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file:v2M102Ch1Fg1.jpg|{{figure number|1}}Schematic diagram of the GEMINI^&trade; electron column (a) and the electron beam path in the column (b).<ref name=Huangetal_2013>Huang, Jason, Timothy Cavanaugh, and Boaz Nur, 2013, An introduction to SEM operational principles and geologic applications for shale hydrocarbon reservoirs, ''in'' W. Camp, E. Diaz, and B. Wawak, eds., Electron Microscopy of Shale Hydrocarbon Reservoirs: [http://store.aapg.org/detail.aspx?id=1197 AAPG Memoir 102], 260 pp.</ref>

file:Figure-1.jpg|{{figure number|1}}Schematic diagram of the GEMINI^&trade; electron column (a) and the electron beam path in the column (b).<ref name=Huangetal_2013>Huang, Jason, Timothy Cavanaugh, and Boaz Nur, 2013, An introduction to SEM operational principles and geologic applications for shale hydrocarbon reservoirs, ''in'' W. Camp, E. Diaz, and B. Wawak, eds., Electron Microscopy of Shale Hydrocarbon Reservoirs: [http://store.aapg.org/detail.aspx?id=1197 AAPG Memoir 102], 260 pp.</ref>

file:M102Ch1Fg2.jpg|{{figure number|2}}Profile view of a typical specimen-electron interaction volume. Note that secondary electrons (SE) originate much shallower in the sample than backscattered electrons (BSE) and therefore provide more surface-driven and less composition-influenced information.<ref name=Huangetal_2013 />

file:Figure-2.jpg|{{figure number|2}}Profile view of a typical specimen-electron interaction volume. Note that secondary electrons (SE) originate much shallower in the sample than backscattered electrons (BSE) and therefore provide more surface-driven and less composition-influenced information.<ref name=Huangetal_2013 />

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[[:file:v2M102Ch1Fg1.jpg|Figure 1a]] shows an example schematic diagram of a state-of-the-art electron column. The column shown here is the Zeiss GEMINI™ column. The ray path of the electrons is illustrated in Figure 1b. Specifically, electrons are emitted from a very fine heated filament with a high electrostatic extraction potential. The electrons are then immediately accelerated to a desired kinetic energy and propagate down the electron column. In the GEMINI column, a very simple two-lens system is employed to focus the electron beam. The upper lens, or the condenser lens, first demagnifies the emitted electrons to some extent. Next, some of the electrons are cut off by an aperture. The focusing strength of the condenser lens and the size of the aperture determine the beam current in the final spot. A final objective lens assembly then focuses the beam down to the desirable spot size. Depending on manufacturer design, the lenses used in the column can be magnetic or electrostatic (or a combination of the two types). In the case of the GEMINI column, the final objective lens is a combination of magnetic and electrostatic lenses. The main advantage of such a design is that there is virtually no magnetic field on the specimen that could distort the imaging if the specimen is magnetic.

[[:file:Figure-1.jpg|Figure 1a]] shows an example schematic diagram of a state-of-the-art electron column. The column shown here is the Zeiss GEMINI™ column. The ray path of the electrons is illustrated in Figure 1b. Specifically, electrons are emitted from a very fine heated filament with a high electrostatic extraction potential. The electrons are then immediately accelerated to a desired kinetic energy and propagate down the electron column. In the GEMINI column, a very simple two-lens system is employed to focus the electron beam. The upper lens, or the condenser lens, first demagnifies the emitted electrons to some extent. Next, some of the electrons are cut off by an aperture. The focusing strength of the condenser lens and the size of the aperture determine the beam current in the final spot. A final objective lens assembly then focuses the beam down to the desirable spot size. Depending on manufacturer design, the lenses used in the column can be magnetic or electrostatic (or a combination of the two types). In the case of the GEMINI column, the final objective lens is a combination of magnetic and electrostatic lenses. The main advantage of such a design is that there is virtually no magnetic field on the specimen that could distort the imaging if the specimen is magnetic.

The earliest work describing the concept of SEM is that of Knoll in 1935.<ref name=Goldsteinetal_2003>Goldstein, J., D. E. Newbury, D. C. Joy, C. E. Lyman, P. Echlin, E. Lifshin, L. Sawyer, and J. R. Michael, 2003, Scanning electron microscopy and x-ray microanalysis, 3rd ed.: New York, Springer, 695 p.</ref> In his work, the focused electron beam was generated to image a metal plate with a transmission signal. The first SEM used to examine thick specimens was described by Zworykin at the RCA Laboratories in 1942.<ref name=Goldsteinetal_2003 /> Everhart and Thornley improved on Zworykin’s original design by employing a scintillator to convert the electrons to light, which was then transmitted by a light pipe directly to a photomultiplier. The Everhart-Thornley detector greatly improves the signal-to-noise ratio of SEM imaging. The first commercial instrument was built in 1963 by Pease at Cambridge Scientific Instruments.<ref name=Goldsteinetal_2003 />  

The earliest work describing the concept of SEM is that of Knoll in 1935.<ref name=Goldsteinetal_2003>Goldstein, J., D. E. Newbury, D. C. Joy, C. E. Lyman, P. Echlin, E. Lifshin, L. Sawyer, and J. R. Michael, 2003, Scanning electron microscopy and x-ray microanalysis, 3rd ed.: New York, Springer, 695 p.</ref> In his work, the focused electron beam was generated to image a metal plate with a transmission signal. The first SEM used to examine thick specimens was described by Zworykin at the RCA Laboratories in 1942.<ref name=Goldsteinetal_2003 /> Everhart and Thornley improved on Zworykin’s original design by employing a scintillator to convert the electrons to light, which was then transmitted by a light pipe directly to a photomultiplier. The Everhart-Thornley detector greatly improves the signal-to-noise ratio of SEM imaging. The first commercial instrument was built in 1963 by Pease at Cambridge Scientific Instruments.<ref name=Goldsteinetal_2003 />  

==SEM contract mechanisms==

==SEM contract mechanisms==

Several types of electrons are generated as the result of the energetic bombardment of the specimen by the primary beam ([[:file:M102Ch1Fg2.jpg|Figure 2]]). All of these electrons carry distinct structural information about the sample and differ from one another in origin, energy, and traveling direction. For example, type I secondary electrons (SE1) emit with a high angle at a close proximity from the impact point and therefore carry high-resolution, surface-sensitive (topographic) information of the sample. Type II secondary electrons (SE2), however, are generated from a deeper and wider volume than the SE1 and reflect at a lower angle, therefore carrying intrinsically lower-resolution topographical information. Similarly, singly scattered backscattered electrons (BSE1) tend to emit at a high angle and are closely linked to compositional contrast, while multiply scattered BSE (BSE2) take off at a lower angle and are used to characterize composition and crystalline structures of a sample. Photons can also be generated from the excitation by the primary electron beam. For example, x-rays with a continuous spectrum in energy (bremsstrahlung) are generated as a result of the deceleration of the electrons. Additionally, characteristic x-ray lines are generated from electron excitation within specimen atoms that interact with the primary electron beam. The latter form of x-ray is particularly useful in chemical analysis with SEM.

Several types of electrons are generated as the result of the energetic bombardment of the specimen by the primary beam ([[:file:Figure-2.jpg|Figure 2]]). All of these electrons carry distinct structural information about the sample and differ from one another in origin, energy, and traveling direction. For example, type I secondary electrons (SE1) emit with a high angle at a close proximity from the impact point and therefore carry high-resolution, surface-sensitive (topographic) information of the sample. Type II secondary electrons (SE2), however, are generated from a deeper and wider volume than the SE1 and reflect at a lower angle, therefore carrying intrinsically lower-resolution topographical information. Similarly, singly scattered backscattered electrons (BSE1) tend to emit at a high angle and are closely linked to compositional contrast, while multiply scattered BSE (BSE2) take off at a lower angle and are used to characterize composition and crystalline structures of a sample. Photons can also be generated from the excitation by the primary electron beam. For example, x-rays with a continuous spectrum in energy (bremsstrahlung) are generated as a result of the deceleration of the electrons. Additionally, characteristic x-ray lines are generated from electron excitation within specimen atoms that interact with the primary electron beam. The latter form of x-ray is particularly useful in chemical analysis with SEM.

Photons with wavelengths in the ultraviolet, visible, and infrared light regions of the electromagnetic spectrum can also be excited by the primary electrons. This phenomenon is called cathodoluminescence (CL). The generation of CL is commonly associated with the presence of certain trace impurities in minerals and therefore is also widely used in geological applications. A few examples are illustrated next to demonstrate several common contrast mechanisms useful for the geological study of shales.

Photons with wavelengths in the ultraviolet, visible, and infrared light regions of the electromagnetic spectrum can also be excited by the primary electrons. This phenomenon is called cathodoluminescence (CL). The generation of CL is commonly associated with the presence of certain trace impurities in minerals and therefore is also widely used in geological applications. A few examples are illustrated next to demonstrate several common contrast mechanisms useful for the geological study of shales.

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file:M102Ch1Fg3.jpg|{{figure number|3}}A shale sample imaged using SE1 signal (left) and SE2 signal (right). Surface-specific information such as pore space and surface roughness is evident in the SE1 image. The SE2 image has more compositional influence, displaying organic matter (OM) bodies that are not evident in the SE1 image.<ref name=Huangetal_2013 />

file:Figure-3.jpg|{{figure number|3}}A shale sample imaged using SE1 signal (left) and SE2 signal (right). Surface-specific information such as pore space and surface roughness is evident in the SE1 image. The SE2 image has more compositional influence, displaying organic matter (OM) bodies that are not evident in the SE1 image.<ref name=Huangetal_2013 />

file:M102Ch1Fg4.jpg|{{figure number|4}}SE2 (a) and BSE1 (b) image of a cross section of a shale rock. Note that the contrast between carbonate (ca) and silica (si) grains is much higher in BSE1; the topographical information is greater in the SE2 image (OM-associated nanopores are not visible in BSE1).<ref name=Huangetal_2013 />

file:M102Ch1Fg4.jpg|{{figure number|4}}SE2 (a) and BSE1 (b) image of a cross section of a shale rock. Note that the contrast between carbonate (ca) and silica (si) grains is much higher in BSE1; the topographical information is greater in the SE2 image (OM-associated nanopores are not visible in BSE1).<ref name=Huangetal_2013 />

file:M102Ch1Fg5.jpg|{{figure number|5}}A BSE2 image of gold (Au) nanoparticles showing crystallographic contrast.<ref name=Huangetal_2013 />]]

file:M102Ch1Fg5.jpg|{{figure number|5}}A BSE2 image of gold (Au) nanoparticles showing crystallographic contrast.<ref name=Huangetal_2013 />]]

===SEM image examples===

===SEM image examples===

[[:file:M102Ch1Fg3.jpg|Figure 3]] shows an SE1 image of a shale sample. At low landing energy (less than 1 keV), the secondary electrons collected with the in-lens detector originate from the very surface of the sample. The SE1 signal is commonly used to image surface details at the highest resolution at the expense of compositional information. On the right side of Figure 3, the SE2 image of the same area as the SE1 image on the left shows the

[[:file:Figure-3.jpg|Figure 3]] shows an SE1 image of a shale sample. At low landing energy (less than 1 keV), the secondary electrons collected with the in-lens detector originate from the very surface of the sample. The SE1 signal is commonly used to image surface details at the highest resolution at the expense of compositional information. On the right side of Figure 3, the SE2 image of the same area as the SE1 image on the left shows the

effects of higher landing energy and deeper specimen interaction, including more compositional and less topographical information.

effects of higher landing energy and deeper specimen interaction, including more compositional and less topographical information.

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file:M102Ch1Fg8.jpg|{{figure number|8}}A secondary electron image of a shale sample with an EDS-derived mineral segmentation overlay. In the segmented region, blue = carbonate, green = clay minerals, yellow = quartz, pink = feldspar, white = pyrite, and gray = organic matter.<ref name=Huangetal_2013 />

file:Figure-8.jpg|{{figure number|8}}A secondary electron image of a shale sample with an EDS-derived mineral segmentation overlay. In the segmented region, blue = carbonate, green = clay minerals, yellow = quartz, pink = feldspar, white = pyrite, and gray = organic matter.<ref name=Huangetal_2013 />

file:M102Ch1Fg9.jpg|{{figure number|9}}At left, a schematic diagram of operation of an FIB-SEM system. The FIB sputters away a thin layer of the sample at a time, while the electron beam/detector system captures an image of each newly exposed surface. At right, a picture of a commercial FIB-SEM CrossBeam™ system, Auriga.<ref name=Huangetal_2013 />

file:Figure-9.jpg|{{figure number|9}}At left, a schematic diagram of operation of an FIB-SEM system. The FIB sputters away a thin layer of the sample at a time, while the electron beam/detector system captures an image of each newly exposed surface. At right, a picture of a commercial FIB-SEM CrossBeam™ system, Auriga.<ref name=Huangetal_2013 />

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To take advantage of the element-specific information caused by x-ray excitation, EDS (or EDX) can be performed. As the electron beam scans the sample surface pixel by pixel, a full x-ray spectrum can be acquired from each pixel. Elemental distribution can therefore be mapped using the relative peak intensity to build an image of the scanned area. This can then be interpreted to estimate mineral phase composition. [[:file:M102Ch1Fg8.jpg|Figure 8]] shows a secondary electron image of a shale sample with EDS-derived mineral segmentation of the same region.

To take advantage of the element-specific information caused by x-ray excitation, EDS (or EDX) can be performed. As the electron beam scans the sample surface pixel by pixel, a full x-ray spectrum can be acquired from each pixel. Elemental distribution can therefore be mapped using the relative peak intensity to build an image of the scanned area. This can then be interpreted to estimate mineral phase composition. [[:file:Figure-8.jpg|Figure 8]] shows a secondary electron image of a shale sample with EDS-derived mineral segmentation of the same region.

==Focused ion beam applications==

==Focused ion beam applications==

Focused ion beam (FIB) systems also find a growing number of applications in geology.<ref name=Goldsteinetal_2003 /> In a typical FIB-SEM system, an extraction field is applied to a gallium (Ga) liquid metal ion source to field emit Ga ions and form a Ga beam. Due to the higher atomic mass, the Ga beam not only can be used to generate electron and ion images, but also may be used to mill samples to remove material. [[:file:M102Ch1Fg9.jpg|Figure 9a]] shows the schematic diagram of an FIB-SEM system where a cross section of the sample is milled by a Ga FIB beam and is imaged simultaneously by the SEM. This milling and imaging process can be automated in a serial fashion to acquire a stack of two-dimensional images, from which a 3-D image volume can be constructed from the data set. This technique is particularly useful in revealing the 3-D distribution of mineral types, organic matter, porosity, and the like in shale (and other rock) samples.

Focused ion beam (FIB) systems also find a growing number of applications in geology.<ref name=Goldsteinetal_2003 /> In a typical FIB-SEM system, an extraction field is applied to a gallium (Ga) liquid metal ion source to field emit Ga ions and form a Ga beam. Due to the higher atomic mass, the Ga beam not only can be used to generate electron and ion images, but also may be used to mill samples to remove material. [[:file:Figure-9.jpg|Figure 9a]] shows the schematic diagram of an FIB-SEM system where a cross section of the sample is milled by a Ga FIB beam and is imaged simultaneously by the SEM. This milling and imaging process can be automated in a serial fashion to acquire a stack of two-dimensional images, from which a 3-D image volume can be constructed from the data set. This technique is particularly useful in revealing the 3-D distribution of mineral types, organic matter, porosity, and the like in shale (and other rock) samples.

Scanning electron microscopy provides different modes and techniques for acquiring high-quality images of shale and other rock samples. The images in this chapter demonstrate their fine resolution and their applicability for the characterization of shale reservoirs.

Scanning electron microscopy provides different modes and techniques for acquiring high-quality images of shale and other rock samples. The images in this chapter demonstrate their fine resolution and their applicability for the characterization of shale reservoirs.