtis et al., 2012), along with the increase in pore volume from

484

the oil-generation stage to the gas-generation stage in the Albany Shales (Mastalerz et

485

al., 2013) and in the Posidonia Shale (Han et al., 2017; Mathia et al., 2016), reveal

486

that the formation of secondary organic pores is strongly associated with the thermal

487

cracking of residual petroleum. The abundance of secondary organic pores is related

488

to the amount of residual oil and solid bitumen, which is mainly dependent on TOC

489

content, kerogen type, and petroleum expulsion efficiency (Katz and Arango, 2018;

490

Ko et al., 2018; Tissot and Welte, 1984).

491

Owing to the difference in maceral composition, different kerogen types show

492

differences in the development of organic pores and the thermal level for the

493

appearance of secondary organic pores. Compared to type III kerogen, type I and II

494

kerogens, which are primarily composed of amorphous material and have a high H/C

495

atomic ratio, generate more oil (Tissot and Welte, 1984). Thus, theoretically, more

496

abundant secondary organic pores are formed in shale reservoirs with type I and II

497

kerogens. This agrees with the observation that fewer organic pores were formed in

498

shales with type III kerogen, such as Chang 7 Shales in Ordos Basin (Loucks et al.,

499

2017) and Emuerhe Shales in Mohe Basin (Hou et al., 2015). On the other hand, the

500

activation energies of type I and II kerogens are higher than that of type III kerogen

501

(Hunt, 1991; Tissot et al., 1987), resulting in the former types requiring a higher

502

temperature or thermal maturity for the petroleum generation and the appearance of

503

secondary organic pores.

504

Based on the above review of the literature and the observation of

505

pyrobitumen-hosted pores in this study, the evolution of organic pores from the

506

immature to the overmature stage is illustrated in Fig. 14. Primary organic pores are

507

mainly present in immature kerogen but would disappear owing to the compaction as

508

the burial depth increases. The secondary organic pore is a bubble formed by the

509

conversion of organic matter into a gas. The gas bubbles formed in the early mature

510

stage are difficult to migrate out owing to the high viscosity of organic matter and

511

originally occupied by the oil in the oil-generation stage. The appearance of secondary

512

organic pores is the result of the petroleum expulsion from the gas bubbles, which

513

occurs at approximately 0.80% VRo. With increasing thermal maturity, the residual

514

oil and solid bitumen crack into gas and form nanoscale pyrobitumen-hosted pores.

515

4.6 Preservation of organic pores

516

With the gradual sinking and covering by younger sediments, the burial depth and

517

overburden stress of the shale strata increase. In this process, the mechanical

518

compaction leads to a significant reduction in the interparticle space and also has

519

effects on the preservations of organic pores (Pommer and Milliken, 2015). The

520

mechanical compaction will destroy the organic pores formed in the kerogen parallel

521

to the bedding direction because of the absence of vertical support. However, organic

522

pores formed in the interparticle pores can be protected from compaction owing to the

523

presence of pressure shadows, which are formed around mechanically competent

524

grains (quartz, feldspar, dolomite, etc.) (Figs. 8A–F). Furthermore, organic pores

525

formed in intraparticle spaces within the early-formed framboidal pyrite (Fig. 6B, D)

526

and dissolution pores (Fig. 7) can also be protected from compaction. The pore size of

527

organic pores also has an effect on their preservations. It was reported that pores of

528

less than 100 nm in diameter are generally resistant to mechanical compaction

529

(Loucks et al., 2009; Milliken and Curtis, 2016). Therefore, owing to the much

530

smaller size of the nanometer-scale organic pores in the Shuijingtuo Shales, they are

531

less likely to be affected by overburden stress. However, we also observed many

532

undeformed organic pores with a pore diameter larger than 100 nm in the

533

Wufeng-Longmaxi Shales in the Fuling shale gas field, Sichuan Basin, China (Yang et

534

al., 2016a). This may be related to the abnormal high pore pressure in the organic

535

pores. The results of the recovery of paleo-pore pressure revealed that at the

536

maximum burial depth (about 6200 m in JY 1 Well), the Wufeng-Longmaxi Shales

537

were in the medium-to-high overpressure state (Gao et al., 2017; Gao et al., 2019).

538

The abnormally high pore pressure can reduce the effective stress (the difference

539

between overburden stress and pore pressure), which is beneficial for the

540

preservations of organic pores. Finally, the preservation of organic pores can be also

541

associated with their formation time relative to the mechanical compaction. Fig. 11B

542

shows the deformed organic pores, which implies that the post-compaction caused the

543

early-formed pores to collapse.

544

5. Conclusions

545

In this study, quantitative and qualitative methods were used to investigate the

546

pore system, especially organic pores, of the overmature Lower Cambrian Shuijingtuo

547

Shale. The following conclusions are drawn:

548

1. The organic matter is the prominent factor controlling the shale porosity for the

549

Shuijingtuo Shales, as evidenced by the strong positive correlation between TOC

550

content and shale porosity, and the abundant nanoscale pores formed in organic matter.

551

The contribution of clay minerals to shale porosity is shadowed by that of the organic

552

matter and can be eliminated by normalizing the pore structure to the TOC content.

553

2. Pyrobitumen-hosted pores are the predominant pore type in the organic-rich

554

Shuijingtuo Shales. The lack of interparticle pores is mainly caused by the

555

precipitation of microcrystalline quartz and filling of the pores by organic matter

556

instead of mechanical compaction.

557

3. The porous pyrobitumen primarily occurred in the original interparticle pores filled

558

by the microcrystalline quartz, which is the primary petrographic evidence to identify

559

the migrated organic matter. Besides, the porous organic matter that filled the

560

intraparticle space within early-formed framboidal pyrite and pre-existing dissolution

561

pores is identified as pyrobitumen.

562

4. Primary organic pores occur mainly in immature kerogen and would diminish as

563

the burial depth increases. The appearance of secondary organic pores is attributed to

564

the petroleum expulsion from the gas bubbles, which occurs at approximately 0.80%

565

VRo. Pyrobitumen-hosted pores are formed by thermal cracking of the pore-filling

566

organic matter, which migrated to the void space in the form of oil or solid bitumen.

567 568 569

Acknowledgments We thank the China National Science and Technology Major Projects (No.

570

2016ZX05034002-003), China Geological Survey Project Grant (No. DD20190561-1)

571

and National Natural Science Foundation of China (Nos. 41830431, 41902140,

572

41690134) for financial assistance to this research. We express our appreciation to Oil

573

& Gas Survey Center of China Geological Survey for providing the shale samples.

574 575 576 577 578 579

Anders, M.H., Laubach, S.E., Scholz, C.H., 2014. Microfractures: a review. Journal of Structural Geology 69, 377-394. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen

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Fig. 1 (A) Geological map of Huangling anticline (Chen et al., 2012) and (B) lithological column and sample distribution of the EYY 1 Well. Nh: Nanhua; Z: Sinian; : Cambrian; O: Ordovician; S: Silurian; D: Devonian; P: Permian; T: Triassic; J: Jurassic; K: Cretaceous.

Fig. 2 Relationships between TOC content and (A) clay mineral content and (B) quartz content.

Fig. 3 Relationships between TOC content and (A) bulk porosity, (B) micropore volume, (C) total pore volume, and (D) surface area.

Fig. 4 Relationships between clay mineral content and (A) bulk porosity, (B) micropore volume, (C) total pore volume, and (D) surface area.

Fig. 5 Relationships between clay mineral content and TOC normalized (A) bulk porosity, (B) micropore volume, (C) total pore volume, and (D) surface area.

Fig. 6 FE-SEM images showing migrated organic matter (pyrobitumen) with spongy pores. (A–D) Intraparticle space (red arrows) within early-formed framboidal pyrite; (E, F) Interparticle space filled with microcrystalline quartz (black arrows). Sample ID Y14, 3026.38 m. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2.

Fig. 7 FE-SEM images showing the dissolution pores filled by migrated organic matter (pyrobitumen) with spongy pores and authigenic quartz. (A, B) Dissolution pores in pyrite grain filled by porous pyrobitumen and authigenic quartz, sample ID Y12, 3020.54 m and Y05, 2996.32 m, respectively. (C, D) Dissolution pores in quartz grain filled by porous pyrobitumen, sample ID Y14, 3026.38 m. The red crosses indicate the EDS locations. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2; BSE = backscattered electron.

Fig. 8 FE-SEM images showing migrated organic matter (pyrobitumen) and artifact-related fractures. (A, B) Interparticle pores filled with porous pyrobitumen and microcrystalline quartz (black arrows), sample ID Y14, 3026.38 m. (C, D) Interparticle pores (red arrows) filled with porous pyrobitumen and microcrystalline quartz, sample ID Y08, 3007.75 m. (E, F) Interparticle pores (red arrows) filled with porous organic matter and microcrystalline quartz (black arrows in F), sample ID Y14, 2016.38 m. (G, H) pyrobitumen with no pores, shrinkage fracture between grain and pyrobitumen (white arrows) and cracks within minerals (black arrows), sample ID Y14, 3026.38 m. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2.

Fig. 9 Histogram of equivalent circular diameter of OM pores and dissolution pores. Samples were quantitatively analyzed with Image J software.

Fig. 10 SEM-CL and SEM-EDS maps showing the petrographic features of detrital and authigenic quartz. A-C are the same areas, sample ID Y14, 3026.38 m; B-F are the same areas, sample ID Y08, 3007.75 m. (A, B) Backscattered electron images of detrital quartz and aggregates of euhedral quartz; (C, D) CL images showing that detrital quartz has a uniform and bright luminescence and authigenic quartz has a dark to non-luminescence. (E, F) EDS maps showing the distribution of minerals. Mag = machine magnification; WD = working distance; BSE = backscattered electron; CL = cathodoluminescene; EDS = energy dispersive X-ray spectroscopy.

Fig. 11 FE-SEM images showing the compaction features. (A) Clay minerals bending toward framboidal pyrite and absence of intraparticle pores between peripheral particles in framboidal pyrite, sample ID Y14, 3026.38 m. (B) Elongated organic pores formed in clay platelets space, sample ID Y17, 3041.33 m. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2.

Fig. 12 FE-SEM images showing dissolution pores. (A, B) Dissolution pores in quartz grains, sample ID Y12, 3020.54 m. (C) Dissolution pores in a calcite grain, sample ID Y20, 3064.90 m. (D) BSE image, same area as C. The red crosses indicate the EDS locations. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2; BSE = backscattered electron.

Fig. 13 FE-SEM images showing artifact-related and natural microfractures. (A, B) Microfractures separating organic matter and clay minerals from rigid grains, sample ID Y12, 3020.54 m and Y19, 3054.20 m, respectively. (C) Natural microfractures filled by migrated organic matter (pyrobitumen), sample ID Y19, 3054.20 m. (D) BSE image, same area as C. Mag = machine magnification; WD = working distance; SE2 = secondary electron 2; BSE = backscattered electron.

Fig. 14 Schematic representation of the evolution of organic pores from immature to overmature stage.

Table 1 TOC content and mineral composition of the Shuijingtuo shale samples. Sample

Depth

TOC

Mineral composition (wt.%)

ID

(m)

(wt.%)

Quartz

K-feldspar

Albite

Calcite

Dolomite

Pyrite

Clay minerals

Illite

Mixed-layer Illite/smectite

Y01

2975.53

1.63

28

-

7

22

11

2

30

25

5

Y02

2978.08

1.58

20

-

9

21

23

2

25

22

3

Y03

2984.98

2.21

48

-

11

4

4

2

31

28

3

Y04

2989.32

1.80

34

-

6

17

14

3

26

21

5

Y05

2996.32

2.27

45

-

13

1

4

3

34

30

4

Y06

3000.18

2.49

49

1

12

2

6

3

27

22

5

Y07

3007.34

2.93

58

1

9

3

2

3

24

20

4

Y08

3007.75

3.02

59

1

10

3

2

3

22

17

5

Y09

3011.50

4.66

62

1

13

3

1

3

17

12

5

Y10

3015.55

8.29

70

1

7

5

2

2

13

10

3

Y11

3017.20

6.83

64

1

14

3

1

3

14

11

3

Y12

3020.54

4.33

57

2

13

6

2

3

17

14

3

Y13

3023.83

5.22

62

1

17

4

2

3

11

8

3

Y14

3026.38

6.20

61

1

14

6

2

3

13

9

4

Y15

3030.18

5.78

60

2

15

5

3

3

12

9

3

Y16

3038.68

5.47

55

2

13

2

4

4

20

15

5

Y17

3041.33

5.88

65

1

10

2

2

2

18

14

4

Y18

3050.80

7.52

77

2

4

4

2

2

9

5

4

Y19

3054.20

7.55

56

17

-

2

3

4

18

14

4

Y20

3064.90

2.37

28

11

-

22

20

2

17

12

5

Table 2 Sample

Results of the reflectance of the solid bitumen. BRo (%)

ID

Number of measuring

Standard

points

deviation (%)

Notes

Y08

2.91

21

0.28

Y09

-

-

-

no solid bitumen particles were detected

Y12

-

-

-

no solid bitumen particles were detected

Y14

-

-

-

no solid bitumen particles were detected

Y19

2.48

4

0.12

few measurable solid bitumen particles

Table 3 Results of bulk porosity, micropore volume, total pore volume and surface area. PV = Pore volume; SA = Surface area; Subscripts, such as <10 and 10–50, represent the pore diameter range (nm). Sample ID

Bulk Porosity

Micropore 3

-2

Surface 2

volume

Volume -2

Total pore

Proportion of pore volume and surface area (%)

area (m /g)

PV< 10

PV10–50

PV> 50

SA< 10

SA10–50

SA> 50

3

(%)

(10 cm /g)

(10 cm /g)

Y01

1.45

0.46

1.30

9.96

45.15

19.02

35.83

93.48

4.92

1.60

Y02

1.82

0.38

1.05

9.25

49.28

17.58

33.14

94.76

3.96

1.28

Y03

1.81

0.53

1.72

15.04

49.43

15.77

34.80

94.95

3.71

1.34

Y04

1.29

0.41

1.25

10.13

42.20

15.35

42.46

93.95

4.23

1.82

Y05

-

0.56

1.82

14.54

53.74

16.18

30.08

94.40

4.24

1.36

Y06

1.80

0.54

1.54

15.65

59.28

15.08

25.65

95.98

3.13

0.89

Y07

2.04

0.67

1.90

19.35

58.69

13.96

27.35

96.31

2.80

0.89

Y08

2.86

0.64

1.81

18.36

60.98

13.78

25.24

96.35

2.79

0.86

Y09

2.53

0.94

2.31

29.10

69.16

10.28

20.56

97.70

1.70

0.61

Y10

4.01

1.33

3.19

44.67

78.99

6.96

14.06

98.61

1.05

0.34

Y11

-

1.19

2.59

37.20

74.91

6.05

19.04

98.78

0.79

0.43

Y12

3.38

0.79

1.84

21.76

74.26

10.55

15.19

97.77

1.76

0.47

Y13

3.40

0.91

2.17

29.23

76.06

8.14

15.80

98.28

1.28

0.44

Y14

3.82

0.95

2.42

32.97

75.95

8.24

15.81

98.37

1.21

0.42

Y15

3.45

0.98

2.29

31.60

76.74

8.27

14.98

98.54

1.10

0.36

Y16

-

0.93

2.29

29.63

70.18

7.12

22.69

98.39

1.04

0.56

Y17

3.52

0.96

2.02

27.02

73.91

9.10

16.99

98.25

1.31

0.44

Y18

-

1.00

2.09

27.69

64.74

9.78

25.48

97.77

1.52

0.72

Y19

3.82

1.16

2.28

32.02

73.44

7.97

18.59

98.41

1.13

0.46

Y20

-

0.45

1.03

11.10

49.05

12.00

38.96

96.28

2.35

1.37

Table 4 Statistical results of the organic pores in Shuijingtuo Shales. Pores were quantitatively analyzed using Image J software. Sample ID

Minimum length

Maximum Length

Median length

Average length

Count

(nm)

(nm)

(nm)

(nm)

Y05

4.98

51.80

11.16

12.6

648

Y05

4.84

24.34

10.17

11.09

220

Y05

4.16

83.88

14.21

16.93

786

Y12

6.94

46.68

15.48

17.1

425

Y14

3.96

21.68

8.33

9.07

247

Y14

3.57

18.26

7.56

8.07

563

Y14

3.61

27.75

7.86

8.5

529

Y14

3.52

30.62

7.77

8.34

685

Y14

7.25

66.22

21.11

22.97

198

Y17

6.17

27.82

12.01

12.64

322

Y17

5.47

30.08

11.51

12.25

516

Y19

5.83

20.33

10.38

10.78

231

Total

3.52

83.88

10.40

12.21

5370

Table 5 Statistical results of the dissolution pores in Shuijingtuo Shales. Pores were quantitatively analyzed using Image J software. Sample ID

Minimum length

Maximum Length

Median length

Average length

Count

(nm)

(nm)

(nm)

(nm)

Y08

15.72

94.67

37.96

41.70

34

Y08

9.29

49.59

17.69

19.42

69

Y12

8.61

47.90

16.59

19.97

87

Y12

17.50

193.75

35.42

50.66

61

Y12

15.81

151.11

37.11

44.10

85

Y12

26.59

219.88

75.40

78.71

35

Y12

33.02

298.66

80.47

91.55

42

Y12

28.83

302.61

122.37

144.43

20

Y14

9.60

304.69

17.80

22.09

109

Total

8.61

304.69

26.55

42.86

542

Highlights 1. Pyrobitumen-hosted pores are the predominant pore type in the organic-rich Shuijingtuo Shale. 2. The porous pyrobitumen primarily occurred in the original intergranular pores filled by the microcrystalline quartz and quartz overgrowth. 3. The lack of interparticle pores is mainly caused by the late cementation and filling by organic matter rather than the mechanical compaction. 4. The organic matter is the prominent factor controlling the shale porosity for the Shuijingtuo Shale studied in this work.

Declaration of Interest Statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, and manuscript is agreed by all authors for publication and no conflict of interest exists in the submission. We would like to announce that the work presented in this article is an original research that has not been published previously.