The U.S. Geological Survey keeps increasing its estimate for the amount of oil under North Dakota. In 2008, the organization estimated that oil deposits in part of the Williston Basin—an area that includes parts of North Dakota, South Dakota, and Montana—had 3.65 billion barrels of oil yet to be discovered. That was 25 times higher than its previous estimate, made in 1995, of about 150 million barrels. Now it’s increased its estimate by a similar amount, raising it 3.75 billion barrels to 7.4 billion barrels. The total is a little more than the amount the United States consumes in one year.

Why is the estimate of the amount of oil going up, even as oil producers do their best to suck that oil out of the ground? Why aren’t the numbers actually going down as that oil is depleted?

There are two main reasons. The first is that geologists’ understanding of oil-containing formations is incomplete. As producers drill more wells, the geologists learn more about the formation.

The second is that the estimate isn’t actually for the total amount of oil underground, but just the amount of oil that can be recovered with existing technology. As technology improves, more oil can be accessed, and the estimate goes up.

The increased estimate this time is because producers have figured out how to access oil in much deeper formations in North Dakota, opening up a huge new potential resource. The 2008 increase had to do with the techniques of hydraulic fracturing and horizontal drilling that made it possible to get oil out of formations that had previously been thought to trap oil too tightly for it to be extracted.

The uncertainty around technological progress makes predicting how long our oil supplies will hold out extremely tricky. It’s not as simple as looking at current estimates of technically recoverable oil, and then calculating how long it will take to use that up. You have to also make guesses about how technology will improve.

Further complicating matters is uncertainty about how much oil will cost. A lot of technically recoverable oil will stay in the ground if prices are too low.

This uncertainty, in turn, makes setting policy extremely difficult. In 2006, Republicans and Democrats alike were worried about what seemed to be an inexorable trend of increasing oil imports. President George Bush declared that “America is addicted to oil” and soon ambitious legislation that would require the production of fuel from cellulosic sources such as switchgrass was signed. In 2008, the year the USGS increased its estimates for North Dakota, U.S. oil production had declined to levels last seen in 1946, half the level seen at the peak of production in 1970.

But, in part because of increasing production in places like North Dakota, decades of declines were reversed–production is again increasing. Americans started using less oil. Oil import levels dropped from 60 percent to 45 percent. Now organizations like the International Energy Agency are predicting that the U.S. could soon be producing more oil than Saudi Arabia (see “Newfound U.S. Oil Wealth Won’t Lower Gas Prices”).

Of course, that prediction could be wrong, since it depends on so many different factors–technology, prices, policy, and so on. 

The bigger point is that those who think we should stop using fossil fuels should stop hoping that we’ll run out. We won’t. The amount of oil and gas in the ground is enormous. The resource in North Dakota is dwarfed by the potential of kerogen oil, which is dwarfed by the amount of natural gas in methane hydrates. We’ll keep spending more on getting more difficult-to-produce hydrocarbons out of the ground–unless viable alternatives are developed. To reduce the use of fossil fuels, the focus should be on developing those alternatives.