Abstract: In the oil fields of northern Iraq, oil is generally produced from more than one reservoir. These reservoirsconsist of Tertiary (one reservoir interval) and Cretaceous (two reservoir intervals) aged units. In this study we triedto find out whether oil in different reservoirs is formed from a single source rock or from different source rocks andwhether they are formed as a result of a single or multi-phase migration.The Khabaz Oilfield is investigated in this study and wells selected in this field are producing from both Tertiaryand Cretaceous reservoirs. Physico-chemical properties of the oils such as specific gravity, API-gravity, amountsof asphaltene, sulfur, wax, ash and Ni, V and chemical composition of reservoir water are used in order to achievethe research purpose. Firstly; these parameters, which were separately defined for each well and reservoir, werearranged as a data base. Then by using computer-aided assessments and comparisons the data were comparedwith each other by the statistical methods such as correlation coefficient (R), factor analysis, and analysis of cluster similarity in order to determine similarities, proximities and differences between the reservoirs.Khabaz oil field has three separate reservoir intervals. These include Tertiary (Jeribe), Cretaceous (UpperQamchuqa) and Cretaceous (Lower Qamchuqa). According to the results of evaluation, all of the wells produce oilfrom a single structure in Khabaz oil field. Only Khabaz-2b is located out of this structure. In this field the specificgravity of oil is high and it contains abundant asphaltic components. Oil produced in the Khabaz Field is originatedfrom the same source rock (Chia Gara Formation). However, according to the result of statistical evaluation it isshown that there are two types of oil (a and b) in the Khabaz oil field. It could be concluded that this compositionaldifference is caused by degradation of oils during migration and/or in reservoirs.