Royal Dutch Shell (RDSA) Plc., Europe’s biggest oil company, said crude prices would on average probably remain at current levels or rise steadily for the next two decades, with periods of volatility.

“There are still tensions in the system that will lead to more of a flat outlook,” Vice President for Global Business Environment Jeremy Bentham said in an Oslo interview today. “So the current type of levels continuing, but with volatilities.”

Brent oil averaged more $100 a barrel for the past three years, tripling from its 2008 low as the world economy recovered from the financial crisis.

In Shell’s “tighter” supply scenarios, crude prices could steadily increase over the 20 next years, while still going through periods of volatility that could see Brent falling to $70 a barrel, Bentham said.

While oil companies are cutting their investments plans to strengthen returns amid falling profitability, spending reductions are not the most important factor influencing the oil price, Bentham said. Increased Iranian exports as sanctions are lifted, Iraq’s ability to boost its production and Libya’s capacity to limit output shutdowns are factors “bigger in totality than the trimming of the investments,” he said.

The possibility of weakening demand growth in emerging markets may be offset by a strengthening U.S. economy, he said.

“Economic development and particularly in the emerging nations, is the biggest driver of demand change,” Bentham said. “That has an important impact on the commodity prices.”