Abstract

This paper revisits the clay mineralogy of the “smectite” alteration zone in the Krafla geo-thermal field via the study of an exploratory well in which temperatures range from 40 °C to 215 °C. The clay alteration consists of several mineral assemblages superimposed in time and space, resulting from different stages of water-rock interaction. Trioctahedral clay minerals (chlorite, corrensite and smectite) are observed throughout the studied section. These minerals can form in nearly closed systems as replacements of groundmass minerals/glass after interactions with resident and nearly stagnant fluids not far from chemical equilibrium (neutral to basic pH conditions) or from direct precipitation from geothermal fluids. They are locally superimposed by Al clay phases (smectite, illite/smectite and kaolinite), which result from intense leaching of the host rocks due to their interaction with low pH fluids under strong W/R ratios. The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics. The current fluid circulation generates trioctahedral clays at depth that cannot be distinguished from pervasive earlier alteration. The only easily detectable signature of current activity and the most relevant signature for geothermal exploration is the presence of Al dioctahedral phases since it indicates leaching and intense hydrothermal activity.

Highlights

  • In the context of global warming, with the stated objective of diminishing greenhouse gases, the geothermal industry appears to be a major target for the production of more renewable and less polluting energy

  • The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics

  • The objective of this study is to re-examine the hydrothermal alteration of an exploratory well drilled in 2007 in the Krafla geothermal field to (1) build a paragenetic model for clay minerals at the drill hole scale and (2) provide clues for the exploration of such systems, to show the implications of this model of clay distribution on electrical sounding approaches

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Summary

Introduction

In the context of global warming, with the stated objective of diminishing greenhouse gases, the geothermal industry appears to be a major target for the production of more renewable and less polluting energy. The two main pros-pecting techniques are geophysical soundings and borehole drilling, this second technique is highly expensive, making geothermal exploration difficult to execute. This is the reason why a large effort has been focused on understanding the dynamics of geothermal reservoirs through geophysical soundings, in particular using electrical methods (e.g., [1]). Studies on electrical soundings and geochemical/mineralogical analyses on core samples have pointed out a correlation between temperature, electrical resistivity, and areas of clayrich hydrothermal alteration This correlation has led to the creation of models describing the architecture of high-temperature geothermal reservoirs ([2,3], Figure 1)

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