DEPOSITIONAL ENVIRONMENTS OF THE UPPER ORDOVICIAN, MAMUNIYAT FORMATION, NW MURZUQ BASIN, SW LIBYA

Abstract

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, on the NW flank 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 are mainly sublitharenites, with some quartz arenites and litharenites. Compositional data for
the three oilfields indicate that they were derived from a similar parent rock. 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. Petrography 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 to the SW of NC
115. Periodic uplift of the basin margin in the SW, and associated base level changes led to the basinward
progradation of braided fluvial systems. That was 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.