Geothermal Exploration in Eritrea

Transcription

1 Geothermal Exploration in Eritrea Ermias Yohannes, Eritrea Ministry of Energy and Mines, Department of Mines Short Course on Surface Exploration for Geothermal Resources UNU-GTP and KenGen, Naivasha, Kenya, November 2007

2 TABLE OF CONTENTS 1. Introduction 2. Regional Tectonic Setting 3. Geologic and Geothermal Setting 4. Alid Geothermal System: Assessment from Faults and Fractures 5. Hydrogeology of Alid 6. Conclusion

3 The African Rift Valley

4 Introduction Electricity consumption by sectors 2% 35% 27% 36% Domestic Commercial S. Industries, LV. Street light The population has an access to electricity is 21%

5 Introduction- Previous works Geothermal assessment was initiated during 1902 by Angelo Marini from the Italian Institute for Military Geography on Alid geothermal manifestations (Marini, 1938). UNDP sponsored work was carried out a reconnaissance survey in 1973 by a Geological Survey of Ethiopia team (UNDP, 1973). In 1992, the late Prof. Giorgio Marinelli and a staff member from the Department of Energy visited Alid area and prepared proposal for detail study. In 1994, Mikhail Beyth of the Geological Survey of Israel surveyed the Alid hydrothermal area for the possibility of epithermal gold deposition. A team of staff from the United States Geological Survey (USGS) and the Ministry of Energy and Mines of Eritrea (MEM) carried out a geological and geochemical investigation at Alid and its surroundings during in 1996 (Clynne et al., 1996). Reinterpretation of the chemistry of water and gas samples (Yohannes, 2004). Fault and fracture analysis (Yohannes et al., 2006), resistivity survey (Berhe et al, 2006) and hydrogeological investigation (Tesfalidet et al., 2006) was carried out on Alid and surroundings.

6 Regional Tectonic Setting zone of crustal extension Down dropped crustal sections, bounded by deep-rooted normal faults (forming grabens) that cut into the basaltic lavas, extruded in the resulting depressions. The geothermal system is mainly associated with volcanics of rhyolitic composition

7 Low temperature areas - springs

8 Samoti spring with high flow

9 High temperature prospects: Geological and Geothermal Setting Location map of Alid and Nabro-Dubbi geothermal areas

10 Geology of Nabro-Dubbi Occurs along the Kod Ali traverse

11 Geologic and Geothermal Setting Outline geology of Nabbro-Dubi area Nabro Lies 2218m a.s.l. and 600m above Mabda Trachytic lava flows and pyroclastic emplace primarily on the Nabro Dubbi the largest reported historical 1861 eruption in Africa Extensive basaltic lava fields cover wide area The major transverse structure separates the Danakil blocks into two geological set up

12 Geological and Geothermal Setting-Alid 112 Km from Massawa 30 Km south of Irafaile village Arid to semi-arid climate C

13 GEOLOGICAL AND GEOTHERMAL SETTING OF ALID ALID STRUCTURAL DOME DOMED RIFT SEQUENCES N m a.s.l. and 700 m uplifted from surrounding

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15 Geological and Geothermal Setting Geological setting: Consists of rifted and faulted young deposits of sediments and volcanic flows. Bounded by metamorphic basement and Stratoid basalt

16 A SECTION OF THE WESTERN SIDE OF ALID CRATER Pumice Lava Shell Sedimentary Sequence Rhyolitic flow Vent Pumice

17 Geologic and Geothermal Setting Geothermal setting- Alid Hot mineralized fluids discharge from many locations. Precipitates in the form of sulpho-salts and clays. Steaming grounds are common. Gas geothermometers indicate a high temperature >220 0 C

18 CHEMICAL COMPOSITION AND POSSIBLE ORIGIN OF SOME GASES OF ALID FUMAROLES Non-condensable gas composition of all samples range from mol% of CO mol% H 2 The N2/Ar /He relations in the fumarolic gases are strongly indicative of a parent geothermal reservoir water that initially was air saturated groundwater.

19 Low temperature areas - springs

20 Alid Geothermal System: Assessment from Fractures and Faults

21 Rose diagram plots of a) Illeghedi, b) Darere, c) Asa-ila, d) Humbebet, e) Abakri, f) Debra Docholo, g) Aerole or Miski-Merehada, and h) Synoptic of the whole area.

22 The systematic examination of faults falls into three major directions Lineaments striking ENE ( ): common lineament, related to major axis of the dome. Lineaments striking NNW ( ): Frequently observed especially at north and south of the dome, related to the trend of the depression. Lineaments striking E-W ( ): These are common as dykes and fractures related to rift tectonics.

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24 Contour plot of Alid area using FFD analysis. Note the high values lie on the Darere-Illeghedi line.

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27 Hydrogeology of Alid

28 Hydrogeology of Alid The Geological and hydrogeological observations showed that: The Wengebo catchment drains directly to Alid and it can be assumed that the catchment is feeder of surface and subsurface water to the Alid geothermal reservoir. The Samoti catchment is a discharge area with very little contribution, if any, in recharging the ground water to the Alid geothermal potential.

29 Conceptual model of Alid geothermal field After Yohannes, 2004

30 Conclusion The tectonic setting and geological make up of the Danakil depression provides a suitable environment for the occurrence of geothermal energy. Alid and Nabbro-Dubbi are the notable prospect A high temperature reservoir (250 0 C) is predicted on the Alid prospect. The source of water and recharge is mainly from the highland water Maximum lineament zone defined by ENE strike is well marked on the FFD analysis inline with the major fracture set of the area. Two high values depicted on the contour map are well accompanied with the thermal manifestation. The resistivity survey carried out marks a 70 0 strike low resistivity anomaly.

31 The current study recommends the following detail works on Alid: Conduct CO 2 mapping of the area to clearly identify zones of heat flow. worth to commence MT and TEM geophysical survey and drill slim hole to map temperature gradient of the area. Perform prospect investigation on Nabbro Dubbi including: Detail geological mapping. Collect water and gas samples and perform analysis and interpretation

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