Characterization and Evaluation of Structural Permeability of the Northern Lake Abaya Geothermal Field, Southern Main Ethiopian Rift
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Date
2019-05-05
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Addis Ababa University
Abstract
Lake Abaya geothermal field is located on the axis of the Southern Main Ethiopian Rift
(SMER), near the western margin where the plateau transitions into the rift floor. The study
area is located approximately 275 km south from Addis Ababa. The main objective of the
research study is to evaluate the structural permeability and characterize the geothermal
system in the Northern Lake Abaya Geothermal Field. To meet the objectives, different
methods have been applied. The methods are; remote sensing, geological field mapping
and sampling, field soil CO2 flux and temperature measurements and petrographic and
laboratory fluid analysis. The results of geological investigations are summarized in a
geologic map (scale, 1:50,000) that shows various lithologies, recent to very young eruptive
features, dense fault systems and active geothermal activities. The exposed volcanic and
sedimentary lithologic units are ignimbrite, trachyte, pumiceous pyroclastic flow and fall
deposits, basalts, scoria, rhyolitic and obsidian lava flows and alluvial and lacustrine
sediments. The volcanic products, except the very recent once, are affected by extensional
tectonics which is manifested by the occurrence of joints, fissures and extensive normal
faulting, all trending NNE-SSW. The thermal fluid analysis results indicate the thermal
waters are primarily of near neutral pH and are primarily of the sodium- bicarbonate type.
The high silica concentration measured in all springs result in estimated reservoir
temperatures of greater than 140°C. The average cation and gas geothermometers suggest
reservoir temperatures of greater than 200°C. The soil CO2 flux measurement results show
elevated fluxes (>>100 g m–2 d–1) along major faults and surface manifestations. The total
CO2 released from the Abaya fault has a total degassing amount of 282.274 ton d-1. The
results of soil temperature survey indicate a higher temperature anomaly up to 98.5 °C. The
structurally controlled surface thermal manifestation, the presence of low resistivity zones
inferred from geophysical data, the anomalous absolute temperatures interpreted from the
thermal imagery, the anomalous soil CO2 flux and temperatures all suggest that the Abaya
geothermal system has favorable deep-seated permeable zones for the fluids to accumulate
and continuously feed the surface thermal emissions.
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Keywords
Lake Abaya Geothermal Field, Enhancement, Thermal Imagery, Soil Co2 Flux, Permeable Zone