In the oil market, not all barrels are created equal. By the end of this year, the US oil industry will be pumping 11m barrels a day of crude, the highest in its history and more than either Russia or Saudi Arabia. These barrels, boosted by the shale revolution and increasingly exported, are seen as critical for keeping the market well supplied as a fast-growing global economy lifts demand for diesel, jet fuel and petrochemicals. But in the industry, debate — some would say, concern — is growing over just how well-suited the shale oil coming out of the US is for meeting this rising demand.

The issue, critics say, is that US shale is far lighter — having been released through narrow fissures in rocks by hydraulic fracturing — than gloopy tarry crudes most people think of when they picture a barrel of oil. This has potentially huge implications because refiners, who turn crude into usable products, have spent decades investing in plants capable of processing far heavier oils that were once expected to dominate supply. The lighter shale barrels, some say, are just not as good for making the products — especially diesel, jet fuel and other so-called middle distillates — that the world increasingly needs. They warn of a potential crunch in years to come caused not by an outright shortage of crude, but by refiners scrambling to compete for more conventional barrels as US shale is found wanting.