OPEC’s Secretary General Mohammed Barkindo on Tuesday called on U.S. shale oil producers to help curtail global oil supply, warning extraordinary measures might be needed next year to sustain the rebalanced market in the medium to long term. “We urge our friends, in the shale basins of North America to take this shared responsibility with all seriousness it deserves, as one of the key lessons learnt from the current unique supply-driven cycle,” said Barkindo. The Organization of the Petroleum Exporting Countries official’s comments came during a speech delivered at a conference.