FIRST ASSESSMENT OF GEOTHERMAL RESSOURCES IN MOROCCO
Abdelkrim Rimi Department of Earth’s Physics
Scientific Institute
Ibn Battouta Avenue, B.P. 703 RABAT 10106 Morocco
Tel: 212-7-774543, Fax: 212-7-774540 Email: rimi@israbat.ac.ma Key Words: Heat Flow, low grade energy, springs, Morocco
ABSTRACT
The Moroccan underground contain geothermal potentialities, mainly low grade energy. The maps of heat flow density and the temperatures distribution down to 3000 m depth, are drawn and area of geothermal resources are defined by the estimation of the heat in place. The more promising regions are northeastern Morocco and the sedimentary basins in the Sahara.
INTRODUCTION
Being entirely dependant as regards energy, an effort is made by Moroccan authorities, since 1970, to develop geothermal research. The purpose of this work is the first reliable assessement, of the geothermal potential covering the whole country. Results are obtained on the base of sub-surface temperatures determinations, and the hydrogeological characteristics of the aquifers containing hot water and which can constitute objectives in geothermics.
1. BASIC DATA
The study of the geothermal field components in Morocco (heat flow density, thermal gradient and subsurface temperatures) was performed using, on the one hand thermal profiles and measured conductivities in shallow equilibrium boreholes, and on the other hand by a simultaneous inversion method (Vasseur et al. 1984) of corrected BHT and estimated thermal conductivities in deep oil wells (Rimi, 1999).
The hydrogeoligical information and the physico-chemical composition of springs are collected, after a significant work of analysis and checking, in the published technical reports.
2. GEOTHERAL FIELD
2.1 Heat flow density trends and geologicla framework
The Precambrian Anti Atlas, belonging to the Northwest African shield is characterized by a flow of 40 ±
5mWm-2 and a gradient of 13.5 ± 2 °Ckm-1, which are
comparable to those observed in the shield itself and the Precambrian rocks worldwide; the stable Mesetas, with Paleozoic and Mesozoic basement, are characterized by an average flow and gradient of about 61 mWm-2 and 29 °Ckm-1
respectively; the northeast of Morocco from the northern Middle Atlas, the eastern Rif until Algeria show a high thermal anomaly with a heat flow and thermal gradients ranges of 80-110 mWm-2 and 30-45 °Ckm-1 respectively. In the Saharan
provinces, the coastal basins present a thermal gradient of 29 ± 7 °Ckm-1 and a heat flow of 81 ± 14 mWm-2 ; finally the
Tindouf basin is abnormally hot with a flow and a gradient of 84 ± 17 mWm-2 and 32 ± 10 ° Ckm-1 respectively.
Two major tendencies are therefore defined on each side of the “south Atlas fault” (figure 1). In the north we observe an
increasing HFD from the northern Middle Atlas, the Rif and the northeastern Morocco toward the Alboran Sea, the Southeast of Spain and western Algeria. These two main geothermal structures could be the result of the same phenomenon of increasing the mantle heat flow in relation with an extensional tectonic regime over northwest Africa and the Betic-Rif orocline, since the Cretaceous rifting and the Alpine orogenesis.
2.2. Subsurface temperatures
The heat flow calculation procedure also provides a posteriori temperatures, interpolated at 500, 1000, 2000, 3000 and 4000 m depth with their standard deviations. Thus the influence of the variations of thermal conductivity, due to lithological contrasts on the thermal gradient are taken into account. The figure 2 presents the computed results as contour maps
2.3. Thermal springs
Among the inventoried hot springs in Morocco, those of north-east are clearly related to the recent volcanic and tectonic activities with a rise in mantellic flow (Rimi, 1999). In Western Rif, the hot temperature and the Artesian rise of thermal springs are ascribed to deep circulation along fracture systems in the area. In central Morocco, the Lalla Haya spring (T=42 °C) is from a structural point of view related to the granit emplacement in the region. The Eastern High Atlas conceals thermomineral springs feeding in calcareous Lias. The hottest one (T>52 °C with a flow rate of 5 litres/s) is located at Foum Zaâbel in the eastern High Atlas. Finally the two springs the more hot in the Southern provinces, Abeino (T=42 °C) and Timoulay (T=40 °C) are located in the hyperthermale zone extending between the Canary Islands and the Tindouf basin
3. MAP OF THE GEOTHERMAL AREA
The variety of the tectonic and lithostratigraphic structures in Morocco created various hydrogeothermal systems whose physical and lithological properties are heterogeneous. For a preliminary description of the Moroccan geothermal zones, two criteria are considered: 1- structural, lithological and hydrogeological context of the rocks to establish the existence or the absence of aquifers. 2- the water temperature up to 3 km.
Regional studies (Lahrach, 1994; Benabidate, 1994; Boukdir, 1994; Zaghloule, 1994 and Bahaj, 1997) gather basic information on the aquifers: lithology, extent, depth, porosity, mode of feeding, gradient and water flow rate. However characteristics such as the permeability, the transmissivity and the salinity are often lacking. According to theses works, the aquifers being able to constitute a geothermal resource in Morocco are of two types: 1) In the Tertiary and Quaternary deposits, the driven back water of porous spaces during the
Proceedings World Geothermal Congress 2000 Kyushu - Tohoku, Japan, May 28 - June 10, 2000
compaction could constitute, up to 700 m of depth, good aquifers of low temperature (50-60 °C). 2) the typical water circulation system for the pre-tertiary aquifers, 1000-4000 m depth, remains that guided by the topographic variations. Meteoric water percolates in the topographic heights and discharges in the zones of low piezometric level, principally in carbonates of the major mesozoïc basins. If the permeabilities are favorable, water can penetrate rather deeply and to be however gradually heated (the average temperature is 100°C). The map in Figure 3 synthesizes the levels of Moroccan geothermal potential as follows: class A characterizes the zone extending since the southern Rif and northern Middle Atlas to eastern Morocco. The temperature of the aquifers existing up to 3000 m of depth can reach 120 °C. The zones of class B are of an average geothermal vocation, the temperature of the aquifers may exceed 100 °C but the aquifers are of moderate importance. In this category, we have the Saharan basins and the southern Rif. The zones of class C are sedimentary basins of great extents where the temperature varies between 30 and 90 °C, the depth to which one can obtain hot water can be so great that this class C cannot be considered like potential resource in the immediate future. Finally the class D shows areas without geothermal possibilities.
3.1. Major hydrogeothermal fields in Morocco Eastern Rif and Eastern Morocco
By the number and the importance of the superficial thermal shows (hot springs, recent volcanic and tectonic activities), in this region is the domain which the most drew the attention. A study of french BRGM (Alsac et al. 1969) concluded that north-eastern Morocco does not constitute really a zone with significant potential. Thereafter a second study entrusted again to the BRGM (Cornet et al, 1974) underestimated the possibilities of Eastern Rif. These two studies could not be conclusive because their thermometries carried out in the first 60 meters of piezometers are strongly disturbed and the HFD could not be measured without thermal conductivity data, in addition their analyses had not integrated deep geophysical data (seismicity, focal mechanisms, electrical conductivity, volcanic xenoliths,...). The positive HFD anomaly in Rif and Eastern Morocco is linked to a lithospheric tectono-magmatic process (Rimi et al, 1998). The sedimentary formations in Eastern Morocco, especially Liasic carbonates which can reach a thickness of 500 m, constitute the most significant aquifer in the area with a low to middle grade geothermal potential. The isobaths of Lias vary from few meters in the south to 1000 m northwards. The salinity of varies with the sense of water movement, 0.5-1% in the south to almost 3% in north. The temperatures of about 60 °C can be reached at 600 m and one could obtain 100 °C at 1000 to 1500 m depth. The first argument which cancelled the conclusions of the BRGM in Eastern Rif was reported by a geothermal anomaly met in a mining borehole drilled in rhyolitic and andesitic series (Kariat Arekman, 35.11N, 2.74W); figure 4). The well, of 680 m depth, revealed from 450 m depth a salted artesian water (20 to 25 g/l and a flow rate of 1.4 to 2 l/s) with a temperature of 42 °C at the discharge and more than 90 °C at the bottom (Demble & Lopez, 1977). The estimated HFD exceeds 200 mW.m-2 (Rimi
& Lucazeau, 1991).
Coastal basins in the Sahara
Belonging to the continental Atlantic margin, these Saharan basins represent a very wide hyperthermal field. The Moroccan
margin consists of mesozoïco-tertiary " post-rift "carbonated sediments, of which the thickness exceeds 10 km. The long wavelength HFD anomalies and the temperatures up to 3000 m depth, from the Canary Islands to the Tindouf basin, show that these Saharan basins are abnormally hot. Oil wells had met hot water and vapor in the liasic aquifer (Hilali and Bouhaouli, 1977). On the other hand, the sub-surface presence in the Moroccan Sahara, of the lower Cretaceous sandy aquifer known as the" Continental intercalaire" (Guiraud, 1988), encourages the exploration of this thick (> 1000 m) resource of good hydraulic properties.
Western Meseta
The western Meseta is a vast basin of 16000 km² consisted of schisto-quartzitic paleozoic series. Hercynien granit and Quaternary basaltic volcanism are reheating lithospheric processes in this zone. Indeed in two wells in the north of this area (Rimi et al, 1998), which attain granitic basement, at 530 to 790 m depth the temperature is of 50 to 60 °C, giving an average thermal gradient of 48 °C.km-1. On the other hand, the
heat production could constitute a good resource in this field, however we do not yet know the contents nor the in-depth distribution of these natural radioelements (U, Th, K).
3.2. Heat in place in the two hot basins
Two basins are chosen in the main geothermal area to give an idea on their potetial energy. Available heat in the selected basins (Table 1), is evaluated by (Muffer & Cataldi, 1978) : H0 = ((1-ϕ)ρmcm + ϕρwcw).(Tt – T0)A.∆z
where H0 = heat of place (J), ϕ = effective porosity
(dimensionless), ρm = average density of rock column (kg/m3),
c = mean specific heat capacity (J/(kg/K)), Tt = temperature at the top of the aquifer (°C), T0 = temperature at the earth’s
surface (°C), A = surface of considered area (m2), ∆z = thickness of the aquifer (m), m = rock matrix, w = water. The porosity values ϕ are deduced from a compaction law of limestones, established in the coastal mesozoïc basin in southern Morocco (Medina & Rimi, 1992): ϕ = 0.64 exp (-1.2 Z), Z in km.
4. CONCLUSION
The use of heat flow data has indicated promising geothermal potentialities in the northeastern Morocco and the sedimentary basins of the Sahara. The hot water may be recovered essentially in Lias series, for geothermal purposes such as space heating. However, the problem with the Liasic aquifers, is that they are formed by intrinsically compact limestones and dolomites, but which can contain water in fractures and even of karsts systems. In some regions such the Rif (Ostapenko, 1985), the reservoirs are located in fractured rocks. The formations are thus transmissives but not very accumulatives. On the other hand, continuous shallow aquifers, even if they are not as hot as Lias, considering their good hydraulic properties such as consolidated sandstone may furnish temperatures less than 50°C which be appropriate for soil heating in greenhouses or fish ponds.
The quantitative use of chimical geothermometers (whose SiO2 silica is the most significant) requires precautions: a
water-rock equilibrium within the geothermal system, and no precipitation of silica, no mixture with nonthermal water during the migration of water towards surface. The temperature – depth profiles, are a diagnosis of the heat transfer processes in subsurface. Therefore, detailed surveys, with a spacing of 2-3 km, are necessary for a fine investigation of the local thermal field. The assessment of these resources could be better specified by combining with other geophysical (geoelectricity and magnetotellury), geological and geochemical (geothermometry and isotopes) determinations.
Aknowledgements
This work has beneficiated partially of the support of the PARS project n° SDU 26 ‘Les Sources thermales du nord du Maroc-Contrôle structural et géothermique-Application pour la recherche de l’eau’.
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Rimi A., Chalouan A. and Bahi L. (1998). Heat flow in the westernmost part of the Alpine Mediterranean system (the Rif, Morocco), Tectonophysics, 285: 135-146 Vasseur, G., Lucazeau, F. and Bayer, R. (1985). The problem
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-16 -14 -12 -10 -8 -6 -4 -2 22 24 26 28 30 32 34 30 50 70 90 110 130 Algeri a Mauritania
Atl
an
tic
Oc
ea
n
mW/m²Heat Flow Density in Morocco (mW/m²) Tindouf Bassin Coas tal S ahar an B asin s Meseta High A tlas Anti Atlas Rif Mid dle
Figure 1 : Heat flow density contours surimposed on the structural sketch of Morocco
-16 -14 -12 -10 -8 -6 -4 -2 22 24 26 28 30 32 34 (a) Temperature at 500 m depth (°C) -16 -14 -12 -10 -8 -6 -4 -2 22 24 26 28 30 32 34 (b) Temperature at 1000 m depth (°C) -16 -14 -12 -10 -8 -6 -4 -2 22 24 26 28 30 32 34 (d) Temperature at 3000 m depth (°C) -16 -14 -12 -10 -8 -6 -4 -2 22 24 26 28 30 32 34 (c) Temperature at 2000 m depth (°C)
Figure 2: Temperature contours (°C) at 0.5 km beneath the surface (a), at 1 km depth (b), at 2 km depth (c) and at 3 km depth (d).
40 60Temperature (°C)80 100 0 100 200 300 400 500 600 700 D ept h ( m )
Kariat Arekman borehole (35.11N; 2.74W) si lt y cl ay sa nd y cl ay ma rl , san d ma rl m arl an d vo lc an ic tu ff rh yo lite Quat erna ry Villa franc hian Pont o-pl ioce ne Mes sini an Pr e-Mes sinian 0 100 200 300 Geothermal gradient (°C/km) Stratigraphy Hydrogeology <--- saturation level Dry residue.: 2 to 16g/l cold and salted water
<--- Confined water cold and salted water
<--- hot and artesian water salt content: 20 to 25g/l PH: 6.5
flow rate: 1 à 2l/s
Figure 4: Thermal and lithological profils of Kariat Arekman borehole
Table 1 : Heat in place in the two main geothermal potential area of Morocco; O-T b: Oujda Taourirt basin represents a N45 furrow between the Middle Atlas and eastern Rif (north eastern Morocco) ; Tarfaya basin belongs to the Coastal Saharan Basins
Region O-T b Tarfaya basin
Reservoir Lias Lias
Lithology Limestone-dolomite Limestone-dolomite
Depth (m) 10003687-4091 Salinity (g/l) 0.1-4l NaCl/ 7,5 Gradient (°C/km) 30-40 20-30 φφφφ 0,3 0,1 ρ ρ ρ ρ (kg/m3) 2,6 2,6 Cw (J/kg/K) 4186 4186 Tt (°C) 50-60 100-140 z (m) 500 400 Α ( Α ( Α ( Α ( m²) 2600 32000 Η Η Η Η0000(J) 1.3e+17 3.2e+18
