Where is all the petroleum coming from?

Coal and oil are both called "fossil fuels."  While the formation of coal from the long-term compression of plant matter seems to be a closed case, the amount of petroleum seems to be much larger than thought.

 

Is this really true?

  - Can you find any evidence for or against the idea that all petroleum was formed from decaying animal matter?

 

 - If true, what possible ways could petroleum have formed in addition to being mainly the remains of dinosaurs?

 

 - Does this have anything to do with how the Earth was formed?

    - Hint:  Are there hydrocarbons in outer space?