In August, Opec’s share of US crude oil imports dropped to 40 per cent – accounting for 2.9m b/d – the lowest since May 1985, according to Financial Times analysis of US Department of Energy data. At its 1976 peak it stood at about 88 per cent.

The impact of the shale boom on Opec members has varied, with African countries such as Algeria and Libya being hit the hardest while Saudi Arabia and Venezuela have remained fairly strong. “It has been Africa that has been severely squeezed,” said Paul Horsnell, an analyst at Standard Chartered.

Nigeria, which produces crude similar to the quality pumped out of North Dakota’s oilfields, has been the biggest victim of the US shale boom. Barrels stopped flowing altogether in July, having reached a 1979 peak of 1.37m b/d.