Characterization And Evaluation Of Structural Permeability Of The Northern Lake Abaya Geothermal Field Southern Main Ethiopian Rift

Lake Abaya geothermal field is located on the axis of the Southern Main Ethiopian Riftrn(SMER), near the western margin where the plateau transitions into the rift floor. The studyrnarea is located approximately 275 km south from Addis Ababa. The main objective of thernresearch study is to evaluate the structural permeability and characterize the geothermalrnsystem in the Northern Lake Abaya Geothermal Field. To meet the objectives, differentrnmethods have been applied. The methods are; remote sensing, geological field mappingrnand sampling, field soil CO2 flux and temperature measurements and petrographic andrnlaboratory fluid analysis. The results of geological investigations are summarized in arngeologic map (scale, 1:50,000) that shows various lithologies, recent to very young eruptivernfeatures, dense fault systems and active geothermal activities. The exposed volcanic andrnsedimentary lithologic units are ignimbrite, trachyte, pumiceous pyroclastic flow and fallrndeposits, basalts, scoria, rhyolitic and obsidian lava flows and alluvial and lacustrinernsediments. The volcanic products, except the very recent once, are affected by extensionalrntectonics which is manifested by the occurrence of joints, fissures and extensive normalrnfaulting, all trending NNE-SSW. The thermal fluid analysis results indicate the thermalrnwaters are primarily of near neutral pH and are primarily of the sodium- bicarbonate type.rnThe high silica concentration measured in all springs result in estimated reservoirrntemperatures of greater than 140°C. The average cation and gas geothermometers suggestrnreservoir temperatures of greater than 200°C. The soil CO2 flux measurement results showrnelevated fluxes (>>100 g m–2 d–1) along major faults and surface manifestations. The totalrnCO2 released from the Abaya fault has a total degassing amount of 282.274 ton d-1. Thernresults of soil temperature survey indicate a higher temperature anomaly up to 98.5 °C. Thernstructurally controlled surface thermal manifestation, the presence of low resistivity zonesrninferred from geophysical data, the anomalous absolute temperatures interpreted from thernthermal imagery, the anomalous soil CO2 flux and temperatures all suggest that the Abayarngeothermal system has favorable deep-seated permeable zones for the fluids to accumulaternand continuously feed the surface thermal emissions.

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