Sedimentary Facies and Depositional Environments of the Upper Palaeocene Harash Formation, Nasser Field, Sirt Basin, Libya

The sedimentary succession of the Sirt Basin reflects its tectonic and structural evolution, which is closely related to the opening of the Atlantic Ocean and the convergence of Tethys in Mesozoic and Tertiary times. Sedimentation during the Upper Palaeocene-Lower Eocene in the central and western Sirt Basin was characterized broadly by shallow-marine carbonates. Six major carbonate lithofacies have been identified within the Landenian/Ypresian carbonate succession in the study area, and these range from mud-supported carbonates to grain-dominated facies, and are interpreted as predominantly deposited under shallow-marine conditions, within the photic zone, as indicated from their richness in phototrophic fauna and flora (thickness ~ 94-169ft / 29-52m). These include restricted marine environment and more open shallow-marine environment and are interpreted as a carbonate platform developed in a homoclinal ramp situation. The type and distribution of the Harash depositional facies were influenced by basin-floor architecture and environmental controls. Porosity is low in the Harash carbonate, reflecting the fine-grained nature and coalescence of the micrite matrix, regardless of intraskeletal pores that are occasionally large. However, higher levels of porosity (10-20 %) were generated where rocks grade into wacke-packstones. Petrographic and petrophysical studies indicate that porosity in the Harash Formation is controlled by depositional environment and tectonic setting.