Use of Field Size Distributions in U.S. Geological Survey's Estimates of Undiscovered Domestic Petroleum Resources

John C. Houghton

The U.S. Geological Survey (USGS) and the Minerals Management Service are updating the assessments in USGS Circular 860, "Estimates of Undiscovered Recoverable Oil and Gas Resources." The onshore areas of petroleum potential in the U.S. will be disaggregated into more than 300 plays, and field-size distributions will be presented for the undiscovered petroleum in each play. A Truncated Shifted Pareto (TSP) distribution is fit to the past discovery record in mature areas, and the quantiles from the fitted distribution are used in a comparative sense to support a subjective assessment of a distribution, usually a TSP, of the potential undiscovered fields. In frontier areas, qualtiles from a TSP that has been fitted to a selected analog play are used. The TSP is "J" shaped, s truncated at both a minimum and a maximum size, and is easy to fit and use. Two degrees of freedom that represent scale and shape are commonly used in fitting the TSP to the past record. Scale refers to the overall size of the fields, and shape refers to the degree of concentration versus dispersion in the habitat. A concentrated distribution is one in which the petroleum is concentrated in the few largest fields. For most plays, an efficiency of exploration is demonstrated by a scale decrease over time and a shape change from concentrated to dispersed, as the larger fields are generally discovered early in the record.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.