International Journal of Coal Geology xxx (xxxx) xxx–xxx

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International Journal of Coal Geology

journal homepage: www.elsevier.com/locate/coal

Identification and nanoporosity of macerals in coal by scanning electron

microscopy
Brian J. Cardotta,⁎, Mark E. Curtisb
a
Oklahoma Geological Survey, Mewbourne College of Earth and Energy, University of Oklahoma, Norman, OK 73019, USA
b
Mewbourne School of Petroleum and Geological Engineering, Mewbourne College of Earth and Energy, University of Oklahoma, Norman, OK 73019, USA

A R T I C L E I N F O A B S T R A C T

Keywords: Recent applications of scanning electron microscopy (SEM) to shale resource plays at magnifications of <
Coal 500– > 80,000 × have reported nanoporosity in organic matter with limited interpretations of organic matter
Maceral type. Macerals, inclusive of kerogen and solid bitumen, are recognized and distinguished in reflected white and
Nanoporosity epifluorescent light in coal and shale samples at magnifications of 200–750 ×. The objectives of this study are to
Scanning electron microscopy
identify macerals by SEM and evaluate which macerals contain primary and secondary nanoporosity. Since coals
Broad ion beam
are organic rich with a better chance of identifying adjacent maceral types than when dispersed in shales, broad
ion beam milled samples of humic and sapropelic (boghead and cannel) coals ranging in rank from peat to
semianthracite were examined in backscattered electron (BSE) mode at low magnification (≤2,500 ×) to
identify maceral type. Once identified, macerals were examined at higher magnifications of 1200–75,000 × to
assess maceral nanoporosity.
Manipulation of the accelerating voltage to 10 kV in BSE mode of a high volatile bituminous humic coal
durain lithotype sample revealed a contrast between maceral groups (vitrinite, inertinite, liptinite), with limited
identification of individual maceral types. Vitrinite maceral subgroups telovitrinite and detrovitrinite are dis-
tinguished based on their relative gray scale appearance compared to other macerals and occurrence as bands or
groundmass, respectively. The liptinite macerals alginite, sporinite and cutinite are distinguished based on dark
relative gray level and their shape. The liptinite maceral bituminite/amorphinite was recognized by dark relative
gray level and occurrence as groundmass in a boghead coal. The inertinite macerals fusinite and semifusinite are
recognized by light gray level appearance compared to other macerals and bogen structure but are not distin-
guishable separately. Macerals dispersed in shale, lacking the subtle contrast of adjacent macerals, are much
more difficult to identify.
Even though porosity is revealed at high magnification in BSE mode, too high of a magnification
(> 15,000 ×) prohibits identification of maceral types. The best approach is to examine samples at a lower
magnification (e.g., 650 ×) at 10 kV accelerating voltage in BSE mode to identify the maceral type and then go
to a higher magnification at 1–2 kV accelerating voltage to observe nanoporosity.
Primary nanoporosity is observed within coal macerals at low rank (peat and subbituminous), but decreases
in amount with increasing rank. Primary microporosity occurs as woody cell lumens in semifusinite and fusinite
macerals. Secondary nanoporosity develops in post-oil solid bitumen in shale beginning below the peak of the oil
window with a lack of nanoporosity at lower thermal maturity. Compared to the abundant nanoporosity of post-
oil solid bitumen in shale, only trace amounts of nanoporosity is observed in other macerals in coals of high
volatile bituminous rank and higher under the SEM.
The emphasis of this study was the identification and nanoporosity of macerals in coal by SEM. The same
results may extend to the same macerals in shale. Knowledge of organic matter porosity distribution by maceral
type and development by thermal maturity provides insight for coalbed methane, shale gas and tight oil pro-
duction potential.

⁎
Corresponding author at: 100 E. Boyd St., Rm. N-131, Norman, OK 73019-0628, USA.
E-mail addresses: bcardott@ou.edu (B.J. Cardott), mcurtis@ou.edu (M.E. Curtis).

http://dx.doi.org/10.1016/j.coal.2017.07.003
Received 28 February 2017; Received in revised form 12 June 2017; Accepted 5 July 2017
0166-5162/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Cardott, B.J., International Journal of Coal Geology (2017), http://dx.doi.org/10.1016/j.coal.2017.07.003
B.J. Cardott, M.E. Curtis International Journal of Coal Geology xxx (xxxx) xxx–xxx

1. Introduction Table 1
Classification of macerals in sedimentary rocks (modified from Stasiuk
et al., 2002). Check mark indicates maceral is identifiable in SEM.
The production of natural gas from coal (i.e., coalbed methane) and
natural gas and oil from shale (i.e., shale gas and tight oil) has re-
volutionized the U.S. petroleum industry in recent years. All hydro-
carbon reservoirs require porosity and permeability to store and pro-
duce oil and gas. In unconventional resource plays (coal and shale),
where the hydrocarbon source rock also is the reservoir, porosity is
most prevalent in the organic matter in organic-rich, siliceous shales
(Loucks et al., 2012; Dong et al., 2017). Scanning electron microscope
(SEM) studies have demonstrated that some organic matter is porous
while others are nonporous at the same thermal maturity (Curtis et al.,
2012). The difficulty in recognizing organic matter types in the SEM
limits the understanding of which types are porous or nonporous
(Hackley and Cardott, 2016).
Organic matter is complex and its classification depends on the
sample type and how it is observed. Organic matter composition varies
by rock type (e.g., coal and shale), depositional environment (e.g.,
humic coal vs. sapropelic coal; marine vs. nonmarine shale), and rank,
among other variables. Taylor et al. (1998, p. 242–243) discussed
several petrographic classifications for organic matter in rocks under
reflected and transmitted light, including the categories of maceral,
kerogen, phytoclast, organoclast, and palynofacies. The maceral clas-
sification was developed for coal and applied later to dispersed organic
matter in shales which contain additional macerals not found in humic
coal (e.g., amorphous organic matter/bituminite/amorphinite, solid
bitumen, alginite, zooclasts). The maceral classification of coal, primary
dispersed organic matter, and solid bitumens in Potter et al. (1998,
their tables 2 and 4) was used in this study. A condensed version of the
maceral classification, modified from Stasiuk et al. (2002), is in Table 1.
All petrographic methods of organic matter observation have lim-
itations. Different sample preparation methods limit maceral identifi-
cation in coal and shale samples under the light microscope. For ex-
ample, polished whole rock coal and shale samples are routinely viewed
in reflected white and epifluorescent light to identify vitrinite, liptinite,
and inertinite macerals, but limit the identification of amorphous or-
ganic matter (AOM). Shale strew slides viewed in transmitted white and
epifluorescent light are optimum to observe AOM and liptinite mac-
erals, but lack a polished surface to identify opaque vitrinite and in-
ertinite macerals. Similarly, the SEM has limitations in the extent that
macerals may be recognized.
Macerals (i.e., all solid organic matter inclusive of kerogen and solid materials can be made in which carbon-rich organic matter appears
bitumen) are recognized and distinguished in reflected white and epi- dark gray while mineral matter displays lighter gray levels under
fluorescent light at magnifications of 200–750 × using 20–50 × oil backscattered electron (BSE) microscopy. Stanton and Finkelman
immersion objectives and 10-15 × oculars (ASTM, 2016). Recent ap- (1979) correlated macerals identified in an optical photomap with
plications of SEM to shale resource plays at magnifications of < 500× identical fields in secondary electron (SE) and BSE images. While
to > 80,000 × have reported nanoporosity in organic matter, with macerals could not be distinguished in SE images, exinite macerals
limited interpretations of organic matter type (e.g., Sondergeld et al., appeared darker than vitrinite and mineral-filled cell lumens dis-
2013; Milliken et al., 2014; Chen and Jiang, 2016). Pore development tinguished fusinite from vitrinite in BSE images. Davis et al. (1986)
by maceral type and thermal maturity is important in evaluating shale examined maceral concentrates of collinite (vitrain), sporinite (durain),
reservoirs for oil and gas (Milliken et al., 2013, 2014). The objectives of and fusinite (fusain) in SE images with limited interpretations of mac-
this study are to evaluate the extent to which macerals may be re- eral types. Lallier-Verges et al. (1991) recognized collotelinite in vitrites
cognized in SEM, which macerals contain primary and secondary por- and collodetrinite in bi- and trimaceral microlithotypes in ultrathin
osity, and at what thermal maturity levels secondary porosity develops. sections of two high volatile C bituminous humic coals by SEM and
TEM.
2. Previous literature Terminologies used for organic matter in shales under SEM include
the general term “organic matter” (Loucks et al., 2009, 2010, 2012;
Early applications of the transmission electron microscope to spe- Curtis et al., 2013; Camp and Wawak, 2013; Er et al., 2016; Zhou et al.,
cialized ultrathin sections of coal distinguished vitrinite, liptinite (e.g., 2016), kerogen and solid bitumen (Bernard et al., 2012, 2013; Chalmers
sporinite, resinite), and inertinite (e.g., micrinite, semifusinite) mac- et al., 2012; Milliken et al., 2013; Cardott et al., 2015; Li et al., 2016)
erals at high magnification (1400–122,500 ×) in appearance much and the related yet different terms “detrital organic matter”, “secondary
different than in the optical microscope (McCartney et al., 1966; Taylor, organic matter”, “depositional organic matter”, and “migrated organic
1966). Based on atomic number contrast, a relative comparison of

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Table 2
Coal samples used in this study (location, type and rank).

OPL no.a Coal bed/location Type Huminite/vitrinite reflectance (% VRo) Rankb

1499 unknown/Ireland Humic peat

1486 Deadman/Wyoming Humic 0.41 subbituminous
106 Santo Tomas/Texas Cannel 0.53 hvCb
1010 Breckinridge/Kentucky Boghead 0.55 hvCb
657D unknown/Kentucky Humic/durain lithotype 0.72 hvAb
1183 Iron Post/Oklahoma Humic 0.58 hvBb
1474 Hartshorne/Oklahoma Humic 0.91 hvAb
1473 Hartshorne/Oklahoma Humic 1.06 hvAb
1437 Stigler/Oklahoma Humic 1.25 mvb
1384 Lower Hartshorne/Oklahoma Humic 1.28 mvb
1470 Lower Hartshorne/Oklahoma Humic 1.67 lvb
1241 Lower Hartshorne/Oklahoma Humic 2.22 sa

a
Oklahoma Geological Survey Organic Petrography Laboratory (OPL) sample number.
b
hvCb, high volatile C bituminous; hvBb, high volatile B bituminous; hvAb, high volatile A bituminous; mvb, medium volatile bituminous; lvb, low volatile bituminous; sa, semi-
anthracite.

matter” (Milliken et al., 2014; Loucks and Reed, 2014; Pommer and importance of organic-matter pores (Loucks et al., 2012; Milliken et al.,
Milliken, 2015). 2013; Loucks and Reed, 2014). Several pore classifications have been
In a landmark paper applying BSE and SE to shales, Loucks et al. proposed for shales; each classification includes an organics category
(2009) reported that intraparticle organic nanopores were the pre- (Loucks et al., 2010, 2012; Slatt and O’Brien, 2011, 2014; Bernard
dominant pore type in siliceous mudstones of the Barnett Shale. Sub- et al., 2013; Loucks and Reed, 2014; Er et al., 2016; Ko et al., 2016;
sequent research by this group and others have confirmed the Milliken and Curtis, 2016; Schieber et al., 2016; F. Yang et al., 2016b).

A B

C D

Fig. 1. SEM BSE images showing poor contrast of macerals (1 kV accelerating voltage). Macerals (organic matter) are dark gray and mineral matter is light gray. A. Medium volatile
bituminous Lower Hartshorne coal showing poor contrast between macerals (OPL 1384; 1091 ×). B. Detrovitrinite (maceral collodetrinite) occurring in matrix of clarain lithotype and
telovitrinite (maceral collotelinite) occurring in vitrain lithotype. High volatile B bituminous Iron Post coal (OPL 1183; sample milled with a focused ion beam). C. Primary porosity in cell
lumens in fusain lithotype (fusinite/semifusinite is indeterminate; 1067×). D. Same field of view as C at higher magnification (6500 ×).

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A B

C D

Fig. 2. Maceral groups differentiated by change in accelerating voltage. High volatile A bituminous durain lithotype (OPL 657D). A. Initial view at 2 kV accelerating voltage showing poor
contrast of macerals. B. Higher magnification (650×) of A showing maceral groups are differentiated at 20 kV accelerating voltage: liptinite macerals appear dark gray to black, vitrinite
macerals appear medium gray, inertinite macerals appear light gray. C. Macerals appear less sharp at 20 kV accelerating voltage due to greater penetration depth of the electron beam. D.
Same field as C showing optimum contrast of macerals at 10 kV accelerating voltage [liptinite macerals (cutinite and sporinite) appear black distinguished by structure; vitrinite macerals
appear medium gray].

Porosity prevalence varies by organic and mineral content and thermal and sapropelic (boghead and cannel) coal samples (Taylor et al., 1998,
maturity (e.g., some clay minerals are more porous than organics; Table 5.1) ranging in rank from peat to semianthracite were examined
Curtis et al., 2012; Sondergeld et al., 2013). Jennings and Antia (2013) in reflected white light and SEM to identify macerals and nanoporosity
noted a lack of organic nanopores in solid organic particles (e.g., (Table 2). The two basic signals in SEM utilize secondary electron and
kerogen) and solid bitumen at thermal maturity < 0.7% VRo (vitrinite/ backscattered electron detectors. Secondary electron (SE) images high
huminite reflectance) in the Eagle Ford Formation. Erdman and resolution sample topography. Backscattered electron (BSE) images
Drenzek (2013) concluded that mineral-based pores are more important reveal organic matter (darker gray) and mineral matter (lighter gray)
than organic-based pores in low-maturity shale plays. Lőhr et al. (2015) by atomic number contrast. As explained below, BSE was preferred to
examined the occurrence of solid bitumen-filling pores in shales and identify maceral type and view nanoporosity. Herein, the term nano-
reported organic pores in AOM and alginite in immature (< 0.4% VRo) porosity is used for pores < 1 μm and microporosity is used for
samples, an absence of organic matter-hosted pores in oil window pores > 1 μm.
(0.5–1.1% VRo) Woodford Shale samples, and AOM-hosted pores (i.e., The coal samples were first examined in reflected white light to
pyrobitumen) in gas-mature (> 1.5% VRo) samples. In an attempt to assign rank based on measured huminite/vitrinite reflectance and to
evaluate the nature of organic-associated pores in shale in SEM, Yang identify maceral types present. Freshly polished whole-rock-particulate
et al. (2016a) erroneously reported vitrinite-reflectance values from pellets, prepared by binding rock particles with cold-setting epoxy in a
Cambrian- and Silurian-age samples, and attributed Type I kerogen and 1 in. diameter mold, were examined on a Vickers M17 Research
alginite and exinite macerals to post-mature samples (> 2.0% VRo), Microscope system adapted for reflected white light in oil immersion
thus negating the maceral identification. and at a total magnification of 500 × following ASTM (2016).
For SEM imaging, rock-chip samples were mounted onto aluminum
3. Methods stubs using Crystalbond™ 509 as an adhesive. The mounted samples
were then mechanically polished with sandpaper at a progressively
Since coals are organic rich with a better chance of identifying ad- smaller grit size to 1500 grit, using water as a polishing fluid. The
jacent maceral types by SEM gray-scale contrast than in shales, humic polished samples were ion milled with argon using continuous

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A B

Fig. 3. A–C. Series of images showing increasing contrast of macerals through manipulation of SEM brightness and contrast controls using the same detector (CBS) and accelerating
voltage (10 kV). Medium volatile bituminous Stigler coal (OPL 1437).

rotational motion in a Fischione 1060 SEM Mill for 3 h at 5 kV with an lighter gray to white and organic matter is darker gray based on atomic
incidence angle of 2°. To reduce charging effects in the SEM, samples number contrast. Identification of maceral types under SEM is depen-
were minimally coated with Au/Pd using a Denton Vacuum Desk V dent on image magnification and contrast. Magnifications in the range
sputter coating system. The coating layer was thin enough that it was of the light microscope (< 1000 ×) using BIB-milled samples enable
non-contiguous and composed of nano-islands only a few nanometers in identification of macerals. Much like the relative gray scale of macerals
diameter. Imaging was performed using a FEI Helios Nanolab dual recognized in reflected white light optical microscopy, gray-scale
beam Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) comparisons of macerals are possible under the SEM. Initial observa-
system. A concentric backscatter (CBS) detector was chosen for imaging tions of humic coal samples at low (1–2 kV) accelerating voltage SEM
to provide atomic number contrast. The accelerating voltage in the SEM settings in BSE mode at 1000 × magnification resulted in all macerals
was altered depending on the information desired with 2 kV accel- having nearly the same gray level with no contrast (Fig. 1A). At best,
erating voltage primarily used to image porosity in the samples whereas vitrinite maceral subgroups telovitrinite (maceral collotelinite) and
10 kV was used to increase contrast for differentiating between mac- detrovitrinite (maceral collodetrinite) were distinguishable based on
erals. occurrence as bands (vitrain lithotype) or matrix (clarain lithotype),
FIB milling cuts an area 10s of microns in width, while broad ion respectively, in humic coals (Fig. 1B). Inertinite macerals were re-
beam (BIB) milling polishes an area up to a few millimeters in diameter cognized based on bogen structure (arc-shaped fragments of preserved
(Klaver et al., 2015). BIB milling was preferred in this study in order to cell walls; ICCP, 2001) in a mechanically isolated, fusain-lithotype
view a larger area at magnifications typical for organic petrology. sample but could not be distinguished separately as a semifusinite or
fusinite maceral (Fig. 1C–D).
4. Results and discussion Durain is a humic coal lithotype that contains vitrinite, liptinite, and
inertinite maceral groups. Fig. 2A shows a poor contrast between
4.1. Maceral identification in coal by SEM maceral groups in the durain sample at an accelerating voltage of 2 kV
and magnification of 250×. Manipulation of the accelerating voltage to
SEM analysis is the best method to observe nanoporosity in organic 20 kV in BSE mode at a magnification of 650× revealed a distinct
matter; however, organic matter types are not easily distinguishable at contrast between maceral groups (Fig. 2B). However, an accelerating
low accelerating voltages of 1–2 kV. In BSE mode, mineral matter is voltage of 20 kV resulted in an unfocused appearance due to the

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A B

Fig. 4. A. Liptinite group macerals are dark gray. Slight difference in appearance between vitrinite and inertinite macerals. High volatile A bituminous durain lithotype (OPL 657D). B.
Good contrast between liptinite (sporinite) macerals (black) and vitrinite macerals (medium gray). High volatile C bituminous Santo Tomas cannel coal (OPL 106). C. Bituminite appears
dark gray in groundmass, Botryococcus alginite appears black, vitrodetrinite appears medium gray, and inertodetrinite appears light gray. High volatile C bituminous Breckinridge
boghead coal (OPL 1010).

increased penetration depth of the electron beam, so 10 kV was selected Semifusinite and fusinite macerals are distinguished from the vitrinite
as the optimal accelerating voltage (Fig. 2C, D). Similar to their ap- maceral mainly by having bogen structure and are distinguishable se-
pearance in reflected white light, liptinite macerals (e.g., sporinite and parately only by side-by-side contrast (Fig. 5A, B). Lacking bogen
cutinite) are distinguished based on dark relative gray level and shape, structure, macrinite/semimacrinite are difficult to distinguish from vi-
vitrinite macerals occur as a matrix maceral with a medium gray level, trinite.
and inertinite macerals have a lighter gray level in BSE mode. In ad- At semianthracite rank, contrast between vitrinite and inertinite
dition to changes in accelerating voltage, further manipulation of the macerals is further diminished, relying strictly on bogen structure,
SEM contrast may be made through differential amplification of the when present, for identification (Fig. 6A, B). Maceral group identifi-
signal waveform using the brightness and contrast controls on the SEM cation works well when the macerals are associated in a coal micro-
(Fig. 3A–C). By manipulating the waveform, the dynamic range of a lithotype in both humic and sapropelic coals, but are difficult to identify
particular region of the video signal (e.g. the organic matter) can be when dispersed in shale.
increased at the expense of clipping the waveform in the region that
carries contrast information about the inorganic minerals in the sample. 4.2. Maceral identification in shale by SEM
Amplification of the video signal does not itself create contrast between
macerals but merely enhances the contrast for observation. Maceral identification by SEM in shale samples is more problematic
Various macerals are illustrated in a high volatile bituminous durain since macerals are dispersed. A smooth curved edge and bogen struc-
lithotype (Fig. 4A) and cannel coal (Fig. 4B). The liptinite maceral bi- ture of inertodetrinite are the best criteria to distinguish inertinite from
tuminite often occurs as the groundmass in boghead coals (Fig. 4C) and vitrinite macerals in shales (Fig. 6C). Lacking these criteria and the
is equivalent to Type A AOM in the classification of Thompson and contrast of adjacent macerals, vitrinite and inertinite dispersed in shale
Dembicki (1986) and fluoramorphinite in the classification of Senftle are not distinguishable by gray level alone under SEM (C. Yang et al.,
et al. (1993). AOM is the most common type of maceral in hydrocarbon 2016a). Table 1 summarizes which macerals in coal and shale may be
source rocks (Thompson-Rizer, 1993) and may possibly be recognizable observed directly in SEM.
by SEM in low thermal maturity shales (Cardott et al., 2015). Several SEM studies on shale (e.g., Loucks et al., 2012; Loucks and

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A B

Fig. 5. Identification of inertinite macerals by SEM. Medium volatile bituminous Stigler coal (OPL 1437). A. Fusinite and semifusinite distinguishable when viewed side-by-side. B.
Inertinite macerals distinguished by light gray appearance but not further differentiated.

A B

Fig. 6. A. Reduced contrast of vitrinite and inertinite at semianthracite rank. Inertinite, with clay minerals filling cell lumens, is distinguished from vitrinite based only on bogen
structure. Semianthracite Lower Hartshorne coal (OPL 1241). B. Vitrinite and inertinite macerals are indistinguishable based only on gray level. Semianthracite Lower Hartshorne coal
(OPL 1241). C. Bogen structure indicates inertodetrinite in Woodford Shale sample (OPL 1481). Lacking bogen structure, inertodetrinite could not be distinguished from vitrodetrinite in
shale sample without contrast of adjacent macerals.

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A B C

D E

Fig. 7. A. High magnification (25,000 ×) prevents maceral identification. Medium volatile bituminous Lower Hartshorne coal (OPL 1384). B. 10 kV accelerating voltage shows good
contrast between maceral groups. High volatile C bituminous Santo Tomas cannel coal (OPL 106). C. Same field of view as B at 1 kV accelerating voltage. Macerals are no longer
distinguishable and all macerals lack nanoporosity. D. Porosity along scratches in vitrinite induced during sample polishing. Low volatile bituminous Lower Hartshorne coal (OPL 1470).
E. Same field of view as D at 2 kV accelerating voltage showing induced nanoporosity along scratches.

B C D

Fig. 8. Induced vs. lack of nanoporosity in vitrinite. Medium volatile bituminous Stigler coal (OPL 1437). A. Polished pellet in reflected white light (500 ×) showing pits in vitrinite
induced during pellet polishing. B. 10 kV accelerating voltage showing contrast of macerals (750×). C–D. Vitrinite at higher magnifications (3500 × 15,000 ×) showing lack of
nanoporosity within the vitrinite maceral confirming that microporosity in reflected white light image was induced.

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A B

Fig. 9. Primary nanoporosity in huminite macerals at low rank. A. Texture seen in huminite macerals in peat at 10 kV accelerating voltage (OPL 1499). B. Same field of view as A at 2 kV
accelerating voltage showing primary nanoporosity in less distinct huminite macerals. C. Primary nanoporosity in huminite in fractured subbituminous coal (OPL 1486).

Reed, 2014; Milliken et al., 2013, 2014) used 10 kV accelerating vol- maceral types (Fig. 7A). Furthermore, nanoporosity is masked by the
tage with limited interpretations of organic matter type. In order to size of the interaction volume at high accelerating voltage (10 kV;
correctly identify the macerals in shale, identification must first be Belin, 1992; Erdman and Drenzek, 2013). The best approach is to ex-
made with the reflecting light microscope and correlated to the same amine BIB-milled samples at a lower magnification (e.g., 650×) with a
field of view in the SEM. Belin (1994) used correlative microscopy solid state concentric backscatter (CBS) detector at 10 kV accelerating
(transmitted, reflected white, epifluorescence, BSE) on non-covered voltage, identify the maceral type, and then go to higher magnification
thin sections of immature shales to better identify organic particles. at 1 or 2 kV accelerating voltage to look for porosity in BSE or SE mode.
Valentine and Hackley (2016) correlated macerals identified in photo BSE mode was used in this study. Fig. 7B shows maceral contrast at
mosaics from a reflecting light microscope to the same field of view in 10 kV accelerating voltage, while Fig. 7C shows the same field at 1 kV
SEM (images for the Bakken Formation are available at https://energy. showing no maceral contrast and no porosity in the macerals.
usgs.gov/Coal/OrganicPetrology/PhotomicrographAtlas/ Coal in general, and vitrinite in particular, is brittle. Scratches and
BakkenFormationShaleGallery.aspx). A more direct approach to mac- pits (erroneously interpreted as porosity) may be introduced during
eral identification in SEM is the application of a specialized sample mechanical sample polishing for both light microscope and SEM sam-
holder (e.g., Zeiss Shuttle & Find, not available for this study) used in a ples (Pommer and Milliken, 2015). Fig. 7D-E shows induced porosity
reflected light microscope and then transfer of the sample holder for (i.e., pits) especially along scratches in vitrinite under SEM. A digital
observing the same field in SEM. camera attached to a reflecting light microscope is more sensitive than
the human eye or 35 mm film. Manipulation of the exposure and con-
4.3. Porosity of macerals trast of the digital camera revealed pits (initially interpreted to be mi-
croporosity) in vitrinite and some inertinite macerals at 500× magni-
Knowledge of organic matter porosity distribution and development fication (Fig. 8A). However, examination of the same coal sample
by maceral type and thermal maturity provides insight for coalbed prepared for SEM by ion milling did not contain the same micro-
methane and shale gas and oil production potential (Giffin et al., 2013). porosity, revealing that the microporosity observed in the light micro-
Even though porosity is revealed at high magnification in SEM, too scope was induced during mechanical sample polishing (Fig. 8B-D).
high of a magnification (> 15,000 ×) prohibits identification of A rank series from peat to semianthracite illustrates porosity in the

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A B C

D E

Fig. 10. A–C. Primary nanoporosity is sparse in vitrinite revealed at 1 kV accelerating voltage (1848 × 10,000 × 75,000 ×). High volatile A bituminous Hartshorne coal (OPL 1473). D–E.
Primary microporosity is common in fusinite/semifusinite while primary nanoporosity within cell walls is sparse as revealed in same field at 2 kV accelerating voltage. High volatile A
bituminous Hartshorne coal (OPL 1474).

maceral groups. Nanoporosity observed in macerals at low rank are Milliken, 2015). Compared to the abundant nanoporosity of post-oil
determined to be primary, consistent with the conclusions of Lőhr et al. solid bitumen in mature shale, only trace amounts of nanoporosity are
(2015). Fig. 9A–B shows primary porosity in peat; the 10 kV image observed under SEM in other macerals at all coal ranks. It should be
shows texture better within the huminite, while the 2 kV image shows noted that some sub-nanometer voids could exist which are below the
the porosity. Fig. 9C shows primary porosity within fractured sub- resolution of the SEM imaging; however, when the size of the void is on
bituminous coal huminite macerals. Primary microporosity occurs as the atomic level, the definition of what void spaces constitute a pore is
open woody cell lumens in bituminous rank semifusinite and fusinite brought into question.
macerals observed in both reflected light and SEM (Fig. 1D). Primary Graptolites occur in Cambrian to Pennsylvanian-age rocks and ap-
nanoporosity is sparse yet somewhat clustered within vitrinite pear similar to vitrinite in the reflecting light microscope (Cardott and
(Fig. 10A–C; similar to the results of Giffin et al., 2013) and inertinite Kidwai, 1991). Luo et al. (2016) described abundant nanoporosity from
macerals (Fig. 10D–E; excluding primary microporosity in cell lumens) high thermal maturity (3.08–4.29% equivalent vitrinite reflectance)
in a high volatile A bituminous coal. Slitted structure in pseudovitrinite Silurian-age, non-granular graptolites. Ma et al. (2016) described por-
is an early oxidation product and is considered to be primary porosity, osity associated with the fine structure in graptolite periderm in over-
while tertiary fractures (i.e., cleat) are secondary porosity. Primary mature samples. The abundance of graptolites and relatively minor
nanoporosity becomes sparser with increasing rank. Baisheng et al. contribution from other types of organic matter made it possible to
(2015) related a decrease in mesopores (2–50 nm in diameter) and recognize the organic matter as graptolites in FIB-SEM. Sparse primary
increase in micropores (< 2 nm in diameter) with increasing rank due nanoporosity occurs in low thermal maturity graptolites (Fig. 12C-D).
to burial compaction. Lack of primary nanoporosity within macerals
occurs in medium volatile bituminous (Fig. 11A–B) and low volatile 5. Conclusions
bituminous coals (Fig. 11C–D). Induced and sparse primary nano-
porosity occur in vitrinite in semianthracite coal (Fig. 12A–B). Recent applications of scanning electron microscopy (SEM) to or-
AOM and post-oil solid bitumen network in shale are identifiable ganic matter in shales have revealed porosity development in organic
under SEM by occurrence as organic groundmass or filling interparticle matter with limited identification of organic matter type. Organic
pores, respectively (Cardott et al., 2015). Under SEM of shales, sec- matter in coals and shales is easily distinguishable from mineral matter
ondary nanoporosity is observed in post-oil solid bitumen beginning as by atomic number contrast at low accelerating voltage (1–2 kV) in the
early as 0.7% VRo (Loucks et al., 2010; Curtis et al., 2012; Bernard backscattered electron mode (BSE), but weakly distinguishable to or-
et al., 2013; Jennings and Antia, 2013; Cardott et al., 2015; Pommer ganic matter type. However, adjacent macerals (solid organic matter
and Milliken, 2015; Reed and Loucks, 2015; Klaver et al., 2016; Misch inclusive of kerogen and solid bitumen) in coals are distinguishable to
et al., 2016). Primary organic matter pores are sparse or nonexistent in the group level, with limited identification to the maceral type level,
lower thermal maturity shales (Loucks et al., 2012; Pommer and under the SEM at low magnification (650–1000 ×) using a solid state

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B.J. Cardott, M.E. Curtis International Journal of Coal Geology xxx (xxxx) xxx–xxx

A B

C D

Fig. 11. A–B. Maceral groups are identified at 10 kV accelerating voltage with lack of nanoporosity revealed in same field at 2 kV accelerating voltage (5000 ×). Medium volatile
bituminous Stigler coal (OPL 1437). C–D. Vitrinite and inertinite macerals are identified at 10 kV accelerating voltage with lack of nanoporosity revealed in same field at 2 kV accelerating
voltage (5000×). Low volatile bituminous Lower Hartshorne coal (OPL 1470).

concentric backscatter (CBS) detector and high accelerating voltage voltage at higher magnification. A coal rank series from peat to semi-
(10 kV) on broad ion beam (BIB) milled samples. Relative to each anthracite illustrated primary nanoporosity primarily in huminite
maceral group under BSE mode, liptinite group macerals appear dark macerals in peat and subbituminous coals, sparse in vitrinite and in-
gray to black, inertinite group macerals appear light gray, and vitrinite ertinite macerals (excluding primary microporosity in semifusinite/fu-
group macerals appear medium gray in coals. Identification of dis- sinite cell lumens) in high volatile bituminous coal, and rare in all
persed organic matter in shales is more problematic, lacking the con- macerals at higher ranks. Although not observed in coal, secondary
trast of adjacent macerals. nanoporosity develops in a post-oil solid bitumen network in shale
Of the three maceral groups, liptinite macerals are most easily dis- beginning below the peak of the oil window. Vitrinite-like graptolites in
tinguishable by their relative dark gray to black appearance and shape low thermal maturity Ordovician-age rocks appear relatively medium
(alginite, bituminite/amorphinite, sporinite, and cutinite). Within the gray in BSE with sparse primary nanoporosity.
vitrinite maceral group, collotelinite may be distinguished from collo- The ultimate solution to identifying organic matter in shale by SEM
detrinite in humic coals by occurrence as monomaceralic bands (vitrain would be to use correlative microscopy to first identify the organic
lithotype) versus matrix (clarain lithotype), respectively. Bogen struc- matter in reflected light and then to view the same field in the SEM.
ture of semifusinite and fusinite macerals aid in distinguishing in-
ertinite macerals from vitrinite macerals, yet are not identifiable to
maceral type. Bituminite/amorphinite in sapropelic coal and post-oil Acknowledgements
solid bitumen in shale are identifiable under SEM by occurrence as
organic groundmass or filling voids, respectively. We thank David Fukuyama for assisting in the preparation and
The best approach to identify porosity in macerals is to first identify imaging of samples. We thank Paul Hackley and two anonymous re-
maceral type at low magnification (< 1000 ×) at 10 kV accelerating viewers for a critical review of this article.
voltage using a solid state concentric backscatter (CBS) detector on BIB-
milled samples and then observe porosity at 1–2 kV accelerating

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B.J. Cardott, M.E. Curtis International Journal of Coal Geology xxx (xxxx) xxx–xxx

A B

vitrinite

vitrinite

C D

Fig. 12. A–B. Primary and induced nanoporosity is sparse in vitrinite (3500 × 15,033 ×). Semianthracite Lower Hartshorne coal (OPL 1241). C–D. Nanoporosity is sparse in graptolite
(7500 × 17,500 ×). Lower Ordovician Polk Creek Shale, 0.66% graptolite reflectance (OPL 644).

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