Role of borehole geophysics in defining the physical characteristics of the Raft River geothermal reservoir, Idaho

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Abstract
Numerous geophysical logs have been made in three deep wells and in several intermediate depth core holes in the Raft River geothermal reservoir, Idaho. Laboratory analyses of cores from the intermediate depth holes were used to provide a qualitative and quantitative basis for a detailed interpretation of logs from the shallow part of the reservoir. A less detailed interpretation of logs from the deeper part of the reservoir is based on much less corroborative evidence. Extensive use was made of computer plotting techniques to arrive at some interpretations. Both the stratigraphic correlation utilizing a full suite of logs and the attitude of bedding calculated from acoustic televiewer logs indicate gentle dips throughout most of the reservoir with some local flexures. Televiewer logs permitted the location and orientation of numerous fractures and several features that may be faults. Temperature and flowmeter logs provide evidence that these fractures and faults are conduits that conduct hot water to the wells. One of the intermediate depth core holes penetrated a hydrothermally altered zone that includes several fractures producing hot water. This altered production zone could be distinguished by several logs. Borehole gamma spectrometry can be used to identify anomalous concentration of uranium, thorium, and potassium which are probably due to transportation by hydrothermal solutions. Computer crossplotting was used as an aid to the identification of such rock types as quartzite, quartz monzonite, and biotite schist in the deeper wells. Alteration of biotite schist to chlorite schist was also recognizable on these logs using computer‐based analysis. Borehole geophysics not only provided much information on the Raft River geothermal reservoir but also permitted the lateral and vertical extrapolation of core and test data and bridged the gap between surface geophysical data and core analyses.

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