I have heard that we are getting all the fossil fuel we are getting until the next catastrophy. I believe it was a special on the discovery or perhaps the history or learning channels (probably disc). It said that when the K-T extinction occured, it left a high concentration of plant and animal life in one layer, which formed into the huge bodies of fuel we have today. That was the first and last time I heard that arguement, and I am no archaeologist, so I don't know. Other sources, such as the foillowing one, seem to suggest that it is a gradual thing, and pretty much everything ends up jumbling together in big vas at some point 9or at least thats what I get from them).
Our fossil fuels formed from life that lived from 300 to 400 million years ago... details in this link. Enjoy!

http://www.fe.doe.gov/education/energy2.html