After half a decade of U.S. oil drillers underinvesting in projects and returning money to shareholders, it could take years to resume pre-pandemic production levels that could further roil oil markets for years to come. "I'm worried that it may get too high, above $100 (per barrel)," according to Scott Sheffield, CEO of shale explorer Pioneer Natural Resources Co., who was speaking to Reuters in an interview at the Petroleum Congress in Houston on Tuesday.