An overview and critique are presented for five basic categories of resource appraisal methods used to generate petroleum resource estimates during the last three decades: (1) areal and volumetric yield techniques, in combination with geologic analogy; (2) Delphi or subjective consensus assessments; (3) historical performance or behavioristic extrapolations; (4) geochemical material balance techniques; and (5) combination methods using geologic and statistical models (e.g., exploration-play analysis). The results of selected resource estimates are compared from several geographic areas in which different methods have been used. Major issues fundamental to resource assessment discussed are (1) the basic requirements and criteria essential in a systematic approach to the se ection of the resource appraisal methods to be used in assessing petroleum resources; (2) a critique of the strengths and weaknesses of the basic resource appraisal methods; (3) the differences among resource estimates that result from different methods being used in the assessments; and (4) the status of resource assessment methodology for the last decade and goals for the future.

A comprehensive analysis is provided for three case studies: nationwide resource assessments for both Canada and the United States and regional resource assessments for the Permian basin of western Texas and southeastern New Mexico. Evidence is presented that suggests (1) that a significant number of the differences among resource estimates result from the use of different appraisal methods, (2) that consistent patterns do occur in the magnitude of the estimates, and (3) that the relative magnitude of the estimates is predictable depending upon the specific method of assessment used.