The Zubair Formation was selected in the X oilfield to determine the tar zones via core, and wells logs including micro-resistivity and NMR. The study focused mainly on the main pay of the Zubair Formation, which contains a thick variable tar mat interval near the oil-water contact area, and it impacts reservoir productivity. Tar mat zones were distinguished using different paradigms such as descriptions of core and study borehole logs behavior. The result indicated that the tar mat thickness increases in the middle part of the X oil field due to the presence of high amounts of shale volume. Therefore, tar mats create barriers between oil columns and formation water, preventing the production of secondary oil in these regions.