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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, SW Libya. In the southwest of the concession area the Mamuniyat Formation comprises braided stream sandstones that pass basinwards, to the northeast, into marginal marine and shallow water marine shelf sediments dominated by sandstone. 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. Sequence stratigraphic analysis of the Mamuniyat Formation, emphasizes the role of unconformity-bounded sediment packages. These are interpreted in terms of regional tectonic events, especially the Taconic orogenic event, and changes in shelf physiography in response to relative fluctuations in sea level, which in turn affected stream equilibrium profiles. Transgressive, highstand and lowstand systems tracts have been identified within the biostratigraphically, poorly constrained Mamuniyat Formation, thereby allowing for a more accurate correlation between the three oilfields. Stratigraphically the Mamuniyat has been divided into a Lower, Middle and Upper part, each assigned informal member status. In terms of this subdivision the Lower Mamuniyat corresponds to a transgressive systems tract when sediment was deposited concomitant with a relative rise in sea level, flooding the entire shelf. During the final stage of this transgression a radioactive shale unit, recording a maximum flooding surface was deposited in the lower part of the Middle Mamuniyat. The upper part of the Middle Mamuniyat corresponds to a highstand systems tract, and the Upper Mamuniyat, which is confined to the B-Field, records lowstand conditions. The Tanezzuft Shale, above the Mamuniyat Formation, records a major transgressive event of regional and economic significance, providing both source and seal for primary Lower Palaeozoic sandstone hydrocarbon reservoir targets. |