Saudi Arabia’s ability, and willingness, to adjust output up or down to offset production volatility from fellow Opec members has steadied the ship in recent months. With market rebalancing delicately poised, and Venezuela, Nigeria and Libya remaining unstable, the need for flexibility from Riyadh is set to grow in 2018.

January’s collective output was 560,000 b/d above the grouping’s nominal 31.92mn b/d production target – based on the agreed upon 32.5mn b/d plus...