HIGH-RESOLUTION ANALYSIS OF PALEOZOIC SHALE USING SCANNING ELECTRON MICROSCOPY AND X-RAY COMPUTED TOMOGRAPHY

Despite the ubiquity of shale in the rock record, the complex interplay among the physical, biological and chemical processes that determines its properties has inhibited our understanding of its origin. In addition, the fine-grained texture and heterogeneity of shale make its analysis challenging. Consequently, the full range of sedimentary processes associated with shale remains a vital subject of scientific endeavor. When compared to coarser grained clastic and carbonate strata, it is apparent that our knowledge of mudrocks lags behind that of other rock types.

Although scanning electron microscopy (SEM) and X-ray computed tomography (CT scanning) have been recognized as useful instruments for observing shale fabric on micro- to nano-scales, they remain underutilized. The objective of this study is to use these tools to investigate the petrologic fabric of shale and its relationship to depositional and diagenetic processes. Preliminary findings show how SEM can be used to observe mineral grains as well as various morphologies of sulfide (e.g., pyrite nodules, framboids, crystals, and coated grains), which are useful indicators of redox conditions in the water column. In addition, the use of energy dispersive X-ray spectroscopy (EDS) coupled with SEM allows for rapid and straightforward determination of mineral grains. CT scanning provides a noninvasive way to image shale samples in three dimensions and will facilitate the determination of heterogeneities and inspections of sedimentary features (e.g., physical sedimentary structures, diagenetic structures, and fossil content). The unit being used has a resolution of ~100 nm and facilitates imaging of micro- to nano-scale features that cannot be observed or analyzed by other methods. By observing these characteristics, there will be a determination of how microfabric is related to macrostructure, sedimentary processes, and depositional environments, and how depositional and diagenetic processes influence shale fabric and pyrite morphology.