Prolific output from US shale formations in recent years have thrown the world oil market off balance. But a long market slump is now forcing shale producers to retreat . US crude oil output hit a 44-year peak of 9.6m barrels a day in April then began to decline. By summer next year it will have fallen by a tenth, the US Energy Information Administration (EIA) forecasts. It took months for US supplies to finally reverse in response to the tumble in oil prices that started in mid-2014. If prices creep higher again, any rebound in shale production will come with a similar lag, analysts say. The US shale energy industry has made a stunning contribution to the oil market glut. Of the nearly 5m b/d in net global crude oil supply added between 2009 and 2014, 3.3m b/d was from the US, EIA data show. World supply stood […]