GEOTHERMAL PROSPECTS I N the DJIBOUTI REPUBLIC Assal Other

227

Proc. 6th Geothermal Workshop 1984

GEOTHERMAL PROSPECTS IN THE DJIBOUTI REPUBLIC

Abdou

Geothermal Institute University of

ABSTRACT Tectonism, volcanism and geothermal zones in Djibouti

Four major areas with geothermal prospects have been The Separation of the continent and the found i n the Djibouti Republic. One of the areas is Arabic Peninsula started in the Eocene which created the Assal an active rift associated with both the Red Sea and of Men and its west- present- day volcanism; the rift constitutes the west- extension, the of Tadjoura (see Fig 1). ward extension of the Gulf of Tadjoura Rift. The This separation can be explained i n terms of a trans- other major areas are associated depressions lation and an anti- clockwise rotation of the Arabic bounded by active faults parallel to the Assal Rift. Peninsula with respect to the African continent. The direction of tectonic features caused by separation The first exploratory well drilled in the Assal Rift is for the Red Sea and about E-W to was a discovery well (heal- 1) which produced hot for the Gulf of Men and the Gulf of Tadjoura (see brine. It is likely that other brine systems also Fig 1). The tectonic features caused by the separ- occur in the Assal Rift. Deep exploration drilling ation of the East Africa Rift Valley is about will commence soon in another geothermal area, the SSW. All these directions can be recognised in the Hanle area, to find out whether hot water tectonic map of Djibouti (see Fig 1 and Fig 2) brine) systems exist which might be more suitable for although a general NW-SE strike direction appears to electric power generation. be dominant i n the W part of the country where most of the geothermal prospects are located (see Fig 3).

INTRODUCTION Analysis of seismic data indicate that the crust beneath Djibouti is thin. has intruded the Djibouti Republic (23,000 is situated in the NE crust along the axis of separation in the Gulf of part of Africa on the Red Sea; it has a common border Tadjoura and also. further to the vest. The most with Ethiopia and the Somali Democratic Republic. It recent basaltic fissure eruption occurred at has a population of about 0.45 million. Ardoukoba (near Lake Assal) i n November 1978.

The country has mountainous relief, two thirds are There are at least larger and smaller areas with covered by volcanic rocks and Quaternary sediments. geothermal prospects, namely: Volcanic rocks are mainly basalts together with rhyolitic rocks, pyroclastics, and trachytes. Only 1. Lake Assal area, (with the separate area of in the southern part of the country, near Ali Sabieh, Allol- Sakolol in the and the Goubbet one finds outcrops of sedimentary rocks of Jurassic area in the E), (Kimmeridgian) and probably Cretaceous These 2. Gaggade area, rocks consist of compact sandstones, which contain 3. Hanle area and about 90% of quartz, massive limestones, silicified 4. Lake Abbe area. limestones and Areas with smaller geothermal manifestations are: The tectonic setting of the country is affected by three major tectonic structures associated with: Arta, 6. Dorra active faulting (separation) in the Gulf of Aden, 7. Tadjoura, and 2) active ocean spreading of the Red Sea, and 8. Obock. 3) active faulting (separation) in the Afar Depression, an extension of the East African Rift. The four major areas are associated with depressions illed by recent sediments and salt deposits. Common surface manifestations are hot springs and fumaroles; the intensity of visible activity is indicated in Table 1. Most geothermal areas have *Present address: I.S.E.R.S.T. been explored either by reconnaissance surveys or by P 0 Box 486, more detailed surveys. The present state of DJIBOUTI exploration is summarised i n Table 1. Geothermal exploration by the Bureau de Recherches et France) was started in 1970 i n the Lake Assal area. A reconnaissance survey of all other thermal prospects listed in Table 1 was under- taken in 1980 by AQUATER (Italy) and the Djibouti Scientific Research Institute (I.S.E.R.S.T.). 228

1: Tectonic structure of the Gulf of Tadjoura and surroundings

Fig. 2: - of Djibouti (taken from H.J. (Ed.) 229

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Table Present stage of exploration and intensity of surface manifestations of geothermal prospects in Djibouti (the degree of coverage or detail of each survey is indicated by crosses)

EXPLORATION STAGE SURFACE MANIFESTATIONS Explor. Hot Area Geology Geophys. Springs

la Lake Assal + Allol- Sakalol + + N Coubbet + + 2 Caggade Area + 3 Hanle + 4 Lake Abbe 5 Arta + 6 Dorra + + + 7 Cadjoura + + + + 8 Obock +

I n 1975 deep exploratory wells were drilled around the lake. Lake Assal covers about 50 the the Lake Assal area vhich will be discussed i n more brine making up the lake is almost saturated vith detail below. An exploratory hole of intermediate respect to minerals (mainly the maximum depth depth was drilled in 1981182 i n the Hanle Area of the lake A salt layer, about 60m thick, where deep drilling will commence i n November 1984. lies to the of the lake and covers an area of about

The Lake Assal - N Goubbet geothermal province 20 to thick layer of gypsum of pure quality occurs at the margin of the salt layer and is As can be seen from Fig the Lake Assal area, suitable for exploitation. together with its extension, Allol- Sakolol, and E extension, the Coubbet at Kharab Bay, forms the Geothermal of t h e Lake Assal area was largest geothermal prospect area in Djibouti. The started 1970 by BRCM and lead to the drilling of 2 area is bisected by the Assal Rift whose axis shifted deep exploration wells i n 1975. Well Assal-1 vas a from an E-W trend in the North Coubbet area to a discovery vell, the important data for both wells are trend in the Lake Assal area. The hssal Rift i n Table 2. occurs within a thick sequence of Pliocene Trapp basalts vhich have been cracked in the axial part of the rift vhich is also associated vith active volcanism and vith seismicity. Arching of the rift Table 2: Summary of exploratory wells and has taken place, presumably as a result of shallov Assal- 2 magma intrusion. The Assal Rift shows similarities to features commonly observed over oceanic spreading centres. Total Max. Well depth temp. Salinity The rift starts in the shallov, 20 long Coubbet Kharab Bay and svings to the towards Lake Assal which lies a t present below sea level. Lake Assal is separated from Coubbet Bay by a km long 1146 258 135 128 silt strip vith recent volcanism (Ardoukoba eruption in 1978). 1554 0

Assal Lake is the lowest point of the African continent. It is now fed by sea water which moves towards the lake along fractures in the axial strip Assal- 1 well was tested during one week i n 1975, the of the Assal Rift. This infiltration, balances in mass flov of 135 consisted of a mixture of steam part the large evaporation taking and brine; the steam rate was equivalent to 1.5 place a t Lake Some groundwater might also to 2 Further in the Lake Assal prospect drain into Lake Assal; it has been postulated that vas started again in 1980 by BRGM vith support most groundwater to the W of Lake Assal is supplied from by infiltration from the Awash River vhich terminates in Lake Abbe. Recharge of most major thermal pros- The prospect has been outlined in great detail pects might be associated with this 1983) by geophysical methods regional groundwater flow from Lake Abbe t o (electric, magnetotellurlc, gravity and magnetic the Tadjoura. Infiltration by meteoric surveys). It appears that both deep wells were waters is probably nil, although some n e t infil- drilled near the edge of a geothermal reservoir with tration might occur in the mountainous areas further lying inside and outside the reser- to the north. voir. The power potential of the field vas estimated to be of the order of 300 if thermal fluids can The Assal Region is potentially the most important be economically. Experience in the €or geothermal development since a geothermal Area (USA) has shown that production hot reservoir been discovered here, but brines with a mineralisation similar to that of resources represented by the be maintained if proper allowance for and is 230

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4 4 . Dorra

Thermal manifestations Areas with geothermal prospects

Fig 3: Areas with geothermal prospects in the Republic of Djibouti 1

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4 .

4 a4 ,,DJIBOUTI

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Dik +++

x + Scale

EOEND 4 high enthalpy agriculture Geothermal high1st enthalpypriority) Resources (2nd priority) A Centres tourism enthalpy industry Fig Location of Geothermal resources and load centres

Energy demand and possible role of geothermal REFERENCES

Electricity in Djibouti is presently Correia, Demange, R.; provided entirely by diesel power plants, the Varet, (1983): Champ production was 119 in The country Rapport du BRCM, CTH, pp. depends on imports to present energy demand; the fuel bill for Varet, (Ed.) de electricity was in Central et 000). Centre predicted that the demand for electricity will reach National de La Recherche Scientifique (France) et about by the year 1990, and 500 by deile Richerche (Italy). year Cost of imported fuel will therefore increase and provide an for the country .