Despite output cuts, Opec’s core Gulf producers actually sent slightly more crude to their core East Asian customers in the first four months of 2017 versus both the same period last year and 2016 as a whole. But this has not been enough to maintain their share of the key Chinese market where they are losing out to long-haul shipments from the Americas.

China moved decisively ahead of the US as the world’s top crude importer in the first four months of 2017. It imported an...