That's what Peak Oil means.
Apparently, some people interpret peak oil as the end of oil. It's not hard to find out what it actually is... Nor is it a difficult concept to grasp. Big fields of cheap oil on production costs aren't discovered with the same frequency like they used to, oil becomes more expensive, and oil that is more difficult and hence more costly to produce becomes economical.
One wonders what part of that is either controversial, or difficult to understand...