Fred A. Rigby
Science Applications, Inc.
Bellevue, Washington
ABSTRACT
Log data from a geothermal well in Surprise Valley, California, were examined as an example of log analysis in a fractured geothermal reservoir in igneous rock. The formation is primarily basalt and basalt breccia with a few layers of andesite, agglomerate, and densely welded tuff. While lithologic transitions recorded on the lithologic log for the well can generally be traced on the well logs, log data do not clearly identify rock types. As has been reported for some other geothermal
wells, neutron porosity readings are affected by the presence of hydrothermal alteration products. In the basalt, neutron porosity values were largely unreliable, although density-porosity cross-plotting appeared to be effective for determining apparent matrix density. The presence of quartz crystals deposited in open fractures produced the unusual feature of exceptionally low porosity readings opposite fractures. Large open fractures in the well could be located using the self-potential log. These points were found to have both low density and low neutron porosity values. A plot against depth of the difference between neutron porosity and porosity found from the density log correlated well with the SP log.