Optimizing nano-dynamic mechanical analysis for high-resolution, elastic modulus mapping in organic-rich shales

An elastic modulus mapping technique based on spatially continuous dynamic nanoindentation is applied to map microscale variations in a fine-grained, kerogen-rich shale consisting of inorganic minerals with an interpenetrating network of microscale pores filled with organic matter. Advantages and limitations of the application of this technique to shales are explored through varying sample preparation and scanning procedures. Filtering techniques are developed to remove data that are negatively impacted by topography and other issues inherent to the mapping technique. As a result, spatial variations of elastic modulus in kerogen-rich regions are seen at substantially higher resolution than has previously been reported. Spatial resolution and continuous mapping across high stiffness-contrast material boundaries are further improved with stringent sample preparation and the use of a sharp tip. Typical modulus values measured by this technique include approximately 10 GPa for kerogen, 15–45 GPa for clay depending on the morphology and orientation, and 50–70 GPa for quartz.