In 1973 the Oil and Gas Resource Appraisal Group was originated within the U.S. Geological Survey (USGS) to develop resource appraisal methodology and apply these methods in assessing the nation's petroleum resources on a regional basis. The resource appraisals were published in USGS Circular 725 on the undiscovered oil and gas resources of the United States for 102 geologic provinces. Since this first assessment, the evolution in the development of petroleum resource appraisal procedures within the Resource Appraisal Group has been significant.

The appraisal methods are designed to evaluate all the known geologic and geophysical data available for a prospective petroleum basin or province. Resource appraisals can be made with any amount of data. However, the amount and kinds of data available will determine the method or methods to be used in the appraisal for any basin or stratigraphic unit. Methods will also change with time, as the amount and nature of the information in a specific area will vary with exploration activity and availability of data.

In frontier areas of exploration (where only gross interpretation of the basin geology is available) by applying the principles from worldwide experience for the occurrence of oil and gas it is feasible to use subjective judgment with minimum data to provide an estimate of the potential petroleum resources. More advanced methods employing objective data and statistical analysis are being employed when increased exploration provides an expanded data base. The methods used in making resource assessments are evolving in complexity to the point that we can deal with exploration plays by stratigraphic units within each prospective province. In areas where data are extremely abundant, the choice of methods used may become more a function of the objectives of the resource assessment and the vailability of staff for the study. If data and time permit, the ultimate approach for a complete resource assessment is to use as many methods as possible as a means of cross-checking results.

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