The Tendaho geothermal system is characterized by the presence of a 50 km wide graben trending NW- SE, dissected on its turn by a series of normal faults which form a sequence of secondary horst and graben. The graben basement is composed of fissural basalts with minor rhyolite of the Afar Stratoid Series, overlain by a 1,200-1,400 m thick sedimentary sequence, consisting of clastic products (mostly siltstone with some sandstone) and intercalated basalt lenses. The occurrence within the graben of continental crust thinning and opening phenomena, accompanied by upward mass and heat transfer, represents a favourable element for the development of important thermal anomalies. According to surface exploration surveys performed during the 1970s and early 1980s, exploration drilling was carried out in the Tendaho Rift, in Central Afar (Ethiopia), from October 1993 to June 1995. Three deep and three shallow wells were drilled in the central part of the Northern Tendaho Rift to prove the existence of a geothermal reservoir and its possible use for electric power generation. The project was jointly financed by the Ethiopian Ministry of Mines and Energy and the Italian Ministry for Foreign Affairs. Project activities were performed by the Ethiopian Institute of Geological Surveys and Aquater S.p.A. In this work, a review and reinterpretation of the available fluid inclusion and isotopic data is presented, fostered by the recent Feasibility Study of Dubti prospect (2013) and by the current developments of the scientific investigation and integrated interpretation of Dubti-Ayrobera geothermal fields and by the renewed interest in the Tendaho basin. Data are mostly from core-drilled samples of wells TD1-4, and after having recomputed their isochrones with a new interpretation taking into account the scientific development occurred after the first work in 1999, they provide an insight into the two reservoir model (shallow and deep) with fault-driven circulation. The possible existence of a double reservoir system was proposed by Battistelli et al. (2002), with an hypothesized deep contributing geofluid. Waters discharged from wells TD-1, TD-2 and TD-4 are low salinity, sodium-chloride geothermal types, with reservoir temperature ranging from 220°C to 270°C. Fluid inclusion data, compared with present-day measured temperatures, indicate either heating (well TD-1) or cooling (well TD-3, Gianelli et al. (1998)) and are used in the present work to reconstruct the thermal history of this reservoir. R/Ra ratios of geothermal fluids discharged from fumaroles, geothermal wells and those trapped in fluid inclusions, suggest a supply of mantle 3He to the geothermal fluids, a mixing between mantle 3He and He of crustal (and/or upper mantle) origin. The 4He/20Ne ratios indicated a moderate or very low input of atmospheric He, except for one sample from Tendaho with the lowest 4He/20Ne ratio both from present-day fluids and paleo-fluids extracted from inclusion in hydrothermal minerals. This low ratio indicates input of atmospheric He from infiltrating water at shallow depths (Ruggieri et al., 1999). In particular, the isotopic signature results support a fault driven circulation model, adding some information on the contributing deep geofluid.
Review of isotopic and fluid inclusion data at Tendaho prospect - Ethiopia
Montegrossi G;Ruggieri G;Magro G
2014
Abstract
The Tendaho geothermal system is characterized by the presence of a 50 km wide graben trending NW- SE, dissected on its turn by a series of normal faults which form a sequence of secondary horst and graben. The graben basement is composed of fissural basalts with minor rhyolite of the Afar Stratoid Series, overlain by a 1,200-1,400 m thick sedimentary sequence, consisting of clastic products (mostly siltstone with some sandstone) and intercalated basalt lenses. The occurrence within the graben of continental crust thinning and opening phenomena, accompanied by upward mass and heat transfer, represents a favourable element for the development of important thermal anomalies. According to surface exploration surveys performed during the 1970s and early 1980s, exploration drilling was carried out in the Tendaho Rift, in Central Afar (Ethiopia), from October 1993 to June 1995. Three deep and three shallow wells were drilled in the central part of the Northern Tendaho Rift to prove the existence of a geothermal reservoir and its possible use for electric power generation. The project was jointly financed by the Ethiopian Ministry of Mines and Energy and the Italian Ministry for Foreign Affairs. Project activities were performed by the Ethiopian Institute of Geological Surveys and Aquater S.p.A. In this work, a review and reinterpretation of the available fluid inclusion and isotopic data is presented, fostered by the recent Feasibility Study of Dubti prospect (2013) and by the current developments of the scientific investigation and integrated interpretation of Dubti-Ayrobera geothermal fields and by the renewed interest in the Tendaho basin. Data are mostly from core-drilled samples of wells TD1-4, and after having recomputed their isochrones with a new interpretation taking into account the scientific development occurred after the first work in 1999, they provide an insight into the two reservoir model (shallow and deep) with fault-driven circulation. The possible existence of a double reservoir system was proposed by Battistelli et al. (2002), with an hypothesized deep contributing geofluid. Waters discharged from wells TD-1, TD-2 and TD-4 are low salinity, sodium-chloride geothermal types, with reservoir temperature ranging from 220°C to 270°C. Fluid inclusion data, compared with present-day measured temperatures, indicate either heating (well TD-1) or cooling (well TD-3, Gianelli et al. (1998)) and are used in the present work to reconstruct the thermal history of this reservoir. R/Ra ratios of geothermal fluids discharged from fumaroles, geothermal wells and those trapped in fluid inclusions, suggest a supply of mantle 3He to the geothermal fluids, a mixing between mantle 3He and He of crustal (and/or upper mantle) origin. The 4He/20Ne ratios indicated a moderate or very low input of atmospheric He, except for one sample from Tendaho with the lowest 4He/20Ne ratio both from present-day fluids and paleo-fluids extracted from inclusion in hydrothermal minerals. This low ratio indicates input of atmospheric He from infiltrating water at shallow depths (Ruggieri et al., 1999). In particular, the isotopic signature results support a fault driven circulation model, adding some information on the contributing deep geofluid.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
