On recent campaign stops, President Obama has taken to summing up America's oil supply problem with the following statistic: We use 20 percent of the world's production, but only have 2 percent of its reserves. Then this past week, the president got pushback from Republicans, who are citing a new government study to allege we have closer to 26 percent of the world's supply.

Who's right? Neither, really. Obama is talking about oil we know is there. Republicans are talking about oil we think is there. 

The president likes to refer to our so-called "proven" reserves -- oil that can be recovered with relative certainty given today's economic, technological, and regulatory constraints. It's oil that companies have already discovered, and that they can drill up profitably without breaking the law. Oil in areas where drilling is banned, such as the Arctic National Wildlife Refuge, isn't included. According to the U.S. Energy Information Administration's most recent estimates, the United States has roughly 20 billion barrels of these reserves, around 2 percent of the global total. But proven reserves are only a small part of the petroleum picture, and don't give us a very accurate picture of future supply. 

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That's where the Republican criticism comes in. This week, the U.S. Geological Survey released a study of the world's "undiscovered, technically recoverable" oil resources. It sounds complicated, but is relatively simple. It's oil we haven't actually found, but believe is there based on geological studies, and think we can get at with current drilling technology, regardless of the legal or economic issues. The new survey did not include the United States, but Republicans have combined its results with previous estimates showing we have 198 billion barrels of this kind of oil. Here's a breakdown. 

By this accounting, the world has about 763 billion "undiscovered" barrels of oil, of which roughly 26 percent indeed belongs to us. These figures don't include the actual reserves currently being drilled across the globe, which are quite significant. Saudi Arabia alone has about 262 billion proven barrels available. 

Now, here's where the USGS figures really fall short: They ignore a massive chunk of the world's potential future oil resources. The study looks at conventional oil. That's regular old black gold, the kind that made Jed Clampett and that nutjob from There Will Be Blood rich. But there are many other kinds of oil, which get lumped into a category called "unconventional  oil." That includes the billions of barrels of tar sands oil in Canada, heavy oil in Venezuela, and shale oil in North Dakota.* 

So Obama's numbers almost certainly underestimate the relative size of America's potential oil supply. The Republicans may be overestimating it. Or, depending on the kinds of unconventional oil discoveries we make in the coming years, they could be massively underestimating it too. But both sides are are oversimplifying the issue. Shocking, I know. 

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*Data for proven reserves usually do not include unconventional oil. However, the EIA specifically includes oil from North Dakota's Bakken shale deposits in its calculations, which in recent years have become a significant source of unconventional crude.