One of our readers wrote in with this question: I read somewhere that there's more oil in the Western part of the United States and Canada than in Saudi Arabia. Is that true?

Here's the answer:

Great question! And the answer is: yes, sort of. The western region of North America, stretching from Mexico through northern Canada, is known as the Western Energy Corridor. And it does indeed contain more oil than Saudi Arabia-about three times more!

But there's a catch: the oil is in the form of oil shale-a type of rock that contains oil-like substances.  Unlike conventional oil that spurts from the ground in liquid form, oil shale has to be mined and then heated to turn it into usable petroleum.

Another catch is that mining oil shale and heating it uses water-lots of water.  In fact, harvesting any substantial amount of oil from oil shale would take around five-percent of the total water in the upper Colorado basin. The American West is pretty dry, so right now there's not enough water there to get oil from oil shale and have enough water for people, plants, and animals to live.

Another way of putting it is that getting oil from oil shale is very expensive, both in dollars and in natural resources. But that doesn't mean it's not worth trying. Oil shale deposits in parts of Colorado, Utah, and Wyoming could yield more than eight-hundred million barrels of oil.

Right now, oil shale doesn't provide much of the oil we use. But as mining technology develops and conventional oil resources decline, oil shale could play an important role in the future.