A Simple and Quick Way to Estimate the Numbers of Undiscovered Hydrocarbon Accumulations in Sparsely Drilled Areas

Richard G. Stanley

The "checkerboard method" is a systematic way of estimating the numbers, but not the sizes, of undiscovered oil and gas accumulations in areas that are sparsely drilled but include at least one discovered oil or gas field. The method was developed during a recently completed USGS assessment of undiscovered resources in the Eel River, Salinas, and other basins of coastal California, and might be of some use in other regions of the world where alternative techniques--such as prospect counting, statistical analysis of discovered fields, and comparison with analog basins--are insufficient or unsuitable.

The area to be evaluated is partitioned into many sectors of equal area (for example, 1 square mile), and each sector is classified as P (producing), D (drilled but not producing), or U (undrilled). Then, a success ratio S is calculated as S = P/(P + D). The number of sectors with undiscovered hydrocarbon accumulations A is calculated as A = US. If one assumes that most undiscovered accumulations are small and occupy surface areas smaller than one sector, then A may be about the same or somewhat less than the number of undiscovered accumulations. Alternatively, if one supposes that there are several large undiscovered accumulations that occupy many sectors each, then A may be larger than the number of undiscovered accumulations.

The checkerboard method should be used with caution because it depends heavily on subjective judgments by the geologist, who must carefully draw the boundaries of the area to be assessed and guess the likely sizes of undiscovered accumulations. The method attempts to measure an uncertain quantity-- specifically, the number of undiscovered hydrocarbon accumulations in a given area--that cannot be known until the area has been thoroughly explored. The numbers obtained by this method are testable hypotheses, subject to revision or rejection as more geologic information is obtained.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California