CSUG/SPE 138975
Imaging Texture and Porosity in Mudstones and Shales: Comparison of
Secondary and Ion-Milled Backscatter SEM Methods
M. Milner, R. McLin, J. Petriello, TerraTek
Copyright 2010, Society of Petroleum Engineers
This paper was prepared for presentation at the Canadian Unconventional Resources & International Petroleum Conference held in Calgary, Alberta, Canada, 19–21 October 2010.
This paper was selected for presentation by a CSUG/SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been
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restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract
Observations from a number of unconventional reservoirs lead us to conclude that four major pore types exist
in fine-grained reservoir and non-reservoir rocks, that they are effectively connected, and that pore sizes from
nanometers to microns must be considered when evaluating size distributions. This paper uses SEM imaging of
Haynesville, Horn River, Barnett and Marcellus Shales to illustrate that pore types other than those hosted by organics
are present in unconventional shale gas reservoirs, and that they are continuous and connected to kerogen-hosted
pores. In addition, we present evidence that the maximum size of pores originating in organic matter is determined by
the size of the kerogen mass (in the case of organic particles) or the geometry of enclosing crystals (in the case of
amorphous, pore-filling kerogen). Pairs of secondary and ion-milled backscatter SEM images address the
misconception that large pores observed in secondary electron images are grain pullouts.
2-D image analysis and 3-D volumetric reconstructions to study pore distributions should take rock
microtexture and the various pore types into consideration. A combined method using thin section textural analysis,
XRD, and SEM imaging is recommended to address scaling issues when choosing samples for 2-D and 3-D
volumetric analysis.
Introduction
Petrologic and petrophysical work in a number of unconventional reservoirs leads us to conclusions similar to
those of Schieber (2010), namely that a number of pore types exist in both reservoir and non-reservoir rocks, that they
are effectively connected, and that pore sizes from nanometers to microns must be considered when evaluating size
distributions. In this paper we confirm and illustrate the relative importance of pore type, size, and arrangement in
examples from four North American gas reservoirs - Barnett Shale, Haynesville Shale, Horn River Shale, and
Marcellus Shale - using combined secondary and ion-milled backscatter imaging.
Attention is given to lithotypes, matrix texture and composition and their influence on organic distribution and
porosity. Whereas some previous work suggests that the organic material makes up most of the gas pore volume, and
that gas calculation can be made considering only pores less than 1 micron (1000 nm) in size (Ambrose, et al., 2010),
the work presented here supports a broader range of effective pore types and sizes, with evidence of probable
conductivity between types. In addition we address the misconception that large pores observed in secondary electron
images are grain pullouts (Loucks et al. 2009; Sondergeld et al. 2010). Ion-milled images suggest that the factors
controlling maximum pore size are the size of the organic mass, or the geometry of the largest intercrystalline void (in
the case of amorphous, pore-filling kerogen).
2 CSUG/SPE 138975 Methods
Samples examined for this paper were cut from whole core collected from gas-productive reservoir rock of the
Jurassic Haynesville Shale, Devonian Horn River Shale, Mississippian Barnett Shale, and Devonian Marcellus Shale.
Two preparation techniques were used for SEM imaging. Secondary electron images represent fresh, minimally gold-
coated surfaces broken normal to bedding. Backscatter images show milled surfaces approximately 1.0 mm by 0.5
mm, created using a JEOL IB-9010 Argon-ion polisher. Both secondary and milled backscatter samples were imaged
using an FEI NovaNano 630 field emission scanning electron microscope. Identification of mineral components in both
methods was confirmed using concurrent energy dispersive X-ray analysis (EDX), performed using an EDAX Genesis
instrument.
XRD data were generated from powdered samples analyzed with a Rigaku Ultima III X-ray diffractometer, and
results were interpreted using JADE software. Thin sections were cut to a standard thickness and stained with a
mixture of potassium ferricyanide and Alizarin Red ‘S’ to aid in identification of carbonate minerals. Petrophysical
measurements including porosity (% of bulk volume), gas-filled porosity and pressure-decay permeability to gas
reported in this paper were produced using a modified GRI Method (Luffel et al. 1993; J. Keller, verbal
communication). Geochemical measurements include weight percent total organic carbon (TOC) measured using
industry standard procedure in a LECO EC-12 or LECO CS-444 carbon analyzer at about 1,000° C; and RockEval II
pyrolysis. Ro values for Marcellus and Barnett samples are calculated from RockEval measurements (Jarvie, 2001),
or in the case of Haynesville samples, vitrinite reflectance analysis was conducted in accordance with ASTM D 2798
specifications.
Pore Types in Selected Unconventional Gas Reservoirs
Studies of numerous gas shale reservoirs suggest that matrix intercrystalline, organic, and intraparticle pores
are the most common pore classes observed under scanning electron microscopy (SEM). Dissolution pores are
found in all mudstones in minor amounts. Both secondary and ion-milled backscatter SEM images of fine-grained
reservoir rocks allow examination of these pore types. Matrix intercrystalline pores are voids between clay flakes and
aggregates, cement crystals, and larger detrital particles; organic pores range from tiny pores in kerogen masses to
larger voids left by alteration of organic particles; dissolution pores result from diagenetic alteration of grains; and
intraparticle pores form connected networks between nanofossil fragments coated with amorphous kerogen (primarily
in fecal pellets). The first three of these are equivalent to phyllosilicate framework (PF) pores, organic matter (OM)
pores, and carbonate dissolution (CD) pores described by Schieber (2010). To these we add a fourth class,
intraparticle porosity, which is unique and volumetrically important in some reservoirs. Intraparticle porosity occurs as
interconnected networks contained primarily in fecal pellets.
The three most common pore classes are identified in Figures 1, 2 and 3. We find the fourth class, dissolution
pores, to be present only in minor amounts in the formations examined. SEM images of this pore type are well
represented elsewhere (Schieber, 2010). Intergranular pores associated with grain-supported laminae in mudstones
are not considered in this paper, but are significant contributors to mudstone and shale porosity where they occur.
[Figures 1, 2, and 3: SEM images illustrating the four most common pore classes observed in gas reservoirs.]
CSUG/SPE 138975
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4 CSUG/SPE
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In rocks where intercrystalline pores are the predominant type, pore size and shape depends on the mineralogy,
geometry and arrangement of adjacent particles. The Jurassic Haynesville Shale example in Figure 1 A, B and C
illustrates a case where intercrystalline pores are dominant. Note that porous kerogen fills some voids (A and B, white
arrows). Compaction, original grain structure, and depositional texture all play a role in determining this fine-scale
arrangement of minerals; however, diagenetic overprints in both clays and other components supply a secondary
control (see also Schieber, 2010).
CSUG/SPE 138975 5
In Figure 1 A quartz silt grains (qtz), calcareous particles (cal), pyrite framboids (py), and organic lenses (o)
form wavy horizontally laminated microtexture slightly disturbed by bioturbation. Figure 1B is a backscatter detail of
the pore system showing intercrystalline pores (black arrows) between illitic clay flakes, adjacent silt grains and calcite
cement crystals. Ker = kerogen blebs, white arrow = intercrystalline pore with kerogen lining, cc = oblique view of a
calcareous coccolith plate. Scale bars inside the kerogen mass at center left measure three organic pores as 0.2, 0.5
and 0.5 microns in size. Figure 1 C is a secondary electron image at approximately the same scale as B. In this case,
the ion-milled backscatter image (B; black arrows) confirms that the flattened voids between clays (C; ixl) are
intercrystalline pores rather than plucked grains (Loucks, 2009; Sondergeld et al. 2010). At center in image C is a
coccolith wheel with a 2 micron diameter central cavity.
In contrast to the Haynesville sample, pores in a typical siliceous mudstone from the Devonian Horn River
Shale (Figure 1, D through F) are mainly of the kerogen-hosted organic type. The matrix is composed of quartz grains
and crystals of various sizes and minor clays, with a less laminated microtexture. Blocky, euhedral quartz crystals and
isolated illitic clay flakes in E and F contrast with darker and spongy textured interstitial kerogen (Images E and F).
Organic pores in kerogen masses (E) are less than one micron in size.
A siliceous mudstone from the Mississippian Barnett Shale (Figure 2 A, B and C) contains varied detrital and
diagenetic particles (A). As in each row, the two leftmost images are ion-milled surfaces viewed under backscatter
SEM. Gray-scale variations in backscatter images reflect contrasts in elemental atomic number (Z-contrast), making
grayscale a proxy for mineralogy. Iron in pyrite or ferroan dolomite shows up as bright spots; clays, quartz cement and
silt grains are lighter shades of gray similar in color. Organic matter has a low Z-contrast and appears as dark gray;
calcite has a higher Z-contrast than illitic clays or quartz and appears lighter gray. Most porosity in the mudstone is of
the kerogen-hosted organic type, although intercrystalline pores are also represented (pores between clays, top part of
B). From these images, most organic pores measure in the tens of nanometers, whereas intercrystalline voids up to a
micron are visible in B. Considering the close proximity of the two pore types, it seems reasonable to expect in a gas-
charged system that both types contain gas. The secondary electron image (C) highlights the rough, spongy kerogen
morphology, indicating gas-window thermal maturity.
Argillaceous mudstone of the Devonian Marcellus Shale is represented in Figure 2 D, E and F. Lenses and
larger particles of organic matter (dark) align with bedding and define sub-millimeter scale lamination (LO type
lamination; Core-Based Mudstone and Shale Classification: A Method for Petrologic Evaluation of Unconventional Gas
Reservoirs, Milner et. al. 2010, in preparation). Alteration of these particles and masses provides an example of a
larger subtype of organic pore. Images E and F, at approximately the same magnification, are details of large altered
organic particles and associated organic pores. In parts of the Marcellus the maximum pore size is determined by the
sizes of the kerogen masses or organic particles and the geometry of enclosing illitic clays and quartz crystals. Pores
of this type up to ten microns in size have been observed. The light-colored crystals at center in E are authigenic
quartz. The secondary electron image (F) shows the rough, granular morphology characteristic of thermally altered
kerogen.
Figure 3 A, B and C show another calcareous/argillaceous mudstone from the Haynesville Shale, this one
dominated by intraparticle porosity. The calcareous fecal pellet outlined in A is enlarged in B and C, and shows
disarticulated coccolithophorid algae plates cemented in an interlocking framework. These pores, individually less
than 1 micron in size, create an interconnected network with high effective porosity. Rectangular, biogenic calcite
crystals in pellets (B) commonly exhibit films of amorphous kerogen. The secondary electron image at the same scale
shows disarticulated coccolith plates extensively coated with nanoporous kerogen.
It is possible with these observations to make some general statements about pore types, their size ranges,
and occurrences. Table 1 below summarizes observations from Figures 1 through 3. Compared with earlier studies of
pore sizes (Nelson 2009; Wang et. al. 2009), images presented here show maximum sizes significantly larger than
those interpreted for shales.
6 CSUG/SPE 138975 [Table 1]
Pore Type Characteristics
voids between clay flakes,
aggregates, cement
crystals, and detrital
particles Size Range less than 1 micron to 5 microns Image Ref Occurrence in Mudstones and Shales matrix intercrystalline Fig1 A,B, C; Fig 2 B (top) dominant in low TOC rocks and mature shales where organics are absent or converted
pervasive in moderate to high TOC systems with gas
window maturity; rocks with finely disseminated,
degraded algal kerogen; abundance increases with
thermal maturity
systems with particulate, lenticular or matty organic
matter and gas window or higher maturity
common in facies with fecal pellets; concentrations of
coccolithophorid algae
grains, microfossils and pellets
laminae in silty or sandy mudstones, and tight gas
siltstones/sandstones; compacted burrows,
bioturbated zones organic (kerogen-hosted) organic (masses and particles) intraparticle dissolution tiny pores in kerogen masses larger voids left by alteration of organic particles connected networks between kerogen-coated fragments voids left by mineral dissolution pores in grain-supported layers, not addressed in this paper 10 nm to 2 micron 50 nm to 10 microns 3 nm to 2 microns 5 microns to 200 microns 10 to 200 microns Fig 1 B, D, E, F; Fig 2 A, B (bottom), C Fig 2 D, E, F Fig 3 A, B, C not shown intergranular not shown
Table 1. Pore Types, Sizes and Characteristics as Observed from Selected Unconventional Gas Reservoirs
CSUG/SPE 138975 7
Rock Microtexture Considerations
The SEM images in Figures 1 through 3 illustrate the relationship between sub-millimeter scale texture and
porosity. Due to the extremely small sample size in SEM work (generally less than 0.5 mm), sampling and correct
representation of larger areas are critical when upscaling results (Sondergeld et. al. 2010). A classification of sub-
millimeter scale texture using thin sections is a logical next step. Figures 4 and 5 present thin section and backscatter
SEM pairs representing common reservoir lithotypes in the same formations examined above.
[Figures 4 and 5]
8 CSUG/SPE
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Each of the thin sections from the Haynesville (Figure 4 A), Horn River Shale (Figure 4 C), Barnett Shale
(Figure 5 A) and Marcellus Shale (Figure 5 C) exhibit textures dependent on mineral composition, grain size, and
organic matter. These microtextures are described in the panels at right of the respective images, along with a two-
letter abbreviation for data collection purposes (Core-Based Mudstone and Shale Classification: A Method for
Petrologic Evaluation of Unconventional Gas Reservoirs, Milner et. al. 2010, in preparation). Wide variations in texture
at thin section scale, mostly related to lamination styles, make careful selection of SEM samples important in order to
assure the best fabric representation. Thin sections provide a method of selection as well as a context for
interpretation of SEM images.
Sub-millimeter scale textural details, when combined with petrophysical and other data (panels to the right of
each pair) provide useful insights into how gas reservoir rocks behave and produce. Key observations from the
sample set presented here are as follows.
• Figure 4 A and B: thin laminae composed of anastomosing pellets (stained pink in A) form surfaces of connected
microporosity in certain pellet-rich lithotypes.
• Figure 4 C and D: nanoporous kerogen distributed in siliceous matrix forms a horizontal to almost boxwork network
of connected micropores.
• Figure 5 A and B: organic matter in Barnett siliceous mudstone is highly dispersed and intimately associated with
matrix authigenic quartz. However, unlike the Horn River sample, the mineral composition is more variable, with
larger grain size and thus a secondary pore network of matrix intercrystalline pores.
• Figure 5 C and D: texture in Marcellus organic argillaceous mudstone is more strongly influenced by aligned clay
fabric than other reservoir lithotypes. The Marcellus samples contain organic matter in larger lenses and particles
than other reservoir rocks, and thus have potentially larger pore sizes in thermally mature facies (up to 10
microns).
CSUG/SPE 138975 9
• In general, lithotypes dominated by matrix intercrystalline pores or larger pores left by organic masses, have higher
porosity and permeability values (i.e., Haynesville and Marcellus Shales).
Relative Importance of Pore Types
Secondary and backscatter SEM images indicate that matrix intercrystalline porosity, defined as voids
between clay flakes and aggregates, cement crystals and larger detrital particles, is the ubiquitous pore type in
mudstones and shales. Where organic porosity is highest (as in the reservoir lithotypes chosen for this paper),
kerogen occupies these voids, as well as forming lenses, stringers, and particles. The greatest amount of open matrix
intercrystalline porosity is present where total organic carbon is low, where minimally compacted matrix contains finely
disseminated grain-coating kerogen, or where thermal maturation has converted most of the kerogen leaving open
voids (organic pores).
A qualitative assessment of the relative abundances of pore types in the four shale gas formations is offered in
Table 2 below. Such an assessment is necessarily subjective and is biased by the availability of reservoir rocks.
However, based on many more than the few images presented in this paper, it is our opinion that these relative
abundances apply in general to the main reservoir lithotypes for the four gas reservoirs listed. In Table 2, pore types
are ranked 1 through 4, from most to least abundant as observed in SEM images.
It should be recognized that changes in the thermal maturity, texture, or mineralogy of the rocks will alter the
observed ranking. Thus, the table is a generalization with many exceptions. As always, it is critical to make
appropriate observations on the reservoir rock to be assessed.
[Table 2]
Formation Dominant Reservoir
Lithotype
matrix intercrystalline
Pore Type Rank organic (kerogen‐hosted) organic (masses and particles) intraparticle
dissolution
intergranular Haynesville calcareous/argillaceous mudstone 1 3 ~ 2 minor 4 Horn River siliceous mudstone 2 1 ~ ~ minor ~ Barnett siliceous/argillaceous mudstone 2 1 4 ~ minor ~ Marcellus argillaceous mudstone 3 2 1 ~ minor 4
Table 2: Relative Abundance of Pore Classes in Selected Gas Reservoirs.
Table 2: Relative Abundance of Pore Classes in Selected Gas Reservoirs. Pore types are ranked by relative
abundance in the main reservoir lithotype named for the four formations compared. 1 = most abundant, 4 = least
abundant, ~ = not observed.
Conclusions
Four major pore types are imaged in four well-known and productive unconventional gas shales via secondary
and ion-milled backscatter SEM. Several pore types occur in close proximity or overlap in type, and therefore can
reasonably be assumed to be connected. The Haynesville, Horn River, Barnett and Marcellus Shales show pore sizes
ranging from nanometers to microns in size depending on matrix microtexture, grain size, mineralogy, organic type and
thermal maturity.
If observed pore sizes are up to 10 microns and the various pore types are connected, 2-D and 3-D models of
shale porosity based on focused ion beam (FIB) SEM cubes with dimension from less than 1 micron to 5 microns
(Sondergeld et. al. 2010, Ambrose et. al. 2010) may not capture and represent all porosity in the rocks. 2-D image
10 CSUG/SPE 138975 analysis and 3-D volumetric reconstructions to study pore distributions should take rock microtexture and the various
pore types into consideration. A combined method using thin section textural analysis and SEM imaging is
recommended to address scaling issues when choosing samples for volumetric analysis.
Secondary electron imaging of broken samples allows assessment of particle and crystal shapes in 3
dimensions, and provides evidence of the condition (maturity) of organic matter. While pores are visible in broken
samples, quantification of size and type is problematic, and intercrystalline pores can be difficult to distinguish from
artifacts. The polished surfaces in ion-milled backscatter SEM allow better pore characterization and introduce the
possibility of image analysis for quantification, if sampling and scale obstacles can be overcome.
Acknowledgements
The authors recognize William Zagorski and Martin Emery of Range Resources, David Spain of BP America, and Ann
Cochran of Encana Canada for permission to publish the images and data presented here, as well as for their
continued input to discussions. Thanks are due to TerraTek-Schlumberger for supporting this work, and to Kelly
Vaughn, John Keller, Dave Handwerger, Tim Sodergren, and our many other colleagues for their ideas and insights.
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