This article originally appeared on BusinessDay.

While much of the upstream sector has struggled mightily since crude oil prices began collapsing in the summer of 2014, refiners have enjoyed a period of relative prosperity.

Falling crude prices buoyed refiners’ margins, even as upstream producers trimmed costs and strained the capacities of their suppliers.

But the easy times for refiners may be fading rapidly. Margins have already tightened as crude oil prices stabilised and refiners began to pass their savings on to customers.

In the next few years, refiners will have to cope with several long-term challenges sure to separate leaders from laggards and likely to force some poor performers from the field.

Challenges include the global supply of crude getting heavier and more sour, as well as unstable supplies from North Africa and the Middle East, with fading production from the North Sea.

These have led to volatility in pricing, forcing refiners to adjust feedstocks more frequently and drastically than in the past and making long-range planning more difficult than before.

The influx of heavier, more sour oils also puts pressure on less complex refineries that must consider investing in conversion units if they want to stay in the game.

These global trends will affect the entire refining sector, but looking at key competitiveness factors—access to market, operating conditions and the quality of the asset portfolio—some groups are better positioned than others to thrive over the next decade.

Joachim Breidenthal is a partner in Bain & Company’s Johannesburg office and leads the firm’s Oil and Gas practice in Africa.