Opec’s inability to tackle global crude stockpiles despite output cuts has seen prices slump in the second quarter of the year. But Opec cuts have fallen largely on sour, heavier grades. As the majority of output gains elsewhere have been of light sweet grades, sweet-sour differentials have tightened considerably. The predominance of sour grades within Opec is therefore somewhat mitigating the impact of price falls on the group’s revenues.

The Opec basket averaged $3/B less than...