Orientational mapping of minerals in Pierre shale using X-ray diffraction tensor tomography.

Fredrik K Mürer,Basab Chattopadhyay,Marco Di Michiel,Pierre Cerasi,Dag W Breiby,Aldritt Scaria Madathiparambil,Kim Robert Tekseth

doi:10.1107/s205225252100587x

Abstract

Shales have a complex mineralogy with structural features spanning several length scales, making them notoriously difficult to fully understand. Conventional attenuation-based X-ray computed tomography (CT) measures density differences, which, owing to the heterogeneity and sub-resolution features in shales, makes reliable interpretation of shale images a challenging task. CT based on X-ray diffraction (XRD-CT), rather than intensity attenuation, is becoming a well established technique for non-destructive 3D imaging, and is especially suited for heterogeneous and hierarchical materials. XRD patterns contain information about the mineral crystal structure, and crucially also crystallite orientation. Here, we report on the use of orientational imaging using XRD-CT to study crystallite-orientation distributions in a sample of Pierre shale. Diffraction-contrast CT data for a shale sample measured with its bedding-plane normal aligned parallel to a single tomographic axis perpendicular to the incoming X-ray beam are discussed, and the spatial density and orientation distribution of clay minerals in the sample are described. Finally, the scattering properties of highly attenuating inclusions in the shale bulk are studied, which are identified to contain pyrite and clinochlore. A path forward is then outlined for systematically improving the structural description of shales.

Highlights

The orientation of nano-crystallites affects the macroscopic physical properties in a wide range of hierarchical materials such as bones (Stock, 2015), polymers (Baer et al, 1987) and shales (Leu et al, 2016)
X-ray micro-computed tomography, based on intensity attenuation of the beam as it propagates through the specimen, has, owing to technical advances during the last decade, become a workhorse for studies of complex natural and manmade material structures, both at home laboratories and at synchrotrons
It is of high importance to develop new methods that can be used at different length scales and inherently contain structural information arising at the nanoscale

Summary

Introduction

The orientation of nano-crystallites affects the macroscopic physical properties in a wide range of hierarchical materials such as bones (Stock, 2015), polymers (Baer et al, 1987) and shales (Leu et al, 2016). Whereas XRD has been used for sample-averaged texture measurements for decades, small-angle scattering and X-ray diffraction tensor tomography (SASTT, XRDTT) have been recently introduced (Skjønsfjell et al, 2016; Liebi et al, 2015, 2018) as techniques to retrieve the spatially resolved 3D nanostructure orientation distributions across millimetre-sized samples. Shales typically split into thin laminae of a few millimetres thickness, reflecting the parallel orientation of clay mineral flakes Sandstones, with their simpler composition and high porosity (10–40%), often exhibit a rather uniform pore-size distribution, and are currently considered well understood in terms of both mechanical properties and fluid permeability (Keelan, 1982). We study the scattering from comparably large high-density mineral grains in the shale sample

Sample

XRD-CT

Attenuation-contrast CT

The mineral composition and clay mineral orientation in Pierre shale

Retrieving the clay mineral orientation by XRDTT

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