Global oil markets are gradually rebalancing, but the pace is more glacial than envisaged by Opec when it agreed on 30 November to implement a six-month production cut. By Opec’s own metrics it is clear that six months will be insufficient to do the job. Will it opt to extend the deal when it meets next month?

Russia and 10 other non-Opec producers early last December agreed to curb output alongside Opec, with the cuts coming into force from the start of this year. But whilst the aim...