Impacts of microfacies type on reservoir quality and pore fabric anisotropy of the Nubia sandstone in the central Eastern Desert, Egypt

Abstract

The Nubia sandstone has a high potential for water and hydrocarbons exploration in its subsurface extensions. The present work aims to study the Nubia sandstone at Gebel Duwi in the central Eastern Desert to characterize its lithostratigraphic setting, microfacies types, and its petrophysical properties. In addition, impacts of mineral composition and diagenetic history of the Nubia sandstone on its storage and deliverability capacities and on its hydraulic and electric pore fabrics will be estimated. The Nubia sandstone is subdivided into three informal members: a lower trough cross‐bedded, coarse‐grained with little mud braided fluvial member; a middle planar cross‐bedded sandstone with ferruginous siltstone intercalations of meandered fluvial member; and an upper gypsiferous coarsening upward clastic coastal to deltaic sediments. The microfacies analysis indicated that six microfacies are present: quartz arenite, subarkose, arkose, sublitharenite, ferruginous greywacke, and ferruginous siltstone microfacies. Several types of diagenetic processes affected the Nubia sandstone; cementation, compaction, and pressure solution are the main reservoir quality‐reducing factors, whereas fracturing, dissolution, and leaching out are the most important reservoir quality‐enhancing factors. For the present study, helium and water porosities, air permeability, and electric resistivities (for plugs drilled in three perpendicular directions for each block sample) were measured. Petrophysically, the studied Nubia sandstone can be summed up into three petrophysical facies: Facies‐1 (quartz arenite and subarkose), Facies‐2 (ferruginous subarkose to greywacke), and Facies‐3 (sublitharenite and arkose). The slight ferruginous and clay content of the second facies has slightly increased the grain density values and reduced its storage capacity and deliverability. Porosity (∅He) values which are measured by the helium injection for the first and third facies samples are good to excellent (14.6 ≤ ∅He ≤ 28.9%), whereas their permeability (k) values are good to excellent (106 ≤ k ≤ 2231 md). The second facies is characterized by relatively less petrophysical potential than the other two facies (av. ∅He = 19.2% and av. k = 557 md). The hydraulic pore anisotropy (λk) of the studied three petrophysical facies is isotropic to moderately anisotropic (1.02 ≤ λk ≤ 1.96), whereas the electric pore anisotropy (λE) is less intensive (1.11 ≤ λE ≤ 1.61).