There could have been a much larger amount of fossil fuels in the ground that was almost used up by the Silurians and now we believe what they left us with is all there ever was.
This makes me wonder: which methods of resource extraction lead to the complete destruction of evidence that resource existed, and which methods lead to a preservation of evidence? If you insert a deep well into a fossil water aquifer, and only use 10% of it, will it all have evaporated after 5 million years due to exposure to the elements?