On a dollar basis, Saudi Arabia was the biggest Opec winner in 2017 with revenues up $23.5bn year-on-year. But as by far the biggest producer in the grouping, its annual revenue swings are typically the greatest.

When looking at revenue changes on a percentage basis the clear winner was Libya, whose earnings soared 61% to $15bn. Perhaps not surprising given that crude output more than doubled from 380,000 b/d to 820,000 b/d. And with an average of 1mn b/d in the first five months of 2018,...