Opec-led production curbs since the start of 2017 have so far failed to bring about the planned market rebalancing. They look likely to be extended to at least the end of 2018. But while global oil inventories remain well oversupplied, this year’s price gains are set to boost Opec revenues by around 23% from last year’s nadir.

Opec agreed to cut production from January by 1.2mn b/d in concert with 10 non-Opec producers who would cut a further 600,000 b/d. But to call it an...