The annual "Outlook for Energy" report issued by ExxonMobil that analyzes world energy needs from now until 2040. The new report has the following to say about shale gas: Natural gas will continue to be the fastest-growing major fuel source as demand increases by about 65 percent. Natural gas is projected to account for more than one quarter of all global energy needs by 2040 and it is expected to overtake coal as the largest source of electricity.