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Nuri Mohamed Fello 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 present study is based on slabbed cores, photographs, core samples, wireline log data and conventional core analysis of the Mamuniyat siliciclastic sediments, which attain a total thickness of about 170 m. The petrology, textural properties, lithology, sedimentary structures, sequence stratigraphy, lithofacies and depositional environments of these sediments have been studied in details. Lithostratigraphically the Mamuniyat Formation has been divided into a Lower, Middle and Upper Member according to grain-size, wireline log response and the proportion of sandstone to siltstone and shale. The three members have been assigned informal status within the Mamuniyat Formation and are the basis for facies analysis of the Mamuniyat sandstones throughout the El-Sharara Field. Throughout the Mamuniyat Formation ten facies have been recognized, based on particular associations of rock types and their lithological, sedimentological and biological characteristics. These facies have been grouped into three different facies associations. The Lower Member of the Mamuniyat Formation is characterized by the presence of a sandstone dominated coarsening-upward sequence accompanied by increased bioturbation, towards the northeast. The sandstone is internally structured by small-scale trough cross-bedding, hummocky cross-stratification and ripple cross-lamination. The Middle Member of the Mamuniyat Formation is dominated by heterolithic-claystone sediments. It is mainly composed of black shales (Radioactive Shale) with subordinate fine- to medium-grained sandstone and occasional coarse-grained siltstone. The black shales are intercalated with wavy, rippled reddish-brown fine-grained sandstone and coarse-grained-siltstone beds. The entire Middle Member shows a general fining-upward trend and passes sharply upwards into fine-grained sandstone, siltstone and shale. The Upper Member of the Mamuniyat Formation also consists of sandstone, showing a coarsening-upward sequence from coarse- to very coarse-grained, which becomes more poorly sorted upwards, accompanied by an increase in bed thickness and the scale of cross-bedding. 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 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 Members of the Mamuniyat. These are attributed to temporal variations in source area uplift, base level change, sediment flux and accommodation space. 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. Sediment composition, regional facies patterns and sequence stratigraphic analysis 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. The NC115 Concession is unlike most other Palaeozoic basins in Libya, in that there is no direct evidence of a glacial influence on deposition during Upper Ordovician (Ashgillian) times. |