Use of pyroclastic rocks as a tool for the evaluation of geothermal systems during the prospecting stage of geothermal reservoirs; case study Naivasha, Kenya

Abstract

Numerous geothermal exploration projects have taught us a fundamental axiom for geothermal exploration in volcanic areas. Many of the complexities and unknown, subsurface characteristics of a volcanic geothermal field can be constrained through logical deduction that is based upon careful field observation, mapping, sample studies, and the integration of related geophysical and hydrogeochemical data. We believe that many geothermal exploration projects in volcanic areas have suffered from the lack of pertinent volcanological observations and interpretations. So many clues regarding the location and magnitude of geothermal systems are available from the volcanic structure and deposits that one might say detailed interpretation of these observations constitutes a type of exploration drill-hole. Therefore, we stress the need for careful field volcanology during geothermal exploration projects in volcanic areas. During the last 10 years, the field of volcanology has been growing rapidly; the resulting new observations and ideas are providing us with numerous hypotheses on volcanic structure and processes. In magma genesis, movement, and eruption phenomena, as well as volcanic structure and thermal histories, there have been many new discoveries that have engendered a better understanding of igneous systems and their relationship to high-grade geothermal systems. These hypotheses and discoveries have important geothermal implications when applied to the interpretation of volcanological observations. Geologists must use what is known about volcanoes, their structure, eruption phenomena, and composition, to reveal necessary information about the heat sources and settings of groundwater—key factors in formation of a hydrothermal system. A basic approach to exploration includes good geological mapping by whatever means is available: topographic maps, aerial photographs, satellite photographs, planetable surveying, tape and brunton traverses, and panoramic viewpoints. Also, systematic descriptions of tephra deposits and rocks are vital, especially for core logs from exploration holes. In applying volcanological observations, one should integrate the observations (for example, mapping and sample analyses) with other information on surface springs and iv fumaroles, water chemistry and hydrology, and geophysical surveys, including gravity, electrical resistivity, seismicity, and heat flow. Any of these surveys by itself, without a geologic framework, is almost useless; integrated with good geological surveys, each is valuable. Hydrochemists, geophysicists, reservoir engineers, and geologists must talk to each other and work as teams to successfully develop geothermal resources. The field approach involves learning everything possible about a volcano or volcanic field, including structure; structural setting, eruption phenomena, composition, and ages of eruptions. Using these data, it is possible to establish hypotheses regarding the location and magnitude of hydrothermal resources. Simply put, the volcano and its products supply information normally gathered from the first drillholes and may provide a view of the volcano's geothermal system or systems. The field approach is usually cost effective and defines the first step towards prescribing the geophysical surveys and exploration drilling that will be of use.