High-resolution large area low vacuum scanning electron microscopy (LV-SEM) imaging for microporosity and diagenesis of carbonate rock systems, and carbonate cemented sandstones

Automated image collection from polished thin-sections of carbonates and carbonate cemented sandstone by scanning electron microscopy (SEM), in conjunction with autonomous stitching can be used to construct high-resolution image maps, with a micron to sub-micron resolution, over areas of several cm2 or greater. Such maps can be formed from any available detector, including backscattered (BSE), secondary electron (SE) and cathodoluminescence (CL). In this study, samples are illustrated that include carbonates, and a number of carbonate cemented sandstone specimens, some of which had previously been studied in relation to laboratory-induced deformation. Imaging was carried out under low vacuum (0.8 Torr), without coating, utilizing both BSE and gaseous secondary electron (GSE) imaging. BSE imaging provides excellent data on variation in particulate and cement composition, general fabric and distribution of features, as well as data on porosity and micro-porosity. While the GSE detector in particular allows for the use of charge contrast imaging (CCI), which is a rapid imaging technique that can provide information within carbonates on cementation, of a similar nature to that characteristically observed through CL studies. CCI montages commonly have a variety of contrast / brightness artifacts due to variation in charge distribution across the scanned image. A variety of remedies are discussed, that can reduce these artifacts, making it easier to apply image analysis techniques across montages. Large area images can be used to later analyze samples in the fashion of a ‘virtual SEM’. As such images can be automatically collected at night or over weekends, and the resulting Images are sufficient for most purposes. This can effectively release SEM time, and provide a much more detailed coverage of samples, which is ideal for archiving. In addition, such large images, or the individually collected tiles can be used as the input parameters for three-dimensional modeling for porosity, permeability and flow analysis utilizing Pore Architecture Modeling (PAM) and Process Analysis Toolkit (PAT) software.