Historically, efforts at petroleum resource assessment have concentrated on assessing amounts remaining to be discovered in unknown fields. Today the most important methodological frontier in petroleum resource assessment is assessing reserve growth in known fields. During the past 10 yr, most of our reserve additions have come from the growth of old fields, not the discovery of new ones. In the contiguous United States, most of our recoverable resource potential of crude oil, and possibly of natural gas as well, is in known fields. To the conventional sources of reserve growth (extensions and new pool discoveries), recent improvements in economics and technology have added such diverse phenomena as infill drilling, well stimulation in tight formations, enhanced oil recov ry, reduced abandonment pressures and production levels, and development of known but previously subeconomic areas and pools. The reserve appreciation models customarily used to estimate reserve growth fail to capture the effects of these and other recent developments. No single method will suffice in future assessment efforts, because the mechanisms of reserve growth have become too diverse. New methods must be tailored to the specific characteristics of each type of reserve growth. Economics must be considered explicitly. Improvements in field data bases are a fundamental prerequisite for the successful development and application of new methods.

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