الفهرس 
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Abstract
The main purpose of this work is to study the stratigraphic setting, facies analysis as well as the depositional environments of the Upper Cretaceous rocks in Gebel El Duwi and correlate it with neighbouring sections. The mineralogic composition and diagenetic history of the present sequences and their influences on the petrophysical potentiality of the studied rocks, and to evaluate their storage capcity potentialities are other important items have to be studied. Based on field observations, lithologic characters, sedimentary facies and petrographic examination the Upper Cretaceous sequence in the studied area is divided stratigraphically into three distinct rock units from older to younger as fallow; Nubia Formation, Quseir Formation and Duwi Formation. The Nubia Formation represents the oldest sedimentary rock unit in the area and rests unconformably over the basement complex. It consists mainly of pebbly, fine to coarse grained, poorly to well sorted, brownish to yellowish sandstone, with mudstone and/or claystone intercalations. The Nubia Formation could be subdivided into three informal members. A lower trough cross-bedded member rest unconformably on basement, marked by light cream-colored coarse sandstone to pebble conglomerate exhibiting lavender and locally yellow staining and great local thickness variations are observed due to basement topography. The middle member is marked by thick sandstone sheets separated by mudstone/claystone layers. It is fairly resistant to weathering and is the most conspicuous unit forming prominent cliffs in outcrops. The sandstone bodies are brown, fine- to medium-grained quartz with prominent tabular cross-bedding. The third upper member consists of reddish brown to greenish-yellow, medium to fine-grained sandstone, sandy siltstone and shale. The basal part of the member is commonly iron stained, developed ripple marks while the topmost part of this member change upward gradually into finer sediments. The Quseir Formation consists of brown to grey siltstone and fine silty sandstone at the base, grading upward to varicolored clayey shales of grey, yellow, brown, and dark red color. The variegated shale consists of multicoloured shale (grey, brownish yellow, green, reddish, violet and blackish) alternating with yellowish sandstone at Gebel El Duwi. The Duwi Formation consists mainly of hard, semi-crystalline or siliceous limestones, phosphatic beds, marls, and shales and contains chert bands, lenses, and nodules. The lower contact is marked by a series of hard porcellanite and siliceous phosphate bands interbedded with marls and coquina limestones. The upper half of the formation is dominated by marls and oysters. The Duwi Formation in the Quseir- Safaga coastal region comprises a heterogeneous suite of shallow marine rocks that lies stratigraphically above the Quseir Variegated Shale and below the Dakhla Shale. The Duwi Formation in the study area contains three phosphate horizons separated by beds of marl, shale, and oyster limestone with flint. The age of the Duwi Formation is Campanian to Maastrichtian. Thirty five representative thin-sections were prepared and blue dyed from the studied section for microscopic investigations and microfacies analysis. The clastic microfacies represent about 90% of the total Nubia Sandstone thickness. Seven clastic subfacies types could be recognized. They are: Quartz arenite, Subarkose, Arkose, Sublitharenite, Greywacke, Siltstone/mudstone and Dolomitic siltstone. Six nonclastic subfacies types could be recognized. They are: Floatstone / Rudstone, Grainstone, Packstone, Wackestone, Dolostone and Phosphate. Cementation, compaction and pressure solution are the principal processes leading to porosity reduction in the Nubia sandstones of the studied area. On the other hand, fracturing, dissolution and leaching out are the most important porosityenhancing factors. The Porosity-reducing diagenetic processes include: a) Compaction and pressure solution: it is indicated by the reducing of the intergranular spaces, cracking and defragmentation of the quartz grains, beside the rearrangement and orientation of the elongated grains, with wavy extinction under the microscope. The decline of intergranular volume during burial is a strong function of initial composition. B) Cementation: is the main porosity-reducing diagenetic process in the Nubia Sandstones. Two forms of cementation are observed in the studied samples. The first one is represented by fine matrix, ferruginous, calcareous sand with gypsiferous cement which completely fill the pore spaces, vugs and fractures. The second form constitutes from iron oxides and/or clay mineral coating and silica overgrowth which reduce the intergranular pore spaces. The pore spaces are completely filled with ferruginous cement. Iron-oxide cements are intergrowth with clay minerals and sometimes form dark thin film around the detrital grains, reducing the primary and secondary porosities. Also, ferruginous cement is found as spot parts to form iron concretions. The silica (authigenic quartz) cement is in the form of overgrowths on the sand-sized grain surface. Sometimes the silica cement is mostly present as meniscus cement and/or filling the pore spaces and vugs. Porosity enhancing diagenetic processes include: Fracturing, several fractures and joint-sets were observed in microscopicscales. This indicates that the Nubia Sandstone of Gebel El Duwi was subjected to multi-story tectonic events from the Eocene to Miocene age. The vast majority of these fractures were filled with iron oxides while others were not filled (nonmineralized). Discontinuous features oriented sub-parallel (horizontal) to bedding with aperture widths. These fractures and joint-set enhanced and increased the porosity and permeability of the Nubia Sandstones. The dissolution and leaching out affecting the detrital components of the studied Nubia Sandstones lead to excellent secondary porosity values (25 %). Most of the dissolved grains were probably feldspars and mica flakes. The relics of the rock fragments, unstable mafic and heavy minerals were also suffered from dissolution effects. The dissolution process in the Nubia Sandstone takes place preferentially along the fracture and joint sets and/or at the cleavage planes of the grains. However, dissolution is not uniformly distributed across the arenite rocks, which lead to the heterogeneity of the Nubia reservoir. The first effect of burial on the grains and matrix of the Nubia Sandstone is the simple compaction, which leads to partial reduction for the primary porosity. Quartz overgrowth is the main process of diagnoses this phase. Fracturing was followed by the invasion of solutions to dissolve and leach out the feldspars, shell fragments and the original cement. Transformation of feldspars precursors into the kaolinite clay mineral accumulated inside the pore channels and throats. Second phase of cementation by silica cement filling pores as meniscus cement or in some cases filling vugs. Based on the petrophysical properties and behaviour and taking into consideration the petrographical and petrographical studies, the Nubia samples were grouped into three facies: Facies-I (Quartz Arenite and Subarkose): Average porosity of facies-I is ranked as very good, average permeability as excellent value, while the average formation resistivity factor is very low. Following the anisotropy scale, parameters of the hydraulic pore fabric indicates, slight anisotropism accompanied with slight elongation, while the electric pore fabric is characterized by slight pore elongation, foliation and slight anisotropism. Facies-II (Siltstone to Greywacke): The formation resistivity factor of the Nubia Sandstone varies from 25.8 up to 138.7 (av. 72.8). Average helium porosity of facies-II is ranked as good to very good, average permeability as very good value, while the average formation resistivity factor is low. Parameters of the hydraulic pore fabric indicate a moderate anisotropism accompanied with slight to moderate elongation and slight foliation, while the electric pore fabric is characterized by slight pore elongation, foliation and slight anisotropism. Facies-III (Sub litharenite and arkose):