Impacts of pore- and petro-fabrics, mineral composition and diagenetic history on the bulk thermal conductivity of sandstones

The present study aims to model the bulk thermal fabric of the highly porous (26.5 <= phi He <= 39.0%) siliceous Nubia sandstones in south Egypt, as well as their pore- and petro-anisotropy. The thermal fabric concept is used in the present study to describe the magnitude and direction of the thermal foliation 'F', lineation 'L' and anisotropy 'lambda'. Cementation, pressure solution, compaction and the authigenic clay content are the main pore volume-controlling factors, whereas the cement dissolution and fracturing are the most important porosity-enhancing factors. The bulk thermal fabric of the Nubia sandstone is raised mostly from the contribution of the mineral composition and the pore volume. The kaolinite content and pore volume are the main reducing factors for the measured bulk thermal conductivity 'k', whereas the quartz content is the most important enhancing factors. The optical scanning technique, which is one of the most accurate and precise techniques, was applied for measuring the bulk thermal conductivity 'k' of the studied samples. For the dry state, the average thermal condutivity 'Icas,' in the NE SW, NW SE and vertical directions, varies from 1.53 to 2.40, 1.54 to 2.36 and from 1.31 to 2.20 W/(mK), respectively. On other hand, 'K-av' for the saline water-saturated state for the NE SW, NW SE and vertical directions varies between 2.94 & 4.42, 2.90 & 431 and between 239 & 3.65 W/(mK), respectively. The present thermal pore fabric is slightly anisotropic, 'lambda' varies from 1.10 to 1.41, refers mostly to the NW SE direction (k(max) direction, elongation direction), whereas the petro-fabric refers to NE SW direction (kmax direction, elongation direction). This gives rise to a conclusion that the pore- and petro-fabrics have two different origins. Therefore, studying the thermal conductivity of the Nubia sandstone in 3-D indicates a pore fabric elongation fluctuating around the N-S direction. (C) 2015 Elsevier Ltd. All rights reserved.