Economic appraisal of frontier and developing basins requires input from an analysis of the distribution of the most likely field size. An analysis of 65 of the world’s basins containing 85% of the world’s discovered and produced petroleum (given as barrels of oil equivalent, or BOE) has concentrated on the larger fields in each basin. Significant variations in the percentage of a given basin’s oil and gas (given as BOE) that is contained in its five largest fields (which contain an average of40-50% of the world’s reserves) appear to be fundamentally related to the architectural form or morphology (a major element in basin classification) and size (volume of sediment in cubic miles) of the basins. Second-order variations in the morphology/size relationships of a basin’s field size distribution by percent-rank appear to be modified by the nature of the sedimentary basin fill, the diversity and variation in the type of “plays,” and the propensity of many basins to overlap or resemble other basin types (hybridism). With few exceptions, variations in the relative richness (BOE recovery factor per cubic mile of basin sedimentary volume) of maturely developed basins of the same class do not alter the percent-rank field size distribution in the five largest fields.
Previous modeling of field size distribution has attempted to consider basins in general through one all-encompassing model. This study tends to support more recent studies suggesting that several distribution patterns should be considered. These apparently consistent variations in the percent-rank of field size distribution might also serve as an additional check on any appraisals or reserve estimates of frontier or developing basins.
Figures & Tables
There is a continual need to update estimates of oil and gas resources remaining to be discovered, and also to refine the methodologies for making these assessments. In 1974, AAPG sponsored a research conference dealing with the above topics, and many of the papers presented there were published in AAPG Studies in Geology 1. As a follow-up to that volume, a U.S. Geological Survey workshop was held in 1983, and many papers from talks presented there, in addition to several other papers, are contained within this volume. The 22 papers have been grouped into two types: those describing methodologies for evaluating resources and those presenting assessments of both conventional and unconventional resources.