| Title | Surface Exploration Results of the Corbetti Geothermal Area in Ethiopia |
|---|---|
| Authors | Gestur GÍSLASON, Hjálmar EYSTEINSSON, Grímur BJÖRNSSON and Vigdís HARÐARDOTTIR |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Corbetti, Ethiopia, , geothermometry, resistivity, MT, TEM, clay cap layer, |
| Abstract | The surface exploration studies that Reykjavik Geothermal has carried out in the Corbetti caldera in Ethiopia, indicate a geothermal resource that exceeds 100 km2, temperatures in the workable range of 250-350°C and expected capacity of over 1000 MW. If proven, this makes Corbetti among the world’s largest geothermal reservoirs. The Corbetti prospect is located some 200 km south of Addis Ababa and hosts a large volcanic complex of basaltic to silicic composition. Numerous fumaroles and thermally altered grounds are seen on surface. Three major geological structures are present, a 155 km2 elliptical caldera ring form, a 50-60 km long and 10 km wide N-S fissure swarm, and a WNW-ESE trending volcanic belt in the middle of the caldera, with summits of Mt. Urji and Mt. Chebi as local magma centres. Shallow exploration wells coupled with lake levels indicate a lateral flow of groundwater from south to north. As this groundwater flows across the northern half of Corbetti, a change is observed in heat flow from a linear gradient type to convective and steaming behaviour. A substantial recharge of heat and mass is therefore taking place from a deep geothermal reservoir underneath Corbetti. A consistent pattern of chemical signature is observed in the 12 fumaroles gas samples. These yield 260-360°C deep reservoir temperatures, based on CO2 type geothermometers. Other studies conducted earlier in Corbetti show similar results. Water samples in the shallow exploration wells also suggest high recharge temperatures. Reykjavik Geothermal has mapped the Corbetti caldera and surroundings by 127 MT- and 119 TEM resistivity soundings. The MT and the TEM data are jointly inverted for a 1-D resistivity model underneath each station pair, and then interpolated into a 3-D resistivity model down to about 25 km depth. Three major resistivity structures are identified. Firstly conductive 500-1000 m thick clay cap layer encountered at about 500 to 1500 m depth inside the northern half of the caldera and deepening to the north. Secondly a deep conductive and probably hot layer is seen at 5-15 km depth. Thirdly there is an abrupt vertical change in the resistivity distribution across a WNW-ESE line through the caldera centre, here attributed to a buried strike-slip structure and recharge of cold groundwater that has prevented the upper part of the southern caldera to develop into a high temperature geothermal resource. Other structures are also seen, like a correlation of the increased thickness of the conductive clay cap layer to the northern caldera rim, and a 50-100 m thick shallow low resistivity tongue away from the Mt Urji summit here interpreted as outflow of hot geothermal fluids. The bottom of the clay cap layer is interpreted to show the boundary of well conductive clay minerals (like smectite) and higher temperature alteration minerals (like chlorite and epidote) which occurs at about 250°C. This interface is thought to represent a 250°C isotherm, provided that the thermal alteration is in equilibrium with the temperature. The depth to this proposed isotherm is about 500 meters beneath Mt. Urji in central part of the Corbetti Caldera and down to 2000 meters outside the caldera. Based on this the reachable area of temperatures more than 250°C is more than 100 km2, which means over 1000 MW using the rule of thumb of 10 MW/km2. |