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العنوان
Petrography and Formation Evaluation of the Bahariya Reservoir, Yasser Oil Field, Western Desert, Egypt \
المؤلف
Abuzeid, Amr Moustafa Abdelaziz.
هيئة الاعداد
باحث / عمرو مصطفى عبد العزيز أبوزيد
مشرف / لبنى محمد شرف
مشرف / أشرف رشدى بغدادى
مشرف / سعيد عبد المعبود على
تاريخ النشر
2019.
عدد الصفحات
126 p. :
اللغة
الإنجليزية
الدرجة
ماجستير
التخصص
الجيولوجيا
تاريخ الإجازة
17/10/2019
مكان الإجازة
جامعة عين شمس - كلية العلوم - الجيولوجيا
الفهرس
Only 14 pages are availabe for public view

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Abstract

This study focuses on the petrographical and petrophysical characteristics of the Bahariya reservoir in Yasser oil field which is located in Khalda Concession between Longitudes 26° 57’ 28” and 26° 59’ 60”, and Latitudes 30° 37’ 54.85” and 30° 39’ 27.63”, about 80 km south of Matrouh governorate.
Bahariya reservoir always considered a primary target in this area, about 80% of Shushan Basin fields are producing from it. The complex lithology of the Bahariya reservoir reveals a lot of secrets which tight the gap between petrography and petrophysics, consequently it could directly help researchers of both specialization for the academic standpoint.
The location of Yasser field reflects its importance in rest of khalda ridge fields which are considered one of the most productive area in the Western Desert, these fields are a little bit similar but structurally compartmentalized.
The used petrographic data was derived from thin section description and scanning electron microscopy for selected representative samples obtained from two wells (Yasser-6, Yasser-4). Also core photos were displayed and described for wells Yasser-4 and Yasser-6. The wireline logging data of four wells (Yasser-46, Yasser-44, Yasser-6, Yasser-4) were created and displayed by Petrel and interactive petrophysics software.
The deepest hole penetrated till unit (3) of Alam Elbueib Formation (Berriasian- Early Aptian) which consists of fluvial sand separated by marine shale beds. This unit is overlained by alternating sand, shale and shallow marine shelf carbonate (units 1 and 2). The end of this transgressive marine cycle is marked by Alamein carbonate (Late Aptian) and a thin shale layer of Dahab Formation (Late Aptian).
The next sedimentation cycle was the deposition of continental and shoreline sands of Kharita Formation (Albian), followed by sandstone beds alternating with shale and limestones of Bahariya Formation which deposited on tidal flat environment during the Early Cenomanian. Continuous deepening of sea level led to the deposition of shales and limestones of Abo Roash Formation (Late Cenomanian- Coniacian). The maximum sea transgression led to the deposition of Khoman chalk (late Santonian- Maastrichtian), this cycle ended by deposition of the Eocene carbonate of Appolonia Formation.
Structurally, Bahariya reservoir in Yasser field is trapped by tilted block bounded by NW- SE oriented normal fault.
Petrography of rocks was studied by core description, thin section and SEM. The core intervals of the upper Bahariya Formation in Yasser field show stacked sequences of fine grained, argillaceous rippled, and bioturbated sandstone and highly glauconitic/ bioclastic sandstone facies with subordinate quantities of carbonaceous siltstone and shale, and thin beds of limestone. Bed contacts vary between sharp and gradual. Core description indicates the presence of clear alternations between laminated and massive sandstone with shale, limestone and subordinate glauconite which suggest tidal flat environment.
The thin section and SEM of sandstone samples define three facies types, Quartz arenite, Subarkose, and Phosphatic lithic arenite. The quartz arenite consists mainly of quartz grains with minor k- feldspar, pyrite aggregates and glaucony pellets. These grains are very fine sand size and well to very well sorted. The detrital components of this rock are well bounded by argillaceous matrix and lozenges of siderite. Some parts show highly glauconitic lamina made up of abundant elliptical sub-to well-rounded glaucony grains. The Subarkose differs from the quartz arenite by higher percentages of k- feldspar, two types of subarkose were found, glauconitic calcareous Subarkose and dolomitic argillaceous Subarkose. Phosphatic lithic arenite characterized by abundance of phosphatic fragments and quartz grains, K-feldspars are less common.
Electrical logging were applied in all the studied wells, but recorded different depths inside the interested reservoir, the shallower logging penetrates only 320ft inside Bahariya Formation in Yasser-44 well, while the logging recorded the complete section of Bahariya Formation in Yasser-46 well (747ft). Logging tools are gamma ray, caliper, bit size, deep, medium and shallow resistivity, density, neutron, photoelectric curves. Sonic log recorded in only two wells (Yasser-4 and Yasser-6). Petrophyical parameters were estimated and correlated in upper Bahariya and the first 150ft inside lower Bahariya Formation. These are total porosity, effective porosity, shale volume, gross sand, net pay, flushed zone saturation, water saturation and hydrocarbon saturation.
Porosity were calculated from the three porosity tools; sonic, density and neutron. Total porosity was derived by the combination of Density and Neutron readings. The effective porosity were calculated to take the shaliness into consideration. As the responding of porosity derived from sonic log to primary porosity was more effective than neutron and density logs. So, sonic readings is recommended for primary porosity calculation. Secondary porosity is derived from subtracting the sonic log porosity from the total porosity.
Hydrocarbon potentiality and saturation determination were obtained later as an interpretation of the petrophysical results. Water saturation was calculated in the uninvaded zone (Sw) and flushed zone (Sxo). The hydrocarbon saturation was also applied.
Two crossplot relations were performed on the Bahariya reservoir to identify lithology by petrophysical parameters. Density/ Neutron Crossplots which were performed in the four wells show that the majority of plotted points are scattered between sandstone and limestone lines, the shale effects tend to be in the southeast quadrant of the crossplots. The other type of crossplot makes lithology characteristics be more obvious, because it combines all three porosity logs to provide the lithology – dependent quantities (M and N). M/N crossplots show that the majority of points are distributed to fill the space between quartz and calcite regions, but they tend to be closer toward quartz.
Distribution of petrophysical parameters were studied vertically and horizontally. The vertical distribution reveals that the reservoir is composed of sandstone and siltstone as a major lithological constituent with appearing layers of carbonate and shale. The formation become more shaly at the expense of sandstone toward deeper depths. The hydrocarbon saturation of upper Bahariya is good in Yasser-46 and Yasser-6, fair in Yasser-4, and very low in Yasser-44 well. Oil saturation is abundant in lower Bahariya of Yasser-6 and Yasser-46.
Horizontal distribution of petrophysical parameters like formation thickness, shale volume, total porosity, effective porosity, gross sand, net pay, water and hydrocarbon saturation were mapped for upper and lower Bahariya separately. These iso-parametric maps show the lateral variation and determine the suitable places for drilling development wells. Thickness of upper Bahariya through wells increases toward the west. On the other side, the thickness of lower Bahariya is not available since only one well completely penetrates the section. Shale volume increases in the upper and lower Bahariya toward northwest and slightly at the central eastern part. Total porosity increases toward south east in both intervals, but the lower Bahariya shows a slight increase toward northwest. Effective porosity in upper and lower Bahariya rises at the central eastern part of the study area. Gross sand appears high at the central and western parts of the area in both intervals of upper and lower Bahariya. Hydrocarbon net pay of upper and lower Bahariya show high values in the central eastern and eastern parts. Hydrocarbon saturation distribution maps show similar directions, while water saturation map display the reverse. Net pay/ gross sand maps revealed high values toward east and north east directions of the study area. According to the previous results, the most favorable parts for accumulation and production are due east of the study area, it is recommended to drill other development wells to better enhance the productivity.
The recorded sandstone facies of the Bahariya Formation revealed the presence of various cementation types and mineral association that reflected in petrophysical crossplots, and logs.
The interpretations of M/N and Density/ Neutron crossplots are concordant with the investigations of thin sections and cores. Both types of data revealed the presence of calcareous cementation in the studied sandstone. Bahariya sandstones demonstrate an abnormal high gamma ray especially in upper Bahariya. This is due to the abundance of argillaceous matrix and/ or lamination. Also, the abundance of Glauconite and k-feldspars in the selected samples led to high radioactive emission detected by the tool. The presence of Fe- bearing minerals like glauconite, chlorite, siderite and pyrite in almost all samples rise the density and photoelectric signals in Bahariya reservoir, especially the upper unit. The signals in the four well analogs suggest the less percent of glauconite in the lower units. The presence of argillaceous matrix and glauconite rise the value of Neutron as well. Secondary porosities are displayed as k- feldspar dissolution and intraparticle glaucony dissolution in Yasser-6 and Yasser-4. These were reflected in the upward shifting points of the M/N crossplots of the same wells.