Auxiliary chemical geothermometers applied to waters from some East African Rift geothermal areas (Djibouti, Ethiopia, and Kenya) for geothermal exploration

Proceedings, 9th African Rift Geothermal Conference Djibouti, 3rd November – 5th November 2022 Auxiliary Chemical Geothermometers Applied to Waters from some East African Rift Areas (Djibouti, Ethiopia, Kenya) for Geothermal Exploration Bernard Sanjuan BRGM, Georesource Division, 3 avenue C. Guillemin, 45060 Orléans, France [email protected] Keywords: Geothermal waters, auxiliary geothermometers, Djibouti, Ethiopia, Kenya ABSTRACT If the temperature values of geothermal reservoirs given by classical chemical geothermometers applied on thermal waters are relatively divergent, it is often difficult to estimate the temperatures of these reservoirs with sufficient accuracy for geothermal exploration, before drilling operations. In this case, some auxiliary chemical geothermometers such as Na-Li, Mg-Li, Na-Rb, Na-Cs, K-Sr, K-F, K-Mn, K-Fe, K-W, etc., existing in the literature can be useful tools to help estimating these temperatures. However, previous studies have shown these geothermometers are not only dependent on temperature, but also on other parameters such as the nature of the reservoir rocks and minerals, the fluid salinity, etc. Consequently, they must be used with caution. Another problem for the use of these geothermometers is that Li, Sr, Cs, Rb, Mn, Fe and W are under the form of trace elements in numerous geothermal waters and that it is often difficult to find analyses of these elements in the literature. In this study, we test some of these auxiliary geothermometers on several waters selected from literature data relative to East African Rift (EAR) geothermal areas, in Republic of Djibouti, Ethiopia and Kenya, where temperature values of deep geothermal reservoirs have been measured into wells or estimated with certainty using chemical classical geothermometers applied on thermal waters. The comparative results are discussed and allow bringing conclusions and recommendations. Among the tested auxiliary geothermometers, this work shows that the Na-Li thermometric relationship defined for the dilute waters from hightemperature volcanic geothermal areas of Iceland is one of the most relevant to estimate the temperatures of deep reservoirs for waters from several geothermal areas from the Republic of Djibouti, Ethiopia and Kenya. In previous studies, this Na-Li thermometric relationship had also given good estimations of reservoir temperatures for high-temperature (T ≥ 300°C) borehole dilute waters from the Los Humeros geothermal area, in Mexico, in volcanic environment. The use of other auxiliary geothermometers such as K-F, K-Sr, Na-Rb and NaCs (when F, Sr, Rb and Cs have been analysed) can be also relevant in some cases. Sanjuan 1. Introduction Several studies about auxiliary chemical geothermometers such as Na-Li, Mg-Li, Na-Rb, NaCs, K-Sr, K-F, K-Mn, K-Fe, K-W have been carried out in the literature for geothermal prospecting in different geological environments, Fouillac and Michard(1981); Kharakha and Mariner(1982); Kharakha et al.(1989); Michard(1990); Sanjuan et al.(2014; 2016a, b). These geothermometers can be very useful when the classical chemical geothermometers such as silica, Na-K, Na-K-Ca, and K-Mg do not give a concordant estimation of reservoir temperature. However, as they do not only depend on temperature, but also on other parameters such as the nature of the reservoir rocks and minerals, their degree of alteration, the fluid chemistry and salinity, etc. (Sanjuan et al., 2014; 2022), they must be used with caution. It is essential to well define the environment in which these geothermometers will be applied before their use. If the different Na-Li thermometer relationships for thermal waters in contact with granite and volcanic rocks, Fouillac and Michard(1981); Michard(1990); Sanjuan et al.(2014) or for geothermal brines in oiland geothermal field sedimentary basins, Kharakha and Mariner(1982); Kharakha et al.(1989) are the most known and used in the literature, very few studies have been carried out using the other auxiliary chemical geothermometers. Michard (1990) showed that chemical geothermometers such as Na-Cs, Na-Rb, K-Sr, K-Mn, K-Fe, K-F, K-W… could also be used for geothermal exploration concerning dilute waters discharged from granite reservoirs between 25 and 150°C in more than sixty areas from Europe (France, Italy, Spain, Bulgaria, Sweden). Sanjuan et al. (2016a, b) defined three new Na-Rb, Na-Cs and K-Sr thermometric relationships using 20 hot natural brines from granite and sedimentary reservoirs, mainly located in the Upper Rhine Graben, France and Germany (70230°C), apart two which were at Salton Sea, in the imperial Valley, USA (320-340°C). The main objective of this paper is to test some of these auxiliary geothermometers on several waters selected from literature data relative to the East African Rift (EAR) geothermal areas, in Republic of Djibouti, Ethiopia and Kenya, where temperature values of deep geothermal reservoirs have been measured into wells or estimated with relative certainty using chemical classical geothermometers applied on thermal waters. The comparative results will be discussed and allow bringing conclusions and recommendations about the use of these new tools for geothermal exploration in EAR geothermal areas. 2. EAR geothermal areas selected in this study The East African Rift System (EARS) is one of the most important volcano-tectonically active regions where heat energy from the Earth’s interior escapes to the surface in the form of volcanic eruptions and the upwelling of heat by hot springs and fumaroles. Therefore, the EARS appears to possess a remarkable geothermal potential, Pürschel et al.( 2013). However, at the present, only Kenya has been able to develop a considerable industrial production of geothermal power (1193-installed MWe in 2020; Huttrer, 2020). There is no geothermal power station in the Republic of Djibouti, despite numerous works of geothermal exploration. Several geothermal areas were selected from studies carried out in volcanic environment in Republic of Djibouti, Ethiopia and Kenya. As the Republic of Djibouti was the country where more geochemical data required for this study were found, the latter were more detailed. Sanjuan 2.1 Republic of Djibouti The Republic of Djibouti is located in the EARS, where three major extensional structures (Red Sea, EAR and Gulf of Aden Rift systems) join to form the Afar depression (Fig. 1A), which is characterized by thinned continental crust. Figure 1: A): A schematic geological map of the Republic of Djibouti (SE Afar Rift) with hydrothermal activity of the Republic of Djibouti. (B): A schematic geological map of the Sakalol-Harralol geothermal area with thermal springs organized in 4 clusters (C1, C2, C3 and C4). In the inset: schematic map of the Afar Depression with the location of Djibouti (black rectangle) (extracted from Awaleh et al.(2017). Sanjuan This depression is bounded by large escarpments to the west and to the south, and by the Danakil Alps to the northeast, Zan et al.(1990). Almost all the Republic of Djibouti is covered by volcanic rocks, mainly represented by basalts, and thermal manifestations are widespread (Fig. 1A). The areas selected for this work were the Sakalol-Harralol Geothermal Field, mainly studied by Awaleh et al. (2017), the Hanlé-Gaggadé Geothermal Field, personal data (1988); Awaleh et al.(2020) and the Lake Abhe Geothermal Field, Awaleh et al.(2015). These areas have been less studied than the high-temperature Asal-North Ghoubbet area where several geothermal wells have been drilled. For this last area and the Tadjourah and Obock areas where the geothermal fluids are mainly derived from seawater-basalt interaction processes at hightemperatures (≥ 160°C) as well as for the Reykjanes area in Iceland, Sanjuan et al. (2014) already determined a specific Na-Li thermometric relationship. a) Sakalol-Harralol Geothermal Field (SHGF) The SHGF (about 200 km2) is one of the largest geothermal fields of the Republic of Djibouti, which is located in the north of the EARS and within the northwestern portion of the GhoubbetAsal, which forms an accretionary rift segment penetrating the Afar depression (Fig. 1A). This rift is one of two emerged oceanic ridges in the world, with the other being Iceland. As noted by Awaleh et al. (2017), similar geological features are the Manda Inakir, the Manda Hararo and the Erta Ale. Detailed geochemical investigations were carried out for the first time by Awaleh et al. (2017) on cold groundwaters (well and borehole waters) and almost all thermal waters (86 thermal springs) from this area (Fig. 1B). The temperature of these thermal springs at surface are ranging from 38 to 78°C and their pH values moderately alkaline from 7.15 to 8.95. Using common statistical analyses and the chemical composition of the waters described in terms of relative concentrations of the main major species, Awaleh et al. (2017) distinguished four clusters of thermal waters in the SHGF (Fig. 1B). The TDS values are increasing from the clusters C1 (minimum TDS of 715 mg/l) to C4 (maximum TDS of 13265 mg/l). Except the cluster C1 for which the waters are mostly of the Na-Cl-HCO3-SO4 type, the geothermal waters of the other clusters are Na-Cl type. The cluster C4 is constituted of the most saline and hottest water at surface (T = 77.7°C).Using different geothermometric approaches, Awaleh et al. (2017) gave a temperature range estimation for the deep geothermal reservoir in the SakalolHarralol area of about 120-160°C, with a mean deep temperature of 143°C. In addition, according to previous hydrological studies, they noted that the presence of a shallow aquifer at 110°C could be also likely. b) Hanlé-Gaggadé Geothermal Field (HGGF) Within the EARS, the HGGF is located southwestwards of the Asal Rift (Fig. 1A). The Hanlé and Gaggadé plains are two of the many tectonic depressions lying parallel to the Asal Rift, between this structure and Lake Abhe, Zan et al.(1990). These two half-grabens and the corresponding intra-basins are 18 km and 10 km-wide, respectively. They have a similar orientation, which is northwest-southeast, and are bounded by a prominent system of master and secondary normal faults, Awaleh et al.(2020). The hanging wall of the Hanlé half-graben is locally disrupted by the emplacement of a large acid intrusion, which forms the domed Baba Alou relief culminating at approximately 972 m and is further dissected by high-angle faults. The Upper Stratoid series (2.2-1.8 Ma) is the main cartographic unit and the basaltic component covers most of its surface (Fig. 1A). Sanjuan The data selected for this study are relative to 10 thermal springs and the deep well H1 in the Hanlé plain, and 2 thermal springs in the Gaggadé plain, personal data (1988); Table 1). The geochemical data from the study of Awaleh et al. (2020) were also selected. The temperature of these thermal springs at surface are ranging from 38 to 70°C. Two geothermal wells (H1 and H2) were drilled in the Hanlé plain in 1987, with depths of 1623 m and 2038 m, respectively, Zan et al.(1990). Maximum temperatures of 72°C and 124°C were recorded in each of these wells. As noted by Zan et al. (1990), temperature logs run in wells H1 and H2 show the Hanlé plain is a system in which temperature seems to be controlled by groundwater circulation (down to 800 m in H1 and to about 1000 m in H2). It was found that the local temperature maximum at shallow depth is connected to aquifers flowing in a lateral direction. The zone with an almost constant temperature, from about 400 to 1000 m in well H2, could be related to the local thermal anomaly generated by the ascent of hot fluids to the Garrabbays fumaroles. All these thermal waters are moderately alkaline with pH values ranging from 7.62 to 8.86. Their TDS values are ranging from 748 mg/l to 2910 mg/l. Except for the Nεinlé thermal water G1, which is Na-HCO3-Cl type, all the other waters are Na-Cl type. Table 1: Chemical composition of geothermal waters from the HGGF, personal data (1988). For the G1 and G2 Nεinle thermal waters, we estimated the reservoir temperature at about 140°C, using the Na-K and silica-chalcedony geothermometers, Arnorsson et al. (1983). Awaleh et al. (2020) proposed a conceptual model of the Hanlé-Gaggadé system, with a mean temperature of 145 ± 15°C for the main geothermal reservoir, which would be located at a maximum depth of 2400 m. This temperature was estimated using different geothermometric approaches. c) Lake Abhe Geothermal Field (LAGF) This geothermal field is located in the Southwestern region of the Republic of Djibouti, on the border with Ethiopia (Fig. 1A). It occurs within a rift basin filled with Pliocene-Quaternary volcanic rocks (mainly basalt) and lacustrine sediments. The lake sediment floor is underlain by a thick sequence of Stratoid basalts dated between 4 and 1 Ma. During the Plio-pleistocene, these basalts were dislocated by extensional faulting, with fault-scarps exceeding 1000 m in height in some areas, allowing the development of deep lakes in the Central Afar and favoring groundwater movement between the different basins like Abhe, Dobi-Hanlé and Asal. The Lake Abhe area is particularly rich with surface hydrothermal features, including fumaroles, hot and warm springs and hydrothermal chimney structures, some of which discharge hot steam at their apex. These are aligned WNW-ESE, parallel to the regional extensional fault network. The surface hydrothermal manifestations are spread over an area of about 100 km2. Latitude Longitude TTDS Na KCa Mg Cl HCO 3 SO 4 SiO 2 Br BLi Sr Rb Cs °C g/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l H1 Galafi borehole 11°42'12.30" 41°51'0.50" 50.0 8.15 1.9 621 37.5 3.81 0.75 635 200 280 120 3.36 0.627 0.031 0.085 0.071 0.082 H2 Boukboukto 11°41'8.25" 41°52'0.84" 48.0 8.01 1.9 621 37.5 4.29 0.80 638 211 275 103 3.40 0.627 0.026 0.093 0.048 0.077 H3 ε asa Mayeb 43.0 8.27 1.9 635 37.5 3.69 1.07 624 220 275 84 3.32 0.627 0.021 0.101 0.036 0.072 H4 Dâli 11°39'0.00" 41°55'4.74" 38.5 8.01 2.9 1039 49.7 6.77 2.67 1064 223 412 63 5.75 1.103 0.012 0.145 0.050 0.070 H5 ε addara 11°38'36.56" 41°55'30.73" 39.0 8.20 2.9 1048 45.4 5.41 1.97 1064 272 407 66 5.75 1.146 0.009 0.136 0.049 0.080 H6 Dahotto 11°37'31.81" 41°57'3.73" 43.0 7.83 2.6 910 25.8 8.50 4.47 964 229 342 70 5.23 0.908 0.011 0.129 0.000 0.064 H7 Minkille 11°39'16.15" 41°56'59.19" 58.0 7.62 2.3 726 14.5 25.17 3.55 922 73 456 87 4.67 0.951 0.036 0.423 0.041 0.070 H8 Daggirou 11°36'30.28" 41°58'37.13" 41.0 7.82 2.4 809 30.1 12.67 4.03 957 177 329 72 4.95 0.822 0.015 0.117 0.000 0.068 H9 Oudgini 11°30'47.17" 41°56'14.88" 40.5 7.89 1.9 644 21.5 3.05 4.23 798 147 226 79 4.27 0.724 0.016 0.158 0.043 0.064 H10 ε agna 11°33'51.05" 41°54'32.18" 40.0 7.81 1.9 602 26.6 6.49 2.84 709 193 244 74 3.80 0.714 0.012 0.077 H11 Hanlé 1 well 72.0 8.86 1.5 483 18.6 8.02 491 206 202 56 0.08 0.000 0.028 G1 Niεinle 42.0 7.88 748 136 5.5 15.43 2.28 138 272 122 58 0.32 0.238 0.017 0.578 0.007 0.011 G2 Niεinle 70.0 7.85 1203 340 18.4 15.83 2.14 383 114 201 129 1.72 0.422 0.031 0.425 0.046 0.071 Sample Sample name pH Sanjuan The sixteen hot waters studied by Awaleh et al. (2015) were selected for this study (SHC1SHC7 group located at the north of the GHC1-GHC9 group). These Na-Cl thermal waters have temperatures at surface ranging from 71 to 99.7°C and pH values varying from 7.61 to 8.79. The hot waters from the SHC1-SHC7 group have TDS values higher than those from the GHC1-GHC9 group (3466 to 3795 mg/l against 1918 to 2236 mg/l). The different geothermometric approaches used by Awaleh et al. (2015) estimated a temperature range of the deep geothermal reservoir of 120-160°C. In spite of the relatively wide range, the three different approaches led to a same mean value of about 135°C. 2.2 Ethiopia Ethiopia is located on the geothermally active East African Rift System (Afar depression and Main Ethiopian Rift, MER; Fig. 2) and therefore has an abundance of sites that are prospective for generation of power. Twenty-four such locations are claimed as is a potential for ultimate generation of 10,000 MWe, Huttrer(2020). The current installed capacity is 7.3 MWe derived from the Aluto Langano field. It is located in the southern part of the rift (Fig. 2). The prospects most advanced include Tendaho and its associated Alalobeda area, Shalla Abiata, Butajira, Meteka, Corbetti, and Tulu Moye (Fig. 2). Exploration at several of these has recorded temperatures greater than 200°C. The Eastern branch, which forms the Ethiopian and Kenyan rifts, marks the boundary between the Nubian and Somalian plates. Through its high regional heat flow due to an underlying basic upper mantle intrusion beneath the thinned crust, it exhibits, by far, the most extensive geothermal resources. Most of the widespread geothermal activity, manifesting itself in the form of numerous hot springs, fumaroles and hydrothermal alteration, is located in the MER and in the Afar depression (Fig. 2). Recent volcanic activity is characterized by Mid-Ocean Ridge Basalt (MORB)-like fissural eruptions. Faulting is a typical and dyke induced. Through the narrow fissures, which have penetrated the crust in the rift axis, the basaltic magma erupted to the surface and formed chains of cones, dykes and sills. In the MER, the water circulates along the dominating NS to NNE trending fault system within the axial valley and originates from the rift flanks. The heat source is provided by dykes and central magma chambers. The major aquifers in this zone are fractured, interlayered basalts and ignimbrites. For this study, the geothermal waters were selected from four main works carried out by Endeshaw (1988), AQUATER (1996), Pürschel et al. (2013) and Minissale et al. (2017). Pürschel et al. (2013) mainly worked on hot springs from three geothermal areas (DofanFantale, Gergede-Sodere and Aluto-Langano; (Fig. 2). The Gergede-Sodere thermal springs emerge along the young faults on axial part of the rift, whereas the Dofan-Fantale and AlutoLangano hot springs are associated with active volcanic centers. Among the waters selected in this study, the geothermal waters Bulga 1 from the Dofan-Fantale area, and Sodere 1 and 2 from the Gergede-Sodere area, are Na-HCO3 type. The Langano 1, Langano 2, L. Spring 84, and L. Spring 10 waters from the Aluto-Langano geothermal field, are also Na-HCO3 type. The water from the geothermal well LA-4 (1987) is Na-HCO3 type and has a TDS value close to 4 g/l. Sanjuan Figure 2: Location map of the Ethiopian Rift Valley showing the main geothermal fields and hydrothermal manifestations (extracted from Endeshaw, 1988). Sanjuan Using different chemical geothermometers, Pürschel et al. (2013) concluded that the Na-K and Na-K-Ca geothermometers provided the most reliable subsurface temperature estimates with 185±20°C for the investigated hot spring samples and 260±15°C for the LA-4 fluid sample. From the study carried out by AQUATER (1996) in the Tendaho geothermal area, the geothermal waters from the deep wells TD-1, TD-2 and TD-4 were selected for this study. After their drilling, these wells yielded a temperature of over 250°C. Ten geothermal waters from different areas were selected in the two other studies: Dallol, Lake Afrera, Hertale, Bilen, Filweha and Lake Abaya 6 waters (Fig. 2; Endeshaw, 1988) and Bilate, Lake Abaya 6, 8, and Dimtu well waters from the northern Lake Abaya area, Minissale et al.(2017). In their study, Minissale et al. (2017) concluded that the application of geothermometric techniques in the liquid and the gas phases suggests the presence of a deep reservoir in which the fluids equilibrated at a maximum temperature of approximately 180°C in the northern Lake Abaya area (for the Bilate sample, for example). For the Lake Abaya fluid samples, the temperature reservoir is estimated at 260°C and at 150°C for the Dimtu well water, using classical geothermometers. For the other study, Endeshaw(1988), the use of different geothermometers give estimations of reservoir temperatures of about 100°C for the Filweha water, 110°C for the Dallol water, 140°C for the Bilen water, 160°C for the Hertale water and 180°C for the Lake Afrera water. 2.3 Kenya There are many geothermal resources in Kenya, most of them in the rift zone where high subsurface temperatures exist due to the young volcanic activity. Kenya’s geothermal capacity growth during the period 2015 to 2019 has been one of the fastest in the world, Huttrer(2020). Installations have totaled 218 MWe, coming from the Orpower4 (45 MWe) and the Olkaria V (173.2 MWe) stations. Current total installed capacity is 865 MWe, which comprises 29% of the national capacity. So far, more than 380 wells have been drilled in several parts of the rift zones. The location of the geothermal resources and young volcanoes is shown in Figure 3. For this work, the geothermal waters were selected from four main studies carried out by Kamondo (1988) in several areas of the Kenyan Rift System (KRS), by Omenda (1998) in the Olkaria field, by Cioni et al. (1992) in the Lake Bogoria area, and by Sekento (2012) in the Menengai field. The Olkaria high-temperature geothermal field, about 100 km NW of Nairobi, is located within the central Kenya segment of the East African Rift System (Fig. 3). It is the greatest geothermal field in Africa to generate electricity. The geothermal area is characterized by Quaternary volcanism of silicic composition of which the youngest is of Holocene age. The rock outcrops are dominated by comendite rhyolites and pyroclastics while in the subsurface are trachytes, basalts, rhyolites and tuffs. Geothermal manifestations include fumaroles, hot-springs and hot grounds. Numerous wells have been drilled to depths of 1000-2600 m and some have encountered temperatures of more than 300°C. The fluid chemistry at Olkaria broadly falls into two types: neutral pH-chloride and bicarbonate-rich waters. High enthalpy, neutral-chloride waters occur dominantly in the area east of Olkaria Hill, and lower enthalpy, bicarbonate-rich waters occur exclusively in the reservoir to the west of Olkaria Hill. For this study, we have selected geothermal waters from 4 wells (301, 305, 306 and 709) reported by Omenda (1998). Sanjuan Figure 3: Location map of the Kenyan Rift Valley showing the main geothermal areas, faults and quaternary volcanoes (extracted from Omenda and Teklemariam, 2010). The Lake Bogoria is found approximately 50-60 km north of the active Menengai volcano, within the main active branch of the KRS (Fig. 3). It is a closed-basin alkaline saline lake typical of African Rifts. A large number of boiling springs and fumaroles are located along the southern half of its shores. Menengai is a major Quaternary central volcano located in the KRS. It hosts one of the high-temperature geothermal fields located in the KRS (Fig. 3). The Menengai caldera is typified by complex tectonic activity associated with the rift triple junction. This is a zone at which the failed rift arm of the Nyanza rift joins the main Kenyan Rift (Fig. 3). The hydrothermal activity in Menengai geothermal field is manifested in the form of fumaroles, warm/ambient temperature boreholes, and hot to warm altered grounds. Sanjuan For some dilute geothermal waters like those from one zone of the Sakalol-Haralol area and from the Lake Abhe area, in the Republic of Djibouti, the Na/Li ratio was rather controlled by the Na-Li thermometric relationship determined by Fouillac and Michard (1981). These relationships complement that defined by Sanjuan et al. (2014) for the geothermal fluids derived from seawater interacting with basalts at high-temperature (≥ 160°C) in Iceland (Reykjanes) and Djibouti (Asal-North Ghoubbet). For all these relationships, it is suggested that Li could be released by biotite dissolution. The new thermodynamic approach using Liminerals as that carried out by Boschetti (2022) could help to better determine the main Licarrier minerals. Concerning the use of the other auxiliary geothermometers, we have shown that very few geochemical data are available in the literature for geothermal waters from EARS areas. Only some interesting trends have been obtained for the Na-Rb, Na-Cs, K-Sr and K-F thermometric relationships, which need to be confirmed, but are rather promising. In the future, we encourage the community of geochemists to perform more analyses of trace elements such as F, Sr, Rb, Cs, Mn, Fe, and W in their studies, in order to develop additional tools for geothermal exploration. 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