Well, the “fossil fuel” theory was never really about “dinosaur goo”, that’s just a popular myth, probably stemming from poorly written books for kids.
Under the theory, coal is supposed to originate from vegetation that got buried on land. Oil and tar, on the other hand, are supposed to originate from simple organisms like plankton buried under the sea floor.
Oh, yeah. I remember that. I don't think 2 billion years is enough time to make that much goo in that manner. That is just me. Having hydrocarbons synthesized somewhere between the surface and the magma makes more sense. The magma is just full chemical goodies waiting to be put to use.