الفهرس 
Location of the Study Area
PETROGRAPHY AND CLAY MINEROLOGYOnly 14 pages are availabe for public view
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Abstract
The Lower Miocene Rudeis Formation is one of the most significant hydrocarbon reservoirs in the Gulf of Suez. The available geological and geophysical data for eight wells (Hb77-5 (A-1), A-2, A-3, A-4, A-10, Hb77-1, Hb77-4 and Hb78-2) were used to detect the reservoir depositional environment of this formation. The majority of the petrographical samples were taken from the Rudeis oil-bearing sandstone reservoirs, in addition to the open hole log response. The petrographic analysis was performed on the middle and lower units of the Lower Miocene rocks in Zaafarana oil field, by using eleven side-wall litho-samples, representing the Rudeis Formation in Hb78-2 well and correlated to core description for Hb77-1 and Hb77-4 wells, 212 ft and 88 ft resinated slabbed cores were obtained from wells Hb77-1 and Hb77-4; respectively to show the lateral facies change, which also confirmed by log response for other wells.
The cored intervals revealed that, the Rudeis Formation comprises mainly of major lithologies which have been sub-divided into 7 sub-units: These are dolomitic feldspathic sandstone intercalated with fossiliferous limestone (R1), dolomitic sandstone intercalated with algal limestone (R2), with shale intercalations, dolomitic sandstone intercalated with dolomitic limestone (R3) and dolomitic pebbly sandstone (R4), bioturbated shales (SH), bioclastic dolomite (DOL) and heterolithic facies (H), correlated with petrographic analysis which based on the samples collected from the 3 wells and the similar log response to build-up complete facies log. The lower and middle units of the Rudeis Formation were deposited as sub-aqueous fan delta. Also, the Nubia Sandstone of Wadi Araba may be acted as a provenance of the studied sandstone, which in turn shows considerable thinning from west to east, where it is represented by more shaly facies. Subsequent diagenetic processes, such as dolomitization and feldspar alteration played a great role in reservoir quality. The well log analysis was used to determine the petrophysical parameters such as porosity, water saturation and clay content. Those parameters revealed the occurrence of considerable change in lithology, and water and hydrocarbon saturations.
Petrophysical log analysis was based on a series of mathematical formula or models. The applied quantitative techniques use the charts and cross-plots, which in fact based on mathematical calculations, using equations, for example Archie’s water saturation equation. Identification of
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lithology is of particular importance in the formation evaluation process. The most useful logs for this purpose are the density, neutron, sonic, gamma-ray and resistivity logs for fluid typing (hydrocarbon or water). An important technique gives accurate result. For lithologic identification is the use of Schlumberger charts and cross-plots between RHOB and NPHI log reading, to obtain the lithologic characteristics of the Rudeis Formation, which is sandstone intercalated with limestone and /or dolomite intercalated with shale. The determination of water saturation is very important to complete the petrophysical parameters of the studied reservoir rocks. There are many equations for calculating the water saturation, such as Archie, Indonesia… etc. In our study; The Indonesia equation has been used for calculating the water saturation for the interested zones, where this equation is more applied for the shaly sand.
”Net Pay” is defined as the thickness of rock that contributes to economically viable production. Net-pay is determined by applying appropriate cutoffs to the reservoir properties, so that nonproductive or noneconomic layers are not counted. This can be done with both the log and core data. The applied cutoffs were: (10% <=PHIE), (0% <VSh<= 35%) and SW (0% <SW<= 60%).
The petrophysical characteristics of Rudeis Formation show that, the shale content ranges from 10 to 16 %, and increases towards the east of the area under investigation. The effective porosity of this formation ranges between 17 and 25 %, and increases towards the west side of the field where the sand thickness increases with low shale content and water saturation range from 20% to 36%, All the calculated petrophysical parameters for the studied wells.
The diagenetic environments of the studied samples were near surface to shallow burial and moderate to deep burial. The former environment was confirmed by bioturbation, micritization of bioclasts infilteration of detrital clays, early carbonate and feldspar dissolution, early dolomitization, minor quartz and feldspar overgrowths. The latter diagenetic environment witnessed the extensive dolomite cementation and dolomite dissolution.
The environment of deposition was classified according to all resulted data, The Middle Rudeis (RM) consists of sandstone, shale sequences with minor dolomite streaks varying in thickness. They were deposited in a sub-aqueous in transgressive shelf type, The Lower Rudeis (RL) is composed mainly of massive sandstone sequences and dolomite. They were deposited as subaqueous agraditional fan delta in a marginal marine alluvial braided plain after that they were subjected to secondary dolomitization.
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Bioturbated shale (clays) was deposited in a subtidal ”lagoonal” setting by sedimentation of suspended clays below storm wave base. Thin interbeds of laminated sands and bioclastic sands record occasional storm activity within the subtidal environment. The claystone facies has no reservoir potentiality and probably acts as laterally extensive permeability barriers within the reservoir section. The dolomite was deposited in association with the subtidal ”lagoonal” facies but in a slightly higher energy nearshore setting as semi-isolated carbonate sand shoals, which has limited reservoir potentiality.
The overall sedimentary environment is probably one of a linear fault-controlled coastline adjacent to a relatively low relief hinterland with small-scale alluvial drainage systems. Despite the subtropical to tropical sea conditions carbonate production was inhibited due to siliciclastic input.