When water is needed for water injection in oil fields of an arid country, far from sea, the water supply is facing various technical and economical problems. The only solution is to find underground problems. The only solution is to find underground waters unattractive for any other possible use. In the selection of water sources, oil exploration is a big chance for underground aquifer surveys and knowledge. This paper shows how it has been used in defining the best way to provide the oil industry water needs and in evaluating possible interferences with other uses such as regional agricultural development plans. plans. First geological studies from oil exploration or development wells allowed to reach a good definition of aquifer extensions and thicknesses. Then water well tests first led to a good knowledge of hydro-geological characteristics. Finally, mathematical models were specially built to define the aquifer deliverabilities, the best water well schemes and the regional consequences on water levels.
Combined use of the two deep aquifers, confined turonian aquifer at an average of 850 meters below ground level with very salty water and artesian albian-barremian aquifer at an average depth of 1400 meters below ground level with fresh water, has been shown necessary to satisfy oil injection water needs and local industrial and domestrical water consumption that together will reach about 1 m /s during the next ten years.
Introduction
Oil from the giant Hassi Messaoud field is extracted from the Cambrian formation since 1956. The field is divided in several parts, called "zones". Many of these zones are passing below the bubble point and will require pressure maintenance as soon as the critical gas saturation will be reached. In two pilot areas, miscible flood has been replaced since 1973 by water injection; this water is supplied from some water wells drilled in the turonian aquifer. But reservoir engineering studies have defined new water requirements for these two zones and some ten others. The strong increase in water needs asked so for a complete and precise study of water supply capabilities of the different existing aquifers. Among the four main water-bearing formations, which are from bottom to the surface, jurassic, albian-barremian, turonian and miopliocene, the study concentrated on the two intermediate deep aquifers as the jurassic formation, tested by oil exploration wells, is not productive and as the miopliocene (water table) productive and as the miopliocene (water table) aquifer, which has a very high deliverability, contains fresh water to be preserved for human consumption and agricultural development.
DESCRIPTION OF WATER BEARING FORMATIONS
Oil exploration, with wildcats scattered in all the northern Sahara sedimentary basin, proved the continuity of geological formations which are water-bearing. Two fundamental works, Busson's geological study and water management ERESS project from UNESCO, have gathered an huge amount of geological and hydrogeological data. General description of the aquifer formations is drawn from their works. Figure 1 gives a rough description of northern Saharian basin, from Morocco to Tunisia; it shows albian-barremian outcrops which are either recharge or leakage areas, the main streamlines and the two great ergs covered by sand dunes. Figure 2 shows the cross-section from Reggane to Matmata (troglodyte village) passing through Hassi Messaoud field; it presents the different water bearing formations and presents the different water bearing formations and gives a rough idea of their depths and thicknesses. Piezometry for albian-barremian aquifer and Piezometry for albian-barremian aquifer and miopliocene-eocene indicates that the first one is artesian and the second one a water table. Jurassic and turonian aquifers, with salty waters, are confined.
