In-situ determination of Qv (effective concentration of clay-exchange cations) of sedimentary rocks using geophysical well logs is discussed with application to geothermal reservoirs. A method is proposed based on the solution of equations relating formation rock and fluid properties and the conductivity of mud filtrate to the data obtained from S.P. (Spontaneous Potential) and resistivity logs.

The application of the proposed method to some wells in the Cerro Prieto Geothermal field in Mexico is discussed.

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