I'm considering a worldbuilding exercise focusing on a "second generation" civilization, which arises after a previous civilization from millions of years ago either died off or left the planet. One of the biggest questions, which I haven't found a good answer for yet, is what would happen to that previous civilization's plastics, specifically what transformation they might undergo after millions of years. If ancient organic material gets us oil, which gets us petroleum, which gets us plastic, what does plastic "get us"?
Would plastic still just be plastic, maybe more condensed or weakened, or would it undergo as dramatic a change as ancient plankton did when it became oil? Would plastic break down and turn back into petroleum?
Though I don't have an exact timeline locked down, for the sake of this question let's assume 20ma between the first civilization beginning plastic production, and the second civilization finding it (or its by-product) and, perhaps, beginning to mine it.
Obviously any answer here would be speculative, but I would like that speculation to be as closely-based on real geological and minerological processes as possible.
Thanks in advance!