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AAPG Bulletin


(Begin page 1883)

AAPG Bulletin, V. 84, No. 12 (December 2000), P. 1883-1891.

Copyright ©2000. The American Association of Petroleum Geologists. All rights reserved.

Swanson's 30-40-30 rule

A. Hurst,1 G. C. Brown,2 R. I. Swanson3

1Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3UE, United Kingdom; email: [email protected]
2Department of Mathematical Sciences, University of Aberdeen, Aberdeen, AB24 3UE, United Kingdom. Current address: Office for National Statistics, 1 Drummond Gate, London, SW1V 2QQ, United Kingdom; email: [email protected]
3deceased early 2000.


Andrew Hurst received a Ph.D. in geology from the University of Reading in 1980. From 1981 until 1990, he worked for Statoil, Norway, in the petroleum technology group. From 1990 to 1992, he was employed as an exploration geologist by Unocal UK. In 1992, he assumed the new Shell Chair of Production Geoscience at Aberdeen University. His main research interest is in reservoir characterization, particularly of turbidite reservoirs. He is the chief editor of Petroleum Geoscience.

Gary Brown received a B.Sc. degree (1989) in mathematics and a Ph.D. (1995) in statistics from the University of Birmingham. From 1996 to 1999 he was employed as a research fellow in statistics at the University of Aberdeen, where he also ran an internal and external statistical consultancy service. His main research interest is in multivariate regression, especially in the associated diagnostics. Since 1999 he has been employed as a methodologist in the Office for National Statistics.

Roy Swanson received a B.S. degree (1953) in geology from the University of Pittsburgh and an M.S. degree (1955) in geology from Michigan State University. He worked for Exxon Corporation first as a geophysicist in Wyoming, Colorado, and Venezuela and then as a geologist in New Orleans. In 1969, he entered the area of exploration economics and in 1981 became coordinator of Inland Exploration analysis for Exxon. He retired from Exxon in 1992.


We would like to acknowledge the helpful comments of Bill James (Exxon), to whom we are also indebted for the neat theoretical justification of the 30-40-30 rule presented in this paper. Peter Rose and Joseph Studlick provided useful review comments for which we are grateful. Through his excellent teaching of this method, Peter Rose provided the impetus to document the theoretical justification of the method.

Our thanks to Roy Swanson for his invaluable contributions to this paper, and to the field of petroleum geology in general. It was with great sadness we heard of his passing away in March 2000.


Evaluation of the possible range of reserves associated with a prospect is a key part of risk taking in hydrocarbon exploration. The challenge of presenting a range of geologically possible models for a range of prospect reserve estimates is addressed using Swanson's 30-40-30 rule. Swanson's rule defines the mean as 0.3P10 + 0.4P50 + 0.3P90, and provides a good approximation to the mean values for modestly skewed distributions. Pragmatic and mathematical justifications for this rule are given. Applications of the rule to a historical field size distribution and a specific prospect evaluation demonstrate its efficacy in handling routine problems in hydrocarbon exploration, with particular reference to use with the lognormal distribution.

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