Transmission Electron Microscopy (TEM) Images of Shale
Bureau of Economic Geology
The University of Texas at Austin
Micro-fabric signatures preserved in fine grain sediment and rock can be used to identify sedimentary environments and processes, thus serving as a distinct signature of depositional and diagenetic environment. Scanning Electron Microscopy (SEM) of cross-sectional Ion milled samples is the major acquisition technique use to characterize the evolution of the mineralogical and organic carbon structure in oil shales. It is very successful in imaging surface features down to 2nm. However, CH4 adsorption data and GRI helium porosity data suggest there is a large porosity contribution from <10nm features not accounted for in SEM measurements. Direct imaging by transmission electron microscopy (TEM) with resolutions of 0.2nm is capable of resolving those fine structures. TEM is a volumetric technique where a high energy electron beam is transmitted through a thinned sample and projected onto a CCD camera. The way the electron beam is scatter or diffracted within the sample volume will dictate the image formed. These images are Nano meter (nm) scale imprints preserved in sediment which can be used to identify sedimentary environments and processes, thus serving as a distinct signature of their depositional and diagenetic environment.
In this study our primary goal was to determine if a stable electron transparent sample could be prepared given the mineralogical complexity and inherent stored stress. In addition we wanted to demonstrate the TEM microanalysis capabilities on previously analyzed geologic samples; To use those capabilities to determine the in situ distribution and character of the organic pore networks; To characterize the mineralogical and clay micro fabric in relation to the organic component; And to characterize the organic matter texture and structure at nano scale. A selected area was prepared using FIB slice extraction technique. The slice was mounted on a TEM grid placed in a TEM for analysis. A slice of 20um x 90nm thick mudstone was analyzed. This is a silty mudstone composited of abundant Chlorite/Illite/Mica clays and silt grain of quartz and albite mixed with organic matter. Most of the pores are found in clay platelets and the organic matter and range in size from a few nano meters to microns.
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