After some experimentation, I notice I'm able to fully deplete dark black spaces over the course of weeks, not years. It was so fast I could watch it in real time. So yes, it seems like a major overhaul is in order... I just don't think "unlimited" is realistic or desirable. I still think 5 game years is a reasonable timeframe - your city shouldn't be Oil Town for dozens of generations but at the same time real oil extraction isn't instantaneous like it appears to be in game.