The Precambrian basement: A major reservoir in the rifted basin, Gulf of Suez

The expected instructions from an exploration manager, to stop drilling and abandon a well where the bit hits the Precambrian basement, no longer applies. The fractured and altered Precambrian basement rocks are the most prolific reservoirs in the southern Gulf of Suez and the northern Red Sea rifts where hydrocarbons are produced from 8 fields, with porosity and permeability values up to 15% and 300 millidarcy, respectively. The surface and subsurface Precambrian basement rocks are related to the final stages of the tectonic-magmatic cycle of the Arabo-Nubian Shield and are composed of quartz-diorite, granodiorite, syenogranite, alkali granites and andesite porphyry, dissected by means of dykes, fractures and joints. Three main directions of fractures, northwest-southeast, northeast-southwest and east northeast-west southwest have been detected in the study area. The porosity and production rates of this reservoir, as well as the oil-water contact movement, depend mainly on the age, intensity and direction of the fractures, diagenetic processes and the dip and direction of the dykes and brecciated zones. The alteration processes reach their maximum intensity in the topmost section, known as the basement cover, where the solution and leaching has led to the enlargement of the fractures and vertical communications. The underlying fracture zone has been affected by differential alteration processes, creating zones of high and low vertical porosity and permeability. Thus, the reservoir potential of the Precambrian basement has been greatly underestimated. (C) 1998 Elsevier Science B.V.