SILURIAN – DEVONIAN OF THE ORIENTAL ALGERIAN SAHARA: implication of new field data from Tassili n'Ajjer outcrops and Berkine Basin (SE, Algeria) for shale gas exploration.

The Silurian – Devonian succession have been deposited in wide sags and sub-basins in a cratonic setting, along the northwestern passive margin of the Gondwana during the opening of the proto-Tethyan ocean.
During this Siluro-Devonian Period, a high subsidence occurs stacked sediment bodies and organic-rich shales were deposited in many places that respectively form important hydrocarbon reservoirs and source rocks throughout North Africa basins and Arabia (Boot et al., 1998 ; Lüning et al., 2000), which lead Algeria to hold the fourth position in terms of the estimates of technically recoverable shale oil and shale gas resources, based on major basin assessment in the world (EIA, 2013).
In the prolific Illizi and Berkine basins (western Ghadames, Algeria) a basin-wide approach is needed, especially with the new “shale gas and shale oil” frontier in the upcoming years, in order to capture regional trends and re-assessment on organic-rich (‘hot’) shale unites which in smaller-scale industrial studies are overlooked.
Berkine Basin covers nearly 102.000 km2. This basin has been a thick sedimentary series of 7000 m corresponding to complex reservoirs but also black shales of the basal Silurian and Frasnian respectively major and secondary source rock systems.
Wireline-logs from more than 146 (Berkine – Illizi Basins) Algerian petroleum exploration wells have been studies (Djouder et al., 2012; Djouder et al., 2014), and recently 03 months extensive field study of outcrops of the Siluro-Devonian sediments was carried out at the south margin (SE, Tassili n’Ajjer) of the basin. The following analysis incorporates biostratigraphic, ichnological, sedimentological and high resolution stratigraphic data, which provides a framework evolution from offshore to deltaic deposition for the Silurian then fluvial to normal-marine depositional conditions for the Devonian.
A total of 06 short-term, third-order sequences can be identified for the Silurian deposits: (i) 'Hot Shale' Oued Imihrou Formation, organic-rich shales with carbonates nodules (Si-1); (ii) above is the Atafaïtafa Formation 500 to 650 m thick with HCS, gutter casts and Cruziana Ichnofacies (Si-2, Si-3); (iii) then overlaid by coarsening-shallowing-upward units. These sequences (Si-4, Si-5 & Si-6) with Storm-wave dominated and Skolithos Ichnofacies are interpreted to reflect deltaic origin.
This delta is the result of a significant sea-level drop during the Upper Silurian (Beuf et al, 1971) but also from the high sedimentation rates prograding northwards (Eschard et al., 2005).
Devonian deposit also can be divided into fourth third-order sequences (Dev-1 to 4), by the same previous analysis. The base of the cycle is the Caledonian unconformity linked to a tectonic event which affected most of the Gondwanan margin at the Silurian – Devonian boundary and a major relative sea-level fall (Eschard et al., 2005).
Previous sequences defined from Tassili n’Ajjer outcrops are well recognized at the basin-scale (Berkine Basin) and can be correlated in the subsurface of the Illizi – Berkine basins (Djouder et al., 2012).
Finally, the full depositional cycle for the Silurian – Devonian is reached and capped by the Frasnian maximum flooding surface, with black shales across North Africa (Algeria, western Libya and southern Tunisia) and which are also developed in many parts of the world (Lüning et al., 2003).