Ghadames and Illizi basins are two of the major oil and gas producing provinces in Algeria. They are located on the Eastern Saharan platform. Their infracambrian basement is overlain by nearly 7500 m sedimentary cover including both Paleozoic and Mesozoic sections. These sediments have been modelled by several teotonic phases and the sedimentary column presents the finger prints of Hercynian, Austrian and Alpine tectonic events. At the North-West, erosion cuts as deeply as the Cambro-Ordovician. Effects of the Austrian phase are characterised by the rejuvenation of ancient lineaments which led to the creation of some structures. The basins achieved their evolution by the Tertiary phase.

The role of the source rock is played by the organic rich formations in the Saharan platform. The most interesting are the basal Silurian and Frasnian/Famennian radioactives shales. Samples are known to have a good to excellent organic content. TOC values range from 2-14% for Frasnian/Famennian and can reach peaks up to 17% within Silurian. The kerogens are type II (oil prone). Additional, secondary, source rocks have been recognised through the entire Ordovician but limited to some horizons within the shaly sections of Microcongiomeratic, Azzei and El Gassi formations. Maturity data integrated with a kinetic modelling indicate that except central Ghadames (Gas window) and the Northern part of Illizi basin (immature). Frasnian/Famennian is practically in oil window. The Silurian is generating oil over all the study area except the central Ghadames and South part of Illizi where it is overmature (gas). The cycles of hydrocarbon generations were realised during the Jurassic to Tertiary/Present for upper Devonian and during the Carboniferous to Cretaceous period for Silurian. Oils from Triassic and Devonian reservoirs are characterised by low asphaltenes, resins and sulfur content.

Oils and extract from probable source rocks where analysed by GC/MS for their detailed geochemical characterisation and maturity assessment.

Plotting of different molecular parameters with a vitrinite reflectance (Ro) of source rock; shows that a vitrinite reflectance calculated (Rc) for Ghadames and Illizi basins oils range from 0.58-0.90% but the majority of oils has Rc less than 0.70%.

The total volumes of hydrocarbons generated in the Illizi and Ghadames basins from the following sourcerocks: Silurian shales and Famennian-Frasnian shales, calculated from the method of the DES/EXP/SH are: 1370 Gm3 oil; 193 Tm3 gas.

GEODYNAMIC EVOLUTION OFTHE ILLIZI/GHADAMES BASINS 1. Sedimentary cycles The sedimentary cover of the Illizi basin which reaches a maximum thickness of 3000 m is mainly constituted of Paleozoic deposits buried in the

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