As China’s biggest oil and gas producer prepares to report what may be its worst-ever earnings, investors are focused on billions of dollars that could be unlocked by a spinoff of its massive pipeline network. PetroChina Co.’s natural gas and crude oil transportation system, stretching from the country’s remote borders with Central Asia to major coastal cities, could be worth at least $85 billion dollars, according to analysts at Sanford C. Bernstein & Co. and Jefferies Group LLC. As President Xi Jinping’s government prepares to unveil long-awaited energy industry reforms, speculation has grown that the company and its parent, China...