Does oil decompose?

As we all remember from elementary school science class, crude oil is made of decomposed dinosaurs.

Question: if we left it in the ground, would it decompose further?

If it will decompose further, why doesn't plastic decompose? Plastic is made from oil.

If it won't decompose further, does that mean that other organic matter (mulch, compost, dead bodies in cemetaries) won't fully decompose either?