The price of crude oil/ petroleum as guoted in the media often refers to the price per barrel (159 liters) of either Brent, witch is traded on the International Exchange or light crude/WTI witch is traded on the New York Mercantile Excange(NYMEX).
There are especially three factors witch determins the price of a barrel of oil. The first is the oils specific gravity or API (The American Petroleum Institutes gravity measurment). The API determines how heavy or light crude oil/petroleum liguid is compared to water. The second factor is its sulfur content and for the third it is where its located.
As we can understand from above, the crude-oil-prices is depending on a lot of different factors wich in the end is determined by demand and shortage or oversupply from OPEC as well as the non -OPEC countries.
Look at the crude oil price graph and the crude oil price history chart below and see how the crude- oil-prices varies in value over time.