Petrologic and geochemical analysis of subsurface materials can be important in assessment of the potential of a geothermal well and in construction of an appropriate reservoir model for the resource. Qualitative information can 'be gained relating to the reservoir's thermal evolution, the nature and distribution of fluid flow at depth, and the geologic/tectonic environment controlling the underlying geometry and mechanisms of heat and mass transfer in the reservoir. Geologic models based on petrologic analysis of the Salton Sea and Cerro Prieto Qeothermal fields (e&, Younker, e t a1 ne=; proven the value 1982;-Elders, G a l., 1982j have such geologic imput in efficient and accurate field assessment and development In addition to providing data for production and drilling decisions, geologic characterization of the resource can assist in appropriate parameter selection for numerical reservoir modelling and aid in interpretation of geophysical, geochemical, and well log data.

The Imperial Valley lies within the Salton Trough (Fig. l), a tectonic depression which represents an extension of the spreading center underlying the Gulf of California. During the Cenozoic this basin was filled by a thick sequence of predominantly fluviatile, lacustrine, and deltaic sediments derived from the Colorado River system. The Salton Trough is an area of crustal thinning and extension, characterized by active tectonism, Quaternary silicic and basaltic magmatism, and high regional heat flow (Elders, 1979), leading to its high potential as a geothermal resource. The magmatic activity in the Imperial Valley has few surface manifestations other than the Cerro Prieto and Sal ton Sea volcanics (Fig. 11, and descriptions of subsurface igneous intrusives from geothermal drill holes have previously been limited and/or proprietary. Recently, a geothermal well near East Brawley intersected a series of thin (3-35111) diabasic to dioritic intrusives. The petrology and chemistry of these meta-igneous rocks can provide insight into the thermal and fluid chemical characteristics of the reservoir and into the processes of magma generation at depth. A description of the rock types and their hydrothermal alteration is presented in order to increase the petrologic data base relating to this important facet of the geothermal potential of the Salton Trough and to provide a case study illustrating how detailed petrologic examination of well cuttings can provide important input in the construction of a geothermal reservoir model.

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