Petrophysical and magnetic pore network anisotropy of some cretaceous sandstone from Tushka Basin, Egypt

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.