EXPLORING MICRO X-RAY COMPUTED TOMOGRAPHY AND SCANNING ELECTRON MICROSCOPY AS TOOLS FOR SHALE ANALYSIS

The fine-grained texture and heterogeneity of shale makes analysis challenging, and this difficulty is compounded by the diverse depositional, diagenetic, and structural factors that determine its properties. Accordingly, the study of shale requires a systematic, multidisciplinary approach. Scanning electron microscopy (SEM) and x-ray computed tomography (CT) are useful instruments for observing shale fabric from mesoscopic to nanoscopic scales, and micro CT, in particular, is underutilized. Micro-CT provides a noninvasive way to image specimens in three dimensions from cm to µm scale, and this work explored its utility for shale analysis and the ways it can be incorporated into workflows for shale evaluation.

Shale samples from several Paleozoic formations in the southeastern and south-central United States were analyzed during this study. Qualitatively, micro-CT facilitated the characterization of heterogeneity and a variety of depositional, diagenetic, and structural features, and the ability to visualize, dissect, map, and filter tomographs in three dimensions is a major asset for shale characterization. Features imaged include micro-scale rock fabric, physical and biogenic sedimentary structures, body fossils, diagenetic minerals, fractures, and faults. Quantitatively, data from CT proved useful for calculating volumetric percentages of porosity and mineral constituents. SEM data were collected from samples used for CT analysis. CT helped pinpoint locations where microfabric could be further resolved by ion milling and SEM analysis. SEM coupled with energy dispersive spectroscopy (EDS) was then used observe and analyze pores, mineralogy, mineral morphology, fossils, and fracture-filling cement. Data gathered from SEM analysis were essential for interpeting CT images. This work demonstrates the utility of incorporating CT and SEM analyses into geologic workflows for the evaluation of shale. By using these technologies, shale properties can be evaluated from mesoscopic to nanoscopic scales, thereby informing a range of sedimentologic, structural, and petrologic interpretations.