The weak value of the US dollar, on the international market, is a factor in current commodity pricing and oil prices to come over at least the short term. It is only one factor, and the only direct economic one I can think of.
The other is that we are simply past peak oil. This is point when supplies are the cheapest they will ever be to extract, using the current technology, over the known existing deposits; its also the point when the few wells that produce, collectively, the largest percentage of world demand, are known to contain less than 50% of oil that can be extracted from them.
There is no need to speculate on oil prices for them to go up. In fact, the one feeds the other. Supply, relative to extraction and refining costs versus world demand (increasing rapidly, largely due to the rapid industrialist of China taking place that is also using cheaper, older, more inefficient means of energy production) is simply shrinking. Or more accurately, what's left is more expensive, and risky (BP's recent deep drilling disaster typifies), to extract while demand continues to rise.
Speculators know this, know the price is going to naturally be set higher as a result (to maximize profits by the oil producing and exporting nations), and that its a good, safe, productive investment.
So there are several factors. Things mentioned play a role, but not in isolation.
-Spyder