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Maps of estimated geothermal parameters from the radially averaged power spectrum of magnetic anomalies superimposed on structures. (a) Geothermal gradient map derived from Fig. 5a, (b) Heat flow map. White circles and white circles with a black dot are the shallow and deep hydrocarbon exploration wells, respectively. The numbers displayed are the numbers of available measured geothermal gradient or heat flow values from boreholes. The red diamonds are the hot springs and the crossed-pickaxe symbols indicate Uranium deposits. The maps are generated using minimum curvature gridding with a grid cell size of 2 km x 2 km.
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Tanzania is one of the several countries intersected by the East African Rift System (EARS) endowed by a geothermal potential that has been explored only to a limited extent. Here we present the first heat flux map over the region based on the Curie point depth (CPD) estimation from aeromagnetic data. We have estimated the base of magnetic sources...
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... been reported by Ravat et al. (2007) and Bouligand et al. (2009) and no peak is visible in the spectra shown in Carrillo-de la Cruz et al. (2020a). Our final choice of a 110 km window is justified because temperature gradients measured in deep petroleum exploration wells are in the range of 22-58 • C/km in different geologic settings of Tanzania (Fig. 6a). Assuming a Curie temperature of 580 • C, this range implies a maximum CPD of 26.4 km. Because this value is <30 km our 110 km window size (approximately 4 times the maximum CPD) seems to be ...
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... results of the CPD, geothermal gradient and heat flow estimated from magnetic data are shown in Figs. 5 and 6. As a reference, in Fig. 5c we have reported the Global CPD estimated by Li et al. (2017). Our estimated CPDs range between 11 and 43 km beneath the geoid (Fig. 5a,b), the geothermal gradient ranges between 14 and 53 • C/km (Fig. 6a) while the heat flow values range between 34 and 132 mW/m 2 (Fig. 6b). As indicated in Fig. 5a,b, shallow CPDs are observed along the eastern branch of the EARS from Kenya through northern Tanzania where they terminate at the Eyasi rift as it diverges into the Archean Tanzanian craton (anomaly 2, Fig. 5a,b). In the western branch, ...
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... estimated from magnetic data are shown in Figs. 5 and 6. As a reference, in Fig. 5c we have reported the Global CPD estimated by Li et al. (2017). Our estimated CPDs range between 11 and 43 km beneath the geoid (Fig. 5a,b), the geothermal gradient ranges between 14 and 53 • C/km (Fig. 6a) while the heat flow values range between 34 and 132 mW/m 2 (Fig. 6b). As indicated in Fig. 5a,b, shallow CPDs are observed along the eastern branch of the EARS from Kenya through northern Tanzania where they terminate at the Eyasi rift as it diverges into the Archean Tanzanian craton (anomaly 2, Fig. 5a,b). In the western branch, shallow CPDs are seen southwest in the Rungwe volcanic province (RVP) and ...
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... our estimated heat flow anomaly is not supported by the mean heat flow value of 34 ± 4 mW/m 2 as reported by Nyblade et al. (1990), Nyblade and Pollack (1993) and Nyblade (1997) and derived from shallow boreholes in the region (Fig. 6b). We suspect that the heat flow from shallow boreholes is not representative of the regional heat flow patterns, possibly because most of the boreholes were originally drilled for mineral exploration or groundwater consumption purposes. Therefore, the standard procedures for heat flow measurements have probably not been implemented. ...
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Citations
... Recent works focused on the spatial variation of the heat flow and the understanding of the anomaly location at several scales, from the African plate to the Malawi country scale, but only deal with terrestrial data and/or estimations (Didas et al., 2022;Gomes et al., 2021;Jones, 2020;Macgregor, 2020). It appears that heat flow data and predictions are highly variable depending on the studies. ...
... It appears that heat flow data and predictions are highly variable depending on the studies. Differences in these results can come from calculation methods, (1) punctual calculations using thermal gradient derived from deep well measurements and lithology-based thermal conductivities (Macgregor, 2020) and (2) predictive map compiling calculations based on gravity data (Didas et al., 2022). Based on a similar approach, Gomes et al. (2021) used geothermal data with a thermal correction using an equation from Hamza (1982). ...
... Indeed, the heat flow estimations can be affected by local effects or local anomalies, like fluid circulations, especially with the use of only deep wells data. At the opposite, the use of only one regional element for calculations, like the Curie point depth or the gravimetry data, does not allow to proceed to a very precise prediction, taking into account of very punctual effects or superficial anomalies (Artemieva, 2006;Didas et al., 2022;Gomes et al., 2021;Li et al., 2017). ...
... Geothermometry from hot springs indicated reservoir temperature of 232 ± 13 • C in Ngozi (Alexander et al., 2016), 140 ± 13 • C in Kiejo-Mbaka and 112 ± 16 • C in Songwe (Alexander et al., 2016;Asnin et al., 2022). Therefore, Ngozi is considered a likely high-temperature volcano-hosted, magmatic geothermal prospect Fritz et al. (2013) and Didas et al. (2022). The East African Rift System (EARS) margins are indicated by heavy dark gray segmented lines and political boundaries by gray continuous lines. ...
The geothermal system related to the Ngozi and Rungwe volcanoes, SW Tanzania, lies at the intersection of the west and east branches of the East African Rift System and has been investigated by many geoscientists for decades. Here we present a 3D electrical resistivity model based on 190 magnetotelluric resistivity soundings that have been integrated with geochemical and geological results to support the development of the geothermal resource conceptual model presented here. The model includes two separate reservoirs, a larger system located beneath the Rungwe volcano and a smaller chloride water reservoir located under the Ngozi caldera, which contains a neutral chloride hot spring with geothermometry >230 • C. An extensive conductive clay cap with variable thickness extends along the 30 km long NW-SE trending Ngozi-Rungwe Fault Zone from the Kiejo area SE of the Rungwe volcano to the Ngozi caldera. The absence of geothermal surface manifestations directly over the inferred Rungwe upflow zone is consistent with effective sealing of the proposed underlying geothermal reservoir by the clay cap. The scarcity of thermal manifestations on the up-dip margins of the low-resistivity clay cap can be explained by coincidence of the base of the clay cap with impermeable Precambrian formations and by structural boundaries. This interpretation implies that the area with the highest geothermal resource potential is the Rungwe volcano where proposed drilling sites might intersect the proposed high-temperature reservoir.
... Information on the modern thermal regime is also used as input data for modeling the state of the Earth's interior in past geological epochs (for example, see (Ismail-Zadeh et al., 2013)). From a practical point of view, estimated lithospheric temperatures can indicate areas suitable for the use of renewable thermal energy sources (Hojat et al., 2016;Didas et al., 2022). In addition, temperature is a significant factor that influences the processes of oil and gas generation (Sorokhtin et al., 2015). ...
... The available terrestrial heat-flow measurements in the Karonga rift, southwest of the Kilambo-Ilwalilo area, exhibit values of only about 30 mW m −2 [57]. For thermal conductivity of 0.7 W m −1 K −1 [58], a geothermal gradient of 40 mK m −1 can be inferred This estimate is substantially in agreement with geothermal gradients recently derived from magnetic spectral analysis and 3D gravity inversion by Didas et al. [59]. Under these thermal conditions, the expected reservoir temperature of 110-140 °C could be reached at a depth of about 3 km. ...
Based on geophysical, geological and geochemical investigations carried out in the last decade, we reviewed three major geothermal plays that well represent the different structural, volcanological and hydrogeological realms that can be encountered in the East African Rift System (EARS). Alalobeda (Ethiopia) and Menengai (Kenya) are examples of typical geothermal plays of the Eastern Branch of EARS. The former is a fault-leakage-controlled geothermal play located in a graben structure. The heat source is likely deep-seated, widespread magmatism, associated with the lithosphere thinning that regionally affects this area. The reservoir temperature of the water-dominated system ranges from 185 to 225 °C. Menengai can be classified as a convection-dominated magmatic play type. The heat source could be a magmatic intrusion located beneath a caldera. A shallow, liquid-dominated reservoir (with temperatures of 150–190 °C) and an intermediate-deep reservoir, hosting steam and liquid (with temperatures of 230–340 °C), were detected. The Kilambo-Ilwalilo play (Tanzania) is an example of geothermal play of the Western Branch of EARS. It is in a half-graben realm where a regional fault controls the ascending groundwater flow. Reservoir temperatures are about 110–140 °C, and the heat source is provided by lithosphere thinning. The results of this study provide helpful guidelines for future studies on the geothermal resources in the rift.
... Geothermometry from hot springs indicated reservoir temperature of 232 ± 13 • C in Ngozi (Alexander et al., 2016), 140 ± 13 • C in Kiejo-Mbaka and 112 ± 16 • C in Songwe (Alexander et al., 2016;Asnin et al., 2022). Therefore, Ngozi is considered a likely high-temperature volcano-hosted, magmatic geothermal prospect Fritz et al. (2013) and Didas et al. (2022). The East African Rift System (EARS) margins are indicated by heavy dark gray segmented lines and political boundaries by gray continuous lines. ...
... To illustrate, inversion modeling using gravity data has been effectually implemented to establish subsurface density models, among them conducted by Tian et al [2] and Witter et al [3]. 3D inversion modeling was [4] to detect regional thermal anomalies for exploring geothermal potential zones. Several approaches have been done in the Sumbersari field, one of them was accomplished by Aprilia et al [5] on geothermal systems based on geochemical analysis. ...
The component of the geothermal system that plays an important role is a heat source. Heat sources can be a magma intrusion from volcanic activity or a geothermal gradient from sedimentation. Identification of heat source can be conducted using gravity anomaly modeling in the Sumbersari Geothermal Field, Southeast Sulawesi. In this study, inversion modeling was undertaken using the occam approach. The input data, Complete Bouguer Anomaly (CBA), was acquired from satellite data processing (GGM Plus). The estimation of heat source depth was established through spectral analysis of CBA data, to obtain regional and residual anomalies, as well as estimates of rock density in the study area. The geometry of the rock body in the inversion process uses a rectangular prism arrangement in the form of a 3D regular grid which represents the distribution of subsurface density. This inversion process uses ZondGM2D software. To improve the subsurface model of the inversion process, data enhancement was also carried out. Fault structure analysis based on geological data and Horizontal Gradient Magnitude (HGM) enhancement data on gravity anomalies was used as a reference for the initial model of subsurface conditions in inversion modeling. Not only horizontal gradient magnitude, but Euler Deconvolution is also carried out to determine the lineament of the geological structure in the study area. Based on the CBA map, low anomalies are associated with sedimentary rock and high anomalies are metamorphic rock with NW-SE and NE-SW trends. The presence of high and low anomalous contrasts in the CBA data indicates the presence of faults that control the geothermal system in the Sumbersari potential area. This is in accordance with the findings of manifestations in the study area which are correlated with geological structures. In addition, the high anomalous contrast resulting from the occam inversion also indicates the presence of altered rock, which is considered as heat source with an estimated very deep heat source. Thus, the results of this 3D gravity inversion can provide comprehensive information and are expected to be a reference in the development of the Sumbersari geothermal field, Southeast Sulawesi.
... From the point of view of the geothermal interest, the Kiejo-Mbaka prospect (Figs. 1 and 2) has the following main characteristics: (1) the occurrence of the recent, basaltic volcanism at Kiejo Volcano (Fontijn et al. 2012), near to the prospect; (2) the presence of hot springs along the Mbaka fault (Kilambo-Kajala) and cold dry gas emissions mainly composed by CO2 (> 95%) on the top of the footwall block of the Mbaka fault (Lufundo); (3) the position of the prospect within the East African Rift System (EARS), and the occurrence of recent seismicity (Fontijn et al. 2010;Ebinger et al. 2019;Camelbeek and Iranga 1996); 4) an unusually thick crust (more than 40 km, Borrego et al. 2018;Didas et al. 2022) and a large seismogenic thickness, up to 35-40 km, that would indicate deepest faults to penetrate to lower crust down to great depths (Ebinger et al. 1989(Ebinger et al. , 1991Camelbeek and Iranga 1996). ...
... suggesting the lack of significant crustal heating controlled by widespread and recent magmatic intrusions in the area. The absence of a thermal anomaly is also documented by Didas et al. (2022), who estimated a geothermal gradient of about 30-35 °C/km by the analysis of aeromagnetic data and CPD (Curie Point Depth) computation. It is worth to mention that Van der Beek et al. (1998) reported a gradient ranging from about 30 °C/ km to 40 °C/km for the adjacent Rukwa-Nyasa rifts. ...
High-resolution magnetotelluric and gravity data have been collected over the Kiejo-Mbaka geothermal field, located along the NW–SE trending Mabka fault, in the Karonga Rift basin (East Africa
Rift System). Such resolution allowed to reconstruct the field structure with unprecedented detail. Resistivity modelling has been obtained by three-dimensional finite-differences inversion of MT data, while density modelling has been accomplished by surface-oriented inversion of gravity data. Geophysical modelling has identified two sedimentary sub-basins separated by the Mbaka fault ridge, exposing the basement; these previously unknown sedimentary fills have a maximum thickness of ca. 1.5 km. The estimation of the clay cation exchange capacity (CEC) from magnetotellurics identifies a layer of low-temperature smectite alteration in the south-western sub-basin sediments, interpreted as a clay cap. The resulting updated conceptual model of the Kiejo-Mbaka geothermal system is therefore a fault-controlled system with lateral leakage into the sediments, expectably implying a
larger reservoir volume than previously estimated.
... From the point of view of the geothermal interest, the Kiejo-Mbaka prospect (Figs. 1 and 2) has the following main characteristics: (1) the occurrence of the recent, basaltic volcanism at Kiejo Volcano (Fontijn et al. 2012), near to the prospect; (2) the presence of hot springs along the Mbaka fault (Kilambo-Kajala) and cold dry gas emissions mainly composed by CO2 (> 95%) on the top of the footwall block of the Mbaka fault (Lufundo); (3) the position of the prospect within the East African Rift System (EARS), and the occurrence of recent seismicity (Fontijn et al. 2010;Ebinger et al. 2019;Camelbeek and Iranga 1996); 4) an unusually thick crust (more than 40 km, Borrego et al. 2018;Didas et al. 2022) and a large seismogenic thickness, up to 35-40 km, that would indicate deepest faults to penetrate to lower crust down to great depths (Ebinger et al. 1989(Ebinger et al. , 1991Camelbeek and Iranga 1996). ...
... suggesting the lack of significant crustal heating controlled by widespread and recent magmatic intrusions in the area. The absence of a thermal anomaly is also documented by Didas et al. (2022), who estimated a geothermal gradient of about 30-35 °C/km by the analysis of aeromagnetic data and CPD (Curie Point Depth) computation. It is worth to mention that Van der Beek et al. (1998) reported a gradient ranging from about 30 °C/ km to 40 °C/km for the adjacent Rukwa-Nyasa rifts. ...
High-resolution magnetotelluric and gravity data have been collected over the Kiejo-Mbaka geothermal field, located along the NW–SE trending Mabka fault, in the Karonga Rift basin (East Africa Rift System). Such resolution allowed to reconstruct the field structure with unprecedented detail. Resistivity modelling has been obtained by three-dimensional finite-differences inversion of MT data, while density modelling has been accomplished by surface-oriented inversion of gravity data. Geophysical modelling has identified two sedimentary sub-basins separated by the Mbaka fault ridge, exposing the basement; these previously unknown sedimentary fills have a maximum thickness of ca. 1.5 km. The estimation of the clay cation exchange capacity (CEC) from magnetotellurics identifies a layer of low-temperature smectite alteration in the south-western sub-basin sediments, interpreted as a clay cap. The resulting updated conceptual model of the Kiejo-Mbaka geothermal system is therefore a fault-controlled system with lateral leakage into the sediments, expectably implying a larger reservoir volume than previously estimated.
Article highlights
Geophysical survey of the Kiejo-Mbaka geothermal field (East Africa Rift System).
Geophysical modelling depicted a secondary sedimentary reservoir.
Outcomes deeply revised the resource assessment.
... This is clearly observed as an anomaly in magnetic maps in high-temperature areas. Magnetic maps are furthermore used to find the Curie point depth (CPD) at which magnetic minerals in the Earth's crust lose their permanent magnetic properties (Didas et al., 2021). The Curie temperature of magnetite, the most common magnetic mineral is around 580 1C (Hunt et al., 1995). ...
This paper describes the main geophysical methods used in geothermal exploration—data acquisition, processing and interpretation of the data—with main emphasis on resistivity methods and resistivity of rocks, passive seismic methods and thermal methods. Finally, a short description is given on conceptual modeling and volumetric resource assessment—the conclusion of surface exploration studies. The Schlumberger, transient electromagnetic (TEM) and magnetotelluric(MT) resistivity methods are broadly discussed, as well as the geothermal meaning of subsurface resistivity structures, particularly the role of smectite. The usefulness of seismic monitoring is detailed.
This article presents the results of a study of the thermal regime of the lithosphere under the TaimyrPeninsula and adjacent territories (70°–80° N, 80°–115° E) based on geomagnetic data. Spectral analysis ofthe lithospheric geomagnetic field given by the EMAG2v3 model was performed using the centroid method.The calculations we performed showed that the minimum depths of the top boundary of lithospheric magneticsources (2.5 km) are typical for the entire Taimyr fold belt and the considered part of the Siberian Platform,and the maximum (6 km) for the North Kara Basin. The position of the top boundary of the magneticallyactive layer of the lithosphere above the bottom of the sedimentary layer under the Yenisei-Khatangaand Khatanga-Lena basins can be associated with the widespread intrusion of basalt traps into the sedimentarylayer. The minimum depths of the bottom boundary of lithospheric magnetic sources (36 km) are confinedto the Eurasian Basin and neighboring territories of the Laptev Sea shelf and the islands of the SevernayaZemlya archipelago, which indicates the greatest heating of the lithosphere under them within theregion under consideration. The depth of the bottom boundary reaches maximum values (≥48 km) under theYenisei-Khatanga and North Kara basins and the Siberian Platform, indicating the existence of the cold and,accordingly, thick lithosphere here, which is confirmed by other independent geophysical data
