Fine-Grained Clay Fraction (<0.2pm): an Interesting Tool to Approach the Present Thermal and Permeability State in Active Geothermal Systems

Abstract

We have investigated by X-ray diffraction the very fine grained secondary minerals (< 0.2 pm) developped in geothermal systems, in relation with their present thermal and permeability state. Because the smallest particles are the most reactive part of a rock, they are the youngest mineral phases of the geothermal fields. This study has been performed on two active geothermal fields: Milos field, Greece (130 < T< 320?C) and Chipilapa field, Salvador (90<T<215"C). In the Milos field, the mineralogical composition of the <0.2pm clay fraction observed in the reservoir strongly differs from the overlying altered metamorphic schists in the presence of abundant quantities of saponite and talc/saponite interstratified minerals at unusually high temperature. These phases are considered to be kinetically control-led "metastable" minerals which rapidly evolve towards actinolite and talc for present temperatures higher than 300 "C. Their occurrence is a good indicator of discharge in highly permeable zones. In the geothermal field of Chipilapa, the mineralogical composition of the < 0.2pm clay fractions fairly agrees with the temperatures presently measured in the wells, whereas several discrepancies may be pointed out from the compositions of coarser clay fractions (<5pm) which contain minerals inherited from higher temperature stages. Permeable zones may be evidenced from an increase of expandable components in the interstratified minerals and a decrease of the coherent domain of the unexpandable clay particles (chlorite).