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, OCTOBER 2019, 70, 5, 373–385

doi: 10.2478/geoca-2019-0021

Deep gravity data interpretation using seismic reflection 

and well data: A case study of the West Gharib-Bakr area, 

Eastern Desert, Egypt 


Egyptian Petroleum Research Institute (EPRI), Ahmad Alzomor Str. 2, Alzohoor District, Nasr City, Cairo, Egypt;

(Manuscript received June 27, 2018; accepted in revised form June 13, 2019)

Abstract: A rigorous processing and analysis of the gravity data with seismic reflection and borehole information  

enabled a general view of the deep-seated regional structures in the West Gharib-Bakr area, Eastern Desert, Egypt. In this 

context, several interpretational techniques were applied to learn more about the supra-basement structures and intra- 

basement sources. The interpretation started with a review of the seismic data to clarify the structural elements on top of 

the Miocene strata, where a number of isochronous reflection maps were constructed and had migrated into depth maps. 

The Bouguer anomaly map was processed using Fast Fourier Transform filtering based on spectral analysis to separate 

the gravity anomalies into its components. Gravity stripping was also performed under the seismic isopachs and density 

controls. The gravity effect of each rock unit was calculated and progressively removed from the original data to obtain 

a new gravity map on top of the Pre-Miocene. To ensure more reliable results, further filtering and analytical processes 

were applied to the stripped map. The results of seismic analysis show simple structural configurations at the Miocene 

level, with a significant increase of evaporite thickness along the Gulf of Suez coast. In contrast, analysis of the stripped 

gravity map reveals a more intricate structure at the Pre-Miocene level, with increasing numbers/lengths of faults on  

the basement surface. Lineament analysis shows two major peaks trending N0–20°W and N50–70°E, produced by two 

main forces in NNW–SSE (compression) and ENE–WSW (tension) directions. The models confirmed a rough and  

ruptured basement surface, with no evidence of any magmatic intrusions penetrating the sediments. The basement relief 

map delineates five basins/sub-basins in the area which are separated from each other by ridges/saddles.

Keywords: West Bakr, Eastern Desert, Egypt, gravity stripping, applied geophysics.


In deep basinal areas which contain excessive thickness of 

evaporite (Evp) rocks, such as the Gulf of Suez (GOS) area, 

the deep structural configurations are not clear enough.  Seis-

mically, the reflection data has several difficulties and large 

deficiencies at depths near the basement. The attenuation and 

dispersion of the energy of waves within the Evp group lead to 

the shortage of seismic data and bad events below the base  

of the Evps. In addition, most of the seismic surveys were 

 acquired in the dip direction, which makes it difficult to deli-

neate locations of the transform faults. Gravitationally, this 

may be attributed to the steepness of the GOS trend or to the 

great gravity effect of the near-surface layers. The structural/

lithological variations of the Evps may create local anomalies 

like those arisomg from the Pre-Miocene (PM) features.  

All this may conceal the deeper sources and make it more dif-

ficult to determine locations of the uplifts and fault patterns. 

To overcome this, an integrated interpretation was carried out 

using all available geophysical data to disclose the hidden 

source structures on or within basement rocks.

The work started with interpreting the seismic sections 

through using the well logging information. The structural 

analysis focused on the shallow features capping the tops of 

the Miocene formations, where seismic lines seem to be very 

good in the upper part but very bad in the lower portion.  

The seismic interpretation is limited by the base of the evapo-

rites, with most basic structural interpretation next to impos-

sible. Therefore, the work has taken another way to interpret 

the deep configurations depending on the Bouguer gravity 

data. The interpretation is based on the fact that the gravity 

anomalies can be regarded as superposition of the anomalies 

reflected from the main density boundaries at varying depths. 

This means that the gravity anomaly includes effects of both 

shallow (residual) and deep (regional) constituents. Isolating 

either of them may enhance the effect of the other. In this con-

text, two different enhancement procedures were applied to 

the Bouguer anomaly map with evaluation of their effecti-

veness in characterizing the deep-seated sources/structures. 

Firstly, the gravity filtering was performed using the Fast 

Fourier Transform technique to separate the gravity anomalies 

into residual and regional components. In fact, gravity filte-

ring results were insufficient to give details about the deep 

configurations. Secondly, the layer stripping technique was 

applied as a more reliable interpretation method, where gra-

vity, seismic and well data interpretation were integrated to 

disclose the hidden potential sources. To do this, the gravity 

effects of all shallow layers (Post-Miocene and Miocene rock 

units) were calculated and subtracted from the Bouguer data. 

The residue provides a stripped map on top of the PM that 

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, 2019, 70, 5, 373–385

conforms more with the deep constituents. This approach 

depends largely on the rock parameters, thickness and density 

of each formation. The formation-density has been derived 

from the drilled wells data, while the geometry of the forma-

tion was identified from the seismic isopach maps. Sub-

sequently the stripped map on top of the PM was subjected to 

further analytical processes. The horizontal gradient filter and 

Euler deconvolution method were applied on top of the PM 

gravity map to map the main fault systems controlling the area. 

The lineaments analysis was performed to define the structural 

trends affecting the basic structure. A stress diagram and a strain 

ellipse were constructed to explain the different tectonic forces 

and associated shear fractures, which controlled the structural 

setup. The gravity modelling was done before and after gra vity 

stripping to confirm the shallower structures, and to deduce 

the PM source structures. Finally, the source parameter ima-

ging method was applied to determine the approximate depths 

of the basins and ridges, producing a map that more closely 

resembles the geology than originally.

The potential data used is the Bouguer anomaly map con-

structed from the ground surface. Askania torsion balance was 

used to construct a map with scale of 1:100,000 and contour 

interval of 0.5–1 mGal, in 1976. The gravity survey was car-

ried out on the western coast of the GOS by Anglo Egyptian 

Oilfield Ltd., for the Egyptian General Petroleum Corporation 

(EGPC). The survey covers the area between Ras Bakr and 

Ras El-Bahar on the western coast of the gulf. The seismic 

data consists of fifteen seismic lines which was shot by 

Geosource Limited, on sept.,1981 under the auspice of the 

General Petroleum Company (GPC), for the (EGPC). 

The aim of this work is to re-interpret all available geophy-

sical data, in terms of structural setting and tectonic fabric, to 

shed light on deep source structures. The interpretation focuses 

on the PM uplifts and cross-faults that may be helpful in loca-

ting deep prospecting. Some emphasis was also placed on  

the distribution of basins and ridges in the area.

Structural considerations

The study area (Fig. 1) lies in the Eastern Desert along the 

western coast of the GOS, in the middle between Suez city to 

the north and Hurghada city to the south. It comprises several 

onshore oilfields, which are confined between the GOS to  

the east and Precambrian basement rocks of the Nubian to  

the west.

Grouping of blocks or sets of blocks having the same direc-

tion of dip lead to subdivision of the GOS into three tectonic 

provinces of contrasting dip attitudes, termed “Araba, 

Belayim, and Amal” from north to south. Each province is 

divided along major north-westerly trending faults into plates 

that almost plunge in the same direction (Mostafa 1976; 

Meshref 1990). The West Bakr oilfields lie in the central pro-

vince of GOS, namely the north-easterly plunging Belayim 

province (Fig. 1) which is bounded from the north and south 

by two major faults or “hinge zones” of a NE–SW trend.  

This province was separated from the Amal province by  

the “Morgan” hinge zone and from the Araba province by  

the “Galala-Zenima” hinge zone. The structure and sedimen-

tology as well as the relationships between tectonics and sedi-

mentation in the active Suez-rift basin were studied by many 

authors (e.g., Gupta et al. 1999; Young et al. 2002; Herkat & 

Guiraud 2006; Khalil & McClay 2008; Pietrantonio 2016; 

Rohais et al. 2016; Bosworth & Durocher 2017; Segev 2017; 

Azab 2018). 

The drillhole information in the study area indicates a thick 

sedimentary section of the PM and younger rocks, unconfor-

mably overlying the basement complex (Ghanim 1972; Scotte 

& Govean 1984). The sedimentary section comprises rocks of 

Carboniferous, Cretaceous, Paleocene, Eocene, Miocene and 

Pliocene-recent ages (Shabaan et al. 1984). The Miocene 

rocks are the main source of oil-bearing rocks and they vary 

from place to place. They are divided into lower clastic rocks 

(Gharandal group) and upper evaporitic rocks (Ras Malsab 

group). The Gharandal group includes the (Nukhul, Rudeis and 

Kareem fms.) that underlie the Evps of the Ras Malaab group 

(Belayim, South Gharib and Zeit fms.). The Evp rocks consist 

mainly of anhydrite, gypsum, salt rocks intercalated with shale 

and sandstone. The PM sediments are generally characterized 

by a wide distribution and great thickness of the clastic rocks. 

The crystalline basement rocks seem to be quite similar to 

those cropping out to the west of the area (Mostafa 1992).

Previous works in the study area (Otsuka & Ogawa 1977; 

Shabaan 1984) indicate that the west Gharib-Bakr area is 

Fig. 1. Location map of West Gharib-Bakr area, with the oilfield con-

cessions (see Appendix for GPS coordinates of the wells). 

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, 2019, 70, 5, 373–385

charac terized by thick layers of Evps that exist in Kareem to 

Zeit fms. and act as sealing rocks. The Evp group shows large 

lithological variations, and consists of anhydrite (high density) 

and salt-halite (low density), contaminated with shale, sands, 

lime (medium density). The percentage of these rocks vary 

laterally/vertically and lead to significant changes in the bulk 

density from one place to another. Generally, the density of  

the Miocene Evps varies widely from 1.9 to 2.9 g/cm


, based 

on the percentage of anhydrite compared to other sediments.

Seismic interpretation

The structural configuration in the study area was studied 

through interpreting fifteen 2D seismic lines vibrated in the NW 

and NE directions. These seismic sections were carefully inspec-

ted to detect the most reliable marker horizons. The available 

well velocity survey of Hashem-1 (Miocene/TD 2909 m), 

EPH-1X (Cenomanian/TD 2659.98 m) and EPG-1X (Nubia/

TD 2915 m), and their composite well logs were used to faci-

litate the identification of the real reflectors in the seismic 

records. The strong/continuous reflectors on the tops of the 

Evp layers were easily traced along the seismic lines. The sub-

surface geological identification was done through picking 

and correlation of reflection times as well as fault detection. 

Analysis of the seismic sections was limited by the base 

Kareem Fm., which is the deepest horizon of seismic interpre-

tation.  Accordingly, several isochronous reflection maps were 

established through picking the two-way times (Fig. 2).  

The average velocity and isochronous reflection maps on top 

of the different formations were used to convert the reflection 

times into depths. The structure contour maps in terms of depth 

and faulting were also constructed on the tops of the Miocene 

formations (Fig. 3). Close correlation between these maps 

shows that the Miocene rocks are affected by a small number 

of faults that increase in number and length with depth.  

The faults run mainly in the NW–SE direction (GOS trend) 

while the NE–SW cross-gulf faults are absent. In addition,  

the isopach maps for post-Zeit, Zeit, South Gharib, Belayim 

and Kareem were constructed from the grid-data of the depth 

contour maps, whilst the isopach map for the Lower-Miocene 

Rudeis Fm. is constructed based on the composite logs of the 

drilled wells. The isopach maps indicate presence of a large 

thickness of Evp sediments in the north-east part, while Lower-

Miocene Rudeis sands increase in a south-west direction.

Seismic profiling was established to show the lateral/verti-

cal subsurface distributions more clearly. A geoseismic section 

BK-1039 (Fig. 4) is displayed as an example in the NE–SW 

dip direction, constructed through transforming the seismic 

reflection time into depth using the well velocity data. 

Generally, the seismic line seems to be very good at shallow 

depths where it shows strong reflections corresponding to  

the high density/velocity Evps. Noticeably, the post-Evps/

Evps decrease in their thickness towards the south-west, and 

significantly increase toward the north-east before dimini-

shing over the offshore ridge (Bakr-Gharib), a fact that was 

verified by well data. The seismic line shows that a small num-

ber of short fractures cut across the Miocene strata. These nor-

mal faults have small throws and do not reach the surface 

where the surface layer seems to be not entirely deformed.  

As expected, seismic evidence is generally very poor at large 

depths, where the continuity of the seismic events is markedly 

bad beneath the Kareem Fm. There is no evidence to support 

the presence of faults below the Miocene Evps. If they are 

present, it is rather difficult to judge whether such bad events 

relate to deep fault-structures or are due to lithological varia-

tions. Generally, seismic interpretation had failed to give any 

exact information about locations, orientations, magnitudes or 

throws of faults below the Evp rocks. Seismic reflection is 

also ineffective for locating the PM high-structures, caused by 

basement uplifts.

The seismic interpretation was confirmed by a SW–NE geo-

logical cross section AA’ (Fig. 5), which extends between four 

wells drilled in the study area. The cross section shows a struc-

tural uplift at the EPH-1, EPG-1 and GN-1 wells, where the 

thickness of the formations is reduced and the formation tops 

have less depth. Eastward, the structure exhibits a real increase 

in thickness of the sedimentary section alongside the GOS 

coast, especially of the Miocene and Post-Miocene sediments. 

This Miocene basin is bounded from the east by an uplifted 

block (Bakr-ridge), where PM rocks are shallower at Bakr-2. 

Westward, the basement rocks were buried under thick sec-

tions of the PM and Lower-Miocene sediments. The Miocene 

layers were affected by a number of normal faults that extend 

downward and are thought to be inherited from older 


Fig. 2. Time-depth curve and velocity analysis of Hashem-1 Well.

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Gravity interpretation

The Bouguer anomaly map (Fig. 6) shows dominance of  

the negative gravity values from 35 to 51 mGal all over  

the area, which may reflect a relatively thick sedimentary 

cover. The gravity field is largely variable from east to west, 

which may be associated with large lithological/structural 

variations in the subsurface. Qualitatively, the map contains  

a number of strong anomalous features of different amplitudes 

and sizes, running nearly parallel to the GOS trend. The high 

and low gravity zones are separated from each other by steep 

gradients, which strike in the north-west direction. Such high 

gravity gradient zones were traced on the Bouguer anomaly 

map by circles of Euler solutions, which tend to cluster along 

geological boundaries. The linear structures are closely asso-

ciated with locations of major bounding faults, which divided 

the area into a number of gravity zones of a north-west orien-

tation. Little or no solutions tend to cluster along the trans-

form-fault traces. Generally, the gravity maxima and gravity 

minima are closely linked with the major structural features in 

the area. For instance, the high gravity anomaly that exists 

along the offshore part corresponds to the pre-existing horst 

blocks extending from Amer to Gharib. The West Bakr oil-

fields (G, H, B and K) could also be correlated with high 

anomalous features. Westward, the gravity low corresponds to 

a broad sedimentary basin of “Khashaba low”, while the gra-

vity maximum in the south-western corner is related to base-

ment outcrop of the Nubian Massif.

To separate the gravity anomalies into their components,  

the Fast Fourier Transform matched filtering technique based 

on spectral analysis was applied to the Bouguer anomaly map. 

The isolation method depends mainly on the slopes of the spec-

tral analysis curve, where the frequency ranges give the most 

efficient estimation of the gravity anomalies to fit the deep and 

shallow constituents. The separation using the energy spec-

trum of potential field data is highly advantageous over any 

other known conventional method (Sadek 1984). The poten-

tial anomalies in the space domain are transformed into 

Fig. 3. Structure-depth contour map: a — on top of Zeit Formation/top Miocene section; b — on top of Rudeis Formation/base of 


Fig. 4. Interpreted geoseismic line BK-1039 along NE–SW direction.

Fig. 5.  Geological cross section A–A', taken in the NE–SW direction, 

passing through EPH-1X, EPG-1X, GN-1 and E. Bakr-2 wells.

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frequency domain and the various parameters of anomaly 

sources are derived from the characteristic properties of the 

amplitude. Moreover, the spectral method provides the possi-

bility of doing depth measurement using gradients when no 

anomalies are expressed in the profile. The theoretical assum-

ption was studied by many authors. Spector & Battacharyya 

(1966) computed the energy spectrum and autocorrelation 

function of FFT for the simple magnetic model. Spector 

(1968) shows that the energy spectrum is greatly affected by 

surface highs above the basement, the greater the high the lon-

ger the wavelengths. The anomalies and autocorrelation func-

tions associated with simple 2-D (two-dimensional) and 3-D 

(three dimensional) magnetic and gravity models were subse-

quently transformed mathematically to the frequency domain 

by Spector & Bhattacharyya (1966). Spector & Grant (1970) 

reported some undesired effects on the calculations of the 

energy spectrum which derive from processes of digitization, 

quality and density of data. Dimri (1992) and Blakely (1995) 

estimated the mean depth of the interfaces considering the log 

of power of the Bouguer gravity spectrum as a function of 

wavenumber/frequency assuming uncorrelated distribution of 

sources (Spector & Grant 1970) or scaling nature of sources 

(Pilkington et al. 1994; Maus & Dimri 1994, 1995, 1996). 

Isolation of the gravity anomalies was carried out using 

Geosoft 2007 software and the results are shown in Figure 7a. 

Two linear segments of the energy decay curve with distin-

guishable slopes were used to isolate the regional and residual 

components, where the regional effects are evident at larger 

wavelengths and the shallow/local effects are evident in short 

wavelengths. Accordingly, the energy spectrum curve invol-

ves two parts: a very steep part at low wavenumbers (0 km



< wavenumber < 0.230  km


) and a less steep part at high 

wavenumbers (0.230 km

< wavenumber < 1.400 km



The depth can be estimated by  

where: (h) is depth; 

(s) is slope of log (energy) spectrum. Accordingly, the slope of 

the line fitted to the upper part of the spectrum curve is used to 

estimate the average depth of deep sources (~24 km), and the 

average depth to shallow sources (~4.5 km). The frequency 

bands related to the regional and residual gravity components 

on the energy spectrum were used through the band-pass fil-

tering. By bandpass, all wavenumbers between limited values 

would be saved or passed while the rest would be cut, so that 

the bandpass filter passes frequencies within a certain range 

while rejecting frequencies outside this range.

The high-pass gravity map (Fig. 7b) shows a set of sharp 

anomalous zones which are closely correlated with the shallow 

components.  The residual gravity field reveals improvements 

of the gravity anomalies linked with near-surface con stituents, 

but still lack details about deep-seated constituents. Thus, it is 

Fig. 6.  Bouguer anomaly map of the study area overlapped with  

the Euler deconvolution solutions (SI = 0, window size = 10).

Fig. 7. a — Power spectrum of the Bouguer anomaly map, with two  

linear segments of the energy decay curve with distinguishable slopes. 

b — High-Pass filtered map of the Bouguer gravity data at the high 

frequency and short wavelength range (230 cycle/km < wavenumber 

<1400 cycle/km).

h = − s / 4π



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, 2019, 70, 5, 373–385

found that the filtered map does not meet the requirements for 

studying the deep source structures.

Combined gravityseismic interpretation

In fact, both seismic and gravity interpretations are unable 

to reflect the deep causative source bodies due to strong 

shielding of the evaporitic rocks. The large thickness of 

Miocene Evps inhibits even the strong seismic energy to 

refract into the deeper horizons. Likewise, gravity anomalies 

are very sensitive to the thickness/density variations of the 

anhydrite-salt rocks. Consequently, it is deemed necessary to 

use an integration of the gravity and seismic in combination 

with well information to interpret deep source structures. This 

was done through applying the gravity stripping technique 

based on Hammer (1963) and Otsuka & Ogawa (1977).  

The gravity stripping depends on computing the gravity effect 

of a certain rock unit or time-rock unit defined by its thickness 

and density, then subtracting the effect from the integrated 

effect of the whole sedimentary section, as has been indicated 

by the Bouguer anomaly map, to get the pre-unit effect, and so 

on for the other units (Parker 1973). This requires an exact 

knowledge of the rock properties (geometry and density) to 

get good results.

The workable equation applied for each rock unit in the 

sedi mentary succession is

where Δg


 is the gravity effect of a certain rock unit in mGal; 

G is the international gravitational constant (6.67 × 10







Δρ  is  the  density  contrast  between  the  formation  density  

and the basement density in g/cm


; H


 is the unit thickness  

in cm.

The depths to the upper and lower surfaces of each rock unit 

have been seismically controlled. The thicknesses of the layers 

were calculated through a number of seismic isopachs (thick-

ness-grids). The formation density was taken from the density 

logs (gamma–gamma) for about eleven drilled wells, which 

were checked and used to construct a set of density gradient 

maps (Fig. 8). For every drilled well, the density log measure-

ments against each formation in the succession were ave raged 

and used as a single value for that formation. Figure 9 is an 

example for density information derived from the Bakr-2 and 

Hashem-1 wells. They represent two different time intervals, 

Bakr-2 Well displays densities of the PM section while 

Hashem-1Well shows the densities of the Miocene section. 

From the obtained thickness-grid and density-grid of each 

rock unit, the gravity effect for every layer was calculated and 

subtracted/removed from the observed data. The process was 

repeated for all formations in the sequence from the surface to 

the datum. In each case, two maps were obtained; the first map 

represents the sum of gravity effects of all formations above 

the datum. The second represents the gravity effect of the rock 

units below the datum. At last, a stripped gravity map on top 


= 2π G Δρ H



of PM was obtained which could be correlated with the 


Generally, the gravity effects of the Evp group are given by 

The stripped gravity map on top of PM sequence can be 

given by the relation

The above relations were applied using Surfer 13 Golden 

Software where the outputs were calculated and contoured.

Figure 10a shows the sum of gravity effects due to sedimen-

tary section above the datum (surface layer, Miocene Evps and 

Lower-Miocene clastics). The map is generally characterized 

by a negative gravity effect (from 1 to 7 mGal), that 

increases progressively from north-east to south-west. 


Figure 10b presents the stripped gravity map on top of the PM 

after removing the effects of the overlying formations from  

the original data. The map shows the largest gravity effects 

with negative values (from 20 to 49 mGal), which gra-

dually increase toward the south-east. This may reveal that  

the gravity effect of the PM is the main source of negative 

gravity present in the Bouguer map. 

The correlation coefficient that measures and determines  

the degree to which two variables are associated, was calcu-

lated between the gravity data before and after the stripping 

process. The results in Table 1 shows a positive correlation 

between the data of Bouguer and data for gravity effects on  

top of the PM. Meanwhile it gives negative correlation versus 

 others, particularly against the Evp group (Fig. 11). The posi-

tive value may suggest that the deep causative sources have 

the highest gravity effect, while the negative value may reflect 

a gravity effect in the opposite direction. Accordingly, it could 

be stated that, despite the large gravity effect of shallower 

structures, the deeper sources are still the most effective.

The horizontal gradient method (Phillips 1998) was applied 

on the stripped map (Fig. 10b) with the goal of revealing  

the hidden faults. The horizontal gradient maxima delineate  

the approximate locations, lengths and trends of the major 

faults on/within the basement rocks. Figure 12a displays two 

main trend patterns, cut cross the study area in the NNW 

(Clysmic) and ENE (Syrian arc). Generally, the structure on 

top of PM in Figure 12a shows dense fractures/faults if com-

pared with the Miocene structure in Figure 3. These linea-

ments were traced and statistically analysed (Affleck 1963) in 

terms of number, length and direction to reveal the basic 

 tectonic trends. Figure 12b shows two major peaks in the 

N0–30°W and N50–70°E directions. These two major peaks 

demarcate two main fault systems (cross-faults) which are 

predominant on the basement surface and divided the area into 

segments. The structure suggests presence of a NE–SW tren-

ding shear fault zone, with a left-lateral displacement, in the 

centre of the map. 



= 2π G ((







) h


+ (







) h




+ (







) h


+ (







) h


).                                     (2)



= Δg





+ Δg






).                                 (3)

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, 2019, 70, 5, 373–385

Stress fields and proposed tectonic model

According to the trend analysis results shown in Figure 12b 

an attempt was made to deduce the stresses and their possible 

relations with the crustal deformation and plate motion which 

affect the area (in-house software). Detected trends interpreted 

from the gravity map after stripping were used in combination 

with the idea of possible reorientation of stress (McKinstry 

1953) to shed light on the direction and intensity of stresses 

explaining the tectonism of the area. Figure 12c shows that 

Precambrian basement is controlled by two different forces 

acting on the region for long periods of time. The area was 

subjected to a regional compressive stress in NNW direction, 

contemporaneous with ENE extension force. The first 

compressive stress is oriented N10°W. It is believed to be 

associated with the collision between the African and European 

plates. This principal axis of stress seems to have shifted 

toward the west during different geological times, with a wide 

azimuth of N0–30°W. Structurally, this stress force resulted in 

two sets of primary and secondary shear fractures. The main 

structural lines produced by the first stress force were formed 

as primary shear fractures in a NNW–SSE direction. The ENE 

folding/thrust faulting was also formed perpendicular to the 

greatest principal axis of this type of stress force. Besides, the 

N35°W (Clysmic) and N15°E (Aqaba) arose as secondary 

strike-slip faults, where the angle between these two sets of 

the sinistral and dextral fault systems is about 50° (Anderson 

1951). The second force affected the area is the tensional 

N85°E, which is attributed to the movement of the Arabian 

plate relative to the Nubian Plate. The structural elements pro-

duced by this tectonic extension primarily take the form of the 

N5°W Clysmic fault system. The N50–70°E trend is consi-

dered as a second order fault system produced by these exten-

sional forces. The rejuvenation of these tectonic trends during 

different episodes may explain the high peak of N50–70°E. 

The principal axis of this tensional force seems to have rotated 

counter-clockwise. A reasonable model explaining the tecto-

nism in the GOS-Red Sea region was proposed by Bayoumi 

(1983), to sketch the expected structural/tectonic development 

during its geological history.

Deep structural elements

The Euler homogeneity relationship was used as an auto-

mated method to derive the plan location and depth estimation 

of buried objects from the gridded-data. Euler’s homogeneity 

equation relates the gravity field and its gradient components 

Fig. 8. Formation density gradient map of the (a) Post-Miocene and Miocene evaporites, and (b) Lower-Miocene Rudeis sands. 

Fig. 9. Density log of (a) Hashem-1 Well, and (b) Bakr-2 Well.

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, 2019, 70, 5, 373–385

to the location of the source with the degree of homogeneity 

expressed as a structural index (SI; Thompson 1982). 


The struc tural index is a measure of the rate of change with 

distance of the field from the source (fall-off rate) and is 

directly related to the source dimension. Depth and structural 

index “solutions” provide information about the source of 

anomalies. The advantage of the Euler Deconvolution (ED) 

technique over other depth interpretation methods is that no 

particular geological model is assumed, and can be directly 

applied to large gridded data sets. The process reduces signifi-

cantly the time required for analysis of the data, it requires no 

prior knowledge of the source of gravitation/magnetization 

direction and assumes no particular interpretation model 

(Barbosa et al. 1999; Hinze et al. 2013). The method can locate 

or outline the confined sources, dykes and contacts with 

remarkable accuracy (Reid et al. 1990). Marson & Klingele 

(1993) applied it to gravity vertical gradients. Zhang et al. 

(2000) applied it to gravity tensor gradient data. The Euler 

deconvolution is used to confirm the modelling results, 

because it is both a boundary finder and a depth estimator 

(Reid et al. 2003).

The ED was applied to the stripped gravity map (Fig. 10b) 

to deduce the major structural elements on top of the PM/ 

basement rocks. Herein, the structural insides of SI = 0 were 

used to identify the rock boundaries and to estimate the depths 

of their sources (Thompson 1982 and Reid et al.1990).  

The calculation is running for window size (10) to obtain solu-

tions for different depths. 

Figure 13 provides the best solution for the block-faults 

with depths ranging from 0 to 4 km on top of the PM (without 

depth of the overlying). The interpretation is based on the 

 linear solutions that mark the geological boundaries and divi-

ded the area into blocks. These linear features tend to occupy 

zones of high gradients between anomalies. The distributions 

and depths were identified using coloured circles, overlapped 

on a shaded relief map. The white circles indicate shallow 

depths while red circles locate deep faults. Generally, the map 

reflects the complex structure on the basement surface through 

a set of major faults that are bordering the main structural fea-

tures in the area. Most of these linear representations (major 

faults) westerly-oriented in the NNW direction, parallel to the 

axis of the GOS. The Clysmic trend is not a long-continuous 

feature, but was dissected and displaced by faults oriented in 

Fig. 10. Gravity effect of the sedimentary section: a — above the datum due to the Post-Miocene surface layer, Miocene evaporites and Lower 

Miocene Rudeis sands; b — below the datum on top of Pre-Miocene.

Correl Coef between


Boug /Pre-Miocene


Boug /Rudeis


Boug /Evaporite


Boug /Post-Zeit


Table 1: Correlation coefficient results of different gravity effects

Fig. 11. Correlation coefficient between the gravity data before and 

after stripping process.

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, 2019, 70, 5, 373–385

an ENE direction. Some circles may be poorly-clustered, 

where there is no significant density contrast, while, the large 

condensation of circles may be due to dipping faults at large 

depths, or upward reactivation of these trends. Generally,  

the fault trends interpreted from ED in Figure 13 agree with 

those shown in Figure 12. In comparison with seismic results, 

the structure on top of the PM (Fig. 13) is more complicated 

than that at the Miocene level (Fig. 3).

Gravity modelling

The profile B–B’ was taken on the Bouguer anomaly map 

(Fig. 6) and on the stripped map (Fig. 10b) cutting across the 

central part in a SW–NE direction. The profile stretches for 

about 35 km, passing through four drilled wells (EPH-1X, 

EPG-1X, GN-1X, AMER-8) which are considered as controls 

and start points. The sedimentary layers were modelled as 2D 

units, where X-axis (strike-length) is long relative to Y-axis 

(depth). The sedimentary sequence consists of three main 

groups with different lithological characteristics; the post-Evps 

(~2200 kg/m


); Evps (~2800 kg/m


); pre-Evps (~2540 kg/m



The rock parameters were constrained by the seismic isopachs 

(geometry) and by information derived from boreholes (den-

sity). The basement density was found to be 2670 kg/m


where there is no magmatic intrusion in the area (Shabaan et 

al. 1984). The modelling process was done using GM-sys, 

included in the Oasis Montage Package (2007).

Figure 14a presents a model for the regional subsurface 

structure (shallow and deep) before doing the stripping pro-

cess. The upper part shows a good fit between the calculated 

and observed gravity profiles. The lower part exhibits great 

changes in the depth to the basement from the west to east, 

with a regional dip regime to the west. The basement surface 

is uneven/rough and loaded by thick and thin sedimentary sec-

tions that conceal beneath it more complicated structures. 

Generally, topography of the basement is in harmony with 

gravity anomalies where the swells and troughs are correlated 

with gravity maxima and minima, respectively. The model 

exhibits two deep basinal areas in both sides, these are; the PM 

basin to the west (Khashaba low) in parallel to the Nubian 

Massif, and Miocene basin to the east (W. Bakr basin) along-

side the GOS. These two basins are correlated with low gra-

vity anomalies, and are separated from each other by a ridge- like 

form (W. Bakr high). The structure exhibits a number of major 

faults separating the main topographic features. These normal 

faults arise on the basement surface, grow upwards into the 

PM section and mostly disappear into Miocene formations.

Figure 14b presents a sketch of the modelled profile after 

doing the gravity stripping. Here, the gravity effect due to the 

upper part of sedimentary cover was neglected since densities 

Fig. 12. a — Maxima of horizontal gradient magnitude of the stripped 

gravity map on top of Pre-Miocene. b — Rose diagram of tectonic 

trends as deduced from the stripped map on top of Pre-Miocene.  

c — stress-strain diagram of the study area based on statistical trend 


Fig. 13. Euler deconvolution results on top of Pre-Miocene as deduced 

from the stripped map.

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, 2019, 70, 5, 373–385

of the Miocene and Post-Miocene were found to be zero.  

The removal of the gravity effect of the shallower portion 

gives a chance to better model the deeper structures. The deep 

source structures were deduced through matching the observed 

and calculated profiles, after fixing the parameters of the upper 

portion. The top of the PM was determined through four 

drilled wells which act as control points, while basement sur-

face configurations were estimated by means of iterative fit-

ting. Generally, the model nearly shows the same structural 

conclusion obtained previously with some modifications on 

the basement surface. The pre-Evp section was divided into 

two sections; the Lower-Miocene Rudies Fm. and PM 

sequence (2440 kg/m


). The structure shows a noticeable 

increase in number of faults on the basement surface more 

than before. The Clysmic faults originate on top of the base-

ment and vanish into the Miocene before reaching the surface. 

The structure exhibits the same two basinal areas present in 

south-western part (Khashaba low) and north-eastern portion 

(W. Bakr basin). The uplifted area between these two basins 

(W. Bakr area) is characterized by the presence of three local 

sub-basins separated from each other by saddles, which 

corres pond to West Bakr fields (K, H, G and B).

Basement relief

Source Parameter Imaging (SPI) function was used here as 

an easy and quick powerful technique for interpreting the 

gravity data to calculate the approximate depths of gravity 

sources. The SPI method utilizes the relationship between 

source depths and the local wavenumber (k) of the observed 

field, which can be estimated for any point within gridded-data 

through the horizontal/vertical gradients. The method was 

 carried out to determine depth to causative sources, edge 

locations, source geometries and density contrast in the area 

(Nabighian et al 2005). One advantage of the SPI method is 

that the depths to the sources can be displayed as an image, 

which makes the task of interpreting the data significantly 

 easier. More elaborate description of the principle, theory, 

application and formulation of the method are given elsewhere 

(e.g., Thompson 1982; Roest et al. 1992; Nabighian & Hansen 

2001, 2002; Fedi 2005, 2007; Nabighian et al. 2005; Reeves 


Practically, the automatic calculations of source depths were 

done by using the SPI software included in the Oasis Montage 

Package 2007. The technique computes source parameters 

from gridded data of the Bouguer anomaly map based on the 

complex analytic signal, where depths can be estimated with-

out assumptions about the thickness of the source bodies.  

The depths to the gravity sources was determined through  

a number of pre-processed grids dx, dy, and dz which were 

calculated and serve as inputs for SPI processing. The first 

order derivative was adhered to remove the deep crustal effect 

from the original data. The upward continuation for small 

height was applied because the SPI is very sensitive to noise 

and interface effect (Nabighian et al. 2005). However, the SPI 

has the advantages that it produces a more complete set of 

coherent solution points and is easier to use. The accuracy of 

SPI  is  about  ± 20 %  in  comparison  with  the  real  data  from 

boreholes (Roest et al. 1992).

The results of the depth estimation obtained from applica-

tion of the SPI method (Fig. 15) exhibits a rough basement 

surface, with great variation in depth to basement rocks which 

ranges between 2.25 and 5.75 km. The map delineates three 

main basinal areas corresponding to the depressions of 

“Khashaba” to the west, and the “W. Bakr” toward the north-

east, and “W. Gharib” to the south-east. These large basins 

were separated from each other by a broad ridge (West Bakr-

Gharib ridge) of relatively low depth. Such basement uplift is 

considered a focal point in trapping oil from the adjacent 

basins. Moreover, three narrow sub-basins (Hoshia, Farag and 

Arta) occupy the northern portion, and are separated from 

each other by structural highs/saddles. All basins/ridges are 

nearly trending in the north-west to south-east direction, paral-

lel to the GOS trend. For comparison, Figure 15b displays  

the approximate depths to gravity sources as deduced from  

the Bouguer anomaly map (i.e., before stripping). Generally, 

the map exhibits nearly the same structural conclusion that 

exists in the former map, but with less details. The disparity 

between these two figures could be attributed to the gravity 

effect of the near-surface constituents.


The seismic lines interpretation has shown a lack of infor-

mation that comes from the deep subsurface section, while 

exhibiting a strong reflection from the relatively shallow part. 

This is logical since the high reflectivity of the Evps makes  

it difficult to trace any reflections at large depths, where  

Fig. 14. Gravity modeling along profile B–B' taken (a) before doing 

layer-stripping on Bouguer anomaly map, and (b) after doing layer- 

stripping on top of Pre-Miocene.

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, 2019, 70, 5, 373–385

the continuity of the seismic events became very bad beneath 

the base of the Evps particularly in the north-eastern side 

where the thickness of the Evps significantly increases toward 

the GOS. Thus, the structural maps reveal few or no faults 

penetrating the base of the Evps, which may be contrary to  

the reality.

The study demonstrates how gravity interpretation can be 

enhanced when integrated with seismic data and borehole 

information in order to reveal the deep structural configura-

tions. The integrated interpretation has overcome the short-

coming of the filtering techniques and also seismic attenuation. 

The crosschecks between the gravity, seismic and well logging 

were worked out to eliminate ambiguity as well as non-unique-

ness of the gravity data interpretation.

The gravity anomalies are positively affected by the anhy-

drite of the Ras Malaab Group and/or dense limestone of 

Gharandal group. Meanwhile, they are negatively influenced 

by the low density halite/salt rocks, and by the Lower-Miocene 

Rudeis sands. The non-linear distribution of the density with 

depth makes most of the filtering methods, especially in space 

domain, ambiguously interpreted when exploring the deep 

prospects. So, it is found that the filtering methods applied in 

this study are rather insufficient in delineating the deep struc-

tures/sources at the PM level, especially because seismic inter-

pretation is uncertain. The filtered maps provide primary 

evidence for deep-seated bodies but add no details. This only 

minimizes the effect of shallow bodies and somewhat enhances 

the major features. 

The correlation coefficient results are very variable with it 

reaching the maximum (0.35) against the PM, while attaining 

the minimum (0.34) against the Evps. The positive correla-

tion suggests that most of the gravity is sourced in the deep-

seated components (PM). However, negative values confirm 

that effects of these rock units act in reverse to reduce the total 


The horizontal gradient filter was able to disclose the deep 

fracture systems affecting the area, which were confirmed 

later by Euler deconvolution solutions that delineated loca-

tions of block-faults well.  In contrast to the Miocene struc-

ture, the PM was ruptured by fractures mainly in the NNW and 

ENE directions. Such cross-faults are definitely expected to 

control hydrocarbon migration and accumulation at the PM 

level, delineating new oilfields.

The statistical analysis of lineaments on top of the PM rocks 

show that the area was affected by two tectonic trends oriented 

in the N0–30°W and N50–70°E. These fracture systems are 

believed to be associated with two stress phases; a compres-

sive phase (NNW–SSE) and an extension phase (ENE–WSW). 

These two different tectonic forces are closely related to the 

Tethyan plate tectonics. The first tectonic trend (NNW to NW) 

is considered one of the oldest tectonic trends in Egypt 

(Meshref 1990). The structural elements belonging to this tec-

tonic phase are believed to be predominant and most probably 

developed in Precambrian compression. It was formed during 

the Pan African orogeny, and rejuvenated during Tertiary–

Quaternary times due to active tensional movement of Arabia 

relative to Africa (Brown & Coleman 1972). It is believed that 

the ENE folding and thrust-faulting stage was primarily 

formed in response to the compression force from the NNW–

SSE direction. They were developed and reactivated during 

the Early to mid-Paleozoic (Caledonian cycle) by rift phases 

resulting from the opening of the Paleo-Tethys under the in -

fluence of NE–SW directional tensional forces (Meshref 1990). 

From the other side, the N50–70°E trend, which is genetically 

related to the Syrian Arc trend, is believed to be one of the 

Precambrian/Paleozoic tectonic trends that affect the Egyptian 

basement and were probably reactivated with the progress of 

Fig. 15. Basement relief map of the study area as deduced by source 

parameters imaging (SPI): a — after gravity stripping; b — before 

gravity stripping

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, 2019, 70, 5, 373–385

geological time (El-Emam 1990). The rejuvenation and inten-

sive deformation of this tectonic trend happened during the 

Late Cretaceous, represented by the NE, ENE & E–W narrow 

elongated domains of folds and faults (Sehim 1993).

In contrast to the seismic analysis, the modelling after gra-

vity stripping reveals complex structural features at the PM 

level. The basement surface is uneven and contains swells, 

troughs and block-faults, of different tilts and throws. The remo-

val of the gravity effects of the upper sedimentary layers, led 

to the appearance of three gravity highs in the mid-area, which 

are linked with three PM uplifts. It is reasonable to state that 

the gravity high anomalies are closely related to basement 

uplifts, while gravity lows are due to deep sources/structures 

not shallower than 3 km.

The source parameter imaging method confirms that the 

actual subsurface situation deduced from the stripped map is 

not as simple as assumed by the Bouguer anomalies. The map 

displays uneven topography of the subsurface with presence 

of several basins and ridges at different depths, which are 

associated with uplifted and down-faulted basement blocks.  

The reversals along the sides of the basin-flank highs may give 

the chance of discovering new oilfields on the PM level.


The results obtained from the different geophysical applica-

tions were integrated to give us a general view on the deep 

source structures and shed more light on geological 


The seismic isopachs indicate that the thickness of the sur-

face layer gradually increases toward the GOS, Miocene Evps 

increase excessively in the north-east direction, whereas the 

Lower-Miocene Rudeis sands are well-developed in the south- 

western part.

The study indicates that the structure on top of the PM is 

much more complicated than that at the Miocene level.  

The integrated interpretation has succeeded in delineating 

 several cross-faults on the basement surface which are diffi-

cult to see on the seismic sections. This indicates dominance 

of the NNW-trending faults (Clysmic trend) which were 

obliquely cut and displaced by ENE-trending faults (Syrian 

Arc trend). 

The structural inferences have depicted that faulting plays 

the main role in complicating the geological setting of the 

area. Lineament analysis delimits two fault systems of first 

order magnitude oriented in the N0–30°W and N50–70°E 

directions. These two major structural-tectonic trends are the 

most widespread on the basement surface, and divide the area 

into segments.

The stress-strain diagram assumes the area has been sub-

jected to two main cycles of tectonism during its geological 

history; the N5°W compressional force and the N85°E ten-

sional force, producing two pairs of primary and secondary 

fracture systems. The impact of such tectonic forces on the 

basement surface may suggest that, although the principal 

phase of opening of the GOS occurred in the Miocene age,  

the forces that caused this opening are older.

The models confirm a simple structure at Miocene level in 

contrast to a complex structure at the PM level. They demon-

strate a highly-fractured basement surface, overlain by a hete-

rogeneous sedimentary cover, with no indications of magmatic 

intrusions penetrating into sediments. They indicate that base-

ment ridges are the source of gravity maxima while the PM 

basins are the main source of gravity minima.

Acknowledgement: Author would like to express his gratitude 

to three anonymous reviewers for their constructive  reviews and 

comments, as well as to Miroslav Bielik for editorial handling 

and suggested improvements to the manuscript.


Affleck L. 1963: Magnetic anomaly trend and spacing patterns. 

 Geophysics 28, 3, 379–395.

Anderson E.M. 1951: The dynamic of faulting and dyke formation 

with application to Britain. 2n


 Ed. Oliver and Boyd, Edinburgh, 


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pretation: a case study from Ras Budran oilfield Gulf of Suez 

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28°21’43.3” N



55’21.9” E




22’17.9” N



52’49.8” E




22’13.2” N



57’23.0” E




25’21.4” N



51’42.8” E




25’51.6” N



53’44.5” E

East Bakr 2



28’28.5” N



59’29.8” E




29’23.2” N



53’44.7” E




25’14.1” N



57’04.8” E




32’15.1” N



01’38.9” E

Amer 8



33’12.8” N



57’55.7” E

HH 83 1



36’36.8” N



56’50.1” E

HH 83 2



38’33.3” N



55’39.9” E