Project Ethiopia Geothermal Description

Introduction

Volcanic geothermal resources are powerful candidates for sustainable electricity generation. In 2010, the top ten volcanic geothermal fields provided 50% of the total worldwide installed power generation capacity (Bertani, 2015).

Active continental rift systems are ideal places for geothermal power generation. Steady, rifting-related magmatic processes result in volcanic heat, that is easily accessible near the surface. An example for such an active continental rift is the Main Ethiopian Rift (MER), which is part of the greater East African Rift System (EARS) (Corti, 2009). The EARS is mostly known for its dramatic landscapes with more than 120 active volcanoes (Global Volcanism Program, 2013) such as the Kilimandscharo and the Erta Ale in the Danakil depression with its bubbling lava lake and multi-coloured hydrothermal fields.

With this project we will conduct a geophysical survey in the MER to understand how magma is spatially distributed in the rift, what controls magma transport and storage under magmatic segments and how it influences the formation of geothermal reservoirs. To accomplish this, we will conduct a regional magnetotelluric (MT) survey covering the central MER (Figure 1). The study area encloses the geothermal fields of Tulu Moye, Aluto and Corbetti. These three geothermal fields have been studied by MT in great detail (Gislason et al. 2015, Samrock et al. 2018, 2020a), a pilot power plant is operating at Aluto and geothermal drilling is ongoing at all sites.

Objectives and previous studies

The new regional-scale MT survey will complement and expand the existing high-resolution MT surveys at Tulu Moye, Aluto and Corbetti. Based on the new data we will compute a multi-scale 3-D electrical conductivity model of the MER that covers depths from the surface of the geothermal fields down to the upper mantle.

The electrical conductivity in the subsurface is predominantly controlled by the presence and composition of fluids and melt and the hydrothermal alteration mineralogy. In volcanic environments MT provides key information about the extent and architecture of magmatic and hydrothermal systems (Samrock et al., 2018): The local models from Aluto and Tulu Moye show both a transcrustal two-level magmatic system with magma ponding in the lower crust, C4, quick rise of magma along weak zones into the shallow crust, C3, and a zone of magma storage and fractional crystallization at 3-5km below the surface, C2 (Figure 2, Samrock et al. 2018, 2020a).

Both models are in striking agreement with petrological findings (Trua et al., 1999; Rooney et al., 2007, Gleeson et al. 2017). Differences in their electrical conductivity reflect different stages of melt evolution and composition (Samrock et al., 2020b). At Aluto and Tulu Moye geothermal wells are drilled through the clay cap, imaged as conductor C1, into the up-flow zone atop of the shallow intrusion C2. However, the big picture that shows the link between continental rifting on a large scale and the formation of hydrothermal reservoirs on a small scale, is still missing.

From the new regional scale 3-D model, we expect to gain new insights into:

    • how the volcanic systems of Aluto and Tulu Moye are connected to the deeper region of melt generation,
    • how magma is spatially distributed beneath the MER,
    • what controls magma transport and emplacement under volcanically active sites and
    • how it influences the formation of prospective geothermal reservoirs.

The potential of geothermal energy for Ethiopia

Even though volcanoes are often regarded as a hazard, they also provide beneficial resources, such as ore minerals and geothermal energy. Various applications from balneology over greenhouse heating to electricity generation, make geothermal energy beneficial not only for smaller communities, but also for the infrastructure of entire countries. Hence, geothermal energy can be a catalysator for sustainable socio-economic development.

Ethiopia´s geothermal energy resources are estimated to have a power generation potential of about 10’000 MWe and thus bearing the potential to play a key role in Ethiopia’s future. While already nearly 100% of the country´s electricity is generated from renewable energies, 43% of the households still have no access to electricity.

To overcome this energy poverty the Government of Ethiopia launched in 2017 the National Electrification Program, which led to a boost in geothermal energy projects. Contracts were signed to install geothermal power generation facilities with a total capacity of more than 1000 MWe.

If the projects succeed, Ethiopia could become one of the worldwide leading countries in geothermal energy, following in the footsteps of its neighbour Kenya. With an impressive increase of 600 MWe geothermal energy in the past 5 years, Kenya is now number 5 of the top 10 leading countries in geothermal energy worldwide. Today, Kenya can be regarded as an enlightening example for large-scale and successful geothermal development.

Publications

REFEREED PUBLICATIONS IN JOURNALS

Samrock, F., A.V. Grayver, H. Eysteinsson, and M.O. Saar Magnetotelluric image of transcrustal magmatic system beneath the Tulu Moye geothermal prospect in the Ethiopian Rift Geophysical Research Letters, 2018. [Download PDF] [View Abstract]Continental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental break-up through lithospheric weakening and enforces upflow of melt to the Earth’s surface. However, the details about melt transport through the brittle crust and storage under narrow rift-aligned magmatic segments remain largely unclear. Here we present a crustal scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3-D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along pre-existing weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs.

 

Samrock, F., A. Kuvshinov, J. Bakker, A. Jackson, and F. Shimeles 3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia Geophysical Journal International, 202/3, pp. 1923-1948, 2015. [Download PDF] [View Abstract]The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field.

 

PROCEEDINGS REFEREED

Samrock, F., A.V. Grayver, B. Cherkose, A. Kuvshinov, and M.O. Saar Aluto-Langano Geothermal Field, Ethiopia: Complete Image Of Underlying Magmatic-Hydrothermal System Revealed By Revised Interpretation Of Magnetotelluric Data , Proceedings World Geothermal Congress 2020, 2020. [Download PDF] [View Abstract]Aluto-Langano in the Main Ethiopian Rift Valley is currently the only producing geothermal field in Ethiopia and probably the best studied prospect in the Ethiopian Rift. Geoscientific exploration began in 1973 and led to the siting of an exploration well LA3 on top of the volcanic complex. The well was drilled in 1983 to a depth of 2144m and encountered temperatures of 320°C. Since 1990 Aluto has produced electricity, albeit with interruptions. Currently it is undergoing a major expansion phase with the plan to generate about 70MWe from eight new wells, until now two of them have been drilled successfully. Geophysical exploration at Aluto involved magnetotelluric (MT) soundings, which helped delineate the clay cap atop of the hydrothermal reservoir. However, until now geophysical studies did not succeed in imaging the proposed magmatic heat source that would drive the observed hydrothermal convection. For this study, we inverted 165 of a total of 208 MT stations that were measured over the entire volcanic complex in three independent surveys by the Geological Survey of Ethiopia and ETH Zurich, Switzerland. For the inversion, we used a novel 3-D inverse solver that employs adaptive finite element techniques, which allowed us to accurately model topography and account for varying lateral and vertical resolution. We inverted MT phase tensors. This transfer function is free of galvanic distortions that have long been recognized as an obstacle in MT inversion. Our recovered model shows, for the first time, the entire magmatic-hydrothermal system under the geothermal field. The up-flow of melt is structurally controlled by extensional rift faults and sourced by a lower crustal basaltic mush reservoir. Productive wells were all drilled into a weak fault zone below the clay cap. The productive reservoir is underlain by an electrically conductive upper-crustal feature, which we interpret as a highly crystalline rhyolitic mush zone, acting as the main heat source. Our results demonstrate the importance of a dense MT site distribution and state-of-the-art inversion tools in order to obtain reliable and complete subsurface models of high enthalpy systems below volcanic geothermal prospects.

 

References

Corti, G. (2009). Continental rift evolution: from rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth-Sci. Rev., 96 , 1-53. doi:10.1016/j.earscirev.2009.06.005.

Gíslason, Gestur, et al. “Results of surface exploration in the Corbetti Geothermal Area, Ethiopia.” In World Geothermal Congress (WGC 2015) , Melbourne, Australia. 2015.

Gleeson, M. L., Stock, M. J., Pyle, D. M., Mather, T. A., Hutchison, W., Yirgu, G. and J. Wade, (2017). Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models. J. Volcanol. Geotherm. Res., 337 , 44-61. doi:10.1016/j.jvolgeores.2017.02.026.

Global Volcanism Program, 2013. Volcanoes of the World, v. 4.9.0. Venzke, E. (ed.). Smithsonian Institution. doi:10.5479/si.GVP.VOTW4-2013

Rooney, T., Furman, T., Bastow, I., Ayalew, D. and G. Yirgu (2007). Lithospheric modification during crustal extension in the Main Ethiopian Rift. J. Geophys. Res. Solid Earth, 112. doi:10.1029/2006JB004916.

Samrock, F., Grayver,A.V., Bachmann, O., Karakas, O. and M.O. Saar (2020b). Integrated magnetotelluric and petrological analysis of felsic magma reservoirs: Insights from Ethiopian rift volcanoes. (In Review at EPSL).

Trua, T., Deniel, C. and R. Mazzuoli (1999). Crustal control in the genesis of Plio-Quaternary bimodal magmatism of the Main Ethiopian Rift (MER): geochemical and isotopic (Sr, Nd, Pb) evidence. Chem. Geol., 155, 201-231. doi:10.1016/s0009-2541(98)00174-0