Reservoir Quality, Structural and Sedimentological Study of the Upper Cretaceous/Lower Paleocene Kalash Formation, Rachmat Oil Field, Concession 13, Eastern Sirte Basin, Libya

In this research, the reservoir quality of the Kalash Formation (Upper Cretaceous/Lower Paleocene) within the Rachmat Oil Field, Concession 13, located in the eastern part of the Sirte Basin, was evaluated. The field, discovered in November 1976, has been producing from the Kalash Formation. The study focused on analyzing the reservoir quality concerning the structural geology and sedimentological characteristics of the Kalash Formation within the Upper Cretaceous level. The formation exhibits a diverse lithology consisting of limestone interbedded with dolomite. Logging data, including Gamma Ray (GR), Resistivity, Neutron, and Density logs, were employed in the analysis. Advanced analysis using Techlog and Surfer software was carried out to assess the petrophysical properties of five wells: L1-13, L2-13, DD1-13, DD2-13, and S1-13. The results revealed that the dolomite within the Kalash reservoir exhibited good quality, with porosity ranging from 5.5% to 13%. Structurally, the sedimentology of the Rachmat oil field is influenced by a legitimate closure, bordered to the west by a down-to-east normal fault intersecting the Hagfa Formation. The spatial relationship between wells L2-13, L1-13, and S1-13 at the Maastrichtian level indicates the presence of a reef at well L1-13, with L2-13 representing the back reef and S1-13 the fore reef area. The absence of hydrocarbons at the Cretaceous level in L2-13, which is up-dip relative to L1-13, suggests a geological barrier in the form of the fault separating the two wells, likely acting as a seal and hindering hydrocarbon migration. Keywords: Reservoir Quality, Structural and Sedimentological System, Sirte Basin, Kalash Formation, Upper Cretaceous/Lower Paleocene, Rachmat Oil Field.