How much oil and gas remains to be discovered in the United States is an important question for petroleum geologists. This paper presents an overview of the methods used by the U.S. Geological Survey in its assessment by play of undiscovered resources of the onshore and state offshore areas of the United States. For the 1989 assessment, new estimation procedures were designed to aid the geologist in the assessment of the size distribution for the remaining population of undiscovered accumulations. This paper presents the details of that procedure and examples of its application for two oil plays in Wyoming and Montana: the Minnelusa play in the Powder River basin and the Basin- Margin Anticline play in the Bighorn basin.

In the new procedure, a truncated shifted Pareto (TSP) distribution was fitted to the size data for the discovered accumulations, which were generally split into three sequential parts: the first, second, and last one-third of the accumulations discovered in each play. Parameters from the fitted curves describing size and shape of the discovered accumulations provided a reference model for use in estimating the size distribution of the undiscovered accumulations in a play. In a given play, these accumulation size distributions multiplied by the estimated distribution for the numbers of undiscovered accumulations produced the resource estimates. This method provides a simple, straightforward, and flexible means for the geologist to use statistics as an aid in estimating undiscovered re ources.