Add Southern Green Canyon and Mars Blend to the growing list of American crude that’s challenging OPEC’s dominance in the world’s biggest oil market. Cargoes of the two varieties produced in the Gulf of Mexico, which are heavier and more sulfurous than supply from U.S. shale fields, are poised to flow into Asia as they turn cheaper relative to similar-quality crudes from nations such as Saudi Arabia and Oman. The deal between producers worldwide to cut output and ease a glut is boosting the cost of Middle East supplies, priced against the Dubai benchmark, because most of the reductions are coming from the region. Meanwhile, U.S. marker West Texas Intermediate is turning relatively weaker as a rebound in global crude prices from the worst crash in a generation is spurring more American rigs into action. Shale oil that was already cheap enough to sail to Asia is now being […]