1 Seismic Reflection Contribution to the Study of the Jerid Complexe Terminal Aquifer ( Tunisia )

According to Kilan (1931) the term « Continental Terminal » concerns the sandy and clayey continental Formations dated Miocene-Pliocene. In 1966, Bel & Demargne highlighted a vertical communication between the aquifer contained in these Formations and the Eocene, Senonian and Turonian aquifers. Consequently, the Continetal Terminal is redefined as a multi-layered aquifer which extends from the Late Cretaceous to the Miocene –Pliocene. Frequently, this hydrogeological system is designated by the term «Complexe Terminal» proposed by Bel & Cuche in 1969.


Introduction
The North African Sahara is characterized by the immense aquifer system of the «Complexe Terminal» (Fig. 1) covering 655.000Km 2 of the Algerian-Tunisian-Libyan domain (UNESCO, 1972c;OSS, 2003).The aquifer thickness is on average 340 m and its reserves are estimated of 11.000 10 9 m 3 (Ould Baba Sy, 2005).
The Jerid area (Fig. 2), located in the Southwestern Tunisia is an arid region where the pluviometry doesn't exceed 200 mm/year.The strong needs in water supply for domestic needs and irrigation render necessary the exploitation of the underground water reserves.
The «Complexe terminal» appears as a potential resource able to provide interesting flows.However, former geological and hydrogeological studies in this region were not sufficient to propose zones and strategies for the exploitation of this resource.Tectonic and sedimentary phenomena and their impact on the aquifer functioning had not been elucidated.The aim of this study is therefore to precise the deposits structures in the Jerid area in view to guarantee a good knowledge of the «Complexe Terminal» geometry and an accurate estimation of the relations between the different hydrogeological units.
The Late Cretaceous is represented by the lithostratigraphic Formations: Zebbag, Aleg and Berda.
The Zebbag Formation dated Late Albian -Cenomanian -Turonian is recognized from deep well data (Fig. 3).It is subdivided in three members.The lower member (Late Albian -Cenomanian), thick on average 150 m, is constituted by dolomites and dolomitic limestones.The Middle member (Cenomanian) is composed of marls, clays, gypsums and thin dolomitic limestones beds.Its power exceeds 600 m in Mergueb and Tazrarit.It varies between 300 and 400 m in the other localities.The upper member (Turonian) is formed of fractured limestones and dolomitic limestones.This member with a thickness ranging from 130 to 300 m is an excellent stratigraphic marker.It corresponds to the «Gattar bar» described in the center and southern Tunisia (Burollet, 1956;Fournié, 1978;Boltenhagen, 1985;M'Rabet, 1987;Abdallah, 1987;Chaabani, 1995;Zouari et al., 1990;Negra, 1994;Ben Youssef, 1999).
The Aleg Formation attributed to the Lower Senonian is represented by clays and marls with limestone and gypsum intercalations.It reveals different thickness: 400 to 470 m in Mergueb, Hezoua and Nefta, 310 to 350 in Degache, Tazrarit and Tozeur and 220 to 280 m in Mahassen, Mzaraa and Hamma.The Aleg Formation constitutes the Sidi Bouhlel anticline core (Fakraoui, 1990).
The Beglia Formation (Miocene), thick on average 100 m, is made up of fine to coarse sands with thin clayey intercalations.At Sidi Bouhlel Jebel, this Formation is underlain by the Senonian deposits (Fig. 4).
The Segui Formation (Miocene-Pliocene) is essentially clayey.It is enriched in sands and this thickness decreases towards the eastern part of the Jerid.Its power attains 500m in Mzaraa well implanted in the western part (Fig. 3).

Hydrogeological context
On the basis of the preceding descriptions, the fractured limestones and dolomitic limestones of the Upper member of Zebbag Formation (Turonian), the friable limestones of Berda Formation (Upper Senonian) and the sands of Beglia Formation (Miocene) are the Jerid «Complexe Terminal» reservoirs (Fig. 5).The clayey and marly deposits of the Lower and Middle members of Zebbag Formation, Aleg Formation, El Haria Formation and Segui Formation are aquicludes (Fig. 5).In the Jerid area, the catchment of the «Complexe Terminal» groundwater is restricted to the Beglia Formation.Piezometric measures (General Direction of Water Resources, 2008), contoured in the form of map (Fig. 6) show a piezometry decrease towards the nord.It is -7/0 sea at Hezoua.It drops to -50 m/0 sea in the southern part of Gharsa Chott.Therefore, south to north is the main groundwater flow direction.
It is interesting to note the packed piezometric contours at the north flank of the Draa Jerid structure; the piezometry falls abruptly to -40 m/0 sea.The water salinity measured for eleven groundwater samples collected from Beglia aquifer ranges from 2, 2 g/l to 5.9 g/l.The salinity map (Fig. 7) reveals a clear increase of values from south to north which coincides with the groundwater flow direction.Additionally, this map exposes high values in the eastern part of the Jerid area, at Sidi Bouhlel Jebel: 3.7 g/l in Hamma well, 4.5 g/l in Kriz well and 5.9 g/l in Tazrarit well.Nearby, Ceddada well expresses a value of 2g/l.This sudden salinity change may indicate an obstructed lateral communication between the aquifers.

Data and methodology
The present study is based on seismic reflection sections associated to deep wells (Fig. 8).These data are provided by the "General Direction of Water Resources" in Tunisia.The seismic reflection (Lavergne, 1986) is a method of exploration geophysics which estimates the properties of the Earth's subsurface from reflected seismic waves (Cagniard , 1962 ;Telford et al., 1976).It requires a controlled seismic source of energy, such as seismic vibrator (Vibroseis) or dynamite explosion.Seismic waves will be reflected when they encounter a boundary between two different materials with different acoustic impedances.They are detected using seismometers ; On land, the typical seismometer is the geophone.In water, hydrophone is used.
In this study, 31 seismic profiles (Fig. 8) covering the Jerid area are interpreted .Realized by oil industry (Tab.1) during four seismic surveys using different parameters of acquisition and processing (Tab.1),these profiles show variable quality.The Miocene sands (Beglia Formation), the Upper Senonian (Berda Formation) and the Turonian (Upper Zebbag Formation) limestones which are the main reservoirs of the «Complexe Terminal» groundwater in the Jerid area constitute good seismic markers.The abrupt lithological change between these deposits and the clays and marly Formations originates a strong acoustic impedance contrast.

Survey
Seismic calibration (Fig. 9) was performed using GNT-1 and to a lesser extent NF-1 petroleum wells.Seismic horizons and facies have been tied using the relation time-depth.
The different reflectors corresponding to the «Complexe Terminal » aquifers and aquicludes have been identified and picked on seismic profiles all over the area.
The interpolation between the interpreted seismic sections leads to the isochron maps construction.These maps display the tectonic structures which affect the « Jerid Complexe Terminal » aquifer.Their analysis specifies the reservoirs geometry.
The geoseismic cross sections, integrating wells data and seismic interpretation, clarify the relations between the different hydrogeological units.They allow the comprehension of the Jerid« Complexe Terminal» aquifer functioning.

Results and discussions
The isochron map (Fig. 10) of the botton of the Beglia Formation (Miocene) reflects the main deformations characterizing the «Complexe Terminal» multilayered aquifer in the Jerid area.In the center, it reveals narrow anticlinals, overfolded towards the south (Fig. 10).These structures situated between 200 and 300 ms extend from Sidi Bouhlel Jebel in the east to Hezoua region in the west.They allow the elongation of the Chotts fold belt, most southern Atlassic structures, until the Tunisian-Algerian boundary.
Major reverse faults oriented E-W to NE-SW, mark the limit between anticlinal structures and two large syncline basins highlighted along the northwestern border of Jerid Chott and in the south of Gharsa Chott (Fig. 11), where the depth of the Beglia aquifer bottom reach respectively 800 and 600 ms.
The northern basin named «Jerid basin» widens and deepens towards the West (Fig. 10).It is limited by a raised structure (400 to 500 ms) characterizing the southern part of Gharsa Chott.Towards the north, this structure evolves to a depression deep of 1600 ms.
In this study, the majority of tectonic accidents are exposed by N-S and NW-SE seismic sections indicating the predominance of E-W and NE-SW directions.Therefore, the resulting structural map of the Jerid area (Fig. 10) reveals that in addition to the tectonic traits described on the geological map, exist in subsurface more important structures which must be taken in consideration for the hydrogeological system characterization.
The obtained results highlighted the tectonics influence on the« Complexe Terminal» aquifer geometry; the Tortonian folding, the reverse faulting during the post-Villafranchian compressive phase compartmentalized the aquifer in the form of tilted blocks.
The geosismic cross sections show the variability of this structure implication.
In Mzaraa-Hezoua sector, western part of the Jerid area , the tectonic deformations affecting the «Complexe Terminal» series control the aquifers depth without influencing the groundwater circulation.In fact, the geoseismic cross section corresponding to the profile E1 (Fig. 12) reveals that the reservoirs formations in Hezoua anticlinal are in communication with their equivalents in Jerid basin, at the north and in Chott Jerid, at the south.The Beglia aquifer expresses similar chemical (chemical composition) and isotopic ( 14 C, 2 H, 18 O tenor) characteristics in Hezoua and Mzaraa regions (Kamel et al., 2005) reflecting this lateral communication.
The geoseismic cross section corresponding to the profile V (Fig. 13) describes the relations between the different hydrogeological units in Tozeur -Mergueb sector, central part of the Jerid area.It is controlled by Tozeur and Mergueb deep wells and Neflayett and Jhim wells which exploit the Beglia aquifer.
The geoseismic cross section shows an important variation of the aquifers depth between the exposed geological structures.In Draa Jerid anticlinal, the Beglia Formation outcrops  The Berda Formation is encountered at 300 m and the Upper Zebbag Formation is recognized at 845 m.In Jerid basin, these reservoirs are respectively reached at 220 m, 460 m and 1180 m.
Additionnaly, this cross section reveals an obstructed communication between the aquifers Formations.In Draa Jerid anticlinal, the Beglia sandy reservoir is isolated.It is wedged between the Segui clays of the Jerid basin and those deposed in the northern edge of the Jerid Chott.
In the same structure (Draa Jerid), the fractured limestones of Berda aquifer are in contact with their equivalents of the Jerid basin and with the Beglia aquifer situated in the Jerid Chott.The Upper Zebbag aquifer collides with the clays and marls of Aleg Formation in the north and in the south.
In the Eastern part of Jerid area, the seismic profiles L and L' interpretation and the data of Tazrarit and Ceddada wells allow the « Complexe Terminal» aquifer characterization.The established geoseismic cross section (Fig. 14) provides informations about the geometry of the aquifer and the groundwater flow.
At Sidi Bouhlel Jebel, the Beglia reservoir exists only in the southern flank , where it outcrops.At the northern edge of Jerid Chott, this reservoir is attained at 600 m.In the basin separating Sidi Bouhlel and Gantass anticlinals, the Beglia Formation depth and thickness decrease towards the south.
Equally, in this sector, the tectonic deformations affecting the «Complexe Terminal» series influence the groundwater circulation, the Beglia aquifer of Sidi Bouhlel anticlinal is opposite the Segui clays deposed in the synclinal basin at the south.This obstructed hydraulic communication between the folded structures explains the significant difference between the salinity of Beglia aquifer at Ceddada well: 2g/l and Tazrarit well: 5.9 g/l (Fig. 7).
The Berda aquifer is exposed at Sidi Bouhlel Jebel.In the north it is in contact with its equivalent which shows an important thickness.In the south it is against the clayey Segui Formation.
A vertical communication between Beglia and Berda aquifers is noticed at the northern edge of Jerid Chott and at Sidi Bouhlel anticlinal.At Gantass-Sidi Bouhlel basin, they are separated by the Eocene-Paleocene deposits, pinched out nearby Sidi Bouhlel Jebel.
At Sidi Bouhlel anticlinal are located the most raised Upper Zebbag aquifer of the Jerid area (300 m).This aquifer is blocked between the clays and marls of Aleg Formation.

Conclusion
This study based on seismic reflection sections and wells data display the tectonic deformations which affect the multilayered «Complexe Terminal» aquifer in the Jerid area (Fig. 15).
The Tortonian folding, the reverse faulting during the post-Villafranchian compressive phase compartmentalized the aquifer in raised and subsided blocks.The geoseismic cross sections reveal that this structure has variable implications; except Hezoua -Mzaraa sector where the reservoirs are in communication, it influences the depth of permeable formations and the circulation of groundwater These results should be useful for choosing the best sites for the «Complexe Terminal» aquifer exploitation in the Jerid area.
Additionally, the present study shows the interest of the seismic reflection method for the hydrogeological systems comprehension when the well data are limited for a precise characterization.Such prospection appeared particularly suitable in this study in view of the great depth of the aquifer and the importance of the tectonic structures which are not easily detectable by the simple well correlations.

Fig. 4 .
Fig. 4. Geological cross-section at Sidi Bouhlel Jebel.The Paleocene is made up of clays and marls.The Eocene is represented by limestones, phosphates and marls.The Paleocene -Eocene sedimentation is absent at Sidi Bouhlel Jebel.It is recognized in Hezoua, Mzaraa, Nefta, Mergueb and Tozeur and its thickness doesn't reach 300 m.The Paleocene -Eocene exposures characterize Metlaoui chain, northern border of the Jerid.

Fig. 5 .
Fig. 5. Deep wells correlation.The deep wells correlation (Fig.5) shows notable variation of the Complexe Terminal aquifer depth.It is characterized by raised and subsided zones (Fig.5).The Beglia Formation, the most superficial reservoir, is encountered at 570 m in Mzaraa well, at 100 m in Hezoua well and at 220 m in Mergueb well.It outcrops in many localities along Draa Jerid and Sidi Bouhlel structures.The Berda Formation reservoir is reached at 790 m in Mzaraa and about 350 m in Hezoua, Nefta and Tozeur.The Upper Zebbag Formation, the deepest reservoir, is encountered at 370 m in Mahassen and Degache.Its depth exceeds 1000 m in Hezoua, Mzaraa, Nefta and Mergueb. 6

Fig. 14 .
Fig. 14.Geoseismic cross section corresponding to the profiles L and L'.