One significant barrier to a major increase in Iran’s oil output could be its membership of the Organisation of Petroleum Exporting Countries (Opec). The 12-member cartel, which controls a third of the world’s oil supply, maintains a quota system that Tehran would have to negotiate. It’s unlikely that Saudi Arabia would be willing to support any increase in Iranian exports, which sets up a tense meeting of the group’s ministers when it next meets in Vienna in June. By then a barrel of oil may already be selling for $20.