Opec’s blind spot when it comes to counting its members’ output of condensate and other NGLs – 7mn b/d and rising – means headline output assessments give only a crude indication of their impact on world oil markets.

It is a peculiarity almost, if not entirely, unique to Opec that its members’ calculated crude oil production entirely ignores well-head condensate. US headline crude oil production figures for instance include this lease condensate, in...