In his energy plan released Thursday, Mitt Romney lamented the fact that a large chunk of federal lands and waters in the United States were off-limits to oil and gas drilling: places such as the Arctic National Wildlife Refuge or the Outer Continental Shelf off the coast of Virginia and the Carolinas. The presumptive Republican presidential nominee promised to open many of these areas for leasing if elected.

(Mac Morrison/Bloomberg)

But how much oil and gas is actually in these areas? And how much extra revenue would the federal government receive from leasing these lands and waters out? As luck would have it, the Congressional Budget Office recently published a report (pdf) on this very subject, and what the CBO found is that the oil and gas benefits from opening these areas up would be fairly limited — at least for the next decade.

The CBO considered a number of Republican proposals: First, that a future administration would open up all the federal lands and waters it possibly could, including portions of the Outer Continental Shelf. Second, that Congress would open up areas like ANWR where drilling is currently prohibited by law.

The report found that opening up all of these areas would bring in $7 billion in tax revenue over the next decade. That's fairly small compared to the $148.9 billion in royalties that the federal government is expected to receive from existing leases.

Why so little revenue? In part because those currently restricted areas don't seem to have much oil and gas compared with what's already available. Here's a chart that might help clarify matters:

The three columns on the left show all of the undiscovered oil and gas that's estimated to exist on federal lands and waters that are currently available for drilling. The middle four columns show oil and gas resources on lands that a Romney administration could open up fairly easily. And the two columns on the right show oil and gas resources that would have to be opened up by Congress.

The basic takeaway here is that the vast majority of oil and gas on federal lands is already available for leasing, particularly in the waters off Alaska and the Gulf of Mexico. There's certainly room to open up further federal lands, but the additional resources appear to be fairly modest in comparison. For instance, opening up the rest of the Outer Continental Shelf to drilling would boost offshore oil and gas production in federal waters by just 3 percent in 2035.

Now, CBO does list some caveats here. For one, these are just estimates of how much oil actually exists. "It is important to note," the CBO adds, "that any projection that involves geologic resources is inherently uncertain." Some of these areas may turn out to be too expensive or geologically difficult to produce. (Many of the Gulf of Mexico resources are in ultra-deep waters that remain technologically daunting.) Other areas may turn out to have larger resources than expected. But there are also political uncertainties. Even if a Romney administration opened up the Outer Continental Shelf, states such as California might not allow leasing regardless.

What's more, the CBO only looked at revenue for the next decade. But if, say, Congress opened up the Arctic National Wildlife Refuge for leasing next year (and that's not likely), oil production wouldn't ramp up for at least a decade. Between 2023-2035, drilling in ANWR might provide the federal government with an additional $2 billion to $4 billion per year in revenue, depending on how the royalties were split with Alaska. But this would still be less than one-sixth of the oil and gas revenue the government was getting from other federal lands.

Now, as always, there are all sorts of other issues involved in opening up these lands and waters. Many politicians in Florida are opposed to opening up the Eastern Gulf of Mexico for drilling because of the risk that oil spills that could tarnish their valuable beaches. And environmentalists are against opening up ANWR because drilling could threaten the fragile ecosystem there. The CBO report doesn't get into any of that. Mostly it just tries to offer a sense of how much oil and gas is actually in the areas being discussed.

Related: Five things to know about Mitt Romney's energy plan.