The basic objective in geothermal well logging is the detection of moderate to high permeability zones containing high temperature fluids. Furthermore, identification of lithology and fluid composition will aid in regional mapping of the resource area.

Most geothermal systems are found in lithological units uncommon in petroleum exploration. To study complex lithologies, one needs calibration data which at the present time do not exist for many mineralogical units. For simpler systems, such as sedimentary type, conventional well logs may have some application.

Consider a sand-shale series type of sediments exposed to hydrothermal fluids. Because of exposure to high temperatures and as a consequence of rock - fluid interaction, the properties of the rock may undergo severe alterations. The degree of such alteration is dependent upon the rock permeability. The higher the permeability, the more surface area is available to the invading hydrothermal fluids. Thus, the nature of alteration in permeable sand is expected to be different than that in the shales.

Hydrothermal alteration may result in the formation of microfractures. It may change the chemistry of the cementation material and various metamorphic reactions may happen including dehydration and decarbonation.

To examine how these changes affect the well log responses in a sedimentary type geothermal field, we studied the well logs from the Cerro Prieto field in Mexico. A fair amount of well logs are available for the field. Copies of the logs were obtained through the Lawrence Berkeley Laboratory.

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