When Opec meets in Vienna on 22 June, it will have to decide on the future of a production deal that provided a $125bn boost in annual revenues last year. Gains are on track to be an even higher $135bn this year, MEES number-crunching suggests.

Of course revenue gains for the remainder of 2018 will hinge on the upcoming discussions. When the deal was initially struck in November 2016, uppermost among concerns were that as the market approached balance the incentive for members to cheat...