الفهرس 
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Abstract
The purpose of the study is to use the available surface and subsurface datasets to model the structural evolution and phases of deformation of the southern part of the Gulf of Suez rift and its impact on the hydrocarbon exploration.
The study area lies in the southernmost part of the Gulf of Suez rift in the southwest dipping half graben.It extends from latitude 28o 10’ 0’’ N to latitude 27o 24’ 43’’ N and from longitude 34o 10’ 0’’ E to longitude 33o 05’ 0’’ E. It is bounded on the east by Sinai Massif on the west by the Red Sea Hills, on the south by Shadwan Island, and on the north by Morgan Oil Filed The most characteristic topographic features of the study area are the exposures of large Precambrian basement blocks at three different localities; namely Gebel El Zeit, Shadwan Island and Esh El Mellaha Range.
The data used in the study are 400 km2 of 3D PSDM (Post Stack Depth Migration) seismic data, 35 wells with geological formation tops, biostratigraphic data, electrical and mud logs, and published data of 5 measured surface sections in Sinai and the Eastern Desert.
The methodology and the approach that were used in the study are the conventional subsurface mapping evaluation techniques. Correlation panels were chosen and constructed to identify the
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missing sections in the wells and assign them to either faults or unconformities.
Isopach maps of different pre-rift and syn-rift units were then constructed along with seismic and structural mapping to reveal the interaction and the relationship between the tectonics and sedimentation. Finally structural cross sections were constructed and then restored at different times in order to reveal the fault history, amount of extension and block rotation and preservation/erosion of pre-rift section at the updip edges of the major fault blocks. The restoration helped in the better understanding of tectonic evolution that has an impact on hydrocarbon exploration as it helps in predicting the reservoir presence at the edges of tilted blocks and defining trap geometry.
The Gulf of Suez rift was opened due to NE-SW oriented extension related to the separation of Arabia from Africa. The Gulf of Suez is a rift zone that runs in a northwest-southeast direction and forms an elongated depression about 320 km long. It includes three half grabens of opposite polarity. The areas of major change in structural dip domain which seem to link the Gulf of Suez half-graben along strike have been termed accommodation zones or transfer zones.
The stratigraphy of the Gulf of Suez rift can be subdivided into two sequences, pre-rift sequence and syn-rift sequence. The pre-rift section comprises the oldest sedimentary record in the rift basin that overlies the granitic Precambrian basement. The Nubia Sandstone is the oldest pre-rift unit. Its age is a matter of debate between the
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surface and the subsurface studies. It ranges in age from the Paleozoic to Early Cretaceous. It consists mainly of sandstone and its thickness changes from north to south (from 1600 ft to 200 ft).
The mixed facies unit or the Nezzazat group overlies the Nubia Sandstone and consists of interbedded glauconitic sandstones, shales and dolomitic limestone. The youngest pre-rift section is the carbonate unit which comprises Sudr chalk (that includes the Brown Limestone member), Esna shale and the Eocene Thebes Formation.
The pre-rift sequence comprises two unconformities, the first recognized unconformity between the mixed facies unit and the carbonate unit. The age of this unconformity is Santonian – Campanian which marks the ceasing of the first deformation phase of the Syrian Arc and therefore it is related to the Syrian Arc deformation.
The second regional unconformity is the rift-breakup unconformity between the pre-rift section and the early syn-rift section. It cuts deep in the Thebes Formation. In some cases, the break-up unconformity is more severe and erodes all the pre-rift section with non-deposition of the Nukhul Formation.
The early syn-rift stratigraphy is represented by clastics deposition in the basin and it consists of the Nukhul, Rudies and Kareem Formations which were mainly deposited in a marine environment. The late syn-rift is characterized by the abundance of evaporation and deposition of salt and anhydrite with minor clastics input.
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The evaporites section consists of the Belayim, South Gharib and Zeit Formations. The Miocene evaporites are considered the prime top seal in the Gulf of Suez and they represent the lateral seal for most of the Miocene producing fields. The thickness and distribution of the evaporites are controlled by young structures, fault movements and the upward salt flow.
The thickness of the Nukhul Formation is strongley controlled by the early rift faults. The thickness in the hanging wall troughs reaches to more than 4000 ft and thins towards the updip edges of the tilted fault blocks with less than 500 ft thickness. At the updip edge of the Zeit and B-trend fault blocks the Nukhul Formation is completely missing due to non-deposition.
Very important observation is that the presence of thin Nukhul Formation or its complete absence is a good indication of the absence or erosion of the pre-Miocene section at the leading edge of the tilted fault block. The absence of the Nukhul Formation is a strong evidence for the likelihood of the eroded pre-rift section. The thickest Nukhul and Rudies sections are present in the downdip areas of the tilted fault blocks.
The isopach map of the Rudies Formation shows a similar thickness trend like the Nukhul isopach map where it has zero thickness due to non deposition at the footwall of the Z Fault. The Mid-Rudies unconformity between the lower Rudies and upper Rudies is considered the peak of the rift subsidence as the thickest formation in
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the syn-rift package is the lower Rudies and the variation in thickness across the rift faults is more pronounced than the younger syn-rift rocks.
Wells correlation, cross sections and seismic mapping have indicated that the southern Gulf of Suez can be subdivided into five major tilted fault blocks named from west to east: Esh El Mellaha Block, West Zeit Block, East Zeit Block, B-trend Block and Ghara Block. They comprise Miocene and pre-Miocene southwest dipping fault blocks bounded by major down-to-the-northeast clysmic normal faults. Each block or half graben is characterized by steep stratal dip (30o-40o) and the main faults are dipping at 30o-20o.
Two synthetic seismograms were created in the study area. Since the provided seismic data are 3D post stack depth migrated, the main objectives of creating synthetic are to tie significant geological markers to seismic data and to decide what to pick, peaks or troughs. The two synthetics show that the top of Kareem formation is a strong trough and top of Basement is a peak.
The seismic and structural mapping demonstrate the structural setting of the southern Gulf of Suez which is characterized by the existence of north-northwest oriented structural trends. These are Esh El Mellaha trend, West Zeit trend, East Zeit trend, B-trend and Ghara trend. They comprise Miocene and pre-Miocene southwest dipping fault rocks bounded by major down-to-the northeast clysmic normal faults
from top basement structural map and top Kareem Formation structural map it is obvious that the B-trend fault and the Ghara fault
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that bound the half grabens are striking NW-SE (clysmic) while the transfer faults that link the clysmic fault segments strike NNE-SSW resulting in the zigzag pattern of the major rift faults.
The change in throw of the B-trend Fault along strike as shown in the structural maps and the fault throw profile (Figure 4.7), where the throw reaches ca. 5500 ft then drops to ca.1800 ft then increase again to a maximum of ca. 7250 feet before it drops again to 3500 feet, suggests that the fault was initiated first as discrete separate segments that were linked later by transfer or linking faults.
The structural style of the southern Gulf of Suez rift is illustrated in four regional cross sections through the five identified regional half grabens extending from the Red Sea Hills to Sinai. The locations of these sections were chosen to optimize well control. Well and outcrop data are combined with seismic data where possible to produce the regional cross sections.
The sections show that the old or early rift faults (i.e. faults initiated at Nukhul time) are gentler in dip angle than the younger faults and bound the early formed tilted blocks that witnessed rotation up to 30o40o. These faults define the area of likelihood of eroded pre-Miocene section. The areas of eroded pre-Miocene section define the updip edges of the early formed half grabens.
The faults in the cross sections are subdivided into three orders. The first order faults are faults initiated at early rift opening and controlled the deposition of the early syn-rift Nukhul Formation. These first order
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faults are approximately planner and extend to the maximum depth of the seismic sections. They may have listric shape at deeper levels joining a basal detachment of the rift-bounding fault. The first order faults are the Rift-bounding fault (RBF), Esh El Mellaha Fault (EF), Zeit Fault (ZF), B-trend Fault (BF) and the Ghara trend Fault (GF). The dip angles of these faults are 25-30o illustrating long history of rotation. On the contrary, the Esh El Mellaha fault experienced less rotation as it stopped rotation since the middle Miocene
The second order faults are faults that are steeper in dip than the early rift faults. Their dip angles are ca. 40o. These second order faults are syn-depositional during deposition of the Rudeis Formation. These faults are not as continuous as the early rift faults and exist as disconnected segments on the maps.
The third order faults are the steepest faults in the study area (dip angles are 45- 55o) and were formed after deposition of the Rudies Formation with the shortest length on the maps.
In areas where there is not enough well penetration to estimate the fault plane dip angle, the dip of the pre-rift section is a useful guide to the fault dip angle. It is important to realize that the angle between a rotated normal fault and the pre-rift sedimentary section is close to 120o.This simple relationship constrains drawing the early rift faults (first order faults) on the structural cross sections to moderate or shallow dips in regions where substantially tilted rocks occur.
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Since the objective of this study is to review the structural evolution of the southern Gulf of Suez rift, restoring the structural cross sections is very important in understating the evolution of the area of study. The benefit of balancing structural sections is recognized in that it validates the interpretation and promotes a better understanding of the geological history of the area of interest.
The constructed structural cross sections were restored using 2DMove software by inclined shear method. Two cross sections were chosen to be restored at multiple time intervals. Both sections were chosen to show the difference between the amounts of extension of the Suez rift from the north to the south in the study area and to illustrate the evolution stages of the rift at different time intervals and their impact on the syn-rift stratigraphy.
The structural sections were restored at top Nukhul Formation and top Kareem Formation in order to see which faults were active during the early rifting stages and how the dip angles of the pre-Miocene rocks progressed with time. In the restored sections, only the fault blocks were rotated keeping the fault plane angles constant at their presentday angle.
The faults that were initiated early at the Nukhul time were responsible for rotating the early rift major fault blocks, which in turn resulted in erosion of the pre-Miocene section at the updip edges of the fault blocks and non-deposition of the Nukhul Formation. These faults continued activity till the deposition of the Miocene evaporites section.
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The pre-Miocene blocks increase in dip angle as the time progresses and the rotation on the old faults increases.
The total regional extension estimates in this study based on the constructed cross sections from wells, seismic and outcrop data and their subsequent restoration in 2D Move software, have been calculated for the two sections to be = 1.43 in the northernmost cross section and = 1.7 in the southernmost cross section.
The different extension ratios have been calculated at Nukhul time, at Kareem time and at present-day.
The Gulf of Suez rift basin is considered one of the world class hydrocarbon bearing basins in the world. The great thickness of the rift fill sedimentary section along with the presence of multiple intervals of source, seal and reservoir rocks have set up an ideal working petroleum system.
Due to the structural style of the Suez rift basin which is characterized by tilted fault block geometry, most of the traps in the Gulf are structural traps. The most common trap is the 3-way tilted block against fault.
The Nubia Sandstone is the most prolific reservoir in the southern Gulf of Suez. Significant discoveries are still being made in the Nubia Sandstone. What also makes the Nubia sandstone an attractive target is its proximity to the active brown Limestone source rock which in turn minimizes the hydrocarbon migration losses. So it is needless to say
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that understanding the preservation of the Nubia Sandstone reservoir and the risk of erosion is very important for hydrocarbon exploration.
One of the main southern Gulf of Suez plays is three-way pre-rift footwall blocks in Nezzazat and Nubia which are charged from the overlying Brown Limestone source rock. One of the main risks of this play is the erosion of the Nubia sandstone reservoir at the tilted block edge and /or the erosion of the pre-Miocene carbonate unit that act as top seal for the Nubia sandstone.
Understanding the structural evolution is very important in exploring for hydrocarbons in South Gulf of Suez, where the risk of preserving the Nubia reservoir at the edges of the tilted fault blocks is primarily constrained by the age of the fault bounding the block which in turn links to the amount of rotation that this block has undergone through the progressing of rifting.
It has been concluded in the present study that the absence of Nukhul formation at the crest of tilted fault blocks may be taken as an indication for the erosion or complete absence of the Nubia reservoir.