Mineralogy Evaluation in a Geothermal Well Using Statistical Probabilistic Log Evaluation Techniques

Abstract

A one well study using open hold well logs was made to identify mineralogy in a geothermal well drilled in igneous rock in Japan which produced two phase fluids. Statistical probabilistic techniques found in the OPTIMA program were used to obtain percentage estimates of a total of 12 minerals with the objective of detecting hydrothermal alteration zones form the mineral assemblage, in groupings of six minerals. In addition, since cuttings were not analyzed in a continuous fashion, the continuous logging results were helpful in detection of lithology changes. The logging suite consisted of the Gamma Ray Log, Spectralog (potassium uranium, thorium), Compensated Densilog, a single detector log, BHC Acoustilog, Dual Laterlog, Electric Log, SP Log, and Temperature Log. Uranium, potassium and thorium along with their rations, were useful in detecting fractures and the producing zones, which were later confirmed by injection production logging. The study demonstrates that open hole logs have a future in geothermal well evaluation. Some suggestions for improvement of the data and further integration of geologic and other information are presented.