The EAGE Conference held in Algiers in April 2005 was the opportunity for 23 attendees to participate in a post-conference field-trip organized in the Tassili N’Ajjer outcrops in southern Algeria. The stratigraphic succession from the infra-Cambrian to the Middle Devonian is exposed in these exceptional outcrops, desert conditions favouring continuous exposure. The field trip started in the city of Djanet in the Infracambrian granites and shales and ended five days later in Illizi in the Lower Devonian Emsian sandstones (Figure 1). During the trip, these outcrops were described in terms of sedimentology, fracturation, and sequence stratigraphy. The outcrops of this area are of particular interest for petroleum geologists as the same sedimentary succession produces hydrocarbons in the nearby Illizi and Berkine basins and in the Hassi- Messaoud area. The Cambrian, Ordovician, and Devonian sandstones observed in the Tassili outcrops have an equivalent in the reservoir units in the subsurface. The Silurian hot shales are also the prolific source rock of the petroleum system in these basins. The succession presented in the outcrops can then be directly compared with the subsurface data of the Illizi and Berkine basins. The area was first surveyed by Killian (1922) who described the sedimentary succession of the region. Then, the lithostratigraphy and the Siluro-Devonian succession in the outcrops were established by Dubois et al. (1967). The reference study of the Lower Paleozoic outcrops in Algeria was published by Beuf et al. (1971). These authors proposed a lithostratigraphic nomenclature for the Lower Paleozoic and a sedimentological description of the formations. They also identified and interpreted the main unconformities in the Lower Paleozoic with a modern approach, discussing the relative role of tectonics and climate during sedimentation. A detailed mapping of the area was also performed by Sonatrach and Beicip under the supervision of the Ministère de l’Industrie of Algeria. However, since these pioneer studies, little has been published about the outcrop geology although hydrocarbon exploration and development has been very active in the nearby Illizi and Berkine basins. The objective of this paper is to propose a reference section for the Lower Paleozoic in the western Tassili with an updated sedimentological interpretation. An interpretation in terms of sequence stratigraphy will also be proposed. We then summarize the lithostratigraphy in use in the outcrops and propose a correspondence with the sub-surface.
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Eschard et al. (2005) studied this question.