Permeability in high-temperature geothermal systems is dominantly fracture controlled. However, fracture characteristics and cross cutting relationships among fracture sets are generally difficult to define from cuttings samples alone. In addition, the sparse number of wells in most systems limits the data that can be obtained on the areal fracture distribution. Despite these inherent difficulties, petrologic techniques and mathematical simulations can be used to develop the models of fracture geometry, permeability, and alteration that must be addressed by reservoir engineers. Our fracture studies have taken two approaches; first, establishing predictive structural models, and, second, documenting fluid-rock interaction using alteration mineralogy and geochemistry. Examples of the use of these techniques from active geothermal fields in the U. S. and Canada are described.

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