A mystery is confounding the US oil market: when inventories rise, prices rise, too. That is not the way it is supposed to work. At 508.6m barrels as of early February, stocks of crude sitting in commercial tanks were 8 per cent higher than a year before and close to a record. In theory, the excess supply should weigh on markets. But this year a loose pattern has emerged after the US government releases weekly oil data: surprisingly large increases in crude stocks have prompted spurts of buying. “The mantra is ‘buy the builds’,” said Andy Lebow of Commodity Research Group, a US consultancy. “It seems to be working, week after week, as inventories build.” The midweek petroleum status report of the Energy Information Administration is watched obsessively by traders, shedding light on supply and demand in the leading oil-consuming nation. The next one is due on Wednesday morning Washington time. Over the last five reports US commercial crude oil stocks rose by a total of 29.6m barrels. Each weekly rise surpassed expectations. While declining immediately after each report, the price of the West Texas Intermediate oil benchmark was trading higher 20 minutes later, often accompanied by a burst of volume. WTI prices also settled higher after four of the past five releases.