ABSTRACT: The George and Stiles Method of Determining Porosity Cutoffs: A Computer Program

POWERS, KATHLEEN C., Chevron, Denver, CO

In 1977 and 1978, C. J. George and L. H. Stiles eluded to a new method for determining porosity cutoffs. Their publications attempted to describe the method in a few paragraphs of text, with no mathematical equations.

In fields where the permeability to porosity relationship is poorly defined, this new method has generated significant interest. However, attempts to define the equations from the limited information have generally met with limited success.

The purpose of this presentation is threefold: first, to delineate what this author believes to be the correct George and Stiles equations and to provide a test as to the accuracy of these equations; second, to compare the results of George and Stiles analysis with "standard" least squares regression analysis using both real and synthetically generated data; and finally, to provide a computer program that will function as a user-friendly tool to derive George and Stiles porosity cutoffs.

Over 20 samples of Permian basin core data were tested. These twenty samples of core data had greater than 0.8 correlation coefficients between permeability and porosity. The George and Stiles and least squares methods both produced nearly equivalent porosity cutoffs (>0.9 correlation between the two methods).

This new method was developed for the specific case where the permeability to porosity relationship in a field is below the statistical confidence level of 0.8. To test the hypothesis that George and Stiles analysis is accurate below the statistical confidence level of 0.8, random

scatter was added to synthetically generated core data. The George and Stiles method proved highly resistant to random scatter in the data.

On the basis of the analysis of both real and synthetic data, it is this author's opinion that in a well-distributed population of data, the George and Stiles method is as accurate as least squares analysis when the correlation of permeability to porosity is greater than 0.8, and furthermore, that the principles of the George and Stiles method are applicable when the correlation coefficient of permeability to porosity is less than 0.8.

Because the George and Stiles method of determining porosity cutoffs is relatively computer intensive, the accompanying PC-based graphics program was found to be extremely useful in applying this method.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)