This paper attempts to unravel the interacting effects of eustacy, subsidence and sediment supply in controlling facies distributions in the Palaeozoic of the Algerian craton. These strata, particularly the Cambro-Ordovician section, contain no diagnostic index fossils, and the identification of sequence boundaries is therefore of paramount importance in defining a stratigraphic framework for the region. The wealth of outcrop and subsurface data in the Algerian Palaeozoic enables us to recognize six major sequences, the boundaries of which correspond to major events such as Taconic and Hercynian tectonism, the Ordovician glaciation and global-scale sea-level falls. Formations within these sequences exhibit thickness variations and lateral facies changes that are traceable and predictable in the subsurface. Unconformities of regional extent generally tie to significant facies changes and to inflection points on the global sea-level curve. These trends stem from the high sensitivity of this area, which was structurally stable over most of its Palaeozoic history, to eustatic sea-level fluctuations. The Cambro-Ordovician strata feature a high proportion of coarse clastic rocks, with subtle lateral and vertical facies variations, which were deposited within continental fluvial and shallow marine environments. The Silurian, Devonian and Early Carboniferous sequences show a typical pattern of marginal shallow marine sand belts, which migrate with changing sea-level and pass rapidly into deep marine shales, many of which are organic rich. The Upper Carboniferous strata consist of sediments deposited in continental and restricted basin settings, created as a result of tectonic movements heralding the Hercynian event. With respect to the occurrence of petroleum, the most important reservoirs are found in lowstand tracts in the Cambro-Ordovician and Siluro-Devonian sequences. Thick radioactive marine Silurian and Devonian black shales deposited during major transgressions provide both source and cap-rocks.

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The Taoudeni Basin is the largest sedimentary basin in Africa. This intracratonic basin, which forms the sedimentary cover of the West African Craton, records episodic sedimentation since the Proterozoic. It is a largely unexplored basin, with only eight exploration wells drilled to date, although its petroleum prospectivity was established in 1974 when gas and liquids were tested in the Abolag-1 well. This paper focuses on the identification and assessment of potential source rocks and reservoirs in the Taoudeni Basin in Mauritania and aims to establish a geochemical genetic link between the occurrence of petroleum and potential sources. Various disciplines and techniques were integrated to reach a better understanding of the petroleum systems potentially existing in this basin. We also define and describe the elements and processes of the Atar-Atar (!) petroleum system. This study is an example of the exploration of an unusual and high-risk play in some of the oldest sedimentary rocks on Earth, with a long, polyphase structural evolution, a complex thermal history (where burial might not be the only controlling factor), a massively diagenetized carbonate reservoir, and large uncertainties on the timing of generation, trap formation and preservation.

The Ordovician of North and West Africa comprises three main transgressive-regressive sequences understood as ‘second-order’ cycles of 10-15 myr duration. Tide- to wave-dominated shallow-marine clastic successions, preserving incidental bryozoan carbonates to the north, include fluvial deposits over the most proximal southern stretches of the platform. The boundary with Cambrian strata remains unclear but the latter are progressively less represented to the south in the undifferentiated ‘Cambro-Ordovician’. To the north, graptolites, brachiopods and trilobites combined with palynomorphs, provide a robust biostratigraphic frame. Maximum flooding intervals occurred in the early to middle Tremadocian, mid-Darriwilian and middle to Late Katian. Two events interfered with an overall long-term transgressive trend. The ‘intra-Arenig’ (late Floian?) tectonic event highlighted palaeo-highs coinciding with Palaeoproterozoic basements. Gondwanan drainage basins were reorganized, which had an impact on sediment sourcing and distribution of detrital material (e.g. zircons) feeding the pre-Variscan Europe. The second event is the end-Ordovician glaciation. The domain supported the greatest part of the Hirnantian glaciers and may also have preserved pre-Hirnantian glacial archives. It is not until the very latest Ordovician that offshore conditions developed far inland; it is however suspected that this inundation benefited from a transient postglacial isostatic flexure.