Opec’s attempts to rebalance the market are being further undermined by sharply rising output from two members exempt from production curbs – Nigeria and Libya. Alongside continued strong non-Opec growth the supply glut looks set to continue into 2018.

Further output gains by both Libya and Nigeria drove a huge 270,000 b/d growth in Opec production last month, and the group’s 32.63mn b/d output was the highest yet this year. This is 710,000 b/d above the group’s...