The company said on its website last Friday that Chen Tonghai resigned as chairman for personal reasons, effective immediately.
Well-placed government sources said that Chen was recently detained by China's Supreme People's Procurator a few days ago.
A report by the Yangzi Evening News said Chen was asked to step down due to a "serious violation of discipline".
Chen is to be succeeded by Su Shulin, who is a former vice president of PetroChina, China's largest listed oil company by assets.
Su,
