Fatih Birol, executive director of the International Energy Agency (IEA), delivers a speech during a session of the 22nd World Petroleum Congress in Istanbul, Turkey, on July 12, 2017. Mohammed Barkindo, secretary-general of the Organization of the Petroleum Exporting Countries (OPEC), said on Wednesday that he is optimistic about oil stocks declining further to heighten the prices at the second half of this year. (Xinhua/He Canling)