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 Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets > 35
Fig.57 : carte de l’activité sismique mesurée entre 1978 et 2001 dans le rift d’Asal et coupes interprétatives à travers la caldera de Fialé perpendiculairement à l’axe du rift (en haut) et le long de cet axe entre le golfe du Goubbet et le lac Asal (en bas). Les isothermes à 600 ° C et 1200 ° C ont été déduits du profil thermique A5. La zone située entre 2 et 4 km de profondeur est sujette à une activité sismique intense qui s’interprète comme résultant de l’interaction eau de mer-magma sous Fialé. (d’après Doubre et al., 2007) Asal: location of wells and surface hydrothermal manifestations
Objective of the present WB project (4 wells)
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Tat’Ali range: transitional basalt-pealkaline rhyolites sequence
Steaming faults with hydrothermal ejecta from open vents
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Dabbahu Teru
Dabbahu magmatic heat source (pantellerite centre topping basaltic shield with all intermediate terms), a leaky transform between Alayta and Manda Hrraro range; important steam vents
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Ma’alata stratovolcano on W Afar margin
• Sitting on streched / intruded basement • Important pyroclatic products • Peralkaline silicic • Caldera elongated transverse • Postcaldera rhyolites • Differentiates but evidence of crustal contamination • Representative of Afar margins • Similitude with EARV volcanism
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets E-W gravity model across Menengai
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Global Volcanism Program, Smithsonian Institution
Silali and Karisimbi calderas and peri/post-caldera trachyte domes and flows
MacKay et al. 1998
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Karisimbi differentiated products
MacKay et al. 1998
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Trachyte flows on eastern flank of Karisimbi totalise 1.8 Km3
- From DEM modeling i.e. half the volume from previous authors (DeMulder…)
MacKay et al. 1998
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Manda Harraro axial range The most « oceanic » range in character: - Volcanology, - Petrology, - Geochemistry - Geophysics
Important geothermal sites available at both ends
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Rectangular calderas on basaltic shield volcano along accreting axis of Manda Harraro
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Two major volcano-tectonic events Manda Hararo Nyira-kivu 2005 2002 L: 70 Km 20 Km H: 9-3 Km 8-4 Km l: 7 m 80 cm
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Conclusion Limiting our presentation to the geoscientific critical datasets required for the successful development of integrated geothermal conceptual models for the Western branch of the rift, we can list the following: > Synthesis of all geological data available from the geoscientific literature, either academic or technical, with a specific focus on those relevant for : • heat source, • reservoir, • hydrothermal surface manifestations • and any borehole. > This includes of course the geographic, climatic and hydrogeological context. > Processing of all data available from satellite and airborne surveys, allowing for precise DEM models and maps to be draws as a support for all data collected during the survey. This includes any available geophysical survey allowing for data reprocessing (gravimetry, magnetics, seismicity, seismic profiles…).
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Conclusion (p.2)
The following field geological approach should be specific and adapted to the site characteristics: - volcanology (including petrology, mineralogy, geochemistry and structural volcanology, study of the inclusions, with an accent on phreato-magmatism and phreatic eruption products) this allowing to determine the characteristics of a possible magmatic heat source; - structural geology with a focus on neo-tectonics in order to determine the fault movements and the characteristics of the fractured reservoir and fluids up-flow from depth; - hydrothermal alterations and deposits studies in the field and in the lab, allowing to map present and past geothermal systems and to precise their characteristics. This concerning the rocks in place as well as the inclusions in the volcanic products mentioned above.
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets Conclusion (p.3, end)
> The geochemical and geophysical surveys will be determined from the geothermal first 3D model that the geological approach has provided, that is both in location and extension as well as for the choice of the most relevant tools. • The fluid geochemistry (water, gas, soil emanations) will allow to precise the origin, temperature and mixing/realimentation models of the fluids in the reservoir • The geophysical methods will even more be adjusted to the characteristics of the target area: if electrical methods (AMT, MT…) are a must for the determination of the low resistivity zones characterizing the reservoir cover of hydrothermal up-flow along fault zones, micro-seismicity, gravimetry, IR & MNT drone surveys and eventually magnetics can also appear as relevant. > These methods will allow to refine and quantify – particularly in depth - the geothermal conceptual model proposed by the geological approach. A few examples – including in Rwanda – show that even the most sophisticated geophysical methods may lead to failure in committing a prospect to exploratory drilling if not based upon a sound geological model obtained using the above described approach. > Study of the local demand around potential sites is to be engaged early, even more for small medium temperature sites where cascade uses should be encouraged Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets
Geothermal serving local development, a social issue requiring a gender approach along EARV
Masaii women
Alutu Ethiopia
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets « Geothermal Village » concept: answer the local needs first with a
cascade use of the geothermal fluid
Artisanal & Water / Electricity distribution to Industrial GeoPower energy neighbouring villages developments storage (drying, cooling…) Africa a PEER Irrigated agriculture project under
Groundwater Fish farming Obama’s Power wells Africa
cattle Kenya (DeKUT) breeding Ethiopia (Un. A.A.) Tanzania (TGS) Site survey for local geothermal developemt in Villages with all Geothermal rural (pastoral) commodities reservoir (water, ORC Plant EARV eletricity…) Magmatic heat source Thermal Cascade use Geothermal station of geothermal wells heat
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets
Ma’Alalta, 1968
Western Rift Wokshop, Kigali, march 2016. J.Varet. Critical data sets