The accusation is absurd on its face. So-called oil and gas industry money includes donations of $200 or more from anybody who works in the industry (even some lowly employee of ExxonMobil, or perhaps a guy who owns a gas station), as well as lobbyists who have had one or more industry clients at one time or another (out of dozens, if not hundreds of clients potentially). The method of accounting makes the numbers completely meaningless. On top of that, even if the accounting were sensible, it would only tell you about correlation, not necessarily causality, let alone about any quid pro quos. For example, maybe Democrats in the oil and gas sector happen to be more anti-gun than pro-gun, and that's why they give more to Hillary than Bernie. Or perhaps (actually far more likely) it's because Hillary has been a viable candidate for longer, so there has been more opportunity for her to receive such donations.
The whole issue is a complete waste of time. I doubt it changes a material number of votes either. The only people who would care about such things have already made up their minds.