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Field Scale Well Log Correlation Using an Expert System

R. A. Startzman, T. B. Kuo, S. A. Wong

Lateral changes in formation characteristics usually observed in well logs can cause difficulty in correlating consistent lithofacies. A computer system can be useful in this work by processing large amounts of information in a short time. Besides shortening the work requirement in a large field study, computer processing may also result in unique and useful correlations not achieved using conventional and manual approaches.

Early computer approaches were derived from methods used to correlate traces that were laterally closely spaced such as those from seismic sections and dipmeters. These methods have been almost entirely numerically based. As lateral spacing between traces increases, the amount of geological inference needed to achieve acceptable correlation also increases. Thus correlation becomes increasingly dependent on expert opinion and less dependent on numbers per se. We have developed a prototype expert system capable of using at least a moderate amount of geological inference to correlate traces between a pair of field-scale wireline logs.

The system attempts to emulate a working geologist's or petroleum engineer's approach to log correlation. Following digitization of all traces from suites of logs run in each well, the first interpretation step is to identify individual lithofacies or zones. The next step is to characterize each zone in sufficient detail to enable correlation with the same zone in another well. Correlation is then accomplished by application of a set of zonal comparison rules intended to reflect the geological environment. Having achieved a tentative correlation, the system then reexamines the original zonation scheme and attempts to automatically improve the correlation until improvement is no longer possible. This paper describes our experience to date with the system. It shows how it works with log data on a Symbolics Model 3640 minicomputer with KEE software. The paper also discusses problems we encountered and the potential for developing expert systems software for log correlation.

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