Barclays analyst Jeanine Wai upgraded Concho Resources (CXO) to Overweight from Equal Weight while downgrading both Devon Energy (DVN) and Callon Petroleum (CPE) to Equal Weight from Overweight and double downgrading Centennial Resource Development (CDEV) to Underweight from Overweight. Sustainable free cash flow with growth continues to be the "holy grail" in Exploration and Production, Wai tells investors in a research note. In 2019 the analyst thinks there will be a "distinct bifurcation" among stocks and the "line in the sand" is sustainable free cash flow yield plus growth. Barring a more imminent oil price recovery, Centennial Resource will likely underperform as valuation climbs due to continued cash burn and decelerating cash flow growth, says the analyst. Further, despite carrying some momentum, Callon's valuation screens rich "given less visibility on its journey" to positive free cash flow, adds Wai.