How do you make petroleum to be rendered useless

I am writing a book in which large amounts of oil reserves are rendered to be useless. Either by converting the oil with a chemical,nanobots or by hitting them with radiation. In other words,make the oil useless for any use.

Does anyone know on how to make oil (petroleum) into a useless substance than cannot be used for humans? It's vital and important to make this as close and realistic as possible.

I like the radiation angle but that would be limited to a specific area unless you wrote about multiple sources of contamination. Since your writing fiction a plausible idea might be to have a bacteria "infect" the oil supply and make it inaccesible. Consider how the oil might be made so viscous that it can no longer be pumped from the ground. How about an engineered strain of bacterial that causes hydro carbons to join into very long chains that intertwine with others. The result is near solid mass of oil . Since you would be writing about a biological event the problem could spread underground and effect large areas and the problem could migrate to other areas compounding the problem.




Just a thought....

Burn it.
Your nanobots or radiation, would have to break the hydrocarbon chain releasing the energy in the bonds. What if you had a bacteria that could feed on this energy. and the oil supply became "infected"
Cool, sounds like the start of a good book to me.
edit; Mark had the same idea as me , cool

When I was a lad, we just added sugar to the fuel tank and that rendered the contents of the fuel tank useless. Simple as that.