With a growing debate on how Opec and its 10 output deal partners will seek to exit their production agreement at the end of 2018, attention is turning to the impact on Opec’s market share.

Opec and the IEA display starkly divergent projections on this in their latest monthly reports, released this week. The IEA sees Opec’s market share falling 420,000 b/d in 2018 to 32.5mn b/d, while Opec sees it rising 340,000 b/d to 33.15mn b/d. This 650,000 b/d difference is worth around...