Innovative Sub-Micron SEM-EDS Mineral Mapping and Analysis Applied to Australian Shale Samples

Abstract

FEI Australia, in collaboration with the University of Adelaide G&G, is applying next generation SEM-EDS based mineralogy analysis (automated mineralogy) to Australian shales traditionally considered too fine grained to result in accurate compositional representations. The challenge of mapping clay bearing formations is the high ratio of mixed mineral spectra generated within the x-ray excitation volume. Here, the latest advances in computationally resolving and visualizing sub-micron phase assemblages with unprecedented accuracy are presented on 6 shale samples from a range of depositional and diagenetic settings. First, the theory, potential, and physical limitations of high-resolution x-ray mineral mapping applied to the analysis of unconventional reservoir rocks are explained. Physical limitations of EDS data (energy-dispersive spectroscopy) related to the minimum x-ray excitation volume are highlighted and juxtaposed with nano-scale BSE (back-scattered electron) data. Next, the innovative ideas for resolving sub-micron mineral assemblages are explained and demonstrated. Example data is presented in the form of colour-coded mineral contribution maps. The data is evaluated in comparison to raw elemental maps, and higher-resolution textural maps. In addition, new spectral match images are computed that can highlight areas of uncertainty in mineral classification. More detailed sub-micron scale mineral data is presented for a massive black shale sample from the Arckaringa Basin. While the sample is macroscopically homogenous, striking compositional lamination of up to 1 mm laminae is highlighted in both colour-coded elemental and mineral maps. The laminae relate to cyclical cementation by kutnohorite (Mn-carbonate) and gypsum, and extend across the entire core width. This observation, including modal mineralogy generated for individual laminae (ROIs), allows for palaeoenvironmental interpretation of deposition occurring within a stratified, restricted basin environment subject to seasonal bottom-water flushing. In a reservoir engineering context, the cemented laminae may form baffles limiting fluid migration to horizontal flow, which is likely to enhance the unconventional reservoir properties. In conclusion, it is discussed how these spatially-resolved mineralogy data are complementary to bulk mineralogy techniques such as XRD and FTIR, and can be integrated into an analytical workflow.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015