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Asal-Fiale Geothermal Project Planning, Drilling and Testing

Electricite de Djibouti Conceptual Model of the Geothermal System for Well

Targeting

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Executive Summary This report presents the current conceptual reservoir model for the Fiale geothermal area and the supporting data and interpretations prepared by Geologica Geothermal Group, Inc. as the Geothermal Consulting Company for Electricite de Djibouti. The conceptual model was developed as a result of a review of historic well information, regional geology, geophysics and geochemistry. Based on the current conceptual model, drilling targets are proposed for four exploration wells, as well as simplified casing design, the surface locations and access roads for the project. The objective of the proposed exploration drilling is to prove a commercial geothermal resource capable of supporting a 20-50 MW electric power generation project for 30 years.

Past geothermal exploration efforts as have proven a heat source capable of supporting electric generation, ample fluid and an impermeable cap. The fourth characteristic needed for a commercial geothermal system is permeability. Historic drilling and has proven permeability in the Asal Rift, southwest of the Fiale Caldera, but there has been no drilling in the Caldera. The locations and wellbore paths of the four exploratory wells have been chosen to increase the probability of intercept commercial permeability.

Additional data collection and analysis is proposed to increase the probability of success of the exploration wells. Geologica proposes to do the following:

• Downhole temperature and pressure survey of Asal 3, 4, and 5. • Fumaroles sampling • CO2 flux shallow temperature survey.

This work should be completed early 2016 before the exploration drilling begins.

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Table of Contents Executive Summary ........................................................................................................................ ii

Table of Figures .............................................................................................................................. v

1. Objectives ............................................................................................................................... 7

1.1. Overview .......................................................................................................................... 7

1.2. Task 1 Objectives ............................................................................................................. 7

2. Background of geothermal exploration in the Asal Rift ......................................................... 8

2.1. Temperature ................................................................................................................... 10

3. Regional geologic setting ...................................................................................................... 13

3.1. Asal Rift Stratigraphy..................................................................................................... 16

3.2. Structural geology .......................................................................................................... 17

3.3. Geologic features of Fiale Caldera ................................................................................. 17

4. Geophysics ............................................................................................................................ 18

4.1. Magnetotelluric Survey .................................................................................................. 18

4.2. Transient Electromagnetic Survey ................................................................................. 23

4.3. Seismicity ....................................................................................................................... 23

4.4. Gravity survey ................................................................................................................ 24

4.5. Geophysical conclusions ................................................................................................ 25

5. Geochemistry ........................................................................................................................ 26

5.1. Geochemical Data .......................................................................................................... 26

5.2. Surface Manifestations ................................................................................................... 27

5.3. Geothermometers ........................................................................................................... 31

5.4. Geochemistry Summary ................................................................................................. 31

6. Conceptual model ................................................................................................................. 32

6.1. Constraints from geochemical, geological, and geophysical observations .................... 32

6.2. Four requirements for a geothermal system ................................................................... 33

6.3. Conceptual model ........................................................................................................... 34

7. Data gaps ............................................................................................................................... 38

7.1. Geothermal exploration drilling at Fiale Caldera ........................................................... 38

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7.2. Temperature transients and permeability changes ......................................................... 38

7.3. Data gap proposal ........................................................................................................... 38

8. Preliminary Drilling targets .................................................................................................. 39

8.1. Proposed Wells ............................................................................................................... 41

8.2. Drilling decision tree ...................................................................................................... 44

9. Summary ............................................................................................................................... 45

Works Cited .................................................................................................................................. 46

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Table of Figures Figure 1. A topographical map showing the study area, previously drilled wells, faults/fractures, the Fiale Caldera, surface geothermal manifestations, and previously proposed drilling sites. The Djibouti N9 road is displayed in gray. .......................................................................................... 10 Figure 2. Temperature-depth profiles from the six Asal wells with A-5 highlighted in red. After Figure 4.2 in BGRM (1993).......................................................................................................... 12 Figure 3. Regional setting of the Asal Rift. Digital elevation model of the Afar Depression and surrounding areas. Inset on the top left shows the extent of the system of rift valley composing the East African Rift. Inset on the bottom right shows the plate kinematic setting of the region; dashed lines indicate plate boundaries and white arrows illustrate plate kinematics. Black and white squares indicate major towns. AA Aleyu–Amoissa; Ado Ado Ale; AnBF Ankober border fault; BG Borkena graben; Bo Borama; D Dabbahu; Da Dallol; De Der’Ela graben; DG Dobi graben; EAR Erta Ale range; GG Gaddale graben; HG Hanle graben; IG Immino graben; MH Manda–Hararo rift; MI Manda-Inakir rift; TG Tendaho graben. The figure is from and the caption is after Corti et al. (2015). ................................................................................................ 15 Figure 4. Geologic setting of Djibouti. Location of the Asal rift indicated in green text. The figure is from Khodayar (2008). ................................................................................................... 16 Figure 5. Geologic map of the Asal rift showing faults and fractures. ......................................... 17 Figure 6. Elevation map with Fiale Caldera outlined. The black triangles mark MT stations, the red squares mark wells, and the lines are cross sections discussed in this section. ...................... 19 Figure 7. Resistivity cross section P 01, including locations of faults and seismic events. True locations of the line, wells, and stations are inset. ........................................................................ 20 Figure 8. Resistivity cross section P 04. True locations of the line and stations are inset............ 21 Figure 9. Resistivity cross section P 02. True locations of the line, well, and stations are inset. . 22 Figure 10. Two cross sections from Doubre et al. (2007). Their locations are highlighted in blue on the map. .................................................................................................................................... 24 Figure 11. The gravity data measured in the Asal Rift and surrounding areas. Reproduced and edited from BRGM (1993)............................................................................................................ 25 Figure 12 Thermal Fluid Sample Locations (Figure 2 from Varet et al., 2014). Blue dots represent selected warm and hot springs, red dots are fumarole locations, green circles are geothermal wells. The large blue arrow represents seawater flow from the Gulf of Ghoubbet NW to Lake Asal. The red arrow represents the active volcanic axis of the rift. Fiale Caldera lies within the red square. Some warm springs extend around the E-NE side of Lake Asal. From west to south the spring areas are known as: l’Oued Kalou, Korilli, and Manda. From east and northeast of Lake Asal the springs are Eadkorar and Alifitta. ...................................................... 28 Figure 13 Location Map of surface manifestations north west (Korilli) and within the Fiale Caldera mapped by M. Khodayar. Reproduced from Figure 34 in ISOR (2008). ........................ 29 Figure 14. Cl-Na-K plot for waters in Asal Rift Area. ................................................................. 30 Figure 15. Cl-Mg plot for waters in Asal Rift .............................................................................. 30

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Figure 16. Map of Asal rift showing fluid flow direction (blue arrows). After Batistelli et al (1991). ........................................................................................................................................... 33 Figure 17. Overview map showing the location of cross section AA’, which is collinear with Doubre et al (2007)’s cross section F3. ........................................................................................ 34 Figure 18. Cross section AA’ through the conceptual model of the Fiale geothermal system. .... 35 Figure 19. Map with proposed drilling targets (dark blue ellipses). Drill pads in green squares. Proposed wells F1 through F4 in magenta. ................................................................................... 40 Figure 20. Conceptual model with proposed wells F1 through F4 overlain. Note that temperature in cross section and in prognosis may not coincide exactly. ........................................................ 41 Figure 21. Pressure vs depth for proposed wells given range of possible salinities. .................... 43 Figure 22. Fiale wells temperature prognosis. Note that wells may or may not encounter a shallow aquifer as depicted, and the depth of the reservoir top is expected to vary between the targets. ........................................................................................................................................... 44 Figure 23. Drilling decision tree ................................................................................................... 45

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1. Objectives

1.1. Overview Geologica Geothermal Group Inc. and its consortium of geothermal industry experts were selected as the geothermal consulting company (GCC) in support of the Project Management Unit (PMU) for Electricité de Djibouti (EDD) on 13 July 2015. The contract was between Geologica and EDD to begin this work was signed in Djibouti on 7 September 2015. The GCC is providing technical oversight and support for each aspect of the geothermal exploration well drilling and testing program at Fiale Caldera and Lake Asal (Fiale-Asal) area. The GCC will review background data, develop a conceptual model, confirm drilling targets, design four exploration wells, prepare a drilling and testing program, assist in the procurement of drilling and testing services, oversee drilling, data collection, and well testing, and evaluate the resource encountered by exploration drilling. Once sufficient data has been collected from drilling and testing the resource, the team will conduct a feasibility study for the development of a geothermal power plant with targeted output of 20-50 MW.

This report is a key deliverable for Task 1, as described in the Terms Of Reference for the GCC. This report represents the Conceptual Model Review/Confirmation Report with data analysis, updated conceptual model, and drilling targets for four exploration wells in fulfillment of Task 1.1 and a Data Gap Closure Plan in partial fulfillment of Task 1.2.

1.2. Task 1 Objectives There are four main resource requirements that a hydrothermal system must possess in order to generate a traditional electrical-grade geothermal system. The system must have 1) a sufficient heat source capable of producing the high temperatures needed for electrical generation, 2) permeability in the form of connected pathways for water to circulate and constitute a reservoir, 3) ample volume of water to recharge the reservoir, and 4) an impermeable cap to protect the system from convective cold water flooding and conductive heat loss. These four conditions must exist simultaneously in one location in order for a traditional geothermal resource to develop. The objective of Task 1 is to review existing geoscientific data on the Fiale-Asal area and confirm the likelihood for these four requirements to occur within the project area, develop a conceptual geothermal reservoir model which supports the occurrence of these conditions at Fiale Caldera and identify drilling targets which explores the Fiale geothermal reservoir as conceived in the model.

Geologica conducted a thorough review of the existing geoscientific data, conceptual model, and the conclusions and recommendations provided in the previously completed reports prepared by the PMU/PIU, PMU/PIU’s consultants, and academia relative to the project as well as reports and articles readily accessible on the web not provided by the PMU/PIU. This review focused on the measured, inferred and interpreted data/information as well as the consistency/evolution of the previous assessment and the data provided. The data/information review included:

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1. Geologic data, including structural interpretation of the geothermal area 2. Geochemical (soil, water, rock) data and interpretations 3. Geophysical survey data and interpretations 4. Hydrological survey data and interpretations 5. Other available geoscientific data and interpretations pertinent to the project

Under Task 1, Geologica collected, organized, and reviewed the above geoscientific data for the Fiale-Asal area. This review is contained in Sections 1 through 5. Based on the existing data Geologica developed a multi-disciplinary, 2-dimensional conceptual model of the project area, described in Section 6. Significant data gaps remain and additional data collection activities are recommended in order to refine the conceptual model, described in Section 7. This model was used to assess and confirm the downhole drilling targets, as well as support the development of a drilling and casing program, which is briefly treated in this report in Section 8 and will be detailed in the Tender documents for Drilling Service Contractor, Civil Works and procurement of Wellhead, Casing and Downhole Equipment.

Once the resource is confirmed by drilling and the geoscientific results integrated into a revised and updated conceptual model, the model will serve as a basis for building a numerical simulation of the resource and support the overall reservoir development program including production and injection well siting.

2. Background of geothermal exploration in the Asal Rift Geothermal exploration in the Asal Rift (Figure 1) has proceeded in three phases. The first phase, in the 1970s, consisted of geological, geochemical, and geophysical surveys conducted in association with the French Geological Survey (BRGM) ( Correia, Fouillac, Gerard, & Varet, 1985) (Árnason, Eysteinsson, & Vilhjálmsson, 2008). Eleven shallow temperature gradient holes (less than 180 m) depth and two deep wells were drilled. The first deep well, Asal-1 (A-1), was productive while Asal-2 (A-2) was not. They were both drilled in the southwestern part of the rift (Figure 1). A-1 produced from a feed zone at a depth of 1137 m. A-2 showed no permeability, but both wells showed temperatures above 260℃.

The second exploration phase occurred in the 1980s, following the 1977 rifting episode associated with the Ardoukoba eruption. At the beginning of the decade, the Italian consultancy Aquater conducted a study in the Hanle area, 60 km to the southwest of the Asal Rift (AQUATER, 1989). Two deep wells were drilled in the Hanle area, showing low temperatures (<124℃). The lack of high temperatures led Aquater to focus on the Asal Rift. Four deep wells were drilled (Asal or A-3, -4, -5, and -6, shown in Figure 1), two of which were productive: A-3 and A-6. Those two productive wells are very close to A-1, intersecting the same productive reservoir as A-1. Unfortunately the produced fluid from wells A-1, A-3 and A-6 were hypersaline. Wells A-4 and A-5 showed temperatures exceeding 350℃, but encountered very little permeability. Geochemical

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testing took place in wells A-3 to A-6. The most extensive testing was completed in A-3, evaluating deliverability over time, scaling, and corrosion.

The third phase of exploration has persisted from the late 1980s to the present. It started in 1988 with a central-loop transient electromagnetic (TEM) resistivity survey completed by the National Energy Association of Iceland (Árnason, Björnsson, Flóvenz, & Haraldsson, 1988). This survey was able to image the water table, as well as a resistivity anomaly under Lava Lake, which was interpreted as a geothermal system hydrologically separated from the deep reservoir in the southwest of the rift. The TEM survey was followed up in 2007 with a 107 station magnetotelluric (MT) survey by Icelandic Geosurvey (ISOR) (Árnason, 2008).

Regional seismic data was recorded from 1979 to 2001 and characterizing four rifting events and provide geometric constraints on the pattern of the seismicity associated with rifting. Detailed micro-seismicity recorded in 2000-2001, including hypocenters, focal mechanisms, and seismic velocity (Doubre et al., 2004, 2007a, 2007b). A persistent cluster of seismic activity was located between 1 and 3 km depth beneath the southern Fiale Caldera, likely related to stresses projected upward from varying magma pressure at 5 or 6 km depth. Previous MT surveys led to the suggestion of a large, shallow magma chamber (Ngoc, Boyer, Le Mouël, & Courtillot, 1981), an assertion which is now viewed as unlikely because of confirmation by Doubre et al. (2004, 2007a, 2007b) that seismic S-waves propagate through the subsurface (S-waves cannot propagate through magma) beneath the caldera.

In 2007 and 2008, ISOR, performed a geological survey of geothermal surface manifestations and related geologic features in addition to the MT Survey mentioned above for purposes of developing an exploration drilling plan for the Fiale Caldera. The results of these studies are described below in the section on Geology (Khodayar, 2008) and Geophysics (Arnason, 2008).

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Figure 1. A topographical map showing the study area, previously drilled wells, faults/fractures, the Fiale Caldera, surface geothermal manifestations, and previously proposed drilling sites. The Djibouti N9 road is displayed in gray.

2.1. Temperature The primary result of the previous geothermal exploration in the Asal Rift which is applicable to the Fiale caldera are the nature of the geothermal fluids and the temperature distribution. The nature of the geothermal fluids is discussed in the section on Geochemistry below. This section discusses the temperature results.

The measured temperatures in the six deep Asal wells are shown in Figure 2. Wells A-1, A-3, and A-6 were drilled within 250 m of each other and had similar productive zones near 240 m depth, from 400 to 550 m depth, and from 1050 to 1316 m depth. Well A-6 appeared to reach a >260oC near-isothermal reservoir. Wells A-2, A-4, and A-5 were unproductive, lacking sufficient permeability in the uncased part of the borehole to flow. However, all wells exceeded 240oC shallower than 1750 m depth. Wells A-5 and A-6 showed profound temperature reversals from roughly (-500 m) to (-1300 m) elevation, consistent with the regional northwesterly flow of cool ocean water that is warmed on its way to Lake Asal.

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Although the results of all six wells in the Asal area have been used to evaluate how surface geoscience data are likely to constrain subsurface resource properties, the greatest attention has focused on how the temperature pattern and rock properties in well A-5, located immediately adjacent to the area of interest, should be extrapolated to constrain targets in the Fiale Caldera using geoscience data sets.

In A-5, the temperature rises to greater than 170°C at 500 m, then experiences a reversal to 60°C at 900 m, and finally recovers linearly to greater than 360°C at 2100 m. The temperature profile implies several likely properties of the source of the 170°C outflow. The linear conductive temperature gradient below 1100 m in A-5 indicates low permeability. Permeability in geothermal reservoirs is commonly low at temperatures over 350°C, close to the temperature at which the solubility of silica reverses.

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Figure 2. Temperature-depth profiles from the six Asal wells with A-5 highlighted in red. After Figure 4.2 in BGRM (1993).

The following conclusions about A-5 can be drawn from the temperature profiles in Figure 2:

• An upflow exists nearby to supply the hot water for the 170oC outflow. If the upflow is over 240oC, as seems likely given the deeper temperatures, boiling is required to reduce the temperature of the water to under 180oC over a short distance in a permeable system. Therefore, the location of the upflow is likely to be marked by fumaroles.

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• A cold cross-flow vigorously cools the well and must originate from a different direction than the shallower 170°C zone. The obvious candidate is the known southeast to northwest flow of ocean water from Ghoubbet Bay to Lake Asal in the rift fissures aligned with this flow.

• The upflow must be at least partially isolated from the cold cross-flow. Candidate types of alteration that might isolate a geothermal upflow in this environment would include smectite clay alteration or anhydrite precipitation from sulfate in sea water encountering a neutral geothermal chloride brine. The MT resistivity pattern does not resolve low resistivity alteration south of the Fiale Caldera at the cold cross-flow depth of about 1000 m in well A-5. No evidence of anhydrite alteration was reported in A-5, although it is located where cross-flow is dominant and therefore may not exhibit the requisite alteration. Structural alignments in the Fiale Caldera just to the southeast of well A-5 differ in strike from the obvious northwesterly trending rift fractures trending past A-5.

• The outflow in well A-5 is capped by smectite alteration, imaged as a low resistivity zone by the MT. The resistivity over much of the Lava Lake zone within the Fiale Caldera is relatively high, suggesting that either fresh lava would have to function as the cap over any upflow located there, or high temperature alteration like chlorite clay is acting as a cap, both unusual circumstances, implying higher risk for a resource in this area. On the other hand, the leakage of steam to the fumaroles implies that some type of hot aquifer exists at depth, although the steam could be supplied from an outflow like the one encountered at 500 m depth in A-5. This ambiguity could be reduced by sampling gas from the fumaroles and estimating the temperature of the origin of the steam. Gas geothermometry of less than 200oC would imply much higher risk for this target than geothermometry greater than 300oC.

These general expectations based on the temperature log of well A-5 have been assessed in the context of the structural geology, evidence of steam leakage to the surface, the seismicity pattern, the MT-TEM resistivity pattern and the alteration pattern to build a resource conceptual model for the Fiale Caldera, as well as alternative models that illustrate the uncertainty in targeting wells in the Asal Rift. Very cold zones like the one in well A-5 are seldom encountered adjacent to high temperature reservoirs, but this specific pattern has been encountered in several basalt-hosted geothermal reservoirs adjacent to open rifts, such as the Theistareykir Geothermal Field (Gautason, et al., 2010).

3. Regional geologic setting Djibouti is located at the intersection of three major extensional structures in the earth’s crust (i.e., a triple junction): the Red Sea, the East African Rift Zone, and the Gulf of Aden (Figure 3) with the Asal Rift being the northwestward extension of the Gulf of Aden (Árnason, 2007; Corti et al, 2015; among numerous others; Figure 4).

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The strike of the Asal Rift is northwest-southeast (NW-SE) with spreading (i.e. extension) in the northeast-southwest (NE-SW) direction. The rift is approximately 9-10 km wide and bounded by impressive normal faults (Árnason, 2007). A series of east-northeast (ENE)-trending generally dextral transform faults extending from the Indian Ocean westward to the Gulfs of Aden and Tadjoura are observed (Figure 4). As reported by Khodayar (2008), the transform faulting within several ENE fractured zones have also been mapped in the Asal Rift and at the intersection of some of the rifting NW-trending faults with these transform fault/fracture zones, calderas, and rare dike outcrops occur along with the geothermal areas at Fiale caldera and Korili/Gale le Kôma. There is recent, ongoing subaerial basaltic volcanism in the rift evidenced by the lava lake at Fiale Caldera (Houmed et al. 2015) and the recent November 1978 fissure eruption at the Ardoukoba Volcano resulting in, as reported by Peltzer et al. (2015), two earthquakes of mb=5+, a basaltic fissure eruption, the development of many open fissures across the rift and up to 80 cm of vertical slip on bordering faults.

In addition to basaltic volcanism, there also occur rhyolites and trachytes at depth along with minor sedimentary deposits (AQUATER, 1989). The tectonics in the region have been ongoing for the past 9 million years ago (Ma) (Doubre et al., 2007), creating rift faults, extensive magmatic activity (i.e. intrusions and extrusions), along with fumaroles and hot springs.

A pre-feasibility study by Reykjavik Energy Invest (REI, 2009) along with Houmed et al. (2015), among others, have identified the Fiale Caldera as a site for further geothermal exploration.

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Figure 3. Regional setting of the Asal Rift. Digital elevation model of the Afar Depression and surrounding areas. Inset on the top left shows the extent of the system of rift valley composing the East African Rift. Inset on the bottom right shows the plate kinematic setting of the region; dashed lines indicate plate boundaries and white arrows illustrate plate kinematics. Black and white squares indicate major towns. AA Aleyu–Amoissa; Ado Ado Ale; AnBF Ankober border fault; BG Borkena graben; Bo Borama; D Dabbahu; Da Dallol; De Der’Ela graben; DG Dobi graben; EAR Erta Ale range; GG Gaddale graben; HG Hanle graben; IG Immino graben; MH Manda–Hararo rift; MI Manda-Inakir rift; TG Tendaho graben. The figure is from and the caption is after Corti et al. (2015).

Asal Rift

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Figure 4. Geologic setting of Djibouti. Location of the Asal rift indicated in green text. The figure is from Khodayar (2008).

3.1. Asal Rift Stratigraphy Deposition in the region belongs to three main rock series (AQUATER, 1989), the oldest of which is the Dalha Basalt Series. It is a sequence of lava flows intercalated with rhyolites, trachytes, and detritic deposits. The age of the Dalha Basalt Series is between 8.9 Ma and 3.8 Ma. The high-temperature productive zones in Asal wells 1, 3, and 6 are productive in the Dalha Basalt Series.

The Afar Stratoid Series overlies (often unconformably) the Dalha Basalt Series in the Asal Rift. It was produced by volcanism from the central Afar rift and is a sequence of basalt-dominated fissure flows that are associated with rhyolitic volcanic centers. Its composition is mainly basalt, with intercalated intermediate products, including trachytes and rhyolites. The Afar Stratoid Series commonly contains lacustrine deposition, suggesting ancient fresh-water lakes in the region between 4 Ma and 1 Ma.

The youngest rock series in the region is the Asal Series, containing the recent basalt lava flows as well as hyaloclastites generated at the onset of the volcanism of the Asal Rift. The Asal Series (1.05 Ma to present) outcrops in ridges on either side of the rift and includes the most recent 1978 volcanic eruption in the inner Asal Rift.

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3.2. Structural geology The Asal Rift is a major WNW-ESE trending extensional regime with numerous faults and fractures (Figure 5). The faulting strikes along the Asal rift, WNW; the area is also traversed by various fractures trending ENE, east (E), north (N), and north-northwest (NNW). Both the faults and fractures show normal slip, but the cross-rift fractures display shear displacement. At Fiale Caldera, the hottest geothermal surface manifestations are aligned on a local NNW open fracture where the cross-rift fractures intersect (Khodayar, 2008).

Figure 5. Geologic map of the Asal rift showing faults and fractures.

3.3. Geologic features of Fiale Caldera The Fiale caldera is situated in the Asal Rift axial graben between two boundary rift faults trending WNW. It is traversed by multiple intersecting N, E, ENE-trending faults and fractures as well as NW-trending fractures having both extensional and shear components. Only the most recent faulting is observed within the Asal Rift as a result of the recent volcanism. As such, some maps

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indicate that the rift faults do not propagate through the caldera. The ENE trending faults and fractures form a transform fault in the area, possibly indicating step-over (Khodayar, 2008).

Most of the surface of the caldera is covered by Lava Lake, which is the surface expression of very young basaltic lava flows. The flows are broken by the WNW, N, E, ENE, NW trending fractures. Local geothermal manifestations (fumaroles and surface alteration) occur along NW, ENE, and WNW-trending faults on the northeast and southwest quadrants of the caldera.

4. Geophysics

4.1. Magnetotelluric Survey A 106 station magnetotelluric (MT) survey was acquired to supplement the earlier TEM survey in order to image the low resistivity clay cap of the underlying relatively resistive geothermal reservoir and to potentially image, as a low resistivity zone, the underlying heat source (Árnason, 2008). The correlation of low resistivity with the clay cap of a geothermal reservoir is the characteristic pattern used to explore most basalt and andesite-hosted geothermal reservoirs worldwide (Ussher, et al., 2000; Árnason, et al., 2000). Deeper 3D imaging of low resistivity using combined MT-TEM has been correlated with very high temperature zones associated with recent magmatism in Iceland and this has been applied to the Asal MT data set by Árnason et al. (2008) and Sakindi (2015) with inconsistent results. Although the results of Árnason et al. (2008) are more interpretable, the reliability of the deep 3D imaging depends on the quality of the MT data, which varies at Asal, especially in the target area. Therefore, the presentation in this report uses cross-sections of resistivity derived from 1D inversions of the MT invariant mode and the TE-mode (Cumming & Mackie, 2010) truncated at depths where noise or resistivity geometry makes the inversions unreliable. The cross sections in Figure 6 illustrate the resistivity pattern among the existing wells at Asal and across the Fiale Caldera.

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Figure 6. Elevation map with Fiale Caldera outlined. The black triangles mark MT stations, the red squares mark wells, and the lines are cross sections discussed in this section.

Resistivity cross section P 01 (Figure 7) is oriented across the strike of the rift connecting wells A-1, A-3, A-4, A-5, and A-6. This cross-section is made by stitching together 1D smooth inversions of the MT TE-mode that are truncated at the depth where they are expected to become unreliable. The color shading of the resistivity contours transitions to white at this maximum reliable depth in the cross-sections.

The smectite clay cap is imaged as the red to orange layer that is continuous across the cross-section, although the elevations of the top and base are highly variable and resistivity values change rapidly laterally. The variability could have several causes. It is unlikely to be caused by the geophysical imaging fallacy called “chain-of-pearls” where lateral variation is over-emphasized. The main issue is likely to be geological conditions that vary rapidly across strike, as is suggested by the differences in the well properties across strike and their similarities along strike, as illustrated in later cross-sections.

In the cross-section in Figure 7, the water table is just below sea level and the resistivity is particularly low where it extends above the water table, probably in areas where boiling has occurred. This also suggests that the rift structures divide the shallow aquifers and clay aquicludes into fault-bounded zones of differing properties. The rapid variation across strike may have led to some under-sampling (aliasing). One aspect of under-sampling is that estimates of the dip of the base of the clay cap across strike are likely to be unreliable, which is unfortunate because mapping

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the base of the clay cap is the standard method of tracing the likely path of a buoyant thermal outflow back to its upflow source, as was planned for the outflow in well A-5.

Figure 7. Resistivity cross section P 01, including locations of faults and seismic events. True locations of the line, wells, and stations are inset.

P 04 (Figure 8) is an MT resistivity cross section oriented along the strike of Asal Rift, crossing the Fiale Caldera. Unlike the resistivity profile in Figure 7, the top and base of the low resistivity zone are much more consistent in this profile. The top of the low resistivity smectite clay alteration is near the water table, smoothly dipping down from SE to NW. The base of the low resistivity is consistently near -500 masl, except southeast of the Fiale Caldera. High resistivity basalt lava (blue) overlies the clay cap, except in the areas adjacent to Ghoubbet Bay to the southeast and Lake Asal to the northwest. The resistivity pattern at the southeast end of the profile likely indicates very shallow thin aquifers hosting seawater within clay-rich volcanics and sediments. A relatively resistive cold seawater aquifer may have been resolved from -250 m to -500 masl at the southeast end of the profile, but does not extend across the Caldera.

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Figure 8. Resistivity cross section P 04. True locations of the line and stations are inset.

Cross section P 02 (Figure 9) runs east to west, crossing well A-5, the Fiale Caldera, and extending to near Ghoubbet Bay. The clay cap thins in the caldera and is truncated to the west at the margin of the current rifting zone. Although not well resolved by the available MT data, the base of the low resistivity seems to dip up from the western caldera past well A-5, with the base of the lowest resistivity zone coinciding with the top of the 170oC outflow in A-5. This suggests that it is at least plausible that an upflow in the Fiale Caldera boils, reduces temperature, and flows buoyantly updip below the clay cap to the northwest past well A-5.

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Figure 9. Resistivity cross section P 02. True locations of the line, well, and stations are inset.

The MT shows that the Asal Rift has an extensive clay cap that varies rapidly in character across strike and is more consistent along strike. It is truncated at the edges of the rift and is thinner and less conductive in the recent lavas that cap the Fiale Caldera. The potential reservoir zone below the low resistivity clay cap is resolved by the MT as being relatively resistive, typical of permeable geothermal reservoir zones. However, the MT does not resolve resistivity variations within the reservoir with enough reliability to infer the properties of the reservoir at the scale of a typical well target zone. In some circumstances, the value of resistivity in a geothermal reservoir may be sufficiently well-resolved to be diagnostic of reservoir properties at a large scale. Unfortunately, the available Djibouti MT data are unlikely to have that specificity because of their relatively high level of noise.

The 3D MT inversions by Árnason et al. (2008) and Sakindi (2015) extend from the surface through the depth of wells in an attempt to resolve a deeper low resistvitiy zone that may be related to the heat source. The smoothing used by the 3D inversion in order to obtain the deeper resolution tends to smooth the low resistivity of the clay cap to greater depth. This means that, although the

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resistivity below the base of the clay cap is resolved by the 3D inversion as a relatative resistor, the specific value of resistivity is probably more dependent on smoothing of the overlying low resistivity zone than on the resistivity of the potential reservoir zone itself. On the other hand, because particularly low resistivity in the clay cap is a potential indicator of an underlying geothermal rservoir, some interpreters rely on the smoothed resistivity of the underlying zone as resrvoir indicator.

In the report by Árnason et al. (2008), they interpret a barrier to northwestern flow based on the resistivity at -75 masl. The 1D TE-mode analysis in our study suggests a smoother transition to the northwest than is indicated by the 3D resistivity map in Árnason et al. (2008). The pattern of the discontinuity in their figure roughly matches the topographic high that is related to recent volcanic deposition. Because of a possibility that this feature may be an artifact of the topography, it was not examined further in our study. However, in a more extended review of the conceptual model, a broader assessment of this feature using both 1D TEM and 1D MT inversions may be relevant to understanding barrier that could isolate the Fiale Caldera from cold cross-flow.

4.2. Transient Electromagnetic Survey The transient electromagnetic (TEM) method resolves resistivity with a shallower depth of investigation than MT and so it typically does not reliably resolve the base of the clay cap of geothermal reservoirs (Cumming & Mackie, 2010), although the reservoir at Asal is sufficiently shallow that the TEM may have successfully resolved the base of the clay cap for an unusually large proportion of the TEM stations. Unfortunately, formatting issues and scope limitations prevented the recalculation of TEM data to produce TEM cross-sections that could have been compared to the MT. However, for geothermal applications, TEM is more routinely used in geothermal exploration to constrain MT static distortion caused by topography and shallow lateral variations in resistivity (Pellerin & Hohmann, 1990). Therefore, the TEM has been applied by Árnason et al. (2008) to constrain statics distortion for the 3D inversion. Unfortunately, the aforementioned formatting limitations prevented the use of the TEM for the 1D MT inversions presented. To evaluate the effect of this on the interpretation, where the MT modes indicated that static distortion was significant, the cross-section was computed by choosing the maximum shifts to higher and lower resistivity consistent with the measured range of the MT. The resolved shallow resistivity patterns were consistent for the cross-sections across the Fiale Caldera, regardless of the static distortion assumption.

4.3. Seismicity There are two main areas of seismicity in the Asal Rift. The first is from 2 to 4 km depth in the southern caldera (Figure 10). Doubre et al. (2007a and b) attributed the seismicity to the rifting event that extended through the 2000-2001 survey and was associated with deeper magmatic movement rather than tectonic fault slipping. The earthquakes are interpreted to occur in the brittle zone just above the transition to higher temperature ductile rocks. The variations in buoyant pressure of the magma push the overlying brittle rocks up and down. Below 3.5 km depth, the

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rocks become ductile, explaining the lack of seismicity at this depth. Propagation of S-waves through this area led to the conclusion that the zone of earthquakes did not overly liquid magma. Doubre et al. (2007a and b) show a low P-wave velocity anomaly in this area, interpreting it as being due to the presence of steam or gas in the rock pores, rather than liquid saturation.

Figure 10. Two cross sections from Doubre et al. (2007). Their locations are highlighted in blue on the map.

There is some seismicity in the older rift to the north that Doubre et al. (2007a and b) interpret as being tectonic in nature, resulting from fault slip that occurs on the ridge margins.

4.4. Gravity survey BRGM conducted a gravity survey in the Asal Region. Árnason et al. (2008) summarized BRGM’s findings. The data (Figure 11) shows gravity highs in the older rocks outside of the rift and in North Ghoubbet bay as well as through the Fiale Caldera and to its immediate south. The high gravity anomalies coincide with recent subaerial volcanism. The basalt lava flows and/or lava intrusions at depth in the volcanic center serve to raise the overall density of the rift region. The higher density rocks are typically more resistant to alteration. Gravity is relatively lower in the

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southwest part of the rift, interpreted as being due to lower density hyaloclastite deposited underwater, as shown in the shallower sections of wells A-3, A-4, and A-6.

Figure 11. The gravity data measured in the Asal Rift and surrounding areas. Reproduced and edited from BRGM (1993).

4.5. Geophysical conclusions The microseismic data indicate that the spreading axis has most recently been active through the Fiale Caldera. Brittle rock exists between 1500 m and 3500 m, where most of the seismicity has occurred. Below 3500 m, the rocks become increasingly ductile with the increase in temperature with depth, explaining the lack of deep seismicity.

The MT data indicates a clay cap persisting throughout the Asal Rift. The clay cap thins around the flanks of the caldera, possibly due to disruption of the clay alteration related to recent eruption of the lavas of the Lava Lake that may not have had time to alter. It is possible that high temperature alteration extended to shallower depth in the past, causing the smectite to alter to chlorite. The dip in the base of the clay cap on the northwestern margin of the Fiale Caldera is consistent with an upflow in the Fiale Caldera boiling and then outflowing beneath the clay cap updip past the 70oC zone found at 500 m depth in well A-5.

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5. Geochemistry

5.1. Geochemical Data The hot springs and geothermal well waters in the Asal Rift have been sampled and analyzed over the last ~30 years. These thermal waters have been analyzed for dissolved solids such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg) and other metals, silica (SiO2), chloride (Cl), bicarbonate (HCO3), sulfate (SO4), and others. Results are presented in Table 1. No geochemical analysis of fumarole steam or gases is available, although a recent JICA report indicates that they collected and analyzed a sample of a fumarole from Fiale Caldera. Fumarole sampling is included in the recommended data gap closure work.

Table 1. Geochemistry for wells (in bold) and selected groundwater samples.

1Sanjuan et al. (1990) 2Correia et al. (1985) 3D’Amore et al. (1998) 4Virkir-Orkint (1990)

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5.2. Surface Manifestations There are two types of surface manifestations in the Asal rift area that are related to the Fiale Caldera geothermal system. These include warm and hot springs at the north end of the rift and fumaroles and hydrothermal alteration within the rift (Figure 12) and fumaroles within the Fiale Caldera (Figure 13Error! Reference source not found.). The thermal springs are clustered where the large NW trending rift faults have been mapped from Gulf of Ghoubbet to Lake Asal. Faulting SW and NE of the Fiale Caldera is predominantly along these major faults which partially form the outer NE and SW edges of the Caldera

Thermal springs around Lake Asal (30°C to >80°C) appear to be seawater which is heated and slightly altered by interaction with basalt (Sanjuan et al., 1990). Despite temperatures up to near boiling, the spring waters retain NaCl concentrations near seawater, suggesting that the waters may not have been significantly hotter than their measured discharge temperatures. Cold seawater appears to flow northwest from the Gulf of Ghoubbet (as observed at shallow locations near wells A-3, A-4, and at hot springs near Lake Asal) both northeast and south west of the Fiale Caldera. The seawater is heated as it flows north but does not become high-temperature brine.

The fumaroles in the Asal rift and in the Fiale Caldera are typically low pressure steam, and probably gas, discharging near boiling from fractures in the surface lavas. There is absent to minor mineral alteration at the vents. The only available data, from an indication from a JICA report, that the helium and helium isotopes indicate a magmatic component in the discharge, as is to be expected. Without any chemistry on the gas and steam discharges, the source of the steam cannot be evaluated. The only conclusion that can be made is that the first water below the fumarole is boiling, i.e. not cold seawater.

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Figure 12 Thermal Fluid Sample Locations (Figure 2 from Varet et al., 2014). Blue dots represent selected warm and hot springs, red dots are fumarole locations, green circles are geothermal wells. The large blue arrow represents seawater flow from the Gulf of Ghoubbet NW to Lake Asal. The red arrow represents the active volcanic axis of the rift. Fiale Caldera lies within the red square. Some warm springs extend around the E-NE side of Lake Asal. From west to south the spring areas are known as: l’Oued Kalou, Korilli, and Manda. From east and northeast of Lake Asal the springs are Eadkorar and Alifitta.

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Figure 13 Location Map of surface manifestations north west (Korilli) and within the Fiale Caldera mapped by M. Khodayar. Reproduced from Figure 34 in ISOR (2008).

The geothermal well waters are also seawater that has been heated through interaction with rock at high temperatures. This depletes the magnesium content, while enriching the potassium, sodium, and chloride contents (Figure 14 and Figure 15). Although similar to Lake Asal waters, the geothermal well waters are enriched in NaCl relative to seawater, and they do not appear to be directly related to Lake Asal waters or to the evaporative processes which produce the higher NaCl content in Lake Asal.

The fluids observed in A-5 are not like the other geothermal fluids previously mentioned, nor are they similar to the warm spring waters which appear to have flowed from the sea northwestward.

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Figure 14. Cl-Na-K plot for waters in Asal Rift Area.

Figure 15. Cl-Mg plot for waters in Asal Rift

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5.3. Geothermometers Geothermometer temperature estimates were made in multiple locations of interest in the Asal Rift, as shown in Table 2. The geothermal wells gave relatively high temperature estimates, staying close to measured values. The fluids with cooler temperature values seem to correlate with spring waters and seawater. Curiously, the tests of A-4 tell two different stories: one sample looks like seawater, while the other looks like geothermal water. The geothermometry results for A-5 were much hotter than measured by the pressure/temperature survey.

Table 2. Geothermometry values for wells and selected groundwater samples.

1Sanjuan et al. (1990) 2Correia et al. (1985) 3D’Amore et al. (1998) 4Virkir-Orkint (1990) 5Fournier and Potter (1982) 6Fournier and Rowe (1977) 7Truesdell (1970) 8Arnorsson et al. (1983) 9Giggenbach (1981) and Giggenbach (1988)

5.4. Geochemistry Summary This interpretation of the geochemical data provides the following constraints for the conceptual model of the Fiale Caldera geothermal system:

• Sea water flows through major NW trending rift faults to discharge at warm/hot springs along Lake Asal. While it is still cool when passing through A-4 (50-255 m) and near A-5

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(800-1100 m), it is heated and slightly altered by the time it reaches Lake Asal. However, the resulting warm and hot waters do not appear to be dilutions or otherwise related to the high temperature well waters nor do they appear to have achieved temperatures significantly higher than the discharge temperatures.

• Fumaroles in Fiale Caldera indicate shallow boiling but it is not clear what fluid is boiling or at what temperature the boiling occurs.

• In the Asal geothermal area to the southwest (wells A-1, A-2, A-3, A-6), sea water has interacted with rock at depth to generate hypersaline brine, a shallow 100-130°C hot zone in scoriaceous basalt and deep 260°C zone of fractured Dahlia basalt underlying clay layers. No shallow cold seawater is reported.

• Although an aseismic zone indicates high rock temperatures (800°C) under Fiale Caldera provide a heat source at depth (>3500 m), there is no clear surface manifestation of this system.

• Although the chemistry of the hot springs at Lake Asal do not indicate discharge of high temperature geothermal fluids such as those from the NW-SE rift faults southwest of Fiale Caldera, they appear to be heated sea water discharging from areas concentrated at the NW end of NW-SE faults which occur both north and south of the Caldera.

• Fractures through Fiale Caldera have different orientations (NNW, ESE).

• A-4 and A-5 are hot (>250°C) at 800 m and 1800 m and have different lithologies than other wells but no permeability below 600 m.

6. Conceptual model

6.1. Constraints from geochemical, geological, and geophysical observations As shown in Figure 16 seawater flows through major rift faults to reach warm springs along Lake Asal. The seawater is still cool as it pass through A-4 (50-255 m depth) and near A-5 (800-1100 m depth). Fumaroles around Fiale Caldera indicate boiling, either shallow or deep, eliminating the possibility of shallow cold water.

In wells A-1, A-3, and A-6, seawater has interacted with rock at depth to generate hypersaline brine. A shallow 100-130°C warm zone in scoriacious basalt and a deep 160°C hot zone in fractured Dahlia basalts underlie clay layers. There is no shallow sea water reported in these wells.

The geology shows that the NW-SE rifting faults do not pass through the Fiale Caldera. Fractures present in Fiale have different orientations, notably NNW and ESE.

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Wells A-4 and A-5 are hot (temperatures above 250°C at 800 m and 1800 m depth, respectively. They have different lithologies from the other wells, but have similar alteration. Neither well has permeability below 600 m depth.

Figure 16. Map of Asal rift showing fluid flow direction (blue arrows). After Batistelli et al (1991).

6.2. Four requirements for a geothermal system Section 2.1 listed the four main requirements necessary for a traditional electrical-grade geothermal system: heat, water, permeability, and a cap. The conceptual model for the Fiale Caldera detailed below is supported by the geoscientific evidence and incorporates these four requirements. The model includes: 1) heat, as evidenced by the aseismic zone containing hot ductile rocks at depths greater than 3.5 km; 2) water, shown by both near-surface and deep seawater cross flows; 3) permeability, demonstrated by the faults, fractures, and microseismic zones displaying brittle fracturing; and 4) an apparent cap, evidenced by the electrical conductor imaged with MT and TDEM.

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6.3. Conceptual model Our conceptual model is displayed in Error! Reference source not found.Figure 1 and described below. This cross-section is collinear with the F3 profile from Doubre et al (2007). The elements of the conceptual model are described below.

Figure 17. Overview map showing the location of cross section AA’, which is collinear with Doubre et al (2007)’s cross section F3.

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Figure 18. Cross section AA’ through the conceptual model of the Fiale geothermal system.

Heat source

The heat source for the geothermal system is likely to be hot rock in proximity to recent cooling intrusions within the center of the rift and beneath the Fiale Caldera. While magma is not expected to be found at drillable depths, small quantities in discrete zones (dikes) have been encountered in geothermal wellbores in similar active rifting environments such as Krafla (Iceland), Puna (Hawaii), and Menengai (Kenya).

Clay cap

The low resistivity region associated with smectite clay alteration, commonly called the clay cap, is included in the conceptual model and approximately coincides with the 5 ohm*m resistivity boundary in cross section. The depth of the top of the clay cap varies from at surface to ~200 m depth. The base of the clay cap is generally at a depth of 800-1000 m corresponding to an elevation of -700 to -900 mMSL. Local concave-down doming of the base of the conductor, such as is observed beneath the Fiale Caldera, are characteristic of upflow zones in geothermal reservoirs, although the aliasing problem previously discussed prevents accurate calculation of concavity.

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This thinning of the clay cap may be due to alteration of the conductive smectite to higher-grade alteration mineral assemblages, including chlorite.

Thermal features

Numerous fumaroles within the rift and around the Fiale Caldera are depicted in the conceptual model as expressions of upflow and outflow zones in different cells. Laterally displaced fumaroles may have similar gas chemistry if they are fed from a lateral outflow originating from the same upflow point. The fumarolic activity is coincident with shallow low resistivity where the clay cap comes close to the surface, although there are not always fumaroles associated with shallow low resistivity because the conduits for fluid flow change often in geothermal systems as stress regimes change direction and mineralization plugs off fluid pathways. Additionally, hydrologic changes caused by sporadic rainfall likely affects the number, location, and prominence of thermal features.

Upflows

There are likely to be multiple upflow zones within the rift, of which the Fiale Caldera may represent only one. The conceptual model posits other potential upflow zones diagrammatically as red arrows and should not be viewed as representing any specific data. Upflow zones may locally exploit normal faults in the rift with enhanced permeability, especially in fining a conduit through the clay cap to the surface for fluid to feed the fumarolic activity.

Downflows

Rising hot buoyant reservoir fluid will lose its heat through three processes: conduction, convection/advection, and boiling. Heat will move through the clay cap and be represented as thermal features and surface heat loss. The cooled reservoir fluid will then descend convectively within its cell, as depicted in the reservoir areaError! Reference source not found. beneath the Fiale Caldera. Also, cold seawater moving through fractures in the rift will be denser than any local groundwater and the thermally buoyant reservoir fluid, and will descend within distributed permeability and locally take advantage of permeable normal faults. As this cold water descends and heats up it will deposit minerals with retrograde solubility (e.g. calcite, anhydrite) which will tend to reduce permeability, restrict the flow of cold water, and possibly lead to the formation of permeability boundaries which can isolate cells. A countervailing effect of the cold water down flow is the thermal volumetric decrease (shrinking) of the host rock, which will tend to open fractures and increase permeability. This permeability stimulation effect is commonly seen in injection wells but declines with time and is often eventually overtaken by the permeability decreasing effects of mineral precipitation.

Outflows

Most geothermal systems include significant later outflows of reservoir fluid, however the expected cellular nature of the reservoirs in the rift as depicted in the conceptual model means that

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what outflows exist may be largely restricted to minor leakage of fluid into permeable horizons in the clay cap which ultimately express as surface thermal features.

Reservoir fluid

The conceptual model shows that the source of water for the system is sea water flowing through the major NW-SE rift faults above and below the conductor, or low-resistivity clay zone. The heat is being generated from the rift-related magmatism which is most active (and at its shallowest) in the Fiale Caldera and its northern edge. The seawater can infiltrate to deeper regions through normal faulting, where it is then heated and encounters permeability, which could lead to the development of a high-temperature geothermal system. Repeated boiling and condensing cycles combined with possible interaction with evaporite horizons in the stratigraphic column may increase the reservoir fluid’s concentration of dissolved solids above that of seawater, perhaps to hypersaline brines such as were encountered in A-3.

Reservoir permeability

The conceptual model includes a permeable reservoir beneath the Fiale Caldera that is related to a body of high seismicity described by Doubre et al (2007). This high seismicity zone is shallower beneath the north of the caldera where it comes to within perhaps 1000 m of the surface and deepens to ~1500-2000 m beneath the southern part of the caldera. The zone extend to a depth of ~3500-4000 m where it becomes aseismic, interpreted as the transition to ductile rock mechanics at high temperature. This zone represent the fracturing of rock which may be associated with cooling of a recent intrusion or intense normal faulting within the rift. In either case, the high seismicity is likely to host enhanced permeability and possibly a convecting geothermal reservoir. The reverse however is not true - aseismic zones are not necessarily low permeability. They may simply represent areas where the state of stress is low, and so good reservoir permeability can be located laterally outside the high seismicity zone.

Permeability boundaries and compartments

The permeable reservoirs are likely separated into cells by impermeable barriers which may be dike intrusions, faults under compression, or secondary mineralization horizons due to processes like boiling or mixing of incompatible fluids. Primary permeability (porosity) of the volcanics is likely to be low. Permeability is most likely to be enhanced in areas where microseismicity indicates brittle, fracturing rocks: e.g., the Fiale Caldera below 1.5 km depth and in the older formations north of the Fiale Caldera. This permeability, though shallow, is indicated by fumaroles and other areas of surface manifestations. Additionally, hyaloclastites below ~500 m depth, formed when the intruding basalts and trachytes were erupted under water, are likely to have high permeability.

The multiple fracture directions within Fiale Caldera suggest that wells drilled to the northeast might encounter the most fractures. The shallow permeability zone in A-5 and its related upflow

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also represent a prospect. Additional data confirming temperature transients in A-5 as well as and confirmation of deeper permeability would help with drill targeting.

7. Data gaps

7.1. Geothermal exploration drilling at Fiale Caldera Drilling done in the Fiale Caldera will be exploratory, as an exploitable geothermal system has not been confirmed. The biggest risk to drilling the shallow system is a possible intersection with the cold seawater intrusion. The biggest risk to successful drilling of the deep geothermal system is a lack of permeability. Therefore, exploratory drilling must avoid the cold seawater intrusion zones and must be drilled directionally across zones to increase the probability of encountering permeability.

7.2. Temperature transients and permeability changes It is important to know if the temperature gradients have yet stabilized after the Ardoukoba rifting event 37 years ago. A-5, the deepest well in the Asal Rift, has mineralogy suggesting higher temperature than measured downhole. It is possible that the temperature reversal that was observed 30 years ago was transient.

Fumaroles in the region provide evidence of leakage of boiling water. It is important to know if this is related to a shallow geothermal system or one that is at greater depth.

Finally, it is important to study the fracture in the region in order to create a permeability model with possible preferential pathways that could affect drilling targets.

7.3. Data gap proposal The first step to resolving the data gap is a pressure-temperature survey of A-5. This will confirm or discredit the existence of the measured temperature inversion in well A-5. If the inversion still exists the conceptual model can be revised to incorporate a large seawater flow through the western edge of the Fiale Caldera, which would significantly alter the targeting of exploratory wells in order to avoid this area. We suggest resurveying the pressure and temperature profiles for the following wells, in order of importance: A-5, A-4, and A-3.

For this scope of work we have solicited quotes from the following vendors: Transmark Drilling (Turkey), Bahadir IPEK (Turkey), Geothermal Development Company (Kenya), and Hades System (New Zealand). Our current recommendation is Hades System, whose contract would include transfer of capital equipment to EdD’s ownership at the end of the project.

Our second recommendation is fumarole sampling. The existing fumaroles, many of which discharge from small fissures in the young flows of Lava Lake, suggest boiling conditions in the subsurface. The chemistry of gases and water isotopes of steam may provide temperature estimates and approximate chemistry of source waters. The GCC recommends collection of approximately

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10 fumarole and hot spring samples feeding Lake Asal. The GCC plans to collect water geochemistry (if possible), gas samples (CO2, H2S, CH4, H2, N2, NH3, O2, Ar, He, hydrocarbons, CO, and 3He/4He), and isotopes (18O, deuterium, and possibly 34S). The analysis will be performed by qualified international laboratories. Training and oversight can be provided by GCC technical staff, which will also provide detailed sampling plans, equipment procurement support, and data interpretation.

The final data gap recommendation is a CO2 flux survey that can be paired with a very shallow temperature survey (e.g. 100 cm) to provide qualitative indications of leaking structures that allow upflow of hydrothermal fluids. Quantitative estimates of heat flux can be derived from CO2/steam ratios in the fumaroles and in the CO2 flux.

To assess the shallow clay alteration with higher resolution than is possible with the MT, the preparation of 1D resistivity cross-sections based on the existing TEM data would supplement the constraints on the outflow in well A-5 and might more reliably indicate its source.

The use of a small civilian unmanned aerial vehicle (drone) with conventional and thermal cameras could provide photogrammetric mapping to develop a high resolution digital elevation model (DEM) and thermal infrared map of the potential target areas. This would support interpretation of structural features, and CO2 flux and shallow temperature surveys.

Our proposal will increase the probability of well success by influencing the direction and location of the reservoir sections of the exploration wells, may increase estimated reservoir sizes, and will confirm changes that may have occurred since 1978.

8. Preliminary Drilling targets Four drilling targets have been proposed within and in close proximity to the Fiale Caldera, as shown in the blue ellipses in Figure 19. These drilling targets can be reached from three drilling pads labeled North, South, and Southwest. Four wells have been proposed to explore these four targets and are designated F1 through F4. It should be emphasized that although these wells are designed to be produced and incorporated into a future geothermal development plan, they are exploration wells. The nearest offset well is A-5, several km to the west, and conditions in F1 through F4 are expected to differ considerably from A-5. All four proposed wells are programmed for a depth of 2500 m, but this may be altered to be shallower or deeper based on the conditions encountered at each site during drilling.

All four wells will be designed conservatively for the possibility of encountering boiling point for depth (BPD) temperatures at shallow depths, and a deep reservoir of at least 300°C. This requires four strings of cemented casing in addition to the cemented conductor pipe: a surface string, two intermediate strings, and production casing string. Since formation stability in the area of investigation is not known, the wells are designed to be completed with slotted liner. Information

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about the reservoir temperature and pressure, fracture gradient, and formation stability may alter casing setting depths after completion of the first well. Proposed well parameters are tabulated in Table 3. The approximate tracks of the proposed wells are shown in Figure 20.

Figure 19. Map with proposed drilling targets (dark blue ellipses). Drill pads in green squares. Proposed wells F1 through F4 in magenta.

Table 3. Proposed Fiale Wells. A-5 shown for comparison.

Well Location Elev. (m)

Depth of “Intermediate” Reservoir (mTVD)

+ (Temp, °C)

Top of Deep Reservoir (mTVD)

Production Casing Shoe

Depth (mTVD)

TD (mTVD)

Throw (m)

F1 Pad North 110 ? 1000-1500 (300°C) 1000 2500 850

F2 Pad North 110 ? 1000-1500 (300°C) 1000 2500 700

F3 Pad West 112 400-600? (180°C)

1200-1800 (300°C) 1200 2500 775

F4 Pad Southwest 136 400-600

(180°C) 1200-1800

(300°C) 1200 2500 475

A5 SW of Fiale Caldera 113 400-600

(180°C) 850? Cold,

1700 (275°C) - 2105 -

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Figure 20. Conceptual model with proposed wells F1 through F4 overlain. Note that temperature in cross section and in prognosis may not coincide exactly.

8.1. Proposed Wells This section details proposed targets and well parameters for F1 through F4. Representative pressure and temperature prognoses are given in Figure 21 and Figure 22.

F1

Proposed well F1 is located on Pad North at an elevation of 110 masl and deviated east-northeast 850 m across the northern rim of the caldera. The primary target is the zone of shallow high seismicity beneath the northern part of the caldera. The secondary target is the major northwest trending normal fault labeled α1 in Doubre (2007) that intersects at the surface with a northwesterly trending fracture set.

F1 is located furthest from offset well A-5 and therefore had the highest level of uncertainty in expected subsurface conditions. Specifically, the shallow 180°C aquifer seen in A-5 may not be present here.

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The zone of high seismicity shallows near the north of Fiale Caldera and so F1 may encounter the reservoir top more shallowly than the other wells, perhaps from 1000-1500 m. For this reason the production casing shoe is proposed at a depth of 1000 m.

F2

Proposed well F2 is targeted from the Pad North location at an elevation of 110 masl and deviated southwest 700 m toward the center of the caldera. The primary target is the zone of high seismicity beneath the central part of the caldera. This target is also overlain by multiple intersecting fracture sets on different azimuths.

F2 is far from offset well A-5 and therefore has a high level of uncertainty in expected subsurface conditions. Specifically, the shallow 180°C aquifer seen in A-5 may not be present here.

The zone of high seismicity deepens near the south of Fiale Caldera, but F2 begins from Pad North so F2 may encounter a similar depth reservoir top as F1, perhaps from 1000-1500 m. For this reason the production casing shoe is proposed at a depth of 1000 m.

F3

Proposed well F3 is targeted from the Pad South location at an elevation of 112 masl and deviated northeast 775 m towards the northern rim of the caldera. The primary target is the zone of high seismicity beneath the central part of the caldera. This target is also overlain by a north trending fracture set which intersects the northern ring fracture of the caldera.

F3 deviates away from A-5, but in the shallow section is less than 1 km from well A-5, therefore the shallow 180°C aquifer seen in A-5 may be present here as well.

The zone of high seismicity deepens near the south of Fiale Caldera, so F3 may encounter a slightly deeper reservoir top than F1 and F2, perhaps from 1200-1800 m. For this reason the production casing shoe is proposed at a depth of 1200 m.

F4

Proposed well F4 is targeted from the Pad Southwest location at an elevation of 136 masl and deviated northeast 900 m across the southern rim of the caldera. The primary target is the zone of high seismicity beneath the central part of the caldera. This target is also overlain by multiple intersecting fracture sets and the southern ring fracture of the caldera.

F4 deviates away from A-5, but in the shallow section is less than 1 km from well A-5, therefore the shallow 180°C aquifer seen in A-5 may be present here as well.

The zone of high seismicity deepens near the south of Fiale Caldera, so F4 may encounter a slightly deeper reservoir top than the northern wells, perhaps from 1200-1800 m. For this reason the production casing shoe is proposed at a depth of 1200 m.

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Figure 21. Pressure vs depth for proposed wells given range of possible salinities.

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Figure 22. Fiale wells temperature prognosis. Note that wells may or may not encounter a shallow aquifer as depicted, and the depth of the reservoir top is expected to vary between the targets.

8.2. Drilling decision tree The order in which the wells are drilled is contingent on initial drilling results in order to incorporate new reservoir data into conceptual model, modify subsequent well targets, and increase the probability of success of the drilling program. Figure 23 diagrams an example decision tree for the first two wells. In this example F3 should be drilled first because it has the highest probability of success. If F3 is successful (high temperature and permeability), then F2 is drilled next and can be assumed to also have a high probability of success since it has similar target. If F2 is unsuccessful (low temperature and/or insufficient permeability), F1 should be drilled next to test a different kind of target.

The decision tree will be updated and revised as data gap information identified in Section 9.1 are incorporated into the model.

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Figure 23. Drilling decision tree

9. Summary A producible geothermal system has been proven by the three wells drilled in a small area in the southeastern Asal Rift, wells A-1, A-3, and A-6. The Fiale Caldera has not been drilled, but appears from surface geoscientific exploration to have the four resource requirements necessary for an electric-grade geothermal system: heat, water, permeability, and a cap. However, the nearest offset well, A-5, has a significant cold water interval incompatible with a producible geothermal system, and therefore drilling in the Fiale Caldera area will require a different approach. Wells near the caldera will need to be drilled directionally to the northeast or southwest in the encountered reservoir section (preferably from south to north). Drilling in the southern caldera may need to be deeper than targets to the north. Surveys to correct the data gap can increase the probability of drilling success.

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Works Cited Correia, H., Fouillac, C., Gerard, A., & Varet, J. (1985). The Asal Geothermal Field (Republic of

Djibouti). Geothermal Resources Council Transactions, Volume 9, 513-519.

Houmed, A. M., Haga, A. O., Abdilahi, S., & Veret, J. (2015). The Asal Geothermal Site, Djibouti Republic (Model Update, 2012). Proceedings World Geothermal Congress. Melbourne.

AQUATER. (1989). Djibouti geothermal exploration project: Republic of Djibouti. AQUATER.

Árnason, K. (2007). Geothermal exploration in the Asal Rift in Djibouti, East Africa. Reykjavic Energy.

Árnason, K., Björnsson, G., Flóvenz, Ó. G., & Haraldsson, E. H. (1988). Geothermal resistivity survey in the Asal Rift in Djibouti. UND-OPS.

Árnason, K., Eysteinsson, H., & Vilhjálmsson, A. M. (2008). The Asal geothermal field, Djibouti: Geophysical surface exploration 2007-2008. Iceland GeoSurvey.

Arnason, K., Karlsdottir, R., Eysteinsson, H., Flovenz, O., & Gudlaugsson, S. (2000). The resistivity structure of high temperature geothermal systems in Iceland. Kyushu-Tohoku.

Arnorsson, S., Gunnlaugsson, E., & Svavarsson, H. (1983). The Chemistry of Geothermal Waters in Iceland III. Chemical geothermometry in geothermal investigations. Geochimica et cosmochimica Acta, volume 47, 567-577.

Corti, G., & Dooley, T. P. (2015). Lithospheric-scale centrifuge models of pull-apart basins. Tectonophysics, Volume 664, 154-163.

Cumming, W., & Mackie, R. (2010). Resistivity imaging of geothermal resources using 1D, 2D, and 3D inversion and TDEM static shift correction illustrated by Glass Mountain case history. Proceedings World Geothermal Congress (pp. 25-29). Bali, Indonesia: World Geothermal Congress.

D'Amore, F., Daniele , G., & Abdallah, A. (1998). Geochemistry of the high-salinity geothermal field of Asal, Republic of Djibouti, Africa. Geothermics, Volume 27, Issue 2, 197-210.

D'Amore, F., Giusti, D., & Abdallah, A. (1998). Geochemestry of the High-Salinity Geothermal Field of Asal, Republic of Djibouti, Africa. Geothermics Vol 27, No. 2, 197-210.

Doubre, C. (2004). Structure et Mecanisms des Segments de Rift Volcano-Tectonics. Etudes de Rift Anciens (Ecosse, Islande) et d'un Rift Active (Asal-Ghoubbet). Phd Thesis.

Doubre, C., Manighetti, I., Dorbath, C., Dorbath, L., Bertil, D., & Delmond, J. (2007). Crustol Structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East

47

Africa): 2. Insights from the 23-year recording of seismicity since the last rifting event. journal of Geophysical Research, V. 112.

Doubre, C., Manighetti, I., Dorbath, C., Dorbath, L., Jacques, E., & Delmond, J. (2007). Crustal structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa): 1. Insights from a 5-month seismological experiment. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112.

Fournier, R., & Potter, R. (1982). A revised and expanded silica (quartz) geothermometer. Geothermal Resources Council Bulletin, Vol 11, 3-9.

Fournier, R., & Rowe, J. (1977). The solubility of amoprhous silica in water at high temperatures and high pressures. American Mineralogist, Vol 62, 1052-1056.

Gautason, B., Guðmundsson, Á., Hjartarson, H., Blischke, A., Mortensen, A. K., Ingimarsdóttir, A., . . . Egilson, Þ. (2010). Exploration Drilling in the Theistareykir High-Temperature Field, NE-Iceland: Stratigraphy, Alteration and Its Relationship to Temperature Structure. Proceedings World Geothermal Congress 2010.

Giggenbach, W. (1981). Geothermal mineral equilibria. Geochemica et Cosmochemica Acta, vol 45, 393-410.

Giggenbach, W. (1988). Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators. Geochemica Cosmochimica Acta, vol 52, 2749-2765.

Khodayar, M. (2008). Results of the 2007 surface geothermal exploration in the Asal Rift and Transform zones, Djibouti. Reykjavik.

Ngoc, P. V., Boyer, D., Le Mouël, J.-L., & Courtillot, V. (1981). Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment. Earth and Planetary Science Letters, 52(2), 372-380.

Pellerin, L., & Hohmann, G. W. (1990). Transient electromagnetic inversion: a rememdy for magnetotelluric static shifts. Geophysics, 55(9), 1242-1250.

Reykjavik Energy Invest. (2009). DRILLING AND TESTING OF GEOTHERMAL EXPLORATION WELLS IN THE ASSAL AREA, DJIBOUTI. Reykjavik.

Sakindi, G. (2015). Three-Dimensional Inversion of Magnetotelluric Data: Geological/Geothermal Interpretation of Asal Geothermal Field, Djibouti, PhD Thesis. University of Iceland.

Sanjuan, B., Michard, G., & Michard, A. (1990). Origine des substances dissoutes dans les eaux des sources thermales et des forages de la région Asal-Ghoubbet (République de

48

Djibouti). Journal of Volcanology and Geothermal Research, Volume 43, Issues 1–4, 333–352.

Sanjuan, B., Michard, G., & Michard, A. (1990). Origine des substances dissoutes dans les eaux des sources thermales et des forages de la région Asal-Ghoubbet (République de Djibouti). Journal of Volcanology and Geothermal Research, Volume 43, Issues 1-4, 333-352.

Truesdell, A. (p 1iii-1xixx). Summary of Section III Geochemical Techniques in Exploration, Section V.U.N. Symposium on the Development and Utilization of Geothermal Resources, Pisa, 1970. Geothermics, vol 1, Special Issue 2, 1970.

Ussher, H. G., Johnstone, R., & Anderson, E. (2000). Understanding the resistivities observed in geothermal systems. Proceedings World Geothermal Congress 2000. Kyushu-Tohoku.

Varet, J. (2014). Asal-Fialé geothermal field (Djibouti Republic): A new interpretation for a geothermal reservoir in an actively spreading rift segment . Proceedings 5th African Rift geothermal Conference. Arusha, Tanzania.

Virkir-Orkint Consulting Group Ltd. (1990). Djibouti: Geothermal Scaling and Corrosion Study: Final Report. Reykjavik.