Cheap oil has finally arrived – and it looks like being a disaster

This oil price slump is turning into a ‘black swan’: one of those economic events that seem to come from nowhere with strange and unforeseen effects. As Brent Crude dips below $70 a barrel and Opec sits on its hands, major banks face losses on financings for US energy companies that must have looked like the safest borrowers in the field in an earlier phase of the shale gas boom.