oa Sedimentology, Diagenesis and Reservoir Characteristics of Eocene Carbonate, Sirt Basin, Libya

Abstract: Hydrocarbons in the Sirt Basin Libya have been found in multiple clastic and carbonate<br>reservoirs from Precambrian to oligocene age. The middle Eocene Nummulite accumulations of Gialo<br>Formation form important hydrocarbon reservoir interval within the Mesozoic-Cenozoic Sirt Basin that<br>originated as large scale subsidence and block faulting commencing towards the end of the Early<br>Cretaceous and continued to develop into the Miocene and perhaps to the present day. Reducing risk in<br>exploration demands an understanding of reservoir facies development, which is governed by the type<br>and distribution of depositional facies and their diagenetic history. Six depositional facies have been<br>identified in respect with the detailed core description, petrographic texture and the faunistic<br>assemblage. These are: Nummulite Facies, Nummulitic Discocyclina Facies, Nummulitic Operculina<br>Facies, Discocyclina-Nummulite Facies, Bioclastic facies and Mollusca Facies. These facies and<br>microfacies can be interpreted as having accumulated in open marine, fore-bank and bank setting.<br>Well preserved large benthic foraminifera dominate the faunal assemblage in the Gialo Formation<br>indicates deposition within the photic zone. Present day reservoir characteristics of the Gialo Formation<br>are the net result of modification to the original depositional characteristics caused by diagenesis.<br>Theses diagenesis took place on the seafloor, under burial, and in the meteoric diagenetic<br>environments. Petrographic and petrophysical studies indicate that porosity and permeability in the<br>Gialo Formation reservoir are the result of the depositional environments of deposition and diagenesis.