When Opec and its allies gather in Vienna next week, the grouping’s fortunes will  largely be dependent on factors outside of its control. Unable to influence US output or global demand trends, Opec+ talks will focus on boosting compliance among the laggards, and whether to act now to extend the production agreement beyond March.

Recent comments by Opec Secretary General Mohammed Barkindo highlight the extent to which Opec is banking on external factors balancing the market,...