Well perhaps they did, and we've only now come to notice them. It may form but there won't be huge reservoirs of it. Those were special circumstances: dead trees lying around that were incapable of rotting because there were no organisms that could break down the lignin. It was only when they were buried and acted upon by sloooow physical forces that they changed into petroleum and natural gas. Today they are acted upon by much speedier biological processes and are recycled into the growth of other organisms. The little bit of oil that forms on garbage--even considering how much garbage we create--is nothing compared to millions of generations of the planet-enveloping forests of the Carboniferous Era. I'm sure the BP spill has piqued many people's interest in the science of oil. Google searches on "oil drilling" have doubtless caused this thread to pop up on quite a few desktops. This is one of the ways we get new members. I fell in here from a Google search on some topic or other in linguistics ten years ago.