So, I'm not a rock scientist. But, it seems like something I should make sure is at least slightly possible before I use it: I have some characters who are digging what's effectively a mineshaft. (It's not a mineshaft, but ignore that for now. It's a deep shaft into a mountain, they're just not mining.) Would it be even remotely possible for them to strike oil under a mountain, assuming that they dug deeply enough? If so, under what conditions? How about methane gas?