- Depositional Environments and Hydrocarbon Significance of the
- Mamuniyat Formation in the NW Murzuq Basin, SW Libya
Nuri M. Fello and Brian R. Turner
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 Upper Ordovician (Ashgillian) Mamuniyat Formation is the primary reservoir target in three oilfields: A, B and H within Repsol Oil Operations Concession area NC115, which lies on the northwestern flank of the Murzuq Basin.
The Mamuniyat Formation is widely distributed on the northwest flank of the basin and it shows good reservoir quality particularly in A, B and H-Fields. The Mamuniyat reservoir shows lateral and vertical variation in petrophysical properties, due to the presence of variable facies types. The main oil generation is believed to have taken place in the Carboniferous immediately prior to Hercynian uplift, with the oil migrating from the NNW into NC115 Concession. Palaeozoic clastics form good hydrocarbon reservoirs, sourced by the Tanezzuft shales. Hydrocarbons may have accumulated in the structural high in the eastern parts of the basin, where combination and stratigraphic traps may exist in association with Palaeozoic unconformities and subsurface folds.
Sediment composition and regional facies patterns 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, which is only 150 km to the southwest of the Concession area NC115. Periodic uplift of the basin margin in the SW, and associated base level changes led to the basinward progradation of braided fluvial systems followed by marine transgressive events emanating from Palaeotethys in the northeast. Both the braided fluvial and shallow water marine sandstones of the Mamuniyat Formation are primary hydrocarbon reservoir targets, with the main source and seal being the eustatically controlled Lower Silurian Tanezzuft Shale (Hot Shale). These same tectonic events influenced facies patterns, sediment deposition and interaction between a variety of shallow water marine and fluvial depositional environments across a NW-SE oriented storm-influenced coastline.