The economics of shipping additional volumes of heavy crude from Iraq to compensate for a potential loss of Venezuela’s supply are just not there at present, Shafaq News reported on Friday, quoting Iraqi economist Nabil Al-Marsoumi. Concerns in the heavy crude market are that the U.S. blockade of tankers carrying Venezuelan oil and heightened tensions in the Caribbean would reduce supply of the extra heavy crude. In theory, the sour and heavy grades from the top producers in the Middle East could be substitutes for Venezuela’s…