Integrated geophysical techniques applied for petroleum basins structural characterization in the central part of the Western Desert, Egypt

: The study area is situated in the central part of the Western Desert of Egypt between latitude 28°00 ′ to 30°00 ′ N and longitude 25°00 ′ to 30°00 ′ E. That region is distinguished by a featureless plain that is divided by depressions in Siwa, Qattara, and Bahariya. The purpose of the current study is to study the predominant struc - tures in the area and how they relate to basin structure. Utilizing aerogravity data ( Bouguer gravity, residual gravity, downward continuation, and Euler deconvolution ) and aeromagnetic data ( reduced to the northern Pole [ RTP ] anomalies and tilt derivative ) is essential to accomplish this purpose. Through both qualitative analyses, these data were submitted to various processing and interpreta - tion approaches. The subsurface structure con ﬁ guration trending in E – W, N – S, NW – SE, NE – SW, ENE – WSW, and NNE – SSW directions has been simulated by utilizing aero - gravity and aeromagnetic data. According to these maps, the study area is divided by around 44 faults. The study's ﬁ ndings indicated that the direction of the basement structure was almost NE – SW and N – S. The optimum Euler depth deconvolution at structure index, SI = 0 shows several features in the study area, including Sill, Dyke, Ribbon, and Step structures.


Introduction
The Western Desert of Egypt covers two-thirds of the total area of Egypt.After the Gulf of Suez region, it is the second most promising region in terms of hydrocarbon potential.The study area is located in the central part of the Western Desert of Egypt.Because of the Nubian aquifer groundwater, which is regarded as the only water supply for sustainable development in such locations, this place is considered a critical water resource in Egypt.Conversely, the Western Desert's northern and central regions make up the second largest oil-producing region.The Western Desert, which includes the Mediterranean littoral zone and the New Valley District, makes up 68% of Egypt's total land and is regarded as a strategic water supply in Egypt due to the Nubian Aquifer's groundwater, which is the only source of water considered necessary for such zones' sustainable development.On the other hand, the second-most significant region for oil production is the northern and central parts of the Western Desert.
A zone of approximately 681,000 km 2 is formed by the Western Desert of Egypt, making up about two-thirds of the whole space of Egypt.
The study area is situated in the central part of the Western Desert of Egypt between latitude 28°00′ to 30°00′ N and longitude 25°00′ to 30°00′ E (Figure 1).That region is distinguished by a featureless plain that is divided by depressions in Siwa, Qattara, and Bahariya.
In the recent study, geophysical shaped data in the form of reduced to the northern pole RTP magnetic map and Bouguer gravity map were gathered from oil corporations, including the Gulf of Suez Petroleum Company (GUPCO), the Egyptian General Petroleum Company (EGPC) [1], British Petroleum Corporation (BPC), and the public data.
Locating faults, minerals, geothermal or petroleum resources, and groundwater reservoirs can be done with the aid of gravity and magnetic investigation.Potential field studies can rapidly and easily cover a huge amount of land at a small cost.The first aim of investigating potential field is to save a better understanding of subsurface geology; the techniques are relatively inexpensive, non-intrusive, and ecologically speaking.They are also passive, meaning no power is required to gather data from the land.Walking traverses are also possible with small portable instruments (gravimeter and magnetometer).
Reduced-to-pole (RTP) magnetic map and a contoured Bouguer gravity anomaly are often the end products of gravity and magnetic surveys.These maps are appropriate for use with all kinds of anomalies for geological interpretation.Bouguer magnetic and gravity anomaly maps resemble topography contour maps in appearance.They are circular, elongated, and round zone of low-and high-gravity or magnetic values.Additionally, they might exhibit linear bands with a steep gradient that are not essentially connected to any topographic characteristics.If the geology of the zone being studied is understood, it is easy to establish a preliminary and qualitative interpretation simply from looking at a map.Gravity or magnetic highs are in several zones linked to uplifted blocks or anticlines; both are structures that get denser older rocks closer to the surface.In other zones, highs might be because of heavy basic intrusions of high magnetic susceptibility.Reciprocally, relatively light acidic intrusions and sedimentary basins sometimes make gravity or magnetic lows.The belts of the remarkable gradient are made by steep contact among various rock types, which occur across fault planes.
A brief of the previous geophysical works carried out in different parts of the Central part of Western Desert is Awad et al. [2] analyzed the gravity data for the northern plateau of Bahariya Oasis and concluded that the main fault trends in this zone are N 35°W, N 45°E, N 75°W, N 5°W, and N 15°E.These trends are supposed to control the foundation mosaic of the Bahariya Oasis zone.
Younes [3]: The Shushan Basin of the northern Western Desert of Egypt showed two kinds of exceptions (A) and (B), as well as two groups of crude oils, based on the organic geochemical features of crude oils and concerning origin rock extracts.
Salim et al. [4]: Modernization of the structural and geological concepts in the old development zone of Abu Sennan resulted in the discovery of two medium sized oil and gas fields in the zone thought before as of lost interest.This study concluded the major stratigraphic play in the trapping hydrocarbon mechanism in the Western Desert.The trap boundaries are found explained by the lateral change of the sandstone facies to the shale barrier.Vertical and lateral separations of the reservoirs put separate hydrocarbon contacts and different reservoir pressures into practice.
Mohamed and Abdel Zaher [5]: The basement surface is not linear (uplift and basin system, and variability in depths from 900 to 7,000 m), according to the interpretation of magnetic modeling and 2-D gravity in the northern Western Desert.
The aims of the present study are to (1) provide detailed subsurface geological structure data in the study area (tectonic framework) and (2) identify numerous structures like Dyke, Ribbon, Sill, and Step in the study area using the best depth of Euler deconvolution at SI = 0. To accomplish these purposes, geophysical data in the form of aerogravity data in the shape of Bouguer gravity, residual gravity, downward continuation, and Euler deconvolution and aeromagnetic data in the shape of tilt derivative of reduced to northern Pole (RTP) maps are utilized.
2 Geology and structural setting

General stratigraphy
The sedimentary sequence in the geologic column of East Bahariya Concession ranges from the Cretaceous to the Miocene.The geologic section is composed of changeable sedimentary carbonates and clastics cycles (Figure 2) [6,7].The north Western Desert's Paleozoic sediments are formed from interbedded shale and sandstone in addition to slight beds of carbonate [8].Norton [9] tried to utilize the names of rock formations from Libya to Egypt, including the Acacus (Silurian), Gara Dahm, and Gargaf (Carmbro-Ordovician) Formations (carboniferous).In the Egyptian section, it was not easy to distinguish between these formations.
The Jurassic comprises a singular transgression cycle consisting of three group rocks that overlap each other, the west and south of the continental Eghei deposits and the corroded Paleozoic sequence.Additionally, the Kattaniya area basement is presumably overlain unconformably by the Jurassic strata, which are, in turn, unconformably overlain by the lower part of the Lower Cretaceous (Barremian-Aptian) layer.The Khatatba Formation permanently lies on top of the Wadi Natrun.Although the lowest portion may be Early Jurassic in age, this formation is Middle Jurassic in age.Only the northern boundaries of the North Western Desert and its eastern portion are known to include the Wadi Natrun Formation.The Khatatba Formations name was suggested by Norton [9] to refer to the back-reef facies which predominated in the Middle Jurassic.A thick carbonaceous shale pattern made it up with interbedded coal seams, limestone streaks, and porous sandstone.The following units are used to represent the Lower Cretaceous: Alamein Dolomite Member is well-marked, and it is a part of the Alam El Bueib Formation.This sandstone unit has sporadic limestone layers in its upper section, and frequent shale interbeds in its bottom portion.In 1967 Norton introduced the Shaltut Formulation to illustrate the argillaceous sandstone rocks of the Neocomian-Barremian-Early Aptian stages that underlie the Masajid Formation in the northern Western Desert.Alamein Dolomite Member is a distribution unit famous throughout Arabia and North Africa, after Norton [9] and Abdine [10] elevated the unit's grade from member to formation.It is composed of vuggy porosity and light brown firm microcrystalline dolomite.A unit of fine to coarse-grained sandstone with supporting carbonate and shale interbeds makes up the Kharita construction.There is always a link between the portion that designates this unit and a specific amount of amorphous silica [9].The following units are used to represent the Upper Cretaceous: At least 170 m of the Bahariya Formation are exposed, and it is divided into three groups (from top to bottom): El Heiz, Gebel Dist, and Gebel Ghorabi.The Abu Roash "G" Group of the underlying Abu Roash Formation may very well be equivalent to the El Heiz Group in numerous wells in the northwestern Desert.Formation of Abu Roash with sandstone and shale interbeds, the sequence is primarily composed of limestone.The typical Abu Roash construction to the north of the Giza Pyramids is the type of locality, and it has been the focus of numerous investigations since the classic study [11].Formation of Khoman is a remarkable unit with abundant chert bands and snow-white chalky limestone.The typical sections are the Ain Khoman, scarp west, and Bahariya Oasis's southwest.The Apollonia formation, a Paleocene to Middle Eocene limestone unit with underlying shale components, represents the Cenozoic in the research zone.By Norton [9] and Abdine [10], this unit was known as the Apollonia Formation and El Gindi Formation, respectively.

Structures and tectonics
A deeper succession of graben belts and low relief horst, detached by big throw master faults, and broad Late Tertiary folds at a shallower depth make up the two structural systems that dominate the Western Desert.Most of the time, the largest structural depressions are half-grabens, which dip north, but they are also broken up into many smaller tilted blocks.The Abu Gharadig basin's edges, interior, and northern regions form a complex situation where flower structures, compression ridges, and reversal faults have been linked to Late Cretaceous wrench faulting [12,13].
The Western Desert, which includes the Mediterranean littoral zone and the New Valley District, records 68% of Egypt's total surface area and is considered a strategic water resource in Egypt due to groundwater from the Nubian aquifer, which is considered the sole source of water for sustainable development in such areas.The Northern and Central Western Desert, on the other hand, is the second most important oil-producing area.The Western Desert of Egypt covers around 681,000 km 2 and comprises over twothirds of Egypt's total land area.

Methodology and data acquisition
The best geophysical exploration tools are the gravity method and the magnetic method, which use surfacedecreasing disturbances in the Earth's gravity and magnetic fields to identify and determine the density and magnetism of rocks as well as to detect subsurface structures.The study's main objective was achieved through the analysis and interpretation of these data (Figure 3).The gravity and magnetic data were used to study the general picture of the subsurface structures of the study 4 Fast Fourier transform (FFT) and power spectrum techniques The Fourier method has been widely used since the development of the FFT computer algorithm.The Fourier Transform is simply a sum of harmonic sinusoids, which are equal to the sampled values at the sampled point.Each harmonic grants a mensuration of its absolute assist to the sum.While the phase angle establishes the site of the coordinate's source of the viable for the particular harmonic concerning the coordinate origin used in the transform calculation.The reverse Fourier Transform allows the easy back and forth between the special data domain and the special frequency or wave number domain [14].
In the current study, FFT mechanism was utilized to obtain the power spectrum of the gravity data.Power spectrum analysis was stratified to the gravity data to gain a moderate depth to the responsible subsurface geological sources (Figure 4).

Qualitative interpretation of the gravity and magnetic data
The objective of the qualitative interpretation of the gravity and magnetic anomalies is to design the subsurface structural features on the basement surface, which influence the overlying sediments.The interpretation process of the Bouguer gravity map or the magnetic map for any zone depends upon the magnitude, gradient, and position of the source of the anomaly.The anomaly's shape will give an idea about the structural nature of the buried body.On the other hand, the size or magnitude of the anomaly reflects some indication about the status of the buried causative body, such as its size, dip, and depth extension.Also, the sharpness of the anomaly is a task of depth since the depth of the source is one of the essential factors in the translation of magnetic data.The sharper anomaly means more rapidly falling from the maximum, and this denotes shallow source, while the deep sources  are characterized by the broader anomaly.Finally, the linearity of magnetic contours reflects the strike lines of elongated intrusive characteristics or the subsurface of huge faults.
5.1 Description of the gravity maps

Bouguer gravity anomaly map
From the Bouguer gravity anomaly map of the study area (Figure 5), the following can be concluded.
In the study area, different closed anomalies of various figures, amplitudes, and trends are predominant.These anomalies could be translated as a consequence of structures within the sedimentary cover and on the basement surface.
The gravity anomaly field in the zone ranges between an upper rate (7.1 mgal) at W and N of the central part of the study area and a lower rate (−39.1 mgal) at SW and the central portion of the study area.The high-gravity anomaly is fundamental because of the rising of denser basement rock, whereas the lower gravity rate refers to sedimentary basins.
The Bouguer gravity map of the study area can classify into positive and negative anomalies, detached by zero Bouguer value.The zone's northern part and extended northwestern border has positive gravity anomaly.The south part has a negative gravity anomaly.These two principle portions comprise two main tectonic units.From Cambrian to the Quaternary, the northern portion represents the faulted block of old sediments [6].

The residual gravity map
The residual gravity anomalies in the area can identify the local irregularities in the gravitational field, which reflect the local and shallow structures.These locals within the residual map can throw more light on small-scale structures that are not easily detected from the Bouguer gravity map.In the present research, the remaining gravity map (Figure 6) was obtained from the least square technique [15] and the wavelength filtering technique of a high pass filter.These techniques were stratified by using the available computer programs [16,17].In the case of applying a great pass filter with dismissing of 0.002837 km −1 , the resulting map (Figure 6b) showed short wavelength and high-frequency anomalies that are considered as remaining compartments.Large numbers of local and small anomalies are observed in the E-W, NW-SE, and NE-SW directions in convenient with the chief construction directions prevalent in the survey zone.Moreover, the residual abnormality map of the order obtained from the square technique (Figure 6b) showed the same anomalies gained from the high pass filter with good resolution.

Downward continuation
Three downward continuation maps were structured using grid spacing of 250, 500, and 1,000 m (Figure 7).

Euler deconvolution
The depth and location of magnetic source anomalies are estimated using the Euler deconvolution.The computerized method was developed by Thompson [18].An instrument for potential field interpretation is Euler deconvolution, also known as Euler depth deconvolution.This technique is utilized to detect the deepness of the anomalies source and structural index of a list of anomalies.Each solution uses the Euler deconvolution method.The best solutions that produce the data are concentrated in certain locations but not dispersed throughout the region.The field rate reduction with space from the source is measured by the structural index [17].
The Euler deconvolution equation is given by Reid et al. [19]: Equation (1) can be rewritten as: In the current study, the structural index used are 0, 1, 2, and 3 to choose the better settling on the remaining gravity anomaly map at SI = 0 shown in Figure 8, SI = 1 shown in Figure 9a, SI = 2 shown in Figure 9b, and SI = 3 shown in Figure 9c.The structural index 0 grants best solution than structural index 1 and 2, as the data are focused at some regions in the study zone not divided the whole zone as SI = 1 and SI = 2.The better solution at SI = 0 refers to several structures like Step, Ribbon, Sill, and Dyke structures in the study zone.Reduction to the northern magnetic pole removes the inclination effects from the magnetic data due to the obliquity of the magnetic field.As a result, connecting the magnetic anomalies to their true reasons becomes simple, making interpretation both simpler and more accurate.The study zone's RTP magnetic map (Figure 10) shows that.
Generally, the source of the positive in the zone may be due to an uplifted basement rock and high magnetic susceptibility.
The shape of the magnetic anomalies in particular was oval to elongate, and some anomalies were nearly rounded.Other magnetic anomalies were uneven and did not reveal any important style, which may refer to lateral homogeneities in magnetic peculiarities.The main magnetic anomaly style is aligned along the N-S direction and the northern portion of the study area, but some magnetic anomalies have NE-SW and NW-SE directions.
The anomalies' sharpness varies from steep at the northwestern and some portions of the north, where the basement rocks are being superficial and gentle and flat at the central and northeastern portions of the survey zone, where thebasement rocks are deeper.
The magnitudes of the magnetic anomalies vary from place to place throughout the study area.It varies from 235.5 to −2.3 nT in the northwestern parts and from 235.5 to −215.7 nT in the central and northeastern parts.
Residual magnetic map: the residual anomalies map carried out by least squares polynomial fitting of second order, and a high-pass filter with cut-off is shown in Figure 11.It performs structural characteristics of intermediate and shallow depths.

The tilt derivative map
Miller and Singh [20] and Verduzco et al. [21] suggested the analytic signal method; the tilt derivative method, also known as the tilt angle method, is a refinement of all that approach.The horizontal gradient amplitude (first horizontal derivative) of the tilt angle is used by the tilt derivative to determine the location and depth of vertical magnetic contacts without knowing the source configuration [22].The advantage of the tilt derivative is its ability to display the zero contour line on or near contact.As defined by the TDR: Figure 12 shows the tilt derivative map (TDR) gained from stratifying the filter on the RTP magnetic map.From this map, it can be seen that the magnitude of tilt anomalies of RTP data ranged from 1.3 to −1.4 radians.
The tilt derivative anomalies have the attractive peculiarities of being positive over sources, crossing via zero at or close the margin of a vertical side source and being negative outside the source zone [20].The tilt derivative anomalies are similar to the first vertical anomalies in the shape of anomalies and major structure trends.The similarity between them reflects and confirms the subsurface structures and their direction are extracted from these maps.

Results and discussion
Gravity and magnetic data have provided useful tools for mapping inaccessible areas or terranes mostly covered by younger sedimentary cover succession.To characterize the first-order structural architecture and the poorly understood basement geology of the study area, we used analysis and interpretation of these data.
The qualitative interpretation of the aerogravity anomaly map and aeromagnetic map of the study area and its derivative from Bouguer gravity anomaly map, residual gravity map, downward continuation, and Euler deconvolution and Reduction to the northern magnetic pole, tilt derivative map (Figures 5-12) showed that main positive and negative anomalies have NE-SW extension as referring to the main structure trend.These anomalies' respective centers were found in acidic and basic rocks.However, the line between these negative and positive anomalies might be a geological contact or a fault.And it is possible that hydrocarbon is present in these anomalies and that the faults influence the reduction of hydrocarbons.
The gravity, magnetic, and derivative trends in the study area were concluded from the Bouguer map and RTP map using the Gay method [23].The trend analysis of the aerogravity and aeromagnetic maps can be explained in Figures 13 and 14.This map indicates that about 44 faults analyze the study zone.It is obvious that the most dominant trends are the E-W, N-S, NW-SE, NE-SW, ENE-WSW, and NNE-SSW trends.The N-S and nearly NE-SW trend is persistent to deeper subsurface standards and is regarded as the regional trend in the study area.
Each solution uses the Euler deconvolution method.The study area's best solutions, which produce the data, are "not dispersed throughout the territory" but rather concentrated in certain locations.The best Euler depth deconvolution at SI = 0 points to several structures such as Ribbon, Dyke, Sill, and Step structure.

Conclusion
In conclusion, the study was conducted by applying the quantitative interpretation of aerogravity and aeromagnetic to the study area using some types of filters in order to detect sedimentary basins and geological structures likely to contain hydrocarbons, the most important results and main conclusions of the current study are: 1.The best Euler depth deconvolution at SI = 0 that pointing to several structures like Ribbon, Dyke, Sill, and Step structure.2. The study area is impacted by groups of fault systems that takes different directions E-W, N-S, NW-SE, NE-SW, ENE-WSW, and NNE-SSW.3. The basement structural direction reveals that the N-S and nearly NE-SW trend are the dominant older fault systems in the survey area.4. In negative anomalies with a thick sedimentary cover, hydrocarbons are probably present. 5. Investigation of petroleum exploration in areas of negative anomalies.

Figure 1 :
Figure 1: Location map of the studied area.

Figure 2 :
Figure 2: Generalized lithostratigraphic column of the Western Desert of Egypt.Modified after Mohamed (2016).

Figure 4 :Figure 3 :
Figure 4: Power spectrum of magnetic and gravity anomaly maps.

Figure 5 :
Figure 5: Bouguer gravity anomaly map in the study area.

Figure 6 : 7 Figure 7 :
Figure 6: (a) Residual and (b) high pass filter of the gravity maps.

Figure 11 :
Figure 11: (a) Residual and (b) high pass filter of the magnetic maps.

5. 2
Description of the magnetic maps 5.2.1 Reduction to the northern magnetic pole

Figure 12 :
Figure 12: Tilt derivative of the RTP magnetic map.