The worst oil market in decades would be hard to spot in West Texas, where two-lane county roads are still jammed with trucks and energy companies are on the prowl for deals. The Permian Basin, the biggest of the shale-oil regions that ignited the U.S. energy boom, is also the only one where production is increasing even as drillers idle more than half the rigs in the country during the longest price slump since the 1980s.