29-01-03 Predictions that oil supplies will dry up within a few years have been common over the last 150 years. The world had produced a total of 900 bn barrels of oil by 2000 -- yet estimates of the total amount of oil resources still in the ground grew throughout the 20th century.
For example: In May 1920, the US Geological Survey announced that the world's total endowment of oil amounted to 60 bn barrels. In 1950, geologists estimated the world's total oil endowment at around 600 bn barrels.

From 1970 through 1990, their estimates increased to between 1,500 and 2,000 bn barrels. In 1994, the US Geological Survey raised the estimate to 2,400 bn barrels, and their most recent estimate (2000) was of a 3,000-bn-barrel endowment.
This is possible because the world's oil endowment is much larger than its oil reserves -- which are identified resources that can be economically extracted and refined using current technology.

As new technologies increase the amount of recoverable oil, and market prices encourage new exploration and development, the world's total endowment goes up. But the world's oil endowment does not include unconventional oil resources. Oil shales, for example, could easily be as large as 14,000 bn barrels -- more than 500 years of oil supply at 2000 production rates. Nor do they include other fossil fuels, such as natural gas and coal.
Unconventional oil resources are more expensive to extract and produce, but we can expect production costs to drop with time as improved technologies increase efficiency.