An in-depth study of the state of the art in Geothermal Well Log Interpretation has been made encompassing case histories, technical papers, computerized literature searches, and actual processing of geothermal wells from New Mexico, Idaho, and California. A classification scheme of geothermal reservoir types was defined which distinguishes fluid phase and temperature, lithology, geologic province, pore geometry, salinity, and fluid chemistry. Major deficiencies of Geothermal Well Log Interpretation are defined and discussed with recommendations of possible solutions or research for solutions. The Geothermal Well Log Interpretation study and report has concentrated primarily on Western US reservoirs. Geopressured geothermal reservoirs are not considered.

In the petroleum industry, well logging is a well developed discipline that has matured over a fifty-year period. Compared to this, geothermal well logging is a very new field of activity. The current practice is to use the same logging equipment and the same log interpretation techniques for geothermal wells as had been used for petroleum wells. However, this approach has proven either inadequate or ineffective in most geothermal areas. The problems here are of two types: (1) those associated with logging equipment and operation, and (2) those connected with log interpretation techniques. This paper focuses on the log interpretation aspects only. 6 refs.

Reservoir types are defined according to fluid phase and temperature, lithology, geologic province, pore geometry, and salinity and fluid chemistry. Improvements are needed in lithology and porosity definition, fracture detection, and thermal evaluation for more accurate interpretation. Further efforts are directed toward improving diagnostic techniques for relating rock characteristics and log response, developing petrophysical models for geothermal systems, and developing thermal evaluation techniques. The Geothermal Well Log Interpretation study and report has concentrated only on hydrothermal geothermal reservoirs. Other geothermal reservoirs (hot dry rock, geopressured, etc.) are not considered.

Progress is presented on the following tasks: review of the state-of-the-art, classification of geothermal reservoir types, data acquisition, problem definition and directions for solution, and refinement of existing interpretation techniques and development of new ones. Computerized literature searches were conducted. The classification system defines five major characteristics which will qualify a potential reservoir. A catalog lists well logs currently available for study. Rock and fluid parameters needed for reservoir studies are listed. A list of matrix characteristics for rocks and minerals is given. (MHR).