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This study testifies the reliability of the electric pore fabric technique applied on the Nubia sandstones in south Egypt through a comparison with pre-assigned magnetic pore fabric and permeability already measured for three perpendicular directions in these rocks. Discrepancies between the different techniques were attributed to the fact that the magnetic pore fabric technique has the ability to detect and define accurately the pore volume distribution in 3-D, whereas the electric pore fabric technique has the ability to detect the shortest paths for the electric current/fluid flow. Although the magnetic pore fabric is a bulk property, the permeability measurements and the electric pore fabric are quantifying transport properties. Both the magnetic and electric pore fabrics refer mostly to slightly and moderately anisotropic fabrics. The integration between both techniques revealed two main directions for fluid migration, swinging to the N-S and the E-W directions. One difficulty of the electric pore fabric technique is due to the surface conductivity effects either from the presence of some clay minerals or cracks. Therefore, interpreting the obtained electric pore fabric must be carried out in care for the cracked and clayey rocks. The surface conductivity effect could be accounted for by using the true formation factor for calculating the electric pore fabric parameters. The electric pore fabric technique can be compared to the 3-D permeability measurements, which show very similar directions. [ABSTRACT FROM AUTHOR]

Pore magnetic fabric is a well-established technique for the determination of pore elongation and preferred directions for migration of the interstitial fluids. This study further exemplify this technique on a set of the Nubia sandstones through a comparison with the pore anisotropy obtained from measuring permeability in three orthogonal directions in a gaz permeameter. The Nubia sandstones are represented in Tushka area (South Egypt) by quartz arenite of large porosity (29–40 per cent) which was measured on thin sections parallel and perpendicular to the bedding plane and petrophysically by helium pycnometry and ferrofluid injection at 1 bar pressure. Petrographically, there is a detectable difference between the porosity values in the bedding plane and in the perpendicular direction indicating inhomogeneity in the pore space network distribution. The petrophysical studies indicate large porosity and permeability values with some differences between the helium and ferrofluid porosity due to presence of micro pore spaces not accessible for the ferrofluid molecules having relatively high diameters and injected at low pressure. An overall agreement is observed between the permeability anisotropy and the magnetic grain and pore fabrics (magnetic anisotropy measured before and after ferrofluid injection). The three fabrics are mainly dominated by a bedding parallel foliation. In a few cases maximum permeability appears to be perpendicular to bedding. Within the bedding plane, maximum pore elongation direction from ferrofluid injection is NNW for Adindan and Kesieba formations and NW for Abu Simbil Formation. The maximum pore elongation direction for Abu Ballas samples showed a direction fluctuating around the E–W direction, the main fault trends in Tushka area. The pore fabric of Abu Ballas formation seems therefore to be structurally controlled, while it would be originated from palaeocurrent directions in the other formations. [ABSTRACT FROM AUTHOR]