Zubair Formation in the giant oil field of North Rumaila in southern Iraq is a good oil reservoir that consists of friable porous sandstone intercalated with thin shale and siltstone layers. The main pay of this reservoir has a thick variable Tar mat (asphaltic) interval near oil-water contact and influences reservoir production and the aquifer support. To delineate Tar mat zones in the reservoir unit of Zubair Formation, different paradigms were followed in this study including description of the cutting and core samples and borehole logs. Numbers of wells were selected to identify the thickness of Tar mat zone in the eastern and western flanks by core and rock cutting samples in addition to the study of borehole logs such as resistivity and nuclear magnetic resonance logs. Asphaltenes are normally dissolved in hydrocarbons in the form of suspension. Then, we study a number of processes can cause these compounds to drop out of suspension at certain zones and the possible mechanisms of Tar mat formation in the considered reservoirs.