Role of permeability versus temperature in the distribution of the fine (< 0.2 μm) clay fraction in the Chipilapa geothermal system (El Salvador, Central America)

Abstract

Detailed X-ray diffraction of clay minerals from the less than 0.2 μm fraction of altered rocks has been carried out on cuttings and cores from three boreholes (CH7, CH9 and CH7B) from the geothermal field of Chipilapa (El Salvador, Central America). In most of the low-permeable horizons, the vertical distribution of I/S and C/S mixed-layers fairly agrees with the general scheme of di-smectite to illite and tri-smectite to chlorite conversion series with increasing depth (max. 2600 m) and temperature (max. 220 °C). The deepest parts of the wells are characterized by chlorite and illite-rich illite/smectite mixed-layers (I/SR ≥ 1) in close agreement with the measured temperature (170–220°C). Within fracture-controlled permeable horizons, the distribution of clay minerals strongly disagrees with the “classical pattern” of di-smectite to illite and tri-smectite to chlorite conversions observed in low-permeable horizons. Great amounts of smectites and/or smectite-rich mixed-layers occur at temperatures which strongly exceed the current range of smectite stability in natural systems (up to 205 °C). Similar discrepancies may also be viewed from chlorite crystallinity in well CH9 and smectite content in smectite-rich chlorite/smectite mixed-layers in well CH7B. These high-temperature smectites are considered as present or very recent metastable phases representing the first step of kinetically controlled conversion series toward illite and/or chlorite. Such phenomena are restricted to the levels in which the present hydrodynamic conditions promote the crystallization process: boiling and mixing of fluids with contrasting temperatures. High-temperature smectites and smectite-rich mixed-layers may be of great interest for the understanding of the very recent evolution of geothermal reservoirs. They do not only indicate the present horizons of active fluid circulations in geothermal wells (smectite coatings at the wall of fractures), but they are also potential guides in the determination of the horizons in which permeability has recently decreased.