Understanding sedimentary and volcanic interactions in the Blue Nile Basin, Ethiopia: Implications for prospectivity along the East African Rift System

The East African Rift System (EARS) is a focus of hydrocarbon exploration. Encouraging results have been gained from the western branch of the EARS. Exploration is, however, limited in the central overlap between Kenya and Ethiopia, where interbedded volcanic and volcaniclastic sequences create complexity and degrade reservoir potential. In this study, we use the Blue Nile Basin, Ethiopia, as an analogue to other rift basins further south. The uplift and subsequent incision by the Blue Nile River has exposed a >1.5 km thick Mesozoic sedimentary succession mantled by the Oligo-Miocene Ethiopian Flood Basalt Province. Lithostratigraphic logs collected across the basin detail the unconformable transition from marine limestones and fluvial sandstones to the subaerial lava flows. Thickness and facies mapping combined with spatial analysis of lineaments and volcanic features are beginning to delineate sub-basins. In some areas, clean siliciclastic sandstones are interbedded with the basal lava flows suggesting volcanism did not shut down pre-existing clastic systems. In other areas, a several hundred metre thick interlava sequence has been recognised in the middle of the lava flow succession and mapped over distances of at least 50 km. The base of the interlava sequence consists of mass flow and fluvial volcaniclastic conglomerates and sandstones that are dominated by felsic material (e.g. quartz, rhyolites) in the north and mafic material (e.g. pyroxene, basalts) in the south. The well rounded nature of the clasts suggests relatively mature systems and that the volcanism was protracted and piecemeal. The variability in composition has implications for drainage development/pathways within the lava field as well as reservoir/sealing properties due to their differing susceptibilities to alteration. The volcaniclastic units are typically lithic greywackes with highly altered matrixes that have high helium porosities (>16%). The upper part of the interlava sequence consists of diatomaceous units and felsic tuffs suggesting the development of large-scale lakes during volcanism. These lacustrine systems may potentially contain Oligo-Miocene source rocks broadly contemporaneous to others known from along the EARS. They may also supplement those in the Upper Jurassic that feed bitumen seeps within the volcanic pile of the Blue Nile Basin. The development in the Blue Nile Basin of intra-volcanic fluvial to lacustrine systems with different volcanic source inputs are analogous to and may help unlock the full prospectivity of the proven petroleum systems in the central overlap of the EARS.