Royal Dutch Shell expects crude oil prices to recover gradually over the next five years, with progress slowed by persistent global oversupply and receding Chinese demand growth.

Shell is betting on Brent crude rising to $90 a barrel by 2020, a key assumption in its move to buy rival BG Group for $70 billion.

"We are not banking on an oil price recovery overnight. It will take several years but we do believe fundamentals will return," Andy Brown, Shell's upstream international director, who oversees the company's oil and gas production outside North America, told Reuters in an interview.

A rise in global supplies, mainly due to a sharp increase in output from U.S. shale, has weighed on oil prices.

In the nearer term, Shell expects Brent crude oil to show only a modest recovery from today's $58 a barrel, with 2016 prices forecast to average $67 a barrel and $75 a barrel in 2017, based on the company's BG offer.

Oil companies rarely reveal the price forecasts that underpin their future strategies. The chief executive of Shell's rival BP, Bob Dudley, said recently he expected oil prices to remain low for "a couple of years most certainly."

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