The ability to visualize and interpret standard scanning electron microscope (SEM) grayscale images is limited by image contrast, magnification level, and the capability of the human eye to distinguish between subtle shades of gray. Pseudocolor conversion using digital image processing software can improve the ability to interpret and communicate information contained within shale and mudstone SEM images. Pseudocolor conversion is most suitable for ion-milled backscattered electron images to enhance mineralogy, as illustrated by examples from the Eagle Ford, Haynesville and Marcellus shales.
Because of the extremely fine-grain size (<62 ???m) of shale and mudstone, shale reservoir core analysis commonly includes the use of a scanning electron microscope (SEM) to produce sufficiently high magnification images to study fine-scale features, such as texture, composition, and porosity. The scanning electron microscope measures various signals emitted from a sample in response to an accelerated electron beam that is scanned in a raster pattern across the specimen surface. Images are produced by assigning grayscale values to represent signal intensity measurements, such as from secondary electron (SE) and backscattered electron (BSE) detectors. The lowest intensity value is scaled black and the highest intensity value is scaled white. Intermediate intensity measurements are thus represented by various shades of gray.
Secondary electrons are low voltage electrons (typically less than 50eV) that are produced primarily by inelastic collisions of electrons from the accelerated electron beam with atoms near the specimen surface. The number of detected electrons that escape from the surface (secondary electron yield) is a function of the orientation of the specimen surface in relation to the orientation of the electron beam and SE detector (Goldstein et al., 2003). This property is used to create highly detailed topographic images of three-dimensional surfaces that are useful for studying rock fabric, texture, and mineral, pore and microfossil morphology. Secondary electron images can also be obtained from smoothly polished or ion-milled surfaces to image and evaluate near surface negative aberrations such as small pores and cracks which are represented as low intensity (dark) regions.