Bio-and Sequence Stratigraphy of Upper Cretaceous – Palaeogene rocks , East Bahariya Concession , Western Desert , Egypt

This work deals with the plankton stratigraphy of the subsurface Upper Cretaceous-Palaeogene succession of the East Bahariya Concession based on planktonic foraminifera and calcareous nannofossils. The examination of the cuttings from fi ve wells: AQSA-1X, KARMA-E-1X, KARMA-3X, KARMA-NW-1X and KARMA-NW-5X is biostratigraphically evaluated. It is possible to identify the planktonic foraminifera as well as the calcareous nannofossil biozones. The analyses of calcareous nannofossils revealed the presence of several hiatuses. Information obtained from well data such as seismic facies analysis for the studied area has enabled classifi cation of the Upper CretaceousPalaeogene succession into fi ve major 2nd order depositional sequences, separated by four major depositional sequence boundaries (SB1, SB2, SB3 and SB4). The Upper Cretaceous-Palaeogene succession in the East Bahariya is divided into 17 systems tracts. These systems tracts are: 7 System tracts of probable Cenomanian age, (the sequence stratigraphic framework as well as the cycles and system tracts of the Cenomanian Bahariya Formation match well with those of CATUNEANU et al., 2006); 4 System tracts of Turonian age, 2 System tracts of Campanian-Maastrichtian age and 4 System tracts of Eocene age.


INTRODUCTION
The Western Desert of Egypt covers an area of approximately 700, 000 km 2 , which represents two thirds of the total area of the country (EGPC, 1992).
The study area (Fig. 1) covers the Karama Field in the East Bahariya concession (EBC), located in the eastern margin of the Abu Gharadig Basin.The Karama Field was discovered in January 2000 by Repsol and Apache, while drilling the Karama 1-X well, which produced oil from the Abu Roash Member.
The Abu Gharadig Basin is located in the central northern part of the Western Desert.Topographically the Abu Gharadig Basin can be subdivided into two main troughs, the western and eastern trough.This basin is recently considered to be the most promising of all as far as hydrocarbon potential is concerned since several oil and gas discoveries had been recorded within its thick marine Cretaceous sequence (AWAD, 1984).
The aim of the present work is the study of the planktonic foraminiferal and calcareous nannoplankton biostratig-

STRATIGRAPHY
The surface of the northern part of the Western Desert is covered by Miocene deposits, except for the Bahariya and Abu Roash areas that are covered with Upper Cretaceous and Eocene sediments (Fig. 2).In the Fayium area both Eocene and Oligocene limestones, clays and sandstones dominate the section.In the Bahariya and Fayium areas Oligocene volcanic basalt makes up a substantial part of the geological succession.The Miocene deposits are subdivided into the lower Miocene Moghra Formation and the Middle Miocene Marmarica Formation (SAID, 1962).
Based on the surface geological map of the Western Desert, the Miocene deposits are separated from the Eocene rocks in the south by a narrow band of Oligocene carbonates at Siwa (HARPE DE LA, 1883) while in Al Arag there is a gradual southerly change into a clastic succession at Bahariya and Fayium (OSMAN, 2003).
The Miocene surface usually doesn't usually refl ect structures at depth as an angular unconformity is observed at many places separating the Miocene and underlying Eocene deposits.The Upper Eocene deposits occupied mainly the  central and the south eastern parts of the Western Desert.These marine deposits are mainly limestone represented by the Thebes and farafra Formations, the former representing an open marine facies while the latter is a shallow shelf lithofacties (SAID, 1960).The Cretaceous deposits are mainly located at the southern part of the Western Desert and they consist mainly of the Nubia Formation.

Subsurface stratigraphy of the Western Desert
The subsurface lithostratigraphic column in the northern part of the Western Desert contains most of the sedimentary section from Pre-Cambrian basement rocks to Recent deposits.The total thickness of the sedimentary section measures about 4270 m (YOUNES, 2003).Figure 2 summarizes the main stratigraphic units in the Abu Gharadig basin of the Western Desert.

Upper Cretaceous rocks
In Egypt, the Upper Cretaceous is marked by the beginning of a major marine transgression which resulted in the deposition of a carbonate dominated section in the study area.According to ISSAWI et al. (1999), the Upper Cretaceous succession is subdivided into three lithostratigraphic units from the oldest Bahariya, to the Abu Roash and the Khoman Formations.
The sedimentary rocks of the Upper Cretaceous occur in three basins of the Western Desert; the Abu Gharadig, Shushan, and Alamin basins, which include the most important hydrocarbon resources in the Western Desert.The formations are discussed from oldest to youngest below.

Bahariya Formation
The type locality of this unit is located in the Gabel El Dist (Bahariya Oasis) where the base of the formation is not exposed at the surface.The maximum thickness of this formation occurs in the Kattaniya-1 well (1143 m) (29° 49 'N and 30° 10'E).The name of this unit was proposed by SAID (1962), STROMER (1914a, b) and SMITH et al., 2006 based on dinosaurs remain for the lower Cenomanian fl uvio-marine (CATUNEANU, 2006) sediments which consist of argillaceous sandstone with some carbonates.

Abu Roash Formation
The type locality of the Abu Roash Formation outcrops to the north of the Pyramids of Giza (30° 2' 44.59"N and 31°14' 24.71"E).This unit is subdivided into seven lithostratigraphic members, termed from top to bottom as A, B, C, D, E, F, and G, which extend from the Santonian to the Upper Cenomanian.Most of the facies were deposited under neritic to open marine, basinal conditions.The Abu Roash represents the alternation of transgressive and regressive phases, characterized by: 1. Shallow to deeper marine carbonates on palaeohighs.The limestones-were dolomitized and have fracture porosity.2. Shallow deposits, consisting of the alternation of shales and sandstones.During Turonian times, an evaporitic sequence was deposited in some areas.
Lithologically, this unit consists of white moderately hard fossiliferous, cherty, massively bedded, micro-to cryptocrystalline chalky limestone.The shales are gray, dark brown and greenish in colour with moderate hardness, containing some glauconite and pyrite.Dolomite is brownish in colour, buff, massive and moderate in hardness.This refl ects a nearshore or Sabkha type of depositional environment.

Khoman Formation
The type section of this unit is located at Ain Khoman, southwest of the Bahariya Oasis.It represents an open marine facies which prevailed in the north Western Desert during the Campanian and Maastrichtian and it unconformably overlies the Abu-Roash Fm., particularly in the structurally highest area.Lithologically this formation consists of two main units: the lower unit is mostly limestone interbedded with shale, the limestone is massive, white dolomitic limestone and the shale is brownish, gray with medium hardness while the upper, unit is of fi ne-grained white chalky massive limestone and massive dolomite and lacks of adequate reservoir properties.

Cenozoic rocks
At the end of Cretaceous, sedimentation in the Western Desert was continuous in the structurally low and subsiding areas while some depositional gaps and erosional truncations were common on the pre-existing highs, which were reactivated, especially during the Palaeocene (SCHLUMBER GER, 1984).
Palaeogene rocks include the Palaeocene to Middle Eocene Apollonia Fm. which consists mainly of shallow to open marine carbonates.The Upper Eocene to Oligocene Dabaa Fm. consists of shallow marine shales and the Miocene Moghra Fm. consists mainly of shallow to marginal marine sandstones with thin shale interbeds and a basalt extrusion near its top.The basal part of the Moghra Fm. laterally changes to carbonates (Shoushan Formation) in the north west portion of the Western Desert.The Oligo-Miocene rocks are also overlain by the middle Miocene Marmarica Fm. to the northwest of the Western Desert, which consists of shallow marine carbonates (NORTON, 1967;ABDEL AAL & MOUSTAFA, 1988).Quaternary deposits unconformably overlie the Miocene rocks and consist of Plio-Pleistocene continental sands.

The Apollonia Formation
This is a widespread limestone with subordinate shale of Palaeocene to Eocene age.The type locality is located south of the village of Susa (32° 53' 48"N and 21° 57' 47"E) in Libya.The type section is 250 m thick and is made up of massive siliceous limestones with numerous chert bands.
The lithology is white, chalky limestone, in many places nummulitic, with some shaly zones, especially where the formation is thick.Glauconite and pyrite are common and widespread.Chert nodules occur, and are common in the lower part.The formation has been subdivided by several operators into units A to D where D is the oldest.Units B and D are thinner and dominanted by shale.

The Dabaa Formation
The type section is the interval 579 to 1021 m interval of the Dabaa-1 well.Lithologically it is composed of gray and greenish-gray clay and claystone with subordinate thin beds of limestone, glauconite and pyrite minerals, of Upper Eoce ne to Oligocene age.
The majority of the wells drilled in the study area revealed the same stratigraphic column of the North-Western Desert with a few exceptions in the variations of thickness and the absence of some rock units.
Calcareous nannofossils were processed by the standard smear slide preparation from raw sediment samples as described by Perch-Nielsen (1985).Smear slides were exam-ined using a light photomicroscope with 1250x objective.A relative abundance of calcareous nannofossils was used as follows: A= abundant more than 5 specimens /fi eld of view (fov), C= common 1-5 specimens/fov, F= frequent one specimen /2-5 fov, R= rare one specimen/6-10 fov and VR= very rare one specimen/more 10 fov.

Planktonic and benthic foraminifera
The planktonic foraminifera were used to date the different successions of the studied wells.Most successions range in age from the Cretaceous (Cenomanian) to the Oligocene.Planktonic and benthic foraminifera are encountered only in the marine upper half of the succession which is represented, in ascending order, by the Khoman Fm, the Apollonia Fm, and the Dabaa Fm.

Preparation technique
Samples were soaked in Hexametaphosphate solution and then washed through a 0.64 mm sieve.The operation was repeated until the residue became clean.Hard samples were then treated by ultrasonic cleaner.Foraminiferal specimens were picked and mounted on squared slides for identifi cation.
The faunal assemblages of the Khoman Fm. belong to the following planktonic foraminiferal zones, from base to top, as follows: Globotruncana ventricosa Zone, G. aegyptia ca Zone (CARON, 1985;CARON et al., 2006 andBER-G GREN &PEARSON, 2006) and Gansserina gansseri Zone which are assigned to the Upper Campanian to Maastrichtian.

Apollonia Formation
The Apollonia Formation unconformably overlies the Khoman Formation.It consists mainly of limestone with occasional argillaceous interbeds in some wells.It is relatively poor in foraminifera and almost barren at some levels.Accordingly, systematic biozonation is diffi cult and impractical to establish, especially due to the absence of some key species (Figs. 3 and 7).However, some other species have helped in establishing different age divisions of the Apollonia Formation Figures 3 to 7 shows the distribution of planktonic foraminifera in the Apollonia Formation of the different wells.The Lower/Middle Eocene boundary has been drawn at the fi rst appearance of Middle Eocene species such as Acarinina bullbrooki especially when associated with Catapsydrax sp and Turborotalia cerroazulensis pomeroli.The Lower Eocene is estimated according to the occurrence of only lower Eocene species such as Morozovella formosa, Acarinina soldanii, & Acarinina pentacamerata.These three species together with Acarinina broedermanni represent the assemblage of the Lower Eocene (BOLLI, 1957;TOUMAR-KINE & LUTERBACHER, 1985;BERGGREN & PEAR-SON, 2006).They are usually followed by a barren interval before the inception of the Middle Eocene species.Accordingly, the Lower/Middle Eocene boundary here is speculative rather than factual.
The Middle Eocene part is the interval richest interval in foraminifera.All the species of the assemblage support this age determination based on comparison with those mentioned in the literature (BOLLI, 1957& BLOW, 1979;TOU-MARKINE & LUTERBACHER, 1985;BERGGREN & PEAR SON, 2006).It is worth mentioning, however, that except for Acarinina and Truncorotaloides, the range of these species extends to the Upper Eocene.Yet, the index species of the Upper Eocene, Globigerinatheka semiinvoluta, is used to delineate the Upper Eocene upon its occurrence.This species appears in some wells (Figs. 3 and 5) and is absent in others (Figs. 4,6 and 7).
The latest Eocene fauna such as Cribrohantkenina, T.c.cocoaensis, T.c.cunialensis are absent from the material sug- gesting that the Upper Eocene when observed is incomplete.Accordingly, the boundary between the Apollonia Fm. and the underlying Khoman Fm. is represented by a hiatus spanning the whole Palaeocene and the basal part of the Eocene with some hiatuses within the formation.The recorded plank tonic foraminiferal zones: from the Morozovella aragonensis zone (P8, BOLLI, 1957 andTOUMARKINE &LU-TERBACHER, 1985) to the Globigerinatheka semiinvoluta Zone (P15, Bolli, 1957) are delineated within the Apollonia Fm.With the pronounced unconformity with the overlying Dabaa Fm.

The Dabaa Formation
The Dabaa Fm. is devoid of identifi able planktonic foraminifera in spite of the sporadic occurrence of benthics species.So, its Oligocene age, as estimated by sporadic fau na, is adopted here.The planktonic and benthic species are shown in Figures 8 to 11.

Nannoplankton biostratigraphy
The zonations of SISSINGH (1977) for the Upper Cretaceous and that of MARTINI (1971) and OKADA & BUKRY (1980) for the Lower Palaeogene were adopted here in the deep wells, contamination by caving is observed through examination of the cutting.In the study wells, nannofossils are frequent to rare and moderately to poorly preserve.

Tranolithus phacelosus Zone (CC23)
Defi nition: It is defi ned from the LO of Reinhardtites anthophorus to the LO of Tranolithus phacelosus.
Remarks: In addition to the zonal markers, the nannofossil assemblage includes the large sized forms of Eiffellithus turiseffelii and Arkhangleskiella cymbiformis.

Arkhangleskiella cymbiformis Zone (CC25)
Defi nition: It is defi ned from the LO of Reinhardtites levis to the FO of Nephrolithus frequens.
Remarks: The CC25 Zone is unconformably overlain by zone NP12 (Early Eocene), with a marked hiatus spanning the uppermost Maastrichtian, as well as the entire Palaeocene and the lower part of the lower Eocene.

Tribrachiatus orthostylus Zone (NP12)
Defi nition: The Base is defi ned by the fi rst occurrence (FO) of Discoaster lodoensis and the top by the last occurrence of Tribrachiatus orthostylus.

Remarks:
In addition to the zonal markers, the nannofossil assemblage includes in the investigated wells the FO of both Scyphosphaera magna and Ericsonia formosa which defi ne the base of NP12.Authors: BRÖNNIMANN & STRADNER (1960) and BUKRY (1973) Age: Early Eocene (Ypresian).

Discoaster lodoensis
Occurrence: Karama-3x (800-1180 m) Remarks: In addition to the zonal marker, the nannofossil assemblage includes (in the base of the zone) the FO of Reticulofenestra dictyoda and Helicosphaera lophota.Its top cannot be defi ned due to the marked hiatus which includes zones NP14 through NP17 in Karama-NW-5x.

Discoaster tani nodifer Zone (NP16)
Defi nition: This zone includes the stratigraphic interval from the LO of Rhabdosphaera gladius to the LO of Chiasmolithus solitus.

Remarks:
The base of the zone is placed at the level of the fi rst appearance of Reticulofenestra umbilica in the studied wells.Rhabdosphaera gladius, the marker of the base of the zone is often of little use for zonation (PERCH-NIELSEN, 1985, p. 439).It is well known that Reticulofenestra umbilica has its FO at the base of the middle Eocene zone NP16 (MARTINI, 1971) and CP14 (OKADA & BUKRY, 1980).
In the study wells, a large scale hiatus is recorded below and above the sediments of Zone NP16, and thus the top and ba se of Zone NP16 cannot be accurately detected in such wells.

Chiasmolithus oamaruensis Zone (NP18)
Defi nition: This zone is defi ned as the interval from the FO of Chiasmolithus oamaruensis to the FO of Isthmolithus recurvus.
Remarks: The base and top of Zone NP18 are well defi ned in Karama-3x, Karama-E-1x and Karama-NW-5x.

Isthmolithus recurvus Zone (NP19/20)
Defi nition: The interval from the fi rst occurrence of Isthmolithus recurvus to the last occurrence of Discoaster sai panensis,and /or Discoaster barbadiensis.

Remarks:
The fi rst occurrence of Isthmolithus recurvus marks the base of the combined zone NP19/20.MARTINI (1971) used the lowest occurrence of Sphenolithus pseudoradians to defi ne the base of zone NP20, but subsequent work (MARTINI, 1976) has shown that S. pseudoradians occurs much earlier and that the two zones cannot be differentiated.Therefore, zones NP19 and NP20 are also combined here, and Zone NP19/20 is defi ned as the interval between the fi rst occurrences of I. recurvus and the last occurrence of Discoaster saipanensis and D. barbadiensis.

SEQUENCE STRATIGRAPHY
The target of seismic stratigraphic analysis is to divide the seismic sections of the East Bahariya Concession into distinct depositional sequences that make up the investigated Late Cretaceous-Eocene succession in the study area.These sequences are bounded from above and below by marked sequence boundaries (EMERY & MYERS, 1996).The following steps were followed to achieve the aim of this study: A) Reorganization of the time-depositional units depending on the detected unconformities or changes in seismic patterns.B) Following these recognized unconformities in the different investigated seismic sections, it was important to defi ne the major sequence boundaries.C) Seismic facies analysis, interpreting depositional environments from seismic refl ection characteristics.D) Sequence stratigraphic analysis of the different formations using the available biostratigraphic data and lithological composition.
The studied Upper Cretaceous-Eocene succession in the East Bahariya concession at the Western Desert is subdivided into fi ve major 2nd order depositional sequences (UCS1,

Upper Cretaceous sequence 1 (UCS1) (Cenomanian)
The fi rst sequence (UCS1) is represented by the Upper Cretaceous rocks including the Upper Bahariya Fm. (Fig. 19).The top of this sequence is bounded by the depositional sequence boundary SB1 which represents the boundary between the Abu Roash-G Member and the underlying Bahariya Fm.It is the deepest identifi ed and selected surface in the study area, which regionally, extends along the East Bahariya Concession, indicating a regional unconformity between the Abu Roash Formation and the underlying Bahariya Formation and the second order depositional sequence boun dary (EL BASSYOUNY, 1970, CATUNEANU et al., 2006).
The base of this sequence can not be followed on the seismic sections because these are badly processed (Fig. 21), while the upper part of the Bahariya Fm. displays discontinuous, moderate to low amplitude refl ectors.Internal confi gurations including onlap, toplap, downlap and parallel refl ectors were recognized.
The external form of this sedimentary body appears either as sheet-like or wedge-shaped units.The principal internal confi gurations appear to be parallel to subparallel.

Interpretation
Clear differences between the high and low to moderate amplitude refl ectors in this formation testify to the existence of at least two different lithological units.One has high amplitude and displays parallel to divergent refl ector terminations.These units are interpreted as shale to siltstone beds.The other lithological units which have a low to moderate amplitude and are characterized by disconnected refl ectors are interpreted as sandstone beds.According to Darwish et al., (1994) andISMAIL et al. (1989) the Bahariya Formation lithologically is subdivided into the lower Bahariya and upper Bahariya units.The Lower Bahariya Formation is composed of a relatively sand rich succession of fi ne to medium grained sandstone and shale while the Upper Bahariya Formation is composed mainly of very fi ne-grained sandstone with shale intercalated with limestone beds.Pyrite and glauconite minerals are common, which were deposited in a shallow marine to fl uvio-marine environment with alternating high and low energy.
Based on the results of the analyzed seismic lines, this subdivision of the Bahariya Fm. into two units has been identifi ed in the all investigated seismic lines thus confi rming the interpretation of the seismic facies.

Abu Roash-G Member (Cenomanian)
Based on the interpretation of the seismic lines, the Abu Roash-G Member can be subdivided into two main parts.
The lower part is characterized by low amplitude and mo dera te continuity refl ectors.Their refl ection geometry at the sequen ce boundaries is considered as erosional truncation (Fig. 23).
From the studied seismic lines the refl ection geometry at the sequence boundaries are considered concordant with lower and upper sequence boundaries of this formation.The seismic facies of the lower part of the Abu Roash-G Member appears as low to moderate amplitude and moderate continuous refl ectors in the internal confi guration as parallel refl ectors, which in the external outline seems to represent elongated wedge-shaped beds.
The seismic facies of the upper part of the Abu Roash-G Member is characterized by high amplitude and very continuous refl ectors.The internal refl ection geometry at the upper sequence boundary is considered concordant and runs parallel to the sequence boundary (Figs.21 and 23).

Interpretation
The characteristic confi guration of the seismic facies in the lower part of the Abu Roash-G Member is interpreted as sand  bars (Fig. 23).The low amplitude and discontinuity of the seismic refl ectors support this interpretation.These sand beds may represent deposits of a prograding delta system, with offshore (pro-delta) silty mudstones at the base overlain by (in ascending order) lower, middle and upper shoreface sandstone, distributary-mouth bar sandstone.
The internal confi guration as parallel refl ectors of the upper part is interpreted as homogenous sediments, which are arranged in parallel thin beds.These features exhibit cycles of mudstone, glauconitic siltstone, limestone, and sandstone deposits.
Based on the depositional environment of the Abu Roash-G Member (MARK et al., 2009) which is conside red  to be delta to shallow marine environment, our interpretation agrees with the statements and characteristic features such as sandstones bodies and shale beds that were re cog nized in the interpreted seismic lines (Figs. 21,23 and 24).

Abu Roash A, B, C, D, E and F members (Turonian)
The Abu Roash Fm. is generally characterized by several strong correlated internal refl ectors representing the contacts between its members.The parallel refl ectors alternate with weak to moderate internal refl ectors, which show chaotic to hummocky internal confi gurations (Fig. 24).The external forms of these members (A, B, C, D, E and F) display different geometries such as sheet-like and wedge-shaped forms, which can be indicated by parallel to divergent internal refl ection confi guration.
Based on Biostratigraphy BARAKAT et al. (1987), these facies features exhibit cycles of shallow marine limestone, sandy limestone and shale of the Abu Roash A, shallow of the Abu Roash B Member, relatively restricted deep marine shale, sandstone and limestone of the Abu Roash C Member, the limestone and shale of Abu Roash D, shallow marine sand and shale of the Abu Roash E Member, and the deep marine limestone and shale of the Abu Roash F Member (Figs. 23 and 25).

Interpretation
The alternation between weak -moderate and high amplitude internal refl ectors suggests at least two different lithological units.The obvious parallel refl ectors are interpreted as shale beds.The low amplitude refl ectors have a chaotic to hummocky internal confi gurations, which indicate impure lime-stone or highly bioturbated sandstone beds.The Abu Roash Fm. of Turonian age consists of limestone with shales and sandstone intercalations providing a good source reservoir and cap rocks for hydrocarbons (BAYOUMI et al., 1987).
According to BARAKAT (1987) the habitats of the faunal assemblage together with the occurrence of detrital quartz grains in the Abu Roash F-A Members suggest a shallow marine environment of the neritic zone.Furthermore the Abu Roash Fm. was deposited in shallow-shelf to open-marine conditions (BARAKAT et al., 1987;BAYOUMI et al., 1994BAYOUMI et al., , 1996)).

Upper Cretaceous Sequence 3 (UCS3) (Campanian-Maastrichtian)
The Khoman Fm. is equivalent to sequence (UCS3).This sequence is bounded by SB2 below and the above.SB2 is at the end of the Turonian.Detailed analysis of SB2 reveals that it is of the type 2 boundary of VAN WAGONER et al. (1988).It is characterized by partial erosion of the Coniacian and Santonian between the Khoman and the top of the Abu Roash Formation, as shown by truncation of the top Abu Roash especially in structurally uplifted places (BAYOUMI & LOTFY, 1989).It is an important disconformity recorded in all the Cretaceous-Tertiary successions of Egypt (HE-WAIDY et al., 2006).
The seismic facies of the lower part of the Khoman Fm. is moderate to low in the continuity and amplitude represented by disconnected refractors.These refl ectors exhibit several characteristic terminations such as onlap, truncations and complicated sigmoidal confi gurations.A group of parallel irregular refl ectors is characteristic for this lower part of the Khoman Fm.The seismic facies of the middle part of the Khoman Fm. is entirely different in character which is dominated by very weak continuity and amplitude with prevailing chaotic seismic confi gurations (Fig. 26).The external geometry of this part represents an inhomogeneous lithological character, which appears to be similar to reef or salt dome geometry (Fig. 26).
The internal configuration of the upper part of the Khoman Formation is characterized by very high continuity and amplitude with parallel refl ectors.These refl ectors extend concordantly with the upper boundary of this sequence and display onlap and oblique parallel confi gurations.The external geometry in the upper part occurs as sheet like units to platform geometry especially to the north of the study area (Fig. 26).

Interpretation
Based on the fossil content and the description of the seismic facies with respect to characteristics of these seismic refl ectors, one could conclude that the logical interpretation of the seismic confi guration, such as very closed and parallel refl ectors, indicate a well bedded and homogenous lithology, which can be shale beds intercalated with limestone or chalk.
Parallel, irregular refl ectors groups are interpreted as isoclinals small folds within the lower part of the Khoman Fm.Fold structures can be attributed to compressional stress in the study area during the Late Cretaceous.
The poorly and weak amplitude refl ectors in the middle part indicate massive chalk limestone.The moderate to low continuity and amplitude signify limestone with a high number content of chert nodules.This interpretation agrees with the lithology of the Khoman Fm. which consists mainly     of chalky limestone with pyritic, cherty strata in the lower part and interbeds of shale in the upper part (BARAKAT et al., 1984, ABDEL HAMID, 1985& GHANEM, 1985).
Facies characteristics, faunal content and planktonic ratio indicate an upward shallowing in the depositional regime from a deep middle/outer to shallow inner shelf during deposition of the Khoman Formation (HEWAIDY et al., 2006).

Eocene sequence 4 (ES)
The fourth sequence is represented by Eocene sediments (ES), which comprise the Apollonia Formation (Figs. 19,20).This sequence is bounded by SB3 in the lower part and by SB4 at the upper part Figs.19 & 20, show the SB3 which appears at the end of the Upper Cretaceous (Campanian -Maastrichtian) and separates between the Eocene sequence (ES) and Upper Cretaceous sequence (UCS3) sequences.The SB3 has moderate to high amplitude refl ectors and is also marked by the radioactivity increase in the gamma ray logs (Fig. 27).
Based on the fossil content, the available data and former geological studies, the lower part of this sequence boundary is affected by submarine erosion causing the erosion causing the removal of the uppermost part of the Maastrichtian and all of the Palaeocene (BAYOUMI et al., 1987).Seismic facies of this sequence which is represented by sediments of the Apollonia Formation have three main features which are also very clear in the gamma logs.
The seismic facies of the lower part of the Apollonia Fm. is moderate to low both in continuity and amplitude, while to the north of the area; refl ectors are moderate to high amplitude and continuity.The internal confi gurations appear to be parallel to divergent (Fig. 28).This part shows the increase in gamma rays (more than the upper part of the Khoman Formation).In general the refl ection geometry at the lower sequence boundary of the Apollonia sequence is considered concordant and exhibits even onlap near to central part of the study area (Fig. 28) The seismic facies of the middle part of the Apollonia Formation is distinguished by high continuity and amplitude.Also the principal internal confi gurations appear to be parallel to the divergent refl ectors.The external form of these sediments is considered to be a wedge like geometry with local irregular bedding features.These structures occur as small isolated and relative high amplitude refl ectors of small domal structures.
The seismic facies of the upper part of the Apollonia Formation is characterized by low to moderate continuity and amplitude.The principal internal confi gurations appear to be parallel to divergent.The external form of these sediments is sheet-like and wedge shaped units.
The foraminiferal assemblage recorded from the upper part of this sequence suggests the end of the restricted environment that prevailed during deposition of the underlying Eocene sediments.

Interpretation
Based on the seismic facies confi gurations the parallel high amplitude refl ectors display a carbonate ramp geometry, which has a very slight dip and occurs as an elongated wedge.The local irregular bedding structures are interpreted as convolute bedding which may be related to slumping.This carbonate ramp extends from north to the south (Fig. 28).It is worth mentioning, that the thicknesses of this carbonate ramp decreases laterally to the south.The low amplitude refl ectors in the lower part indicate a heterogeneous lithology, which may be related to mixed pure carbonate sediments contaminated with cherty nodules and lenses parallel to bedding.
The lithological composition and the characteristics of the seismic refl ectors of this sequence suggest that its depo-sitional conditions range the inner shelf at the base to the outer shelf at the top of this sequence.

Oligocene to recent sequence 5 (ORS)
The Oligocene to recent facies in the study area are represented by the sediments of the Dabaa Fm. in the lower part and the Moghra Fm (Miocene) at the top of the investigated seismic lines.Both formations build up a complete sequence the lower boundary of which is represented by the sequence boundary (SB4), whereas the upper boundary represents the top of all interpreted seismic lines in the study area.The sequence boundary represents the unconformity surface due to erosion of sandstone and siltstone from type section defi ned in the Dabaa-1 well (TAWADROS, 2001).
Based on the analyzed lines, the seismic facies of the Upper top facies are characterized by: Internal confi guration of the lower part gradually increasing in both the continuity and frequency of seismic refl ectors upward showing several seismic terminations such as toplap with parallel to sigmoidal confi gurations.These refl ectors run parallel to each other and display a high amplitude and frequency in the middle part of this sequence.The external form of this unit resembles sheet-like units.
The upper part of the sequence is characterized by chaotic, discontinuous, high frequency and low amplitude refl ectors (Fig. 29).The internal confi guration of the seismic facies is characterized by seismic termination features: onlap, toplap and erosion truncations.The external form is of the sheet-like and wedge shaped units with observable channel-like geometry.

Interpretation
A gradually increase in both the continuity and frequency of the seismic refl ectors in the lower part of this sequence indicates changes in the lithology.Hence the lower part of this sequence is interpreted as shale beds of the Dabaa Formation intercalated with evaporites concentrated in the lower part of the sequence and characterized by low amplitude refl ectors and very chaotic seismic confi guration (Fig. 29).
The upper part of this sequence is interpreted as sandstone beds of the Moghra Formation because of the relatively low seismic refl ectors, in addition to the occurrence of typical sandstone features such as channel fi ll structures.
These characterize the Upper top facies of the Dabaa and Moghra Formations suggesting a change from shallow marine inner shelf environment at the base to fl uvial facies at the top.This assumption agrees in some points with the interpretation of the seismic lines.

Gamma ray response
The change in the gamma ray responses in the various units can be recognized.The boundary between units UCS1 and UCS2 is marked by a decrease in the gamma ray values (Fig. 30).
The presence of sandstone and shale intercalations in many parts of the UCS1 unit causes oscillation of the gamma ray curve (Figs. 29,30).The contact between the units UCS2 and UCS3 can be detected by a decrease in the gamma ray values (Figs. 20, 29) accompanied with the change from shale to limestone lithofacies.
The boundary between the unit UCS3 and the ES is marked by a slight increase in the gamma ray values.UCS3 shows no change in gamma ray values because it is composed only of chalk limestone.The ES unit shows some high values in the gamma ray log which are ascribed to the shale intercalations in this unit (Fig. 29).The upper part of the ORS unit is characterized by a marked decrease in the gamma ray values with respect to the underlying unit ES.

DISCUSSION OF THE SYSTEMS TRACTS
The Upper Cretaceous -Palaeogene succession in the East Bahariya concession is divided into 17 systems tracts.These systems tracts are outlined below:

Lowstand systems tract (LST1)
This systems tract represents the fi rst one studied in this work.It comprises the upper part of the first sequence (UCS1), which represents the upper part of the Bahariya Formation.The lower boundary of this tract is not traceable on seismic profi les.This lowstand system is characterized by low GR and relatively high resistivity (Fig. 30).
Sediments of the LST1 are mainly sandstones with siltstone interbeds.These deposits are overlain by the clastic and non-clastics deposits of shale and limestones, which belong to the lower part of the Abu Roash-G Member (Fig. 31).
The available data indicate that this interval represents the maximum drop in sea level.This drop started with the deposition of the coarse clastics (sandstone) of the Bahariya Formation and continued and increased with deposition of more clastics and evaporites.This systems tract is widely recognized across the Western Desert.
The TST is missing, due to either non-deposition or subsequent erosion.The HST of this sequence is also missing, most likely due to subaerial erosion (fl uvial incision) where the upper (LST1) is directly overlain by the 2 nd order depositional sequence of the Abu Roash Formation, being truncated at the top by the sequence boundary (subaerial unconformity).
The age of this lowstand system is Early Cenomanian.The LST1 deposits are limited by the fi rst sequence boundary (SB1), which delineates the end of the 1st sequence and the beginning of the second.

Highstand systems tract 1 (HST1)
Based on the available data and their resulting interpretations LST and TST system tracts in the investigated well sections are entirely missing.The absence of these may be attributed to the erosion cycle, which is indicated by truncation at the top by the sequence boundary due to subaerial unconformity.
This highstand system tract occurs in the Lower part of the Abu Roash-G Member, and represents the maximum phase of transgression after deposition of the Bahariya Formation (Fig. 32).
The progradational HST1 sediments are characterized by an upward increase in GR and moderate to low resistivity.Lithofacies of the HST1 sediments are entirely different from those of the LST1 sediments (Fig. 31), and includes mainly shales and limestones.The limestone is laterally variable in thickness whereas the shale is concentrated mainly in the central part of the studied area.The total thickness of this high system tract reach a maximum (30 m) in the Karma-SW 1x well and is minimal (18 m) in the in Karama-NW 5x well (Fig. 31).

Lowstand system tract (LST2)
This systems tract represents the second studied low system.It consists of a small part of the Abu Roash-G Member.The progradational LST2 sediments are characterized by low GR and relatively high resistivity.
Sediments of the LST2 are present all over the study area.The lithological composition of these deposits is mainly sandstone, mudstone and siltstone.The thickness of the LST2 sediments is quite small and uniform (about 10 m) (Fig. 31).
Seismic facies analysis and Gamma ray log confi rm that the sandstone sediments indicate that, the prevalent environments of depositions during deposition of this system tract were estuaries or shorelines.
The sandstone units were most likely deposited as bayhead deltas, tidal channels and barrier beaches because they display the characteristic features of tidal environments with a high dynamic regime of and wave infl uence.

Transgressive systems tract (TST1)
The retrogradational deposits of TST1 are marked by high GR and represents the onset of transgression and sea-level rise (NUMMEDAL & SWIFT, 1987).It overlies the transgressive surface (TS1), which tops the lowstand systems tract (LST1) (Fig. 31).The TST1 sediments are composed of shale and limestone deposits in most of the area and mainly shale with a few limestone interbeds in the West of the study area.
The maximum fl ooding surface (MFS1) separates the deposits of the retrograding interval of the TST1 below from the prograding deposits of the HST1 above (Fig. 31).This surface, together with the TST1 sediments, is associated with high peaks in the GR logs (SCHAFFER 1987;ARMEN-TROUT & CLEMENT, 1990).
The thickness of this system tract shows an increase in the middle part of the investigated area and decrease laterally in both sides (Fig. 31).Interpretation of seismic facies and Gamma ray log indicate that open marine conditions prevailing during the deposition of this system tract.

Highstand systems tract 2 (HST2)
This highstand system tract overlies the sediments of the transgressive systems tract TST1.Lithofacies of the HST2 sediments are clearly different to those of the HST1 sediments.HST2 sediments contain more shale in comparison with the HST1 (Fig. 31).The thickness of the HST2 sediments is quite variable.The maximum thickness (about 10 m) is detected at Aqsa-1X and K-SW-1X wells, while the minimum thickness (4 m) in K-NW-1X well (Fig. 31).From GR and lithology the prevailing environmental conditions during deposition of this system tract are deep marine conditions.

Lowstand system tract 3 (LST3)
The aggradational LST3 sediments have quite a similar well log response to those of the LST2 sediments (Fig. 31).The lithology of these deposits is mainly pro-delta silty-mudstone, lower, middle and upper shoreface sandstone, distributary-mouth bar sandstone, and distributary-channel sandstone, silty-muddy interdistributary or coastal plain deposits.The thickness of these deposits is quite variable with maximum thickness (about 12 m) drilled in K SW-1X and K SW-10X wells and minimum thickness (about 5 m) in K-NW-1X well.The log shape and the lithofacies characteristics of these sediments indicate that the LST3 sediments are deposited in similar conditions to those of the LST1.

Transgressive system tract 2 (TST2)
The retrogradational TST2 sediments have a quite different lithological composition and log response characteristics to those of the TST1.These sediments include the upper parts of the Abu Roach-G Member, composed of shale, limestones and sandstone respectively.
The upper boundary of the TST2 sediments is marked by an abrupt increase in the GR where the lower part shows low to moderate GR values.The increase of GR values in the upper part is due to an increase in the shale ratio within this system tract.The high shale content is attributed to the fact that, the Early Turonian started with the great transgres- sion of the Late Cretaceous, which was recorded as the maximum fl ooding surface in the Western Desert (MFS2) (BAR-AKAT, 1987).The thickness of these deposits is quite va riable with the maximum thickness of 60 m in well K-SW-1X.

Highstand systems tract 3 (HST3)
This highstand system tract comprises the Abu Roash F, E and D Members.With the advent of the Turonian, the maximum fl ooding surface (MFS2) covered the northern part of the Western Desert (BARAKAT, 1987).This system tract tops the transgressive systems tract TST2 and represents the upper part of the Abu Roash-G Member.
Lithofacies of the HST3 sediments are similar to the HST1 sediments, composed mainly of shale and limestone (Figs. 26 and 29) where the Abu Roash members E, F and G are the most prolifi c and outstanding source rocks in the Western Desert (Lüning, 2004).

Lowstand systems tract 4 (LST4)
This lowstand systems tract is relatively thin representing part of the Abu Roash Fm.In the Turonian time (Figs.26  and 29).It overlies the high stand system tract (HST3).The intervals of LST4 are characterized by a relatively decrease in gamma ray values.
This systems tract is part of the Abu Roash-C Member, composed of shale, sandstone and limestone indicating a shallow shelf marine (sublittoral) environment (ABDEL KHALEK, 1989).

Transgressive system tract 3 (TST3)
TST3 sediments have quite similar lithological well log characteristics to those of TST1, which is exclusively composed of shale and limestone sediments.
The upper boundary of the TST3 deposits is marked by moderate GR values.The second maximum fl ooding surface (MFS3) in Turonian time in the Western Desert is encountered directly above this system tract (Figs.27 and 30).
This system tract is apart of the Abu Roash-D Member, which consists of limestone alternating with thin fl int bands refl ecting a shallow shelf marine environment (ABDEL KHALEK, 1989).

Highstand systems tract 4 (HST4)
This highstand systems tract occupies the top of the upper part of the Abu Roash Fm. and consists mainly of shale, limestone and sandstone intercalations and its deposits display moderate to high GR values, because the main lithological component is shale.
HST4 overlies the transgressive surface (MFS3), which tops the second transgressive systems tract of Turonian age.The predominant environmental conditions of deposition were those of shallow marine shelf (ABDEL KHALEK, 1989).

Transgressive system tract 4 (TST4)
This transgressive systems tract represents the beginning of the sequence boundary (UCS3), which covers the high stand system tract sediments (HST4) high stand system tract sediments whereas the LST was truncated during the Coniacian and Santonian at the top of Khoman Formation due to subaerial exposure represented by an unconformity.
Lithofacies of the LST4 sediments are composed of sediments of limestones intercalated with shale which denotes the lower part of the Khoman Fm. (Figs. 27 and 30).The relatively deep sea conditions that prevailed by the offset of the Early Cenomanian persisted during the Late Cenomanian, Turonian and attained their peak in the Maastrichtian (BARAKAT, 1987) as refl ected by the interpreted MFS4 surface.The TST4 is characterized by a strong increase in the gamma ray, which indicates an increase in sea level in the Western Desert (MFS4).

Highstand systems tract 5 (HST5)
This highstand systems tract represents the upper part of the Khoman Fm. and consists of open-marine white chalky limestone with chert bands.It overlies the transgressive surface (MFS4), which tops the transgressive systems tract of Campanian-Maastrichtian age.
Data such as GR and fossil content indicate that the environmental conditions, which were prevailing during the deposition of the system tract, were indicative of a shallow inner shelf of marine environment.The fossil content and available composite logs indicate that, this lowstand systems tract is composed of shallow marine limestone with numerous chert intervals.The cherty intervals of LST5 are characterized by a relative increase in gamma ray values.The high values are attributed to the concentration of organic matter within the chert beds.
The lithological, foraminiferal data and characteristics of the seismic refl ectors of this sequence suggest that the depositional conditions in this systems tract range from outer shelf to inner shelf at the base of the Eocene period.

Transgressive systems tract 5 (TST5)
This transgressive systems tract includes the middle part of the Apollonia Formation.It overlies lowstand systems tract sediments (LST5) of the Eocene Sequence (ES).The MFS5 surface delineates the upper boundary of the retrograding sequence of the TST5 sediments.This surface is marked by the maximum GR response and the foraminiferal content (Figs. 27 & 30).The MFS5 is overlain by the prograding sequence of HST6 sediments.This systems tract represents the maximum transgression recorded in the Western Desert area during the Middle-Late Eocene.Based on various types of available data the environmental conditions prevailing during the deposition of the system tract are outer shelf to inner shelf.

Highstand systems tract 6 (HST6)
This highstand systems tract consists of the upper part of the Eocene Sequence (ES) and represents the upper part of the Apollonia Fm.It is characterized by the abrupt increase of the Gamma ray values (Figs. 28 & 31).This increase in Gamma values is due to the high mud and silts fractions.
Highstand systems tract (HST6) overlies the maximum fl ooding surface MFS5, which tops the transgressive systems tract of Eocene age, where it includes limestone with shale interbeds.Limestone beds are rich in foraminifera which may indicate a deeper water habitat.

Transgressive systems tract 6 (TST6)
This transgressive systems tract includes the Dabaa Fm.It overlies the highstand systems tract (HST6) of the Eocene Sequence (ES).Transgressive system tract 6 (TST6) is characterized by an increase of gamma values with oscillation, but in the upper part gamma values suffer a moderate decrease compared to the lower part of this transgressive system tract (TST6).The available data suggest that the LST was removed due to marine erosion (TAWADROS, 2001).The environmental conditions, which were predominant during the deposition of this transgressive systems tract, were marine outer shelf to deep marine environments.
The aforementioned interpreted palaeo-environments of the study area exhibited echoes of the tectonic events affecting the southern margin of the Tethys Ocean.The study area, however, is located 80 Km offshore southern Tethys Ocean in the Abu Gharadig Basin, which is situated on the North African Plate.This basin developed in conjunction with a complex series of E-W and Northeast-Southwest oriented normal and wrench faults (Fig. 33) that formed during the Late Jurassic rifting and continued into, or were rejuvenated during the Early Cretaceous along the east Mediterranean margin resulting in the oceanization of the eastern Mediterranean basin.Rifting resulted from the initiation of the closure of the Tethys (GUIRAUD, 1998).

SUMMARY AND CONCLUSIONS
The calcareous nannofossils identifi ed belong to CC22, CC23, CC25 of the Campanian -Maastrichtian age and zones NP12, NP13 of Early Eocene age and NP16 of Middle Eocene age and NP18 and NP19/20 of Late Eocene age.The ages of the different rock units were identifi ed by means of planktonic foraminifera as well as calcareous nannofossils and some hiatuses were detected within the studied wells as follows: 1.The latest Cretaceous and the all of the Palaeocene are probably missing in the whole area, resulting in a major unconformity which may suggest tectonic instability due to pulses of epeirogenic movements affecting the area.

Figure 1 :
Figure 1: Location map of the area under study (a, b).

Figure 3 :
Figure 3: Distribution chart of the planktonic and benthic foraminifera of the Well Aqsa-1x.

Figure 4 :
Figure 4: Distribution chart of the planktonic and benthic foraminifera of the Well Karama-3.

Figure 5 :
Figure 5: Distribution chart of the planktonic and benthic foraminifera of the Well Karama-E-1x.

Figure 6 :
Figure 6: Distribution chart of the planktonic and benthic foraminifera of the Well Karama-NW-1x.

Figure 7 :
Figure 7: Distribution chart of the planktonic and benthic foraminifera of the Well Karama-NW-5x.

Figures
Figures 12 to 16 show the distribution of the nannoplankton in the different wells and some stratigraphically important taxa are represented in Figures17 and 18which are identifi ed from the Aqsa-1x, Karama-3x, Karama-E-1x, Karama-NW-1x and Karama-NW-5x wells respectively.

Figure 12 :
Figure 12: Distribution chart of the nannofossils of the Well Aqsa-1x.

Figure 13 :
Figure 13: Distribution chart of the nannofossils of the Well Karama-3x.

Figure 14 :
Figure 14: Distribution chart of the nannofossils of the Well Karama-E-1X.

Figure 16 :
Figure 16: Distribution chart of the nannofossils of the Well Karama-NW-5x.

Figure 21 :
Figure 21: Lithologic section of the Upper Cretaceous-Eocene Succession correlated with seismic profi le 1219.

Figure 22 :
Figure 22: Shows the lower part of seismic line 5667(middle of the study area), (b): The interpreted seismic line 5667 displaying the main internal confi gurations which characterized the seismic facies of Upper Cretaceous Sequence 1 (UCS1).

Figure 23 :
Figure 23: Shows the lower part of seismic line 5727(middle of the study area), (b): The interpreted seismic line 5775 illustrates the internal confi gurations of sandstone in the lower part of Abu Roash-G Member as a part of Upper Cretaceous Sequence 2 (UCS2).

Figure 24 :
Figure 24: Showing the lower part of seismic line 5930 (east of the study area), (b): The interpreted seismic line 5930 illustrates the internal confi gurations of sandstone in the lower part of Abu Roash-G Member as a part of Upper Cretaceous Sequence 2 (UCS2).

Figure 25 :
Figure 25: Relative sea level fl uctuations in the Upper Cretaceous, Western Desert.

Figure 26 :
Figure 26: Showing one of the internal confi guration which distinguish the seismic facies of Upper Cretaceous Sequence 3 (UCS3) (upper part of line 5667).

Figure 28 :
Figure 28: Showing the lower part of seismic line 5630 (middle of the study area), (b): The interpreted seismic line 5630 illustrating the internal confi gurations of Eocen Sequence.

Figure 29 :
Figure 29: Showing the upper part of seismic line 5455 (west of the study area), (b): The interpreted seismic line 5455 displaying the main internal confi gurations which characterized the seismic facies of Upper Eocene to Recent Sequence (UERS).

Figure 31 :
Figure 31: Schematic diagram for W-E correlation of the system tracks in the Abu Roash-G Member.

Figure 32 :
Figure 32: Formations, Sequences, Systems tracts and sea-level changes of the Upper Cretaceous-Eocene age of the subsurface East Bahariya succession.
This lowstand system tract represents the lower part of the Apollonia Fm. (Figs.27 and 30) wich overlies the sequence boundary (SB3), where It is tops the high stand systems tract (HST5).

Figure 33 :
Figure 33: Illustrates the major structural trends in the Western Desert (after EGPC, 1992).
2. The boundary between the Apollonia Fm. and the overlying Dabaa Fm. is represented by an unconformity with corresponding hiatuses of different time spans.While the latest Eocene planktonic foraminifera such as Cribrohantkenina, Turborotalia cerroazulensis cocoaensis, Turborotalia cerroazulensis cunialensis are absent, it is possible to identify the Upper Eocene by means of the index calcareous nannofossils.The Apollonia Fm. (Campa nian-Maastrichtian) rests unconformably on the Khoman Fm. (Eocene) in the study wells.The upper part of the Maastrichtian, as well as the entire Palaeocene, the basal part of the Eocene, and the upper part of the Middle Eocene are missing.The Eocene/Oligocene boundary could not be delineated due to the absence of nannofossil assemblages in the Dabaa Fm. 3. The Upper Cretaceous-Palaeogene succession for the studied area allows classifi cation into fi ve major 2 nd order depositional sequences, separated by four major depositional sequence boundaries (SB1, SB2, SB3 and SB4).The Upper Cretaceous-Palaeogene succession in the East Bahariya is divided into 17 systems tracts.The se systems tracts include: 7 System tracts of Cenomanian age; 4 System tracts of Turonian age, 2 System tracts of Campanian -Maastrichtian age and 4 System tracts of Eocene age.