Pore Fabric Anisotropy of the Cambrian–Ordovician Nubia Sandstone in the Onshore Gulf of Suez, Egypt: A Surface Outcrop Analog

Abstract

Integrated studies on pore fabric anisotropy have increased the general understanding of fluid flow patterns through reservoir rocks. In this study, pore anisotropy was studied based on measuring permeability and formation resistivity factors in vertical and horizontal directions for 130 plug samples from a total of 65 oriented block samples. These samples were representatively collected for the Nubia sandstones C and D in the western southern onshore of the Gulf of Suez. In addition, the porosity was measured using two techniques, namely water and helium injection. The effective pore radius r35 and the pore radius of the displacement pressure (rdp) were also measured. Petrographic studies of some representative thin sections and scanning electron microscope studies (SEM) were applied to study the mineral compositions of the studied samples to declare the most important porosity-reducing and porosity-enhancing diagenetic factors. The results show that the studied samples can be categorized into three rock types (RRTs), namely quartz arenite (RRT1), quartz wacke (RRT2) and mudstone (RRT3). The best storage and flow capacity was assigned to the RRT1 samples, whereas the least quality was assigned to the RRT3 samples. This could be attributed to wide pore throat distributions and the anisotropy of pore spaces due to the presence of a vertical subsidiary fracture system that dominated in many samples. This system was enhanced by introducing authigenic kaolinite as pore-filling clay minerals, causing the reduction in vertical permeability but supporting the vertical electric current flow. For the RRT3 samples, this vertical micro-fracture system was reduced by silica cementation which caused the reduction in both fluid and electric current flow. The pore anisotropy of most of the studied samples is in the range of slight anisotropy with some exceptions in the RRT3 samples which are characterized by moderate anisotropy.