I would say that it can be abiotic but not always. Methane is an easy molecule to form, like water. Have you ever walked through a peat bog and seen bubbles rising up? That is methane from organic decomposition. But there is no way oil can come from the bowels if the earth. It is too hot and too much pressure.
So, mostly abiotic. The next question is whether new methane is still being generated? If so, then it is a renewable energy source.