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

• We studied the thermal and petrophysical properties of the Nubia sandstones.
• Thermal properties are mostly controlled by lithology and pore volume.
• Thermal conductivity is affected by the grain and pore fabrics.
• The pore fabric is composed of 2 pore systems; main and subsidiary fabrics.
• Thermal foliation ‘F’ is the main contributor to the bulk thermal anisotropy.

The present study aims to model the bulk thermal fabric of the highly porous (26.5 ≤ øHe ≤ 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 ‘λ’.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 ‘kav’ 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, ‘kav’ for the saline water-saturated state for the NE–SW, NW–SE and vertical directions varies between 2.94 & 4.42, 2.90 & 4.31 and between 2.39 & 3.65 W/(mK), respectively.The present thermal pore fabric is slightly anisotropic, ‘λ’ varies from 1.10 to 1.41, refers mostly to the NW–SE direction (kmax 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.