answer:They won’t “run out”. That’s the thing about oil in the ground: there will always be “some amount more”. It might be deeper, it might be more dispersed, it might be mixed with more contaminants or it might simply be in a part of a country’s territory (jungle, arctic ice cap or ocean, for example) that is inconvenient (read: difficult and expensive) to access. In fact, even the accessible oil fields that will have been considered “used up” will not be “empty” in the way that your car’s fuel tank can be emptied and dried, for example. What will happen is that the marginal deposits will be made more and more expensive due to the cost of extraction and refinement into usable product. So what that means is that more and more labor and expense will go into producing less and less product until it becomes economically non-viable. There’s no telling when that point might occur. It could be pretty soon, in fact, if competing energy sources are developed which undercut the cost / price of the marginal petroleum so developed. At that point, there would be no incentive to explore for deeper deposits or more complex refining methods.