Your dog was consumed by worms and other burrowers. Just as on land, there are burrowers and terd rustlers in the oceans. They mainly live in oxygenated waters. Petroleum source rocks occur in anoxic waters where there is not apply oxygen to support such life and thus organic matter can be preserved without being consumed. Concentrate the organic material, bury it and heat it, then you have expulsion. Migrate it to a trap and you have a field. Most oil and gas misses a trap and comes to the surface.
Hey! No fair bringing up geology - that’s for pointy-headed scientists!