If shale does deliver on its promise and keeps prices low for decades, it will be a mortal threat to OPEC states that rely on oil revenue to cover social spending and placate fast-growing populations. The “break-even cost” for their budgets has doubled over the last decade, reaching $140 (£91) in Iran, $115 in Bahrain, $100 in Iraq, $80 in Saudi Arabia, and $75 even in the lightly-settled Emirates. Russia too has topped $100.