there's technically an infinite amount of oil if you manage your well pressure properly, you tap a well into a reservoir, use it until its run out, then switch the well off and drill a new one in another reservoir and allow the first one to build up pressure again, keep switching to new reservoirs until you have used all the shallow ones, then research deep wells and continue moving from reservoir to reservoir then by the time you have used up the last one the first one should be back up to pressure again and you can switch it back on