The best news is that the age of petroleum has only just begun. For more than 80 years, geological estimates of the world's endowment of oil have risen faster than humanity can pump it out of the ground. In 1920, the U.S. Geological Survey estimated the total amount of oil remaining in the world amounted to only 20 billion barrels. By the year 2000, the estimate had grown to 3,000 billion barrels.
Geologists are continually forced to revise their estimates upward, because every year technological advances make it possible to draw upon petroleum resources whose extraction were once unthinkable. We can now drill wells up to 30,000 feet deep. The amount of oil that can be recovered from a single well has been enhanced by a technology that allows multiple horizontal shafts to be branched off from one vertical borehole. The ability to drill offshore in water depths of up to 9,000 feet has opened up the vast petroleum resources of the world's submerged continental margins.
The world also contains immense amounts of unconventional oil resources that we have not yet begun to tap. For example, tar sands found in Canada and South America contain 600 billion barrels of oil, enough to supply the U.S. with 84 years of oil at the current consumption rate. Worldwide, the amount of oil that can be extracted from oil shales could be as large as 14,000 billion barrels ? enough to supply the world for 500 years