As I wrote, the Wikipedia link refers to a source - which is a report from the US government. The cheapest and most efficient sources have already been found.

As I wrote, the Wikipedia link refers to a source - which is a report from the US government. The cheapest and most efficient sources have already been found. They're in the middle east, and many of them have recently reached the point where production is declining despite all technical efforts. And those efforts to sustain a very high production rate from a diminishing source mean that the decline could indeed become quite steep. As for new discoveries, the problem is that they, too, are declining (and have done so for over 2 decades).