The hills of Titan, that is. Or rather, there isn’t so much oil “in” the hills, as oil “raining down on the hills”. Hydrocarbon rain, to be precise.
This raises the question of non-biologically originated hydrocarbons – currently accepted knowledge is that hydrocarbons that we slurp out of the ground have a biological origin (although “Oil comes from Dinosaurs” is a vast oversimplification), but there has been research to indicate that some of the hydrocarbons in the mantle of the earth originate from non-biological sources (thanks to Cecil Adams of The Straight Dope for succinct summaries). I’m not a geologist, it would be interesting to hear what sort of an influence the extraterrestrial data of the space program is having on current geologic theory.