HIGHLIGHTS OF OLIGO-MIOCENE SILICOCLASTIC HYDROGEOLOGIC RESERVOIR SYSTEMS IN NORTHEASTERN TUNISIA AFRICAN MARGIN

Geophysical wireline logging, seismic analyses combined to sedimentologic and structural studies in outcrops analogues in addition to hydrogeochemical groundwater analyses had permitted to highlight new potential deep water fractured carbonate and sandstones reservoirs in the eastern margin of Tunisia. Deltaic Oligocene and Miocene siliciclastic deposits had been identified and present 500 meters of several multi-layers sandstone aquifer reservoirs composed by five to six Oligocene sandstone extended lenticular bodies and seven to nine Miocene ones exhibiting each one an average thickness of 20 to 50 meters with porosity of 25 to 40 %. Some Oligocene and Miocene exploited deep groundwater sansdstone levels are characterized by respectively an average values of transmissivity and permeability of 2 10^-3 m²/s and 4 10^-5 m/s and 3.7 10^-3 m²/s and 6 10^-5 m/s. Water Flow exploitation of Oligocene and Miocene aquifers vary respectively from 10 to 40 l/s and from 1 to 20 l/s. Several sub-basin piezometries show a water flow of phreatic and deep aquifers from West to East to the Mediterranean sea and/or to the Sebkhas of the Sahel and Saouef basins. Hydraulic gradient of groundwater vary from 1 to 15 %0. The average salinity of these aquifers is ranging from 1 to 4 g/l near outcrops and with great values reaching 75 g/l for deepest aquifers. The shally rate in sandstone reservoir bodies is less than 20% and Water saturation is about 80 %. Intensive fracturing by deep seated faults bounding the different sub-basins play a great role for groundwater recharge and inter-layer circulations between affected reservoirs. Deep water characterisation by piper, stiff and Gibbs diagrams and PCA analyses highlights a variable sulfate, sodic, bicarbonate and magnesic waters, useful for drinking and/or irrigation. The total estimated amount of Oligo-Miocene geologic sandstone porous reserves in the Saouaf and Sahel areas is about 10 10¹² m³ equivalent to 1.500 10⁹ m³ of water, if we consider in pessimistic scenario all aquifers as captive ones with an average value of storage coefficient of 1.5 10^-4 along a superficy of about 40 000 km². Taking into account the annual renewable rain water supply of 5 10⁹ m³ in Tunisia, the estimated deep hydrogeologic potential in these areas could serve as a strategic reserves for decades.