A handful of U.S. shale oil producers are pushing up their production forecasts, saying efficiency gains from drilling in prime rock are helping them eke out more crude in the middle of the worst price crash in six years. The slightly bolder outlooks this week from Oasis Petroleum Inc ( OAS.N ), Devon Energy Corp ( DVN.N ), Pioneer Natural Resources Co ( PXD.N ) and Diamondback Energy Inc ( FANG.O ) show that the confident swagger that typified the U.S. shale boom’s early days has yet to be fully tempered by the more than 50 percent drop in oil prices CLc1 since last year. Though the consensus view is that rig productivity in U.S. shale basins is stalling to portend a drop in national output as companies struggle […]