Now that oil prices have dropped to levels not seen since 2009, helped by a flood of oil flowing from hydraulic fracturing or fracking wells in North Dakota and Texas, it's time to ask the question: How long can the U.S. oil boom last?

In the short term, the price drop threatens profits from fracking, which is more expensive than conventional drilling. Sure enough, permit applications to drill oil and gas wells in the U.S declined almost 40 percent in November.

But in the long term, the U.S. oil boom faces an even more serious constraint: Though daily production now rivals Saudi Arabia's, it's coming from underground reserves that are a small fraction of the ones in the Middle East.

That geologic reality is easy to forget in the euphoria of the boom. Output from oil fracking in the U.S. has tripled in the past three years, from about one million barrels per day in 2010 to more than three million barrels per day at the end of 2013. Total U.S. oil production has risen to more than nine million barrels a day, a level close to 1970's historic high and nearly as high as the 9.6 million barrels of daily oil production from Saudi Arabia.

View Images Years of oil prices above $100 per barrel have driven a boom in oil production from shale, providing thousands of oil field jobs and boosting U.S. production to near-record levels. Photograph by Eugene Richards, National Geographic

While the U.S. still relies on imports for about 40 percent of its petroleum, oil imports have dropped since 2005 because of improved domestic supply from oil fracking, better vehicle fuel efficiency, and depressed fuel demand as a result of the 2008 economic crash. The U.S. Department of Energy reports a growing surplus of domestic oil.

Breaking Even With Fracking

Fracking oil or gas from mile-deep shales is expensive: It requires deep vertical and horizontal drilling and injections of chemicals, sand, and water at high pressure. Until now, high oil prices have nonetheless made fracking a lucrative investment. More than a million fracked oil or gas wells exist in the U.S.

With oil prices down, so are profits. Recent analysis by Scotiabank estimates that frackers need $69 per barrel of oil to make money. One oil executive quoted in the Economist said he can cope as long as the oil price is above $50. Another said the industry is "not healthy" below $70.

Businessweek reports that the "dirty secret" of the shale oil boom is that it may not last. Fracked wells are short-lived, with a well's output typically declining from more than 1,000 barrels a day to 100 barrels in just a few years. New wells must be drilled frequently to maintain production.

While wells currently pumping can survive low market prices because they have already incurred startup and drilling costs, low oil prices diminish the incentive to invest in new well investments.

Of course, as Michael Webber of the University of Texas at Austin told the New York Times, price fluctuations are part of a repeating cycle in the oil business over the past century. No one thinks the current low prices are permanent.

"This is what commodity markets do," Webber said. "They go to high price, and high price inspires new production and also inspires consumers to use less. After a couple of years of that, prices collapse. Then low prices inspire consumers to consume more and encourage suppliers to turn off production. Then you get a supply shortage and prices go back up."

View Images Saudi Arabia remains the world's largest oil producer. Known Middle East oil reserves are vastly bigger than U.S. oil shale reserves. Photograph by Reza, National Geographic

Geological Limits

While low prices may only temporarily throttle expansion of oil fracking, the underlying geology—deeply buried shale rock that contains diffuse hydrocarbons—looms as a more fundamental limit on fracking's future. Recent projections indicate that by decade's end or a few years after, U.S. oil production from fracking will likely flatten out as supplies are depleted.

"A well-supplied oil market in the short-term should not disguise the challenges that lie ahead," International Energy Agency (IEA) chief economist Fatih Birol said in releasing the IEA's 2014 World Energy Outlook.

The IEA report projects that U.S. domestic oil supplies, dominated by fracking, will begin to decline by 2020. "As tight oil output in the United States levels off, and non-OPEC supply falls back in the 2020s," the report says, "the Middle East becomes the major source of supply growth."

Earlier this year the U.S. Energy Information Agency (EIA) also forecast a plateau in U.S. oil production after 2020.

The basis for these forecasts are estimates of shale oil reserves. A 2013 Energy Department report on technically recoverable shale oil—the amount that's recoverable without regard to cost—puts U.S. potential at 58 billion barrels. That's equivalent to a little more than eight years of U.S. consumption at the current rate of almost 19 million barrels a day.

The Energy Department's estimate of "proved reserves" of shale oil—those that can be recovered economically today—is only about ten billion barrels. That's about a sixth of technically recoverable reserves, and less than a year and a half's worth of current consumption. Proved reserves include all currently known U.S. oil shale resources-North Dakota Bakken, Texas Eagle Ford, Colorado and Nebraska Niobrara, Texas Barnett, and others.

In contrast, the proved reserves from just three Middle East nations—Saudi Arabia, Kuwait, and the United Arab Emirates—total more than 460 billion barrels. That's 46 times U.S. shale oil reserves, and more than 12 times the total U.S. oil reserves.

Those estimates help explain why the IEA projects the Middle East as "the major source of future supply growth," long after the U.S. shale oil boom has run its course. Price is important, but whether oil exists at all is even more so.