GEOPHYSICAL (GLOBAL GRAVITY FIELD MODEL) AND FIELD (WELL DATA) CONSTRAINTS ON GROUNDWATER FLOW IN THE NUBIAN SANDSTONE AQUIFER IN EGYPT, SUDAN, LIBYA, AND CHAD

The Nubian Sandstone Aquifer System (NSAS) is the largest groundwater aquifer in the Sahara spanning the political boundaries of four countries in north-eastern Africa; Egypt, Sudan, Libya, and Chad. The NSAS represents as a potential water resource for future development programs in these countries. Thickness of the aquifer ranges from tens of meters to over three kilometers in the center of the Kufra Basin. These variations could be structurally or lithologically controlled. Using the global gravity field model (EGM 2008) and well data (~2000 groundwater and oil wells), the thickness of the sedimentary cover and the distribution of both the structural trends and the basement uplifts across the NSAS were delineated. Findings include: (1) thickening of the sedimentary sequence from less than 0.5 km in the south (Uweinat and northern Sudan) up to 5 km in the north (proximal to Mediterranean coastline), (2) shallow basement depth (< 400 m) in southern Egypt along the Uweinat-Bir Safsaf-Aswan uplift that separates the deep intracratonic Dakhla Basin north of the uplift from the shallow basins of northern Sudan in the south, (3) the observed gravity anomalies are largely controlled by basement; similar gravity anomalies and patterns are observed before and after the removal of the contributions of overlying sediments, (4) several structural trends were delineated, the most prominent of which is the NE-SW Pelusium trend, a system of an echelon left-lateral mega shears; (6) The Pelusium trend runs sub-parallel to the eastern margin of the Mediterranean Sea, and curves in a NE-SW direction cutting across western Egypt, southeastern Libya, and northern Chad. Investigations are underway to investigate whether the Pelusium structures were responsible for the formation of deep basins (aquifers) bound by basement uplifts in Libya (Kufra Basin) and Egypt (Siwa and Farafra oases).