Integration and Modeling of Geoscience Data from the Tendaho Geothermal Area, Afar Rift, Ethiopia

The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the Tendaho geothermal area, Ethiopia. The geothermal area is located within the NW-SE trending Tendaho Graben (TG), which includes the southern part of the younger and active Manda Hararo Rift (MHR) in the Afar region. Rifting and volcanic activity within TG occurred mostly between 1.8 and 0.6 Ma but extended to at least 0.2 Ma. Extension gave rise to about 1600 m of vertical displacement verified by drilling of the Afar Stratoid sequence, over a crust with a mean thickness of ~23 km. The main emphasis of our study was on the Dubti and Ayrobera geothermal areas, near the central axis of the TG, although we also reviewed data for Allalobeda, located near its western bounding faults. New data from these areas include magnetotelluric (MT) and time domain (TDEM) surveys completed in 2013-2014, as well as radon and shallow temperature surveys. The digital enhancement and modeling of existing potential field data revealed previously unrecognized structural lineaments trending mainly NW, NE, and WNW. New MT and TDEM data have been integrated with the existing electromagnetic data set, obtaining >200 MT stations covering the study area. 1D and 2D modeling of the data set has revealed local updoming of a widespread shallow conductor associated with thermal areas at Dubti and Ayrobera, and a possible relation to intrusive heat sources. Updoming is attributed to conversion of smectite clay to higher-temperature mixed-layer clays and chlorite due to hydrothermal fluid circulation. A review of the linkages between faulting and volcanism was also undertaken. Mapping the main axis of young volcanism and the base of the shallow electrical conductor provides input to a 3D temperature model constructed from well data (6 geothermal, 9 shallow gradient and multiple shallow water), near surface temperature measurements, liquid and gas geothermometry, and assumptions regarding conductive heat flow from young intrusion. We have compiled the most relevant data for Tendaho and organized it in a way that it can be viewed and interpreted in 3D using Leapfrog© Geothermal Software. We integrated all the key data to visualize patterns and develop internally consistent subsurface temperature and conceptual models. The model for Dubti indicates deep upflow to the SE of the drilled wells that migrates up NW-trending structures and then spreads laterally in shallow permeable horizons. At Ayrobera, deep upflow is likely localized by NW-trending faults on the eastern flank of the main axis of youngest volcanism. The temperature model, along with conceptual considerations, was used to define risked areas of interest for further exploration activities including targeting and drilling of deep wells. A key uncertainty is to what extent large regions of deep (>2 km) low resistivity are real, and how to model them in terms of temperature and permeability.