Abstract Scanning electron microscopy (SEM) and its auxiliary technologies have been exploited by various industries for more than 50 years. One of its main applications has been to characterize materials at very high magnification. Recent interest by the petroleum industry to better characterize and understand shale hydrocarbon reservoirs has let to an increased interest in utilizing SEM technology for shale reservoir studies. The purpose of this paper is to review basic SEM operational principles and applications that are particularly useful for characterizing shale hydrocarbon reservoirs.

Abstract To better understand the influence of microscale geochemical and microstructural relationships on the bulk petrophysical properties of unconventional shale systems, core samples from four producing North American formations were cross-sectioned with an argon ion polisher and imaged with a field emission scanning electron microscope (FE-SEM) using a variety of complementary detectors. We demonstrate distinct advantages of the ion-polishing technique for the preservation of the internal shale structure. Moreover, we show how such preparation affords a wider choice of imaging options for both chemical and structural characterization, such as backscatter electron observation at varying beam potentials coupled with x-ray and cathodoluminescence spectroscopic techniques.

Abstract The ability to visualize and interpret standard grayscale scanning electron microscopic (SEM) images is limited by image contrast, magnification level, and the capability of the human eye to distinguish between subtle shades of gray. A pseudocolor conversion technique is described using the freely available ImageJ digital image processing and analysis program that can improve the ability to interpret and communicate information contained within SEM images of shales. This technique is most suitable for backscattered electron images to enhance contract variations and aid compositional interpretations, as illustrated by examples from the Eagle Ford, Haynesville, and Marcellus shales.

Abstract Porosity in shale samples is often at such a small scale that analysis with a focused ion-beam scanning electron microscope (FIB-SEM) is needed to resolve the nanometer-size pores in three dimensions. This gain in resolution, however, comes at the expense of analysis volume (typically no more than 1000 um3), which may not be representative of the larger sample. This paper describes a method of creating high-resolution, large-scale SEM images to aid in selecting more representative locations for FIB-SEM imaging and analysis than by current industry practice.

Abstract This paper describes and illustrates features of shales and mudstones at the nanometer and micrometer scales using standard scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) techniques. Microfabric observations at these scales not only provide insights into sedimentary and postdepositional processes but also offer evidence useful in understanding storage and primary migration patterns in unconventional shales. The images illustrated are suggested as references to guide future shale studies related to shale porosity and permeability. Examples are provided of various shales (Barnett, Woodford, Eagle Ford, Rhinestreet, Fayetteville, and Marcellus). Microfabric and pore features illustrated include clay flakes related to open-network floccules and clay-alighed fabric, plus other features produced by nonclay minerals. Organic matter produced by zooplankton and algae (e.g., coccolithophores, Tasmanites) is described because it may form organic mucus that adheres to and helps aggregate clay flakes. Organic matter is also common within porous fecal pellets. Coccolithophores, sponge spicules, and foraminifera tests contain hollow internal chambers, which provide porosity and probably permeability, even when filled with clay minerals. Conventional SEM provides a rapid and relatively inexpensive way of evaluating pores and microfabrics in shales.

Abstract Secondary electron (SE) images of rough three-dimentional shale surfaces and backscattered electron (BSE) images of smoothed argon (Ar)-ion-milled surfaces provide valuable information to the study and understanding of texture, composition, and pore systems in unconventional shale reservoirs. This paper describes a method to acquire both traditional SE and Ar-ion-milled BSE images from the same sample by acquiring images from adjacent sample splits. This method allows side-by-side comparisons of traditional SE and Ar-ion-milled BSE images, providing much more information than can be obtained from either method alone. This dual method of imaging representative sister samples, capturing the same features at the same scale, gives powerful information that could not be effectively addressed before. The traditional scanning electron microscopic (SEM) method images secondary electrons that are ejected from the k-orbitals of the specimen atoms by inelastic scattering interactions, with beam electrons giving a large depth of field on high-relief surfaces. This methodology cannot fully assess the exact location of the dominant porosity type or the nature and distribution of the organic matter on the high-relief surface. These considerations are essential to petrographic evaluation of reservoir and completion quality in unconventional shale reservoirs. SEM samples prepared using Ar-ion milling with BSE detection allow us to effectively evaluate the nature and distribution of the organics and the porosity by looking at electron density contrast or Z-contract. Lower-density material, such as organic matter, appears darker, and higher-density material, such as quartz, appears brighter in SEM images. Void space, such as porosity, appears black. However, this method cannot accurately determine texture or differentiate some mineral cement because many minerals are of similar densities; hence, they have similar Z-contrasts. Using both techniques in tandem for representative sister samples on similar sample features and textures makes up for the deficiencies of each method alone. Mudstone samples from the Haynesville Shale are characterized with this technique and illustrate the complex texture, dominate porosity types, and the nautre and distribution of organic matter, including bitumen, the solid residue resulting from the thermal conversion of kerogen to hydrocarbons.

Abstract Although shale gas systems constitute a new target for commercial hydrocarbon production, only a little attention has been paid to the evolution of these unconventional systems with increasing thermal maturation. This study reports the characterization of samples of the Lower Toarcian (Lower Jurassic) Posidonia Shale from northern Germany at varying levels of thermal maturity. Observations were made using an original combination of focused ion beam-scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM). The paper documents the formation of microfracture-filling bitumen in close assotiation with kerogen residues with increasing maturity. Porosity evolves from mostly submicrometric interparticle pores in immature samples to intramineral and intraorganic pores in overmature (gas mature) samples. This intraorganic nanoporosity has most likely come about by the exsolution of gaseous hydrocarbon and been hydrocarbon wet during the thermal maturation processes. Although FIB-SEM and TEM images are small compared to field size, this study emphasizes the need for nanoscale imaging to better constrain hydrocarbon generation processes in gas shale systems.

Abstract Porosity and pore size distributions are key attributes for characterizing shale gas storage capacity and flow in shale. In this study of shale samples from the Horn River Formation, analysis of field emission scanning electron microscopic (SEM) images of argon-ion-milled samples, nitrogen adsorption and desorption experiments, and mercury injection capillary pressure were applied to characterize pore volumes, pore morphology, and pore sizes.

Abstract Scanning electron microscopy (SEM) of shales from three unconventional gas/liquid plays (Nordegg, Montney, Duvernay) of the Western Canadian Sedimentary Basin were combined with routine analytical investigations to characterize mineral composition, mineral assemblage, morphology, organic content, and porosity. The investigations demonstrated that despite marked diagenetic differences between these shales, some common textural and pore characteristics occurred in all samples. The study showed that SEM morphological investigations of unconventional shale reservoirs provided important information about mineral aggregates, cementation, and clay mineral distribution, which allows interpretations about diagenetic history.

Abstract Pore system and matrix constituents of the Eagle Ford Shale are described from 86 representative samples from southwestern and eastern Texas. Argon-ion-milled samples were imaged using a field emission scanning electron microscope (FE-SEM) to document matrix constituents and pores. Samples were chosen based on lithology, x-ray diffraction mineralogy, matrix porosity, and effective permeability measurements.

Abstract Laboratory measurements of porosity, permeability, and fluid saturation are routinely acquired from core samples to quantify petrophysical data for shale reservoir studies. However, these measurements do not provide information regarding the microstructural character of the mineral matrix, organic matter distribution, and pore network. New imaging techniques using focused ion beam scanning electron microscopy (FIB-SEM) were applied to a subset of prepared core samples from four wells to study the microstructural character of the Cenomanian-Turonian Eagle Ford Formation in the Mavrick Basin, south Texas, in the United States. The studied wells span a wide range of organic thermal maturity from the oil window to the dry-gas window. Sixteen digital rock physics (DRP) models were prepared from three-dimensional (3-D) SEM image volumes to quantify volumes of mineral matrix, organic matter, and pore space. The results of this study revealed organic matter pore type, pore size, and permeability vary by stratigraphic interval and the degree of thermal maturity. Pendular organic matter with bubble pores was more common in the oil window. Both pendular and spongy organic matter types with bubble and foam pores were common in the gas/condensate window. Organic matter pores from samples within the dry-gas window had smaller average pore diameters and lower permeability than samples within the oil window. Comparison of core porosity and permeability measurements derived from DRP models with laboratory core measurements showed broad similarities, indicating that despite the small analysis volume involved with FIB-SEM 3-D image volumes, the DRP models prepared in this study were generally representative of the larger core samples.

Abstract The distribution of nanometer-size pores in ten selected Eagle Ford Group, Haynesville, Marcellus, and Barnett shale samples was similar when comparing relative numberical abundances of maximum pore diameters but not when comparing relative abundances of pore areas (pore sizes). Differences also existed between units in the association of pores with organic material. Pores were measured on argon-ion-milled (AIM) samples and examined with a field emission environmental scanning electron microscope (SEM). One Haynesville sample was also evaluated using a focused ion beam (FIB) SEM to compare to the AIM results. With the AIM samples, pore types were subdivided into three categories—organic pores, mixed matrix/organic pores, and matrix pores—based on the amount and type of material (organic or inorganic) surrounding the pores. Organic pores are pores generally associated with kerogen macerals, whereas mixed matrix/organic pores are pores that are probably associated with bitumen or pyrobitumen. Matrix pores are not associated with any organic matter. Within the sample set studied, only the Barnett samples contained pores almost exclusively within organic particles. The majority of the maximum pore diameters were less than 100 nm within all the samples examined. Only the Barnett samples, however, had a majority of their pore areas (or porosity) comprised of pres less than 10,000 nm squared (which is the area of an equidimensional pore with the maximum pore diameter of 100 nm).

Abstract Middle Devonian Marcellus Shale and late Devonian New Albany Shale samples were argon-ion milled and studied by scanning electron microscope for petrographic features and pore development. Petrographic observations revealed that a finite number of pore types exist in spite of considerable variability in composition, depositional setting, and compaction history. The four major pore types are framework pores, framework shelter pores, dissolution pres within inorganic grains, and organic matter pores.