Geological studies of subsurface Paleozoic strata of northern Western Desert, Egypt

Journal of African Earth Sciences,

Vol. 7, No. 1, pp. 103-111, 1988

0731-7247/88 $ 3 . 0 0 + 0 . 0 0 P e r g a m o n Journals Ltd.

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Geological studies of subsurface Paleozoic strata of northern Western Desert, Egypt M. W. EL DAKKAK Department of Geology, Universityof Alexandria, Egypt

(Receivedfor publication 16January 1987) Abstract--The paleozoic rocks encountered in several wells located at the northern part of the Western Desert consist of distinctive series of carbonate and non-carbonate strata representing changes of environmental conditions. The carbonate sequences were deposited during late Cambrian, late Devonian, late Carboniferous and early Permian. The major non-carbonate sequences were deposited during the major span of lower Paleozoic, major span of Devonian, early Carboniferous and early Permian. Fivemajor facieswere recognized: quartz arenite, quartz wacke, argillite, biosparite and biomicritereflectingdifferent environmentalconditions. The original sediments were subjected to post depositional changes notably compaction, cementation, overgrowth, neomorphism and micritization.

INTRODUCTION

further south as far as Bahariya Oasis. Marine Ordovician and Silurian strata were not met, while an Upper Paleozoic basin is proved occupying the area of Faghur. Said and Andrawis (1961) separated well preserved Lower Carboniferous (Visean) fossils from Faghur Well No. 1 and Mamura Well No. 1. Said (1962) reported the presence of Paleozoic rocks in some wells drilled by Shara Petroleum Company in the northern part of the Western Desert, where thick clastic Lower Paleozoic section has proved to exist in the western reaches of the Western Desert. Cambrian rocks were reported in Gibb Aria, Betty, Ghazalat and Bahariya wells, and Devonian-Lower Carboniferous successions were encountered in Faghur well. Soliman and El Fetouh (1970) presented isopach and lithofacies maps for Carboniferous at the northern part of Egypt. El Hashemi (1972) studied some Paleozoic cores of some wells drilled at

THE PRESENT work is a stratigraphic, petrographic, environmental and diagenetic study of the Paleozoic rocks encountered at several wells (Faghur No. 1, Gibb Aria No. 1, Ghazalat No. 1, Betty No. 1 and Bahariya No. 1 of Shara Petroleum Company) located at the northern part of the Western Desert (Fig. 1). The present work is based on the study of 44 cores (21 cores from Faghur Well, seven from Gibb Aria Well, seven from Ghazalat Well, five from Betty Well and four from Bahariya Well), ditch samples and lithologic logs which are kindly provided by E G P C . Little has been published on the subsurface Paleozoic of the northern part of the Western Desert. Amine (1961) concluded that the Cambrian Sea covered the northern part of the Western Desert and extended

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Subsurface Paleozoic strata of northern Western Desert, Egypt with associated chert fragments pass into a very micaceous silty shale. The bottom of the Cambrian has not been reached until 10.066 ft depth. A core from depth 91299160 ft (core 39) yielded numerous linguloid brachiopods and a single trilobite which has been identified as Ptychaspis sp. of Upper Cambrian age. This part of the Cambrian succession is composed of 75% sandstones, 17% argillites and 8% carbonates with a clastic ratio of 11.5 and sandstone/argillite ratio of 4.30. At Ghazalat Well, a Cambrian succession (432 ft thick) was recorded on lithologic and paleontologic evidences. The top of the Cambrian is located at 9.650 ft where the lithological and electrical characters of the sediments bear a similarity to those of the upper part of the Cambrian succession of Gibb Aria Well. The base of the Cambrian has not been reached till the depth of 10.082 ft. This part of the Cambrian succession is composed of 82% sandstones and 18% argillite with sandstone/argillite ratio of 4.47. At Bahariya Well, a Cambrian succession (1500 ft thick) with Upper Cambrian brachiopods and trilobites as well as distinctive lithological and electrical characters is reported between depths of 4480 and 5980 ft from the ground level. The Cambrian succession at Bahariya Well is composed of 74% sandstones, 23.3% argillites and 2.7% carbonates with a clastic ratio of 36.5 and sandstone/argillite ratio of 3.17. At Betty Well, a part of Cambrian succession (243 ft thick) between depths of 14.200 and 14.643 ft from the ground level was determined on lithological and electrical similarity with the nearby Gibb Aria and Ghazalat Wells. No fossils were recorded to support this determination. The Cambrian succession of this well is composed of 93% sandstones and 7% argillites with sandstone/argillite ratio of 12.84.

Undifferentiated Lower Paleozoic At Faghur Well, a part of undifferentiated Lower Paleozoic (260 ft thick) between depths of 10.665 and 10.925 ft is composed of 31.2% sandstones, 68% argillites and 0.8% conglomerate with a sandstone/argillite ratio of 0.46.

Devonian

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level) are composed of 69.89% sandstones, 20.52% argillites and 9.59% carbonates. The clastic ratio is 9.43 and the sandstone/argillite ratio is 3.41. The Carboniferous mega fossils are not abundant. The top of the Carboniferous succession is fossiliferous with Polytaxis sp. of Upper Carboniferous age.

Permian At Faghur Well, a Permian succession (230 ft thick between depths of 6.040 and 6.270 ft below the ground surface) is composed of 24.48% sandstones, 8.69% argillites and 67.83% carbonates with a clastic ratio of 0.47 and a sandstone/argillite ratio of 2.70. The Permian strata are fossiliferous with Waagenocencha montepelierensis and Anisopyge cf perassulata of Lower Permian age.

Undifferentiated Upper Paleozoic At Betty, Ghazalat, Gibb Aria and Bahariya Wells, no fossils were found in the sandstone succession over the Cambrian, so it is difficult to differentiate this rock type.

PETROGRAPHY

Sandstones predominate the studied Paleozoic succession. Argillites are represented at different levels while carbonates are represented to a less extent notably near the tops of Devonian, Carboniferous and Permian successions.

Sandstones The sandstones encountered at the different studied wells are made up largely of quartz grains and argillaceous material, in addition to minor amounts of microcline, orthoclase and opaque minerals. Based on the clay percent, the Paleozoic sandstones are classified into two main types: quartz arenite and quartz wacke (Pettijohn et al. 1973).

Quartz arenite (Figs. 4 and 5). The Cambrian and Pre-Devonian arenites are composed of more than 90% At Faghur Well, a Devonian succession (3.825 ft non-undulatory quartz grains, range in grain size from thick) between depths of 6.840 and 10.665 ft is composed medium to coarse grained. Most of the grains exhibit of 72.47% sandstones, 21.41% argillites and 6.12% finely pitted surface. A low percentage of feldspar, carbonates with a clastic ratio of 15.32 and a sandstone/ mainly microcline, and some opaque heavy mineral argillite ratio of 3.38. The most important recorded grains are observed scattered throughout the matrix. Devonian fossils are: Theodessia aff hungerfordi; ProFew representative samples were investigated for their ductella cf. hallana; Leptostrophia magnifica; heavy mineral contents. It was found that opaques, Platyracheela cf. mesastrialis; and Fenestrillina aft zircon, rutile and tourmaline are the dominating mineromaciata. The first two genera indicate an Upper Devoals. The quartz grains are mostly rounded and well nian age. sorted, these features indicate their maturity (Folk 1951, 1968). The quartz grains are cemented chiefly with silica, Carboniferous sometimes with iron oxides either coating the quartz At Faghur Well, Carboniferous strata (570 ft thick grains or in the form of small irregular patches. Carbobetween depths of 6.270 and 6.840 ft below the ground nate cements are rarely recorded. Glauconites are

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M . W . EL DAKKAK

recorded throughout the Cambrian succession either as rounded grains or as irregular patches. The irregularity may be due to the result of compaction where the glauconite grains were squeezed into the pore spaces. The Devonian arenites of Faghur Well differ in texture and composition from one level to another. The arenites of the lower level (cores 24 and 25) range in grain size from medium to coarse grained. They are composed of about 90% of non-undulatory quartz grains beside low percentage of argillaceous material and calcite cement. The quartz grains at that level are angular to subangular. The arenites of higher level (cores 22, 21, 20, 19 and 17) are formed of fine to very fine, angular to subangular quartz grains. Medium rounded grains and irregular patches of glauconite are scattered throughout the matrix. Iron oxides and calcite cements are also present between the quartz grains. The arenites at the top of the Devonian (cores 16, 15, 13 and 11) are medium grained, well rounded and consist almost entirely of non-undulatory quartz grains. Calcite, iron oxides and glauconite grains are locally prominent. Few altered calcitic o61ites (core 15) are scattered throughout the matrix (Fig. 5). Using the textural maturity order of Folk (1951, 1968) the Devonian arenites at the base of the succession are submature since the quartz grains are neither well sorted nor well rounded, while the arenites at the top of the Devonian succession are mature. The marked variations in the textural maturity of the Devonian arenites are seen to have been related to environmental factors. Lower energy environment is predicted to be dominating during the deposition of the lower part of the Devonian succession while high energy environment is supposed to be responsible in the upper part of the succession. The Carboniferous arenites (core 9) are fine grained, well sorted and composed of about 90% of non-undulatory quartz grains. Iron oxides are present as cementing material between the grains.

Quartz wacke (Fig. 6). The quartz wackes are less abundant than the quartz arenites in the studied Paleozoic successions. The main constituents of this type of sandstone are ill-sorted, non-undulatory quartz grains. The quartz grains are variable in size and shape; they are generally angular to subangular and range in size from very fine to medium. Few microcline grains are present. The matrices consist of clay and silt size particles of quartz. The clayey matrices are concentrated in some quartz wackes as patches or as parallel laminae. These features may be due to the squeezing of the clay by the effect of compaction. The quartz wackes are considered texturally immature (Folk 1951, 1968). This type of facies is represented by cores 3, 4, 5 and 6 (Bahariya), 36 and 37 (Gibb Aria), 47 (Betty) and 18 (Faghur). Provenance The Paleozoic sandstones are composed mainly of non-undulatory monocrystalline quartz grains. The degree of undulatory extinction is an indicator of the

provenance of the quartz. According to Folk (1966), non-undulatory quartz grains are derived from plutonic igneous rocks. Therefore, the quartz grains of the Paleozoic sand-stones appear to have been derived from the southern terrain of Pre-Cambrian igneous rocks.

Environment of sand deposition The Cambrian sandstones encountered in the subsurface wells located in the north-western part of the Western Desert are believed to be deposited under shallow marine condition as evidenced by the presence of Upper Cambrian marine trilobites and brachiopods. Reducing conditions have prevailed during the Cambrian deposition as indicated by the common occurrence of the glauconite grains. Two main facies are distinguished on the basis of the clay percent: quartz arenite and quartz wacke. Quartz arenite appears to have been deposited under a high energy environment documented by the absence of mud sedimentation and the high sorting and roundness of the grains. Quartz wacke indicates deposition under a quiet energy environment. According to Pettijohn et al. (1973) the wackes are generally marine and considered as turbidite sands. No marine evidences are recognized in the undifferentiated Paleozoic sandstones that overly the marine Cambrian succession in Betty, Ghazalat, Gibb Aria and Bahariya Wells and underly the marine Devonian in Faghur Well. Samples from the undifferentiated sandstones, cores 32 (Gibb Aria), 25 (Ghazalat), 39 (Betty) and 27 (Faghur) were mechanically analysed. The data were used for the calculation of statistical grain size parameters using Folk and Ward's (1957) formulas. Values of these parameters are applied on some scatter diagrams e.g. standard deviation (a/) vs mean size (Mz) of Friedman (1967) and mean size (Mz) vs skewness (SK1) of Moiola and Weiser (1968). The studied samples fall in the field of mixed dune and river according to Friedman's (1967) boundary and in the dune field according to the boundary of Moiola and Weiser (1968). The Upper Paleozoic quartz arenites in Faghur Well are believed to be deposited in marine depositional environments as evidenced by the presence of marine fauna, glauconite grains and some o61ites. The Upper Paleozoic marine succession present in Faghur Well is not recorded in the other studied wells, its equivalent may be in the continental undifferentiated Paleozoic. This indicates that the Upper Paleozoic basin of deposition was restricted in the area of Faghur. The Devonian quartz arenites in Faghur Well are submature at the base of the succession while at the top they are mature. The change of maturity reflects the fluctuation of environmental energies where the energy was low at the lower succession and relatively higher at the upper succession. The Carboniferous and Permian quartz arenites are mostly mature. The maturity of these sandstones reflects the long distance of transport and reworking where the environmental energy was high enough.

Subsurface Paleozoic strata of northern Western Desert, Egypt

Fig. 4. Quartz arenite: detrital quartz grains show concave--convex boundaries indicating pressure-solution. Cambrian, core 29, Ghazalat Well. Fig. 5. Quartz arenite: moderately well sorted sandstone with distinct o61ite grains. Devonian, core 15, Faghur Well. Fig. 6. Quartz wacke: fine angular quartz grains in argillaceous matrix. Cambrian, core 3, Bahariya Well. Fig. 7. Quartz overgrowth: secondary enlargement of quartz grain. The boundary between the grain and the overgrowth made very distinct by coat of iron oxide on the original detrital grain. Lower Paleozoic, core 28, Faghur Well. Fig. 8. Quartz overgrowth and cracking: secondary enlargement of the quartz grain. The quartz grains are heavily cracked due to the effect of compaction. Cambrian, core 46, Betty Well. Fig. 9. Quartz grain partly replaced by calcite probably due to the solution of the silica and redeposition of carbonate. Cambrian, core 27. Ghazalat Well. In each figure, bar scale is equal to 0.5 ram.

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Fig. 10. Mature quartz arenite shows the effect of pressure-solution between the grains (concave-convex boundaries). Cambrian, core 27, Ghazalat Well. Fig. 11. Quartz arenite shows fractured quartz grains due to the effect of compaction. Cambrian, core 29, Ghazalat well. Fig. 12. Biosparite: rock consists of bryozoan and crinoidal fragments with interstitial material composed of sparite. Carboniferous, core 9, Faghur Well. Fig. 13. Biomicrite: rock is composed of micritic limestone with skeletal fragments of bryozoan, brachiopods and echinoderms. Devonian, core 10, Faghur Well. Fig. 14. Diagenetically altered biosparite, showing syntaxial overgrowth around an echinoderm fragment. Permian, core 8, Faghur Well. Fig. 15. Diagenetically altered biomicrite showing the aggrading neomophism process in the form of irregular patches of sparry calcite. In each figure, bar scale is equal to 0.5 mm.

Subsurface Paleozoic strata of northern Western Desert, Egypt Argillites Argillites are recorded at different levels throughout the Paleozoic succession. They are present either as thin beds intercalated within the sandstone or form aggregate units up to 50 ft thick. At Faghur Well, argillites form 23.18% of the total Paleozoic succession. Concerning the provenance of the argillite constituents, it is suggested to be derived from the southern crystalline igneous rocks. The identification of the clay minerals in some argillite samples, cores 14 and 26 (Faghur), 4 (Bahariya) and 36 (Gibb Aria), by means of infra-red spectroscopy reveals the presence of illite and chlorite minerals. These two minerals represent the stable assemblage where the other clay minerals alter with depth of burial and possibly with the passage of time (Friedman and Sanders 1978). Carbonates Carbonate rocks were deposited during late Cambrian, late Devonian, late Carboniferous and early Permian. Two major facies can be identified within these carbonates. Biosparite (Fig. 12). Formed of about 70% of skeletal grains (bryozoans, echinoderms, brachiopods and few percentage of foraminiferal grains). The cement is formed of sparry calcite which appears to be formed by direct precipitation in the interstitial voids. The skeletal fragments show a relatively high degree of sorting and roundness which suggests that the fine interstitial materials were washed out by strong current under a turbulent high energy environment (Plumley et al. 1962). This facies is represented by cores 8 and 9 (Faghur) and core 38 (Gibb Aria). Biomicrite (Fig. 13). Formed of about 40% of skeletal grains (fragments of brachiopods, echinoderms and bryozoans). The matrix is formed of micrite with few patches of sparry calcite. Signs of burrowing are clearly observed throughout the rock constituents. This facies suggests deposition under a marine environment of low energy level (Plumley et al. 1962). This facies is represented by core 10 (Faghur).

DIAGENESIS

109

original grain margin. Grain overgrowths are well developed among very clean quartz arenites, while in quartz wackes, due to the presence of clay matrix, the formation of grain overgrowth was inhibited due to the lack of pore spaces between the grains (Friedman and Sanders 1978). Silica necessary for pore filling and overgrowths may have been derived from solution of fine quartz silt, from pressure-solution among grains with accompanying silica reprecipitation or from upward migration of connate water charged with silica (Von Engelhardt 1967). The carbonate cement is mainly in the form of microcrystalline calcite. In few cases, carbonates penetrate some quartz grains (Fig. 9). This feature may be due to solution of quartz grains and redeposition of carbonate or due to carbonate replacement process (Pettijohn et al. 1973). Iron oxides frequently exist coating the quartz grains or filling the pores between the grains. Compaction. The compaction of the Paleozoic sands decreased the porosity and caused the formation of grain intergrowth (Fig. 10) and grain fracturing (Fig. 11). By compaction the sand grains are rearranged with a complete or partial decrease in porosity. As a result of compaction, the contacts between the grains are modified, the grains have developed a variety of grain contact ranging from simple line contacts, concave-convex contacts and sutured contacts. These are characteristic features of clean quartz arenites. A large amount of detrital matrix or cement inhibits the formation of this feature (Aalto 1972). Clays The original detrital clays of the Paleozoic succession altered to black argillites may be due to the effect of compaction in response to pressure from the overlying rocks. The black color of the argillites resulted either from the reduction of iron compound present within the detrital clays throughout the diagenetic processes (Hatch and Rastal11965) or from the organic matter that attains 2.4% in some analysed samples, cores 26 (Faghur) and 36 (Gibb Aria). Carbonates Diagenetic alterations within the studied carbonate rocks have obliterated many of the original depositional textures. The prominent diagenetic features recognized within the Paleozoic carbonate rocks are as follows.

Sands The Paleozoic sands have undergone a variety of diagenetic alterations as follows. Cementation. Sands are commonly cemented by silica, carbonate, iron oxides or a combination of these cements. The silica cement is in the form of pore-filling and quartz overgrowths (Figs. 7 and 8) as evidenced by the occurrence of traces of iron oxides marking the

Cementation. Cementation is clearly developed within the interparticles and intraparticles cavities (Fig. 12); The calcite cement is significantly coarser toward the pore center, which is a distinctive character of early cementation in a fresh phreatic environment (Longman 1980). Oldershaw and Scoffin (1967) mentioned that the inversion of aragonite to calcite would produce an excess of calcium carbonate to act as inter- and intra-particle cement.

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Overgrowth. Syntaxial overgrowth rims are of common occurrence particularly around echinoderm fragments (Figs. 12 and 14). Evamy and Shearman (1965, 1969) have described in detail the evolution of rim cement on echinodermal grains. Such overgrowths can be assigned to a specific diagenetic environment. Longman (1980) restricted this feature to the fresh water phreatic environment. Neomorphism. Aggrading neomorphism has affected the micritic matrix of the biomicrites. It is in the form of irregular patches of a mosaic interlocking crystals (Fig. 15). Randozzo et al. (1977) mentioned that the aggrading neomorphism of the biomicrite must have occurred within the supratidal or subtidal environments. Micritization. Many skeletal grains especially echinoderms were altered to homogeneous patches of micrite, where their original internal structures are completely or partially obliterated. Micritization may have been caused by a variety of processes. Bathurst (1971) suggested that the fossil grain may become micritized by an extension of the process which creates micrite envelopes by the effect of boring algae. Wolf (1965) mentioned the possible influence of organic decomposion by the effect of bacteria.

SUMMARY AND CONCLUSION The study of several subsurface sections distributed in the north-western part of the Western Desert has resulted in the recognition of proven marine Paleozoic sequence starting with the Cambrian and terminating with Lower Permian with remarkable absence of marine Ordovician and Silurian. These two systems may have their equivalents in the undifferentiated Paleozoic part of the succession. The Paleozoic rocks encountered in the different studied wells consist of distinctive series of carbonate and noncarbonate strata. Five rock types are recognized: quartz arenite, quartz wacke, argillite, biosparite and biomicrite. The thickest and more complete Paleozoic succession is recorded in Faghur Well No. 1 among the studied wells. During the Cambrian, the area was invaded from the north by the Tethys. Mature quartz arenite and quartz wacke with black shale intercalation were deposited in shallow reducing marine environment. Linguloid brachiopods and a single trilobite which has been identified as Ptychaspis sp. of Upper Cambrian age are recorded in some core samples. Glauconite grains are commonly present in the quartz arenite and the quartz wacke. The undifferentiated Paleozoic clastic sequence encountered in the different studied wells consists of unfossiliferous mature quartz arenite, barren of any marine evidences. It was considered to be deposited during a regressive phase in a continental depositional

environment as evidenced from the petrographical and mechanical studies. Several stratigraphic and petrographic features suggest that during the Ordovician and Silurian periods, the area under consideration was subjected to some disturbance echoing to the world wide crustal movement of the Caledonian orogeny. As a result, the Tethys was regressed and terrestrial condition has prevailed. The Devonian deposits are generally considered transgressive in relation to the underlying Paleozoic sequence. They consist mainly of immature quartz arenite at the base grading upward to mature with intercalation of argillites. Limestones of biomicritic facies are present near the top of the succession. The maturity changes of the Devonian sandstones reflect fluctuation of the environmental energy, where the immature sandstone was deposited under low energy environment and the mature sandstone was deposited under relatively higher energy environment. Reducing conditions have prevailed during the deposition of the Devonian sandstones as indicated by the occurrence of glauconite grains. The biomicrite facies of the Devonian limestone is the predominant facies and reflects low energy and sheltered marine depositional environment. The most important recorded Devonian fossils are Theodessia aft hungerfordi and Productella cf hallana of Upper Devonian age. The Carboniferous sequence consists mainly of alternating mature quartz arenite and black argillite with limestone of biosparitic facies near the top. Glauconite grains are common within the quartz arenite which reflect the reducing marine environment. The biosparite is considered to be formed under high energy shallow water depositional environment. Polytaxis sp. of Upper Carboniferous age is present in abundance at the top of the succession. The Permian succession started at the base with mature quartz arenite deposited under a marine reducing environment, followed by limestone of biosparitic facies which reflects deposition under a high energy marine environment. The Permian strata are fossiliferous with Waagenocencha montepelierensis and Anisopyge cf. perassulata of Lower Permian age. In late Permian, regression of the Tethys took place as a result of the Hercynian orogeny and a terrestrial conditions have been prevailed during the Permo-Triassic time as indicated from the type of sediments that accumulated over the marine Lower Permian in Faghur Well. From the above discussion, it is concluded that the Cambrian basin of deposition has been covered the whole studied area, while the Upper Paleozoic basin was restricted to the area of Faghur where marine deposits of Permian, Carbiniferous and Devonian were recorded. Cementation and compaction are the main diagenetic processes that influenced the Paleozoic sands. Compaction is responsible for the change of the shale to argillite. Cementation, overgrowth, neomorphism and micritization are the main diagenetic processes that affected the Paleozoic carbonates.

Subsurface Paleozoic strata of northern Western Desert, Egypt The Paleozoic marine transgression from the Tethys appears to be restricted to the northern part of Egypt except the area of Gulf of Suez where succession of marine Carboniferous strata crop out. This suggests that an embayment from the Tethys penetrated the Gulf area during the Carboniferous age. The age, stratigraphy and distribution of the Paleozoic succession in the Gulf of Suez region has been discussed and reviewed by several workers (Shata 1955, Kostandi 1959, Said 1962, Abdallah et al. 1963, Omara and Conil 1965, Omara and Schultz 1965, Omara and Kenawy 1966, Omara 1967, Hassan 1967, Soliman and El Fetouh 1969, 1970 and Issawi and Jux 1982). In the extreme south western part of Egypt, Lower Carboniferous sandstones with Lepidodendron plant remains are recorded (Mechikoff, 1926, Said, 1962). Klitzsch (1978) and Issawi and Jux (1982) considered the clastic succession of the south western part of Egypt to be of Ordovician to Carboniferous age. REFERENCES Aalto, K. R. 1972. Diagenesis of orthoquartzites near Bogota, Colombia. J. Sedim. Petrol. 42,330-340. Abdallah, A. M., E1 Addindani, A. and Fahmi, N. 1963. Stratigraphy of Upper Paleozoic rocks, western side of Gulf of Suez. Geol. Surv. Egypt, paper No. 25. Amine, M. S. 1961. Subsurface features and oil prospects on the Western Desert, Egypt. 3rd Arab Petroleum Congress, Alexandria. Andrawis, S. F. 1972. New biostratigraphic contribution for the Paleozoic rocks of Gibb Aria Well No. 2, Western Desert, Egypt. 8th Arab Petroleum Congress, Algiers. Andrawis, S. F., El Afify, F. and Abd E1 Hameed A. T. 1983. Lower Paleozoic trilobites from subsurface rocks of the Western Desert, Egypt. N. Jb. Geol. Palaeont. Mh. 1983(1), 65--68. Barakat, M. G. 1982. General review of the petroliferous provinces of Egypt. Petroleum and natural gas project, Cairo Univ./M.I.T. Technology Planning Program. Bathurst, R. G. 1971. Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam. Dapples, E. C. 1967. Diagenesis of sandstones. In: Diagenesis in Sediments (edited by Larsen, G. and Chilingar, G. V.). Elsevier, Amsterdam. El Hashemi, M. M. 1972. Sedimentation and oil possibilities of the Paleozoic sediments at Siwa basin, Western Desert, Egypt. 8th Arab Petroleum Congress, Algiers. El Sweify, A. 1975. Subsurface Paleozoic stratigraphy of Siwa-Faghur area, Western Desert, Egypt. 9th Arab Petroleum Congress, Dubai. Evamy, B. D. and Shearman, D. J. 1965. The development of overgrowths from echinoderm fragments. Sedimentology 5, 211233. Evamy, B. D. and Shearman, D. J. 1969. Early stages in development of overgrowths on echinoderm fragments in limestones. Sedimentology 12,317-322. Flawn, P. T., 1953, Petrographic classification of argillaceous sedimentary and low grade metamorphic rocks in subsurface. Bull. A.A.P. G. 37,560-565. Folk, R. L. 1951. Stages of textural maturity in sedimentary rocks. J. Sedirn. Petrol. 21,127-130. Folk, R. L. 1966. A review of grain size parameters. Sedimentology 6, 73-93. Folk, R. L. 1968. Bimodal super mature sandstones. Product of the desert floor. 23rd Internat. Geol. Congress, Paragu: Proc., sec. 8, pp. 9-32.

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