الفهرس 
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Abstract
The study area, Siwa Oasis, is located at the North Western Desert, about 300km South of the Mediterranean Sea Cost (Matrouh City) and extended in an E−W direction, occupying an area of about 980 km2.
The present study deals with interpretation of the available Vertical Electrical Soundings (VESes), as well as analysis of the given well log data (gamma-ray, short and long normal resistivities, self potential, caliper, neutron and density) within the study area. These cumulative data were used for determining the geoelectric (for the shallow section), geologic and hydrogeologic features of the water-bearing rocks and finally to solve the over-pressuring problem of the groundwater in Siwa Oasis area.
Thirty Vertical Electrical Sounding (VESes) stations were measured throughout the study area. Quantitative interpretation was done through the analysis of the field curves in terms of electric layers of specified thicknesses and resistivities using both Zohdy’s method (1989) and Rinvert’s method (1999), then constructing the geoelectric cross-sections along the layout of the electrical profiles. The quantitative interpretation revealed the presence of five geoelectric layers, forming the shallow section of the study area down to 225.5m. These layers are varied in their resistivities and rock constituents from very low (0.5.m) to very high (3485.m) resistivities for the five layers, which consist of calcareous sand deposits or composed of argillaceous limestone and limestone, shale and marl (Moghra Formation), carbonate rocks (upper layer) and thin clay layer, respectively. Along these sections, especially at the fourth geoelectric layer or upper carbonate layer, the flow direction of the groundwater is expected and drawn. There are different true resistivity maps constructed for the second, fourth (upper carbonate layer) and the base of the fourth geoelectric layers. Also, the depth map to the bottom of the carbonate aquifer, the thickness map of the confined clay layer and the depth map to the top of the Nubia Sandstone aquifer, in addition to the structural setting map are constructed.
All the available well log data, used in the evaluation for the rock units encountered in eleven wells selected in Siwa Oasis area, are corrected by using Hocoor1 program. Principally, the matrix volume (Vma), shale volume (Vsh), total porosity (t), effective porosity (eff), reducible water saturation (Swr), irreducible water saturation (Swirr), relative permeability (Krw), absolute permeability (Kabso), and effective permeability (Keff) are calculated as petrophysical parameters for the two aquifers. Accordingly, the results of evaluation were represented in iso-parametric maps for the carbonate and Nubia Sandstone aquifer. But, the detection of the vertical and horizontal fracture apertures is carried out only for the carbonate aquifer.
The Dia-porosity cross plots (shale model) and the neutron-density cross plots are constructed in each well for the two aquifers. The Litho-saturation cross plots are used for showing the integration among Vma, Vsh, Swr, and Swirr of the two formations (carbonate and sandstone).
The piezometric head, pressure and gradient maps are constructed to show the engineering properties changes for the Nubia Sandstone aquifer. These maps reflect that, the piezometric heads, pressures and gradients decrease to the western direction along the oasis.
The resistivity, neutron and density logs are used for defining the different zones of over-pressuring in the Nubia Sandstone aquifer. These logs reveal a decrease of the depth to the top of over-pressuring from the areas parallel to the coordinate 25°30\ E directed to ward the western and eastern directions. The water pressure gradients increase from the previous areas to ward the western and eastern directions too, where the maximum calculated value is about 1psi/ft in El−Awaff well, and they are expected within 0.05psi/ft along the oasis. The formation pressure, which is calculated at the first depth in the zone of over-pressuring, increases generally to ward the eastern direction and decreases to ward the western direction, where the maximum calculated value is about 2661.2psi in El−Awaff well.
From the statistical analysis using MINITAB package, the following deduced linear regressions for the carbonate aquifer; in case of estimating the effective porosity (eff) from the total porosity (t) and the reducible water saturation (Swr) from the effective porosity (eff) can be used. But for the Nubia Sandstone aquifer; estimating the effective porosity (eff) from the total porosity (t), the reducible water saturation (Swr) from the effective porosity (eff), the relative permeability (Krw) from the reducible water saturation (Swr) and the piezometric pressure from the effective porosity (eff) can be used. Also from this analysis, the eff, Swr, Swirr, and Keff can be interpreted around 54.1%, 17.8%, 3.6% and 3.4%, respectively, from the piezometric pressure. But around 21.1% of this pressure can be interpreted from the shale content, compaction factor, formation depth, and density of the rock constituents and temperature of the formation.