Impact of laminations on physical properties of the Bahariya ormation, Egypt

Abstract

The type locality of the Bahariya Formation in Egypt is the Bahariya Oasis, which lies about 280 km southwest of Cairo, Egypt. It is composed mainly of sandstones which deposited in fluvio-marine to shallow marine environment. The high grain density values recorded for some parts of the Bahariya Formation is due to the existence of iron oxides and pyrite presented as pore filling and/or cementing materials. The primary sedimentary structures like wavy and zigzag laminations severely affect the majority of rock petrophysical properties. The rock laminations are considered as the main source of reservoir rock anisotropy. The relation between pore specific surface (Spor) and permeability is positively improved in the laminated rock samples rather than in the non-laminated. There is a robust and close relationship between porosity and bulk density for non-laminated samples (R2 = 0.86), while for laminated samples, the relation is very weak due to the presence of iron oxides and pyrite laminas. Permeability anisotropy is highly scattered, but generally it increases with depth. Higher permeability anisotropy supports the horizontal migration of reservoir fluids. Permeability anisotropy is most likely due to the existence of microfractures and/or laminations which are mostly existed in the upper part of the Bahariya Formation. The low values of electrical resistivity are showing by laminated samples in depth more than 3516m. It most likely due to the presence of conductive solids like iron rich cement or matrix and rock pore space architectures. However, the mineral composition of some few laminas is free of iron oxides (depth=3515m). The electrical resistivity anisotropy has high values for laminated samples relative to the non-laminated ones. It increases, in general, with depth as a trend of highly scattering data points. This anisotropy is responding by the magnitude of the magnetic susceptibility anisotropy. The non-laminated samples have low magnetic susceptibility relative to the samples with laminations. The magnetic susceptibility of the Bahariya samples is apparently not related to irreducible water saturation (Swi). The main reason for irreducible water saturation variations is the amount and type of clay minerals inside the rock pore spaces. The clay mode of occurrence is usually pore filling, lining, bridging or grain coating. The displacement pressure (Pd) measured for the Bahariya samples are ranged from 10 up to 5000 Psi. The pore surface area of non-laminated samples shows a close and reliable relationship with displacement pressure (R2 = 0.97), while the pore surface area of the laminated samples has no relation with the displacement pressure (R2 = 0. 09). The samples expose mean hydraulic pore radius (R50 &R35) less than 0.04 µm are considered as bad reservoir or even sealing rocks or permeability barrier irrespective to laminations.