contribution to the hydrogeological study of limestone...

CONTRIBUTION TO THE HYDROGEOLOGICAL STUDY OF LIMESTONE TERRAINS IN U.A.R.

ISMAIL MAHMOUD EL RAMLY Senior Hydrogeologist

Hydrogeology Department - Desert Institute Mataria, Cairo. Egypt.

ABSTRACT

Studies done by the author for exploration and exploitation of ground water reservoirs in fissured limestones lead to some interesting results in different areas in U.A.R.

Recent results from the hydrogeological and geochemical investigations undertaken during the search for ground water in several hydrostratigraphic units ranging in age from upper Cretaceous chalks and limestones to Miocene dolomitic limestones and even up to Pleistocence formations opened many interesting points which should be taken in consideration while these studies are undergoing.

It was found that there is a very close relationship between the distribution of the jointing system and the ground water flow in a certain area. Field pumping test analyses in fracture8 limestones revealed many results in such aspects and helped in locating areas of higher transmissivity from those of lower ones.

Geochemical analyses showed also the relationships between the water contained in different fractures.

This paper also deals with the origin of some natural lakes surrounded by fractured Eocene limestones, distributed along the southern rim of the extensive Qattara Depres- sion in the Western Desert of Egypt.

RÉSUMÉ

Contribution à l’étude hydrogéologique des terrains calcaires en RPpublique Arabe Unie Les études effectuées par l’auteur en vue de l’exploration et de l’exploitation des

réservoirs d’eaux souterraines situés dans les calcaires fissurés font apparaître des résul- tats intéressants pour différentes régions de la République arabe unie.

Les résultats de travaux hydrogéologiques et géochimiques auxquels on a procédé au cours de recherches d’eaux souterraines dans plusieurs unités hydrostratigraphiques échelonnées des craies et calcaires du crétacé supérieur aux calcaires dolomitiques. du Miocène ont permis récemment de préciser nombre de points intéressants qu’il convient de ne pas négliger dans ces recherches.

11 a ainsi été découvert qu’il existe un rapport très étroit entre la répartition du sys- tème de diaclases et l’écoulement des eaux souterraines dans une certaine région. Les analyses des tests de pompage effectuées dans les calcaires fracturéS.donnent à cet égard de nombreux résultats et aident à localiser les zones de transmissivité relativement forte et a les distinguer des zones de faible transmissivité.

Des analyses géochimiques ont aussi montré les rapports qui existent entre les eaux contenues dans différentes cassures.

Cet article traite aussi de l’origine de certains lacs naturels entourés de calcaires éocènes fracturés, répartis le long de la bordure méridionale de l’immense dépression de Qattara, dans le désert occidental d’6gypte.

1. I N T R O D U C T I O N

Increase in the exploration and exploitation of groundwater reservoirs in U.A.R. (figure i) led to many problems concerning the development of the different reservoirs in the country. As an example, exploitation of the extensive Nubian Sandstone artesian aquifers in the Western Desert of Egypt (Dakhla and Kharga Oases) showed that extensive development and utilization of this hydrostratigraphic unit by drilling so many wells in these 2 areas have caused depletion in the discharges and pressures

348

ofthe flowing wells during the last five years since the project was executed. Accordingly, the author encouraged the idea of exploring and exploiting other hydrostratigraphic units such as the limestones (a) quifers which is the aim of this paper.

Studies done by the author in different areas of U.A.R. (Western Desert, Eastern Desert, and North and Central Sinai) for the limestone reservoirs (surface water reservoirs in the form of natural lakes, karst springs, artificial reservoirs along dam sites built on fissured limestones, and ground water reservoirs in the form of flowing and non-flowing wells), gave a lot of informations and interesting results concerning their hydrogeological and geochemical characteristics.

Thus, this study can be divided into two categories, namely groundwater limestone reservoirs and surface water in the form of natural lakes and springs. Also, the study includes part of the studies undergoing now for artificial lakes formed due to the con- struction of control dams along wadi channels (e.g. EI Rawafa D a m along Wadi El Arish, North Sinai) having its banks surrounded by fissured limestones.

2. PHYSIOGRAPHY AND CLIMATE

2.1. PHISIOGRAPHY AND THE EFFECT OF RAINFALL ON DIFFERENT CARBONATE ROCK UNITS

An appreciation of the major surface features of the land (physiographic units) can be of considerable strategic, and to a lesser extent, tactial significance. Mention of a particular physiographic unit should, for example, convey a picture of the general terrain. Certainly the term “basin and range” suggests a definite pattern of mountains plains, drainageways, soil types, and other elements of the terrain. The concept of geometry is adhered to in the present study in establishing the major physiographic units (mountains, plain-and-mountain complexes, hill lands, plateaus, plains, as illustrated in figure 2); however, it became clear early in the study that exceptions would have to be made in the subdivision of these units. Certain plains and hill lands, for example, exhibited important distinguishing characteristics not adequately defined in terms of form or geometry. ln view of this fact, origin or genesis was chosen as a secondary bases of physiographic breakdown. However, a maximum of form or geometric criteria are included in all unit definitions as shown in the legend attached to figure 2.

The nature of the effect of rainfall along the whole country varies according to the geological character of the districts visited. It is obvious from the surface features along the northern part of U.A.R. where the Miocene and younger limestone beds predominate, that the effect of rain reaches its maximum. Deep ravines and sinkholes were encountered along the extensive Lybian Plateau which extend along the northern, northwestern, and southwestern part of the Qattara Depression. Also, it was noticed along the escarpment overlooking the Mediterranean littoral of U.A.R. It can be seen along the Miocene and younger limestone formations exposed at the eastern and western coasts of the Gulf of Suez and at some parts of the Western Coast of the Red Sea.

O n the other hand, those districts which are occupied by limestone strata of Middle and upper Eocene are, as the effect of rainfall, largely characterized by (1) plateau surfaces; (2) valleys bordered by steeply sloping or vertical cliffs; (3) abrupt steps in the valleys, or sidds, giving rise to fine waterfalls during spates; (4) undercutting by back splash of the waters; (5) formation of water pools or depressions at the foot of the waterfall (in the form of sinkholes); (6) the side streams form boulder valleys filled with blocks (slumps) broken off from the bordering cliffs. All these features are well seen in the Tih and Egma plateaus in central Sinai, the Maaza plateau in the Eastern Desert north of Qena, in the neighbourhood of Cairo in the hills to the east of Helwan, and in the Western Desert along the plateau bordering the Nile Valley on its western side and extending from Cairo to Aswan. As the Upper Eocene formations are formed

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Fig. 2 RANGES: Elongate belts of massive mountains. MASSIFS: Roughly circular aggregations of massive mountains, RANGDOM HILLS: Hills with moderate to steep slopes. SAND DUNES: Eolian sand, change shape and position rapidly. PLATEAUS: Elevated masses of land bounded on one or more sides by scarps. ALLUVIAL PLAINS: Flood plains, terraces and subaerial delta of the Nile River. COASTAL PLAINS: Plains bordering the sea. DEPRESSION PLAINS: Low lying plains of interior drainage bounded by scaprs. DESERT PLAINS: Interior plains formed or modified by eolian deposition or erosion,

of thin limestone bands and softer strata, the results are the same, but on a smaller scale, and the whole country is broken into minor drainage lines of no great vertical depth.

The cretaceous limestone strata differ from the Eocene series in the greater abundance of soft members in the succession. South of Qena, where the softer Cretaceous shales and white limestone rise from underneath the Eocene limestones, the regular terrace arrangement is replaced by confused foothills, due in large measures to the sliding of the heavy masses of limestone over the slippery surfaces of the moistened shales and clays. The effect of these land slides is everywhere visible on both sides of the Nile between pena and Esna, around Farafra Oasis, north of Dakhla Oasis, north and east of Kharga Oasis, and also along the southern part of Central Sinai, being further exagerated by the presence of faulting. They are obviously connected with the geological structure, being best developed at the junction of the Eocene limestone and Cretaceous shale series. Every important hard bed tends to form a plateau, but as the softer formations are rapidly denuded beneath it, the unsupported portions are speedily broken off and carried away during heavy shower storms. The result is the production of typical bad-land scenery, where the topography becomes of extraordinary complexity. These features were observed in many places in Central and northern Sinai; along the central part of the Eastern Desert and also at the southern Oases of the Western Desert.

In regions formed of Jurassic, Triassic and Carboniferous Limestone rocks as in northern and centrai Sinai, the general features produced by rain differ but little from those noted in areas occupied by Miocene, Eocene and Cretaceous formations. This can be illustrated along El Maghara major fold (Jurassic), Arif El Naga dome (Triassic), in northern and east central Sinai respectively. Um Bogma district (Carboniferous) is an example for West Centrai Sinai.

2.2. CLIMATE

It can be noticed that U.A.R. lies for the most part within the temperate zone and less than a quarter of it, is south of the tropic of Cancer. The whole country forms part of the great desert belt that stretches eastwards from the Atlantic across the whole of North Africa, and onwards through Saudi Arabia; and like all the other lands lying within this belt it is characterized by a warm and almost rainlessclimate (figure 3). The air temperature in Egypt frequently rises to over 40' centigrades in the daytime during the summer, and seldom falls as low as 0' centigrade even on the coldest nights of winter, and the average rainfall over the country as a whole is only about a centimeter a year. It is clear from figure 3 that even along the Mediterranean littoral, where the most rain occurs, the average yearly precipitation is less than 200 millimeters, and the amount decreases very rapidly as one proceeds inland from the coast. Thus while Alexandria, on the Mediterranean Coast, has an average annual rainfall of 190 millimeters, Cairo, some i70 kilometers inland has only 30 millimeters; Asyût, which lies some 300 kilometers south of Cairo, has but half a centimeter; and Aswan, some 600 kilometers south of Cairo, has practically no rain at all.

With so scanty a rainfall it is not to be wondered at that by far the greater part of U.A.R. consists of barren and inhospitable deserts. Indeed, if the rain, that falls within her own borders were the sole source from which U.A.R. could derive water-supplies, the whole land would be one vast uninhabitable desert. But fortunately the Nile, tra- versing the entire length of the country on its northward course to the Mediterranean, continually brings dowp into U.A.R. large volumes of surface water derived from the heavy rainfall of tropical highlands lying far to the south; and the supplies thus fur- nished, being led by artificial canais over the narrow strips of alluvial land on either side of the river within its trough-like valley, and over the broad alluvial expanses of

3 52

)-

Y S E A

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the Fayûm depression and the delta, serve to render these tracts (which comprise in all, however, only about 3 % of the total area of the country) as fertile as any lands in the World, and thus capable of supporting a dense agricultural population.

3. HYDROGEOLOGY

3.1. GENERAL GEOLOGY

Ground water occurrence in limestone formations is governed by the geology, and a thorough knowledge of the stratigraphy, lithology, and structure of the area is required toward its evaluation.

Stratigraphy

Ancient calcareous sedimentary rocks were observed by different geologists to be exposed in a region from 100-150 kms due east of korosko oat: 22' 45' N and long.: 31' 04' E). This was an indurated quartzose or gritty limestone which have great simi- larities with the squeezed muddy limestones of the Alps being of probable pre Carbonif- erous age. These Egyptian calcareous rocks shows evidence of far earlier date, being formed anterior to the Carboniferous period. Series of crystalline limestone which had been metamorphosed by the pink granite with which they were associated, were also encountered in Western Sinai. In Eastern Sinai; no calcareous schists were found. As there is an almost complete absence of fossils in them, it is impossible to fix their precise position in the stratigraphical column. No trilobites, graptolites, or other organisms of well defined geological age have ever been recorded from them, and this negative evidence weighs heavily in favour of the vast majority of these formations being pre- Cambrian.

Carbonate rocks encountered either during surface geologic mapping or from subsurface drilling information of deep wildcat wells range from Cambrian to Pleistocene.

Figure4 shows clearly the carbonate rocks exposed on the surface all over the country Surface and subsurface data for the different carbonate rock units ranging in age from Jurassic to Pleistocene are illustrated by figures 5-1 1 inclusive.

It is worth to mention that the sedimentary column resting over the pre-Cambrian basement complex is composed of the following lithological divisions: 1. Lower clastic division: Pre-Cenomanian, predominantly clastic with intercalated

calcareous sediments; 2. Middle calcareous division: Cenomanian to top of Eocene, mostly calcareous; 3. Upper clastic division: Oligocene to Recent, predominantly clastic but with organo-

genic limestones.

From this classification, it is clear that predominance of the carbonate percentage in the middle calcareous division supports the idea that Northern and Central Egypt was a part of the greater Tethys geosyncline during Cretaceous and early Tertiary times as the sediments found in north and Central U.A.R. are similar to the other Cretaceous and early Tertiary sediments of the Mediterranean area.

Figure 12 which is a cross section along the southwestern district of Qattara Depres- sion shows clearly the distribution of the upper Eocene, and lower and Middle Miocene having a predominancy of calcareous material than other lithological units. An uncon- formity between the lower Miocene and the upper Eocene limestones can be noticed, where the Oligocene is completely missing in this area. Both the upper Eocene and lower and middle Miocene limestones are two important hydrostratigraphic units in this area of the Western Desert.

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Structure

Structurally, northern and central Egypt are part of the Syrian Arc, showing a system of folds which crosses Egypt and Syria in a NE-SW direction.

Plate 13 shows the general tectonic picture of an Arabian craton overlapped inter- mittently by shallow seas which spread out from elements of the Tethyan geosynclinal system, north of the country. This craton (the Arabo-Nubian massif) is surrounded by a shelf area which can be divided into three units: the stable shelf, the Gulf of Suez taphrogeosyncline, and the Unstable Shelf-Stable and unstable shelves are similar but in the latter the formations are thicker and more disturbed. The unstable Shelf is characterized by having a series of asymmetrical folds with steep side to the south- east. In Sinai the folds are strongest in the north but fade away towards the Stable Shelf into the Tih Plateau of Upper Cretaceous and Eocene rocks. There are few iso- lated faults and faulted-up blocks running in a NW-SE direction, which are probably caused by the graben and horst movement of the Gulf of Suez region. In the Gulf of Suez area, the body of water occupies a graben as is attested by faults on both sides which have most of their downthrown sides toward the Gulf. This faulting was initiated before the Miocene, and continued probably up to Recent time. The structure of the Gulf of Aqaba, except for its width, is undoubtedly similar to the Gulf of Suez but faulting is difficult to see because of the uniformity of the basement rocks on both sides of the gulf.

In the Eastern desert folding and faulting seem to have occurred along EW or NNE- SSW directions. The latter direction is probably the result of the clysmic or Eritrean tectonical activity, while the former is of the Mediterranean direction. Folding, e.g. is in line with those of Northern Sinai and Western Desert but is not as well developed here and is much complicated by the faulting.

The Western Desert is more characterized by the presence of folding than faulting except in its eastern part towards the Nïle Valley and the Delta Basin where faults predominate. Folding here have the directions of NE-SW, with the Syrian arc or swell system and are more of less parallel to the geanticlipal welt. Folding seem to have become stronger at or near, the end of Turonian lime. This may have been the period when movements took place on the greater number of anticlines. It can be noticed that not all stresses were relieved by folding, but in places, for example Abu Roash and Wadi El Natrun area, competent beds broke instead of bending and faulting parallel to the lines of folding took place.

From the impression gained from field work, the author is inclined to believe that the faulting is secondary to the folding, coming at a time when the rocks could no longer stand the stress of folding, so that they sheared during folding, this is borne out by the fact that some of the faults are almost entirely faults of horizontal displacement. However, most of the faults are apparently normal, although they arc hidden in nearly every case by alluvium deposits.

Joint surveys from tectonically complicated arcas are mostly too complex to allow any trustworthy interpretation, although this has often been attempted. When we consider regions which are almost unfolded, the patterns are much more simple and show obvious relations to structural features, and even they may be random strike or poorly developed.

In different parts of U.A.R. it was noticed that near pronounced anticlines well- defined joints are formed parallel to the strike of the axial plane of the anticline. Also there are belts of closely spaced parallel joints which presumably mark zone of shearing.

Studies done by the author in Farafra Oasis in the Western Desert of Egypt showed that along the steeper side of the anticlines, larger proportion of fractures occur. Fis- sured Maestrichtian chalk of Farafra Oasis show clearly the great similarity between the total joints measured, joint sets and the airphotos linear features. Survey studies

3 64

around Qasr El Farafra, in the Farafra depression indicated the presence of a very marked consistency of joint patterns and they slowly swing from N W north of Qasr El Farafra to NE south of it. The joint systems are perpendicular to the bedding plane. The sets of tension-joints observed are perpendicular to the main stress and might have developed as an effect of elastic release of the compression of the chalk. A valuable contribution to the sets of joints in this area which are perpendicular to the bedding- plane, is that they are joined by “en échelon” gash (tension) points, filled by calcite which stand on the ground surface as small ridges having a height of about 40 centi- meters in some places.

Other studies completed in Qara Oasis, Qattara Depression in the Western Desert, together with the natural lakes existing along the southern rim of the Qattara Depres- sion showed that jointing is entirely related to the ground water movement in such districts.

3.2. HYDROLOGICAL REGIME

General Remarks

U. A. R. is charactised by prevailing arid conditions which is marked by the low amount of precipitation and the extremely high evaporation capacity. The high temper- atures together with the low relative humidity values and the active wind account together for the high rate of evaporation. Due to lack of rainfall, aridity increases and accordingly scanty moisture content and surface vegetation can be accounted for. NO direct percolation from the ground surface takes place as moisture in the top soil never exceeds the amount required for infiltration through the dry soil. During occasional floods, downward percolation takes place in the Wadi beds through fissured rocks and Wadi fill. In Centrai and North Sinai together with the northern part of the Western Desert (EI Diffa Plateau), percolation is the only source of ground water recharge, thus plays an important role in the hydrological regime of these areas.

3.2.1 Groundwater regime

Ground water exploration and exploitation of fractured limestone reservoirs in U.A.R. has not yet reached its climax. Drilling operations by oil companies and other organizations in the country showed the existence of proliferous amounts of ground water in the different formations encountered. Limestones and dolomites having an age ranging from Jurassic up to Pleistocene comprise several groundwater reservoirs of great magnitude. The water from each limestone aquifer differ qualitatively from that of other reservoirs.

From field studies it was noticed that water in fissured carbonate rocks, as in other aquifers, moves in the direction of the hydraulic gradient, but the direction of movement was not easy to be determined exactly, especially in the fault zones, because the configuration of the water surface was not possible to be determined accurately. It was noted also that the different limestone aquifers contain openings ranging in size from minute cracks, in which the movement of water is accompanied by a large loss of head, to caverns through which the water moves freely.

In Farafra Oasis and the area surrounding the Qattara Depression from its western, southern and southeastern sides the flow lines indicate that the flow is in different directions. There is a great relationship between the direction of flow and of jointing in these areas. Water entering the cavernous and honeycomped limestones in these certain areas of outcrop undoubtedly moves downdip through interconnected solutional cavities. The large storage volumes in the different formations of these areas permit a long-term overdraft policy to be adopted. The development of such reservoirs in future should

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~

Fig. 13 - Simplified Tectonic Map of Egypt (after Said, Geology of Egypt, 1963).

366

follow an intense hydrogeological investigation by which safe yield estimates and eco- nomical mining of the stored water can be done to prevent the permanent depletion of these reservoirs.

Table 1 shows the hydrogeologic properties of some carbonate reservoir rocks in selected areas in U. A. R.

Worth to mention, while undergoing the pumping tests in the fissured Maestrichtian chalk of Farafra Oasis, it was noticed that the fissures supplying the pumped well are separated from those which supplies the observation wells as there was no effect noted on the water levels of the observation wells during pumping. This case was not noticed in Fuka where the water levels in the observation wells were affected while pumping.

3.2.2. Surface Water regime (figure 14) (Natural lakes, Karst Springs and Artificial reserooirs)

Natural Lakes

One of the interesting scientific problems of the northern part of the Western Desert of U.A. R. is the presence of 5 natural lakes namely: Sitra, Nuweimisa, Bahrein (2 lakes) and Arag, which are located south of the Qattara Depression. Their permanence and origin led the author to make a detailed study as to reach an answer for the question of their presence. Geologic, hydrologic and geochemical studies were done by the author in the last 2 years on these lakes.

These lakes are located in an almost rainless region and of a high magnitude evaporation rate. They cover an area of 15.6 square kilometers for Sitra, 1.6 square kilometer for Nuweimisa, 2 square kilometers for Bahrein east, 3.2 square kilometers for Bahrein west, and 0.2 square kilometer for Arag lake. Their surface elevations ranges between-9 meters and-58 meters relative to sea level. The lakes are surrounded by fissured limestone of Middle Eocene age. They are situated along a huge syncline parallel to the significant Bahrein-Hassanein Anticline (figure 12). Recent studies by the author on the shoreline deposits (peat and silt deposits) indicated the presence of the index fossil Melanin (Eumelania) Tuberculata of a definite Pleistocene age. It is a brackish water fossil and indicates that these lakes have a pre-pleistocene age. The author is in favour of the view that these lakes originated from the springs issuing from the fissured limestones covering this district. These springs were observed by him in I956 while working there, as they can be noticed to be located in the center of the lakes.

Karst Springs

Springs issuing from Karstic limestones were encountered in many areas in U.A. R. In the Western Desert these springs are found in the Oasis depressions of Kharga, Dakhla, Farafra, Siwa, Qara, and Wadi EI Rayan depressions. Examples for the Eastern Desert is Ain Sukhna, located west of the Gulf of Suez, Helwan sulphur springs, and springs of St. Paul and St. Antoneys Monastries. In Sinai, the famous Ain Gudeirat, Ain Qadis, Ain Sudr and Ain Ratama are examples together with some other springs along El Tih and Egma plateaux.

Springs of the Oases depressions get their water from deep horizons and the water temperature range from 28OC (e.g. Ain El-Màmal, Siwa Oasis) to 4loC (Ain EI Gabal, Dakhla Oasis). Siwa springs get their water from middle Miocene fissured limestones, while at Dakhla, Ain El Gabal gets its water from upper Cretaceous limestones.

Ain EI Gudeirat, Ain Qadis and nearly all the springs of Central and North Sinai differs from those of the Oases depressions in having their waters from infiltration of heavy shower storms through the fissured limestone surrounding them. Estimated discharges for Ain Gudeirat in July 1941 was 871 cubic meters per day and in July 1964 it was estimated to be i O0 cubic meters per day. For Ain Qadis measured discharge for

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37 1

July 1941 was found to be 76 cubic meters per day, while in July 1964 it was found to be 20 cubic meters per day. Ain Sudr discharge was found to be in August 1941 to be 18 cubic meters per day. In the area of Ain EI Gudeirat and Ain Qadis, it can be con- cluded that meteoric waters percolate through joints and solution channels of the Gebe1 El Ain (a mesalike upland of considerable areal extent capped by Middle Eocene limestone), probably to the top of the Esna Shale, which forms an impervious surface (aquitard), and thence by gravity down dip in a northwest direction, to issue eventually at both springs.

Artificial Reserooirs

In north Sinai and along Wadi EI Arish channel (north of Gebe1 EI Hala1 by about 8 kilometers, there is an arch dam named El Rawafa Dam. This dam was constructed in 1946, to be used as a storage reservoir for water conservation and flood control. The dam has a height of 20 meters above its base and is built on a lower Eocene (Nekhl formation) fissured flinty limestone. Its reservoir capacity in 1946 was 3 million cubic meters, but now it never holds more than a million cubic meter as it is already filled by a great amount of silt carried out by floods along Wadi El Arish during rainy seasons and depositcd in front of the dam. Studies undertaken by the author recently showed that a great loss of the water preserved in front of the dam is undergoing from the reservoir through the fissured limestones surrounding it together with other losses caused by evaporation. The fissures have 2 main directions; mainly NE and NW. Interesting results gave a valuable idea about losses from artifical reservoirs surrounded by fissured limestones.

3.2.3. Formafion of Caves (figure 14)

Caves are not, as a rule, common features in U.A.R. Where they do occur, they are often of some size, though usually only single chambers extending for short distances into the hiII-side. Some caves were discovered by various geologists in the Eastern and Western Deserts. Of most significance are those caves recorded northwest of Qena in the Eastern Desert along El Maaza limestone plateau and directly north of Nag Hamadi at Wadi Nefukh and Wadi Gasab. (36'30' N and 32'15' E). Other caves in the Western Desert, previously recorded in the literature, are those along the limestones plateau on the camel trail between Idfu and Dush (South of Kharga Oasis) and also on the Camel trail between Asyût and Farafra Oasis. Those caves were found to be rising in the Oper- culina Iybgca and Aheolina limestones of lower Eocene age.

During his work in Farafra Oasis, the author discovered another cave in lower Eocene limestone, near Qaret Sheikh Mohammed Abdalla, on the camel trail Connec- ting Farafra and Bahariya Oasis (27'40' N and 28'30' E'. Old travertine deposits surrounds this cave and some calcite and aragonite crystals can be seen hanging from the ceiling of the cave in the form of beautiful stalactites.

Drilling operations showed many troubles while drilling cavernous limestone formations due to loss of circulation. As an example is Gibb Afia water well, northwest of Qara Oasis; in which a great loss of circulation was encountered in Middle Eocene limestone between depths 228 mts and 246 mts where the hole penetrated through top of a cavern having a height of 18 mts. Also at Bir El Nusswater wells and Burg El Arab wildcat well, caverns were encountered. To the south-west of Asyût by 100 kilometers, on the higyway connecting Asyût and Kharga Oasis, a well drilled to a depth of 1141 mts (in L. Senonian) encountered some caverns in lower Eocene limestones between depth 67 and 299 mts from ground surface.

Accordingly, future drilling in limestones in U. A. R. will show clearly many areas having these cave features which owe their origin to the solution of the limestones by the movement of ground water through its fissures and crevices.

312

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Fig. 15 - M a p showing location of water sampled from different sources, U. A. R.

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TAI Selected Chemiral Analyses for djffe

Temper-

(CO)

No. location Source Hydrostratigraphic unit ature p H EC

1. Burg El Arab Dug well 2. El Dabaa D u g well 3. Fuka Bore Hole 4. Ras EI Hekma D u g well 5. Siwa Oasis Bore Hole 6. Qara Oasis Bore Hole 7. Sitra Lake Surface Water 8. Nuweimisa Lake Surface Water 9. Bahrein East

Lake Surface Water 10. Bahrein West

Lake Surface Water 11. Arag Lake Surface Water 12. Farafra Oasis Dug well 13. Ain EI Gabal- Spring

14. El Maghara Bore Hole 15. El Themada Bore Hole 16. Ain El Gudeirat Spring 17. Ain Qadeis Spring

Dakhle Oasis

Pleistocene oolitic L.S. M . Miocene L.S. Miocene L.S. Pleistocene M. Miocene L.S. U. Eocene L.S. M . Eocene L.S. M . Eocene L.S.

M . Eocene L.S.

M. Eocene L.S. M. Eocene L.S. Maestrichtian chalk U. Cretaceous L.S.

Lower Jurassic L.S. U. Senonian Chalk M . Eocene L.S. M. Eocene L.S.

25.3 - 11,( 23.4 7.5 3,. - 7.7 3,< 23.0 7.7 3,: 26.0 8.3 34 24.5 7.8 - 20.0 7.2 360,i 21.6 7.4 260,l

19.0 6.7 470,l

20.0 7.7 350,i 19.0 - 28.9 8.4 - 41.0 7.65 8

-

- 30.0 - 29.5 8.2 15,i 23.3 - 23.5 - -

-

4. G E O C H E M I S T R Y

Table 2 shows clearly the chemical analyses of selected and representative water samples from different limestone reservoirs all over the country (figure 15).

From field observations, it was noticed that different gases accompany the flow of water from either springs or water wells issuing from carbonate rocks of the oases depressions of the Western Desert and also the deep water wells of the Sinai Peninsula. Between the most important gases are COZ, H2S and methane. As we know, CO2 is considered to be one of the principal agents of chemical change on the different minerals contained in the mother rock. Pressure of this gas if even dissolved from atmo- spheric air by rain at 0.0003 have the power to dissolve 50 mg of CaC03. Carbonic acid gas content in ground water varies between 180 and 550 mg. liter and rarely exceeds 600 mg. per liter. It was found that this gas has great effect on the decomposition of limestones and silicates. Its presence in natural ground waters increases its acidity. Organic matters (e.g. lignite, Coal and hydrocarbons) have always a tendency to oxidise and generate carbonic acid gas in great amounts. Also, sulphuric acid produced by the oxidation of sulphides plays a great role in the decomposition of the limestones and its effect should noi be ignored. In addition reduction of the sulphates produce H2S gas and it is considered to be one of our problems as a severe corrosive agent for pipes and screens of the Oases wells.

Temperature measurements for ground waters from carbonate rocks was found to be ranging from 19OC to 41OC. This wide range shows clearly if the water is meteoric

314

ural waters in Limestone Reservoirs in U. A. R.

T.S.S. Parts per million Trace 1.p.m.) Elements

Ca Mg N a K cos HC03 sog CI B Fe

8,170 2,100 2,500 2,500 2,280 4,880 04,295 41,294

56,696

98,756 37,000 1,813 288

6,000 9,606 1,640 1,100

393 536 71.2 64.4 125 96.7

107.2 107.0 130 65 296 180

2,500 8,679 2,143 7,594

1,786 8,138

1,429 9,013 1,650 1,650

12 45 36 56 136 120 107 108 150 177 160 56

2825 - 750 - 838 - 838 -

430 1,150 52

67,000 3,340 37,000 2,000

72,000 2,960

42,000 3,200 - - 55 - 71 33.5

4,000 140 491 - 230 -

- -

nil 122 nil 366 12 348 21 537 - 110 - 195 6 153 30 171

nil 189

15 207

18 103 nil 106

1.5 17

- -

- - - - - -

4,006 97 1

- 1,032 - 971 324 780 862 59

2,674 112,500 3,732 76,000

-

2,955 143,500 57 0.24

5,411 24,600

47 16.5 1,270 nil 161 171

96,000 260,300

124 49

2,600 3,700 503 355

0.34 O. 3 0.39 1 .O3 - - - -

or deep seated one, except for the temperature measurements taken along the shorelines of the natural lakes south of Qattara Depression as it was not possible to measure the actual temperature of the springs issuing from the center of these lakes, which are expec- ted to be higher than the values tabulated (table 2).

It can be noticed from table no 2 that mostly all the water from our limestones reservoirs have a high salinity. From these analyses and other more ones obtained from such ground water reservoirs in different areas of U. A. R., it is clear that there are different types of waters, as follows:

A. Chloride type waters:-which can be represented by those waters obtained from carbonate rocks along the Mediterranean littoral zone, due to see water encroachment. It is also noticed that waters of the natural lakes have a higher chloride content than

any other anion. These waters which can be considered as brines and of a high N a cl content does not necessarily indicate fossil sea water, but may simply mean that there has been salt concentration by evaporation. The author is in favour with the latter case as evaporation factor is high in this region of the Western Desert.

In the case of EI Maghara and EI Themada waters (Sinai), they have also a higher N a cl content and this case might be due to dissolution by stagnant groundwater.

B. Bicarbanate type waters:-examples for this case are waters from Ain EI Gabal (Dakhla Oasis) and Ain EI Balad (Farafra Oasis) as their waters have a high sodium bicarbonate content.

375

C. Sulphate type waters:-This can be represented by waters from Qara and Siwa, Oases, Western Desert, as the limestones from which these waters issue contain considerable amounts of gypsum and anhydrite.

Analyses done for the estimation of boron in some of the water samples tabulated (table 2) indicated that the natural lakes, south of Qattara Depression contain a great amount of boron ranging between 33.5-57 p.p.m., while for waters from El Themada well indicated the presence of 16 p.p.m. boron. Generally speaking, boron is one of a group of elements whose abundance in marine sediments and sea water is too great to be accounted for by a source from weathered igneous rocks. Boron is removed from sea water by illite, which is the most abundant of the clay minerals. Illite contains 459 p.p.m. B. The average boron content of shales is close to 120-130 p.p.m.B. Some authors reached a conclusion that boron content in primordial ocean is more than in the present ocean, and that its content in sea water has not varied significantly for the last 2 to 3 billion years. The addition of Juvenile water and boron to the ocean has presumably been most rapid during periods of tectonic activity. However, erosion and sedimentation and hence boron removal, have also been most active during periods of increased tectonism.

It is worth to mention that boron can be used as a paleosalinity indicator and quan- titative boron paleosalinity data can be applied to the reconstruction of paleoenviron- ments. Thus it gives rise to the possibility of its being a reflection of conditions in the groundwater origin.

5. ACKNOWLEDGEMENTS AND BIBLIOGRAPHY

5.1. ACKNOWLEDGEMENTS The author’s acknowledgement is due to his colleagues at the Desert Institute, in

particular Dr. A. Shata and Dr. Ahmad Amin for their benefit discussions and for the facilities provided during the preparation of this work. M y thanks are to m y colleagues at the general Petroleum Co of Egypt for the valuable informations obtained. Also the author forwards his thanks to Miss Khairia Amir, for her help in the chemical analyses of some of the water samples. Collection of water samples from the natural lakes, south of Qattara Depression by the staff of the Geology Dept. of PAN AM E R I C A N OIL CO of Egypt, in their concession area, is greatly appreciated as it added valuable informations to this work.

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