Suez Basin Research Articles

Effective porosity determination in clean/shaly formations from acoustic logs with applications

Reliable evaluation of porosity in clean and/or shaly formations requires knowledge of matrix (type and nature), fluid, shale volume and shale transit time affecting the determination of such parameter. If all are linked together in a compatible and quantitative way, good results for porosity are obtained. However, the objective assigned to this paper is to introduce an equation for estimating effective porosity in clean/shaly formations from sonic logs. This is considered as an acoustic- V sh model that represents a few advantages over the other equations. In addition to being a simpler expression, it treats the shale effect as a correction term subtracted from the clean–sand term (i.e., the equation clearly points out the practical aspect of the shale effect and its correction.). Also, it gives reliable and acceptable values for porosity with Δ t ranging between 70 to 110 μs/ft and Δ tsh from 70 to 95 μs/ft (i.e., good results could be achieved with variable shale transit time). After being tested among a wide variety of samples representing different lithologies, the equation gives acceptable values of porosity with reasonable accuracy specifically where no porosity tools other than sonic logs are available. Successful application to real field data in the Gulf of Suez Basin of Egypt and Williston Basin in the USA, is also demonstrated.

BVW as an indicator for hydrocarbon and reservoir homogeneity

Bulk Volume Water (BVW) which is defined as the product of water saturation and the porosity is one of the most important reservoir parameters in: (1) Defining the state of rock whether homogeneous or not, (2) Determining reservoir permeability, and (3) Indicating shaliness effect. The purpose of this paper is mainly two folds: (1) To evaluate such parameter using Resistivity and Gamma ray data in shaly sand reservoirs and in absence of the porosity data, especially in old wells that suffer from lake of data; as a function of formation water resistivity ( R w), corrected true resistivity ( R t), gamma ray readings (GR) and shale resistivity ( R sh), and (2) To use such parameter as a hydrocarbon indicator. Such method was checked and successfully applied in three examples; one from San Juan Basin, USA is used to test the proposed formula in determining such parameter and the other two from Gulf of Suez Basin of Egypt, the first to serve the second fold of the paper, the use of (BVW) as a hydrocarbon indicator; whereas the second well is used to test and apply the two folds of this paper to illustrate how far such treatment is more or less reliable and accurate.

Porosity estimation using a combination of Wyllie–Clemenceau equations in clean sand formation from acoustic logs

An equation relating acoustic travel time to the matrix and fluid times with the aim of quantifying sonic porosity in clean formations is introduced. However, this can be done by multiplying the well-known time average equation of Wyllie et al. [Geophysics 21 (1956) 41] by the acoustic formation factor equation of Raiga-Clemenceau et al. [Log Anal. (Jan.–Feb. 1988)] in an objective manner. This equation, which was satisfactorily tested among a wide variety of samples representing different lithologies, gives very close results with the measured values within an acceptable range of error. With the exception of core measurements and when no other porosity tools are available, the proposed equation is considered as a good predictor for estimating porosity, at least for the studied intervals considered here. What makes this equation more reliable and advantageous over another is that it integrates most of the parameters affecting porosity in a compatible way, which enhances the accuracy of estimating such parameter. Also, it gives reasonable values for rocks, which have a transit time between 40 and 120 μs/ft. Above this range, the usefulness of this equation must remain in doubt. Its successful application to real field data in the Gulf of Suez Basin of Egypt is also demonstrated.

RELATIONSHIP BETWEEN PETROLEUM COMPOSITION AND DEPOSITIONAL ENVIRONMENT OF PETROLEUM SOURCE ROCKS FROM THE GULF OF SUEZ AND THE WESTERN DESERT OF EGYPT

ABSTRACT Crude oils from the Gulf of Suez and the North Western Desert of Egypt have been analyzed for geochemical biomarkers using GC and GC-MS techniques. The biomarker compositions of the crude oils have been used to differentiate crude oils of non-marine, normal marine and marine carbonate sources. The geochemical features of Zaafarana crude oil from the Gulf of Suez Basin indicate a marine carbonate depositional setting. One crude oil from Budran however, possesses geochemical characteristics consistent with an origin from source rock deposited in normal marine conditions. Bahar and Morgan show normal marine source rock deposition environment with terrigenous organic matter input. On the other hand, the crude oils from the North Western Desert have bulk and biomarker characteristics cosistent with non-marine depositional setting, with the exception of one oil sample which appears to have a mixed marine/terrestrial sources. The presence of oleanane in some of these oils suggests source rocks deposited in deltaic or near shore environment in Post Cretaceous Basin.

Source-dependent biomarker properties of five crude oils from the Gulf of Suez, Egypt

Five crude oils from some producing wells in the southern part of the Gulf of Suez were characterized using a variety of organic geochemical parameters. The results reveal significant differences within the oils from the Gulf of Suez basin that suggest two oil types and one mixed type. Type 1 oils from Ras Fanar and East-Zeit fields show a high relative abundance of gammacerane indicating a marine saline-source depositional environment. Another related feature of these oils is the predominance of C 35 over C 34 17α( H)-homohopanes. Type 2 oils from Gama and Amal-9 wells show oleanane indices over 20%, indicating that they originated from an angiosperm-rich, Tertiary source rock. Type 3 oil from Amal-10 well has geochemical characteristics intermediate between those of types 1 and 2. This may reflect mixing of oils from the sources of both types 1 and 2. The geochemical characteristics of the oils vary sufficiently to suggest multiple sources. The type 1 oils possess geochemical characteristics consistent with Upper Cretaceous Brown Limestone or Lower Eocene Thebes Formation carbonate source rocks. Type 2 oils are very similar in geochemical character to the Lower Miocene Rudeis Formation source rocks.

Continental Rifts: Evolution, Structure and Tectonics

Twenty one “friends of continental rifts” wrote Continental Rifts: Evolution, Structureand Tectonics. They define the object of their passion as elongate tectonic depressions along which the entire lithosphere has been modified by extension. Strictly speaking, passive margins and highly extended terranes such as the Basin and Range are not included in this definition, but the authors consider them to be related to continental rifts. The authors hail from academia and set as their main goal “an improved understanding of the fundamental lithospheric processes of rifting, with primary focus on deep structures and processes associated with rifting.” Consequently, many well‐known extensional systems, for example, the North Sea grabens, the Suez Basin, onshore and offshore eastern China, and large areas of southeast Asia, are barely considered. Rift aficionados from the petroleum industry will find very little to interest them in this book.

Sequence Stratigraphy and High-Frequency Cycles: New Aspects for a Quantitative Evaluation of the Gulf of Suez Basin, Egypt

A new development in the application of sequence stratigraphic concepts in marine as well as continental basins is the recognition of high-frequency cyclic patterns in rock successions in the subsurface. Studies of six wells from the northern, central, and southern parts of the Gulf of Suez show the presence of well-preserved, high-frequency cycles with periodicities similar to the orbitally forced Malankovitch parameters. Subsurface rock successions, third-order sequences, and high-frequency cycles were compared with outcrops. After establishing the biostratigraphic framework for the above-mentioned wells, a sequence analysis was performed. Sequence boundaries and maximum flooding positions in each well were calibrated with the occurrences and evaluation of the high-frequency cycles. It became obvious that there is an intimate relationship between these high-frequency Milankovitch cycles and sequence organization. In addition, a close relationship can be observed in the subsurface as well as in outcrops between high-frequency climatic changes (connected to the Milankovitch cycles) and (litho)facies variability. Quantitative evaluations of each sequence and/or systems tract can be computed with the International Geoservices' cyclicity analysis tool (MILABAR). The results are summarized in a well composite chart, rate (NAR), and ratio of preserved time. In correlations between the wells, an accuracy of 500-100 Ka can bemore » obtained. The quantitative evaluation of the sequence and high-frequency cycle analysis gave some new aspects concerning the (litho)facies and geodynamic development during the pre- as well as the synrift stages of the Gulf of Suez Basin.« less

The Red Sea — Gulf of Aden: re-assessment of hydrocarbon potential

The results and conclusions of the UNDP/World Bank Red Sea — Gulf of Aden Regional Hydrocarbons Study Project (June 1989 – January 1992) are summarized. These derive from detailed examination of pooled public sector data provided to the project by the participating bordering states in the region; these included samples and logs from about 56 wells drilled in the two basins (out of a total of 71) and about 30 000 line km of reflection seismic data (selected from 110 000 km available). Well logs were digitized and correlated, and basinwide biostratigraphic, lithostratigraphic and palaeoenvironmental studies, source rock organic richness, quality and maturity studies and sedimentological analyses studies were performed by contracted specialist staff in the UK and France. These were integrated with basinwide seismic structural and isopach mapping for selected regional reflectors/intervals by national task forces in Cairo under project supervision; academically acquired crustal seismic and other geophysical data and oil industry velocity and other borehole data were also integrated for the Red Sea. Modelling utilizing recently acquired geophysical data indicates crustal asymmetry for the Red Sea rift with oceanic crust of pull-apart basin type flooring some portions of offshore Egypt and Sudan on the west and stretched continental crust underlying the eastern Arabian flank. These interpretations furthermore suggest that sinistral strike-slip processes controlled the initial break up of the Arabian plate, shaping the African flank as a sharp plate boundary; similar processes appear to have initially shaped the Somalia flank of the Gulf of Aden rift. Only later did true seafloor spreading propagate; in the eastern Gulf of Aden 10–12 Ma, about <5 Ma in the western Gulf of Aden, Afar and the southern/central Red Sea. Biostratigraphic work shows that deep subsidence and ocean water inundation commenced in the late Middle Oligocene in the Gulf of Aden, in the Late Oligocene in the southern Red Sea and in the Early Miocene in the Central Red Sea/Gulf of Suez. A regional lithostratigraphic nomenclature scheme for the Miocene-Recent syn- and post-rift Red Sea successions keyed to detailed biostratigraphical distribution charts and microfaunal assemblage characteristics was erected, tied to the well documented and understood petroliferous Gulf of Suez basin scheme; this allows facies variations within the basin to be compared and allows consistent interpretation and correlations across the basin to which local informal nomenclature can be related. Pre-rift (pre-Oligocene) marine sediments are present in most of the Gulf of Aden (Jurassic-Eocene) but only incompletely on the basin margins in the Red Sea (Jurassic in the south, Upper Cretaceous — Early Palaeogene in the north). Significant pre-rift oil/gas prone Upper Jurassic, Cretaceous and Palaeogene source rocks occur in Gulf of Aden sub-basins initiated by Late Jurassic — Early Cretaceous tensional tectonics, with maturity levels ranging from early-late to post-mature, whereas early synrift source rocks are generally immature. In the Red Sea good quality pre-rift source rocks outcrop in the north and south but are probably overmature for oil in the basin except in the extreme south (if present). Good syn-, pre- and post-salt and post-rift source levels occur throughout the basin but in the central/southern sectors only the post-salt remains within the oil window because of high geothermal gradients. Both basins are little explored. Attractive pre-rift Jurassic-Eocene exploration plays in rotated fault and horst blocks in predominantly carbonate but subordinate sandstone reservoirs are present in the Gulf of Aden (confirmed by an important undeveloped Eocene light oil discovery in offshore Yemen) with subordinate synrift potential where maturity is established. Relative continental shelf narrowness is economically limiting, but wider sectors occur. In the Red Sea the principal exploration plays are in Miocene synrift, pre- and post-salt successions. Rotated fault blocks occur in the pre-salt, but maturity for oil in the deeper levels is restricted to the northern sector. These levels in the central and southern sectors are overmature except in the southernmost portion but excellent potential exists in the synrift post-salt and early post-rift sequences with a number of attractive play types. Reservoirs for synrift plays are mainly sandstones and potential is confirmed by several undeveloped gas and condensate discoveries and numerous oil seepages throughout the basin; relatively wide continental shelves occur in the south and along much of the eastern flank.

Geology of the Warda Discovery, Gulf of Suez, Egypt

British Gas, as operator for its partners, Yukong Ltd., and Union Pacific Petroleum Suez Ltd., recently discovered the Warda field in the Gulf of Suez in November 1990. Discovery well Hb78-2, located 4 km off the mouth of the Wadi Araba on the west side of the north-central Gulf of Suez, reached a total depth of 8,400 ft in Nubia clastics. Oil was tested in the Hb78-2 from five separate clastic intervals at a cumulative flow rate of approximately 7,000 BOPD of 20-24{degree} API with no water. Currently, an appraisal well program is underway to extend the limits of the field. This significant new-field discovery is located in a part of the Suez basin that previously considered to have limited oil potential. Geophysical assessment of the structure used exploration 3-D seismic coverage and concurrently acquired potential-fields datasets. The Warda structure at Kareen-Rudeis levels is a broad four-way closure, trending northwest-southeast, which drapes over older pre-Miocene tilted fault blocks. The Warda structure lies downthrown to a major Clysmic shoulder fault and is bounded on the northeast by another Clysmic down-to-the-basin normal fault. Cross faulting is noted at the deeper levels, and particularly influences the rift boundary faults. According to burial-history modeling,more » oil generation occurred within 4-5 million years after the Eocene-Senonian carbonate source rocks achieved onset of oil generation at a birthline of 10,000-11,000 ft subsea. The Cretaceous-Carboniferous Nubia sandstones apparently have acted as a conduit for oil migration from the adjacent Lagia hydrocarbon kitchen a few kilometers to the northeast.« less

Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence

Fifty-six apatite fission-track ages and 52 horizontal confined track-length measurements are reported from Precambrian crystalline rocks along the western margin of the Gulf of Suez, Egypt. Ages fall in the range of ca. 11–385 m.y. and older ages often occur within very close geographic proximity to younger ones, indicating non-uniform uplift. The wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths. On the basis of apatite fission track ages and their length distributions, data fall into three distinct groups. Group I: ages ranging from 43 to 385 m.y. Length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length. Group II: ages ranging from 23 to 31 m.y. Length distributions are negatively skewed with either a distinct tail or a small peak of short tracks. Group III: ages ranging from 11 to 20.5 m.y. Length distributions are al unimodal, narrow, negatively skewed and have the longest mean lengths among samples studied. Apatite ages from groups I and II are interpreted as “mixed ages” as a result of cooling during uplift from different levels within the apatite partial track annealing zone. Ages from Group III are interpreted as “cooling ages” due to uplift from the apatite total track annealing zone with minor partial annealing. Correcting the ages of the two oldest samples in this group for track-length reduction yields ages of 21 ± 2.2and23 ± 1.5m.y. It is proposed that the onset of rift-flank uplift in the Gulf of Suez—northern Red Sea area occurred between 21 and 23 m.y. ago. Fission-track analysis in combination with subsidence data from the Gulf of Suez basin, indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the Gulf of Suez. Furthermore, this uplift is contemporaneous with, and is directly related to, the process of extension and subsidence at the Gulf of Suez.

Neogene tectonic and stratigraphic events in the Gulf of Suez rift area, Egypt

The Gulf of Suez Neogene (syn-rift) section contains at least five datable intra-basinal unconformities or stratigraphie hiatuses. These events and associated changes in sedimentary sequences seen in the basin can be related to changes in Suez rift subsidence, Miocene eustasy, and the tectonic effects of adjacent Red Sea and Dead Sea rifting. The first hiatus spans Oligocène to earliest Miocene time and separates syn-rift from pre-rift strata throughout the region. This erosional event resulted from regional emergence during the Oligocene and the initiation of Suez rifting. A second hiatus, from ~ 21-19 Ma, separates the poorly dated, shallow-marine Nukhul Formation of Aquitanian to early Burdigalian age from the overlying upper bathyal shales of the middle Burdigalian to Langhian (NN3–5) Rudeis Formation. This hiatus resulted from both a brief lowstand in early Burdigalian sea level and increased Suez rift-related tectonism. This event climaxed with the rapid tectonic subsidence that established deep-marine conditions by 19 Ma. A third major event occurs within the Rudeis at ~ 17 Ma (NN4, N7) and marks the initiation of uplift of I he rift shoulders and margins, and decelerated subsidence, or even minor uplift, in the axial trough. Hiatuses are seen over paleohighs e.g. tilted fault blocks, whereas coarse elastics are deposited in more basinal areas in submarine fans and as turbidites. A fourth unconformity, at ~ 14 Ma, separates the Middle Miocene Kareem Formation from overlying anhydrites and open marine shales of the Belayim Formation. Minor tectonic uplift throughout the Suez region resulted from this event, and a sill was established between the Mediterranean and Suez basins, allowing cyclic anhydrite-marl deposition from ~ 14 to 11 Ma. This “post-Kareem event” may be the sedimentary response to initiation of movement along the Dead Sea-Aqaba transform. A sharp drop in sea level at ~ 11 Ma restricted the Suez basin even more, and marked the initiation of massive halite precipitation in the central and southern Suez, well south of the sill area. A final major Neogene event occurs at the Miocene-Pliocene boundary, and is marked by a major unconformity caused by tectonic uplift. This tectonic pulse is related to initiation of Red Sea sea-floor spreading, and acceleration of sinistral motion along the Dead Sea-Aqaba transform at ~ 5 Ma. This event establishes the marine connection with the Red Sea and the sedimentary environments seen in the Gulf of Suez today.

Oil Window in the Gulf of Suez Basin, Egypt: ABSTRACT

Oil Window in the Gulf of Suez Basin, Egypt: ABSTRACT

Gulf of Suez-Rift Basin Stratigraphy: An Interplay of Subsidence and Eustatic Sea Level: ABSTRACT

The Gulf of Suez and Red Sea rift basin underwent a period of rapid subsidence from the early Miocene to the Pliocene during which time a thick (up to 4 km) series of marine evaporites accumulated within the basin. The evaporitic sequence interfingers with carbonates and clastics over structural highs within and along the margins of the basin. Evaporite deposition was also interrupted basin wide by short periods of normal marine sedimentation. Timing and paleo-oceanographic aspects of evaporite deposition within the rift is controversial. A change over of marine source waters within the basin from the Mediterranean Sea to an opening of the rift to the Indian Ocean occurred sometime between the earliest Messinian and earliest Pliocene. Preliminary data suggests that anhydrites from this evaporite sequence retain original Miocene sea water Sr/sup 87//Sr/sup 86/ values which can be compared to Neogene strontium isotope versus time curves in order to further constrain the age of the nonfossiliferous evaporite group. This, combined with currently accepted biostratigraphies for the normal marine strata, enable us to refine rift stratigraphy in order to examine basin subsidence, evaporite accumulation rates, and the correlation of rift tectonics, sedimentation, and associated paleo-oceanographic events. Initial fragmentation and subsidence propagatedmore » from the south to the north in the Gulf of Suez during the Aquitanian to Burdigalian (20-25 Ma), and mixed clastic, carbonate, and evaporitic sediments (Nukhul Formation) up to 700 m thick were deposited in isolated subbasins within the rift. This episode was followed by renewed uplift of the rift shoulders, rapid subsidence, and increased clastic influx (late Rudeis Formation) during the Burdigalian (ca. 20-17 Ma).« less

Structural Style, Stratigraphic Variability, and Hydrocarbon Traps in Gulf of Suez: ABSTRACT

Hydrocarbon traps in the Gulf of Suez, whether of a structural or a stratigraphic type, fundamentally are all products of the highly complex late Cenozoic fault-block mosaic. Understanding the geometry and the evolution of this tilt-block mosaic is the key to further successful exploration in the Gulf. The major faults forming the Gulf of Suez basin are listric normal faults broken along strike by synchronous cross faults that serve as transform or transfer structural elements. The cross faults are a consequence of the inability to form uniform and laterally continuous listric fault surfaces within the mechanically anisotropic and inhomogeneous crystalline basement terrane of the Gulf. The combination of contemporaneous Gulf-parallel listric faults and cross-strike transfer faults gives rise to the complex mosaic of polygonal tilt blocks. Vertical variations in the structural behavior result from a synrift stratigraphy dominated by thick marls and evaporites. The ductile synrift rocks exhibit shallow secondary detachments, drape folds, and compaction and/or diapiric structures t at are decoupled from, but appear controlled by, the brittle underlying basement-rooted tilt-block mosaic. Lateral stratigraphic variability observed in the Gulf is the consequence of sea level rising and falling relative to the higher structural elements in the tilt-block mosaic. This has resulted in deeply eroded basement highs, clastic apron and perched reef reservoirs associated with the highs, and thick organic-rich marl accumulations in the intervening lows that serve as kitchen areas. End_of_Article - Last_Page 305------------

Oil and Gas Prospects in Egypt

Accumulations of oil were first known in Egypt since 1968, some ten years after the famous Drake well in Pennsylvania in August 1859. These oil accumulation were found as seepages in tunnels dug for extracting sulphur in the area of Gemsa on the West coast of the Gulf of Suez. Since that time exploration activites were conducted on and off throughout a long history of operations which witnessed the use of the latest developments in exploration tools and techniques. So far, the main oil province in Egypt is the Gulf of Suez Basin, where the bulk of oil reserve have been found. Other hydrocarbon provinces are the Nile Delta Basin, the Abu Gharadig Basin and the Alamein ridge, both in the Western Desert. Three important gas discoveries were made in the Nile Delta Basin two of which are offshore, in addition to other so for non commercial gas finds. In addition to the known oil and gas discoveries the exploration activities throughout that long period contributed volumenous geological and geophysical material and data which added appreciably to the geology of Egypt at large, and in particular to the petroleum geology of the country. The Gulf of Suez oil province displayed very favourable conditions for the generation and trapping of hydrocarbons in Miocene and Pre-Miocene pools though at the same time it displayed very complicated geological setting which renders the exploration work a difficult task. The Nile Delta Basin, on the otherhand, is a relatively very recently explored area. Accordingly, extensive exploration work is still needed in order to evaluate the hydrocarbon possibilities of that basin. So for, gas has been discovered at a number of locations in lower Pliocene-Miocene pools. Whether future work would prove that oil generation took place remains to be seen. The Western Desert did not yet display the required response with regards to the exploration work conducted till now. To date, only few small discoveries have been made with oil and gas pools mainly vvithin the Upper and Lower Cretaceous. Recent advances in seismic techniques are helping in locating deeper structures, which might prove instrumental in the upgrading of the oil possibilites of the region. At present, the exploration activities are going on in the different oil and gas provinces in Egypt. These activilles are expected to increase and are also expected to expand to new areas in the eastern part of the Gulf of Suez Basin, offshore and onland, in Northern Sinai and in the Offshore Mediterranean. These new areas are believed to house very good prospects which will be tested not before long. It is also hoped that progress in seismic tools and techniques will add to the possibilities of finding more oil and gas.

Generation, Migration, and Entrapment of Petroleum in Extensional Basins: ABSTRACT

The interplay of tectonics, thermal regime, and depositional history generally determines the petroleum potential of individual basins. The application of these factors to a review of several extensional basin systems (Rhine graben, North Sea basin, Reconcavo basin, Gulf of Suez basin, Red Sea basin, Bass basin, and Gippsland basin) provides the basis for this analysis. Extensional tectonic systems provide a mechanism for thinning the crust and upper mantle (the lithosphere). The result is a progressive rift-basin evolution starting with graben formation and ending in the development of continental margins. However, if tensional stress stops at any stage, a post-rift phase of subsidence, regulated by thermal decay, begins. Sedimentary rocks deposited prior to extension (pre-rift sediments) are preserved in the graben areas but eroded from the horsts. Rift sediments (usually clastics) deposited during extension are eroded from the uplifted areas, but carbonate rocks and evaporites may be deposited if the climatic environment is favorable. Sediment deposition is continued after extension stops (post-rift sediments). Source beds can be in the pre-rift, rift, or post-rift sediments but generation does not occur until they are buried to the depth of the generative window. Generation is aided by the high heat flow caused by the thinned crust. The kind of petroleum generated is dependent on the type of organic material present and the deepest zone of generation reached. Extensive normal faults contribute to trap geometry but inhibit long distance migration. Consequently, except in the post-rift section, entrapment of petroleum requires that the source and reservoir rocks are in close proximity, which can be accomplished by faulting, interfingering, or the superposition of source rocks on reservoir rocks at unconformities. The most favorable conditions for generating and trapping large oil fields are commonly in or near the deepest part of the basin. End_of_Article - Last_Page 779------------

Evolution of Eocene-Miocene Sedimentation Patterns in Parts of Northern Egypt

An analysis of sedimentologic data of Eocene, Oligocene, and Miocene rocks from northern Egypt has resulted in a synopsis of the depositional systems operative. A complex pattern of dominantly carbonate and minor associated terrigenous facies was deposited during Eocene time in narrow and elongate tectonic basins that were arms of the Tethys sea. The basins were separated by elongate structural ridges that resulted from Late Cretaceous tectonic activity. Infill of the ancestral topography during early and middle Eocene time, as reflected by the distribution of facies, ultimately provided the depositional base for seaward progradation of terrigenous sedimentation beginning in the late Eocene. Progradation during the Oligocene and Miocene was from southwest to northeast, ov r an open shelf of irregular bathymetry. This resulted in deposition of a thick terrigenous section in the western and central parts of the northern Western Desert. Longshore currents aided in the redistribution of sediments in an easterly direction toward the present Nile delta and resulted in an easterly shift of terrigenous depocenters from Oligocene through Miocene time. As the terrigenous clastic depocenters shifted eastward, carbonate environments reappeared in the west and onlapped abandoned parts of the fluvial-delta systems. By the early and middle Miocene, carbonate environments had become extensive and continued to shift eastward with shifting terrigenous clastic deposition. Upper Miocene strata have not been recognized in Egypt by some workers, but terrigenous clastic facies ear and north of Lake Manzala may represent late Miocene deposits. The embryonic Gulf of Suez, which developed as an arm of the Mediterranean during the Oligocene and early Miocene, was the site of deposition of arkosic sands, fanglomerates, and some evaporites, which originated within or from along the edge of the basin. However, fine-grained terrigenous sediments were transported by longshore currents to the Gulf of Suez from ancestral Nile deltas near the present Nile delta. Large volumes of evaporites, contemporaneous with the Nile delta, accumulated in the Gulf of Suez basin during the middle and late Miocene as a consequence of construction of a terrigenous clastic dam at the north end of the gulf.

New Evidence of Transcurrent Movements in Red Sea Area and Petroleum Implications

New evidence, derived from the displaced positions of two pairs of prominent shear zones--Abu Masarib (Arabia)-Duwi (Africa) and Wadi Alhamd (Arabia)-Wadi Hafafit (Africa)-indicates that Arabia has moved northward about 150 km relative to Africa along the Red Sea rift. The displacement of the shear zones was observed initially in photographs of the Red Sea area taken during Gemini earth-orbiting missions and has been substantiated subsequently by data derived from the geologic literature. The movement also must have displaced the basement block in northwestern Arabia (Maqna block) from its original position at the southern end of the Gulf of Suez to its present position at the Gulf of Aqaba. Hence it is evident that Maqna block and the adjacent offshore area south of Sinai Peninsula were part of a structural unit (Greater Gulf of Suez basin) and thus are similar in sedimentary and tectonic history and petroleum potential. Gemini pictures show that the Red Sea structures are dominated by five major fissure systems; many show evidence of being transcurrent. Certain fissures trending WNW-ESE appear to bear a spatial relation to most oil occurrences in the Gulf of Suez.