Opec’s successes in reducing its crude output came to a juddering halt in May, ending a run of five consecutive monthly falls. Output rose 280,000 b/d primarily due to the two members exempt from the deal – Nigeria and Libya. With further gains possible in June, Saudi statements that Opec’s 12 other participants will “take the slack” from such gains may be put to the test soon.

Output of the rejigged grouping – Equatorial Guinea joined last month –...