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The Murzuq Basin, SW Libya, is one of a series of Palaeozoic intracratonic sag basins on the North African Saharan Platform. The structural fabric of the basin was developed during the Late Proterozoic Pan-African orogenic event, which has strongly influenced the stratigraphy and depositional patterns within the predominantly Palaeozoic clastic basin-fill. The main focus of this study is the Upper Ordovician (Ashgillian) Mamuniyat Formation, which is the primary reservoir target in three oilfields A, B and H within Repsol Oil Operations Concession area NC115, on the NW part of the Murzuq Basin. A major problem with the Mamuniyat is the location of the sediment provenance, due to the lack of adequate subsurface and outcrop data, and the relationship and controls on sediment flux and the depositional systems. Petrographic data derived from sandstone samples from cored intervals through the Mamuniyat Formation show that they can be classified mainly as sublitharenites, with some quartz arenites and litharenites. Compositional data for the three oilfields indicates that they were derived from a similar parent rock, but with differences in modal composition, textural attributes and porosity of the lower, middle and upper parts of the Mamuniyat attributed to temporal variations in source area uplift, base level change, sediment flux and accommodation space. Facies patterns suggest that these same tectonic events influenced sediment deposition and interaction between shallow marine and fluvial depositional systems across a NW-SE oriented coastline, subjected to frequent storms. Sediment composition and grain-size parameters, together with regional facies patterns all suggest that the Mamuniyat sandstones were derived from a nearby, tectonically active, granitic basement source terrain, which was most probably the uplifted Ghat/Tikiumit Arch to the SW of NC 115 Concession. Periodic uplift and base level changes led to the basinward progradation of braided fluvial systems followed by marine transgressive events from the NE, across a storm-influenced shallow marine shelf. The braided fluvial and shallow marine sandstones are the primary reservoir target, with the main source and seal being the Lower Silurian Tanezzuft Shale. |