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W Rift Workshop Kigali Varet Technical Workshop on the Geological Development and Geophysics of the Western Branch of the Greater East African Rift System with Emphasis on Factors that Control the Development of their Geothermal Systems. Kigali, Rwanda 9-11 March 2016 Critical datasets required for the successful development of integrated geothermal conceptual models: Summary of supportive information required before committing a prospect to exploratory drilling Jacques Varet SARL Géo2D (France) Ressources géologiques pour le Développement Durable Associate Professor, DeKUT, GETRI (Kenya) www.geo2d.com Western Rift Wokshop, Kigali, march 2016. J.Varet Gisenyi Diversity of geothermal systems - solutions > A very large range of geothermal resources available, which vary considerably according to the geological context on one side, and on the nature of the demand on the other side. > There is no generalized approach in committing a prospect that would fit as diversified applications as district heating in graben areas, heat pumps for household thermal regulation, engineered deep EGS, as well as high enthalpy geothermal exploration in geodynamically active areas. Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Do not just search for power for the grid! Geothermal can do more, diverse… > We will focus on the approach to geothermal systems in active rift zones, including volcanically active as well as tectonically active rifts, as expressed along the eastern and western branches of the EARV respectively. > Even though, there are still different types of possible geothermal developments – from community based small-zine systems to search for large electrical production serving the electric grid - implying quite different approaches. > Deciding upon exploratory drilling rely upon the quality of the expected resource (possibly high flow rate of high temperature fluids of compatible chemistry) but obviously also on the demand of the communities on and around the site, implying socio-economic as well as geoscientific studies. Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Any geothermal project necessitates transdisciplinary approach Géosciences: - Geology - Geophysics - Geochemistry - Drilling - Reservoirs Social Engineering - Thermics Sciences: - Mecanics - Sociology - Thermodynamics - Ethnology - Electricity - Economy - …. - Politics - …. Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets General methodology Development scheme Energy needs 2020 et 2050 Technical choice for geothermal production system Scenario 3 Area of production / distribution Licences Scenario 2 Scenario 1 Restricted areas (protection….) Various scenarios of Geothermal Resources development schemes for geothermal energy at local/regional level. Defining objectives Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Terrestrial heat flow higher along plate boundaries, notably expansion zones due to mantle melting (decompression) * 60 à 100 mW/m2 in stables zones (60-100 kW/Km2) * up to ten times more in active zones (of the order of the MW/Km2 i.e. renewable) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Part of the Earth energy is dissipated along diverging plate margins, unfortunately most are immerged under oceans But most of the geothermal energy stock is available and economically valuable in normal gradient areas provided high permeabilities zones are present Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets EAR: the two asthenospheric domes of Ethiopa and Kenya : supposedly due to mantle plume, i.e. inducing anomalous hot upper-mantle Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets But rate varies from 2 cm/year in Afar to 5mm in Ethiopian rift and 2 mm in Western rift EARV From oceanic To continental rift Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Magmatic variations of continental rift systems: source magma as well as basic/salic proportions Barberi, F., Santacroce, R., & Varet, J. (1982) Chemical Aspects of Rift Magmatism. In. G.Palmason (Editor). Continental and Oceanic Rifts. Geodynamics Ser.8: 223-258. Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Continental rifts in Europe also associate graben tectonics and local volcanic units, some of them evolved (French Massif central) Perialpine Europe geodynamic context of continental rifts and volcanism Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets European rift zones : volcanic units and tectonic graben co-exist Graben, frequently assymetric, may display interesting thermal anomalies (ex. Limagne, France) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Positive thermal anomalies coexist with negative ones due to thermal convection along faults (ex. Rhine graben, Soultz) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Looking for heat sources along EARV There are 2 lava lakes in the world: Erta Alé and Nyiragongo i.e. emerged magma chambers Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets But surface hydrothermal manifestations have not the same importance Ma’alalta N. Afar Ethiopia Garrabayis Djibouti Rep. Gustave Courbet Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets L’Origine du Monde Magma genesis Repeated diking along single axis from shallow source (nearly tholeiitic) Fissural random emissions from deep source (K alkali basalt) Karisimbi Erta Ale, Afar southern flank, Ethiopia Rwanda Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets No possible heat source in such fissural basaltic lava fields (very suitable for potatoes plantations) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Heat source northern Afar, parallel to Red Sea Erta Ale range provides from S to N a textbook petrological and volcanological evolution of oceanic type basalts from typical fissural stage up to elongated shield and central cumulovolcanoes with very shallow (1-3 Km deep) magma chambers. Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Afar volcanic axial ranges : emerged oceanic ridge segments Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Elliptic calderas along rift axis : Erta Ale vocano > 20 Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Erta Ale shield volcano > From picritic basalts to plagioclase rich intermediates > Up to ferrobasalts and dark trachytes Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Basaltic elliptic caldera collapseroof of very shallow magma chamber Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Central volcanoes : peralkaline rhyolites > Eruptive cycle up to trachytes and commendites (slightly peralkaline rhyolites) > Volcanological and petrological expression of a very shallow magma chamber Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Central volcanoes peralkaline rhyolites > Displaying shallow magma chambers > Low pressure & fO2 crystal fractionation > Confirmed by recent interferometry studies (following 2008 eruption ) 1 Km depth (Pagli et al. 2012) > Numerous fumaroles including HT Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Dalla filla rhyolite cone cumulo-volcano (Erta Ale range) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Shallow heat sources (1 to 3 Km) Successive steps in volcanoe’s construction in Erta’Ale range from fissural to shield and stratovolcano, with development of a shallow magma chamber. From south to North due to decreasing spreading rate Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Erta Ale axial range volcano-magmatic evolution Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Erta Ale range: a complete magmatic sequence from basalts to rhyolites > Strong iron enrichment > Very low oxygen fugacities > Low depths magmatic differenciation > i.e. 1-3 Km heat sources Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Other extremity of Afar (Djibouti) Asal range first merged segment of Aden ridge Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets the Asal rift axis seen from lake Asal (260m bsl) > 30 Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets OCEANIC RIFT CONTEXT: STEAMING OPEN FISSURES IN FIALE CALDERA, ASAL Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Geological map of the Asal rift hyaloclastites in orange, recent basalts in deep blue and violet; early rift basalts (300.000 – 100.000 y.) in pale blue ; stratoid series (3 to 1 My) in very pale blue. Lacustrine deposits (diatomite) in yellow Ardoukoba 1978 (after Koreli Stieltjes, hotsprings ed. BRGM, Zone Nord 1978) Ghoubbet Project site Baie des requins Site Asal 1- 4 Fiale caldera Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Asal axial range nearer to tholeiites, compared with Erta Ale (low fO2 shallow fractionation) Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Plagioclase segregation in porphyritic basalts (near surface lava lake conditions) explains the iron enrichment in later liquids Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Geophysical data (sismicity, MT restivity) allow to map the location & depth of the heat source (magma chamber) and of the reservoir zone (brittle magma-water interaction) Western
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