TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Upper Zubair Formation is a giant reservoir consisting of 350 ft of excellent quality sandstone. This main producing reservoir has a 100 thick tar mat historically encountered near oil water contact. Recently, a well completed in the crestal part of structure above tar mat zone and in good quality sand did not contribute to oil production and presence of tar was suspected. A re-look at identification, origin and distribution of the tar mat has become critical for formulating future development strategy of the reservoir.

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