The Ultimate is Cumulative Production when production ends. It is very important to recognize that there is an ultimate, even if the number cannot be determined exactly. The number proposed here is a rounded 1750 Gb which can be used to drive the depletion model for the next ten or twenty years through peak production into decline. It refers, like all the other numbers to conventional oil only. It may well be due for some upward revision in the far future in a high or very high price environment when some of what is here treated as non-conventional becomes conventional. The 1750 Gb is in fact approximately the same as the 2300 Gb proposed by the US Geological Survey, if that is redefined to take into account only Median Probability Reserves and the low end of the range of Undiscovered which is all that meets relevant time and economic constraints. It also is suggested by the trend of published estimates. Those desiring a higher number often show the same plot but take the scatter rather than the trend. Here weight is given to the trend because the more recent estimates have the advantage of modern technology and a colossal database from worldwide drilling. Care must however be taken in checking the definitions applied. Some of the higher reported numbers include what is here treated as non-conventional oil. Another very compelling argument comes from the plot given in Figure 15, showing discovery with reserve revision backdated to the discovery date. It shows a marked inflection to falling discovery in the 1960s. Extrapolating the subsequent curve gives a very clear intimation of future discovery and hence the Ultimate. If indeed the reserve estimates of particularly the Middle East prove to be higher than here assessed, it would mean that the inflection would be even more pronounced, reducing further the recent trend. It is accordingly difficult to argue convincingly for an Ultimate much higher than that proposed, given especially the fact that virtually the entire world has been explored sufficiently to indicate the existence of the major provinces. Only a new province with a crop of giant fields could have any global impact.
The Middle East Gulf Region has the largest endowment with 687 Gb, followed by Eurasia with 295 Gb and North America with 238 Gb. Saudi Arabia (280 Gb), the FSU (230 Gb) and the United States (210 Gb) are the three most richly endowed countries