Etude Statistique Et Géostatistique Des Éléments Chimiques Majeurs Des Eaux Du Continental Intercalaire.

Résumé: The term "Continental Intercalaire" is used to describe a continental episode interbedded between two marine sedimentary cycles: at the base, the Paleozoic cycle which ended with the Hercynian orogeny and at the top, the Upper Cretaceous cycle resulting in the Cenomanian transgression Busson (1970). The Continental Intercalaire (CI) hosts the lower Cretaceous formations represented by sandy and clayey sandstone from the Neocomian, Barremian, Aptian, and Albian ages. This aquifer is continuous from Saharan Atlas in North to the Tassili of Hoggar sothward, and from the Guir and Saoura valley in West to the Libyan desert Eastern. The objective of this work is to highlight the geochemical evolution of these waters. To achieve such a goal, a methodology of various approaches were used and is consists of geological and geostatistical investigations. The geological modeling was carried out to establish 3D models and stratigraphic sections which can explain the limits, thicknesses and different geological facies in the studied aquifer. The geostatistical modeling was carried out to determine the spatial variability of chemical elements in waters from the CI aquifer in the two basins of the northern Sahara. The main results of this thesis can be summarized as follows: The chemical elements of the CI waters in the two eastern and western hydrogeological basins reveal three families that are influenced by the lithology of the CI aquifer and are represented by: : - The first family that is linked to evaporites (Cl-, SO42-and Na +); resulting from the dissolution of evaporite formations and in particular Halite (NaCl) and Gypsum (CaSO4.2H2O) - The second family is characterized by the presence of the carbonate pole (Ca2+, Mg2+ and HCO3-) resulting from the dissolution of calcite (CaCO3) and Dolomite (CaMg (CO3) 2) minerals. - The third family represents waters influenced by silicates (SiO2 and K +) resulting from the dissolution of silica (SiO2) of sands, sandstones and clays. The obtained variograms of the chemical elements of the western part display a zonal anisotropy which explains the distribution of the elements in a zonal behavior which along in agreement with the distribution of the lithological facies . The higher contents of all chemical elements are located to the South and southwestern part of the western sector, excepting SiO2 contents which are more enhanced in the East and South-East. On other hand, in the eastern part of the CI aquifer, the eigenvector F1 variography shows a nested structure which represents three infrastructures from a surface point of view which controls the distribution of the elements K, Si, temperature and pressure. Similarly, the same variographic structure obtained from the CI top and bottom levels of the Eastern region. Such structures are mainly controlled by tectonic and geological phenomena; requiring further confirmation by other analytical methods. The CI stratigraphic model at Ouargla constitutes a compartmentalized aquifer, the structure of which in raised and collapsed blocks is, essentially, controlled by the folding of the Lower Cretaceous series into anticlinal and synclinal structures in which the play reactivation of these NE – SW accidents can allow a water communication between aquifers (drainance).

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