First, doesn't the US import most of its oil from South America? Maybe I'm remembering outdated information, but I could have sworn that was true..
The price of oil is based on the global market, balancing global supply with global demand. Most places tend to get their oil from the closest source to save a little on transportation costs. They pay the same price as people on the other side of the world getting their oil from the other side of the world, though. The cost savings of having your oil tankers take shorter trips is not enough to justify paying very much more for the oil.
Second, aren't oil prices sort of artificially controlled by OPEC?
Presumably, OPEC still has the power to raise oil prices by artificially limiting the supply of oil, the way they have done in the past. I don't believe they are doing so right now. Our current oil supply is limited by oil companies' decisions made over the past decade about how many oil wells to dig, based on the cost of digging each individual well, weighed against the projected price of oil.
For most oil producers, once they have sunk the money into building the oil well, it makes economic sense to suck the oil out of that well as fast as possible and sell it at the market price, whatever that happens to be. When the time cost of money is factored in, it is more profitable to sell your oil today at $100/barrel than ten years from now at $200/barrel. In contrast it frequently makes economic sense to delay drilling the oil well for a decade, until the price of oil rises enough to justify the drilling costs.
To summarize, the oil market is very efficient in the short term, using price fluctuation to exactly balance instantaneous supply with instantaneous demand. In the long term, it is highly speculative, with investors losing their shirts or making huge fortunes based on their ability to predict oil prices years in the future.