Halliburton Co. , the biggest provider of fracking, warned that the world is headed into a period of scarcity for oil after seven years of underinvestment following crude’s plummet from $100 a barrel in 2014. “For the first time in a long time, we’ll see a buyer looking for a barrel of oil as opposed to a barrel of oil looking for a buyer,” Jeff Miller, chief executive officer at the Houston-based oilfield contractor, told an audience at the World Petroleum Congress on Monday. Halliburton has seen spending by crude explorers outside the U.S. and Canada cut by roughly half compared with historical norms, Miller said. The hired hands of the oil patch have also been squeezed this year by rising costs, while producers have sought to return profits to investors rather than boost spending.