Impact of image resolution on quantification of mineral abundances and accessible surface areas

Abstract

Imaging has emerged as a valuable means of geological sample characterization and parameterizing reactive transport simulations where image analysis can provide porosity, mineral composition and mineral accessible surface area data, for example. Images can be collected using a variety of techniques and at a range of resolutions, yet the impact of image resolution on measured properties is largely unknown. In this work, the impact of 2D image resolution on the calculated mineral abundances, accessibilities and effective surface areas are examined for a sample from the Paluxy formation, Kemper County, Mississippi. Scanning electron microscopy (SEM) backscatter electron (BSE) images of thin sections were captured at resolutions ranging from 0.3 μm to 6 μm. Images were segmented into pores and discrete mineral phases using ImageJ and algorithms developed in MATLAB. Porosity, mineral abundances and mineral accessibilities were calculated by counting pore and mineral pixels in the segmented image where accessible minerals were deemed as those adjacent to the pore space. A 3D X-ray computed tomography (CT) image of a core sample was collected, segmented, and analyzed to evaluate the 3D connected porosity. Cuboids with the same total area as the 2D image were randomly sampled and used to calculate the 3D connected surface area. This was then multiplied by mineral accessibility to calculate accessible mineral surface areas for non-clay minerals. Minimum variations were observed for mineral abundances calculated from images with varying resolutions. For high resolution images, 0.3 μm to 1 μm, mineral accessibilities agreed relatively well. For images with resolutions from 1 μm to 6 μm, the calculated accessibility of smectite/illite decreased with decreasing resolution while quartz accessibility increased. This in turn resulted in higher effective surface areas for quartz with decreasing resolution. No significant variations were observed for calcite, siderite and K-feldspar.