الفهرس يوجد فقط 14 صفحة متاحة للعرض العام
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المستخلص
The study part of Abu Mady area is located in the northern section of the Nile Delta, between latitudes 31º 23̀ - 31º 27̀ N and longitudes 31º 19̀ - 31º 23̀ E. There are two ways to reach the study area from Ismailia. The first and short one, is from Ismailia, Port-Said (Round Way), Damietta, Gamasa (International Coast Way) and finally to Abu Mady. The other and long one, is from Ismailia, Faqus, El-Mansoura, Shirbin and finally to Abu Mady.
The Nile Delta area is mostly represented by stippled area or a blank space described as Quaternary. The disclosure of its past geologic history become possible with activities of oil companies that started to work in the Nile Delta in the early sixties of last century. The oil companies got interested in its area in the light of possible oil genesis and trapping in convergent margins with the plate tectonic context published at that time. With the success in discovering the first gas field of Abu Madi north Nile Delta in 1966 more exploration work has been done. The Nile Delta is most fertile agricultural region in Egypt. It represents about 95% of the cultivated area of the whole country, besides it is one of the most heavily populated regions in the world.
Topographically, the investigated area lies between the Damietta and Rosetta branches of the River Nile including some large shallow lakes and marshes in the north. It is generally flat with a maximum relief of about 10 feet. Its surface is almost covered by Recent-Quaternary sediments.
In Abu Mady area, Oil Companies face a problem of finding water of less salinity than the soil water. Soil water has very high salinity and can not be used in water-processing stations, fire tanks or agricultural purposes. So, finding groundwater of less salinity is very interesting not only for Oil Company in the area but also for agricultural purposes. For this reason, the hydrogeological and geophysical investigations of the study area are chosen as the main objectives of this dissertation.
The present study is an attempt to achieve hydrogeological information necessary for the assessment and management of the groundwater in the area. It is also intended to evaluate the hydrogeologic characteristics of the main aquifer. Accordingly, geophysical investigations including geophysical well-logging and electrical resistivity soundings have been conducted to dissolute the subsurface conditions in the area, especially in the locations free from water wells. The coordination of the geophysical measurements with the hydrogeological information assists in putting a comprehensive picture for the future development of the area.
On the other hand, buried gas pipes represent a problem for resistivity survey. So Geomagnetic survey, representing in total magnetic field intensity and vertical magnetic gradient measurements, has been carried out in the part of the area which is interested in resistivity survey.
VMG contour map and stacked profiles show anomalies of the vertical magnetic gradient, which were surveyed in a rectangular grid of 200m X 200m in area over a buried gas pipes. The border of buried gas pipes can be derived from contour map and stacked profiles.
The geophysical surveying includes two methods for hydrogeological investigation in the study area. These are geoelectric survey and the well logging methods.
The well logging technique is considered as one of the most reliable and accessible geological-geophysical method for determining rock petrophysics and areas of high porosity and permeability (aquifer) that would produce the most suitable water.
Three types of well logging tools have been used in this study for logging most of the drilled wells. These tools are Conventional resistivity, Gamma ray and Self-potential. The well-logging data are subjected to a comprehensive Qualitative and Quantitative interpretation for formation evaluation. This interpretation has been constructed to be familiar with the specific drilling logging characteristics of the water wells. The results of formation evaluation interpretation are represented in the form of lithology column illustrating the vertical variation of lithology with depth and the main water bearing layer. However, a number of iso-parametric maps have been constructed to illustrate the lateral variations inherited in the well-logging deduced parameters of the main water bearing layer. These parameters include the volume of shale, resistivity, Gamma ray, Self-potential, porosity and total dissolved solids of the groundwater. Estimation of these illustrations indicates the following:
The Quaternary section in the study area is mainly composed of sands with varying percentage of clay interferences that range from 8 % to 34 %.
The petrophysical properties of the main water bearing layer determined from well-logs. This layer has a natural radiation ranges from 5 to 17 API with an average value 11 API. It indicates that water bearing sand is nearly clean from shale interferences. This layer has also self-potential ranges from -20 to -26 mv with an average value equals -23 mv. It indicates its permeability and medium of its water salinity. Main water bearing layer has a resistivity as measured by 16ً resistivity tool ranges from 11 to 27 Ω.m with an average value equals 16 Ω.m, while for 64ً resistivity tool, ranges from 22 to 28 Ω.m with an average value equals 25 Ω.m. This value of resistivity is the highest value of the water bearing layers inside wells. It means that the main water bearing layer has the lowest value of water salinity than the other water bearing layers. Its porosity ranges from 19.09 % to 32.37 % with an average value equals 25.73 %.
Salinity of the main water bearing layer ranges from 2000 to 4000 ppm, while the upper and lower layers have salinity more than 10000 ppm and reaches 30000 ppm in the soil water. Gamma ray logs correlation shows two shale layers separating the first three water bearing layers and preventing interference between different water types.
To study the high salinity phenomenon in some wells than the others, we construct many contours maps of the main water bearing layer and correlate between logs. Contour maps proved that there are similarities between petrophysical properties of the main water bearing layer and there is no big difference between them in the study area. Correlations proved also that there are no big differences between these wells, especially in the main water bearing layer. Then there is no big difference in water salinity of the main layer. This also proves that there is no principle reason to increase water salinity but the reason may be due to interference between high salinity water (30000 ppm) of an upper layer and brackish water of the main layer through walls of some near wells which have no good cement of casing. So when these wells have good cement cased, salinity of water samples returned to normal values. So it is recommended that when drilling wells again in the area, high layers have to be separated from the main bed.
Cross-plot technique was applied on data which obtained from wells drilled at the study area. Results of cross plots are presented as the following:
Lithology (sand – shaly sand – shale)
Water bearing layers
-Brackish water bearing sand
-Saline water bearing sand
-Very saline water bearing sand
On the other hand, fourteen vertical electric soundings (VES’es) were conducted in the study area, especially in the sites of no water wells. This is for giving much information about the subsurface geological and hydrogeological framework of the study area.
The geoelectric surveying was done using the Schlumberger array and the resistivity meter RSp-6. The maximum current electrode separation was 1400 m where the spread layouts are governed by the accessibility of the terrain. The measured resistivity values are interpreted automatically. The results are presented in the form of pseudo- and geoelectric cross-sections and iso-resistivity maps in addition to true resistivity map of the main aquifer.
However, the results of the geoelectric surveying are calibrated with the near-by wells of known lithological and hydrogeological information to derive a resistivity spectrum for the subsurface lithofacies for a reliable and acceptable interpretation. Estimation of the geoelectric sections and maps indicates the followings:
These sections contain different geoelectric layers which are assumed to be; from top to bottom as follow:
1- Dry sand which has resistivity values varying between 85-102 Ω.m. Its thickness ranges from 2-4 m.
2- Very saline water bearing sand which has resistivity values ranging from 20-27.5 Ω.m. Its thickness range from 14-17 m.
3- Shale layer which has resistivity values varying between 7.7-9.9 Ω.m. Its thickness ranges from 2.2-4 m. It represents a sail that prevents very saline water to invade the lower layers.
4- Saline water bearing sand and shaly sand which have resistivity values varying between 31-39 Ω.m. Its thickness range from 66.1-68 m.
5- Shale layer which has resistivity values varying between 7.7-9.9 Ω.m. Its thickness ranges from 10.7-12 m. It represents a sail that prevents saline water to invade the lower layer of brackish water.
6- Brackish water bearing sand (main aquifer) which has resistivity values varying between 40-48.4 Ω.m. Its thickness ranges from 53-58.3 m.
7- Sandy shale to shaly sand which has resistivity values varying between 31-39 Ω.m.
Expected NW-SE fault affects through some geoelectric sections. The effect of fault appears through these geoelectric sections because it permits saline water in the upper layer to invade the main aquifer and changes its salinity (from brackish to saline), while there is no effect through other geoelectric section in the south as there is not any change in salinity of water along this section. These are also clarifying in pseudo-sections. Accordingly, the southern part of the study area is recommended for drilling water wells.
Generally, the main water bearing layer in the study area could be used in the future guided by the priority map (Fig.6.1). This map is constructed using all the deduced results and effective geophysical and hydrogeological parameters through this research work.