The Illinois basin has produced oil for nearly a century and few scientists anticipate significant new accumulations will be discovered in the future. Drilling activity and exploration, although greatly diminished from past levels, continue at an impressive rate largely in response to an enhanced economic condition over the past 5 years. What petroleum reserves and likely sizes of new fields remain to be discovered in the Illinois basin?

Estimates of remaining undiscovered hydrocarbon reservoirs can be made in a variety of ways. Total hydrocarbon production data for the basin are expected to be guassian; thus, future production can be predicted from past production. Frequency distributions of discovered field sizes permit estimates of those field sizes remaining to be discovered. These distributions may be either cumulative or annual plots which when coupled with production by field size data allow estimates of undiscovered reserves to be made.

Conclusions resulting from data for the Illinois part of the basin confirm anticipated, intuitive expectations that only small fields, less than 100 acres (40 ha.) reasonably can be expected in the future although a few medium fields, 100 to 500 acres (40 to 200 ha.) apparently still remain to be found. Most of these undiscovered fields will be stratigraphic accumulations. Total reserves discovered in the future will be small.

Although the days of petroleum exploration by major and minor oil companies, as well as large independent operators, have passed completely or are rapidly disappearing in the Illinois basin, opportunities for small independent operators and individuals are perhaps better today than ever before. More geologic information now is available to them and economics will continue to become increasingly more favorable to them.

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