sign in sign up
<<
Home , Erythraean Sea, Gulf of Tadjoura

Proceedings, 8th African Rift Geothermal Conference Nairobi, Kenya:2 – 8 November 2020

Preliminary Geological Exploration For Geothermal Resources With Emphasis On Faults A Case Study Of Nord-Ghoubbet, A Geothermal Field In

Samod Youssouf1, 2, Kotaro Yonezu2 and Saadi Nureddin2 1Office Djiboutien de Development de l’Energie Geothermique (ODDEG). 1, 2, Department of Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395 Japan Email: [email protected]

Keywords: Afar Depression, Nord-Ghoubbet, Fault, Fault density, geothermal manifestation.

ABSTRACT

Djibouti is located in Afar Depression (AD), resulting from the separation between Arabian, Nubian and Somalian plates followed a genesis of the oceanic and continental rift. The and the Red constitute an oceanic rifting while the system (EARS) constitutes a continental rifting. In the presence of this tectonic structure, Djibouti presents suitable conditions for geothermal resource. The country has several geothermal occurrences that are expressed either as hot springs, or fumaroles, or alteration zones, silica sinter or travertine environment. Nord-Ghoubbet area is among the major geothermal potential areas and is located near the NW-SE trending axial volcanic range of Asal- Ghoubbet rift which is considered as a high enthalpy geothermal prospect. The major reason behind being favourable geodynamic and hydro-geological aspects. Besides, the area has an intrinsic quality and well-developed fracture network. Tectonically, the study area is affected by transverse faulting ensuing from the transition zone between Asal range and Ghoubbet submarine rift segment and the oceanic ridge identified along the axis of the Gulf of Tadjourah. This study was conducted to assess whether there is any relationship between faults systems and the occurrence of surface geothermal manifestations. Here, most manifestations are fumaroles. Therefore, it is essential to examine whether these faults are linked to geothermal manifestations and can be used in geothermal resources exploration. The main objectives of this study was to confirm whether the surface manifestations mainly controlled by fault zones. Results structure analysis showed that there are three sets of faults with an NW-SE oriented Asal-Ghoubbet, ESE to EW trending of Gulf of Tadjourah and the N-S Makarassou transform fault. The entire area is structured by NW-SE and NNW-SSE normal faults. These orientations of faults are closely associated with the geothermal manifestation occurrences in the study area. In addition to the relationship between fault systems and geothermal manifestations, the fault density in the area is as high as the frequency of surface geothermal manifestations. Fault density analysis can be also used to estimate and assess the potential “Samod et al.” areas for the geothermal occurrence and is a tool for geothermal exploration using similarity of geological setting, especially in areas where geothermal occurrences are fault-controlled.

1. Introduction A geothermal system is generally characterized by its geological, hydrogeological and heat transfer characteristics, while a geothermal resource is formed by an economically sufficient amount of heat concentration in the drillable depth of the Earth's crust. Among the potential geothermal sites in Djibouti Republic considered as suitable for high enthalpy geothermal energy development, the Nord-Ghoubbet area was reported to be one of the most promising for geothermal energy developments based on the detailed surface exploration (Varet, 2010). Geothermal explorations in Djibouti started since the 1970s. The dominant surface manifestations observed are fumaroles in Nord-Ghoubbet. Different methods of exploration applied, include analysis chemical data from the condensate of the fumaroles. They have given insights on the possible reservoir temperatures which are ranging from 260°C to 280°C and 100°C to 160°C (CERD, 2011). Compared to the fumaroles of the Asal rift zone, Nord- Ghoubbet fumaroles have significantly lower δD and δ18O (Geothermica, 1987). It is suggested that no primary steam from undiluted nor diluted deepwater exist in this zone. Fumaroles are more likely the result of secondary steam from previously condensate steam or boiling groundwater (Geothermica, 1987). Previous studies indicate that the fumaroles observed in the Analé wadi in Nord-Ghoubbet are nicking a main N170E fault zone and suggest that these faults are still active (CERD, 2011). The current study aims to use faults to delineate potential areas for geothermal resources which will serve as a tool for geothermal exploration. Therefore, the main objective of this study is to assess whether there is any relationship between faults systems and the present surface geothermal manifestations.

2. Study Area The Nord-Ghoubbet area is located in the central part of the Republic of Djibouti on the northern flank of Ghoubbet Al Kharab Bay, between Lake Asal and the Gulf of Tadjourah (Figure 1). On the other side, the study area is located on the North East of the Asal high- temperature field. Although the area is present at different geological structures from... Which are very active and located at a similar distance from Djibouti as Lake Asal. The interests in this zone have been recognized in the late 1980s, with exploration studies in the Asal geothermal field. Previous studies on geothermal exploration indicate that this area has a lot of potentials as compared to the rest of the country. The geothermal reservoir of Nord- Ghoubbet and should also be beneficial due to relatively low salinity fluids compared to the Asal fluids certainly one of the best for the Djibouti Republic by the long-lasting flow of source waters from Day mountain (Goda massif) downstream towards the gulf for the last few Million years (Varet 2010). The origin of this geothermal fluid in the reservoir was estimated meteoric water (CERD, 2011) as it has been confirmed by geochemistry data. This relief and the particularly steep area is located about twenty kilometres far from Nord- Ghoubbet. The average annual rainfall in this of the Goda massif is the highest in the country and varies between 300 and 350 mm per year (CERD, 2011). “Samod et al.”

Figure 1: Map of Djibouti showing the study area in dashed lines 2.1 Geothermal surface manifestations in the study area The area has several well-known fumaroles at ebullition temperature and at least 12 new fumaroles at the east zone were discovered in 2019 by ODDEG (Djiboutian Office for Development of Geothermal Energy) with different surface temperature ranges (72 to 99 ºC) (Figure 2). For instance, geothermal fumaroles with highest temperatures in the Republic of Djibouti were recorded also in this area. On the one hand, the silica geothermometers show the reservoir temperature between 100 and 170°C for Nord-Ghoubbet. On the other hand, the study using other geothermometers such as Na/K, Na/K/Ca suggested around 260°C to 280°C (CERD, 2011). In this study, the fumaroles location was taken from the 1983 Geological map by ISERST-ORSTOM and the reconnaissance field conducted by ODDEG in 2019.

Figure 2: Fumaroles distribution in the study area. 2.2 Geological setting of the study area The study area is dominated by three major geological formations which (Figure3). “Samod et al.”

First, the formations outcropping at the Horst de Moudou’oud, and undergoing intense hydrothermal alteration events. Their outcropping thickness is more than 300 meters (BRGM, 1982). In the proximity of Analé wadi, the rhyolitic rock type is intercalated in the Dalha basalt. Second, the Gulf basalt flows are mainly outcropping to the east and constitute toggling blocks with a thickness of 200 meters and thinned out towards the west (BRGM, 1982). Those relatively thin Gulf basalt flows (several tens of meters) covered the lower ancient Dalha series whose age is between 3.6 and 5.9 Ma (CERD, 2011). Third, to the west, the formation of the external margin of Asal Rift (0.7 Million years) outcrops and essentially forms stacks. These are basalt flows over a total thickness of 150 to 200 meters (Gasse et al. 1985).

Figure 3: Geological map in Nord-Ghoubbet (modified of Carte Geologique de la Republique de Djibouti « », Gasse et al, 1985) 2.3 Structural setting of the study area The study area is located at the triple junction between the Nubian plate (Northeast), the Somali plate (Southeast) and the Arabian plate. This triple junction forms the convergence area of the three rifts also called the Afar Depression (Figure 4): - The (oceanic rift) defines the Arabian plate of . - The Gulf of Aden (oceanic rift) extends from the Indian to the Red Sea. - The East African rift (rift) is the frontier between Nubian and rift and extends up to Tanzania. “Samod et al.”

Figure 4: The rifting system in Afar depression, the black square indicate the Republic of Djibouti. The recent geological history of this region is the result of the spread of the Gulf of Aden ridge (The Gulf of Aden is a young oceanic crust) resulting in the formation of a series of disjointed rift segments in which tectonic deformation and magmatic activity are still active (Figure 5). This ridge seems to end abruptly to the west on the of Africa. However, a narrow Gulf extends landward: the Gulf of Tadjourah is enclosed within the Republic of Djibouti has been recognized as a portion of the oceanic rift in the prolongation of the Gulf of Aden (Gaulier, 1991). The western part of this oceanic axis is the famous Asal rift (Needham et al., 1976). The rift of Asal-Ghoubbet is one of the youngest and most western segments of this ride. It constitutes the onshore extension of the Tadjourah Rift (Manighetti et al., 1997, Daoud et al., 2011). To understand the structural and tectonic evolution of the region, it is important to locate it in its geodynamic context for that the area of Nord-Ghoubbet situated under the influence of several different tectonic patterns and affected by several faulting systems:

- The Asal-Ghoubbet rift normal faults trending NW (Tapponnier and Varet, 1974), affect all formation in this area. - The Makarassou N-S trending transverse faults (Tapponnier and Varet, 1974), continues in particular in the old basalts and the basalts of borders (Gulf basalt) - Older trends identified in the Dalha and older Mabla. - The opening of the Tadjourah Gulf which started 3 Million years ago and continues until today, affects the submerged axis but also the basalts of the borders (WNW- ESE), Ruegg in 1975 estimates that this fracturing is also active in the Asal axis from seismic data analyses. The Nord-Ghoubbet area acts as a block and stops the extension of Tadjourah Gulf to the landward and change the direction to NW-SE in 0.9 Ma (Asal rift). This tectonic evolution and variations indicate that the active faults are resulting from the transform system linking Ghoubbet with ridge but it also results (Manighetti et al. 2001) from the fast rotation of this brittle block. Numerous fumaroles and some hot springs, affecting the whole block, testify the leakage of the geothermal reservoir due to the fairly active tectonics of the site. The fracture network in Nord-Ghoubbet was well developed. “Samod et al.”

Figure 5: Geological map with faults patterns distribution.

3. Relationship between Faults and Fumaroles Distribution Faults are known to have a strong impact on the feasibility of geothermal projects, both in high enthalpy and medium-low enthalpy settings, as the impact, they can have on the permeability of the subsurface (Younger et al, 2012.). The structures are potentially permeable zones that can act as pathways for geothermal fluids to flow from the reservoir at depth to the shallower level (Suryantini and Hendro H. W., 2010). As such, it was necessary to establish their relationship with occurrences of geothermal manifestation such as fumaroles in the study area. The relationship between faults distribution and fumaroles occurrences was evaluated by using fault density and rose diagram. Rose diagrams were used to assess the general orientation of faults and fault density is a simple method to locate highly fractured areas. 3.1 Methodology The faults were digitized from the geological map of Tadjourah of 1983 by ISERST- ORSTOM using Arc GIS Pro 2018 software and a shapefile was created. It was displayed on DEM data to see how it correlates spatially with the structural features. The shapefile containing faults was from saved as AutoCAD vectors in a data interchange file (DXF) format to be compatible with Rockworks software version 17 where the rose diagrams were plotted. The rose diagram (Figure 7a) was plotted in rockwork using the linear algorithm under the utility function. “Samod et al.”

Figure 6: Methodology flow chart 3.3 Results and Discussion A rose diagram is a circular histogram plot which displays directional data and the frequency of each class (Prabu et al., 1964). These diagrams have been used in many studies to define the structural trend. For instance, Charleson, (2016) used rose diagrams to interpret geological structures in Northern Malawi. In general, a rose diagram was used to analyze the orientation of lineaments, faults and fracture. The results from the rose diagram have shown that one main NW-SE (direction of rift Asal- Ghoubbet) orientation of the faults in the study area. The N-S (Makarassou fault transform direction) and WWN-SSE to EW (the opening of Tadjourah Gulf direction)(Figure 7b) are less pronounced. This means that most of the faults in the study area are oriented in the NW- SE direction. As mentioned in the structural setting of the study area, these trending patterns were influenced by the three major fault systems which affected the area. In addition, the fault pattern associated with fumaroles distribution in the study area can be described as NW- SE, WWN-EES as well as N-S. Therefore, those faults will be considered as active faults. The results show that the geothermal fumaroles are mainly located within the major faults with NW-SE direction.

“Samod et al.”

Figure 7:a) Digitized faults overlay DEM image and, b) Rose diagram, showing the faults the direction NW-SE are the main prominent, and WWN-EES to EW, NS is less prominent trending pattern in the study area. Fault density refers to the total number of faults per unit area. Thus areas with more faults per unit area are considered as high fault density hence high-density zone while those with few or fewer faults per unit area are considered as low fault density hence low-density zones (Charleson, 2016).

Figure 8: Fault density map showing high, medium and low-density zones. Fault systems are important in geothermal studies because they are considered as potential permeable zones where thermal fluids flow from the reservoir at depth to the upper levels. As such, areas with more faults and cracks are considered as high fault density areas. The Nord- Ghoubbet is located within the triple junction as we know this depression of Afar which was tectonically active with many faults. “Samod et al.”

For the reason, the fault density analysis is used here to evaluate the relationship between the presence of faults and the surface geothermal manifestations as fumaroles. Fault density analysis was calculated using the line density option under spatial analyst tool in arc GIS. The software simply counts the number of faults per unit area. Areas with a high number of faults per unit area were identified as high-density areas followed by medium and low-density zones (Charleson, 2016). The results from the fault density analyses shows that the study area has high, medium and low fault density zones (Figure 8). When fumaroles were mapped, it was shown that the fumaroles of zone A occur generally in high-density areas, and fumaroles located in zone B occur in high and medium density areas and 1 hot springs in the low- density zone. In terms of percentage, it means that out of surface manifestation in Nord- Ghoubbet, almost the fumaroles occur in high-density areas, and some in medium density areas and at least one in low-density areas (hot spring).

4. Conclusion The present study shows that faults in the study area have several trends with NW-SE as the dominant orientation, and Ns transform fault. The remaining faults are WWN-EES oriented and less pronounced. However, it can be noted that the general occurrence of surface geothermal manifestation mainly fumaroles, follows a similar trend. Moreover, fault density analysis shows that fumaroles mainly occur in medium and high fault density areas (Figure 8). This study suggested that it is possible to use geological structures like faults to delineate potential areas during geothermal exploration.

5. Recommendations - Detailed structural analysis basing on field works. - Mapping of hydrothermal alteration zones. - Further geochemical and isotopes analysis for the Nord-Ghoubbet East fumaroles. - Detailed mapping of the dykes, cones etc… to localise heat source.

REFERENCES

BRGM, Service Géologique National, "GEOTHERMIE REPUBLIQUE DE DJIBOUTI, EXPLORATION GEOLOGIQUE ET GEOTHERMIQUE DE LA ZONE NORD GHOUBBAT", A.GADALIA, F.IUNDT et R.FABRIOL,82 SGN 022 GTH, 1982.S. CERD Présidence de la République, INSTITUT DES SCIENCES DE LA TERRE, Mai 2011. "Étude de Préfaisabilité Prospect Géothermique du Nord Ghoubbet République de Djibouti". Charleson Namandwa Eliyasi, Exploration For Geothermal Resources Using Geological Structures With Emphasis On Faults - A Case Study Of Northern Malawi Proceedings, 6th African Rift Geothermal Conference Addis Ababa, Ethiopia, 2nd – 4th November 2016. Gasse, F., Fournier, M., Richard, O. & l C., R. 1985. Carte géologique de la République de Djibouti à 1:100 000. Tadjoura. Notice explicative. ISERST, Ministère français de la Coopération, Paris. Geoterrnica Italian, 1987." Geochemistry of North Ghoubbet Asal Region". Rapport préliminaire. “Samod et al.”

Manighetti, I., Tapponnier, P., Courtillot, V., Gruszow, S. & Gillot, P.-Y., 1997. Propagation of rifting along the Arabia-Somalia plate boundary: the gulfs of Aden and Tadjoura, J. geophys. Res., 102, 2681–2710. Manighetti, I., P. Tapponnier, V. Courtillot, Y. Gallet, E. Jacques, and Y. : Gillot Strain transfer between disconnected, propagating rifts in Afar, J. Geophys. Res.,106 (2001) 13,613– 13,665. NEEDHAM H.D., choukroune P., Cheminee J.L, Le PICHON X. Franche-TEAU J.& Tapponnier Ardoukoba rift (Northeast Africa) and the oceanic rift valley. –Earth planet.Sci.Lett., 28, 439-453. Prabu, P., & Rajagopalan, B. (2013), Mapping of Lineaments for Groundwater Targeting and Sustainable Water Resource Management in Hard Rock Hydrogeological Environment Using RS- GIS, In Climate Change and Regional/ Local Responses. Richard, O. & Varet, J. Study of the transition from a deep oceanic to emerged rift zone: Gulf of Tadjoura, République de Djibouti. Int. Symp. Geodyn. Evols. Afro-Arabian System, Roma (1979). Ruegg, J.-C., 1975, Structure profonde de la croûte et du mateau supérieur du Sud-Est de l’Afar d’après les données sismiques: Annals of Geophysics, v. 31, p. 329–360. Suryantini and Hendro H. Wibowo, Application of Fault and Fracture Density (FFD) Method for Geothermal Exploration in Non-Volcanic Geothermal System; a Case Study in Sulawesi-Indonesia, Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010. Tapponnier P. et Varet J. : La zone de Makarassou en Afar: un équivalent émergé des "failles transformantes" océaniques. C.R. Acad. Sc. Paris, D 278 (1974)209-212. Varet J.: Contribution to favorable geothermal site selection in the , ARGEO Meeting, Djibouti, 17p. (2010). Younger PLJ, Gluyas J, Stephens WE. Development of deep geothermal energy resources in the UK. Energy 2012; 19-32