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Abstract: The Structural Evolution of the Ghadames and Illizi Basins During the Paleozoic, Mesozoic and Cenozoic: Petroleum Implications

Francois J. Gauthier, Arezki Boudjema, Rabia Lounis

The Ghadames and Illizi basins cover the majority of the eastern Sahara of Algeria. Geologically, this part of the Central Saharan platform has been influenced by a series of structural arches and "moles" (continental highs) which controlled sedimentation and structure through geologic time. These features, resulting from and having been affected by nine major tectonic phases ranging from pre-Cambrian to Tertiary, completely bound the Ghadames and Illizi Basins.

During the Paleozoic both basins formed one continuous depositional entity with the Ghadames basin being the distal portion of the continental sag basin where facies and thickness variations are observed over large distances. It is during the Mesozoic-Cenozoic that the Ghadames basin starts to evolve differently from the Illizi basin. This separate evolution was a result of the Ghadames basin becoming the southern extension of the "Triassic" basin whose history was controlled by the opening and closing (towards the north) of the Tethyan ocean. The Paleozoic sedimentary succession (ranging in thickness from 0-3600m) was influenced by: the Pan-African controlled fracture network; the tectonic activity related to plate movements, and Eustatic sea level variations. Eustatic low-stand periods resulted in continental deposition yielding the major petroleum-bearing reservoir horizons (Cambrian, Ordovician, Siluro-Devonian and Carboniferous). High-stand periods correspond to the major marine transgressions covering the majority of the Saharan platform. These transgressions deposited the principal source rock intervals of the Silurian and Middle to Upper Devonian.

The Mesozoic and Cenozoic section (ranging from 0 to 3800 m in thickness) is restricted to the Ghadames basin and the northernmost part of the Illizi basin. The main reservoirs of the Mesozoic and Cenozoic are Triassic sandstone sequences which are covered by a thick evaporite succession forming a "super-seal". Structurally, the principal phases affecting this sequence are the extensional events related to the breakup of Pangea and the Alpine compressional events.

The Ghadames and Illizi basins, therefore, have been controlled by a polyphase tectonic history influenced by Pan African brittle basement fracturing which resulted in complex structures localized along the major basin bounding trends as well as several subsidiary trends within the basin. These trends, as

demonstrated with key seismic data, have been found to contain the majority of hydrocarbons trapped.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France