I thought it might be useful to review some of the issues surrounding future reserve assessment, especially given the fact that so many here have obviously read Campbell and Laherre’s various discussions. This is a very important issue to my mind, as what view you take might make the difference in the real peak by several decades. If you’ve seen this before just hit the BACK button.
There is a USGS Open File report (03-137) by Ronald Charpentier that talks a bit about the different methodologies and some of the problems associated with each. I believe that M. Lynch has talked in general terms about this issue in one of his presentations. Given space constraints I’ll focus on the potential issues around projections from current distributions.
The common method by which the so-called “curve fitters” come up with URR for a basin is by plotting creaming curves. This is simply a plot of cumulative oil discovered (ordinate) versus year (abscissa). The recognition is that in mature basins one sees a relatively steep rise in the curve at some point in time, which is followed by a shallowing and an eventual asymptotic relationship with the year axis. Campbell and Laherre have both used this relationship to forecast potential URR in basins at a given point in time. The belief that this is a valid method requires the assumption that in all basins the largest oil fields are always discovered first even with random drilling (documented in the literature by Arps and Roberts (1958), AAPG Bulletin, V42, 11, p. 2549-2566). In reality, however, there are a number of factors that often lead to discovery of large fields late in the exploration history of single basins (eg. access, data quality, politics, economics). Charpentier uses the Ghadames/Berkine basin as a good example where there was an early exploration history in the seventies and eighties that discovered relatively small fields, followed by improvements in seismic technology in the nineties and subsequent discovery of several giant fields. Plotting the data indicates a creaming curve approach conducted in 1990 would have missed the potential for the addition of some 5 billion barrels of oil in the mid-nineties.
A very good example of this is seen in creaming curves constructed for the Niger Basin in Nigeria. This is a single geologic basin, albeit with several petroleum systems and playtypes present. The following is a creaming curve created including all data to present (source is the most recent IHS database and thanks to Pete Rose for the plotting routine).
Chart
What is interesting to note is there are several inflection points in the curve. If one were to have attempted creaming curve analysis in the early sixties and seventies, the late eighties or mid-nineties the result would have been a much more pessimistic assessment of URR than was actually achieved. In reality the various steepened portions of the curve represent improvements in technology such that deeper and deeper water depths could be explored efficiently and cost-effectively. The question you now are left with is will the curve remain flat or take another jump? There are a number of other examples of this, including the Michigan basin, Gulf of Mexico etc.
The proper analysis at any given point in time needs to take into account first what the remaining geologic potential is in any given basin, and then what should be reasonably recovered ultimately given the notion that economics and politics will become more favorable with a dwindling resource base. It is for this reason I believe the USGS methodology is a superior way of predicting URR. The issue of course is that the USGS can afford to acquire the large priority databases and spend hundreds of man-hours of exhaustive analysis whereas consultants and small contractors can't.
I’ll follow this up with a related discussion on probabilistic distribution of field size as a predictive tool and the associated pitfalls.