Nanoscale Pore Changes in a Marine Shale: A Case Study Using

Aug 1, 2018 - Cipher Consulting, Limited, 6 Stardust Street, Kenmore , Queensland 4969 , Australia. # Organic Geochemistry Section, GFZ German Researc...
0 downloads 0 Views 4MB Size
Subscriber access provided by READING UNIV

Fossil Fuels

Nanoscale pore changes in a marine shale: A case study using pyrolysis experiments and nitrogen adsorption Shangbin Chen, Zhaoxi Zuo, Tim A Moore, Yufu Han, and Clementine Uwamahoro Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01405 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Nanoscale pore changes in a marine shale: A case study using

2

pyrolysis experiments and nitrogen adsorption

3

Shangbin Chen a, b, Zhaoxi Zuo c, Tim A. Moore d, e, Yufu Han f, Clementine

4

Uwamahoro a, b

5

a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process

6

of the Ministry of Education, China University of Mining and Technology, Xuzhou

7

221116, China

8 9 10 11

b. School of Resources and Geoscience, China University of Mining and Technology, Xuzhou, 221116, China c. School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

12

d. Queensland University of Technology, Brisbane, QLD 4001, Australia

13

e. Cipher Consulting Ltd, 6 Stardust Street, Kenmore, Brisbane, QLD 4969, Australia

14

f. Organci Geochemistry Section, GFZ German Research Centre for Geosciences,

15

Potsdam D14473, Germany

16

Abstract: Nanoscale pores have an important role in the accumulation of gas in shale

17

gas reservoirs. Indeed, the formation of nanopores is critical for the characterization

18

and evaluation of shale reservoir. Moreover, the effect of pyrolysis on the

19

modification of nanopores is not clear. Therefore, this paper focuses on pyrolysis and

20

nitrogen adsorption experiments to examine nanoscale pore structure and evolution in

21

marine shale strata with low total organic carbon (TOC). All the examined samples

22

contain micropores, mesopores, and macropores. The results show that the number of

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23

micropores increased as a result of artificial maturation (i.e. pyrolysis), which resulted

24

in a significant increase in the surface area and the total pore volume. The openness of

25

the pores significantly increased when the maturity was higher than 2.5%Ro (vitrinite

26

reflectance). The 1.5-7.5 nm and 60-70 nm pores are the most pronounced to change

27

after pyrolysis. Furthermore, liquid hydrocarbons produced during heating were

28

shown to influence pores of approximately 41 nm width. In the over-mature stage (Ro

29

= 2.77%), the numbers of pores and pore volume significantly increased during

30

pyrolysis. The pore structure of the over-mature shale was different from that of the

31

shale during the mature and high maturity stages. Pores less than 20 nm wide nearly

32

provided 90% of the surface area and at least 50% of the pore volume. The

33

transformation of organic matter from the solid state to the liquid and gas states is

34

most closely related to the number of mesopores. The pores with sizes less than 10 nm

35

in width have the greatest change in the proportion of surface area to pore volume

36

with increasing maturation.

37

Keywords: nanoscale pore evolution; pyrolysis; thermal maturity; low TOC marine

38

shale; China

39

1. Introduction

40

The general burial depth of shale gas reservoirs in the United States does not

41

exceed 3000 m, and typically ranges between 800 and 2600 m.1-3 Conversely, the

42

burial depth of Paleozoic shale gas reservoirs lies between 3000 and 4000 m in the

43

Sichuan Basin of China; the maximum burial depth of these reservoirs is believed to

44

exceed 7000 m.4, 5 Paleozoic shale gas reservoirs in southern China are generally in

ACS Paragon Plus Environment

Page 2 of 43

Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

45

the high- and over-maturity stages. Unfortunately, the pore structure and capacity of

46

these reservoirs are not well understood.6-13 Research has shown that high maturity

47

leads to nanoscale pores, which can improve shale gas storage capacity.14-21 Due to

48

significant heterogeneity in shale gas reservoir, the mineral composition and pore

49

structure vary at a nanometric to macrometric scale.22

50

Pore structure in shale gas reservoirs is primarily influenced by the thermal

51

maturation processes of organic and inorganic materials. The frequency of pores in

52

organic matter varies depending on the level of thermal maturity.23-27 Moreover, the

53

pores in organic matter are well developed when the maturity is greater than 0.9% Ro

54

(vitrinite reflectance), which indicates relatively high pore volume.28 However, there

55

is a non-monotonic evolution trend in pore volume during shale maturation processes

56

29

57

with increasing maturity.25, 27 The increase in the number of pores within organic

58

matter is closely related to the initial hydrocarbon generation and secondary cracking

59

of organic matter. As the maturity of organic matter reaches the gas generation

60

window, porosity continues to increase and is characterized by isolated and complex

61

mesopores and macropores.30

; indeed, the pore size and quantity of organic matter did not significantly increase

62

Pyrolysis studies on organic-rich shales showed that a large number of nanopores

63

are formed from organic matter as maturity increases.27, 31-41 With increasing pyrolysis

64

temperature, the number, volume, and surface area of micropores, mesopores, and

65

macropores increase in organic-rich shales.36 The number of nanoscale pores in

66

organic-rich mudstone and shale increases with increasing temperature and pressure,

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

67

and the peak temperature is consistent with the yield peak temperature of gaseous and

68

liquid hydrocarbons. In contrast to micropores, the number of mesopores and

69

macropores decrease with increasing temperature and pressure.40 Therefore, the

70

processes concerning the evolution of pores are debatable among researchers.36, 40 The

71

variation in number and structure-type of pores less than 100 nm in width is more

72

complex than those having a width superior to 100 nm.40 Whether the evolution of

73

organic matter is too high obscures other phenomena. In low TOC shales, the change

74

in nanoscale pores as a function of thermal maturity is not well understood.

75

Consequently, and in order to understand nanoscale pore evolution, low TOC marine

76

shale samples were collected and pyrolysis experiment, and nitrogen adsorption

77

experiments were conducted.

78

2. Materials and methods

79

2.1 Samples and geologic background

80

The Xiamaling Formation (~1.37Ga), located in North China, is characterized by

81

black shales of relatively low thermal maturity (Tmax is 445°C) and has been identified

82

as a potential petroleum source rock.42 We selected the Xiamaling Formation marine

83

shales from the Zhaojiashan profile in the Xiahuayuan area in Zhangjiakou city, Hebei

84

Province in China, which is located in the Yanshan Meso-Neoproterozoic rift basin.

85

The Yanshan rift basin is an active tectonic unit on the North China platform, also

86

known as the "Yanshan subsidence zone", which has an area of 80,000 km2 (Figure

87

1a).43 The Yanshan subsidence zone is divided into 7 structural units including two

88

uplifts and five depressions, which are the Shanhaiguan uplift, the Mihuai uplift, the

ACS Paragon Plus Environment

Page 4 of 43

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

89

Xuanlong depression, the Jingxi depression, the Liaoxi depression, the Jibei

90

depression, and the Jidong depression (Figure 1b). The study area is located in the

91

Xuanlong depression.

92

The strata within the study area belong to the Yanshan stratigraphic division of

93

the North China-type stratigraphic area. The stratigraphy, from the Archaean to the

94

Cenozoic include the Archaean Qianxi Group, the Middle Proterozoic Changcheng

95

System and Jixian System, the Upper Proterozoic Qingbaikou System, the Cambrian

96

System of the Paleozoic, the Jurassic System of the Mesozoic, and the Quaternary

97

system of the Cenozoic (Figure 1c). The Meso-Neoproterozoic strata are divided into

98

12 groups of 3 series (from bottom to top), and overlie the Archean Qianxi Group in

99

an angular unconformity, with the top covered by Lower Cambrian strata (Figure

100

2a).43 The Xiamaling Formation belongs to the Upper Proterozoic Qingbaikou System,

101

which unconformably sits atop the Tieling Formation.44 The Xiamaling Formation

102

mainly consists of shale and is divided into four lithological members; moreover, the

103

Xiamaling Formation ranges from 135 to 335 m in thickness in the Yanshan region.

104

Member 1 is approximately 145 m in thickness and is composed of gray-black shale,

105

yellow-green foliated sandy shale, and variegated shale containing iron concretions.

106

Member 2 is approximately 66 m in thickness and is composed of purple and green

107

mudstone, dark-green and turquoise sandstone, and purple shale with yellow-green

108

shale interbedded near the top. Member 3 is about 250 to 280 m in thickness and is

109

composed of gray-black siliceous shale in the lower part and interbedded

110

yellow-green shale and black shale intercalated with sandstone lenses in the upper

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

111

part. Member 3 is characterized by high TOC content black shales and has been

112

identified as potential oil source rocks.42,

113

thickness and is composed of gray and dark gray mudstone, laminated limestone, and

114

gray-green shale and contains abundant stromatolites (Figure 2b).

45

Member 4 is approximately 60 m in

115

The Xiamaling Formation was deposited in a marine environment and contains

116

type I organic matter with TOC contents between 1% and 5%.45-49 The organic matter

117

maturity of the Xiamaling Formation in the Xiahuayuan region is reported to be in the

118

range of 0.6%-0.7%,50 0.46%-0.76%,51 and 0.6%;52 the reported values indicate low

119

maturity. Therefore, the Xiamaling Formation is suitable as a sample for pyrolysis

120

experiments.43, 52

121

The original block sample from the third member of the Xiamaling Formation

122

was collected from a fresh profile (the Zhaojiashan profile) exposed by excavation

123

and not affected by oxidation (Figure 1 and Figure 2). The Zhaojiashan profile shows

124

that the Xiamaling Formation in this region is extensively developed. The Xiamaling

125

Formation is relatively shallow with a present burial depth of 0-1000 m. Therefore,

126

vitrinite reflectance (Ro) for organic matter in the Xiamaling Formation ranges from

127

0.6% to 0.7%. However, there is a large igneous body exposed in the study area that

128

may have affected the maturity of the organic matter. Eight cylindrical samples, of 2.5

129

mm × 50 mm dimensions, were drilled from the collected block sample, which

130

were labeled A0 as the original samples. Of the 7 original samples A0 were then

131

subjected to different pyrolysis conditions; the products were labeled A1, A2, A3, A4,

132

A5, A6, and A7.

ACS Paragon Plus Environment

Page 6 of 43

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

133

2.2 Experimental analysis and methodology

134

The pyrolysis experiments were conducted in the Lanzhou Institute of Geology,

135

Chinese Academy of Sciences (LIGCAS). The sample was subjected to different

136

lithostatic and hydrodynamic pressures associated with burial by a WYMN-3 HTHP

137

instrument (developed by Wuxi Institute of petroleum geology, China Petroleum and

138

Chemical Co., LTD). The instrument maintains isothermal heating of the samples by

139

automatic pressure compensation and eventual expulsion of hydrocarbons

140

immediately after the pyrolysis temperatures are reached. The samples were treated

141

for 24 hours at the temperatures and pressures included in Table 1. Upon the

142

completion of the experiment, the expelled oil, water, and gaseous hydrocarbons were

143

collected and analyzed, as detailed by Zhong et al. 53 In order to ensure full pyrolysis,

144

the samples were placed in the reactor for a full 24 hours. The experimental

145

parameters were designed considering the current main burial depth and the

146

hydrostatic pressure of shale gas reservoir of the Sichuan Basin (Table 1).43, 54, 55 The

147

rock and fluid densities are 2.6 g/cm3 and 1.0 g/cm3, respectively. The 7 original

148

samples (A0) were heated to 350, 400, 420, 450, 480, 520, and 550°C.

149

The Average pore size, pore size distribution, and surface area were analyzed via

150

nitrogen adsorption. The tests were carried out using a Quadrasorb surface area and

151

pore size analyzer; the Quadrasorb instrument uses the static volumetric method.

152

After crushing and splitting, samples were degassed in vacuum with temperatures less

153

than 150°C for 4 hours (vacuum to 266.664 Pa). The isothermal adsorption-desorption

154

experiment was carried out at -195.8°C (77.3K) with pure nitrogen gas. The

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 43

155

instrument’s aperture detection ranges from 0.35 to 500 nm; the relative pressure

156

ranges from 0.001 to 0.998. Density functional theory (DFT) analysis and multi-point

157

Brunauer Emmett Teller (BET) analysis regression models were used to acquire the

158

pore size distribution and specific surface.56

159

In this study, the investigated geochemical parameters include the organic matter

160

richness (present-day TOC) and thermal maturity. The TOC was determined by a

161

carbon and sulfur analyzer. Thermal maturity was estimated using optical microscopy

162

and spectroscopy. Moreover, vitrinite reflectance is the most common optical method

163

used; it is performed through microscopic inspection of kerogen and an analysis of the

164

reflectivity of the particles via a photomultiplier.57 The vitrinite-like material

165

reflectance (Rom) is derived in the marine sediments below the Permian strata.58 The

166

vitrinite-like material reflectance can indicate the Early Paleozoic maturity index.57

167

Furthermore, Zhong et al.60 established the correlation between the vitrinite-like

168

material reflectance (Rom) and the equivalent vitrinite reflectance (Ro). The correlation

169

between Rom and Ro is shown in the following equations:

170

Ro = 1.042 Rom + 0.052 (0.30% < Rom < 1.40%)

171

Ro = 4.162 Rom - 4.327 (1.40% ≤ Rom < 1.60%)

172

Ro = 2.092 Rom - 1.079 (1.60% < Rom < 3.0%)

173

Using the above arithmetic correlations, the Rom can be converted into equivalent

174

Ro, which has been used for reservoir evaluation. The correlation between maximum

175

Rom and random vitrinite-like reflectance (Rran,o) is given by Rom = Rran,o

176

58 value)*1.064.

(mean

Laser Raman spectroscopy was performed with a Raman spectroscopy

ACS Paragon Plus Environment

Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

RmcRo

= 0.0537×d(G-D)-11.2161 (where

177

Senterra instrument, using the equation:

178

RmcRo

179

the Raman method is more accurate in the high and over- mature stage, 62 and can be

180

used as a verification method for optical light microscopy tests.

181

3. Results

182

3.1 Maturity and total organic matter content

is the value tested from the Laser Raman method). It is important to note that

183

The results of vitrinite reflectance and laser Raman spectroscopy are given in

184

Table 2. The vitrinite reflectance values of the samples A0, A1, A2, A3, A4, A5, A6, and

185

A7 are 0.69%, 0.93%, 1.12%, 1.50%, 1.92%, 2.23%, 2.48%, and 2.77%, respectively.

186

Vitrinite reflectance values indicate that A1 and A2 are in the mature thermal stage

187

(middle catagenesis stage; 0.7% < Ro < 1.3%), A3 and A4 are in the high maturity

188

thermal stage (later period of catagenesis stage; 1.3% < Ro < 2.0%), and A5, A6, and

189

A7 are in the over-mature thermal stage (metagenesis stage; Ro > 2.0%).54 The

190

indicated levels of thermal maturity suggest that A5, A6, and A7 are in the (dry) gas

191

generation window. Moreover, Laser Raman spectroscopy yielded similar results to

192

the reflected light microscopy, especially in the high and over-mature thermal stages.

193

The TOC values after pyrolysis range from 0.56% to 1.30% (Table 2). The TOC

194

values decrease as the Ro values increase; this is explained by the conversion of some

195

organic matter into hydrocarbons as a result of heating. Nonetheless, the decrease in

196

TOC is not linear with the increase of the pyrolysis temperature.

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

197

3.2 Pore structure parameters

198

The values for multi-point BET (MBET) surface area, total pore volume, and

199

average pore diameter for A0 are 8.2 m2/g, 0.019 ml/g, and 9.1 nm, respectively. The

200

pore structure parameters of the other samples (A1-A7) are given in Table 3. The

201

surface area values range from 5.9 to 12.9 m2/g, with an average of 9.6 m2/g. The

202

surface area fluctuated in a ‘w’-like pattern with increasing maturity (Figure 3a). The

203

total pore volume values range from 0.016 to 0.033 ml/g, with an average of 0.023

204

ml/g. The total pore volume, similar to the surface area, also fluctuated in a ‘w’-like

205

pattern as a function of increasing maturity (Figure 3b). The pores that formed by

206

pyrolysis are mainly related to the changes in the shale composition, especially by the

207

evolution of organic matter. The TOC content of the sample is small, so the number of

208

pores formed by the organic matter is limited. Similarly, the pore volume formed by

209

the hydrocarbon generation effect is restricted due to the constraint of pressure.

210

Therefore, the pore volumes of A1-A7 are close to that of A0. The average pore

211

diameters range from 7.4 to 12.3 nm, with an average of 9.9 nm. With increasing

212

maturity, the average pore diameters also fluctuated in a pattern roughly opposite to

213

the trend of the surface area and total pore volume (Figure 3c). Moreover, there is a

214

positive linear correlation between the total pore volume and the maximum

215

hydrodynamic pressure (R2 = 0.5446) (Figure 3d). The pore volume of A6 is less than

216

those of the other samples. The degree of condensation and aromatization of kerogen

217

increases and superimposed pressure after the over-mature stage is reached; this

218

results in the compaction of the mineral pores and the inhibition of hydrocarbon

ACS Paragon Plus Environment

Page 10 of 43

Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

219

generation, which leads to the reduction in the total pore volume. Then, with further

220

increase in pressure, the pores are damaged, and the conduction and adjustment of

221

pores, especially mesopores, significantly increase; this causes the total volume of the

222

pores to increase again.

223

4. Discussion

224

4.1 Conversion rate of organic matter

225

The Ro increased from 0.69% to 2.77% when the temperature increased from 350

226

to 550°C. The organic matter is converted to liquid and gaseous hydrocarbons;

227

therefore, the TOC contents of the samples A1, A2, A3, A4, A5, A6, and A7 decreased to

228

1.30%, 1.05%, 0.56%, 1.09%, 1.13%, 0.9%, and 0.96%, respectively (Figure 4a).

229

From these results, the conversion rate of the organic matter Cn (where n = 1, 2, 3, 4, 5,

230

6, and 7) can be defined by:

231

Cn = (TOCA0-TOCAn)/TOCA0*100

(1)

232

The calculated conversion rates of the seven samples (A1-A7) are thus 5.11%, 23.36%,

233

59.12%, 20.44%, 17.52%, 34.31%, and 29.93%, respectively (Figure 4b). The results

234

showed the absence of a significant positive linear correlation between conversion

235

rate and temperature. Different temperatures and pressure conditions have different

236

organic conversion rates. The conversion rate of A3 showed to be the highest

237

(59.12%). At this high maturity thermal stage (late catagenesis stage), oil generation

238

and oil cracking simultaneously occur; therefore, the residual organic matter in A3 is

239

the lowest.

240

With increasing thermal maturity, there exists a roughly linear correlation

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 43

241

between surface area on one hand and TOC (Figure 4c) and TOC conversion rate (Fig.

242

4e) on the other. Similarly, there exists a linear correlation between the total pore

243

volume on one hand and TOC (Figure 4d) and TOC conversion rate on the other (Fig.

244

4f). The increase in the trend of the pore volume is weaker than that of the surface

245

area (Figure 4e and f). However, the evolution of organic matter produces a large

246

number of micropores15-20, which significantly increases the specific surface area. Due

247

to the pressure effect in our case, the increase in pore volume is limited. If the

248

parameters of A0 were used as reference values, the TOC conversion rates of A3 and

249

A4 would be 59.12% and 20.44%, respectively; moreover, the surface area rates of

250

change of A3 and A4 would be 48.90% and 57.63%, respectively. We also defined the

251

conversion rate of surface area Surfacen (n = 1, 2, 3, 4, 5, 6, and 7) as:

252

Surfacen = (SurfaceA0-SurfaceAn)/SurfaceA0*100)

(2)

253

All these characteristics suggest that in the process of organic matter generation, the

254

sizes of micropores increase, which results in a disproportionate increase in the

255

surface area with respect to the total pore volume.

256

4.2 Pore structure characterization

257

Pore characteristics can be estimated by using nitrogen adsorption and desorption

258

isotherms (Figure 5a-i). All the curves were similar to TypeⅡisotherm according to

259

the BET classification. Although there are differences among adsorption isotherms,

260

they generally formed reverse “S” shaped curves. At a low-pressure stage (p/p0 =

261

0-0.2), the volume of adsorption slowly rises; this indicates that the adsorption

262

process is both monomolecular and polymolecular. During the intermediate stage

ACS Paragon Plus Environment

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

263

(p/p0 = 0.2-0.8), the adsorption volume slowly increases with increasing pressure; this

264

suggests that the adsorption process is multimolecular. Finally, during the last stage

265

(p/p0 = 0.8-1.0), the adsorption volume sharply rises, but the samples were not close to

266

saturation adsorption despite the relative pressure reaching the saturated vapor

267

pressure; this implies the presence of mesopores and macropores in the sample.

268

Hysteresis loops were detected in all the samples (Figure 5). Hysteresis loops are

269

formed from mismatching between adsorption and desorption isotherms in the

270

high-pressure stage (p/p0 = 0.4-1.0). Furthermore, hysteresis loops indicate that the

271

samples contain open pores. Different hysteresis loop types reflect the shape and

272

characteristic of the pore structure. Type A hysteresis is attributed to cylindrical pores;

273

type B hysteresis is associated with slit-shaped pores; type C hysteresis is attributed to

274

wedge-shaped pores with open ends; type D hysteresis loops result from

275

wedge-shaped pores with narrow necks at one or both open ends; and type E

276

hysteresis loop is attributed to “ink-bottle” pores.63 With the exception of the presence

277

of micropores, the hysteresis loops in all the isotherms close before reaching a relative

278

pressure of 0.3 in the desorption process. Indeed, the fact that all the hysteresis loops

279

are not closed in our case indicates that the samples contain micropores.63, 64

280

All the adsorption isotherms are steep near the saturated vapor pressure and at

281

moderate pressure (Figure 5). This feature is similar, but not identical, to the class B

282

hysteresis loop provided by De Boer; 61 moreover, it is also similar to the H3-type and

283

H4-type hysteresis loops recommended by IUPAC. The characteristics of the

284

hysteresis loops indicate that the pore structure of the shale is relatively complex and

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

285

is composed of nanopores with a semi-amorphous structure. Pores have peculiar

286

shapes and mainly consist of cylindrical pores with two open ends and parallel-plate

287

pores with four open sides. However, the shapes of the pores vary among samples, as

288

indicated by the different hysteresis loops. The hysteresis loops of A0 and A1 are

289

narrower than those of A4, A5, A6, and A7, which suggest that the wedge-shaped pores

290

are changed to slit-shaped pores upon increasing the thermal maturity (Figure 5b and

291

f-i).

292

The degree of pore openness is positively correlated with the slope of the

293

adsorption lines.63 The slope of the adsorption line of all the samples, except A1,

294

increased with increasing thermal maturity (Figure 5j). The slope of the adsorption

295

line A1 decreased because organic matter is consumed in the process of hydrocarbon

296

generation during maturity; 27, 28, 30 thus, liquid hydrocarbons are mainly produced,

297

which plug some of the pores and consequently limit pore openness. The A7 isotherm

298

showed an increase in the slope (Figure 5j), indicating that there are more open pores

299

in comparison to the other samples. The openness of the pores is significantly

300

increased when the maturity is greater than 2.5%.

301

4.3 Nanoscale pore size distribution

302

Pore volume distribution was obtained by the DFT method (Figure 6). The DFT

303

model is suitable for calculating the pore size distribution in mesopores and

304

micropores.64, 65 Moreover, each peak on the graph represents a proportion of the

305

pores. Five features can be seen in figure 7. Firstly, the peaks are divided into two

306

types, which are consistent and inconsistent peak shapes. The first inconsistent peak

ACS Paragon Plus Environment

Page 14 of 43

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

307

shape consists of multiple peaks; the peaks are located between 1.5 and 7.5 nm

308

(Figure 6a). The second inconsistent peak shape is located between 60 and 70 nm

309

(Figure 6b). In the other regions, the pore widths and peak shapes were consistent.

310

Secondly, there is a distinct peak at 1.5 nm, indicating that a large number of pores

311

have a width less than 1.5 nm. Thirdly, there are multiple peaks between 2 and 50 nm,

312

indicating that the pores within this range are predominantly present in the samples.

313

Fourthly, A7 has markedly larger pores in comparison to the other samples; indeed, A7

314

indicated an over-mature stage. Furthermore, A7 showed the maximum ordinate

315

change in the pore width range between 9.5 and 70 nm (Figure 6a). Finally, A1’s

316

ordinate is less than that of A0 when pore width was below 41 nm; this changed after

317

41 nm (Figure 6c). All these features suggest the predominance of pores of width less

318

than 10 nm and the high sensitivity of pore width to temperature. Changes in pore

319

structure were observed in the width range of 1.5-7.5 nm and 60-70 nm. The liquid

320

hydrocarbons produced during heating influence the pores with 41 nm width. In the

321

over-mature stage (A7, Ro = 2.77%), the distribution of pore structure does not vary,

322

but the number of pores and pore volume significantly increase. This indicates that the

323

pore structure of the over-mature shale is different from that of the mature and high

324

maturity shale. The peaks indicate that the pores in this region occupy a certain

325

proportion, and the differences in peak numbers and peak maxima can reflect the

326

differences in the distribution of pores at each region. Since the samples A1-A7

327

evolved from the original sample (A0), the pore structures are similar at the

328

macroscopic scale. Nonetheless, there are differences at the microscopic level due to

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

329

changes in the composition of materials caused by pyrolysis. Additionally, changes in

330

the pore structure are concentrated in the two regions, 1.5-7.5 nm and 60-70 nm,

331

indicating that the most prominent changes caused by pyrolysis mainly occur in these

332

two pore intervals.

333

4.4 Evolution of nanoscale pore surface area and volume

334

The surface area and total pore volume vary with increasing maturity (Figure 3

335

and Figure 8a and b). The sizes of the micropores, mesopores, and macropores are 50 nm, respectively. The trends in micropore and mesopore surface area

337

are similar to that of the total surface area, while the trend in volume within

338

mesopores is similar to that of the total pore volume (Figure 7a and b). The

339

proportions of micropore surface area range between 8.3% and 13.2% when the

340

maturity is less than 1.2% (i.e. for A0, A1, and A2; Figure 7c). The proportions of

341

micropore surface area range between 30.9% and 36.2% when the maturity is greater

342

than 1.2% (i.e. for A3, A4, A5, A6, and A7). Moreover, the proportion of mesopore

343

surface area varies between 84.7% and 88.9% when the maturity is less than 1.2%.

344

When the maturity is greater than 1.2%, the proportions of mesopore surface area

345

fluctuate between 20.3% and 67.3%. The proportions of macropore surface area do

346

not change when the maturity ranges between 1.1% and 2.8%. Furthermore, there is a

347

correlation between the trends of the proportions of the surface area of micropores

348

and mesopores. However, the distribution of the total pore volume is different from

349

that of the surface area (Figure 7c and d). Mesopores and macropores account for

350

80.9%-85.9% and 10.5%-16.3% of the total pore volume, respectively (Figure 7d).

ACS Paragon Plus Environment

Page 16 of 43

Page 17 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

351

These percentages hardly vary with increasing maturity. When maturity is greater

352

than 1.2%, the proportions of micropore volume account for 0.7%-2.1%; when

353

maturity exceeds 1.2%, the proportions of micropore volume increase to 3.9%-5.9%

354

(Figure 7d).

355

To further discuss the variation in the distribution of micropores, mesopores, and

356

macropores in the context of the total surface area and total pore volume, we defined

357

the coefficient K An (n = 1, 2, 3, 4, 5, 6, and 7) as:

358

K An = (proportion of An-proportion of A0/proportion of A0*100)/ conversion rate

359

of TOC An *100

(3)

360

where “proportion A0” is the ratio of pore volume (or surface area) of micropores,

361

mesopores, or macropores to the total pore volume (or surface area) of the sample A0;

362

“proportion An” is the ratio of pore volume (or surface area) of micropores, mesopores,

363

or macropores to the total pore volume (or surface area) of the sample An.

364

The coefficient KAn reflects the consistency of pore change and the

365

transformation of organic matter. Organic matter transformation into one kind of pore

366

is reflected by stable K values. Figure 7 (e and f) shows that the K values of mesopore

367

surface area and volume are the most stable, indicating that organic matter

368

transformation mainly concern mesopores.

369

Distribution histograms (Figure 8a and b) were generated for surface area and

370

pore volume using < 10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, and > 70 nm width

371

sizes. The pore surface area is predominantly distributed in pores inferior to 20 nm in

372

size. The proportions of surface area within the < 10 and 10-20 nm pores are in the

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

373

range of 68.8%-84.6% and 8.7%-16.8%, respectively; the mean values are 76.0% and

374

13.3% for the pore sizes < 10 nm and 10-20 nm, respectively. Furthermore, the

375

proportions of pore volume range between 27.5% and 44.3% for pore sizes inferior to

376

10 nm (with an average of 34.8%), and between 18.5% and 23.8% for pore sizes

377

10-20 nm (with an average of 21.2%). Interestingly, pores less than 20 nm in width

378

account for approximately 90 and 50% of the total surface area and total pore volume,

379

respectively. During maturation, pores less than 10 nm wide experience the most

380

change. Finally, the correlation between maturation and surface area of pores less than

381

10 nm in width is opposite to that of larger pores (Figure 8c and d).

382

5. Conclusions

383

During heating, the number of micropores increases, which results in a

384

significant increase in the surface area. Isotherm and hysteresis loops indicate that

385

samples contain nanoscale pores as well as micropores, mesopores, and macropores.

386

The number of open pores significantly increases when maturity is greater than 2.5%.

387

Pores between 1.5 and 7.5 nm and between 60 and 70 nm wide are most sensitive to

388

pyrolysis. Liquid hydrocarbons are produced in pores approximately 41 nm wide. In

389

the over-mature stage (Ro = 2.77%), the distribution of pore structure does not vary;

390

yet, the number of pores and pore volume significantly increases, which indicates that

391

the pore structure of the over-mature shale is different from that of the mature and

392

high maturity shales. Micropores and mesopores are the main contributors to surface

393

area; the latter is the main contributor to the total pore volume. Moreover, pores less

394

than 20 nm wide nearly provide 90% of the total surface area and exceed 50% of the

ACS Paragon Plus Environment

Page 18 of 43

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

395

total pore volume. The transformation of organic matter is notably related to

396

mesopores. The proportions of surface area and pore volume for pores less than 10

397

nm show the most change after pyrolysis. Finally, pores less than 10 nm in width

398

change in size according to thermal maturation, while larger pores change in an

399

opposite manner.

400

Acknowledgements

401

The authors would like to give sincere thanks to the funding agencies that

402

supported this research. This work was supported by the National Natural Science

403

Foundation of China (No. 41772141; No. 41402124), the Fundamental Research

404

Funds for the Central Universities (2017CXNL03), and the Priority Academic

405

Program Development of Jiangsu Higher Education Institutions (PAPD).

406

References

407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

(1) (2) (3) (4) (5)

(6)

(7) (8)

(9)

Curtis, J. B. Fractured Shale-Gas Systems. AAPG Bull. 2002, 86 (11), 1921–1938. Hill, D. G.; Lombardi, T. E.; Martin, J. P. Fractured Shale Gas Potential in New York. Annual Conference- Ontario Petroleum Institute; 2002. Lemos, W. Shale Gas Revolution. Icis Chem. Bus. 2011, 280 (15), S24. Fang, H.; Zou, H. Y.; Lu, Y. C. Mechanisms of Shale Gas Storage: Implications for Shale Gas Exploration in China. AAPG Bull. 2013, 97 (8), 1325–1346. Xiao, X. M.; Song, Z. G.; Zhu, Y. M.; Tian, H.; Yin, H. W. Summary of Shale Gas Research in North American and Revelations to Shale Gas Exploration of Lower Paleozoic Strata in China South Area. J. China Coal Soc. 2013, 38 (5), 721–727(7). Ross, D. J. K.; Bustin, R. M. Shale Gas Potential of the Lower Jurassic Gordondale Member, Northeastern British Columbia, Canada. Bull. Can. Pet. Geol. 2007, 55 (1), 51– 75. Chen, S. B. Study Review on Microstructure and Adsorption Heterogeneity of Shale Reservoir. Coal Sci. Technol. 2016, 44 (6), 23–32. Tang, X. L.; Jiang, Z. X.; Jiang, S.; Li, Z. Heterogeneous Nanoporosity of the Silurian Longmaxi Formation Shale Gas Reservoir in the Sichuan Basin Using the QEMSCAN, FIB-SEM, and Nano-CT Methods. Mar. Pet. Geol. 2016, 78, 99–109. Tian, H.; Li, T. F.; Zhang, T. W.; Xiao, X. M. Characterization of Methane Adsorption on Overmature Lower Silurian–Upper Ordovician Shales in Sichuan Basin, Southwest China:

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469

(10)

(11)

(12)

(13)

(14) (15) (16)

(17)

(18)

(19)

(20)

(21)

(22)

(23) (24)

Experimental Results and Geological Implications. Int. J. Coal Geol. 2016, 156, 36–49. Zhao, W. Z.; Li, J. Z.; Yang, T.; Wang, S. F.; Huang, J. L. Geological Difference and Its Significance of Marine Shale Gases in South China. Pet. Explor. Dev. Online 2016, 43 (4), 547–559. Zou, C. N.; Yang, Z.; Pan, S. Q.; Chen, Y. Y.; Lin, S. H.; Huang, J. L.; Wu, S. T.; Dong, D. Z.; Wang, S. F.; Liang, F. Shale Gas Formation and Occurrence in China: An Overview of the Current Status and Future Potential. Acta Geol. Sin. Ed. 2016, 90 (4), 1249–1283. Hu, H. yan; Hao, F.; Lin, J. feng; Lu, Y. chao; Ma, Y. quan; Li, Q. Organic Matter-Hosted Pore System in the Wufeng-Longmaxi (O3w-S11) Shale, Jiaoshiba Area, Eastern Sichuan Basin, China. Int. J. Coal Geol. 2017, 173, 40–50. Li, J. J.; Yin, J. X.; Zhang, Y. N.; Lu, S. F.; Wang, W. M.; Li, J. B.; Chen, F. W.; Meng, Y. L. A Comparison of Experimental Methods for Describing Shale Pore Features — A Case Study in the Bohai Bay Basin of Eastern China. Int. J. Coal Geol. 2015, 152, 39–49. Reed, R. M.; Loucks, R. G. Imaging Nanoscal Pores in the Mississippian Barnett Shale of the Northern Fort Worth Basin. Am. Assoc. Pet. Geol. Annu. Conv. 2007, 16, 155. Reed, R. M.; Loucks, R. G. Low-Thermal-Maturity (