Sedimentary History and Stratigraphic Sequences

Rocks dating from Early Devonian are made up of sandstones with plant remains resting discordantly on the Silurian deposits. Throughout the Devonian, sedimentation is characterized by a repetition of transgressive/regressive cycles with an alternation of mudstones and sandstones. The reservoirs in Devonian sandstones represent important productive levels in the Illizi Basin. Uplift occurred during the upper Devonian and even part of the upper Silurian in the southern part of the Eastern Erg Basin. 

The post-Hercynian subsidence was much less pronoimced in the Illizi Basin than in those farther north. With the Devonian reservoirs at rather shallow depths (800-1000 m) in the Illizi, the Jurassic deposits follow concordantly on those of the Triassic (Busson 1967). The Liassic is made up of evaporites, the overlying Dogger becoming increasingly argillaceous upwards, and the Malm comprises essentially mudstones. 

The Cretaceous sediments consist of evaporites, limestones, dolomites and thin sandstone beds. The calcareous littoral facies of the Apt and Alb are oil-bearing in Tunisia and sandstones of the same age represent the main aquifer of the northern Sahara. The subsequent Tertiary sedimentation in Tunisia led to great thicknesses of up to 7 000 m. These sediments represent the succession from the Paleocene to Pliocene in the east and in the littoral parts of the Gulf of Gabes in Tunisia. The Alpine orogeny of the Tertiary raised the unstable parts of the platform and resulted in a multitude of faults and other complex structures. 

At the end of the Cretaceous the northern basins were uplifted simultaneously at the onset of the Alpine orogeny, to become covered by thin deposits of sands and limestones dating from the Mio-Pliocene. 

---