at about 22 billion barrels. But here's the strange thing: the United States also had proven oil reserves of 22 billion barrels through much of the 1940s. How is that possible? New sources of oil kept getting found, more-difficult-to-obtain oil suddenly became more economically viable, new oil-extraction techniques gained favor, and so forth. This brings us to our next category of oil: undiscovered technically recoverable resources. Oil companies cannot consider this oil an asset. Whether that oil will be recovered depends in part on technology and/or the price of oil. These estimates change over time. The Bakken Formation in North Dakota, South Dakota, Montana, and southern Canada was discovered in the 1980s and 1990s, but because as much as 500 billion barrels of oil was scattered through layers of shale and sandstone, little could be recovered at the time. But with new production techniques, new estimates suggest that 3.65 billion barrels could be extracted from the Bakken Formation. "Using horizontal drilling and hydraulic fracturing, operators increased Bakken production from about 3,000 barrels per day in 2005 to 137,000 barrels per day in 2009 and 225,000 barrels per day in 2010,"  noted Richard Newell, the EIA administrator in testimony to Congress last year. "The domestic crude-oil and natural-gas industry has undergone a technological revolution that has revitalized the resource base in the onshore lower 48 states," Newell added. "The use of horizontal drilling in conjunction with hydraulic fracturing has greatly expanded the ability of producers to profitably produce crude oil and