economics , why oil prices increase?

They haven't. Today's price for light crude was $71/barrel.

At the beginning of April, the price was $85/barrel.

http://futures.tradingcharts.com/chart/CL_/70

Doesn't sound like an increase to me.

But note that it is all about supply and demand. Some oil wells are profitable at $20/barrel, while others need $80/barrel to be profitable.

http://www.businessweek.com/news/2010-06-02/produc...

So as soon as demand is high enough that it can't be met by the cheaper sources, the price rises so the more expensive sources are worth using.

There is some factor of speculation, but it is minor compared to the real demand for real use.

Demand is increasing (or expected to increase) as economic growth begins picking up.

Supply is in jeopardy due to the uncertainties around the BP disaster and the increasing cost of extracting oil from the ground.

Substitutes, in the form of renewable fuels or nuclear, are a long way off from being able to supply the energy needs oil currently supplies.

Lifting....;

The international price of oil is determined by the number of US $ is required to purchase a certain quantity. Therefore, if the value of the US $ decreases, relative to other broadly available currencies, it requires more US $ to purchase a certain quantity of oil, and the price is said to increase.

Worldwide demand for petroleum products, obviously, causes a fluctuation in price, relative to the supply/demand curve.