Just one article but there are plenty of dry academic papers you can wade through here and there...
http://en.wikipedia.org/wiki/Abiogenic_petroleum_origin
Surprisingly, the idea that oil comes from old biomatter, for instance, swamps and the like, is still a hypothesis...they don't actually know and haven't proved for sure what made (and possibly
still makes) our oil reserves.
They
could just be a natural part of planetary formation, as evidenced by the huge hydrocarbon reserves on Saturns moon Titan, which certainly hasn't had forests and swamps in the past to rot down and produce petroleum, and in the atmospheres of Jupiter, Saturn, and Neptune.
It's a really interesting subject. But one thing is for sure...whatever oils source, we keep finding more and more of the stuff every year...