Thesis (Ph.D)-Cairo University, 2023.

Bibliography: pages 169-185.

In summary, rift basins that result from orthogonal or oblique rifting are
characterized by a segmented border fault system. These faults are parallel to
the rift axes (McClay et al., 2002). Preexisting zones of weakness in the
Precambrian basement were reactivated during rift initiation in Upper Egypt.
This rifting may be attributed to the far field stress associated with the
opening of the South Atlantic that caused the reactivation of the NW-SE and
WNW-ESE-oriented Precambrian faults and shear zones (Moustafa, 2019).
Therefore, the axis of rifting in the Upper Egypt Rift is strongly controlled
by an inherited NW-striking basement fabric (Mostafa et al., 2016). Hence,
this work shows that the Komombo, Nuqra, and Kharit basins were
previously studied by many authors as separate half-grabens (e.g. Ali et al.,
2018; Said and Sakran, 2019) due to the lack of subsurface data to support
the presence of one major rift. Our regional seismic mapping revealed the
existence of a NW-striking rift system that bounds the three half-grabens
through two major accommodation zones.
The interpretation of 6,500 km of 2D seismic reflection data allowed the
author to recognize a NW-oriented Early Cretaceous rift system crossing the
Nile River between latitudes 23° N and 25° N, which was previously masked
due to the lack of subsurface data. It is named here as the Upper Egypt Rift
and, structurally, comprises the Komombo, Nuqra, and Kharit basins, which
are hereafter referred to as the Nile Valley basins.
CHAPTER-6 SUMMARY AND CONCLUSIONS
156
6.1.1. Stratigraphy
The outcropping clastic successions in southern Egypt, including the study
area, northern and central Sudan, have been the subject of some controversy
about their names, origins, stratigraphy, distribution, and ages (El Naggar,
1970; Hermina et al., 1989, Klitzsch, 1990, Said, 1990; Issawi et al., 2009).
Issawi et al. (2016) have extended this debate to the lithostratigraphic
classification of the drilled units in wells Komombo-3, Nuqra-1 (Nugra-1),
and Kharit-1. In addition, the occurrence of Jurassic sediments in these
basins (Bisewski, 1982; Kerdany and Cherif, 1990; EREX, 2006; Fathy et
al., 2010) has been refuted by the results of paleontological analyses of
several wells (EREX, 2011; Wood et al., 2012; Abdelhady et al., 2016;
Mostafa et al., 2016).
Multiple names were given to the same stratigraphic units by different
authors (e.g., Said, 1990; Issawi, et al., 2009). However, this study uses the
stratigraphic nomenclature and ages proposed by Hermina et al. (1989), El
Naggar (1970), and EREX (2011), for the reason that the ages of drilled rock
units are confirmed by recent paleontological analyses by EREX Petroleum
Consultants in the Upper Egypt Rift (e.g. wells of Al Baraka-1, Narmer-1,
and Kharit-1). The Precambrian Nubian Shield in the study area and its
surroundings remained tectonically stable until the Early Cretaceous. Later,
the area acted as an interior continental basin for fan, delta, lake, and fluvial
sediments fed from the higher cratons (Wood et al., 2012; Selim, 2016).
A. Precambrian basement
The Precambrian Nubian Shield in the study area and its surroundings
remained tectonically stable until the Early Cretaceous. Later, the area acted
as an interior continental basin for fan, delta, lake, and fluvial sediments fed
CHAPTER-6 SUMMARY AND CONCLUSIONS
157
from the higher cratons (Wood et al., 2012; Selim, 2016). The country rock
is made of gneisses and schists (e.g. mica -and hornblende-schists and
gneisses, feldspar augengneisses, and amphibolites) that run roughly north
and south on the east side of the Nile (Said, 1962). In wells drilled in study
area, the average depth to top basement ranges from 1,800 to 2,600 m. It is
composed mainly of basic volcanic igneous andesite rocks. It is highly
fractured with common fracture filling calcite and epidote.
B. Syn-rift successions
The pre-rift phase was characterized by the existence of a prominent
platform area. This relatively high platform area provided sediments to the
adjacent subsiding areas. However, no significant amount of sedimentary
successions existed in the area of study prior to rifting. The syn-rift phase is
represented by Lower Cretaceous successions that unconformably overlay
the Precambrian basement rocks and were reported only from wells.
The post-rift phase began with major marine flooding of the Late
Cretaceous-Eocene systems and is characterized by an intense compressional
event synchronous with late Cretaceous-Eocene Syrian Arc tectonics
resulting from the closing of the Neotethys Ocean (Sehim, 1993; Selim,
2016).
C. Reservoirs
Stratigraphic successions drilled in the Komombo basin contain several
sandstone intervals with multiple pay intervals (Fathy et al., 2010; Wood et
al., 2012), and oil shows at variable depths. Oil was discovered in Al Baraka
field in two pay zones, including the shallow level of Six Hills Formation
(total vertical depth subsea: 914-1,220 m) and the deeper parts of the
Komombo sandstone (2,440 m). The Basal Komombo Formation pay
CHAPTER-6 SUMMARY AND CONCLUSIONS
158
intervals in units A and C are intercalated with the Komombo-B source rock
interval (Sea Dragon Energy, 2010).
Clastic sediments are derived from elevated footwall blocks and exposed
basement ridges. In the latter case, axial drainage systems may link separate
basins (Faulds and Varga, 1998). Accommodation zones provide the entry
points for large alluvial fans, fan deltas, and sand-rich turbidite deposits
(Morley et al., 1990). Shales near reservoir intervals act as intra-formation
seals, while thick shales within the upper parts of the Komombo and Six
Hills formations offered top seal. The thick shale interval of the SabayaQuseir Formation provides a regional top seal and lateral seal near downthrown.
6.1.2. Regional surface geology
The Upper Egypt Rift is well exposed at the surface, and marked by
topographic depressions and eroded fault scarps, which are distinct from its
uplifted rift shoulders to the south in the Kharit basin, and the region covered
by Nile sediments to the north in the Komombo and Nuqra basins. The prerift phase was characterized by the existence of a prominent platform area.
This relatively high platform area provided sediments to the adjacent
subsiding areas. However, no significant amount of sedimentary successions
existed in the area of study prior to rifting.
The remotely sensed data that have been used through this study includes
four Landsat-8 scenes covering the study area. These images were acquired
in August 2020. Numerous NNW to NW-oriented normal dip-slip faults
were recorded in the study area. These faults are characterized by steeply
dipping angles (60-70ᵒ
) towards the southwest and northeast directions.
These faults form several horst and grabens juxtaposing the Precambrian
basement against the Late Cretaceous strata or younger successions.
CHAPTER-6 SUMMARY AND CONCLUSIONS
159
The Kalabsha fault zone is located to the southwest of Aswan (~60 km to the
south of the Aswan High Dam), in the southern Western Desert of Egypt. It
is located at ~5 to 10 km to the south of Gebel Al Baraka, and is still active
today in the area surrounding the Aswan Reservoir. The Seiyal fault is a
dextral wrench faults recorded in the study area. It is parallel to the Kalabsha
Fault with an E-W strike and associated with sag synclines
Circular ring structures form domes over the Sin El Kaddab plateau that
affect the Early Paleocene surface on the geological map and on the Landsat
images along the western side of the study area.
The interpreted longitudinal folds can be subdivided into drag folds, reverse
drag folds and roll-over folds. Several NW-SE orientated kilo-meter scale
broad, open folds are developed parallel to the NW-oriented fault segments.
These folds are described as hanging wall forced folds e.g. NW-oriented
broad, open folds recorded in the eastern margin of Wadi Natash.
6.1.3. Structural interpretation and basin geometry
The structural architecture at the top basement level shows a consistent
pattern with two major fault sets: N115-145°E and N-striking faults. The
NW-striking faults are oblique to the Red Sea-rift axis and dip to the SW and
NE. Seismic lines across the rift show that the Cretaceous beds pinch out and
thin toward the SW. The major N-S boundary fault (F-2) defines the eastern
rift shoulder of the central Nuqra basin. It has a maximum displacement of
about 2,250 m at the depocentre of the Nuqra basin, where thick Cretaceous
strata are observed on its hanging wall. This major fault is characterized by
fault linkages amongst N-S and N150°E fault segments. Therefore,
displacement variations are observed along its strike.
CHAPTER-6 SUMMARY AND CONCLUSIONS
160
In the Nuqra basin, the strata thickening near the N-S fault segments
indicates their earlier development relative to the shorter N120°E and N150-
160°E segments. These N-S fault segments are represented as transfer faults
between the NW-striking fault segments.
In the Komombo and Nuqra basins, NW-SE and N-S striking faults (F-1 and
F-2) splay out into several faults (F-1a, F-1b, F-1c, and F-1d; F-2a and F2b). These splay faults are oblique to the rift axis and distribute the total fault
offset recorded over a wide area. On map view, fault splays are arranged in
en echelon geometry with relay ramps in between. The relay ramps roughly
tilt toward the basin, where the contours on the marker horizon are at a high
angle to the strike of the faults. On the seismic sections across the rift basins,
master faults dip to the west in an opposite direction to the rotation of the
faulted blocks. The interference between NW-striking and splay faults is
generally revealed on seismic sections as seismic attenuation area due to
sub-seismic faults. Although oblique faults to the NW-striking rift axis are
relatively short (10-20 km), they are characterized by their large heaves (e.g.
F-1a and F-1b). At basement level, the NW-striking master faults F-1 and F2 display hard-linked zigzag patterns with the oblique N120°E, E-W and NS faults. Faults F-3 and F-4 are moderately sinuous.
Two overlapping synthetic transfer zones (zones A and B) separating the
tilted half-grabens were identified in our interpretation. The along-strike
displacement variations of the fault F-1result in multiple structural styles in
the Komombo basin. Resulting variations in subsidence controlled the
distribution of the Early Cretaceous syn-rift sediment (Komombo
Formation), including proved source rocks in the Komombo basin (Dolson et
al., 2014; Mostafa et al., 2016). The bottom strata of Komombo source rock
are encountered in the wells drilled in the deeper areas of the basin. Seismic
CHAPTER-6 SUMMARY AND CONCLUSIONS
161
interpretation reveals these sediments to be restricted to early NW-striking
normal faults.
The structural high separating the Komombo and Nuqra basins was drilled
by well Diwan-1, which reached the basement without encountering source
rocks. The younger fault F-1 propagated in the footwall of an earlier oblique
fault during a phase of rift widening. This resulted in a broadening of the Six
Hills depositional area farther to the northeast. The confinement of
Komombo source rocks to near the older faults resembles the same setting at
well Diwan-1, and explains the absence of source rocks in wells Diwan-1
and Narmer-1. Younger fault activity is recognized in the basal Pliocene Nile
sediments.
The southeastern extension of the Komombo basin, east of the Nile River,
developed on the hanging wall of the master fault F-1, forming an elevated
ridge on its footwall side. The two faults (F-1 and F-2h) forming this
structure are shown in the map view as conjugate divergent overlapping
transfer zones (Morley et al., 1990) with a soft linkage in the middle part of
the study area. Two depocentres are formed on the hanging walls of these
faults, with a structural high in the area of fault linkage.
The Nuqra basin is developed near a N-S striking master normal fault (F-2).
This fault extends for 90 km through a linked fault system composed of NW
and N-S fault segments, and led to the presence of Upper Cretaceous
sediments and volcanic rocks on its footwall. This half-graben structure is
additionally crossed by several faults in different directions. These faults
have a maximum length ~3 km with minor displacements. The Nuqra basin
is steeply dipping toward the bounding fault system in the east, where the
basin sediments of the Santonian to Early Campanian Um Baramil
Formation are exposed at ~15 km from the rift axis. Therefore, it forms an
CHAPTER-6 SUMMARY AND CONCLUSIONS
162
asymmetric half-graben with growing syn-rift sediments toward fault F-2.
Although well Narmer-1 encountered oil, it was drilled on a structural high
and, consequently, did not encounter the Komombo source rock.
Seismic mapping shows that the Kharit basin is divided into two
depocentres. The northern depocentre is bounded from the east by the NWstriking fault F-3. The basement reveals a more basinal subsidence along the
eastern master faults F-3 and F-4 that delimit the Kharit basin to the top east,
with the depth to the basement reaching 3,000 m in the southeastern part of
the basin. The general rotation of the fault blocks is toward the NE.
Therefore, the growth of the syn-rift strata indicates younger fault
movements on the eastern side of the basin. The southern depocentre takes
the geometry of the graben structure bounded by F-4 and F-8.
Seismic profiles across the Kharit basin reveal the deeper basal rift
sediments to be restricted to the trough where the Komombo Formation was
possibly deposited during early rifting. Furthermore, the Sabaya and Abu
Ballas formations show wrench-related anticlines trending ENE and
associated with basin inversion. Smaller antithetic faults are recognized on
the hanging walls of the major faults. It is worth mentioning that the well
Kharit-1 was drilled in a deep graben structure rather than on a structural
high. The well reached the basement at a measured depth of 2,225 m and did
not encounter potential reservoir rocks.
CHAPTER-6 SUMMARY AND CONCLUSIONS
163
6.1.4. Tectonostratigraphic evolution
The tectonic evolution of the Upper Egypt Rift was mainly influenced by the
Mesozoic evolution of the Neotethys Ocean due to major plate
reorganizations generated by the breakup of Pangea. Additionally, the
opening of the Atlantic Ocean complicated the geodynamic settings due to
the changes in stress fields during different stages of rifting (Stampfli et al.,
2002; Guiraud et al., 2005; Berra and Angiolini, 2014).
The propagation and tectonostratigraphic context of the Upper Egypt Rift is
related to the Cretaceous tectonics, and was achieved through poly-phase
NE-oriented extension during the Berriasian, Aptian, and Albian (Selim,
2016). It ceased in the Late Santonian (83-85 Ma) synchronously with other
Syrian Arc structures in northern Egypt (Sehim, 1993). The main tectonic
phases that mark the evolution of the Upper Egypt Rift, in the framework of
North Africa evolution, can be summarized as follows.
A. Early Berriasian-Late Barremian
The first rifting episode occurred from the Berriasian to the early Aptian
(145-121 Ma), and was triggered in response to the opening of the South
Atlantic (Guiraud et al., 2005; Sekatni Aïch and Gharbi, 2019; Gharbi et al.,
2022). It was accompanied by rifting along the future continental margin of
West Africa, from Angola to Cameroon (Guiraud and Bosworth, 1999). In
the Upper Egypt Rift, this period witnessed the deposition of source rocks in
the Komombo and the Six Hills formations. The earliest syn-rift lacustrine
sediments of the Komombo Formation are restricted to depocentres on the
hanging walls of NW-striking fault segments. The early formed syn-rift
depocentres extended for ~20 km long with sedimentary package
characterized by its wedge shape, which contains strata up to 75 m thickness
with limited areal extent (Selim, 2016). Seismic and geochemical data show
CHAPTER-6 SUMMARY AND CONCLUSIONS
164
that the Berriasian-Barremian recorded maximum subsidence in the Nuqra
and Kharit basins, with a high possibility of main depocentres preserving
thick source rocks (Mostafa et al., 2016). Contemporaneous subsidence of EW-trending half-grabens in the northern Western Desert and northern
Cameroon occurred at this time (Maurin and Guiraud, 1993). The end of this
rifting phase is generally marked by a regional unconformity in the study
area.
B. Early Aptian-Late Albian
The Aptian-Albian period is known for its regional extensional setting across
the north-facing southern Tethyan continental passive margin of northern
Africa (Guiraud et al., 2005; Gharbi et al., 2022). This resulted in high
subsidence rates along NW-SE trending troughs; e.g., the Sudan basins
where up to 3-5 km of continental sandstone and shale were deposited
(Schull, 1988; Bosworth, 1992) and the Upper Egypt basins where 2-3 km of
continental sediments were deposited.
Sedimentation continued during the Aptian-Albian in the form of nonmarine sand and shale intercalations of Abu Ballas and Sabaya formations,
associated with rift widening as syn-rift depocentres extended for ~70 km
long and ~20 km wide.
The Upper Egypt basins are characterized by an angular unconformity
between Aptian-Albian sediments and their overlying successions (Wood et
al., 2012). The development of the angular unconformity is generally
associated with tectonic activity and block faulting. Two transgressions
occurred, around the mid-Aptian and Late Albian, resulting in the
development of large marine gulfs in southern Egypt (Guiraud et al., 2005).
Seismic data indicate that the Komombo, Nuqra, and Kharit basins subsided
at the same rate during the deposition of Aptian-Turonian successions.
CHAPTER-6 SUMMARY AND CONCLUSIONS
165
C. End of the Cenomanian
During Cenomanian, Tethyan margins recorded a major global sea-level
transgression with amplitude of more of 225-250 m above present-day sea
level (Haq, 2014; Tassy et al., 2015). At the end of the Cenomanian (92 Ma),
there was a major plate reorganization and the principal direction of
extension shifted from NE-SW to ENE-WSW (Guiraud et al., 2005).
In the study area, several episodes of volcanism are recognized in the
Cenomanian succession, as marked by the Natash bialkaline basalts, and
lasted with the formation of Campanian-Maastrichtian trachyte plugs
(Meneisy, 1990). The earliest stage of oil generation occurred during the
Cenomanian, while expulsion and migration occurred in Eocene times.
D. Early Turonian-Late Maastrichtian
The second rifting episode locally developed in the study area from the
Turonian to Late Maastrichtian (94-66 Ma). Rifting was accompanied by
minor volcanism represented by Natash volcanics in the eastern side of the
Nuqra basin, which is equivalent to the minor volcanism encountered in
wells of the northwest Muglad basin, and the Upper Cretaceous andesitic tuff
in the central Melut basin (Schull, 1988).
The early Turonian was a period of uplift, related to the initial opening of the
Red Sea and a worldwide drop in sea level. At this time, the entire Upper
Egypt area was sub-aerially exposed except for its deeper basins. This was
followed by a major marine transgression represented by the Turonian Abu
Aggag Formation, which continued to the Santonian. By the Late Santonian,
several E-W trending fault zones registered dextral transpression, for
example the Guinean-Nubian lineaments and Kalabsha fault in southern
Egypt (Bosworth et al., 1999).
CHAPTER-6 SUMMARY AND CONCLUSIONS
166
6.1.5. Impact of inversion tectonic on trapping system
The hydrocarbon traps related to tectonic inversion and wrenching may be
associated with fractures and faults that break the top seal and breach the
trapping system (e.g. well Memphis-1 in northern Komombo basin, Mostafa
et al., 2016). Seismic sections across the Al Baraka field in the central part of
the Komombo basin show mild tectonic inversion. The inversion-related
anticlines are interpreted in sequences younger than the Paleocene Esna
shale. This suggests a long span of Syrian Arc tectonics in the study area,
more than the recognized period of inversion tectonics of the northern
Western Desert.
Rotated fault blocks and inversion anticlines comprise the main hydrocarbon
traps in the Upper Egypt Rift, as successfully tested in the Komombo basin.
The relatively young age of the wrench-forming anticlines (Early Eocene,
Sehim, 1993) suggests a charging time during the Mid Eocene or later and,
except for secondary migration, the oil retention remains risky. However,
further work is required to define the timings of oil migration and trap
development in the Upper Egypt Rift .
Oil expulsion at Ro values of 0.7% is documented at an average depth of
1,220 m in the wells of the Komombo basin, whereas a shallower expulsion
level was encountered at 610 m in well Narmer-1 in the Nuqra basin.
Thermal Alteration Index (TAI) and Ro maturity curves in the Komombo
basin extrapolated to Ro of 0.2% indicate an exhumation of around 500 m
(Dolson et al., 2014), and 1,220 m in roof successions, as calculated by
Abdelhady et al. (2016). Hence, the thickness of the subsurface Cretaceous
succession in the northern segment of the Upper Egypt Rift reaches 3-5
times of its equivalent strata in the uplifted rift shoulders (Issawi et al., 2016;
Selim, 2016).
CHAPTER-6 SUMMARY AND CONCLUSIONS
167
The Nuqra and Kharit basins on the east side of the Nile experienced twice
the erosion of the basin roof sediments, where denudation exposed the
Sabaya Formation at the surface near well Kharit-1, and the Maghrabi
Formation in well Narmer-1. The latter well has a Ro of 0.62% at 366 m and
the extrapolated maturity curve to the Ro level of 0.2 % indicates
exhumation of 1,220 m. The burial history curve shows the Cenomanian as
the critical time for oil generation, while expulsion and migration occurred in
the Eocene.

تم استخدام البيانات السيزمية ثنائية الأبعاد مع تسجيلات الآبار والجيولوجيا السطحية لتقديم رؤى جديدة حول الهندسة الهيكلية والتطور التكتوني لصدع طباشيري مبكر يعبر وادي النيل في صعيد مصر. يمتد هذا الصدع إلى حوالي 260 كم في اتجاه شمال غرب-جنوب شرق، وعرض حوالي80 إلى 100 كم، ولم يتم دراسته سابقًا بسبب نقص البيانات تحت السطحية. تم تسميته هنا باسم صدع صعيد مصر. وهو مقسم إلى ثلاثة احواض، أو أقسام، من الجنوب الشرقي إلى الشمال الغربي: أحواض خريط ونقرة وكوم اومبو، وهي ممتلئة بطبقات قارية سميكة (حوالي 3700 م) والطبقات البحرية من العصر الطباشيري السفلي والكامباني/ماستريخت. أحواض خريط وكوم اومبو عبارة عن أحواض نصف مدمجة ومحدودة بصدوع تضرب في اتجاه شمال غرب وتحد صخور ذات ميل في اتجاه شمال شرق. وهي مرتبطة بصدع عادي رئيسي ذات اتجاه شمال - جنوب ويحد الجانب الشرقي من حوض النقرة وينتهي في منطقتين متداخلتين من مناطق النقل الاصطناعية. هندسة التصدع بالمنطقة نشأت عن طريق إعادة تنشيط منطقة التصدع الموازية لصدع النجد والتي كانت موجودة في عصر ما قبل الكمبري أثناء بدء الصدع. تشير استعادة المقاطع السيزمية العرضية الى تاريخ تمدد متعدد الأطوار لصدع صعيد مصر. حدث التصدع الاول خلال فترة البريازيان والفلانجيان و والابتيان المبكرة نتيجة لشق جنوب المحيط الأطلسي، وتأثر أيضًا بالتطور التكتوني لمحيط النيوتيثي. تم نشوء التصدع الثاني محليًا في المنطقة خلال العصر السينوماني-التوروني وتميز بوجود البراكين القلوية والصخور البركانية المتداخلة. تتواجد براكين الناتاش ذات الصلة بالصدع وقطاعات التراكيت ذات العصر السينوماني/التوروني في الجانب الشرقي من حوض النقرة ولكن لم يتم تسجيلها في حوضي كوم أمبو وخريط. بعد ذلك بفترة وجيزة، شهد الحوض بأكمله انقلابًا تكتونيًا خفيفًا متزامنًا مع أحداث القوس السوري الانضغاطية. تشكل كتل الصخور الصدعية المائلة والتراكيب المقلوبة قليلاً هياكل المصائد الهيدروكربونية الرئيسية في حوض كوم امبو. هناك حاجة إلى المزيد من العمل لتحديد العلاقة المعتمدة على الوقت بين هجرة البترول وتكوين المصائد البترولية في صدع صعيد مصر.

Issues also as CD.

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