-
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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15
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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18
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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25
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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27
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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30
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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31
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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32
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.