Occurrence and significance of hydrothermal alteration in active geothermal systems : a case study of well HN-7, Hellisheidi geothermal field, SW-Iceland and surface exploration at Karisimbi geothermal system, NW-Rwanda

Abstract

Various methods and techniques are used in geothermal exploration to evaluate geothermal systems and understand geothermal reservoir zones. Investigating hydrothermal alteration is one of these methods which can provide direct information about geothermal reservoirs because geothermal fluids, by interaction, can change the composition and properties of rocks. This results in the formation of hydrothermal alteration minerals, some of which are known to form at specific and stable temperature regimes. Thus, they can provide a base for mapping the temperature regimes of a geothermal system. Analysis of hydrothermal alteration can start during surface exploration where hydrothermal deposits are found. Generally, however, this method is more commonly used during exploration/production drilling of geothermal wells.

Petrographic analysis of drill cuttings from well HN-7, Hellisheidi high-temperature field, SW Iceland, was conducted to analyse the hydrothermal alteration in the uppermost 1200 m of the well. Identification and interpretation of hydrothermal alteration is an important part of geothermal exploration, either on the surface or the subsurface, as it gives insight into the present and past conditions of geothermal reservoirs. Based on the petrographic analysis of samples from rock formations drilled through in the uppermost 1200 m of well HN-7, the rocks consist of fine- to medium-grained basalt, overlaying consecutive layers of basaltic breccia and tuff, referred to as hyaloclastite, and pillow basalt. The study of hydrothermal alteration minerals revealed a medium temperature environment (maximum 140°C) for the upper 700 m. The alteration minerals in this zone are mainly zeolites like chabazite, thomsonite, mesolite and scolecite along with fine-grained clay, evaluated in XRD as being smectite. Fine-grained clay changes to coarse-grained clay with depth, and certainly with increased temperature, and along with other minerals like laumontite, the clay constitutes a transition to a high-temperature environment which is evidenced by high-temperature hydrothermal alteration minerals such as quartz (>180°C), wairakite (>200°C), prehnite (>240°C) and epidote (~250°C).