Waning compliance with the Opec/non-Opec “declaration of cooperation” to curb crude output is severely hampering efforts to rebalance the crude market. Allied with strong production gains elsewhere, this poor compliance has helped keep global stock levels high, putting a lid on prices in 2017.

A Saudi-led committee held meetings this week with production laggards in a bid to boost compliance. But the latest efforts look to be falling on deaf ears and the group’s August...