The eyes of the world will once again be on Vienna when the 15 Opec members and their 10 partners meet there next week. The 6 December meeting comes at a critical juncture, with oil prices having lost some $25/B since early October. Its outcome will have a critical impact on whether prices firm or test new lows.

After Brent settled below $60/B on 23 November for the first time since October 2017, prices have fluctuated either side of the line.

The 2016 “Declaration of...