It’s a popular myth that oil and natural gas are the products of dead dinosaurs. However, the sheer quantity of hydrocarbons found in the depth of our planet is sufficient evidence to invalidate this hypothesis. The vast majority of dead dinosaurs decomposed completely. What few remains we have, apart from some very rare exceptions, are skeletons only. Their flesh were eaten and rotted away as is the normal fate of dead animals.
Brontosaurus: by MCDinosaurhunter Own work, CC BY-SA 3.0
The popular myth likely had its origin with the coal fields that display clear evidence of having once been forests. If trees can be turned into coal, why not dinosaurs too? However, much of the oil and gas that have been discovered come from rock strata far below where life has ever existed. There is also methane and ethane lakes on Titan, Saturn’s largest moon. Those hydrocarbons were surely not produced from dinosaurs.
There are also molecular clouds in space containing methane and ethane together with simpler molecules such as water and carbon monoxide. Hydrocarbons are everywhere. There is no need for dead dinosaurs to explain their existence in the depth of our planet.
The fact that our planet has hot internals, far warmer than would be the case if nothing was going on, is seen as proof that there must be radioactive processes taking place. This in turn explains the expanding Earth, and also the abundance of salt, water and hydrocarbons found deep below Earth’s surface.
Radioactivity produces lighter elements from heavy elements. This is known as radioactive decay. It produces heat together with hydrogen, carbon, oxygen, chloride, sodium, nitrogen, etc. These lighter elements combine in turn to produce organic compounds of various kinds. Salt, hydrocarbons and water are all produced continuously inside Earth as byproducts of radioactivity, and this too produces heat. The heat inside our planet comes from both radioactivity and chemical reactions.
Our planet is not a dead and inert body. It shivers with earthquakes, often in response to solar flares in our direction, it erupts lava, it expands and it produces organic compounds. Earth is very much a living planet, connected to a living universe.