Laherrere's graph was produced by a mathematical technique called Hubbert Linearisation in which annual production is plotted against cumulative production to that date. King Hubbert used it in his prediction in 1956 that US oil production from the Lower 48 would peak in 1973, which it duly did. A very powerful technique when it works. Well before even half total reserves have been produced, it can tell you what total reserves will be.