A statistical method for estimating rock properties from well logs has been developed and implemented in two computer programs: OPTIMA and OPTIMA-RT. Both programs use the set of available logs to estimate best values and confidence intervals of rock properties.

These programs are based on a non-linear weighted least-squares method, which combines actual measurements, theoretical values, and uncertainties into an objective function whose minimum yields the most probable set of results. The OPTIMA-RT program utilizes resistivity logs from devices having different depths of investigation to estimate diameter of invasion (d{) and true resistivities of the virgin (Rt) and flushed (Rxo) zones. The OPTIMA package combines different types of log data to estimate porosity, fluid saturations, and mineral volumes.

We have applied the OPTIMA and OPTIMA-RT programs to a well located in the northern North Sea. Acoustic, radioactivity, and resistivity logs covering a depth of about 1000 ft were available for the study. Geologic information from the area indicates the presence of feldspar-bearing sands, silts, clays, mica, and other accessory minerals.

Resistivities calculated with the OPTIMA-RT program are used as input for OPTIMA. In spite of the lithologic complexity, mineral compositions calculated by OPTIMA agree with available geologic data. Variations in the confidence interval of the calculated values emphasize the importance of being able to measure the reliability of the estimation.

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