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Seismic – Wireline logs sequence stratigraphic analyses and geologic evolution for the Upper Cretaceous succession of Abu Gharadig basin, Egypt
Accepted Manuscript Seismic – Wireline logs sequence stratigraphic analyses and geologic evolution for the Upper Cretaceous succession of Abu Gharadig basin, Egypt Mohammad Abdelfattah Sarhan PII:
S1464-343X(17)30048-1
DOI:
10.1016/j.jafrearsci.2017.02.004
Reference:
AES 2805
To appear in:
Journal of African Earth Sciences
Received Date: 13 January 2017 Revised Date:
31 January 2017
Accepted Date: 1 February 2017
Please cite this article as: Sarhan, M.A., Seismic – Wireline logs sequence stratigraphic analyses and geologic evolution for the Upper Cretaceous succession of Abu Gharadig basin, Egypt, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.02.004. 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.
ACCEPTED MANUSCRIPT
Seismic - Wireline logs Sequence Stratigraphic Analyses and
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Geologic Evolution for the Upper Cretaceous Succession
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of Abu Gharadig Basin, Egypt.
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BY
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Mohammad Abdelfattah Sarhan
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Geology Department, Faculty of Science, Damietta University, Damietta, Egypt.
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[email protected]
ABSTRACT
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The Upper Cretaceous megasequence in the northern part of the Egyptian Western Desert
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has been classified into four 2nd order depositional sequences. These sequences started with
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the Cenomanian SQ-I topped by the Turonian – Santonian SQ-II. However, both SQ-III and
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SQ-IV represent the Campanian- Maastrichtian time span. The interpreted 2nd order SQ-I and
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SQ-II have been further subdivided into six smaller 3rd order sequences (SQ-1 to SQ-6). The
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depositional history started during the Early Cenomanian times, characterized by wide marine
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invasion enabled the deposition of the shallow marine Bahariya Formation (SQ-1). The Upper
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Cenomanian times, witnessed a rapid subsidence, simultaneously with new marine
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transgressive phase. This is resulted in the deposition of SQ-2, consuming the entire
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sediments of the Abu Roash G Member. During the Turonian – Coniacian times the northern
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parts of Egypt showed successive oscillating transgressive – regressive marine cycles led to
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equivocal sedimentary bodies of the Turonian-Coniacian Abu Roash Formation (SQ-3, SQ-4,
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and SQ-5). During the Santonian age, the northern parts of Egypt were subjected to tectonic
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crustal shortening, producing large scale folds. As a result, a new tectonically-overprinted
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marine depositional cycle started and marked by rapid phase of basin subsidence. This was
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ACCEPTED MANUSCRIPT accompanied by a deep marine invasion covered most of the northern parts of Egyptian lands,
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depositing the lower parts of the Khoman- B (SQ-6) under transgressive depositional
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conditions. By the end of Santonian cycle, the upper parts of the Khoman Formation B
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Member was deposited during a gradual, and slow relative sea level drop ending the
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deposition of SQ-6. At the beginning of the Campanian – Maastrichtian times, a new
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widespread sea-level rise associated with basin subsidence. Accordingly, two successive
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depositional cycles were thus initiated, forming SQ-III and SQ-VI sequences separated by
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unconformity type-2 boundary (SB-8).
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Key Words:
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Seismic - Wireline logs - Sequence Stratigraphy - Geologic Evolution- Upper Cretaceous
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- Abu Gharadig Basin - Egypt.
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1. INTRODUCTION
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Abu Gharadig basin is the largest basin in the northern part of the Egyptian Western
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Desert. It is considered to be one of the most important sedimentary basins in terms of
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petroleum potentials (Bayoumi and Lotfy, 1989), covering an area of 17500 km2. This basin
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is tectonically representing an E-W trending rift formed in the Late Jurassic - Early
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Cretaceous times, followed by an inversion tectonic event during the Late Cretaceous age
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(Bayoumi and Lotfy, 1989; Guiraud, 1998; Bosworth; 1994; Moustafa, 2008 and Wescott et
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al., 2011).
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Regarding to the economic importance of Abu Gharadig Basin, it represents one of the
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most hydrocarbon productive basins in the Northern Western Desert. It has more than 20
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fields currently produce About 10000 BOPD oil from the Upper Cretaceous Abu Roash and 2
ACCEPTED MANUSCRIPT Bahariya Formations (Ahmed, 2008 and El Ayouty 1990). However, most of the
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hydrocarbons discovered were drilled as a structural prospect and a few traps are stratigraphic
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in nature (Sultan and Abd El Halim 1988).
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Since the Upper Cretaceous sediments of the Egyptian Western Desert including Abu
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Gharadig Basin acquire the required conditions for hydrocarbon occurrences (source rocks,
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reservoirs, and both the structural and stratigraphic traps in addition to the cap rocks). It
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encouraged many workers to pay much attention and intensive studies in different approaches.
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For example, Boukhary et al (2014) have classified the Upper Cretaceous–Tertiary succession
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in the eastern Abu Gharadig basin into five major depositional sequences controlled by global
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eustasy and local tectonics.
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According to the available composite logs, the Upper Cretaceous rock units of Abu
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Gharadig basin are summarized in Fig. (1). Generally, the Upper Cretaceous lithostratigraphy
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of Abu Gharadig basin starts at the base with the Cenomanian Bahariya Formation which
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consists mainly of fine to very fine sandstone with some shale and limestone intercalations.
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This Formation is overlain by Abu Roash Formation which composed essentially of
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alternations of carbonates and clastics. It has been further subdivided into seven members
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named from top to base; (A to G). B, D and F Members are relatively clean carbonates
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however the other units contain variable contents of clastics with the limestones. The Khoman
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Formation (Maastrichtian) overlies Abu Roash Formation which subdivided into two
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members; the lower named Khoman B Member which is composed of shale and argillaceous
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limestone however the upper Khoman A member composed mainly of chalky limestone.
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The Bahariya Formation was deposited in shallow marine environment and also the
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overlain Abu Roash Formation. It was deposited under shallow marine shelf conditions within
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positive accommodation space less than 200 m (Catuneanu et al. 2006).
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However, the Maastrichtian Khoman Formation deposited under open marine to outer shelf
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condition (EGPC, 1992). Accordingly, the Upper Cretaceous megasequence in the northern
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part of the Egyptian Western Desert including the area of study within Abu Gharadig basin
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represent a major marine transgressive depositional cycle. The present work aims to use the seismic and well logging data of the examined
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sedimentary basin to analyse the subsurface Upper Cretaceous sedimentary succession in a
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sequence stratigraphic framework. Based on the foregoing sequence stratigraphic analysis, the
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depositional evolution along the Northern part of the Egyptian Western Desert through the
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Upper Cretaceous time will be deduced.
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2. STUDY AREA, AVAILABLE DATA AND TECHNIQUE The study area is located at the central part of Abu Gharadig Basin in the northern
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Western Desert and lies between latitude 29.5o and 30o 00 N and between longitudes 28.3o
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and 28.8o E (Fig.2).
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Thirty two 2D seismic reflection profiles in SEGY format with 2 km interval space
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extracted from 3D seismic cube were kindly provided from Khalda Petroleum Company after
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the permission of the Egyptian General Petroleum Corporation (EGPC). These seismic
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profiles can be classified into three groups; nine in-lines extend in ENE-WSW trend
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(Depositional strike trend) and each one extends for 55.5 km in length and nineteen cross-
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lines extend in NNW-SSE trend (Depositional dip trend) of 25.5 km in length in addition to
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four arbitrary seismic lines connecting the available wells covering the study area as shown in
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Fig. (3). Moreover, composite logs are available for five wells (AG-2, AG-5, AG-6, AG-15
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and SAWAG-1) with different logging data in the form of; Gamma ray, Sonic and Density for
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AG-2, AG-5, AG-6, AG-15 wells, in addition to the Check-shot data for only SWAG-1 well.
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To achieve the previous aim of the work, the following procedures are followed:
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(1) Picking and tying of the clearest sequence boundaries from seismic and well data to
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classify the Upper Cretaceous succession into 2nd (3 – 50 my) and 3rd order depositional
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sequences (0.5 – 3 my) according to Emery and Myers (1996).
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(2) The interpreted seismic sequence boundaries have been mapped over the grid of 2D seismic lines to construct time depth maps.
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(3) Preparing isochron maps of each interpreted seismic sequence.
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(4) The interpreted 3rd order depositional cycles then subdivided into their included systems
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tracts using the available well logging data.
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(5) Construct the relative sea level curve of the Upper Cretaceous time in the study area
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according to the foregoing sequence stratigraphic analysis. This curve is expected to
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reflect the successive changes in the accommodation space during the deposition of the
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Upper Cretaceous sedimentary succession. This in turn reflects the depositional history
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along the northern part of the Egyptian Western Desert during the Upper Cretaceous.
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3. SEQUENCE STRATIGRAPHIC ANALYSIS
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The Transgressive-Regressive Model (T-R Model) of Embry (1993) and Embry &
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Johannessen (1993) has been applied in the present work to perform the sequential
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subdivision for the Upper Cretaceous sedimentary succession. According to this model, the
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sequence boundaries represent the maximum regressive surfaces (MRS), however the entire
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sequence is subdivided into lower regressive systems tracts (RST), topped by transgressive
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systems tracts (TST) and both of them are separated by the maximum flooding surface
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(MFS).
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ACCEPTED MANUSCRIPT Based on the Above concept, the investigated Upper Cretaceous megasequence in the study
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area has been classified into four 2nd order depositional sequences named, from base to top, as
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follow: Sequence-I (SQ-I), Sequence-II (SQ-II), Sequence-III (SQ-III) and Sequence-IV (SQ-
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IV). Moreover, the interpreted SQ-I and SQ-II have been subdivided into smaller 3rd order
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depositional sequences, however the lack of adequate well logging data made it hard to
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subdivide both SQ-III and SQ-IV into smaller subdivisions.
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Each interpreted 3rd order sequence has been distinguished into lower regressive systems
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tracts (RST) topped by transgressive systems tracts (TST) and both systems tracts separated
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by the maximum flooding surfaces (MFS).
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It is worth mentioning that the clear offlap break which appears in the cross- seismic line
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(Depositional Dip Line) (Fig. 4a) is considered herein as apparent offlap break (not actual)
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because of the seismic reflectors are curved upward again further to the NW direction. This
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apparent offlap break may be attributed to a later tectonic activity that led to an uplift of an
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igneous intrusion beneath the pre-deposited Upper Cretaceous sedimentary package leading to
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the existence of the anticlinal shape for the upper sedimentary succession as shown in Fig
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(4.a). On the other hand, the Above mentioned upward curvature in the seismic reflectors may
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also be caused by normal tilting of the Upper Cretaceous strata against a major fault which
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unfortunately doesn’t appear here, and may exists further to the NW direction (i.e.: to the left
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side of the present seismic profile).
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The following section represents the discussion of each interpreted depositional sequence:
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3.1. Depositional Sequence-I (SQ-I):
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The depositional sequence-I (SQ-I) is a 2nd order depositional sequence. It is the basal
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interpreted depositional sequence in the examined Upper Cretaceous sedimentary succession.
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ACCEPTED MANUSCRIPT It represents the Cenomanian depositional phase (˃ 6.0 my) in the study area. SQ-I is
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composed of the entire sediments of the Bahariya Formation in addition to the G Member of
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the overlying Abu Roash Formation. The lower sequence boundary has not been interpreted
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from the present seismic data because of the low resolution of the seismic profiles at this great
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depth, and also because of the inadequate well data. The upper sequence boundary, on the
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other hand, is the top surface of the Abu Roash G Member as defined from the available
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wireline logs. This sequence can be subdivided into two 3rd order depositional sequences;
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from bottom to top (SQ-1) and (SQ-2):-
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153 3.1.1. Depositional sequence-1(SQ-1):
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Age and Lithology:
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The 3rd order depositional sequence (SQ-1) represents the lower part of the major depositional
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sequence-I (SQ-I), encompassing the entire sediments of the Cenomanian Bahariya
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Formation. The composite logs of the Bahariya Formation indicate that it is composed mainly
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of sandstones with rarely shale intercalations.
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Sequence boundaries:
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The lower sequence boundary of the present sequence (SB-1) coincides with that of the
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depositional sequence SQ-I which has not been interpreted herein (Fig. 4a &b). However, the
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upper sequence boundary (SB-2) with overlying (SQ-2) represents the maximum regressive
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surface (MRS-1) of the SQ-1. It
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where along the seismic profile, SB-2 displays high amplitude reflector due to the contrast in
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acoustic impedance between the sandy facies of Bahariya Formation and the upper shale
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facies of Abu Roash G Member.
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onlapping terminations of the overlying seismic reflectors, and by the toplapping terminations
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of the underlying reflectors (Fig. 4a &b). However, this seismic surface is sometimes
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has been easily detected from both seismic and log data
SB-2 has been easily defined seismically due to the
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ACCEPTED MANUSCRIPT represented by significantly discontinuous reflector due to the many normal faulting
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processes, reflecting the tectonic disturbances affected the lower part of the Upper Cretaceous
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megasequence in the study area (Fig. 4b). On the other hand, the SB-1 (MRS-1) also has been
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easily proved from wireline logs by the abrupt increase in gamma ray values, and the ∆T
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values (sonic logs) and the abrupt decrease in density log values characterizing the
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transformation from the sandstone of the Bahariya Formation to the shale of the overlying
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Abu Roash G Member (Fig.5).
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As for the 3rd order depositional sequence (SQ-1), it is represented by well-defined two
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systems tracts; a transgressive systems tract (TST-1), and a regressive systems tract (RST-1).
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Both tracts are separated by the well-defined maximum flooding surface (MFS-1) that
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separates the lower transgressive systems tracts (TST-1) displaying high gamma ray values
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due to its relative richness in shale content, unlike the clean sandstone unit of the upper
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regressive systems tracts (RST-1) displaying low gamma ray values (Fig. 5a, b, c & d).
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3. 1.2 Depositiona sequence-2 (SQ-2):
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Age and Lithology:
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The 3rd order depositional sequence-2 (SQ-2) overlies the 3rd order depositional sequence-1
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(SQ-1). It encompasses the entire sediments of the Abu Roash G Member belonging to the
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uppermost Cenomanian age (Fig. 6). It is essentially composed of shale, siltstone and
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sandstones with limestone interbeds. This mixed lithology reflects a relatively lower energy
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depositional regime than those prevailed during the deposition of the previous (SQ-1),
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including the sandstones of the Bahariya Formation. The presence of the limestone beds
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reflects more effective shelf marine deposition.
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SQ-2 rests on SB-2 and topped by SB-3 representing the maximum regressive surface (MRS-
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2) of the examined sequence and the upper surface of Abu Roash G Member (Fig. 6). SB-3
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has been interpreted only on the basis of the well logging data. It is delineated when an abrupt
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reduction in gamma ray values and ∆T values, coupled with an abrupt increase in density log
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values, proving the passage from the mixed clastics/carbonates succession of the Abu Roash
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Formation G Member to the almost pure limestone succession of the overlying Abu Roash
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Formation F Member (Fig. 5).
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Systems Tracts:
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The lower transgressive systems tracts (TST-2) of the investigated SQ-2 and the upper
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regressive systems tracts (RST-2) of the present sequence have been easily separated by a
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clear maximum flooding surface (MFS-2). This surface lies approximately at the middle of
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the SQ-2 and displays the maximum increase in gamma ray values attributed to the maximum
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increase in shale facies (Figs. 5 &6).
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3.2. Depositional Sequence-II (SQ-II):
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The 2nd order depositional sequence SQ-II overlies the 2nd order depositional sequence (SQ-I).
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The upper sequence boundary of SQ-II is the upper surface of the Khoman B Member (SB-7).
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SQ-II, as 2nd order depositional sequence, encompasses the entire time span extending from
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the Turonian, Coniacian and Santonian age in the area of study (~10 my). It includes the
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entire sediments of the F, E, D, C, B and A members of Abu Roash Formation, in addition to
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the lower member of Khoman Formation (Khoman B Member). The present 2nd order SQ-III
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is herein subdivided into four 3rd order depositional sequences; namely from the base: SQ-3,
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SQ-4, SQ-5 and SQ-6 (Fig. 6).
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Age and Lithology:
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The 3rd order depositional sequence-3 (SQ-3) encompasses the sedimentary successions of the
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F Member and E Member of Abu Roash Formation consuming the Early Turonian age.
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Limestone is the only rock constituent of the Abu Roash F Member with rarely shale
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interbeds, whereas the overlying Abu Roash E Member consists of shale, sandstones and
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siltstone with minor limestone interbeds form (Figs. 1 & 6). This variation in lithology
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between the two members indicates a clear shift from relatively deeper marine depositional
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environments, with wider accommodation space prevailed below and produced the limestones
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of F Member, to the overlying shallow marine environments produced the clastics of E
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Member deposited in markedly reduced accommodation space due to the continuous clastics
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supply.
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Sequence boundaries:
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SQ-3 is bounded by SB-3 discussed before and by SB-4 along the uppermost surface of E
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Member of Abu Roash Formation. SB-4 boundary has been interpreted on both seismic and
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well logging data. On seismic profiles, (SB-4) exhibits high amplitude reflector due to the
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acoustic impedance contrast between the lower clastic unit of E Member, Abu Roash
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Formation, and the upper carbonate unit of D Member, Abu Roash Formation. (SB-4) is
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emphasized on seismic sections on the basis of the onlapping termination patterns of the
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overlying reflectors and by the toplapping terminations of the underlying reflectors (Fig. 4).
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Like SB-2, SB-4 sometimes represented by discontinuous reflectors due to the extensional
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normal faults affected SB-2 below. On well-logs, on the other hand, SB-4 has been interpreted
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on the basis of the abrupt decrease in gamma ray values and sonic values, coupled with abrupt
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increase in density values, proving the passage from the lower clastic unit of Abu Roash
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ACCEPTED MANUSCRIPT Formation E Member to the upper carbonate unit of Abu Roash Formation D Member (Fig.
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5).
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Systems Tracts:
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The whole carbonate Abu Roash F Member has been interpreted in this study as the
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transgressive systems tracts (TST-3) of the examined SQ-3 however, the total entire clastic
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sediments of the Abu Roash E Member has been suggested to represent the regressive
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systems tracts (RST-3) of the this sequence. Accordingly, the maximum flooding surface
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(MFS-3) which separates the previous two systems tract is considered to be the uppermost
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surface of Abu Roash F Member (Fig. 6).
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3.2.2 Depositional sequence-4 (SQ-4):
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.Age and Lithology:
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The 3rd order depositional sequence-4 (SQ-4) includes the entire sediments of both Abu
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Roash D and C Members of the Turonian age (Fig. 6). D Member is mainly composed of
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limestone with minor shale interbeds, whereas the overlying Abu Roash Formation C
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Member is essentially formed of shale, sandstones and siltstone with minor limestone
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interbeds (Fig. 1). Like (SQ-3), this remarkable variation in lithology between C and D
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Members again reflects another shift from a relatively deeper marine environment of wider
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accommodation zone to shallower marine environment enables deposition of the clastic facies
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of Abu Roash Formation C Member deposited within relatively reduced accommodation
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space receiving high clastics input from the nearby lands.
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Sequence boundaries:
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The 3rd order depositional sequence-4 (SQ-4) rests on the sequence boundary (SB-4), (See
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SQ-3). It is topped by Sb-5 which represents the uppermost surface of Abu Roash Formation
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C Member. SB-5 has been only interpreted depending upon the well logging data when a
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sharp decrease of both gamma ray and sonic values coupled with intense increase in density
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C Member to the overlying carbonate facies of B Member (Fig. 5).
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Systems Tracts:
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The entire limestone of Abu Roash D Member has been interpreted in this study as the
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transgressive systems tracts (TST-4) of the SQ-4 topped by the maximum flooding surface
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(MFS-4) representing the uppermost surface of Abu Roash D Member however; the Abu
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Roash C Member of clastic in nature has been interpreted as the regressive systems tracts
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(RST-4) (Figs. 5 & 6).
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3.2.3 Depositional sequence-5 (SQ-5):
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Age and Lithology:
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The 3rd order depositional sequence-5 (SQ-5) includes the entire sedimentary succession of
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the Abu Roash Formation B Member of uppermost Turonian age, in addition to the Abu
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Roash Formation A Member of Coniacian age (Fig. 6). B Member is mainly composed of
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limestone with minor shale content, whereas, the overlying Abu Roash Formation A Member
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consists of a thick intercalations of shale and limestone beds (Fig. 1). Like SQ-3 and SQ-4,
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the change in lithology from non-clastic-rich facies of the lower B Member to the clastic-rich
284
facies of the upper A Member further proves a new shift of the deeper marine to shallower
285
marine environments.
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Sequence boundaries:
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The 3rd order depositional sequence-5 (SQ-5) is underlain by SB-5, discussed before, and is
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topped by SB-6 which represents the uppermost surface of A Member (i.e the uppermost
289
surface of the Coniacian age). SB-6 has been interpreted on seismic profile on the basis of the
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onlapping relation of the overlying seismic reflectors and toplapping relation with underlying
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seismic reflectors (Fig. 4). SB-6 is relatively similar to both SB-2 and SB-4 on seismic
292
profiles by displaying a comparatively strong, moderate amplitude and discontinuous
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ACCEPTED MANUSCRIPT segmented reflectors cut by some normal faults as reported for SB-2 and SB-4, although with
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less effect (Fig. 4b). SB-6 has been traced in well-logs by the sharp increase of both gamma
295
ray and sonic values and the abrupt decrease in density values indicating the passage from the
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underlying A Member (mixed shale and limestone facies) to the overlying Khoman Formation
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B Member having more shale composition (Fig. 5a : d).
298
Systems Tracts:
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The Abu Roash B Member of limestone in nature has been interpreted in present work as the
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transgressive systems tracts (TST-5) of the examined SQ-5 and topped by the maximum
301
flooding surface (MFS-5) which lies on the uppermost surface of the Abu Roash B Member
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however; the whole Abu Roash A Member of mixed sediments (but with more clastic shales)
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considered as regressive systems tracts (RST-5) (Fig. 6).
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3.2.4 Depositional sequence-6 (SQ-6):
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Age and Lithology:
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The 3rd order depositional sequence-6 (SQ-6) represents the entire sedimentary succession of
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the whole Khoman Formation B Member of Santonian in age and composed of intercalations
309
of shale and limestone beds (Figs. 1 & 6).
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The seismic facies of SQ-6, it displays parallel to sub parallel pattern with moderate to high
311
continuity however, it contains some high amplitude reflectors alternate with low amplitude
312
reflectors due to the alternations between shale and limestone beds (Fig. 4a &b).
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Sequence boundaries:
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The 3rd order depositional sequence-6 (SQ-6) is bounded by below by the sequence boundary
315
SB-6, and by is topped by the sequence boundary (SB-7) representing the uppermost surface
316
of Khoman Formation B Member of the Santonian age. SB-7 has been interpreted on the
317
seismic profiles by the onlapping termination relation with the overlying reflectors and by the
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strongest and the most continuous reflector on the seismic profiles owing to the marked
320
passage from the underlying shale / limestone intercalations beds of Khoman Formation B
321
Member to the overlying clean chalky limestone for the Khoman Formation A Member (Figs.
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1 & 4). On the other hand, SB-7 has been easily interpreted from the wire-line logs, due to a
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sharp reduction of gamma ray and sonic values coupled with an abrupt increase in density
324
values appropriate the passing from the lower Khoman B Member of the mixed shale /
325
limestone beds to the upper unit of Khoman-A Member of pure limestone in composition
326
(Fig.1).
327
Systems Tracts:
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According to the available well logs data, the Khoman B Member (SQ-6) has been divided
329
into lower transgressive systems tracts (TST-6) topped by the maximum flooding surface
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(MFS-6). It is characterized by increase in gamma ray reading suggesting the maximum shale
331
content deposited during the ultimate sea level rise. The upper regressive systems tracts (RST-
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5) exhibits decrease in gamma ray values reflecting the low shale content and the infilling of
333
the accommodation space by sediments which led to a progradational pattern (Figs. 5 & 6).
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3.3. Depositional Sequence-III (SQ-III):
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The 2nd order depositional sequence (SQ-III) overlies the 2nd order depositional sequence
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(SQ-II). It consumed the earlier times of the Campanian-Maastrichtian age (~8.0 my) (Fig. 6).
338
The sequence is entirely composed of carbonates sediments of the lower part of Khoman
339
Formation A Member. This sequence is bounded below by the sequence boundary SB-7
340
(discussed before), whereas it is topped by the sequence boundary (SB-8). The latter has been
341
traced on seismic profiles due to the onlapping relations with the overlying reflectors and the
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relatively strong and continuous reflector on the seismic sections. Owing to lack of well log
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data, it is not possible to further subdivision of SQ-III into smaller 3rd order depositional
345
sequences.
346
SQ-III displays uniform parallel and relatively high continuous reflectors indicating the
347
deposition of the present limestone within a uniform depositional regime without any
348
disturbance from nearby clastic input.
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3.4. Depositional Sequence-IV (SQ-IV):
351
The 2nd order depositional sequence-4 (SQ-IV) includes the limestone beds for the almost
352
upper half of the Campanian-Maastrichtian Khoman Formation A Member age (~9.0 my),
353
(Fig. 6). It is bounded below by SB-8, whereas it is topped by the sequence boundary SB-9.
354
The sequence boundary SB-9 is indicated by relatively strong and continuous reflector that is
355
onlapped by the overlying seismic reflectors, while it toplaps the underlying seismic reflectors
356
(Figs 4a and b).
357
Like the depositional sequence (SQ-III), SQ-IV has only been interpreted depending on the
358
investigation of the seismic profiles. It displays uniform, well-stacked simple parallel
359
configuration with relatively high continuity and high amplitude seismic reflectors proving
360
the continuation of the uniform depositional started before during the deposition of the chalky
361
limestone of the Khoman Formation A Member (Fig. 4).
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4. GEOLOGIC EVOLUTION
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The geologic evolution of the Upper Cretaceous megasequence in the examined area can be
368
summarized as follows:
369
(1) By the Early Cenomanian times, a wide marine invasion covered the northern parts of
370
Egypt, and possibly vast areas of the Upper Egypt (Said, 1990). As a result, a marked
371
increase in the relative sea level over these territories including the study area took place
372
accompanied with noticeable increase in the depositional accommodation space. These
373
conditions have enabled the deposition of the shallow marine Bahariya Formation (SQ-1)
374
as the lowermost rock unit in the examined area. The deposition started by the rapidly-
375
growing basal transgressive systems tracts (TST-1) in form of thick retrogradational-
376
aggradational parasequences. The continuous basin-filling and the upward growth of
377
sedimentary bodies resulted in a continuous reduction of the accommodation space, till
378
reach minimum toward the maximum flooding surface (MFS-1). Accordingly, the
379
depositional regime was changed to produce of progradational parasequence forming the
380
lowermost regressive systems tracts (RST-1).
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(2) By the final stages of the Upper Cenomanian times, a rapid subsidence, simultaneous to a
382
new marine transgressive phase took place, resulting in the deposition of SQ-2,
383
consuming the entire sediments of the Abu Roash Formation G Member. Accordingly, the
384
increase in the depositional space was filled by the lower part of Abu Roash Formation G
385
Member (TST-2) as rather thick retrogradational parasequences. When the depositional
386
rate reached minimum, conditions of (MFS-2), a phase of a gradual sea level fall began,
387
starting to develop the overlying RST-2 (the upper part of Abu Roash Formation G
388
Member).
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390
area, witnessed a uniformly depositional regime in form of successive oscillating
391
transgressive – regressive marine cycles. These oscillating marine cycles have resulted in
392
the production of almost equivocal sedimentary bodies of the Turonian-Coniacian Abu
393
Roash Formation. During the transgressive phases of these cycles, up-growing
394
retrogradational to slightly aggradational parasequences of limestones or/and relatively
395
deep marine siliciclastic deposits were developed as TSTs (TST3, TST4, and TST5) of the
396
depositional sequences SQ-3, SQ-4, and SQ-5. By the ultimate basin filling, remarkable
397
reduction of the hosting accommodation spaces took place commonly accompanied with
398
the development of the successive regressive phases of the concerned marine cycles. At
399
these regressive phases, the successive RSTs (RST3, RST4, and RST5) of the depositional
400
sequences SQ-3, SQ-4, and SQ-5were developed.
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(4) By the Santonian age, the northern parts of Egypt and Sinai were subjected under tectonic
402
disturbances associated with crustal shortening, producing large radii undulations and
403
block faulting (Smith, 1971; Said, 1992; El-Fawal & Gaber, 2005; Mostafa, 2008
404
and Abd El-Aal et al., 2015). As a result, a new tectonically-overprinted marine
405
depositional cycle started by that time. Hence, a markedly rapid phase of basin
406
subsidence accompanied with a deep marine invasion covered most of the northern
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parts of Egyptian lands, depositing the lower parts of the Khoman Formation belonging to the (B) Member. Deposition was accomplished in form of thick retrogradational-aggradational limestone and chalky limestones of the TST-6. By the
410
end of that Santonian cycle, the upper parts of the Khoman Formation B Member was
411
deposited as RST-6 during a gradual, and slow relative sea level fall ending the
412
deposition of SQ-6.
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414
with significantly tectonically influenced basin oscillating subsidence took place.
415
Accordingly, a long (17 my) tectonically forced depositional conditions prevailed during
416
this phase. Hence, two successive depositional cycles were thus initiated, forming the
417
interpreted SQ-III and SQ-VI separated by unconformity type-2 sequence boundary (SB-
418
8). Thick chalky limestone of Khoman Formation (A) Member was deposited, under
419
deeply submerged basin with strong oscillating sea level rise accompanied by an effective
420
basin subsidence.
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(6) By end of the Maastrichtian age, world-wide active tectonic events took place prior to the
422
deposition of the Tertiary rocks (K/T Boundary). The echo of such events in the study
423
area was in form of significant basin uplifts associated with the erosional phase, managed
424
to expose the pre-deposited Upper Cretaceous succession. Accordingly, the type-1
425
sequence boundary (SB-9) was developed prior to the deposition of the overlying Eocene
426
Apollonia Formation and closing the depositional history of the Upper Cretaceous
427
megasequence in the study area, and the northern part of the Egyptian Western Desert.
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The foregoing sequence stratigraphic analysis has been used to construct the relative sea level
432
curve in order to summarize the successive sea level fluctuations during the deposition of the
433
Upper Cretaceous megasequence, especially in the study area, and the northern part of the
434
Egyptian Western Desert as well. Relative similarities do exist between the suggested relative
435
sea level curves in the present work with the global sea-level curve stated for the same time
436
span (Fig. 6). Generally, the global sea-level curve of Haq et al, (1987) displays a gradual
437
increase in the sea level from the Cenomanian time up to the Maastrichtian age however, this
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general sea level rise was subdivided into smaller sea level cycles by the higher resolution
439
sequence stratigraphic analysis which may be correlated to the present minor cycles suggested
440
by this work.
6. CONCLUSIONS
442
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The detailed sequence stratigraphic analysis using the Transgressive-Regressive Model (T-R
444
Model) has been applied to the available seismic and well logging data for the Upper
445
Cretaceous megasequence of Abu Gharadig Basin which represents the largest basin in size
446
and the oldest basin in hydrocarbon exploration activities in the northern part of the Egyptian
447
Western Desert has revealed that this megasequence can be classified into four 2nd order
448
depositional sequences. These sequences started with the Cenomanian Sequence-I (SQ-I)
449
topped by the Turonian – Santonian Sequence-II (SQ-II). However the interpreted Sequence-
450
III (SQ-III) together with Sequence-IV (SQ-IV) represents the Campanian- Maastrichtian
451
time span.
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The interpreted SQ-I and SQ-II have been subdivided into smaller 3rd order depositional
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sequences however, the lack of the well logging data prevented the finer subdivision of both
455
SQ-III and SQ-IV which represent together the Khoman-A Member of Campanian-
456
Maastrichtian age. Each interpreted sequence has been defined by two bounded surfaces
457
(sequence boundaries) displaying the maximum regressive surfaces (MRS). However the
458
entire interpreted 3rd order sequence has been distinguished into lower regressive systems
459
tracts (RST) topped by transgressive systems tracts (TST) and both systems tracts separated
460
by the maximum flooding surfaces (MFS). The 2nd order SQ-I encompasses the Bahariya
461
Formation in addition to the lowermost member of Abu Roash Formation (G Member). This
462
sequence has been classified into two 3rd order depositional sequences; Sequence-1(SQ-1)
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464
Member. However, the SQ-II has been subdivided into four 3rd order depositional cycles;
465
Sequence-3 (SQ-3) which comprises the Abu Roash F and E Members, Sequence-4 (SQ-4)
466
which consists of the Abu Roash D and C Members, Sequence-5 (SQ-5) which represents the
467
Abu Roash B and A Members and Sequence-6 (SQ-6) which represents the Khoman-B
468
Member.
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The relative sea level curve of the Upper Cretaceous time in the concerned area of study has
471
been deduced according to the foregoing sequence stratigraphic analysis. This curve reflects
472
the successive changes in accommodation space during the deposition of the Upper
473
Cretaceous sedimentary succession in the northern part of the Egyptian Western Desert.
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The Upper Cretaceous basin evolution along the northern part of the Egyptian Western Desert
476
was controlled by a repetition changes in the relative sea level (accommodation zone)
477
resulting in successive eight depositional cycles. The relative sea level rise was controlled by
478
the actual sea level rise coupled with the rapid basin subsidence however; the reduction in the
479
accommodation zone was due to the basin in-filling status. These cycles can be summarized
480
as follow:
481
1- The first depositional cycle (SQ-1) represents the lower part of the Cenomanian age within
482
which the lower part of the Bahariya Formation started to be deposited during a wide marine
483
invasion covered the northern part of Egypt as a transgressive systems tract (TST-1) however,
484
its upper part deposited as a regressive systems tract (RST-1).
485
2- The second depositional cycle (SQ-2) contains the upper part of the Cenomanian age
486
within which the lower part of Abu Roash G Member formed as TST-2 and its upper part
487
represents the RST-2.
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489
Abu Roash F Member deposited and forming TST-3 and the deposition of the mixed clstic-
490
carbonate Abu Roash E Member forming RST-3.
491
4- The fourth and fifth depositional cycles (SQ-4 and SQ-5) consumed the rest of the
492
Turonian age in addition to the whole Coniacian age resulted in the deposition of the entire
493
carbonate sediments of both Abu Roash D and B Members as TST-4 and TST-5, respectively.
494
However, the mixed clastics-carbonate sediments of both Abu Roash C and A Members
495
deposited forming TST-4 and TST-5, respectively.
496
5- The Sixth cycle (SQ-6) took place in the Santonian age. During this depositional sequence,
497
the lower part of the Khoman-B Member deposited as TST-6 while the upper part of the
498
Khoman-B Member deposited as RST-6.
499
6- Regarding to the Campanian – Maastrichtian age during which the chalky limestone of
500
Khoman A Member deposited, the present work assumed that this deposition took place
501
within two 2nd order successive depositional cycles forming the interpreted SQ-III and SQ-VI
502
separated by type-2 sequence boundary without exposing the shelf area in the basin.
503
7- By end of the Maastrichtian age, an active tectonic uplifting took place led to the erosion
504
for the emerged Upper Cretaceous strata resulted in forming type-1 sequence boundary (SB-
505
9) ending the depositional evolution of the Upper Cretaceous megasequence in the area of
506
study and generally in the northern part of the Egyptian Western Desert.
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ACKNOWLEDGEMENT
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The author is grateful to the Khalda Petroleum Company and the Egyptian General Petroleum
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Corporation (EGPC) for providing the necessary subsurface data presented in this work.
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Abd El-Aal, M. H., Attia, T. E. and Aboulmagd, M. A. (2015): Structural Analysis and
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Tectonic Evolution based on Seismic Interpretation in East of Nile valley, BeniSuef
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basin, Egypt. IOSR Journal of Applied Geology and Geophysics.Vol. 3, No. 5, PP 51-
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60.
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Ahmed, M. A. A. (2008): Geodynamic evolution and petroleum system of Abu Gharadig
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Basin, North Wastern Desert, Egypt. PhD Thesis, RWTH-Aachen University,
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Germany.
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Bayoumi, A.I., and Lotfy, H.I. (1989): Modes of structural evolution of Abu Gharadig Basin,
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Western Desert of Egypt, as deduced from seismic data. Review Journal of African
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Earth Sciences, 9, 273-287.
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Bosworth, W. (1994): A model for the three-dimensional evolution of continental rift basins, north-east Africa. Geologische Rundschau, 83(4), 671-688. Boukhary, M., El Nahas, S., El Naby, A. A., Aal, M. H. A., Mahsoub, M., & Faris, M. (2014):
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Seismic and sequence stratigraphy of Upper Cretaceous–Tertiary succession, eastern
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Abu Gharadig Basin, Western Desert, Egypt. Stratigraphy, 11(2), 109-141.
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Catuneanu, O., Khalifa, M. A., & Wanas, H. A. (2006). Sequence stratigraphy of the Lower
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Cenomanian Bahariya Formation, Bahariya Oasis, Western Desert, Egypt.
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Sedimentary Geology, 190(1), 121-137.
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EGPC (Egyptian General Petroleum Corporation), (1992): Western Desert, oil and Gas fields,
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A comprehensive overview. 11 EGPC Expl. and Prod. Conf. Cairo., pp: 1-431.
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El Ayouty, M.K. (1990): Petroleum geology. In Said, R. (Ed.), Geology of Egypt (567-599), Balkema, Rotterdam.
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El-Fawal, F. and Gaber, A. (2005): El-Gafton El-Kabir Reef Island, NW Red Sea:
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Sedimentomorphic Analysis and Genesis, Journal of Petroleum & Mining
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Engineering, Suez Canal University, Vol.8, No. 1, pp. 33-53. Embry, A. F. (1993): Transgressive-regressive (T–R) sequence analysis of the Jurassic
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succession of the Sverdrup Basin, Canadian Arctic Archipelago. Canadian Journal of
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Earth Sciences, Vol. 30, pp. 301–320.
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Embry, A. F., and Johannessen, E. P. (1993): T–R sequence stratigraphy, facies analysis and
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reservoir distribution in the uppermostTriassic-Lower Jurassic succession, western
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Sverdrup Basin, Arctic Canada. In Arctic Geology and Petroleum Potential (T. O.
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Vorren, E. Bergsager, O. A. Dahl-Stamnes, E. Holter, B. Johansen, E. Lie and T. B.
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Lund, Eds.), p. 121–146. Norwegian Petroleum Society (NPF), Special Publication 2.
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Emery, D. and Myers, K (1996): Sequence stratigraphy. Blackwell Scie. Ltd., Oxford, 297 p.
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Guiraud, R. (1998): Mesozoic rifting and basin inversion along the northern African Tethyan
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margin: an overview. Geological Society, London, Special Publications, 132(1), 217-
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Haq, B.U., Hadenbol, J. and Vail, P.R. (1987): Chronology of fluctuating sea levels since the Triassic:
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Sedimentary Basins of Libya, The Geology of East Libya, vol. 3, pp. 29-46.
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Said, R. (1990): The geology of Egypt. A.A. Balkema, Rotterdam, p 734.
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Schlumberger (1984): Well evaluation conference: Geology of Egypt. 1-64.
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Schlumberger (1995): Well evaluation conference: Western Desert. 56-71.
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Smith, A. G. (1971): Alpine deformation and oceanic areas of the Tethys, Mediterranean and Atlantic. Bull. Geol. Soc. Am. 82: 2039-2070. Sultan, N., and Abd El Halim, M. (1988): In Tectonic framework of Northern Western Desert,
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Egypt and its effect on hydrocarbon accumulations. Paper presented at the 8th
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Exploration and Production Conference, Egyptian General Petroleum Corporation,
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Cairo. Novemebr, 20-23.
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Wescott,W. A., Atta, M., Blanchard, D.C., Cole, R. M., Georgeson, S. T., Miller, D. A.,
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O’Hayer, W. W., Wilson, A. D., Dolson, J. C., Sehim, A. (2011): Jurassic Rift
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Architecture in the Northeastern Western Desert, Egypt. Search and Discovery Article
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#10379 Posted December 19, 2011 *Adapted from poster presentation at AAPG
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International Conference and Exhibition, Milan, Italy, October 23-26, 2011
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FIGURES CAPTIONS
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Fig. (1): Location of the study area within Abu Gharadig basin in the northern Western
607
Desert.
608
Fig. (2): Available seismic data with well locations.
609
Fig. (3): Lithostratigraphic log of the Upper Cretaceous megasequence of Abu Gharadig
610
Basin.
611
Fig. (4.a): Interpreted Seismic Profile (Line No. 1790) representing an example of the
612
depositional dip trend along NNW-SSE direction shows the subdivision of the Upper
613
Cretaceous megasequence into seismic sequences.
614
Fig. (4.b): Interpreted Seismic Profile (Line No. 5240) representing an example of the
615
depositional strike trend along WSE-ENE direction shows the subdivision of the Upper
616
Cretaceous megasequence into seismic sequences.
617
Fig. (5): Displays the sequence stratigraphic analysis for the Upper Cretaceous megasequence
618
as interpreted from the electric logging data of AG-2 well (a), AG-5 well (b), AG-6 well (c)
619
and AG-15 well (d) with the related depths below sea level in feet unit.
620
Fig. (6): Summary table displays the interpreted depositional sequences with the suggested
621
relative sea level curve for the Upper Cretaceous megasequence in the northern Western
622
Desert.
624 625
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Fig. 1
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NW
SE
NW
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AG-6
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AG-6
Fig. 4a
5 km
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SB-9 SB-8 SB-7 SB-6 SB-4
SB-2
Onlap Toplap
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WSW
5 km
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AG-15
WSW
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SB-4 SB-2
Onlap Toplap
Fig. 4b
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5a
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5b
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5d
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ACCEPTED MANUSCRIPT 1- Sequence stratigraphic analysis based on seismic and well logging data for the Upper Cretaceous megasequence in the northern part of the Egyptian Western Desert.
of the Egyptian Western Desert.
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2- Tectonic evolution of the Upper Cretaceous megasequence in the northern part
3- Relative sea level fluctuations during the deposition of the Upper Cretaceous
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megasequence in the northern part of the Egyptian Western Desert.
S1464-343X(17)30048-1
DOI:
10.1016/j.jafrearsci.2017.02.004
Reference:
AES 2805
To appear in:
Journal of African Earth Sciences
Received Date: 13 January 2017 Revised Date:
31 January 2017
Accepted Date: 1 February 2017
Please cite this article as: Sarhan, M.A., Seismic – Wireline logs sequence stratigraphic analyses and geologic evolution for the Upper Cretaceous succession of Abu Gharadig basin, Egypt, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.02.004. 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.
ACCEPTED MANUSCRIPT
Seismic - Wireline logs Sequence Stratigraphic Analyses and
2
Geologic Evolution for the Upper Cretaceous Succession
3
of Abu Gharadig Basin, Egypt.
4
BY
5
Mohammad Abdelfattah Sarhan
6
Geology Department, Faculty of Science, Damietta University, Damietta, Egypt.
7
[email protected]
ABSTRACT
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The Upper Cretaceous megasequence in the northern part of the Egyptian Western Desert
11
has been classified into four 2nd order depositional sequences. These sequences started with
12
the Cenomanian SQ-I topped by the Turonian – Santonian SQ-II. However, both SQ-III and
13
SQ-IV represent the Campanian- Maastrichtian time span. The interpreted 2nd order SQ-I and
14
SQ-II have been further subdivided into six smaller 3rd order sequences (SQ-1 to SQ-6). The
15
depositional history started during the Early Cenomanian times, characterized by wide marine
16
invasion enabled the deposition of the shallow marine Bahariya Formation (SQ-1). The Upper
17
Cenomanian times, witnessed a rapid subsidence, simultaneously with new marine
18
transgressive phase. This is resulted in the deposition of SQ-2, consuming the entire
19
sediments of the Abu Roash G Member. During the Turonian – Coniacian times the northern
20
parts of Egypt showed successive oscillating transgressive – regressive marine cycles led to
21
equivocal sedimentary bodies of the Turonian-Coniacian Abu Roash Formation (SQ-3, SQ-4,
22
and SQ-5). During the Santonian age, the northern parts of Egypt were subjected to tectonic
23
crustal shortening, producing large scale folds. As a result, a new tectonically-overprinted
24
marine depositional cycle started and marked by rapid phase of basin subsidence. This was
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26
depositing the lower parts of the Khoman- B (SQ-6) under transgressive depositional
27
conditions. By the end of Santonian cycle, the upper parts of the Khoman Formation B
28
Member was deposited during a gradual, and slow relative sea level drop ending the
29
deposition of SQ-6. At the beginning of the Campanian – Maastrichtian times, a new
30
widespread sea-level rise associated with basin subsidence. Accordingly, two successive
31
depositional cycles were thus initiated, forming SQ-III and SQ-VI sequences separated by
32
unconformity type-2 boundary (SB-8).
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Key Words:
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Seismic - Wireline logs - Sequence Stratigraphy - Geologic Evolution- Upper Cretaceous
36
- Abu Gharadig Basin - Egypt.
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1. INTRODUCTION
38
Abu Gharadig basin is the largest basin in the northern part of the Egyptian Western
40
Desert. It is considered to be one of the most important sedimentary basins in terms of
41
petroleum potentials (Bayoumi and Lotfy, 1989), covering an area of 17500 km2. This basin
42
is tectonically representing an E-W trending rift formed in the Late Jurassic - Early
43
Cretaceous times, followed by an inversion tectonic event during the Late Cretaceous age
44
(Bayoumi and Lotfy, 1989; Guiraud, 1998; Bosworth; 1994; Moustafa, 2008 and Wescott et
45
al., 2011).
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Regarding to the economic importance of Abu Gharadig Basin, it represents one of the
47
most hydrocarbon productive basins in the Northern Western Desert. It has more than 20
48
fields currently produce About 10000 BOPD oil from the Upper Cretaceous Abu Roash and 2
ACCEPTED MANUSCRIPT Bahariya Formations (Ahmed, 2008 and El Ayouty 1990). However, most of the
50
hydrocarbons discovered were drilled as a structural prospect and a few traps are stratigraphic
51
in nature (Sultan and Abd El Halim 1988).
52
Since the Upper Cretaceous sediments of the Egyptian Western Desert including Abu
53
Gharadig Basin acquire the required conditions for hydrocarbon occurrences (source rocks,
54
reservoirs, and both the structural and stratigraphic traps in addition to the cap rocks). It
55
encouraged many workers to pay much attention and intensive studies in different approaches.
56
For example, Boukhary et al (2014) have classified the Upper Cretaceous–Tertiary succession
57
in the eastern Abu Gharadig basin into five major depositional sequences controlled by global
58
eustasy and local tectonics.
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According to the available composite logs, the Upper Cretaceous rock units of Abu
60
Gharadig basin are summarized in Fig. (1). Generally, the Upper Cretaceous lithostratigraphy
61
of Abu Gharadig basin starts at the base with the Cenomanian Bahariya Formation which
62
consists mainly of fine to very fine sandstone with some shale and limestone intercalations.
63
This Formation is overlain by Abu Roash Formation which composed essentially of
64
alternations of carbonates and clastics. It has been further subdivided into seven members
65
named from top to base; (A to G). B, D and F Members are relatively clean carbonates
66
however the other units contain variable contents of clastics with the limestones. The Khoman
67
Formation (Maastrichtian) overlies Abu Roash Formation which subdivided into two
68
members; the lower named Khoman B Member which is composed of shale and argillaceous
69
limestone however the upper Khoman A member composed mainly of chalky limestone.
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The Bahariya Formation was deposited in shallow marine environment and also the
71
overlain Abu Roash Formation. It was deposited under shallow marine shelf conditions within
72
positive accommodation space less than 200 m (Catuneanu et al. 2006).
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However, the Maastrichtian Khoman Formation deposited under open marine to outer shelf
74
condition (EGPC, 1992). Accordingly, the Upper Cretaceous megasequence in the northern
75
part of the Egyptian Western Desert including the area of study within Abu Gharadig basin
76
represent a major marine transgressive depositional cycle. The present work aims to use the seismic and well logging data of the examined
79
sedimentary basin to analyse the subsurface Upper Cretaceous sedimentary succession in a
80
sequence stratigraphic framework. Based on the foregoing sequence stratigraphic analysis, the
81
depositional evolution along the Northern part of the Egyptian Western Desert through the
82
Upper Cretaceous time will be deduced.
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2. STUDY AREA, AVAILABLE DATA AND TECHNIQUE The study area is located at the central part of Abu Gharadig Basin in the northern
86
Western Desert and lies between latitude 29.5o and 30o 00 N and between longitudes 28.3o
87
and 28.8o E (Fig.2).
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Thirty two 2D seismic reflection profiles in SEGY format with 2 km interval space
89
extracted from 3D seismic cube were kindly provided from Khalda Petroleum Company after
90
the permission of the Egyptian General Petroleum Corporation (EGPC). These seismic
91
profiles can be classified into three groups; nine in-lines extend in ENE-WSW trend
92
(Depositional strike trend) and each one extends for 55.5 km in length and nineteen cross-
93
lines extend in NNW-SSE trend (Depositional dip trend) of 25.5 km in length in addition to
94
four arbitrary seismic lines connecting the available wells covering the study area as shown in
95
Fig. (3). Moreover, composite logs are available for five wells (AG-2, AG-5, AG-6, AG-15
96
and SAWAG-1) with different logging data in the form of; Gamma ray, Sonic and Density for
97
AG-2, AG-5, AG-6, AG-15 wells, in addition to the Check-shot data for only SWAG-1 well.
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To achieve the previous aim of the work, the following procedures are followed:
99
(1) Picking and tying of the clearest sequence boundaries from seismic and well data to
100
classify the Upper Cretaceous succession into 2nd (3 – 50 my) and 3rd order depositional
101
sequences (0.5 – 3 my) according to Emery and Myers (1996).
103
(2) The interpreted seismic sequence boundaries have been mapped over the grid of 2D seismic lines to construct time depth maps.
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(3) Preparing isochron maps of each interpreted seismic sequence.
105
(4) The interpreted 3rd order depositional cycles then subdivided into their included systems
106
tracts using the available well logging data.
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(5) Construct the relative sea level curve of the Upper Cretaceous time in the study area
108
according to the foregoing sequence stratigraphic analysis. This curve is expected to
109
reflect the successive changes in the accommodation space during the deposition of the
110
Upper Cretaceous sedimentary succession. This in turn reflects the depositional history
111
along the northern part of the Egyptian Western Desert during the Upper Cretaceous.
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3. SEQUENCE STRATIGRAPHIC ANALYSIS
113
The Transgressive-Regressive Model (T-R Model) of Embry (1993) and Embry &
115
Johannessen (1993) has been applied in the present work to perform the sequential
116
subdivision for the Upper Cretaceous sedimentary succession. According to this model, the
117
sequence boundaries represent the maximum regressive surfaces (MRS), however the entire
118
sequence is subdivided into lower regressive systems tracts (RST), topped by transgressive
119
systems tracts (TST) and both of them are separated by the maximum flooding surface
120
(MFS).
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122
area has been classified into four 2nd order depositional sequences named, from base to top, as
123
follow: Sequence-I (SQ-I), Sequence-II (SQ-II), Sequence-III (SQ-III) and Sequence-IV (SQ-
124
IV). Moreover, the interpreted SQ-I and SQ-II have been subdivided into smaller 3rd order
125
depositional sequences, however the lack of adequate well logging data made it hard to
126
subdivide both SQ-III and SQ-IV into smaller subdivisions.
127
Each interpreted 3rd order sequence has been distinguished into lower regressive systems
128
tracts (RST) topped by transgressive systems tracts (TST) and both systems tracts separated
129
by the maximum flooding surfaces (MFS).
130
It is worth mentioning that the clear offlap break which appears in the cross- seismic line
131
(Depositional Dip Line) (Fig. 4a) is considered herein as apparent offlap break (not actual)
132
because of the seismic reflectors are curved upward again further to the NW direction. This
133
apparent offlap break may be attributed to a later tectonic activity that led to an uplift of an
134
igneous intrusion beneath the pre-deposited Upper Cretaceous sedimentary package leading to
135
the existence of the anticlinal shape for the upper sedimentary succession as shown in Fig
136
(4.a). On the other hand, the Above mentioned upward curvature in the seismic reflectors may
137
also be caused by normal tilting of the Upper Cretaceous strata against a major fault which
138
unfortunately doesn’t appear here, and may exists further to the NW direction (i.e.: to the left
139
side of the present seismic profile).
140
The following section represents the discussion of each interpreted depositional sequence:
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3.1. Depositional Sequence-I (SQ-I):
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The depositional sequence-I (SQ-I) is a 2nd order depositional sequence. It is the basal
144
interpreted depositional sequence in the examined Upper Cretaceous sedimentary succession.
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ACCEPTED MANUSCRIPT It represents the Cenomanian depositional phase (˃ 6.0 my) in the study area. SQ-I is
146
composed of the entire sediments of the Bahariya Formation in addition to the G Member of
147
the overlying Abu Roash Formation. The lower sequence boundary has not been interpreted
148
from the present seismic data because of the low resolution of the seismic profiles at this great
149
depth, and also because of the inadequate well data. The upper sequence boundary, on the
150
other hand, is the top surface of the Abu Roash G Member as defined from the available
151
wireline logs. This sequence can be subdivided into two 3rd order depositional sequences;
152
from bottom to top (SQ-1) and (SQ-2):-
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153 3.1.1. Depositional sequence-1(SQ-1):
155
Age and Lithology:
156
The 3rd order depositional sequence (SQ-1) represents the lower part of the major depositional
157
sequence-I (SQ-I), encompassing the entire sediments of the Cenomanian Bahariya
158
Formation. The composite logs of the Bahariya Formation indicate that it is composed mainly
159
of sandstones with rarely shale intercalations.
160
Sequence boundaries:
161
The lower sequence boundary of the present sequence (SB-1) coincides with that of the
162
depositional sequence SQ-I which has not been interpreted herein (Fig. 4a &b). However, the
163
upper sequence boundary (SB-2) with overlying (SQ-2) represents the maximum regressive
164
surface (MRS-1) of the SQ-1. It
165
where along the seismic profile, SB-2 displays high amplitude reflector due to the contrast in
166
acoustic impedance between the sandy facies of Bahariya Formation and the upper shale
167
facies of Abu Roash G Member.
168
onlapping terminations of the overlying seismic reflectors, and by the toplapping terminations
169
of the underlying reflectors (Fig. 4a &b). However, this seismic surface is sometimes
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has been easily detected from both seismic and log data
SB-2 has been easily defined seismically due to the
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ACCEPTED MANUSCRIPT represented by significantly discontinuous reflector due to the many normal faulting
171
processes, reflecting the tectonic disturbances affected the lower part of the Upper Cretaceous
172
megasequence in the study area (Fig. 4b). On the other hand, the SB-1 (MRS-1) also has been
173
easily proved from wireline logs by the abrupt increase in gamma ray values, and the ∆T
174
values (sonic logs) and the abrupt decrease in density log values characterizing the
175
transformation from the sandstone of the Bahariya Formation to the shale of the overlying
176
Abu Roash G Member (Fig.5).
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177 Systems Tracts:
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As for the 3rd order depositional sequence (SQ-1), it is represented by well-defined two
180
systems tracts; a transgressive systems tract (TST-1), and a regressive systems tract (RST-1).
181
Both tracts are separated by the well-defined maximum flooding surface (MFS-1) that
182
separates the lower transgressive systems tracts (TST-1) displaying high gamma ray values
183
due to its relative richness in shale content, unlike the clean sandstone unit of the upper
184
regressive systems tracts (RST-1) displaying low gamma ray values (Fig. 5a, b, c & d).
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3. 1.2 Depositiona sequence-2 (SQ-2):
187
Age and Lithology:
188
The 3rd order depositional sequence-2 (SQ-2) overlies the 3rd order depositional sequence-1
189
(SQ-1). It encompasses the entire sediments of the Abu Roash G Member belonging to the
190
uppermost Cenomanian age (Fig. 6). It is essentially composed of shale, siltstone and
191
sandstones with limestone interbeds. This mixed lithology reflects a relatively lower energy
192
depositional regime than those prevailed during the deposition of the previous (SQ-1),
193
including the sandstones of the Bahariya Formation. The presence of the limestone beds
194
reflects more effective shelf marine deposition.
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SQ-2 rests on SB-2 and topped by SB-3 representing the maximum regressive surface (MRS-
197
2) of the examined sequence and the upper surface of Abu Roash G Member (Fig. 6). SB-3
198
has been interpreted only on the basis of the well logging data. It is delineated when an abrupt
199
reduction in gamma ray values and ∆T values, coupled with an abrupt increase in density log
200
values, proving the passage from the mixed clastics/carbonates succession of the Abu Roash
201
Formation G Member to the almost pure limestone succession of the overlying Abu Roash
202
Formation F Member (Fig. 5).
203
Systems Tracts:
204
The lower transgressive systems tracts (TST-2) of the investigated SQ-2 and the upper
205
regressive systems tracts (RST-2) of the present sequence have been easily separated by a
206
clear maximum flooding surface (MFS-2). This surface lies approximately at the middle of
207
the SQ-2 and displays the maximum increase in gamma ray values attributed to the maximum
208
increase in shale facies (Figs. 5 &6).
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3.2. Depositional Sequence-II (SQ-II):
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The 2nd order depositional sequence SQ-II overlies the 2nd order depositional sequence (SQ-I).
212
The upper sequence boundary of SQ-II is the upper surface of the Khoman B Member (SB-7).
213
SQ-II, as 2nd order depositional sequence, encompasses the entire time span extending from
214
the Turonian, Coniacian and Santonian age in the area of study (~10 my). It includes the
215
entire sediments of the F, E, D, C, B and A members of Abu Roash Formation, in addition to
216
the lower member of Khoman Formation (Khoman B Member). The present 2nd order SQ-III
217
is herein subdivided into four 3rd order depositional sequences; namely from the base: SQ-3,
218
SQ-4, SQ-5 and SQ-6 (Fig. 6).
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220
Age and Lithology:
221
The 3rd order depositional sequence-3 (SQ-3) encompasses the sedimentary successions of the
222
F Member and E Member of Abu Roash Formation consuming the Early Turonian age.
223
Limestone is the only rock constituent of the Abu Roash F Member with rarely shale
224
interbeds, whereas the overlying Abu Roash E Member consists of shale, sandstones and
225
siltstone with minor limestone interbeds form (Figs. 1 & 6). This variation in lithology
226
between the two members indicates a clear shift from relatively deeper marine depositional
227
environments, with wider accommodation space prevailed below and produced the limestones
228
of F Member, to the overlying shallow marine environments produced the clastics of E
229
Member deposited in markedly reduced accommodation space due to the continuous clastics
230
supply.
231
Sequence boundaries:
232
SQ-3 is bounded by SB-3 discussed before and by SB-4 along the uppermost surface of E
233
Member of Abu Roash Formation. SB-4 boundary has been interpreted on both seismic and
234
well logging data. On seismic profiles, (SB-4) exhibits high amplitude reflector due to the
235
acoustic impedance contrast between the lower clastic unit of E Member, Abu Roash
236
Formation, and the upper carbonate unit of D Member, Abu Roash Formation. (SB-4) is
237
emphasized on seismic sections on the basis of the onlapping termination patterns of the
238
overlying reflectors and by the toplapping terminations of the underlying reflectors (Fig. 4).
239
Like SB-2, SB-4 sometimes represented by discontinuous reflectors due to the extensional
240
normal faults affected SB-2 below. On well-logs, on the other hand, SB-4 has been interpreted
241
on the basis of the abrupt decrease in gamma ray values and sonic values, coupled with abrupt
242
increase in density values, proving the passage from the lower clastic unit of Abu Roash
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ACCEPTED MANUSCRIPT Formation E Member to the upper carbonate unit of Abu Roash Formation D Member (Fig.
244
5).
245
Systems Tracts:
246
The whole carbonate Abu Roash F Member has been interpreted in this study as the
247
transgressive systems tracts (TST-3) of the examined SQ-3 however, the total entire clastic
248
sediments of the Abu Roash E Member has been suggested to represent the regressive
249
systems tracts (RST-3) of the this sequence. Accordingly, the maximum flooding surface
250
(MFS-3) which separates the previous two systems tract is considered to be the uppermost
251
surface of Abu Roash F Member (Fig. 6).
252
3.2.2 Depositional sequence-4 (SQ-4):
253
.Age and Lithology:
254
The 3rd order depositional sequence-4 (SQ-4) includes the entire sediments of both Abu
255
Roash D and C Members of the Turonian age (Fig. 6). D Member is mainly composed of
256
limestone with minor shale interbeds, whereas the overlying Abu Roash Formation C
257
Member is essentially formed of shale, sandstones and siltstone with minor limestone
258
interbeds (Fig. 1). Like (SQ-3), this remarkable variation in lithology between C and D
259
Members again reflects another shift from a relatively deeper marine environment of wider
260
accommodation zone to shallower marine environment enables deposition of the clastic facies
261
of Abu Roash Formation C Member deposited within relatively reduced accommodation
262
space receiving high clastics input from the nearby lands.
263
Sequence boundaries:
264
The 3rd order depositional sequence-4 (SQ-4) rests on the sequence boundary (SB-4), (See
265
SQ-3). It is topped by Sb-5 which represents the uppermost surface of Abu Roash Formation
266
C Member. SB-5 has been only interpreted depending upon the well logging data when a
267
sharp decrease of both gamma ray and sonic values coupled with intense increase in density
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269
C Member to the overlying carbonate facies of B Member (Fig. 5).
270
Systems Tracts:
271
The entire limestone of Abu Roash D Member has been interpreted in this study as the
272
transgressive systems tracts (TST-4) of the SQ-4 topped by the maximum flooding surface
273
(MFS-4) representing the uppermost surface of Abu Roash D Member however; the Abu
274
Roash C Member of clastic in nature has been interpreted as the regressive systems tracts
275
(RST-4) (Figs. 5 & 6).
276
3.2.3 Depositional sequence-5 (SQ-5):
277
Age and Lithology:
278
The 3rd order depositional sequence-5 (SQ-5) includes the entire sedimentary succession of
279
the Abu Roash Formation B Member of uppermost Turonian age, in addition to the Abu
280
Roash Formation A Member of Coniacian age (Fig. 6). B Member is mainly composed of
281
limestone with minor shale content, whereas, the overlying Abu Roash Formation A Member
282
consists of a thick intercalations of shale and limestone beds (Fig. 1). Like SQ-3 and SQ-4,
283
the change in lithology from non-clastic-rich facies of the lower B Member to the clastic-rich
284
facies of the upper A Member further proves a new shift of the deeper marine to shallower
285
marine environments.
286
Sequence boundaries:
287
The 3rd order depositional sequence-5 (SQ-5) is underlain by SB-5, discussed before, and is
288
topped by SB-6 which represents the uppermost surface of A Member (i.e the uppermost
289
surface of the Coniacian age). SB-6 has been interpreted on seismic profile on the basis of the
290
onlapping relation of the overlying seismic reflectors and toplapping relation with underlying
291
seismic reflectors (Fig. 4). SB-6 is relatively similar to both SB-2 and SB-4 on seismic
292
profiles by displaying a comparatively strong, moderate amplitude and discontinuous
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294
less effect (Fig. 4b). SB-6 has been traced in well-logs by the sharp increase of both gamma
295
ray and sonic values and the abrupt decrease in density values indicating the passage from the
296
underlying A Member (mixed shale and limestone facies) to the overlying Khoman Formation
297
B Member having more shale composition (Fig. 5a : d).
298
Systems Tracts:
299
The Abu Roash B Member of limestone in nature has been interpreted in present work as the
300
transgressive systems tracts (TST-5) of the examined SQ-5 and topped by the maximum
301
flooding surface (MFS-5) which lies on the uppermost surface of the Abu Roash B Member
302
however; the whole Abu Roash A Member of mixed sediments (but with more clastic shales)
303
considered as regressive systems tracts (RST-5) (Fig. 6).
304
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3.2.4 Depositional sequence-6 (SQ-6):
306
Age and Lithology:
307
The 3rd order depositional sequence-6 (SQ-6) represents the entire sedimentary succession of
308
the whole Khoman Formation B Member of Santonian in age and composed of intercalations
309
of shale and limestone beds (Figs. 1 & 6).
310
The seismic facies of SQ-6, it displays parallel to sub parallel pattern with moderate to high
311
continuity however, it contains some high amplitude reflectors alternate with low amplitude
312
reflectors due to the alternations between shale and limestone beds (Fig. 4a &b).
313
Sequence boundaries:
314
The 3rd order depositional sequence-6 (SQ-6) is bounded by below by the sequence boundary
315
SB-6, and by is topped by the sequence boundary (SB-7) representing the uppermost surface
316
of Khoman Formation B Member of the Santonian age. SB-7 has been interpreted on the
317
seismic profiles by the onlapping termination relation with the overlying reflectors and by the
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319
strongest and the most continuous reflector on the seismic profiles owing to the marked
320
passage from the underlying shale / limestone intercalations beds of Khoman Formation B
321
Member to the overlying clean chalky limestone for the Khoman Formation A Member (Figs.
322
1 & 4). On the other hand, SB-7 has been easily interpreted from the wire-line logs, due to a
323
sharp reduction of gamma ray and sonic values coupled with an abrupt increase in density
324
values appropriate the passing from the lower Khoman B Member of the mixed shale /
325
limestone beds to the upper unit of Khoman-A Member of pure limestone in composition
326
(Fig.1).
327
Systems Tracts:
328
According to the available well logs data, the Khoman B Member (SQ-6) has been divided
329
into lower transgressive systems tracts (TST-6) topped by the maximum flooding surface
330
(MFS-6). It is characterized by increase in gamma ray reading suggesting the maximum shale
331
content deposited during the ultimate sea level rise. The upper regressive systems tracts (RST-
332
5) exhibits decrease in gamma ray values reflecting the low shale content and the infilling of
333
the accommodation space by sediments which led to a progradational pattern (Figs. 5 & 6).
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3.3. Depositional Sequence-III (SQ-III):
336
The 2nd order depositional sequence (SQ-III) overlies the 2nd order depositional sequence
337
(SQ-II). It consumed the earlier times of the Campanian-Maastrichtian age (~8.0 my) (Fig. 6).
338
The sequence is entirely composed of carbonates sediments of the lower part of Khoman
339
Formation A Member. This sequence is bounded below by the sequence boundary SB-7
340
(discussed before), whereas it is topped by the sequence boundary (SB-8). The latter has been
341
traced on seismic profiles due to the onlapping relations with the overlying reflectors and the
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ACCEPTED MANUSCRIPT toplapping nature with the underlying seismic reflectors (Fig. 4). Moreover, SB-8 displays a
343
relatively strong and continuous reflector on the seismic sections. Owing to lack of well log
344
data, it is not possible to further subdivision of SQ-III into smaller 3rd order depositional
345
sequences.
346
SQ-III displays uniform parallel and relatively high continuous reflectors indicating the
347
deposition of the present limestone within a uniform depositional regime without any
348
disturbance from nearby clastic input.
349
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3.4. Depositional Sequence-IV (SQ-IV):
351
The 2nd order depositional sequence-4 (SQ-IV) includes the limestone beds for the almost
352
upper half of the Campanian-Maastrichtian Khoman Formation A Member age (~9.0 my),
353
(Fig. 6). It is bounded below by SB-8, whereas it is topped by the sequence boundary SB-9.
354
The sequence boundary SB-9 is indicated by relatively strong and continuous reflector that is
355
onlapped by the overlying seismic reflectors, while it toplaps the underlying seismic reflectors
356
(Figs 4a and b).
357
Like the depositional sequence (SQ-III), SQ-IV has only been interpreted depending on the
358
investigation of the seismic profiles. It displays uniform, well-stacked simple parallel
359
configuration with relatively high continuity and high amplitude seismic reflectors proving
360
the continuation of the uniform depositional started before during the deposition of the chalky
361
limestone of the Khoman Formation A Member (Fig. 4).
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4. GEOLOGIC EVOLUTION
366
The geologic evolution of the Upper Cretaceous megasequence in the examined area can be
368
summarized as follows:
369
(1) By the Early Cenomanian times, a wide marine invasion covered the northern parts of
370
Egypt, and possibly vast areas of the Upper Egypt (Said, 1990). As a result, a marked
371
increase in the relative sea level over these territories including the study area took place
372
accompanied with noticeable increase in the depositional accommodation space. These
373
conditions have enabled the deposition of the shallow marine Bahariya Formation (SQ-1)
374
as the lowermost rock unit in the examined area. The deposition started by the rapidly-
375
growing basal transgressive systems tracts (TST-1) in form of thick retrogradational-
376
aggradational parasequences. The continuous basin-filling and the upward growth of
377
sedimentary bodies resulted in a continuous reduction of the accommodation space, till
378
reach minimum toward the maximum flooding surface (MFS-1). Accordingly, the
379
depositional regime was changed to produce of progradational parasequence forming the
380
lowermost regressive systems tracts (RST-1).
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(2) By the final stages of the Upper Cenomanian times, a rapid subsidence, simultaneous to a
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new marine transgressive phase took place, resulting in the deposition of SQ-2,
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consuming the entire sediments of the Abu Roash Formation G Member. Accordingly, the
384
increase in the depositional space was filled by the lower part of Abu Roash Formation G
385
Member (TST-2) as rather thick retrogradational parasequences. When the depositional
386
rate reached minimum, conditions of (MFS-2), a phase of a gradual sea level fall began,
387
starting to develop the overlying RST-2 (the upper part of Abu Roash Formation G
388
Member).
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area, witnessed a uniformly depositional regime in form of successive oscillating
391
transgressive – regressive marine cycles. These oscillating marine cycles have resulted in
392
the production of almost equivocal sedimentary bodies of the Turonian-Coniacian Abu
393
Roash Formation. During the transgressive phases of these cycles, up-growing
394
retrogradational to slightly aggradational parasequences of limestones or/and relatively
395
deep marine siliciclastic deposits were developed as TSTs (TST3, TST4, and TST5) of the
396
depositional sequences SQ-3, SQ-4, and SQ-5. By the ultimate basin filling, remarkable
397
reduction of the hosting accommodation spaces took place commonly accompanied with
398
the development of the successive regressive phases of the concerned marine cycles. At
399
these regressive phases, the successive RSTs (RST3, RST4, and RST5) of the depositional
400
sequences SQ-3, SQ-4, and SQ-5were developed.
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(4) By the Santonian age, the northern parts of Egypt and Sinai were subjected under tectonic
402
disturbances associated with crustal shortening, producing large radii undulations and
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block faulting (Smith, 1971; Said, 1992; El-Fawal & Gaber, 2005; Mostafa, 2008
404
and Abd El-Aal et al., 2015). As a result, a new tectonically-overprinted marine
405
depositional cycle started by that time. Hence, a markedly rapid phase of basin
406
subsidence accompanied with a deep marine invasion covered most of the northern
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parts of Egyptian lands, depositing the lower parts of the Khoman Formation belonging to the (B) Member. Deposition was accomplished in form of thick retrogradational-aggradational limestone and chalky limestones of the TST-6. By the
410
end of that Santonian cycle, the upper parts of the Khoman Formation B Member was
411
deposited as RST-6 during a gradual, and slow relative sea level fall ending the
412
deposition of SQ-6.
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ACCEPTED MANUSCRIPT (5) As the Campanian – Maastrichtian times started, a widespread sea-level rise associated
414
with significantly tectonically influenced basin oscillating subsidence took place.
415
Accordingly, a long (17 my) tectonically forced depositional conditions prevailed during
416
this phase. Hence, two successive depositional cycles were thus initiated, forming the
417
interpreted SQ-III and SQ-VI separated by unconformity type-2 sequence boundary (SB-
418
8). Thick chalky limestone of Khoman Formation (A) Member was deposited, under
419
deeply submerged basin with strong oscillating sea level rise accompanied by an effective
420
basin subsidence.
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(6) By end of the Maastrichtian age, world-wide active tectonic events took place prior to the
422
deposition of the Tertiary rocks (K/T Boundary). The echo of such events in the study
423
area was in form of significant basin uplifts associated with the erosional phase, managed
424
to expose the pre-deposited Upper Cretaceous succession. Accordingly, the type-1
425
sequence boundary (SB-9) was developed prior to the deposition of the overlying Eocene
426
Apollonia Formation and closing the depositional history of the Upper Cretaceous
427
megasequence in the study area, and the northern part of the Egyptian Western Desert.
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5. RELATIVE SEA LEVEL FLUCTUATIONS
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The foregoing sequence stratigraphic analysis has been used to construct the relative sea level
432
curve in order to summarize the successive sea level fluctuations during the deposition of the
433
Upper Cretaceous megasequence, especially in the study area, and the northern part of the
434
Egyptian Western Desert as well. Relative similarities do exist between the suggested relative
435
sea level curves in the present work with the global sea-level curve stated for the same time
436
span (Fig. 6). Generally, the global sea-level curve of Haq et al, (1987) displays a gradual
437
increase in the sea level from the Cenomanian time up to the Maastrichtian age however, this
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general sea level rise was subdivided into smaller sea level cycles by the higher resolution
439
sequence stratigraphic analysis which may be correlated to the present minor cycles suggested
440
by this work.
6. CONCLUSIONS
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The detailed sequence stratigraphic analysis using the Transgressive-Regressive Model (T-R
444
Model) has been applied to the available seismic and well logging data for the Upper
445
Cretaceous megasequence of Abu Gharadig Basin which represents the largest basin in size
446
and the oldest basin in hydrocarbon exploration activities in the northern part of the Egyptian
447
Western Desert has revealed that this megasequence can be classified into four 2nd order
448
depositional sequences. These sequences started with the Cenomanian Sequence-I (SQ-I)
449
topped by the Turonian – Santonian Sequence-II (SQ-II). However the interpreted Sequence-
450
III (SQ-III) together with Sequence-IV (SQ-IV) represents the Campanian- Maastrichtian
451
time span.
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The interpreted SQ-I and SQ-II have been subdivided into smaller 3rd order depositional
454
sequences however, the lack of the well logging data prevented the finer subdivision of both
455
SQ-III and SQ-IV which represent together the Khoman-A Member of Campanian-
456
Maastrichtian age. Each interpreted sequence has been defined by two bounded surfaces
457
(sequence boundaries) displaying the maximum regressive surfaces (MRS). However the
458
entire interpreted 3rd order sequence has been distinguished into lower regressive systems
459
tracts (RST) topped by transgressive systems tracts (TST) and both systems tracts separated
460
by the maximum flooding surfaces (MFS). The 2nd order SQ-I encompasses the Bahariya
461
Formation in addition to the lowermost member of Abu Roash Formation (G Member). This
462
sequence has been classified into two 3rd order depositional sequences; Sequence-1(SQ-1)
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Member. However, the SQ-II has been subdivided into four 3rd order depositional cycles;
465
Sequence-3 (SQ-3) which comprises the Abu Roash F and E Members, Sequence-4 (SQ-4)
466
which consists of the Abu Roash D and C Members, Sequence-5 (SQ-5) which represents the
467
Abu Roash B and A Members and Sequence-6 (SQ-6) which represents the Khoman-B
468
Member.
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The relative sea level curve of the Upper Cretaceous time in the concerned area of study has
471
been deduced according to the foregoing sequence stratigraphic analysis. This curve reflects
472
the successive changes in accommodation space during the deposition of the Upper
473
Cretaceous sedimentary succession in the northern part of the Egyptian Western Desert.
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The Upper Cretaceous basin evolution along the northern part of the Egyptian Western Desert
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was controlled by a repetition changes in the relative sea level (accommodation zone)
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resulting in successive eight depositional cycles. The relative sea level rise was controlled by
478
the actual sea level rise coupled with the rapid basin subsidence however; the reduction in the
479
accommodation zone was due to the basin in-filling status. These cycles can be summarized
480
as follow:
481
1- The first depositional cycle (SQ-1) represents the lower part of the Cenomanian age within
482
which the lower part of the Bahariya Formation started to be deposited during a wide marine
483
invasion covered the northern part of Egypt as a transgressive systems tract (TST-1) however,
484
its upper part deposited as a regressive systems tract (RST-1).
485
2- The second depositional cycle (SQ-2) contains the upper part of the Cenomanian age
486
within which the lower part of Abu Roash G Member formed as TST-2 and its upper part
487
represents the RST-2.
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489
Abu Roash F Member deposited and forming TST-3 and the deposition of the mixed clstic-
490
carbonate Abu Roash E Member forming RST-3.
491
4- The fourth and fifth depositional cycles (SQ-4 and SQ-5) consumed the rest of the
492
Turonian age in addition to the whole Coniacian age resulted in the deposition of the entire
493
carbonate sediments of both Abu Roash D and B Members as TST-4 and TST-5, respectively.
494
However, the mixed clastics-carbonate sediments of both Abu Roash C and A Members
495
deposited forming TST-4 and TST-5, respectively.
496
5- The Sixth cycle (SQ-6) took place in the Santonian age. During this depositional sequence,
497
the lower part of the Khoman-B Member deposited as TST-6 while the upper part of the
498
Khoman-B Member deposited as RST-6.
499
6- Regarding to the Campanian – Maastrichtian age during which the chalky limestone of
500
Khoman A Member deposited, the present work assumed that this deposition took place
501
within two 2nd order successive depositional cycles forming the interpreted SQ-III and SQ-VI
502
separated by type-2 sequence boundary without exposing the shelf area in the basin.
503
7- By end of the Maastrichtian age, an active tectonic uplifting took place led to the erosion
504
for the emerged Upper Cretaceous strata resulted in forming type-1 sequence boundary (SB-
505
9) ending the depositional evolution of the Upper Cretaceous megasequence in the area of
506
study and generally in the northern part of the Egyptian Western Desert.
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ACKNOWLEDGEMENT
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The author is grateful to the Khalda Petroleum Company and the Egyptian General Petroleum
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Corporation (EGPC) for providing the necessary subsurface data presented in this work.
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REFERENCES
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Abd El-Aal, M. H., Attia, T. E. and Aboulmagd, M. A. (2015): Structural Analysis and
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Tectonic Evolution based on Seismic Interpretation in East of Nile valley, BeniSuef
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basin, Egypt. IOSR Journal of Applied Geology and Geophysics.Vol. 3, No. 5, PP 51-
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Ahmed, M. A. A. (2008): Geodynamic evolution and petroleum system of Abu Gharadig
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Basin, North Wastern Desert, Egypt. PhD Thesis, RWTH-Aachen University,
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Bayoumi, A.I., and Lotfy, H.I. (1989): Modes of structural evolution of Abu Gharadig Basin,
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Bosworth, W. (1994): A model for the three-dimensional evolution of continental rift basins, north-east Africa. Geologische Rundschau, 83(4), 671-688. Boukhary, M., El Nahas, S., El Naby, A. A., Aal, M. H. A., Mahsoub, M., & Faris, M. (2014):
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Catuneanu, O., Khalifa, M. A., & Wanas, H. A. (2006). Sequence stratigraphy of the Lower
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EGPC (Egyptian General Petroleum Corporation), (1992): Western Desert, oil and Gas fields,
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El Ayouty, M.K. (1990): Petroleum geology. In Said, R. (Ed.), Geology of Egypt (567-599), Balkema, Rotterdam.
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El-Fawal, F. and Gaber, A. (2005): El-Gafton El-Kabir Reef Island, NW Red Sea:
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Engineering, Suez Canal University, Vol.8, No. 1, pp. 33-53. Embry, A. F. (1993): Transgressive-regressive (T–R) sequence analysis of the Jurassic
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Embry, A. F., and Johannessen, E. P. (1993): T–R sequence stratigraphy, facies analysis and
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Sverdrup Basin, Arctic Canada. In Arctic Geology and Petroleum Potential (T. O.
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Vorren, E. Bergsager, O. A. Dahl-Stamnes, E. Holter, B. Johansen, E. Lie and T. B.
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Lund, Eds.), p. 121–146. Norwegian Petroleum Society (NPF), Special Publication 2.
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Emery, D. and Myers, K (1996): Sequence stratigraphy. Blackwell Scie. Ltd., Oxford, 297 p.
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Guiraud, R. (1998): Mesozoic rifting and basin inversion along the northern African Tethyan
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Haq, B.U., Hadenbol, J. and Vail, P.R. (1987): Chronology of fluctuating sea levels since the Triassic:
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Said, R. (1990): The geology of Egypt. A.A. Balkema, Rotterdam, p 734.
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Schlumberger (1984): Well evaluation conference: Geology of Egypt. 1-64.
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Smith, A. G. (1971): Alpine deformation and oceanic areas of the Tethys, Mediterranean and Atlantic. Bull. Geol. Soc. Am. 82: 2039-2070. Sultan, N., and Abd El Halim, M. (1988): In Tectonic framework of Northern Western Desert,
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Wescott,W. A., Atta, M., Blanchard, D.C., Cole, R. M., Georgeson, S. T., Miller, D. A.,
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O’Hayer, W. W., Wilson, A. D., Dolson, J. C., Sehim, A. (2011): Jurassic Rift
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FIGURES CAPTIONS
606
Fig. (1): Location of the study area within Abu Gharadig basin in the northern Western
607
Desert.
608
Fig. (2): Available seismic data with well locations.
609
Fig. (3): Lithostratigraphic log of the Upper Cretaceous megasequence of Abu Gharadig
610
Basin.
611
Fig. (4.a): Interpreted Seismic Profile (Line No. 1790) representing an example of the
612
depositional dip trend along NNW-SSE direction shows the subdivision of the Upper
613
Cretaceous megasequence into seismic sequences.
614
Fig. (4.b): Interpreted Seismic Profile (Line No. 5240) representing an example of the
615
depositional strike trend along WSE-ENE direction shows the subdivision of the Upper
616
Cretaceous megasequence into seismic sequences.
617
Fig. (5): Displays the sequence stratigraphic analysis for the Upper Cretaceous megasequence
618
as interpreted from the electric logging data of AG-2 well (a), AG-5 well (b), AG-6 well (c)
619
and AG-15 well (d) with the related depths below sea level in feet unit.
620
Fig. (6): Summary table displays the interpreted depositional sequences with the suggested
621
relative sea level curve for the Upper Cretaceous megasequence in the northern Western
622
Desert.
624 625
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Fig. 1
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NW
SE
NW
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AG-6
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AG-6
Fig. 4a
5 km
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SB-9 SB-8 SB-7 SB-6 SB-4
SB-2
Onlap Toplap
ACCEPTED MANUSCRIPT ENE
WSW
5 km
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AG-15
WSW
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SB-9 SB-8 SB-7 SB-6
SB-4 SB-2
Onlap Toplap
Fig. 4b
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5a
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5b
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Lithostratigraphic Surface
Equivelent Interpreted Surface SB-7
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5c
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Khoman-B ( top of Khoman-B Member) AR/A (top of Abo Roash A Member) SB-6 AR/B (top of Abo Roash B Member) MFS-5 AR/C (top of Abo Roash C Member) SB-5 AR/D (top of Abo Roash D Member) MFS-4 AR/E (top of Abo Roash E Member) SB-4 AR/F (top of Abo Roash F Member) MFS-3 AR/G (top of Abo Roash G Member) SB-3 BAHARIYA (top of Baharyia SB-2 Formation) G/W CONTACT (top of Gas / water contact)
Fig. 5d
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ACCEPTED MANUSCRIPT 1- Sequence stratigraphic analysis based on seismic and well logging data for the Upper Cretaceous megasequence in the northern part of the Egyptian Western Desert.
of the Egyptian Western Desert.
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2- Tectonic evolution of the Upper Cretaceous megasequence in the northern part
3- Relative sea level fluctuations during the deposition of the Upper Cretaceous
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megasequence in the northern part of the Egyptian Western Desert.