How do you explain the vast methane ocean discovered on Titan?
In an environment without sufficient oxygen, Carbon and Hydrogen will eventually form Methane as it is the lowest energy state of those atoms. Ethane and the like will also be formed depending on the ratio of atoms that exist.
But the simple methane molecule is a far cry from the complex long-chain hydrocarbons (Alkanes, Naphthenes, Aromatics and Asphaltics) found in crude oil.