That's funny: do you know why there is petroleum at all?
When plants evolved to have line, they where able to become trees.
Sadly, no bacteria was able to decompose linine. It took a looooot of time (I don't remember how much, but a whole lot). So trees wouldn't decompose when falling down, so they would grow on top of each other, burying the oldest one more and more.
And the end, you have wood very deep, under a lot of pressure and I'm a hot environment: it created the petroleum.
So what you're expecting can take millions of year and it would be ironic if petroleum had to be twice in such a cycle.
Yes, indeed. It took a while, but not that long in the grand scheme. A few million years; plastic is more concentrated and more delicious, energy-wise.
I forgot to mention my theory, which is that we will be the ones who dig up the plastic to either recycle it or use it for its energy -- it will have been conveniently concentrated in landfills after all :)
Sadly, no bacteria was able to decompose linine. It took a looooot of time (I don't remember how much, but a whole lot). So trees wouldn't decompose when falling down, so they would grow on top of each other, burying the oldest one more and more. And the end, you have wood very deep, under a lot of pressure and I'm a hot environment: it created the petroleum.
So what you're expecting can take millions of year and it would be ironic if petroleum had to be twice in such a cycle.