Title: Depositional Environment Analysis and Revaluation of Petrophysical Properties of Reservoir Formation . Summary: Understanding of depositional environment of any reservoir formation has an impact on the successful development and exploitation of that reservoir, as it can affect the drilling practices, evaluation programs, and completion method. Knowledge of depositional environments is important for reconstructing earth history, understanding earth processes, and helping humans survive and prosper on earth. Geologists use analyses of depositional environments to help locate inside the earth, sources of oil, natural gas, and coal, deposits of valuable metals, minerals, rocks, and aquifers, which are usable sources of groundwater. Therefore, this graduation project will focus on the study of depositional environment of Sabatayn basin in general and Alif reservoir formation in particular. A special part of this graduation project will be dedicated to perform a re-evaluation of the petrophysical properties of the reservoir using available logs data. This project is focusing on describing the depositional environment for the petroleum system in Al-Nasr Field-Block 5 by interpretation of well logs. GR curve shape was the available source for interpretations. It also focused on re-evaluating of the petro-physical properties of the reservoir by using also well logging data and will be interpreted using Techlog 2015.3. Techlog 2015.3 will be used for extract GR curve for depositional environment analysis. Re-evaluation of petro-physical properties for the available well logs Re-zoning process was done for the main reservoir using GR log where thin shale layers were delineated separately. GR log represents vertical profile of grain size, as the shaly content in sandstone increases with decreases of grain size. Similarly, GR also shows deflection in trend as clay content decreases with increase of sand grain size.