The belief is that crude is formed by organic compounds when intense heat and pressure are added to them.

The oceans are full of fish, plankton, algae, etc. When these die and eventually sink to the bottom of the ocean, over many many years they can seep into the earth, forming oil. The same thing can happen with land-based organic materials. It takes a long, long time.

If you want to read more, you can look at this: http://en.wikipedia.org/wiki/Petroleum
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