Here are some of the points of circumstantial evidence for that theory:
1) Oil was found on Mars2) Oil was found in Sweden at the rim of a meteorite crater that punctured the earth's crust millions of years ago. There were none of the porous coral-reef ocean-sediment formations that normally hold oil were found, and are postulated to be where oil must be formed by ancient lifeforms.
3) The Earth's core contains a large amount of silicon carbide, as well as radioactive elements. In theory, hydrocarbons should be a bi-product of the radioactive decay process, and being very light, should rise towards the surface, where it would be trapped by porous reservoirs and sealed in by impervious overburden.