Old king coal

There is a theory that oil, gas and coal are abiotic. That is, they aren’t made from dead stuff but formed deep down (like a hundred miles) in the earth’s mantle then rise through vents and fissures to be trapped by impervious rock… Where coal undergoes further transformation into the solid material I guess…
The more I look into it the less barmy it sounds than the idea that hydrocarbons are made from piles of dead stuff that somehow didn’t decompose before it was miraculously buried under layers of silt and mud.
Seeing as there is abundant methane within our solar system (the gas giants for a start) it seems silly to presume that earth would be a special case for the formation of hydrocarbons.