Where does petroleum come from?

Petroleum comes from oil fields, which exist both on land and offshore. Generally, these are called reservoirs or oil deposits. If you are asking about the origins of petroleum, then petroleum comes from deposited organic material (plant and animal), generally deposited more than 10 millions years ago. The transformation of deposited organic material into oil, requires the sufficient organic material, millions of years of time, confinement and sufficient temperature. Something like making a cake, you need a container, the ingredients, and the right temperature applied for a certain amount of time. Except with oil, we talk about millions of years instead of hours. You may want to read more from this link on Wikipedia: http://en.wikipedia.org/wiki/Oil_field http://en.wikipedia.org/wiki/Petroleum