9Chapter IIGeological Setting

This chapter is devoted to reviewing the structural, tectonic andstratigraphic framework of the northern Western Desert and Razzak oilfield, tounderstand their control on the oil accumulation in the study area.

2.1Introduction:In the past, the north Western Desert was intermittently submerged by

epicontinental seas. Several tectonic events affected the north Western Desert.The early Paleozoic and the late Paleozoic events were mild and are representedby regional uplifts of moderate magnitude producing disconformities within thePaleozoic and between the Paleozoic and the Jurassic.

The presence of wide spread continental Jurassic deposits indicates thatthe late Paleozoic event could not have produced major structural ortopographic irregularities. During the Jurassic, which was accompanied bymajor plate movements including the separation of the Apulian microplate,many of the emerging land masses of north Egypt became submerged by thenewly formed Neotethys. The end of the Jurassic witnessed a major orogenicmovement which resulted in the emergence of the land.

The most important tectonic event occurred during the late Cretaceous andearly Tertiary and was probably related to the movement of the North Africanplate toward Europe. It resulted in the elevation and folding of major portions ofthe north Western Desert along an east-northeast west-southwest trend (SyrianArc system) and in the development of faults of considerable displacements.(Said, 1990).

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2.2Regional Tectonic:The general tectonic evolution of Egypt was governed by the tectonicmovement of the African and Laurasian plates (Twadros, 2001).

According to Said (1962) he classified the Western Desert into three majortectonic units (Figure 2.1).

Figure 2.1 Regional tectonic devisions of Egypt. (EGPC, 1992)

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The tectonic features of these unite, from south to north, are reviewed, asfollows:

The stable shelf occupies the southern area of Western Desert, south of

latitude 28o 00" N. It is characterized by high basement relief. Thin sedimentarycover of mainly Mesozoic fluvial-continental clastics section overlies these

basement rocks.The unstable shelf is located directly north of the stable shelf . It is

characterized by the northward thickening of the sedimentary section underlainby low basement relief. The sedimentary section in this area reaches thousandsof meters in thickness and is of Paleozoic to recent in age. It is characterized byhigh organic richness, faulting and folding geometry which is favorable forhydrocarbon accumulations. All oil and gas fields have been located in thisshelf.

The hinge zone is very narrow in width and is parallel to the

Mediterranean Sea coast to the south. It is the area lying betweenMiogeosyncline and the unstable shelf . It is responsible for the rapid

thickening of Oligocene to Pliocene sediments that forming the Nile delta to thenorth.

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2.2.1 Geotectonic Cycle:

Figure 2.2 The geotectonic cycles of Egypt. (Meshref, 1990).

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According to Meshref (1990) six major geotectonic cycles or phases can berecognized in the Western Desert (Figure 2.2). These are:

1- The Caledonian cycle (Cambrian Devonian).2- Variscan Hercynian (Late Paleozoic).3- Cimmerian / Tethyian (Triassic Early Cretaceous).4- Sub Hercynian Early Syrian Arc (Turonian Santonain).5- Syrian Arc main phase (Paleogene).6- Red Sea phase (Oligocene Miocene).

During the Paleozoic, mild tectonism prevailed, characterized by broad tabularuplifts and block faulting. This resulted in the development of extensive shelfplatforms and some shallow epicontinental basins. The hydrocarbon potential ofthe Paleozoic sequences is mainly associated with broad, but subtle structuraltraps.

In the Triassic to Early Jurassic times, the break-up of Pangaea and progressiveopening of the Neo-Tethys were associated with the development of extensionalintra-cratonic rift basins. The structural orientations were mainly NE, EW andWNW. The southern rim and the rift shoulders of the uplifted broken Africancontinental shelf were rimmed by fluvial sandstones. The rift basins were filledin the early stage by estuarine deposits, which were sand-rich in many placesdue to the active syndepositional tectonics. Shallow marine shales andcarbonates subsequently draped the sandy estuarine fills providing perfect sealsas well as source rocks. To the northwest, newly compiled information aboutthe structural and depositional history of major hydrocarbon provinces, indicatea rift-related depositional regime.

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By the Middle to Late Jurassic, the tectonic relief had disappeared and extensiveplatform carbonates were deposited.

Between the Late Jurassic and the Tertiary, successive phases of rotation andcollision between the African Plate and Eurasia controlled structuration andbasin development.Shelf conditions resumed throughout the Middle to Late Cretaceous with mixedclastics/carbonate platform deposition. Intra-Cretaceous mild compressionaltectonic phases are recognized as local unconformities. These culminated in amajor regional angular unconformity at the Cretaceous - Tertiary boundary,indicating a Late Cretaceous (Campanian-Maastrichtian) structuration climaxassociated with both compressional and wrench tectonics.

Several continental plate collision phases are recorded between the PangeanMega segments of Laurasia and Gondwana through phanerozoic . These phasesare interrupted by extensional rift phase associated oceanic crust formation andflooding of continental plate margins.

A further important factor was the sinistral or dextral rotation of the northAfrican plate relative to Laurasia, (Figure 2.3) which had a strong modifyingeffect on the local basin tectonic styles encountered in northeast Africa and inparticular the Western Desert (Smith, 1972; Said, 1990; EGPC, 1992).

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Figure 2.3 Motion between Africa and laurasia (modified by Said, 1990).

Said (1990), illustrated the fault trends located in north Africa during Jurassicand Cretaceous. In the Jurassic period, the fault trends are NE-SW but duringthe Cretaceous they are NW-SE.

Extensional tectonic activity was terminated in the Late Cretaceous by theSyrian arc inversion phase (MacGregor and Moody, 1998).

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2.3Regional Structure:The Western Desert can be divided into a number of large scale

structural provinces in response to lateral movements between Europe andAfrica (Figure 2.4).

So far, all the hydrocarbon discoveries in the Western Desert have beendrilled as structural prospects, either in the form of three or four-way closurestructures or as fault blocks structures. The development of the finds indicatesthat the structural element was the main factor determining the trapping of theoil in almost all of the discoveries. However, in some fields the stratigraphicelement in hydrocarbon trapping is evident in the pinching out of some sandpays in the Cenomanian-Turonian sequence, as well as in the facies changesfrom clastics to carbonates.

Figure 2.4 The regional structure framework of the Western Desert,Egypt (EGPC, 1992).

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Most of the Western Desert structure in which oil and/or gas accumulationswere discovered indicates that late Cretaceous-early Tertiary movements wereinstrumental in their formation. The accumulation of hydrocarbons in thesestructures took place after early Tertiary times. This conclusion is confirmed bythe study undertaken on the burial history of a number of horizons in the MersaMatruh area which show that oil generation (and consequently accumulation)must have taken place some time after the end of the Cretaceous (Taylor, 1984).

The dominant structural style of the Western Desert comprises twosystems: a deeper series of low-relief horst and graben belts, separated bymaster faults of large throw, and broad Late Tertiary folds at shallower depth(Sestini, 1995).

The regional structural elements of the Western Desert have been dealtwith by many authors since the early decades of the past century:

Krenkel (1925), introduced the name Syrian Arc for a series of foldstrending NE-SW and running into the hinterlands of eastern Mediterraneanacross Syria and terminating at the Taurus ranges of southern Turkey, theseseries continue Westwards into Sinai and further in the Western Desert.

Hume (1929), recognized north-south folds in the Western Desert withgreat amplitude and gentle dips. He visualized Upper Egypt as a block cutacross by two anticlines separated by a syncline: the anticlines are worked byKharga Oasis to the west and Wadi Qena to the east, the syncline is occupied bythe Nile valley north of Luxor.

Sandford (1934), recorded two distinct anticlinal crests separated by asyncline between Samalut and Minia along the Eastern bank of the Nile.

Shata (1953), described some of these surface folds between Maghara inSinai and Cairo.

Shukri (1954), enumerated some folds Syrian Arc parallel to the faultsthat were active during deposition of late Cretaceous. He pointed out that thedomal structures are characterized by a break in sedimentation between the late

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Cretaceous and early Tertiary while the troughs in between are characterized bycontinuous deposition.

Knetsch (1957 and 1958), related the N-E folds to the germanotype.Youssef (1958) reported that, on the western bank of the Nile south of

Esna in upper Egypt, the upper Cretaceous rocks form a series of anticlines andsynclines.

Knetsch (1958) discussed in detail the folding mechanics of the AbuRoash uplift, one of the classic structures of the mobile belt of Egypt.

According to Said (1990), the north Western Desert structures aredominated by faults many of which can be identified from seismic and welldata. The majorities are steep normal faults; some of which suffered strike slipmovements during part of their history. The strike slip movements wereprobably related to the lateral movements which the African plate underwentduring the Jurassic and late Cretaceous. Faults of north-south trend are knownonly in the area to the southwest of Matruh. There are also a large number ofhanging faults affecting the shallower parts of the section and usually of limitedthrow:

1- Faults with displacements of magnitude range from 1500 m to 3000 mare limited to Kattaniya horst and Abu Gharadig graben.

2- Faults with displacement of magnitude range from 750 m to 1500 m arepresent in the northern parts of the region but are widely spaced.

3- Faults with displacements of magnitude less than 750 m throw are morefrequent. Their trend is east-west in the Abu Gharadig basin, northeast-southwest in the Kattaniya high and northwest-southeast over the rest ofthe north Western Desert.

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Faults of north-south trend are known only in the area to the southwest ofMatruh.

Most folds owe their origin to compressional movements which affected thearea during the late Cretaceous-early Tertiary tectonic event. These folds have anortheast-southwest trend.There are other folds which owe their origin to normal or horizontally

displaced faults.According to Said (1962) these folds could be divided into three major groups:

a- The north-south folds, these exhibit themselves mostly in the subsurfaceand have their marked effect on the Paleozoic sediments.

b- The northeast folds, these were especially active during the Cretaceousand Eocene ages.In the subsurface the northern half of the Western Desert is crossed bylarge number of these folds arranged in lines having the same trend asthe Syrian Arc system which is related to the late Cretaceous-earlyTertiary movement (Laramide). This folding system affected thenorthern part of Egypt up to latitude 23 0 north.

c- Northwest folds, these affected the Oligocene and younger sedimentsand are exposed on the surface and as well as being found in thesubsurface with gentle dips.

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2.4 Regional Stratigraphy of the Western Desert:2.4.1 Depositional Basins:

The sediments recorded by deep drilling in the northern Western Deserthave shown that this large area is differentiated beneath its flat cover of youngersediments into a number of major paleogeographic basins.These sedimentary basins were the scope of numerous investigations byregional geologists as Amin (1961), Said (1962), Norton (1967), Issawi (1972),El Gezeery et al. (1972), Metwalli and Abd El Hady (1973, 1974, 1975), Abu ElNaga (1984), Elzarka (1983), Schrank (1983), Taylor (1984), Said (1990)Abdine et al. (1993), Sestini (1995), Guiraud (1998), Mahmoud and Schrank(2003), El Beialy (2005).

They delineated, discussed the geologic history and followed thedistribution pattern of the sedimentary basins in Egypt. These basins originatedas a result of structural effects and divided into six basins (Figure 2.5):

Figure 2.5 The sedimentary basins located in the North Western Desert, Egypt.(Meshref, 1982).

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1- Faghour Plateau:

At the eastern part of Egypt between Egypt and Libya, the province ofplateau is about 9000 feet (2743.2 meters) of Paleozoic strata overlayingbasement rocks.

2- Siwa Basin:Is a northeast continuation of Kufra basin in Libya, formed by gentle

crustal downward and faulting and thickness over 9000 feet (2743.2meters) of Paleozoic strata.

3- Abu Gharadig Basin:The deepest basin area in the north of theWestern Desert and divided

into northern and southern sub-basins. The southern sub-basinssedimentary section exceeds 15000 feet (4572 meters). The northern sub-basin has in excess of 35000 feet (10668 meters). The Abu Gharadig basinis an oriented asymmetrical graben or half graben. The margin of the basinis marked by a major border fault zone which up thrown basement toabout 10000 feet (3048 meters) forming Sharib-Sheiba ridge. The AbuGharadig basin is a tensional normal fault and then developed as strongright lateral component. It resulted from a stress pattern related to theopening of north Atlantic from Turonion time 90Ma to Paleogene 60Ma.

- Sharib-Sheiba High:The Abu Gharadig basin was developed as a rift and coastal basin

formed as a pull-part to the south and north (Paleozoic, Jurassic and partof cretaceous). These high separated Abu Gharadig basin from northbasin, with E-W trend.

4- Ghazalat Basin:This is a seismically defined coast parallel rift or graben. The basin

exceeds 7000 feet (2133.6 meters) of Jurassic strata, and a total thicknessof Mesozoic and Tertiary rocks believed to exceed 19000 feet (5791.2meters), located in northwest of Abu Gharadig basin.

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5- The Northern Basins:These basins were formed by the breakup of the northern coast of

Pangea as the present day continental fragment of Greece and Turkeybroke away from the northern edge of the coastal basin. Thepredominance of flower structure along the coast is interpreted asevidence of strike slip fault resulting from compression caused by shear,and divided into four sub-basins (EGPC, 1992) which are:

a- Matruh Sub-Basin.It is pronounced through the trend from the coastline near Mersa

Matruh. It was developed from pre-middle Jurassic until early cretaceous.b- Shushan Sub-Basin:

It is westerly located northeast-southwest trending basin. Thesedimentary cover within the Shushan basin is about 25000 feet (7620meters).

c- Dahab-Mireir Sub-Basin:This basin is central of coastal basin, bounded by Sharib-Sheiba high to

the south and dabaa ridge to the northwest. Two ENE ridges are cuttingthis basin namely Qattara-Alamein Ridge and Washka Ridge.

d- Natrun Sub-Basin:This sub-basin is the eastern end of the coastal basin. It was subsided

during Jurassic time where more 9000 feet (2743.2 meters) of shallowmarine-deltaic sediments were deposited. It is overlained to north andnortheast by deltaic and to south by the Kattaniya horst.

6- Gindi basin.The Gindi basin is bounded to the north by the Kattaniya horst, and has

a series of NW-SE faulting throwing down to the SW and contains severalstrongly faulted anticline structure, generally bounded by reverse faults.

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2.4.2 Stratigraphy:A brief review of the stratigraphic succession penetrating the north

Western Desert of Egypt.

Figure 2.6 Stratigraphic section penetrating in north Western Desert. (Abu E1Naga, 1983).

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Paleozoic:The Paleozoic sediments of the north Western Desert are of monotonous

composition and are made up of interbeded sandstone and shale with a fewcarbonate beds. This monotony makes the identification of workable rock unitsdifficult.

The Paleozoic age was divided into Cambrian, Ordovician, Silurian,Devonian, Carboniferous and Permian ages, Hantar (1990):

1- Cambrian:Cambrian strata are made up of sand stones of various colors,

glauconitic and shale of reddish, brick and gray colors.The presence of marine fossils in the Cambrian strata gave a point to amarine environment of deposition. Cambrian strata rest unconformablyover the basement rocks which provide a clear boundary.The upper boundary, is less certain and is usually marked by an arbitrarystratum of Silurian, Devonian, Carboniferous or younger age.

2- Ordovician:No fossil-bearing strata of Ordovician age were indentified in the region.

3- Silurian:Silurian strata are made up of shale, siltstone and thin limestone beds

intruded by a gabbroic sill.4- Devonian:

Devonian strata are made up of a lower sandstone unit with minorshale interbeds and an upper shale unit with minor siltstone andsandstone interbeds. The sandstone is fine to coarse-grain and its colorranges from white to brown or pink. The shale is mainly grey orgreenish grey. The thickness of the Devonian strata is in the range of

900 to 1000 m. The lower and upper boundaries are poorly defined andare usually arbitrarily marked. The presence of marine foraminifers,ostracods, condonts, acritarchs, brachiopods, bryozoans and

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echinoderms in the upper shale unit, suggests a marine environment ofdeposition. The sands of the lower unit may have been deposited underfluvial conditions.

5- Carboniferous:The lower boundary of the Carboniferous strata is marked by

the disconformable contact with the underlying Devonian strata. Theupper boundary is marked by the unconformable contact with theoverlying marine or continental late lower or early middle Jurassicstrata. The presence of rich micro and macro-fossil assemblagespoints clearly to the marine nature of the sediments (Said andAndrawis, 1961; Abd El Sattar, 1983).

6- Permian:The Permian strata are made up of dolomitic limestones with a few

thin shale and sandstone interbeds. The Permian occurrences seem tohave been deposited in littoral to sublittoral environments.

Mesozoic:The Mesozoic age is divided into Triassic, Jurassic and Cretaceous,

Hantar (1990). There is no Triassic or early Jurassic marine sediments known inthe region in spite of the fact that early Jurassic continental sediments arerecorded in most parts of the region.

1- Jurassic:The deposits of the Jurassic are classified into the following units

from top to bottom:a- Sidi Barrani Formation.Is a thick carbonate section of middle Jurassic to early Cretaceous age.The carbonates are mainly dolomitic. A few interbeds of sandstone,shale and anhydrite occur at the base of the formation.

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b- Masajid Formation.The name Masajid formation was proposed by Al Far (1966).

Typically a massive limestone sequence of middle to late Jurassic age.The formation is clearly marked in most of the region. It overliesconformably the clastic Khatatba formation.c- Khataba Formation.

The name Khataba formation was proposed by Norton (1967). Theclastic section of the Khatatba formation has a few limestone interbedsand is made up of sandstone and shale. The shale is grey to brownishgrey and the sandstone is fine to medium grained and is brown in color.The formation rests conformably over the Wadi Natrun formation in thenortheastern and eastern parts of the area. It underlies the Masajidformation conformably in most areas except in the south where itunderlies the lower Cretaceous Burg El Arab formation unconformably.The contact with the Masajid is sharp and is marked by the change offacies from dominantly clastic section of the Khatatba to the morecalcareous section of the Masajid.

d- Wadi Natrun Formation.The name Wadi Natrun formation was also proposed by Norton

(1967). It includes marine carbonate-shale sequence of middle Jurassicage. The carbonates of the section are mostly dolomitic. Wadi Natrun isalways overlain by the Khatatba formation. Wadi Natrun formation hasa limited distribution and is known only in the eastern part of the areaand along its northern borders.

e- Bahrein Formation.

The formation is made up of red color clastics. The name replacesthe Eghi group which was proposed by Norton (1967) for the continentalsection above the Carboniferous. The name is gaining acceptance andwas used in the RRI report (1982). The Bahrein formation is of early to

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middle Jurassic age and possibly older. The Bahrein formation liesunconformably below the marine Khatatba formation. In places alongthe southern and western stretches of the area, the Bahrein restsunconformably below lower Cretaceous Betty formation or the Alam ElBueb member.

2- Cretaceous:According to Hantar (1990), Cretaceous is divided into:1- Lower Unit: made up of clastics and belonging to lower Cretaceous.The lower unit includes an important carbonate bed of great areal extent,the Alamein dolomite which provides the reservoir rock for threeimportant oil fields in the region. The lower Cretaceous is representedby the Burg El Arab formation that made up of a thick sequence of fineto coarse-grained clastics.Burg El Arab formation is divided into two units:

a- Alam El Bueb.b- Kharita.

Burg El Arab formation is divisible into four members form top to bottom:a) Kharita. This is a unit of fine to coarse-grained sandstone with shale and

carbonate interbeds. The Kharita member is assigned an Albian toCenomanian age. The unit was deposited in a high energy shallowmarine shelf. In the extreme north, the unit seems to have been depositedin deeper water, while in the south it was under the influence ofcontinental conditions.

b) Dahab. It is a grey to greenish grey shale unit with thin interbeds ofsiltstone and sandstone. Faults with a throw exceeding the thickness ofthe Dahab shale will adversely affect the underlying Alamein reservoir.The age of the Dahab shale is Aptian to early Albian.

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c) Alamein dolomite member: It is made up of light brown hardmicrocrystalline dolomite with vuggy porosity. A few thin shaleinterbeds are present. The unit seems to have been deposited in ashallow marine, low to moderate energy environment. Some operatorscombine the Dahab with the Alamein dolomite in one unit, the Alameinformation. Earlier classifications (Metwalli and Abdel Hady, 1975)combined the Dahab and the overlying Kharita under the name AbuSubeiha.

d) Alam El Bueb or its lateral equivalent the Matruh: It is a sandstone unitwith frequent shale interbeds in its lower part and occasional limestonebeds in its upper part. The Alam El Bueb member includes units thatwere given different names by different operators such as:Matruh group, Aptian clastics, Alamein shale, Dawabis, Shaltut,Umbaraka, Mamura and operational units A, B, C, D1, D2, E, F1 andF2. The member ranges in age from Barremain to Aptian. Theenvironment of deposition was shallow marine with more continentalinfluence towards the south.

2- Upper Unit:

Made up of carbonates and belonging to the upper Cretaceous.In Egypt the upper Cretaceous marks the beginning of a major marinetransgression which resulted in the deposition of a dominantly carbonatesection (Said, 1962). In the north Western Desert, the mainly calcareousdeposits of the upper Cretaceous developed in the Abu Gharadig basinwhere they form a number of oil reservoirs. These sediments are dividedinto three rock units (from top to bottom):a- Khoman formation: It is made up of snow white chalky limestone

with abundant chert bands. The Khoman formation overliesunconformably different units of the Abu Roash or the Bahariya and

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underlies unfonformably the Apollonia or Dabaa formation. It wasdeposited in open marine outer shelf conditions. The deposition ofthe Khoman was associated with a rise of the sea level whichextended the sea to the south of the north Western Desert.

b- Abu Roash: This is mainly a limestone sequence with interbeds ofshale and sandstone. The unit is divided into seven membersdesignated from top to bottom: A, B, C, D, E, F and G. Members B,D and F are relatively clean carbonates while members A, C and Eare largely fine clastics. The lower unit G is made up of interpeddedcarbonates and clastics. The Abu Roash overlies conformably theBahariya formation. The Abu Roash underlies the Khomanformation where the contact is determined by the change of lithologyfrom crystalline limestone of the Abu Roash to the chalky limestoneof the Khoman.

c- Bahariya: The Bahariya formation is of late Cenomanian age, wasdeposited first under fluviatile conditions. Operators previously gaveseveral names to Bahariya formation: Razzak sand, Meleiha sand orMedeiwar member of the Abu Subeiha formation. The Bahariya isexposed along the floor and both sides of the Bahariya depression.The exposed section measures at least 557.7 ft (170 meters) and isdivisible into three members from top to bottom:

El Heiz: It is made up of dolomites, sandy dolomites andcalcareous Rich in fossils.

Gebel Dist: Is made up of fine-grained, well bedded,ferruginous Clastics carrying a large number of fossils.

Gebel Ghorabi: It is made up of cross-bedded, coarse-grained, and seemingly non fossiliferous sandstones.

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CenozoicPaleocene deposits are mainly mudstone which, in early to middle

Eocene, were reduced or mainly removed in north Egypt.During the late Eocene-Oligocene, thick open marine calcareous shales were

deposited (Dabaa formation) in north Western Desert.Marine Pliocene deposits are in the form of shallow marine pink

limestones or sandy limestones and evaporites.Paleocene to middle Miocene rock units from top to bottom, Hantar (1990):

a) Marmarica formation: Is made up of a limestone, dolomite andshale sequence of middle Miocene age.

b) Mamura formation: Is a limestone and calcareous shale sequencewhich is the marine equivalent of the Moghra. It rests above theDabaa formation and is conformably overlain by the middleMiocene Marmarica formation.

c) Moghra formation: Is made up of a clastic fluvio-marine delta-front sequence of early Miocene age.

d) Dabaa formation: Is made up of marine shales of upper Eocene-Oligocene age. This formation had previously given severalnames: Qasr El Saga, Maadi, Birqet Qarun and Gehannan. Theformation rests with minor disconformity on the apollonianformation. It is conformably overlain by the Moghra or theMamura formation.

e) Apollonia formation. This is a Paleocene to middle Eocenelimestone unit with subordinate shale members. It overliesunconformably the Khoman chalk. The Paleocene section isusually made up of limestone with thin layers of shale beds. Theformation is conformably overlain by the Dabaa formation. Theformation has previously been described as the Gindi formationor as the Esna, Thebes and Mokattam formations.

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2.5 Local Stratigraphy of the Razzak Oil Field:The Razzak oil field stratigraphic column (Figure 2.7).

Figure 2.7 Stratigraphic section penetrating in Razzak field. (EGPC, 1992).

The stratigraphic succession penetrated in the Razzak field (Figure 2.7) rangesin age from Miocene to Early Jurassic and has a total thickness of more than13,000 ft (3963 meter).

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The Jurassic section has been divided by Robertson Research International(RRI, 1982) into three units:

1) A lowermost unit is the Bahrein Formation, which consists of a massive,predominantly continental sandstone section.

2) A middle section consists of interbedded siltstones, sandstones, shales,and some carbonates of the Khatatba Formation. The depositionalenvironment is shallow marine and the Khatatba Formation represents amarine transgression into the area. It is conformable with both theBahrein Formation below and the overlying Masajid Formation.

3) An upper carbonate unit, the Masajid Formation, is about 80 ft (24 m)thick and consists of dense dolomites, limestones, and dolomiticlimestones with shale intercalations. The unit has an open marine

depositional environment and represents the maximum extent of LateJurassic marine transgression in the area. A regional unconformity

separates the Jurassic from the overlying Lower Cretaceous section.The Cretaceous section consists, in a broad sense, of four alternatingsedimentary cycles (RRI, 1982).The first and third cycles, from the bottom, consist of predominantly massivesandstones with thick interbeds and intercalations of shales in some places. Thesecond and fourth cycles consist primarily of open to shallow (and possibly inpart restricted) marine carbonates, which represent relatively quiet shelfconditions.

1) The first cycle has been divided into the following units in ascendingorder:

a) The Neocomian Betty Formation, which is about 460 ft (140 m)thick and consists of interbedded varicolored shales, sandstones,and sandy shales. It was deposited under marine conditions.

b) Overlying the Betty unconformably is the Alam El BueibFormation (Barremian), 1050 ft (320 m) thick and composed of

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marginal marine to deltaic sandstones with some carbonate andgreenish-gray shale intercalations.

c) The upper part of the Alam El Bueib Formation is the Aptiansand member (lower Aptian), which is some 1650 ft (350 m)thick and consists of a sequence of thick sandstones with someshale interbeds, especially toward the base. The depositionalenvironment is considered to be mainly shallow marine tooccasionally sublittoral.

2) The second cycle consists of the Alamein Formation that has beendivided into two members:

a) The Alamein dolomite of the Alamein Carbonate Memberconsists mainly of carbonates with subordinate shales. The unitranges from 235 to 245 ft (72 to 75 m) thick. The depositionalenvironment is shallow to possibly, in part, restricted Marine.

b) The Dahab Member, some 260 ft (79 m) thick, consists ofshallow marine to sublittoral interbeds of shales, sandstones,siltstones, and dolomitic limestone. This member generallyforms the seal for the underlying carbonate reservoir.

3) The third (clastic) cycle has been divided in ascending order into thefollowing:

a) Conformably overlying the Alamein formation, the AlbianKharita formation, some 898.95 ft (274 m) thick, is composed ofmarginal marine to deltaic sandstone with shale and rarecarbonate interbeds.

b) Unconformably overlying the Kharita Formation is the upperAlbian to lower Cenomanian Bahariya Formation (including theRazzak Member), which is 700 ft (213 m) thick in the area.

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c) The formation is made up of interbedded sandstones, shales, andsandy shales, with occasional limestone stringers. Theenvironment of deposition changes from shallow marine at thebase to deep marine toward the top.

4) The fourth (carbonate) cycle consists of the Upper Cenomanian toConiacian Abu Roash Formation, which is up to some 2100 ft (640 m)thick in the Razzak area and conformably overlies the BahariyaFormation. The Abu Roash is, in turn, unconformably overlain by theCampanian to Maastrichtian age, open marine, fine-grained chalkylimestones of the Khoman formation.

The Abu Roash has been subdivided into the members "A and B", "C""D and E", "F" and "G." These represent cyclic shallow marine to openmarine depositional environments.The Abu Roash "A and B" at the top of the formation is composed oflimestones with shale and fine-grained lime silt and mud interbeds andwas deposited under fluctuating high energy, shallow marine torelatively deep marine, transgressive conditions.The Abu Roash "C" member is composed of calcarenites with silty shaleinterbeds. The Abu Roash "D and E" consists of dense, glauconitic,chalky limestones with dolomitic crystalline limestone and shaleintercalations. The Abu Roash "F" member consists of a thick pyritic,cream-colored calcarenite with abundant open marine fauna and forms awidespread marker in the Western Desert. The lower Abu Roash "G"member consists mainly of interbedded limestones and shales with a thindolomite unit at the base that is oil bearing in the West Razzak andRazzak Main fields.

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2.6 Structure of Razzak Oil Field:Razzak field separated into three main fields (West Razzak, Main

Razzak, and East Razzak) so its named Razzak field Complex (Figure 2.8).

Figure 2.8 Razzak field complex main fields. (Abdine et. al. 1993).

Razzak field area lies on a northeast plunging anticlinal nose among one ofthree conspicuous mapped anticlinal features within Razzak area.These three anticlinal noses are aligned with the Alamein-Yidma trend on theCenomanian and Aptian seismic horizons, having the same trend of the SyrianArc system which continued during the Eocene time. (Said, 1962, and Norton,1967).

1- The first anticlinal nose lies at the extreme southwestern part of the

Razzak area, with two producing wells (RZK-4 and RZK-12). Drilled onits crest (West Razzak).

2- The second anticlinal nose trends northeast and lies on the extremenortheast part of the study area (East Razzak).

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3- The third anticlinal nose is the most important structural feature forhydrocarbon trapping and oil production in the Razzak area. Ten wellswere drilled on both its crest and flanks. This trap acquires the form of anortheastern plunging anticlinal nose lying on the central part of the

Razzak area. This nose is dissected into several blocks by two sets ofintersecting normal faults. These two sets of faults are trendingnorthwest-southeast and northeast-southwest following what are knownas Erythrean and Aualitic trends. Most of the northeast-southwest faultsare parallel to the plunging axis of the anticlinal nose.

Consequently, Razzak structure could be considered as a trap formed by bothfolding and faulting. The active faulted blocks were continuously subsidedduring sedimentation indicating that faulting played the great role in oil trappingand establishing the present structural configuration of the field.

2.7 Razzak Oil Field Reservoirs:According to EGPC, (1992) there are three main reservoirs in the

Razzak oil field as follows:

1) Abu Roash G.The producing horizon is sand of 16 ft (4.88 m) thickness, 32%

porosity, 23% water saturation, and 145 millidarcy (md) permeability.The initial and current reservoir pressures are 2400 Pounds per SquareInch (PSI) and 800 PSI, respectively. Bubble point pressure is estimatedto be 1800 PSI. Water oil contact was observed at -5370 ft (1636.78 m).Production increase, and water cut and the Gas/Oil Ratio (GOR)increase, indicated that the driving mechanism is a combination ofdepletion and partial water drive. Original oil in place and ultimatereserves are 104 Million Stock Tank Barrels (MMSTB) and 18.72MMSTB respectively, the remaining reserves are 10.36 MMSTB.

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2) Bahariya reservoir.The producing horizon is sandstone of 45 ft (13.72 m) net pay

thickness, 25% porosity, 15% water saturation, and 400 mdpermeability. Oil water contact was observed at -5680 ft (-1731.26 m).Initial and current reservoir pressures are 2500 PSI and 2300 PSIrespectively. Bubble point pressure was measured to be 300 PSI.Drop in the reservoir pressure is small (200 PSI) after cumulative oilproduction of MMSTB for over 20 years of production, water cut is veryhigh (90%), also GOR is low.This performance indicates that the driving mechanism is active waterdrive. Original oil in place and ultimate reserves are 45.5 MMSTB and16.38 MMSTB, respectively, the remaining reserves are 1.38 MMSTB.

3) Aptian dolomite.The producing horizon is dolomite of 120 ft (36.58 m) thickness,

7% porosity, 15% water saturation, and unknown permeability value.The oil water contact was observed a -7279 ft (-2218.64 m). The initialand current reservoir pressures are 3250 PSI and 3180 PSI. Bubble pointpressure was estimated to be 350 PSI. Reservoir pressure drop is small(70 PSI) after cumulative of 20.66 MMSTB over 20 years, and the watercut is very high (92%), also GOR is low.This performance indicates that the driving mechanism is active waterdrive. Original oil in place and ultimate reserves are 90 MMSTB and30.6 MMSTB, respectively, the remaining reserves are 9.94 MMSTB.

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2.8 Exploration and Development Concepts:The Razzak field was discovered primarily as a result of improved

reflection seismic data resolution of pre-Tertiary events that became available inthe late 1960s and early 1970s. The ability to map in greater detail and accuracythe Cretaceous and earlier events with up-to-date seismic techniques has led to anumber of significant discoveries in recent years.

Understanding the relationships between basin development history andstructural growth of specific prospective areas with respect to timing of

hydrocarbon generation and migration from the basinal areas is necessary forefficient exploration and development. This, with other tools used in basinanalysis, such as satellite imagery, magnetics, gravity, and geochemistry,narrowed the search for prospective structures. The well density distribution ispresently too sparse to provide the number of subsurface control points neededfor the detailed analysis required to locate future prospects.

Well data, provide the basic general stratigraphy, depositional history, andframework for the seismic interpretations. Wells also give essential information

regarding the geographic and vertical stratigraphic distribution of seals,reservoirs, and source rocks in the section and the locations of the general areasof regional highs and basinal lows. Detailed facies maps and seismicallycontrolled isopach maps, combined with sedimentation rate and maturationhistory profiles, help isolate the likely mature source areas in the Western

Desert and provide estimates of the timing of expulsion and direction ofhydrocarbon migration.