Regional geology and petroleum systems of the Illizi-Berkine area of the Algerian Saharan Platform: An overview

The Berkine and Illizi basins are Palaeozoic-Mesozoic intraplate depressions that preserve an over 7000 m thick sedimentary rock record and contain world-class petroleum systems with over 39 BBOE EUR hydrocarbon reserves. Regional seismic transects; and a wealth of well data are used to review the tectonostratigraphic evolution of the area. The Gondwana (Palaeozoic) and Tethys (Mesozoic) supercycles make up the bulk of the Phanerozoic succession. These 1st order cycles record extensional basin formation, followed by a protracted period of overall net subsidence, and conclude with basin inversion and regional uplift during the Hercynian and Alpine orogenies respectively. Structural styles were dictated by transpressional or transtensional reactivation of mostly NNE oriented basement structural grain of Late Proterozoic-Ea.Cambrian (Pan-African) age and vertical movement of long wavelength regional highs and lows. Local tectonism is considered to be the far-field effect of plate-tectonic processes that affected the North African plate-boundaries, such as the Caledonian and Hercynian Orogenies, the opening of the Tethys and Atlantic oceans, and the Alpine Orogeny. These were accompanied by localised thermal mantle processes. Regional unconformities subdivide the Gondwana and Tethys supercycles into eight and seven megasequences respectively. These regional units often record 2nd order transgressive-regressive cycles that constrain the mapping of reservoir-seal pairs and of main source rock intervals. The lower Palaeozoic megasequences are characterised by sand-rich laterally extensive coarse-grained fluvial and shallow marine reservoirs interbedded with thin and widespread marine shales. They were mostly deposited in shorelines of shallow epeiric seas and continental braided river systems, or during the Late Ordovician, in a short-lived continental glacial system. Third order sequences display ramp-style geometries with largely parallel to low-angle sigmoidal stratal patterns. Early Mesozoic sedimentation followed the Late Palaeozoic uplift and peneplanation in the form of hinterland sequences containing fluvial and alluvial sandstones and playa-lake mudstones and evaporites interbedded with thin lacustrine or shallow marine shales and dolomites. The prolific petroleum systems that evolved within these series are associated with the Early Silurian and Late Devonian world-class source rocks and numerous Paleozoic and Triassic reservoirs. This regional review provides a renewed comprehensive insight into the geological evolution of the area that can help guide local exploration efforts, but also serves as a model for exploration in other intra-cratonic basins of the world. (C) 2009 Elsevier Ltd. All rights reserved.