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