Sedimentary microfacies and reservoir quality of the Nubian sandstone Formation, southern Gulf of Suez province, Egypt

Abstract

The present work considered a combination study between analysis of core petrophysics and sedimentary microfacies to estimate the reservoir quality of the studied Nubian sandstone reservoir in Saqqara oil field. The SEM observation of the studied Nubian reservoir displays quartz, feldspars and detrital clays are the dominant framework grains. Patchy dolomite, pyrite, kaolinite and illite are the common authigenic components. Euhedral syntaxial quartz overgrowths are relatively less common. Generally, all the samples are well cemented with relatively close grain packing and poorly developed primarly intergranular porosity. Most of the samples are fine grained and moderately to well sorted. Point counted porosity ranged from nil to 34%. The studied reservoir sequence could be sub divided into two reservoir rock types (RRTs). RRT1 is the best rock type forming the most upper parts of the Nubian Formation which composed from well sorted, fine grained, moderately cemented sandstone with minor primary intergranular porosity, minor dispersed pore filling detrital clay matrix, minor poorly crystallized illite and highly fractured framework feldspar grains. Whereas the RRT2 composed from moderately sorted, fine grained, well cemented with rare primary intergranular porosity, common dispersed pore filling detrital clay matrix and kaolinite plates, well crystallized hairy-like illites locally replace clay matrix. The petrophysical properties of the Nubian sandstone are more affected by significant quantities of Kaolinite and locally patchy siderite that are the main factors in reducing point counted porosity. RRT1 represent the best reservoir quality which has fair to excellent horizontal permeability (1.9–2719 md), poor to good porosity (7.3–19%), tight to good reservoir quality index RQI (0.69–1.98 µm), tight to very good flow zone indicator FZI (3.21–10.52 µm). Whereas the RRT2 is the tightest reservoir rock type, it is composed of quartzwackes microfacies and has fair to good porosity (1.7–16.6%), tight to very good horizontal permeability (0.01–560 md), tight to good RQI and FZI values (0.02–0.86 µm and 0.66–6.07 µm, respectively.