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Assessing a Petroleum System on the Frontier of Geological Time: the Mesoproterozoic of the Taoudeni Basin (Mauritania)

Abstract

The Taoudeni Basin is the largest sedimentary basin of Africa extending over almost 2 million square kilometres in six countries (mainly Mauritania and Mali, and marginally Algeria, Burkina Faso and Senegal). It is one of the major structural units of the West African craton but remains largely unexplored. The basin contains up to 5–6 km of Proterozoic to Cenozoic sediments. Although being a typical intra-cratonic basin (large extension and long-lived slow subsidence), its sedimentary succession recorded the major tectonic events that affected this portion of the continent and allowed outcropping of the infill at its northern edge. Sedimentation was dominated by shallow marine clastics but a thick stromatolitic unit with interbedded organic-rich claystones and shales, the Atar Group, was deposited during the Mesoproterozoic. The basin underwent several phases of deformation since the Proterozoic; the Hercynian orogeny defined its present configuration, and the subsequent Mesozoic subsidence was very slow. During the Late Triassic to Early Jurassic times, the Taoudeni Basin suffered an intense phase of volcanic intrusion. The hydrocarbon prospectivity of this frontier basin was established in Mauritania early in its exploration history. The Abolag-1 well, drilled in 1974, flowed about 0.48 MMscf per day of gas (and possibly some condensate) from the Atar Group carbonates, which constitute the main play. Abundant bitumen and other hydrocarbon evidences are widespread across the basin. Organic-rich intervals of the Atar Group constitute a world-class source, containing oil-prone organic matter with excellent generative potentials and TOC contents ranging from 5 to 25%. The long and multi-phased tectonic evolution of the basin, and its complex thermal history, makes the Mesoproterozoic-Mesoproterozoic (!) petroleum system difficult to assess even if the volume potentially generated is encouraging. A first hypothesis we stated is that the main generation phase took place during the pre-Hercynian subsidence phase, and therefore the long residence time of the hydrocarbons in the reservoirs would imply that the preservation of the traps (breaching) and fluid phase (cracking) are critical risks. However an alternative hypothesis can be proposed after re-examination of the exploration concepts in the light of new thermal, maturity and geochemical data. Younger processes could have occurred, resulting more favourable in terms of exploration risk.