It is interesting in the graph that Iraq, followed by Kuwait are the first to bump their reserves numbers. Then those two are the same as each other in what, 1983? Then kuwait is better than Iraq from 1983 to what looks like 1989 when Iraq catches up again. Sometime around 1986-1988 it looks like UAE and Iran start going head to head with the original two, and then all four of them are close to each other the same for 7 years straight. Almost at the same time these 4 get to be each around the same level, and each of them at over half of Saudi Arabia. Then all of a sudden Saudi Arabia goes way up. Funny to me :)
I believe it is broadly accepted the the increases in cited reserve numbers came about in anticipation of OPEC allocating production limits based on reserves in the mid-1980s. The convergence at 100 GB would seem to reflect a political driven negotiation to equalize production limits - not any kind of systematic reassessment of actual reserves.