The oil drilling holes carried out on the Saharan Platform (southern Tunisia) produced evidence of the presence of Paleozoic bedrock. The dominant lithology of the Lower Paleozoic is clays and sandstones whereas the Upper Paleozoic is marked by carbonated sandstone and clayey marine series. The qualitative and semiquantitative mineralogical compositions (bulk rock and clay fraction) were evaluated by X-ray diffraction, using Siemens Kristalloflex 810 diffractometer. The chemical analyses of major elements were carried out by Atomic Absorption Spectrophotometry. Trace and rare earth elements were determined by ICP-MS Perkin-Elmer SCIEX Elan-5000. Chemical microcomposition was determined using Phillips CM20 STEM. The mineralogical studies of Paleozoic clays showed the predominance of illite accompanied by small amounts of kaolinite and chlorite. The geochemical data put in evidence the relatively high amounts of Fe₂O₃ and Na₂O which are most probably associated with aluminosilicates. The flat pace of the rare earth curve analyzed in relation to the NASC indicates the tendency to the impoverishment in HREE.