Oil and gas will remain central to the world's supply of energy for decades to come, the chief executive of giant producer Saudi Aramco said Monday, urging market players to ensure energy security by bolstering faltering investment. Oil major bosses and energy ministers are meeting at the World Petroleum Congress in Istanbul, with the outlook clouded by the low price of crude oil, currently trading at around 45 dollars a barrel compared with a peak of over 145 dollars in 2008. This has driven down investment to record lows, while oil majors are also grappling with the new importance of unconventional sources like shale and renewables.