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?
The Biggest Oil Discovery In 50 Years? Bottom line, you can't write what you want, it'll be credible.