THE PRECAMBRIAN BASEMENT - A MAJOR RESERVOIR IN THE RIFTED BASIN, GULF OF SUEZ

The expected instructions from an exploration manager, to stop drillin g and abandon a well where the bit hits the Precambrian basement, no l onger applies. The fractured and altered Precambrian basement rocks ar e the most prolific reservoirs in the southern Gulf of Suez and the no rthern Red Sea rifts where hydrocarbons are produced from 8 fields, wi th porosity and permeability values up to 15% and 300 millidarcy, resp ectively. The surface and subsurface Precambrian basement rocks are re lated to the final stages of the tectonic-magmatic cycle of the Arabo- Nubian Shield and are composed of quartz-diorite, granodiorite, syenog ranite, alkali granites and andesite porphyry, dissected by means of d ykes, fractures and joints. Three main directions of fractures, northw est-southeast, northeast-southwest and east northeast-west southwest h ave 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, diageneti c processes and the dip and direction of the dykes and brecciated zone s. The alteration processes reach their maximum intensity in the topmo st section, known as the basement cover, where the solution and leachi ng has led to the enlargement of the fractures and vertical communicat ions. The underlying fracture zone has been affected by differential a lteration processes, creating zones of high and low vertical porosity and permeability. Thus, the reservoir potential of the Precambrian bas ement has been greatly underestimated. (C) 1998 Elsevier Science B.V.