Structural evolution of the Abu Gharadig field area, Northern Western Desert, Egypt

Accepted Manuscript Structural evolution of the Abu Gharadig field area, northern western desert, Egypt A.M. El Gazzar, A.R. Moustafa, P. Bentham PII:

S1464-343X(16)30321-1

DOI:

10.1016/j.jafrearsci.2016.09.027

Reference:

AES 2685

To appear in:

Journal of African Earth Sciences

Received Date: 20 June 2016 Revised Date:

3 September 2016

Accepted Date: 22 September 2016

Please cite this article as: El Gazzar, A.M., Moustafa, A.R., Bentham, P., Structural evolution of the Abu Gharadig field area, northern western desert, Egypt, Journal of African Earth Sciences (2016), doi: 10.1016/j.jafrearsci.2016.09.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structural evolution of the Abu Gharadig field area,

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Northern Western Desert, Egypt

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A. M. El Gazzar1, *, A. R. Moustafa2, and P. Bentham3 * Corresponding author

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1: BP Egypt, Maadi Egypt.

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E-mail: [email protected]

2: Geology Department, Ain Shams University, Cairo, Egypt.

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E-mail: [email protected] 3: BP Egypt, Maadi Egypt.

E-mail: [email protected]

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Keywords: Abu Gharadig Field; Northern Western Desert; Egypt; Inverted Basins;

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Inversion Folds.

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Abstract

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Discovered in 1969, the Abu Gharadig (AG) Field was the first large hydrocarbon

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discovery in the Abu Gharadig Basin of the Western Desert of Egypt. Oil production

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began in 1973, with gas brought into production in 1975. The field produces mainly from

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upper Cretaceous clastic reservoirs.

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The AG Basin is an E-W trending intracratonic rift basin, about 330 km long and 50-75

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km wide. It was initially formed as a large half graben basin during the Jurassic time in

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response to Tethyan rifting and continued to subside throughout the Cretaceous time.

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The half graben was subsequently inverted during the Late Cretaceous as part of the

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Syrian Arc deformation which affected northern Egypt. The Mid-Basin Arch, the AG

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Anticline, and the Mubarak High are three NE-SW oriented main inversion anticlines

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located within the AG Basin and are controlled by inversion of pre-existing Jurassic rift

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faults. The AG Anticline has an overall NE-SW orientation with a gentle plunge towards

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the NE and SW. It is locally bounded by two NE–SW-trending inverted faults on the

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southwest and northeast, accounting for the asymmetry of the anticline. Reverse offset

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of Cretaceous horizons is obvious at these inverted faults. Fault propagation folding is

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developed above the tips of the inverted faults at the Late Cretaceous Abu Roash and

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Khoman Formations. Based on thickness changes and stratigraphic relationships,

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inversion started during the Santonian time and continued into the Campanian-

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Maastrichtian. Inversion continued during deposition of the Paleocene–Middle Eocene

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Apollonia Formation and the Late Eocene–Oligocene Dabaa Formation.

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1. Introduction: The northern Western Desert of Egypt (Fig. 1) has proven to be a prolific hydrocarbon

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province despite the monotonous character of it surface which is generally a featureless

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plain. The first commercial hydrocarbon discovery was found in the Aptian Alamein

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Formation in 1966 by Phillips Petroleum (EGPC, 1992). Today the Western Desert is an

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important hydrocarbon province with more than 3 bnboe produced to date (published

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online basin report, IHS and WoodMac). Most of this production comes from Jurassic

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and Cretaceous rift basins. Although the amount of produced hydrocarbons is not very

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big, the northern Western Desert is attractive to explorationists due to its relatively low

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cost of hydrocarbon exploration and production as it is an onshore area almost free

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from any surface urban occupancy and generally has very low relief. In addition, the

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prospective Cretaceous reservoirs include several targets that are generally not very

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deep. Similarly, the northern Western Desert is attractive to geoscientists as its

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subsurface portrays valuable geological information pertinent to the Mesozoic and older

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evolution of this portion of northern Africa which is generally masked by flat-lying

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Tertiary rocks.

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Within the featureless plain of the northern Western Desert there are few prominent

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topographical or geological features like the faulted and folded Abu Roash complex to

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the north of the Giza Pyramids that reflect its intricate geological history (Said, 1990).

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Most of the surface is covered with gently-dipping Neogene strata showing significant

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lithological uniformity. The intensive exploration activities since the 1960’s have

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demonstrated that the subsurface is much more complex than what the surface geology

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shows, and this simplicity hides a complex structurally-controlled sedimentary basins

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that reach up to 8–9 km in depth, like the (AG) Basin (Said, 1990), Fig. 1. These

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sedimentary basins were developed during the Mesozoic times and extend across

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northern Egypt to Sinai (Moustafa and Khalil, 1990). Located within an overall

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northward-thickening continental passive margin they form a series of discrete E-W to

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ENE-WSW and NE-SW oriented half-graben basins (Sultan and Halim, 1988; Emam et

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al., 1990; Taha, 1992; Moustafa, 2008; Bevan and Moustafa, 2012) and include besides

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the AG Basin the Alamein, Kattaniya, Matruh, and Shoushan basins (Fig. 1). Other

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Mesozoic basins have also been documented within Egypt, including the WNW-ESE

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oriented Tiba–Natrun basin and the NW-SE oriented Asyut and Khomombo basins

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(Nagaty, 1988; Taha, 1992; Bosworth et al., 2008, Fig.1).

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The AG Basin is an E-W trending intracratonic basin located at the northern part of the

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Western Desert of Egypt (Fig. 1). It is about 330 km long and 50-75 km wide

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(Demerdash et al., 1984, Abdel Aal and Moustafa, 1988) and is bounded on the north

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by the Qattara Ridge (also known as the Sharib-Sheiba High) and on the south by the

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Sitra Platform (Fig. 1). The basin is crossed by three NE-SW structural highs, known

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from west to east as the Mid-Basin Arch, the AG Anticline, and the Mubarak High. The

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subject of this work, the AG Field occupies the AG Anticline (Fig. 1).

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The AG Basin was first revealed during the earliest exploration activities in the Western

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Desert by gravity, magnetic and reconnaissance seismic surveys carried out in the

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Western Desert in the mid-1950s and early 1960s. At that time only two dry wells (Betty-

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1 and Bahariya-1) had been drilled within its confines (El Ghoneimy and El Gohary,

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1988).

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The AG Field was the first oil and gas discovery situated in the central part of the AG

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Basin. It was discovered in 1969 by drilling of the AG-1 well. The AG-1 well was located

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on the crest of a NE-SW oriented anticline, which had been mapped on the basis of the

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early seismic program (EGPC, 1992). The discovery well reached a total depth of

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13,445 ft and encountered wet gas in sandstones of the Cenomanian Bahariya and the

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Albian Kharita Formations (Fig. 2). In addition, oil shows were found in sandstones of

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the Turonian Abu Roash Formation (“C” Member). Several exploration and appraisal

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wells were drilled to evaluate the whole field potential and to confirm the first exploration

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well results. The AG Oil Field was brought on stream in March 1973 with production

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from the Abu Roash “C”, “D”, and “E” Members (Fig. 2) in nine naturally flowing wells.

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Even though the gas market in Egypt was not developed at that time, but the Bahariya

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gas reservoir discovered in AG-1 was put on stream in August 1975 with offtake being

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controlled by local demand. By July 1987 a total of nine wells had encountered gas in

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the Bahariya Formation (Fig. 2), and were used to satisfy the growing gas market in

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Egypt. By 1992, a total of 40 wells of all categories had been drilled in the AG Field.

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Many other oil and gas fields have since been discovered in the AG Basin.

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The objective of this paper is to describe in detail the geometry and timing of various

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deformations events that developed and altered the AG basin with a main focus on the

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AG field area. We will also discuss the impact of the tectono-stratigraphic evolution on

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petroleum habitat across the field area and in similar structures inverted basins across

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the region.

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The data used for this study includes approximately 850 km2 of 3D seismic data, in

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addition to borehole data of 15 wells including basic wire-line logs (GR and lithology).

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Seismic-to-well ties were developed and being utilized in seismic mapping of Top

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Bahariya, Top Abu Roash/A, Top Khoman/A and Top Apollonia. The 15 wells cover an

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area larger than that shown in the present study. Out of these 15 wells, 7 wells are

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located in the AOI (Table 1). Formation picks and lithology data were utilized to

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determine the potential reservoir zones. The available seismic data are TWT pre-stack

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time-migrated (PSTM) reprocessed data of two different merged seismic vintages (1998

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and 2002) acquired over areas of (600km2 and 250 km2) respectively by

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BP/WesternGeco through land acquisition campaigns. Data fold number is 40 and 87.5

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for the 1998 and 2002 surveys respectively with record length of 4 seconds. The data is

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considered to be of good enough quality for detailed structural mapping.

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2. Regional Setting and Basin Evolution:

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The Mesozoic extensional rift basins of northern Egypt (Fig. 1) are considered to be part

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of the greater Tethyan passive margin (Keeley, 1994; Guiraud, 1998; Guiraud and

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Bosworth, 1999) that initiated in Permo-Triassic time during the onset of break-up of

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Gondwana and early opening of Neotethys Ocean (Şengör, 1979; Stampfli et al., 2001;

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Garfunkel, 2004) or during the Late Triassic–Liassic (Biju-Duval et al., 1979; Argyriadis

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et al., 1980). The breakup of Gondwana continued into the Triassic with the

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development of extensive systems of generally E-W trending rift basins located inboard

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from the continental margin (Bosworth et al., 2008). Onshore and offshore surface and

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subsurface data in northern Egypt and the eastern Mediterranean indicate an initial

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Jurassic basin-opening due to NW-SE extension evidenced by the existence of NE-SW

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oriented faults showing abrupt changes in Jurassic thickness across them (Moustafa,

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2008, Bevan and Moustafa, 2012, and Longacre et al., 2007). On the other hand, Keely

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and Wallis (1991) imply extension in the NE-SW direction concluded from their

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proposed westward propagating WNW-ESE oriented Jurassic rift in northern Egypt.

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During subsequent Early Cretaceous the extension direction rotated clockwise toward

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the NE-SW direction, resulting in the formation of new basins of NW-SE trend (Beni

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Suef, Asyut, and Khomombo, Fig. 1) and driving continued extension and subsidence in

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the E-W (e.g. Abu Gharadig) and NE-SW (e.g. Gindi) oriented rift basins (Bosworth et

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al., 2008). More isolated Mesozoic age rift-basins also occur further south, near the

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border with Sudan, such as Misaha (Fig. 1) and Upper Egypt Basins (Nagati, 1988;

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Taha, 1992; Dolson et al., 2001).

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The apparent change in extension direction during the Early Cretaceous was perhaps

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related to a change in the direction of plate movements. The motion of Africa relative to

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Eurasia in the Jurassic was eastward and this changed to northeastward during Early

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Cretaceous to Santonian time (Guiraud and Bosworth, 1997). These two different

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directions of plate movement are consistent with the two different basin orientations

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namely NE-SW and WNW-ESE to NW-SE (Moustafa, 2008). The extension events

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described above are also equivalent to the major Mesozoic extensional and subsidence

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episodes described previously by Guiraud, 1998 and Guiraud et al., 2005 in northern

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Africa.

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The Mesozoic extensional phases were followed by subsequent compressional pulses

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during Late Cretaceous to Early Tertiary time (Shukri, 1954; Moustafa, 1988; Guiraud

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and Bosworth, 1997; Guiraud, 1998; Guiraud et al., 2005; Bevan and Moustafa, 2012).

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These compressional pulses occurred as a result of major plate reorganization and

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convergence between Africa and Eurasia and were transmitted across the entire African

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plate resulting in the structural inversion of appropriately oriented basins (Bosworth et

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al., 2008). These inverted structures extended as a fold-belt (Fig. 1) across the whole

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North African and Arabian margins. The pronounced inversion of the Jurassic and Early

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Cretaceous Cyrenaica Basin resulted in formation of the Jabal al Akhdar anticlinorium

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(uplift) in northeast Libya (Röhlich, 1978; Papanikolaou et al., 2004; El-Hawat and

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Abdulsamad, 2004). Smaller inverted structures and folds developed from north-central

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Egypt (Sehim, 1993), across Sinai (Bartov et al., 1980; Jenkins, 1990; Moustafa and

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Khalil, 1990), and northward along the Levant-Lebanon (Walley, 2001) margin to the

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Palmyrides of Syria (Lovelock, 1984; McBride et al., 1990; Best et al., 1993). Krenkel

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(1925) joined these folds with those in Syria and coined the name “Syrian Arc System”

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to this large S-shaped fold belt. In Egypt; the exposed folds in northern Egypt were

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mapped in Gebel Maghara (Moustafa and Khalil, 1989 and Moustafa, 2014), Gebel.

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Halal (Abd-Allah et al., 2004), Gebel. Yelleg (Moustafa and Fouda, 2014), Abu Roash

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(Moustafa, 1988), and Wadi Araba (Moustafa and Khalil 1995).

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In the northern Western Desert, subsurface data indicate that Late Cretaceous

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sedimentation within the Mesozoic rift basins was interrupted by the development of the

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inversion folds (Moustafa, 2008; Bevan and Moustafa, 2012). Although basin

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subsidence continued during the Late Cretaceous and Early Tertiary times in the AG

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Basin, a prominent folding phase started during Santonian time forming several NE-SW

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oriented inversion anticlines. These anticlines are compartmentalized by NW-SE

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oriented normal faults. These anticlines along with tilted fault blocks bounded by early

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rift WNW-ESE and E-W oriented faults form the main hydrocarbon traps in the AG

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basin.

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Subsurface studies in the northern Western Desert have also shown the presence of

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other inverted basins forming important hydrocarbon-producing structures (e.g.

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Moustafa et al., 1998). Seismic and well data have shown that the inversion continued

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through to the Oligo-Miocene Epoch (Bevan and Moustafa, 2012). Late Cretaceous to

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Eocene carbonate sequences in particular show considerable thickness variations in the

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vicinity of the inverted structures, due to the development of the folds as growth-

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anticlines during relative high sea level conditions at these times (Bevan and Moustafa,

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2012).

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3. Stratigraphy:

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The stratigraphy of the northern Western Desert is characterized transgressive and

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regressive cycles of mixed carbonate and clastic sedimentation throughout the

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Mesozoic and Cenozoic sections. Fig. 2 summarizes the stratigraphic column of the

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study area. The deepest penetration in the available well data of the AG field is the

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Bahariya Formation (Cenomanian age). Older stratigraphic sequences are mainly

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described from general Western Desert stratigraphic columns and nearby fields.

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3.1. Jurassic–Coniacian (Syn-Rift) Mega-sequence: The Jurassic–

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Coniacian mega-sequence represents the main syn-rift assemblage of the

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AG Basin during the opening of the Tethys and the development of rift

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basins on the greater Tethyan passive margin. Within this syn-rift tectono9

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sequence there are three distinct cycles of transgressive sedimentation

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marking three sub-sequences, in the Jurassic, Early Cretaceous and Late

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Cretaceous (Bevan and Moustafa, 2012).

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The Jurassic sub-sequence comprises

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represented at the base by the Ras Qattara Formation, also known as

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Bahrein Formation, which represents non-marine clastics of Early Jurassic

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age (Hantar, 1990). This formation is made up of fine to coarse-grained

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red sandstones with thin pebble interbeds, siltstone and shale,

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occasionally carbonaceous or pyritic in nature (Hantar, 1990). The

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formation unconformably overlies different units of the Paleozoic or even

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basement and is overlain by the shallow-marine mixed clastics and

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carbonates of the Middle Jurassic Khatatba Formation (Keeley et al.,

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1990; Norton, 1967). The clastic section of the Khatatba Formation shows

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a few limestone interbeds and is made up of sandstone and shale with

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thin coal seams present at different levels of the section (Hantar, 1990).

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These coals are equivalent to those exposed and mined in the Gebel

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Maghara area of northern Sinai which were described by Al-Far, 1966 in

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the Safa Formation. The Middle Jurassic Khatatba Formation was

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a transgressive sequence

deposited in broad fluvial flood plains, in which there are overbank swamps and small lakes transitional to estuarine or lagoonal settings (Bosworth et al., 2015). The Khatatba Formation is a major reservoir and

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source rock in the Northern Egypt Basins (El Sisi et al., 2002; Dolson et

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al., 2001; Bakr, 2009) and is believed to include the main gas- and oil-

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prone shale and coal source-rocks in the northern Western Desert

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hydrocarbon province. This was confirmed by source rock studies and

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geochemical comparison of the molecular characteristics for the wellhead

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oils to solvent extract GC (Gas Chromatography) and GCMS (Gas

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chromatography Mass spectrometry) data from organic-rich Jurassic core

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material (Abrams et al., 2012). Similar conclusions have been made in

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published oil and source rock petroleum geochemistry studies (Abdine et

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al., 1993; Alsharhan, et al., 2008; Ghanem, et al., 1999; Maky and

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Ramadan, 2008; Bakr, 2009; Sharaf and El Nady, 2003; Shalaby et al.,

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2011; Shalaby et al., 2012, El Diasty and Moldowan, 2012 and 2013). The

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Khatatba Formation is conformably overlain by the massive shallow-

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marine carbonates of the Upper Jurassic Masajid Formation (Al-Far,

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1966). The latter represents the acme of the Jurassic marine

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transgression (Moustafa, 2008).

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The base of the Early Cretaceous sub-sequence is marked by a major

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regression. This tectono-sequence begins with non-marine to shallow-

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marine clastics of the Alam El Bueib Formation (Fig. 2). There is a major

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unconformity between the Masajid and Alam El Bueib Formations (Sultan and Halim, 1988), and the clastics of the Alam El Bueib Formation generally show dramatic thickening into basin-bounding faults than the

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underlying Jurassic, indicating that growth and extension on the faults was

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greater in the Early Cretaceous than during the Jurassic. Early Cretaceous

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continental rifting was active in N. Africa and Arabia and produced

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predominantly E-W oriented rift-basins (Guiraud and Bosworth, 1999).

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Subsequent marine transgression reached its maximum with the

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deposition of the Aptian-age Alamein Dolomite and Dahab Shale

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(Moustafa, 2008). Both the Jurassic and Early Cretaceous sediments are

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commonly deposited in half-grabens as syn-rift wedges across the

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northern Western Desert. The Alamein Dolomite provides a reliable

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seismic marker defining the top of the Early Cretaceous syn-rift tectono-

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sequence. The Alamein Dolomite and Dahab Formations were deposited

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across the region and are only absent due to non-deposition or erosion

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over the Sharib-Sheiba High.

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The overlying Albian to Coniacian tectono-sequence is marked at the base

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by the fluvial to shallow-marine Kharita Formation (Fig. 2), followed by the

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shallow-marine clastics of the Bahariya Formation (Fig. 2), which are Late

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Albian–Early Cenomanian in age. Transgression continued into the Late

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Cenomanian when carbonates of the Abu Roash Formation (Fig. 2) were

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deposited (the Abu Roash “G” Member). This was the first in the series of

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carbonate, shale and sandstone cycles of the Abu Roash Formation. The Abu Roash Formation is divided into 7 Members termed Abu Roash A to G Members (Norton, 1967; Aadland and Hassan, 1972) from top to

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bottom. According to Hataba and Ammar (1990) Members A to E are

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probably the lateral equivalents of the exposed units in the Abu Roash

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area described under the names Plicatula, Flint, Acteonella, Limestone + 12

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Rudistae, and Sandstone series respectively (Beadnell, 1902 and Said,

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1962). In the subsurface the members are well defined and are easily

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traced. Members B, D and F are relatively clean carbonates whereas

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Members A, C, E and G are largely fine-grained clastics (Hantar, 1990).

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The Abu Roash “F” Member is an oil-prone carbonate source-rock

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(Barakat et al., 1987; Hegazy and Tammam, 1990; Maky and Sayed,

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2007; Bakr, 2009) which is 100 to 130 ft thick (Bayoumi, 1996) and is

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composed of massive bedded, dolomitic, shelly wackestone, with

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laminated bituminous wackestone and microbioclastic mudstone. It was

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deposited in a restricted neritic to deep marine environment (Helba and

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Bakry, 1996). The Abu Roash “F” Member has a high organic content

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(regionally 1 to 5 weight percent) and is one of the main source rocks for

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the oil in the AG Basin (Abdine et al., 1993; Aly et al., 2003; Alsharhan, et

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al., 2008; Maky and Saad, 2009).

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In many areas of the northern Western Desert, the Albian to Coniacian

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tectono-sequence is considered to be a post-rift sequence to the earlier

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Jurassic and Early Cretaceous syn-rift sequences as it typically has

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uniform isopach, thickening generally to the north towards the open marine Tethys basin (Bevan and Moustafa, 2012). However, on some major extension faults such as the Mubarak and Abu Gharadig Faults

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(NE–SW trend) this tectono-sequence continues to expand into the

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immediate hanging-walls. Hence, localized extension and fault-related

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subsidence therefore remained active even during the Late Cretaceous

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(Guiraud and Bosworth, 1997 and 1999).

3.2. Santonian–Middle Eocene (Syn-Inversion) Sequence: The

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base of the Santonian Middle Eocene tectono-sequence is marked by a

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regressive phase of deposition in the Santonian. The Khoman B Member

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shales (Fig. 2) were deposited in relatively restricted basins that were

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previously tectonically active (Said, 1990 and Hantar, 1990). The major

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phase of transgression started in the Campanian and led to increased sea

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levels in the latest Cretaceous to Eocene, and to the deposition of the

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snow white chalks and limestones of the Khoman A and Apollonia

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Formations (Norton, 1976; Fig. 2) in open marine outer shelf environment

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(Hantar, 1990). The Khoman chalk also has abundant chert bands

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(Hantar, 1990). Following the deposition of the Khoman B Member, the

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first Syrian-Arc compressional event occurred which resulted in folding,

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faulting, uplift and subsequent erosion of the Santonian and older

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formations in the cores of some of the inversion anticlines such as at Wadi

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Araba Anticline on the western side of the Gulf of Suez (Moustafa and

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Khalil, 1995) and the Kattaniya inversion anticline in the northern Western Desert (Nemec, 1996; Abdel Aziz et al., 1998), as well as the inversion folds of northern Sinai (Moustafa, 2010). Generally there is a major

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angular unconformity between the Khoman B and Khoman A Members

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and the chalks and limestones of this tectono-sequence show dramatic

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thickness variations above the inverted structures indicating that structural 14

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growth occurred throughout this period of continuing deposition. The

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Apollonia isopach map of the Kattaniya inverted basin (Abd El Aziz et al.,

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1998) indicates non-deposition of the Apollonia on top of the Kattaniya

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inverted basin. Several inversion anticlines also stood high above the level

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of the Eocene sea in the northern Western Desert (Salem, 1976) and

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other areas in northern Egypt (Shukri, 1954).

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3.3. Late Eocene–Miocene (Late Syn-inversion) Sequence: The

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Late Eocene–Oligocene marine shale of the Dabaa Formation (Norton,

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1967 and Hantar, 1990) and the marine clastics of the Miocene Moghra

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Formation (Said, 1962) unconformably overlie the Apollonia Formation

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(Fig. 2). In the study area very gentle folding of the Dabaa and Moghra by

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the AG Anticline indicates last phase of the inversion during the Late

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Eocene-Early Miocene time. Similarly, the growth geometry of the

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inversion structures and stratal growth of the Oligocene and Early

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Miocene is also recorded in some areas such as the Mubarak anticline

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(Bevan and Moustafa, 2012) as evidenced by the relief on the structure in

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the Dabaa and Moghra Formations.

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Contemporaneous with the continued growth of the anticlines, OligoMiocene intrusive and extrusive basalts are found in several locations, some of which appear near to major inversion structures. These basalts

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were synchronous with the initiation of the Gulf of Suez rift with regional

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NE-SW extension as the Arabian Plate began to separate from the African

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Plate and are related to a small-plume (Cairo mini-plume of Bosworth et 15

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al., 2015). These basalts have been recorded in northern Fayoum and the

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southern edge of the Kattaniya inversion (Gebel Qatrani), and 200 km to

332

the southwest of Cairo in the Bahariya Depression (Meneisy, 1990).

333

4. Structural setting of the AG Fold:

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Seismic structure mapping of the AG field area, from the deepest mappable horizon

335

(estimated Top Jurassic) to the shallowest horizon (Top Apollonia) has been completed

336

using 3D seismic data tied with the well data. Mapping of the structure below the Top

337

Jurassic was not possible due to the inadequate resolution of seismic data and it was

338

also limited by seismic record length beneath that level. In addition, the deepest

339

penetration of the available well data in the area only reaches the Bahariya Formation,

340

so even the Top Jurassic surface is estimated based on a regional understanding.

341

Seismic to well ties were developed and utilized to map the top Bahariya and shallower

342

horizons. In contrast, the Top Alamein Dolomite and Top Jurassic (Top Masajid) were

343

mapped based only on the seismic character of these two carbonate horizons, making

344

use of their well-recognized seismic character over many parts of the northern Western

345

Desert, and without actual well penetrations in the studied wells to validate them.

346

Seismic mapping shows that the AG fold is about 12.5 km long asymmetric, doubly

347

plunging, fault-bounded anticline with about 400 meters of structural relief. It is oriented

348

NE-SW and the northwestern flank has an average dip of 6° and the southeastern flank

349

has an average dip of 12° at the level of the Abu R oash Formation. The fold is bounded

350

by two NE-SW oriented reverse faults (R1 and R2) at the Alamein Dolomite level. R1

351

Fault is bounding a portion of the northeastern limb of the anticline whereas the R2 fault

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is bounding a portion of the southwestern limb (Figs. 3 – 7). The R1 fault extends for

353

about 8 km at the northeastern part of the anticline and dips toward the SE at an angle

354

of 45˚. The R2 fault extends for about 8 km at the southwestern part of the anticline and

355

dips toward the NW at an angle of 55°. The flanks o f the AG Anticline are steep only at

356

the two reverse faults (Fig. 7). The crest of the anticline is dissected by number of

357

normal faults oriented NW-SE perpendicular to the fold axis (Figs. 3, 4 and 7). Both of

358

the R1 and R2 faults have thicker Early Cretaceous sediments in their hanging walls

359

compared to their footwalls indicating normal slip on these faults during Early

360

Cretaceous time. The Late Cretaceous – Eocene section is relatively thinner at the

361

crestal area of the AG Anticline compared to the flanks (Figs. 5 and 6). At the Top

362

Jurassic surface, normal fault offset is obvious at the R1 and R2 faults, in contrast to the

363

Top Alamein Dolomite surface where the two faults show reverse offset (Figs. 5 and 6).

364

The Bahariya - Apollonia section is folded above the tips of both faults by fault-

365

propagation-folding representing inversion of the two NE-SW oriented R1 and R2 faults.

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5. Timing of structural deformation:

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Seismic-derived isochron (time thickness) maps (Figs. 8 and 9) and several flattened

368

seismic sections were constructed to analyze the structural evolution of the AG field

369

area. The Jurassic and older history is mainly derived from published literature as no

370

subsurface data were available in the study area.

371

The isochron map of the Early Cretaceous section between the Top Jurassic to the Top

372

Alamein Dolomite (Aptian) shows thickening wedges in the hanging walls of R1 and R2

373

faults (Fig. 8). Flattened Alamein Dolomite in seismic sections across the two faults

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clearly shows these thickened wedges (Figs. 10-A and 11-A). This indicates that the R1

375

and R2 faults were initiated as normal faults before and continued through the

376

deposition of the Early Cretaceous section with growth sedimentary packages in their

377

hanging walls.

378

The flattened seismic sections across the R1 and R2 faults at Top AR/A (Figs. 10-B and

379

11-B) show a relatively uniform thickness of the Late Cretaceous section between the

380

Top Alamein Dolomite and Top AR/A in the vicinity of both faults, indicating a passive fill

381

in a post extension phase at this time interval. However, continuous subsidence is

382

known on the fault bounding the Qattara Ridge/Sharib-Sheiba High during the same

383

time, indicating that some local extensional movements existed during the Late

384

Cretaceous time. This matches observations made by Guiraud and Bosworth (1997 and

385

1999) and Moustafa (2008).

386

The isochron map of the Santonian – Maastrichtian section between the Top AR/A and

387

Top Khoman/A shows significant thinning above the crest of the AG Anticline (Fig. 9).

388

The flattened seismic sections also show these thickness variations. Seismic data show

389

an angular unconformity within the Khoman Formation at the Top of the Khoman/B

390

Member (Santonian section), Figs. 10-C and 11-C and provide evidence for the start of

391

inversion in Santonian time. Moustafa (2008) also reported the same intra-Khoman

392

unconformity on seismic data in another portion of the AG Basin. These observations

393

match well with Guiraud and Bosworth (1997) dating of the structural inversion in North

394

Africa. Seismic sections show a broader thinning of the Paleocene – Eocene section

395

(Apollonia Formation) above the AG Anticline (Figs. 10 and 11). This indicates

396

continuous growth of the AG Anticline until Middle Eocene time. Similarly, very gentle

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folding of the Dabaa and Moghra by the AG Anticline (Figs. 5 and 6) indicates last

398

phase of the inversion during the Late Eocene-Early Miocene time.

6. Discussion:

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Based on the previous observations, the R1 and R2 faults bound the steeper parts of

401

the AG inversion fold limbs. These faults experienced normal slip during the Early

402

Cretaceous extensional phase leading to deposition of thick Early Cretaceous wedges

403

on their hanging walls. The Jurassic rock units fall below the seismic record length and

404

no data was available to constrain the Jurassic and older history of the study area.

405

However, when compared to other NE-SW inverted faults in the northern Western

406

Desert; e.g. Kattaniya Basin (Abd El-Aziz et al., 1998), Alamein-Razzak Basin (Bakr,

407

2009), and Mubarak inverted Basin (Bevan and Moustafa (2012); it is possible that the

408

R1 and R2 faults could also have been active as normal faults during the Jurassic time.

409

The R1 and R2 faults are oriented NE-SW indicating a NW-SE extensional direction.

410

This fault orientation is also aligned with the known Jurassic extensional faults

411

extending across northern Egypt, which may indicate that these faults were initiated

412

earlier during the Jurassic time and continued subsidence during the Early Cretaceous.

413

In the AG field area, the two NE-SW oriented faults have left-stepping en echelon

414

arrangement. This apparent en echelon arrangement of the faults during the Jurassic-

415

Early Cretaceous extension phase has not been noticed in other inverted basins in the

416

northern Western Desert. The stratigraphic section between the Bahariya and AR/A

417

(Cenomanian–Turonian age) shows continuous subsidence with extensional growth

418

toward the north along the Qattara Ridge/Sharib-Sheiba bounding faults (Fig. 1). Away

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from these two main faults the Cenomanian–Turonian section is essentially isopachous

420

everywhere across the study area.

421

Cretaceous subsidence was interrupted by a compressive phase during the Late

422

Cretaceous time which continued into the Early Tertiary (Paleocene–Eocene). This

423

compressional pulse reactivated the pre-existing NE-SW oriented extensional faults

424

leading to positive structural inversion and development of the AG inversion anticline in

425

the hanging walls of the R1 and R2 faults. The pre-existence of an asymmetric graben

426

between the R1 and R2 faults during the Early Cretaceous and probably earlier in the

427

Jurassic time, has closely controlled the asymmetric shape of the Fold formed during

428

inversion. This is demonstrated by the change in fold vergence and steep limb along the

429

Fold’s length. Isochron maps and flattened seismic sections show thickness variations

430

of the Khoman section at the crest of the inversion anticline during the Campanian–

431

Maastrichtian time. The maps show significant thinning of the Khoman section above

432

the crest of the inversion fold and thickening toward its limbs. Also seismic sections

433

show the existence of the Intra-Khoman unconformity between Khoman/B (Santonian)

434

and Khoman/A (Campanian–Maastrichtian) Members, indicating that the compression

435

started at Santonian time and continued through the Campanian–Maastrichtian. This is

436

aligned with Guiraud and Bosworth (1997) where they demonstrated that the inversion

437

started in the Santonian across all of North Africa, including Egypt.

438

(Paleocene–Middle Eocene) section also shows thickness variations in the vicinity of

439

the AG Anticline, and thinning above crest of the fold indicating that the compressional

440

phase continued through the Early Tertiary Paleocene–Middle Eocene time.

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The Apollonia

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A number of NW-SE oriented normal faults were also mapped across the crest of the

442

AG Anticline. These faults accompanied the AG folding and formed at a nearly

443

orthogonal direction to the AG Anticline axis as a result of local extension of the fold

444

crest. Moustafa (2013) compared similar NW-SE normal faults of several inverted

445

basins in northern Egypt and found out that such faults have consistent NW-SE

446

orientation in all inverted basins but are not exactly aligned transverse to the inversion

447

folds. He postulated that the inversion folds are forced to form parallel to the underlying

448

inverted faults whereas the NW-SE oriented normal faults were formed parallel to the

449

maximum compressive stress axis at the time of inversion.

450

Macgregor (1995) presented a brief analysis of hydrocarbon distribution in inverted rift

451

basins where he classified three types according to the intensity and lateral extent of

452

inversion-related uplift: a. simple rifts without significant inversion, b. locally-inverted rifts

453

which show restricted areas of uplift along faults, and c. regionally-inverted rifts where

454

the majority of the original basin area has undergone uplift accompanied by regional

455

scale erosion. Based on Macgregor (1995) the locally-inverted basins usually show high

456

degrees of petroleum retention and associated exploration success, with the key factor

457

often being the relationship between timing of trap formation and petroleum charge.

458

Regionally-inverted basins show lower exploration success rates, with failures often

459

being attributed to a redistribution of hydrocarbons during inversion with losses

460

occurring through biodegradation, surface erosion or seepage along surface-penetrating

461

faults. Macgregor (1995) concluded that inversion is a process which, dependent on its

462

severity and its relation to hydrocarbon generative phases, may have a positive or

463

negative effect on the hydrocarbon systems of a rift-basin.

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464

In the Abu Gharadig field area, we described a locally-inverted rift basin where

465

structural growth occurred synchronous with continuing regional subsidence and

466

sedimentation.

467

erosion is not observed. The geograhically-limited structural inversion in the AG basin

468

clearly occurred after deposition of the key reservoir and source rock intervals.

469

Continuing subsidence during and after inversion served to preserve laterally-extensive

470

reservoirs and their top seals. In other adjacent areas of such as the Kattaniya Basin

471

and Northern Sinai, the contemporaneous Late Cretaceous and early Tertiary inversion

472

structures, both reservoirs and top seals have been strongly breached by erosion

473

reaching down into Cretaceous and sometime even to Jurassic strata. Furthermore, the

474

AG fold forms a sub-regional culmination within the larger extensional basin setting, so

475

lateral migration would naturally be focused from adjacent basinal areas towards the

476

emergent structural high. Continued post-inversion sedimentation not only preserve

477

reservoirs and their top seals, but also deposited additional Tertiary strata that maintain

478

key source rocks at burial depths where they have been able to continue to generate

479

hydrocarbons.

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7. Summary and conclusions:

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There was no broader regional uplift and evidence for widespread

481

The AG inversion anticline and associated reverse faults R1 and R2 were mapped

482

across the AG field area. The R1 and R2 faults were formed as en echelon normal

483

faults during the Jurassic-Early Cretaceous time due to NW-SE extension. These faults

484

were subsequently inverted during Late Cretaceous–Middle Eocene time forming the

485

AG Anticline via positive structural inversion. The anticline crest is following the R1 and

486

R2 faults strike in a NE-SW orientation indicating NW-SE shortening during the Late 22

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Cretaceous time. The AG Anticline shows changing vergence along-strike with its steep

488

limb, controlled by the R1 reverse fault at its northeastern flank and the R2 reverse fault

489

at its southwestern flank. The fold crest is also dissected by a number of NW-SE

490

oriented normal faults formed parallel to the regional direction of compression causing

491

the inversion.

492

The limited inversion intensity in the AG Field area and the timing of inversion relative to

493

continued subsidence of the AG Basin and the timing petroleum generation and

494

migration have combined to make the AG Field one of the larger producing hydrocarbon

495

fields in the northern Western Desert. Most importantly, the preservation of multiple

496

reservoirs and seals within the main structural high followed by continued post-inversion

497

burial of key source rocks and lateral migration of hydrocarbons from adjacent basinal

498

areas into the main inverted trend have all served to allow the accumulation of a large

499

amount of oil and gas. (Fig. 12).

500

The conclusions reached in this study are of relevance to explorationists working both in

501

the Western Desert of Egypt and elsewhere. We believe there is remaining hydrocarbon

502

potential in the AG Field area, either vertically below or spatially-offset from the main

503

inverted structural trend and its existing production. Additional detailed structural

504

mapping of the type undertaken in this study will help to identifying additional

505

hydrocarbon traps resulting from Jurassic-Early Cretaceous rifting phases but largely

506

unaffected by the subsequent local inversion. Furthermore, the structural and tectonic

507

settings of the AG Basin may offer useful geological analogues for explorationists

508

working in other areas of the Western Desert of Egypt, in North Africa (e.g. eastern

509

Libya or offshore North Sinai), and in other petroleum provinces around the world where

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510

local structural inversion may have significantly influenced the distribution of

511

hydrocarbon fields.

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Acknowledgements: We would like to thank the Egyptian General Petroleum

514

Corporation, Apache Egypt Companies, and BP Egypt for releasing the subsurface data

515

for this study. We also thank the journal reviewers Dr. William Bosworth and Dr. Samir

516

Khalil for constructive comments that helped improve the manuscript.

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Figure Captions

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Table 1: AG Field well data summary.

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Figure1: Mesozoic and Cenozoic Basins in Egypt (modified after, Moustafa, 2008; Bevan and Moustafa, 2012; Dolson et al., 2001. Upper Egypt basins added after Bosworth et al., 2008).

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Figure 2: Generalized stratigraphic column of the AG field study area.

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Figure 3: Top Alamein Dolomite TWT structural map (contour interval = 50 msec).

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Figure 4: Top AR/A TWT structural map (contour interval = 50 msec).

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Figure 5: Seismic section across R1 Fault. See Figs. 3 and 4 for location.

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Figure 6: Seismic section across R2 Fault. See Figs. 3 and 4 for location.

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Figure 7: Seismic section along the AG Fold. See Figs. 3 and 4 for location.

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Figure 8: Top Jurassic - Top Alamein Dolomite isochron map (contour interval=100 msec).

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Figure 9: Top AR/A - Top Khoman/A isochron map (contour interval=100 msec).

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Figure 10: Flattening of seismic section of Fig. 5 at Top Alamein Dolomite (A), at Top AR/A (B), and at Top Khoman/A (C).

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Figure 11: Flattening of seismic section of Fig. 6 at Top Alamein Dolomite (A), at Top AR/A (B), and at Top Khoman/A (C).

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Figure 12: AG Field petroleum elements summary chart.

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37 Figure 1: TWT seismic section 1 across R1 Fault. The figure is showing uninterpreted and interpreted versions of the same line. See Fig. 4 for line location.

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Tectono-stratigraphic evolution of the AG field, northern Western Desert, with full description of AG Fold and bounding faults; including timing of structural inversion and the geometry of inversion-related structures. How does AG Fold structural evolution compared to other inversion folds across northern Egypt, specifically northern Sinai. Effect of inversion fold on petroleum system and hydrocarbon exploration not only in AG field but also in northern western desert as whole and other similar inverted rift basins.

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