Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Comparison of Factors Influencing Pore Size Distributions in Marine, Terrestrial, and Transitional Shales of Similar Maturity in China Wenjing Mao†,‡ and Shaobin Guo*,†,‡ †
School of Energy Resource, China University of Geosciences (Beijing), Beijing 10083, People’s Republic of China Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, Beijing 10083, People’s Republic of China
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ABSTRACT: This paper compares pore size distributions in different sedimentary facies shales for similar maturity in typical areas, China. Pore size distributions were analyzed using low-pressure gas adsorption, mercury intrusion capillary pressure, and field-emission scanning electron microscopy techniques. Results showed that pore volume and surface area of shales clearly covary with sedimentary facies, total organic carbon (TOC) contents, clay minerals, maturity, and kerogen type. Influenced by the compaction and clay minerals, the immature terrestrial Jurassic Yanan shales exhibited the highest pore volume and surface area, which are much higher than those of overmature transitional Permian Shanxi and marine Longmaxi shales. The pore volume of shales mainly consists of meso- and macropores, whereas micro- and mesopores comprise most of the surface area. For similar immature shales, pore size distributions are mainly influenced by the degree of compaction. For similar overmature shales, pore size distributions are mainly influenced by kerogen type and TOC content.
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INTRODUCTION A significant majority of the shale gas in the United States is of marine origin.1−3 Relative to the United States, shale gas in China is unique, as it occurs mostly in marine, terrestrial, and transitional strata.4−6 In comparison to other natural gas resources, shale gas is mainly stored as free or adsorbed gas in pores. Therefore, studies about the pore structure of shale have been heavily reported in the literature.7−9 The International Union of Pure and Applied Chemistry (IUPAC) classifies pores according to size as micropores (50 nm). Yang et al.10 used nitrogen (N2) adsorption techniques to study pore size distributions in shales among different sedimentary facies. Results showed that the surface area and pore volume of the overmature marine shales are much higher than those of the highly mature transitional and less mature terrestrial shales. However, liquid N2 temperatures (−196 °C) can affect micropore detection. Klaver et al.11 used broad ion beam polishing and high-resolution scanning electron microscopy (BIB−SEM) and mercury injection capillary pressure (MICP) to analyze the pore characteristics of Germany’s Posidonia shale. Differences between porosity inferred by MICP and BIB−SEM are interpreted to the fact that mercury intrusion can dissolve barriers between unconnected micropores. Curtis et al.12 used the focused ion beam−scanning electron microscopy (FIB−SEM) method to describe changes in the organic matter pore of Woodford shale samples across a maturation gradient. The lack of porosity in the highly mature shale sample suggests that thermal maturity alone is insufficient at predicting porosity and other factors, such as total organic carbon (TOC) content, also influence pore development. Even within the same sample, basic petrographic analysis can reveal variation in organic matter pore distributions as a result of the differences in macerals. Mastalerz et al. 13 performed quantitative analysis of pore development in five New Albany shale samples spanning a maturity from immature to © XXXX American Chemical Society
overmature and indicated that pore size distributions are almost entirely restricted to maturity and mineralogical composition. Different from the results in some previous studies, no significant relationship between TOC contents and pore volume means that the shale samples were subjectively selected and large differences in maturity obscure the relatively minor role of TOC contents affecting porosity. Most studies have focused on shale pores in a sedimentary environment.14−16 Given this gap in understanding, this paper analyzes shales from the terrestrial Jurassic Yanan, transitional Permian Shanxi (both from the Ordos Basin), and the marine Silurian Longmaxi (of the Sichuan Basin) to compare their pore size distributions. A range of analytical methods, including low-pressure gas adsorption, MICP, and fieldemission scanning electron microscopy (FE-SEM), are used to describe and quantify the pore structure for shale samples among different sedimentary facies. The study also interprets how TOC and mineral contents influence the pore space with maturity similar to the control variable. Comparing pore size distributions for different sedimentary environments from key areas of China can help constrain shale reservoir evaluations and support shale gas development in China.
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GEOLOGICAL SETTING AND SHALE SAMPLES The samples analyzed here consisted of shales in marine, transitional, and terrestrial depositional environments. Ordos Basin samples were collected from wells J601 and ZK15-1 sampling the Jurassic Yanan Formation as well as from well Y88 sampling the Permian Shanxi Formation. The Sichuan Basin sample came from well PY1 sampling the Longmaxi Formation (Figure 1). Received: April 19, 2018 Revised: June 15, 2018 Published: June 25, 2018 A
DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Map showing the locations of shale samples in the Ordos Basin and Sichuan Basin, China. varied. Tables 1 and 2 list basic experimental data. Shale samples from J601-1, Y88-1, and PY1-1 were selected for FE-SEM imaging.
Marine shales in China deposited in the Early Paleozoic. The Sichuan Basin is located in southwestern China and hosts seven sets of organic-rich shales.17−19 The Lower Silurian Longmaxi shales are formed in a deep marine shelf depositional environment, with the thickness of 20−150 m, and occur at subsurface depths of less than 4000 m. The marine Silurian Longmaxi shales exhibit a relatively high organic matter content and brittle lithologic properties, which are beneficial to hydrocarbon generation of shale gas. Transitional shales occurred primarily in the Carboniferous−Permian of northern China and the Permian Longtan Formation of southern China. The delta front−coastal marsh environment resulted in rapid changes in lithofacies and thin monolayers interbedded with coal and dense sandstone or limestone.20−22 Terrestrial shales deposited during the Mesozoic−Cenozoic in the Ordos, Tarim, Bohai Bay, and Junggar Basins of northern China and part of the Sichuan Basin of southern China. Previous studies have shown that organic-rich terrestrial shales can form thick layers and host a complex mixture of organic matter types. These factors create abundant reservoir space and optimize preservation to create favorable conditions for shale gas enrichment.23−25 The Jurassic Yanan Formation in the Ordos Basin is a typical deep to semi-deep lacustrine deposit. The coal-bearing stratum consists of gray, gray and white, fine-grained sandstone and dark gray siltstone as well as black shales and coal seams. Maturity is relatively low.
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Table 1. Depth, Kerogen Type, TOC Content, and Maturity of Shale Samples from Different Sedimentary Facies sedimentary facies marine Silurian Longmaxi shale transitional Permian Shanxi shale terrestrial Jurassic Yanan shale
sample ID
depth (m)
kerogen type
TOC (wt %)
Ro (%)
PY1-1 PY1-2 PY1-3 Y88-1 Y88-2 Y88-3 J601-1 J601-2 ZK15-1
2084.3 2125.16 2139.23 2397.15 2400.05 2427.16 1134.63 1149.53 438.8
I I I III III III III III III
1.77 0.841 2.85 0.969 2.1 1.901 2.516 2.764 3.093
3 3.4 3.64 2.63 2.64 2.68 0.5 0.52 0.47
The marine Silurian Longmaxi shale samples are comprised of type I kerogen, with an average TOC value of 1.82 wt % and an average maturity of 3.35%. The transitional Permian Shanxi shales are comprised of type III kerogen, with an average TOC value of 1.65 wt % and an average maturity of 2.65%. The terrestrial Jurassic Yanan shales are comprised of type III kerogen, with an average TOC content of 2.79 wt % and an average maturity of 0.50%. Sample data show that the TOC content and maturity vary with different sedimentary facies shales. The transitional Permian Shanxi and terrestrial Jurassic Yanan shales include more clay minerals. The marine Silurian Longmaxi shales contain a small amount of carbonate minerals (Figure 2). Pore Size Distribution. The pore size distributions of shale pores were measured by different techniques with N2 adsorption to obtain information about mesopores (2−50 nm), whereas carbon dioxide (CO2) and MICP were used to characterize micropores (50 nm), respectively. MICP analysis was performed using an Autopore IV9500 instrument. Measured values ranged from 3 nm to 1000 μm with injection/ejection volume precision of less than 0.1 μL. The pore size
METHODOLOGY
Sample Material. The TOC content, maturity, and mineral composition of the samples were initially determined to select three sets of shale samples among different sedimentary facies. These were analyzed for pore size distributions using low-pressure gas adsorption and MICP. Each shale sample of the same sedimentary facies showed similar maturity and kerogen type, but TOC and mineral composition B
DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. Mineral Composition of Shale Samples from Different Sedimentary Facies mineral composition (wt %) sedimentary facies marine Silurian Longmaxi shale
transitional Permian Shanxi shale
terrestrial Jurassic Yanan shale
sample ID
clay
quartz
PY1-1 PY1-2 PY1-3 Y88-1 Y88-2 Y88-3 J601-1 J601-2 ZK15-1
45 23 26 55 57 52 55 64.1 55
41 49 48 33 39 42 33 12.2 32
K-feldspar 1 4 3 1 2 4 7.1 4
plagioclase
calcite
dolomite
pyrite
6 15 8 1 2 1 7 16.6 9
3 3 7
2 5 4
2 1 4
siderite
10 5 1
Figure 2. Mineralogical ternary diagram summarizes the composition of shale samples from different sedimentary facies.
Table 3. Micro-, Meso-, and Macropore Volume of Shale Samples from Different Depositional Environments pore volume (cm3/g)
pore volume ratio (%)
sample ID
total pore
micropore
mesopore
macropore
micropore
mesopore
macropore
PY1-1 PY1-2 PY1-3 Y88-1 Y88-2 Y88-3 J601-1 J601-2 ZK15-1
0.0165 0.0083 0.0160 0.0288 0.0170 0.0119 0.0434 0.0418 0.0627
0.0018 0.0009 0.0015 0.0001 0.0014 0.0019 0.0021 0.0022 0.0024
0.0071 0.0036 0.0037 0.0083 0.0084 0.0045 0.0288 0.0289 0.0550
0.0077 0.0038 0.0108 0.0203 0.0072 0.0055 0.0126 0.0107 0.0053
10.72 10.51 9.25 0.51 7.99 15.95 5.43 5.43 3.92
42.66 43.46 23.10 28.78 49.49 37.69 75.52 68.58 87.43
46.62 46.04 67.64 70.71 42.53 46.36 19.04 25.99 8.64
was calculated using the Washburn equation.26 Shale samples were vacuum-treated and dehydrated prior to analysis. Shale samples were cut into 3−10 mm sized cubes, vacuum-treated, and dehydrated prior to analysis. Low-pressure ( illite− smectite mixed layer > kaolinite > chlorite > illite.5,37−39 The two clay mineral types with the least volume, kaolinite and chlorite, are far lower than that of illite and illite−smectite mixed layer. As a result of less illite and more illite−smectite mixed layer content, the transitional Permian Shanxi shales should exhibit the higher surface area than that of the terrestrial Jurassic Yanan shales, which is different from the result in our study. This correlation likely arises from the large F
DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 7. Relationship between clay minerals, TOC content, and surface area: (a) total pore, (b) micropore, (c) mesopore, and (d) macropore.
Figure 8. Comparison of (a) pore volume and (b) surface area among different sedimentary facies shales.
area of the marine Silurian Longmaxi shale pores exceeds that of the transitional Permian Shanxi shales.
The marine Silurian Longmaxi shales are comprised of type I kerogen. Algal content promotes organic matter pore development.10,33 Along with the higher maturity level, hydrocarbon generation will produce a large number of pores, which gradually connect to form larger pores (Figure 5b). At the same time, compaction affects layers; meso- and macropores are filled by feldspar and other minerals (Figure 5a).12 The pore volume of the transitional Permian Shanxi shales therefore exceeds that of the marine shale. The inner surface of clay mineral pores is generally simple and smooth, while the inner surface of organic matter pores is complex, with many folds or bulbous protrusions. The surface
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CONCLUSION
The pore volume of the immature terrestrial Jurassic Yanan shale is in the range of 0.0418−0.0627 cm3/g, which is much higher than that of the overmature transitional Permian Shanxi and marine Silurian Longmaxi shales in ranges of 0.0117− 0.0288 and 0.0083−0.0165 cm3/g, respectively. The surface area of terrestrial Jurassic Yanan shale is in the range of 18.17− 27.12 m2/g, which is much higher than that of the overmature transitional Permian Shanxi and marine Silurian Longmaxi G
DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 9. (a) Pore volume and (b) ratio in shale samples from different depositional environments.
Figure 10. (a) Surface area and (b) ratio in shale samples from different depositional environments.
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ACKNOWLEDGMENTS This study received support from the National Science and Technology Major Project of China (Grant 2016ZX05034). The authors thank three anonymous reviewers for their comments and suggestions.
Table 5. Average Clay Composition among Shales from Different Depositional Environments average clay composition (%) sedimentary facies
kaolinite
chlorite
illite
I/S
%S
marine Silurian Longmaxi shale transitional Permian Shanxi shale terrestrial Jurassic Yanan shale
20 43
15 8 11
66 21 11
19 51 35
7 15 65
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shales in ranges of 4.81−10.73 and 5.19−10.13 m2/g, respectively. The pore volume of the shale samples consists mainly of meso- and macropores, while micro- and mesopores account for most of the surface area. Among micro-, meso-, and macropores, the relative contributions to the pore volume and surface area vary with sedimentary facies. Pore distributions are influenced by the kerogen type, TOC content, maturity, and clay minerals. For immature shales, pore development is mainly affected by the limited degree of compaction rather than the clay minerals or TOC content. For overmature shales, kerogen type and TOC content promote pore development.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Shaobin Guo: 0000-0003-3391-8806 Notes
The authors declare no competing financial interest. H
DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.8b01373 Energy Fuels XXXX, XXX, XXX−XXX
