Saudi traders at the Saudi Investment Bank in Riyadh in 2011. REUTERS/Fahad Shadeed Oil will stay cheap.

In an in-depth report on long-term energy trends, OPEC, the 13-member cartel of oil producers, said a barrel of oil would not be worth $100 until after 2040.

But the good news for oil states is that the price will go up, albeit slowly.

OPEC said oil companies' spending cuts planned for next year will limit supply in the medium term, raising the price slowly even if demand for oil stays low.

The calculations are based on a basket of different oil products from OPEC nations (the OPEC reference basket) and are adjusted for inflation based on 2014 dollars. In nominal terms oil will hit about $160 a barrel in 2040.

"Reflecting on all of these considerations, the long-term value of the ORB in this Outlook is assumed to rise from more than $70/b in 2020 to $95/b in 2040 (both in 2014 dollars)," the group said in its report.

"Correspondingly, nominal prices reach $80/b in 2020, rising to almost $123/b by 2030 and more than $160/b by 2040," OPEC said.

Here's the chart from OPEC: