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SPATIAL ORIENTATION AND SHAPE OF PORES IN A SANDSTONE: A CASE STUDY FROM THE OUTER CARPATHIANS BASED ON X-RAY COMPUTED MICROTOMOGRAPHY

Piotr Jan Strzelecki

Abstract

The pore space is an important part of a rock that affects its mechanical and petrophysical properties. Among numerous methods for pore space characterization X-ray computed microtomography (?CT) is one that provides information on the spatial distribution of pores within rocks. Followed by image processing and analysis, detailed information on orientation and shape of voids can be obtained. Hence, it is a good method for microstructural studies on the porous space. The aim of this paper is to present spatial variability of pores within a sandstone based on ?CT. The study was performed on the flysch sandstone sample representing Magura beds from the Magura Nappe, Polish segment of Outer Carpathians. The study was supplemented by polarized light microscopy and cathodoluminescence studies of corresponding thin sections. The voxel size of ?CT images was equal to (0.01 mm)3. The ?CT images were processed with Fiji software and 3D ImageJ Suite plugin. The processing consisted of thresholding, filtering and segmentation of pores. For each void an ellipsoid was fitted. Obtained orientations and lengths of ellipsoids? radiuses provided information on the pores spatial orientation and shape. The spatial data were projected on stereonets with contour plots, whereas the shape of pores was classified using Zingg diagram. Detailed study of the porosity was performed on four selected zones of the sample, which were selected on the basis of visual difference in porosity and presence of tectonic feature i.e. deformation band. The average porosity obtained for each zone was respectively: 0.5%, 1.2%, 2% (for the zone with deformation band) and 2.7%. The shape of voids is diversified. Rod-shaped voids dominate (ca. 42%) whereas voids of sphere, blade and disc shape consist respectively 25%, 17% and 16%. The size of pores are up to 0.24 mm in radius length. There are visible trends in orientations of pores. The orientations of the minor axes tend to align perpendicular to the bedding plane, while the orientations of the major and medium axes align parallel to it. In the zone with the deformation band, slight shifting of the major and medium axes maxima is recorded. It seems that the pores spatial orientation bears record of compaction, sedimentary and tectonic processes. It could be concluded that ?CT derived data are useful in the microstructural studies of the porosity.

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DOI resolver: doi.org/10.5593/sgem2018/1.4/s06.104

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