The world has tapped only 18 percent of the total global supply of crude, a leading Saudi oil executive said Wednesday, challenging the notion that supplies are petering out. Abdallah S. Jum'ah, president and CEO of the state-owned Saudi Arabian Oil Co., known better as Aramco, said the world has the potential of 4.5 trillion barrels in reserves - enough to power the globe at current levels of consumption for another 140 years. Jum'ah challenged oil ministers and petroleum executives at an OPEC conference in Vienna to step up exploration "and leave the minimum amount of oil in the ground." "The world has only consumed about 18 percent of its conventional potential," Jum'ah said, contending that should lay to rest fears that the world is in danger of being tapped out within a few decades. Many experts estimate that Earth's recoverable oil resource is at least 3 trillion barrels and potentially more than 4 trillion barrels. If global consumption rises about 2 percent a year from today's levels of about 85 million barrels a day, they say, the low end of that range would only be enough to last until roughly 2070.