1 Introduction

Egypt comprises three primary petroleum provinces: the Gulf of Suez, the Western Desert, and the Nile Delta. The Egyptian Nile Delta is a remarkable sedimentation system in which the complicated interactivity between the Nile and the Mediterranean Sea determined its geologic history for over thirty-five million years [1]. It has a unique architectonic feature, with specific sedimentation patterns owing to processes of sediment transportation and post-depositional diagenesis processes [2, 3]. Shapes, orientation, sizes, continuity, net-to-gross, and reservoir characteristics are the outcome of the transportation identity of sediments with considering other factors such as basin configuration, depositional environment, and fluctuation of the eustatic, tectonics, and climate changes [4].

West El-Burullus (WEB) field is a gas discovery in the Pliocene sequence, located in the western offshore Nile Delta basin, south of the west Delta deep marine (WDDM) concession in the Mediterranean Sea (Fig. 1). It was discovered through the drilled WEB-1 well in the WEB area. The WEB field is located between latitudes 31° 30′ 50″ and 31° 38′ 30″ N and longitudes 30° 21′ 35″ and 30° 31′ 20″ E. It exhibits a combined stratigraphic-structural trapping style within the Pliocene Kafr El-Sheikh Formation. The reservoir is confined to marine turbiditic deposits. It is formed of silty and sandy levees sediments that are sealed vertically and horizontally (pinch out) by the Kafr El-Sheikh shale formation.

Fig. 1
figure 1

Location map of West El-Burullus area, showing the interpreted seismic survey and well location

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Pliocene sediments are represented by Kafr El-Sheikh Formation unconformably overly the Rosetta Formation (evaporites). Kafr El-Sheikh Formation consists of thick shale with sand beds deposited in an outer shelf marine setting [5]. These represent the main gas reservoirs deposited in low-stand systems as turbidites. Delineating the depositional system and constructing a rigorous geological-depositional model needs integrating various disciplines. In this study, multidisciplinary techniques are applied for mapping the concealed geologic features to clarify the WEB canyon characteristics and origin and its fairways that controlled the depositional system in the West El-Burullus gas field. The multidisciplinary approach used in this study comprises seismic structural setting, seismic geomorphology, sequence stratigraphy, and sedimentology.

Spectral decomposition is an efficient analytical method, which has significant success in clarifying turbidite systems, basin floor fans, and fluvial-deltaic environments and as a powerful tool for delineating subtle stratigraphic plays such as channels, overbank, levees, and fans deposits. Although stratigraphy resonates with a specific frequency depending on bedding thickness, the interpreter can visualize subtle changes, heterogeneity in thickness, and discontinuities, additionally, can predict bedding thickness quantitatively and precisely [6].

2 Geological Setting

The offshore Nile Delta is characterized by the presence of three major structural features consisting of a triangular megastructure. In the south, the Delta is restricted by an E–W trending fault named the Hinge line, which separates the continental shelf to the south destination, from the Nile Delta sedimentation system to the north [7]. The western Nile Delta is cut across by a major slip fault that is striking NE–SW direction namely the Rosetta fault. It splits the West El-Burullus concession into two uneven areas, Burullus high to the southeast while at the northwest a big half-graben has a regional dip to the southeast with a crest lying on Nile Delta offshore anticline (Fig. 2a) [8, 9]. The eastern Nile Delta is cut across by a major NW–SE strike-slip fault namely the Temsah fault which represents the counterpart of the Rosetta fault. On its hanging wall, it shows a prolific series of parallel tilted fault blocks [10]. All the trends are aligned parallel to the circum-Mediterranean plate boundaries and are thought to be genetic from the Tethy's history. This can be understood as follows: the Hinge line is a paleo passive margin, the Rosetta fault is a paleo transform fault, and the Temsah fault is a paleo rifting system [11].

Fig. 2
figure 2

Structural pattern of North Nile Delta sub-basins (a) [6], Paleo-geographic map illustrating the gross depositional environments at the boundary between the lower and middle Kafr El-Sheikh Formation successions (b) [17], and Regional geological cross section showing the tectonic framework and the stratigraphic aspects affecting the study area (c) [16]

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The Nile Delta Pliocene stratigraphy was discussed by many authors [12,13,14,15,16,17,18,19]. The Late Miocene-Early Pliocene Rosetta anhydrite is environmentally deposited in a sabkha condition. The Rosetta Formation is overlain by Abu Madi Formation which complies with fluvial-deltaic deposition to marine coastal sedimentation, generally deposited in SSE-NNW trending valleys. It is a fining-up formation composed of quartzose sands, infrequent conglomerates later becoming dominated by increasing clay interbeds toward the maximum flooding surface at the base of Pliocene. Otherwise, the lower-middle Pliocene sediments of Kafr El-Sheikh Formation correspond to deep water sediments deposited in outer shelf or slope environments related to the study area (Fig. 2b, c) [19, 20]. It is composed of silts and muds with infrequent thin interbedded sands (turbidites). Kafr El-Sheikh Formation is of Lower-Middle Pliocene age and comprises deep water sediments deposited in outer shelf and slope environments. It constitutes of muds and silts with occasional thin interbedded sands. The general stratigraphic column of the Nile Delta is shown in Fig. 3 [21]. It comprises sediments of the Miocene to the Holocene age. This sequence is a thick pile of clastic sediments in which the West El-Burullus discovery has been made. The sand reservoirs of these fields are of the middle Pliocene age and form part of the Kafr El-Sheikh Formation.

Fig. 3
figure 3

Chronostratigraphic chart showing depositional environment of the Miocene, Pliocene and Pleistocene ages of the study area, correlated with the drilled Pliocene Kafr El Sheikh Formation in WEB-1 well modified after Stirling [18]

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The West El-Burullus area is located on the Burullus horst block dipping to the north and limited toward the northwest by the regional Rosetta fault and toward the southeast by the Burullus normal fault. Both regional faults are oriented NE-SW (Fig. 2a). The Burullus regional high is displayed with the WEB canyon put in the Pliocene slope settings of the Delta (Fig. 2). The faults are thought to have provided an active structural control also during the deposition of the sediments throughout the Pliocene time. The horst itself was faulted by minor normal faults (Fig. 2c). Growth faults have induced syn-sedimentary faults by gravity and compaction of the sediments. They create local topography and accommodation for ponded turbidites during the depositional history.

3 Dataset and Methodology

The West El-Burullus area was covered in 2006 with a three-dimensional seismic survey. The seismic cube available in our study is normal polarity and has a zero-phase wavelet. The vertical scale of the seismic cube is in terms of time (two-way time in ms) (Fig. 1). The area was penetrated in 2008 with the WEB-1 well that targeted the Pliocene sediment of Kafr El Sheikh Formation with a water depth of 19.2 m below sea level at the well location. The WEB-1 well penetrates a higher amplitude seismic behavior which is a direct indication of hydrocarbon occurrences such as gases. A complete log set is available for the WEB-1 well (resistivity, spontaneous potential, gamma-ray, neutron, sonic, and density) in addition to pore-pressure data and geological reports. The formation tops were defined using the bio-stratigraphical reports.

This study comprises an integration of multidisciplinary studies including seismic geomorphology, seismic facies analysis, sequence stratigraphy, and well-log analysis. These methods are summarized as follows:

  1. (1)

    The seismic geomorphology combines seismic root mean square and coherency attributes and the spectral decomposition attribute to better understand the paleo-depositional environment that is preserved with its signatures in the rocks and detected in the seismic cube.

  2. (2)

    Seismic facies analysis illustrates how the amplitude behaves with changing the geological facies.

  3. (3)

    Sequence stratigraphy explains the lithological boundaries through the WEB canyon evolution stages.

  4. (4)

    Well-log analysis accompanied by a petrophysical assessment helps in detecting the depositional environment and the reservoir quality.

  5. (5)

    Finally, deducting the geological depositional model and WEB canyon architecture by tracking the structural/stratigraphic settings of the WEB canyon reservoir.

3.1 Seismic-Well Tie

Seismic interpretation starts with creating a synthetic seismogram by convolving the reflectivity acquired from acoustic and density logs and the wavelet extracted from seismic data for making well tie to link the stratigraphic and formation tops to the seismic reflectors. Synthetic seismogram creation is the first and vital procedure in the seismic interpretation process. As an outcome, the convolution of reflectivity created from the density log, sonic log, and the extracted wavelets is synthetic seismic traces [22]. A preparatory step for controlling well log quality was conducted for correcting the log spikes to a preferable construction of synthetic seismic traces used for the seismic well tying. Miss-tie occurs due to the signal-to-noise ratio (S/N ratio), and when the actual seismic trace wavelet mismatch the synthetic wavelet [23]. When reaching a satisfaction percentage of well-to-seismic tie correlation coefficient of about ~ 0.7, the stratigraphic tops picked across the seismic data, making output synthetic traces thereafter matched with authentic seismic data [24].

3.2 Seismic Attribute Analysis

The seismic geomorphology is tracked by spectral decomposition, RMS amplitude and coherency attribute to depict the geomorphology and sedimentary facies distribution at all evolution stages for the WEB canyon. Channel evolution is viewed in terms of slope gradient reinforced by increasing or decreasing its size and shape, at all stages of its deposition more than expected due to the increasing or decreasing the source feeding for its sediments along with the age of the channel [21]. The coherency (variance) attribute measures the similarity in every point in a seismic cube to its contiguous points, by computing positional waveform similarity in both crossline and inline, which is used here to detect the discontinuity and muddy facies.

The classical attribute (RMS) is used to detect the bright spot, which is an indication either the lithology change or the lithology with the gas effect. It is produced by measuring the reflectivity through a seismic window, as the square root of the sum of the squared amplitudes divided by the number of samples., The channels can be detected by the amplitude intensity changes with its distinctive meander shape with bright spots compared to its surroundings that detect a good indication of channels and its gas content. Although RMS is often utilized, spectral decomposition is a vigorous appliance analysis that is used with weighty success in identifying the channel fairways, fans, lobes, over-bank deposits, and other architectural elements, especially for clastic depositional systems. Spectral decomposition is used successfully in fluvial-deltaic and basin floor turbidites channels and fan systems. Since it is known that each material has its frequency, the same scientific concept has been applied to the stratigraphic types, which resonate with each stratigraphic feature at a particular frequency, depending on the lithology type, and bedding thickness.

It is an essential element for seismic geomorphology to analyze the lithology, fluid indication, and paleo-depositional environments to provide a litho-geomorphological mapping for sedimentary facies, especially when integrated with the various related disciplines such as seismic inversion [25], well-log analysis [26], seismic attributes [27], amplitude versus offset [28], and multi-seismic component [29].

The spectral frequency decomposition is an appreciable professional tool for seismic-based geological interpretations. A long history of utilizing spectral decomposition in detecting the low frequency underneath the hydrocarbon reservoirs. Castagna et al. [30] matched undertaking decomposition for instant spectral analysis, while Peyton et al. [31] used coherency and spectral decomposition to confidently interpret the incised canyons. Further, Partyka et al. [6] used spectral windowed analysis to build discrete-frequency cubes for investigating reservoir characterization. Spectral decomposition is used in mapping the shallow water by Peyton et al. [31] and the deep water using a tuning model of thin-bed to the very thin stratigraphic features made by Bahorich et al. [32]. The results encompass tuning cubes and a mixture of discrete prevalent frequency cubes that make the interpreter discern the amplitudes tuned with specific wavelengths.

Spectral decomposition is used for temporal mapping of bed thickness [6], determining stratigraphic traps [33], and delineation of hydrocarbon distribution [30]. Spectral decomposition encompasses the processing of setting apart seismic traces into several bands of frequencies and thereafter merging panels of frequencies to upgrade the discernible effect to the geometries of sediments [34] and to depict and determine the stratigraphic settings as channels. Frequency blending/fusion diminishes the tuning effect of thin beds through multi-band tuning in widened thickness, which generates a pragmatic view of litho-stratigraphic geometry [29].

In this study, the spectral decomposition with frequency blending in RGB (red, green, and blue) colors, is used for time slices at different successive levels between the canyon top and base to track the vertical and horizontal changes of the reservoir for depicting planer lithofacies geomorphology. Discrete fast Fourier transform (DFFT) is used with a time slice considering seismic horizon interpretation of which phase or amplitude data will be transformed to a frequency domain. After extraction of frequency cubes, three main frequency cubes are selected and visualized in RGB color and blended, considering the desired thickness interval for the visualization, which took into consideration that the lowest frequency is tuned with the thickest beds, otherwise the highest frequency is tuned with the thinnest beds.

The tuning frequency of channels could be either less or greater than the synoptic dominant frequency. Before implementing spectral decomposition, the power spectrum of the seismic cube is conducted. The first step of the spectral decomposition is implemented by extracting the different frequency cubes from the full stack seismic cube, then the displayed window for the different frequency cubes is used to select the best three frequency cubes that will give a better stratigraphic indication. The second step is coloring the selected frequency cube with RGB colors. All these colored frequency cubes are blended making it easier for specifying stratigraphic quality and thickness indications. Each color reflects a part of the channel as it appears in red with a high-thickness channel part. The blue color demonstrates its small thickness, and the medium thickness is reflected in the green one. If the channel is tuned in white it is mean tuned with all colored frequencies “Red, Green, Blue” which means it has a wide range of multi-thickness of channels at all stages of its evolution.

3.3 Seismic Facies Analysis

Seismic facies analysis as a clarification for seismic reflectors and their parameters, such as amplitude behavior, shape, and continuity, at each stratigraphic type to define all dissimilarity and the lateral lithofacies of the seismic. This approach is used before by other researchers in other petroleum provinces such as Gong et al. [35].

3.4 Seismic Sequence Stratigraphy

Sequence stratigraphy and depositional facies analysis based on borehole logs (e.g., gamma-ray) for tracking WEB canyon during its forming time to define any sequence boundaries during its evolution. More than one principle is used for deducing the geomorphological depositional environment via the integration of borehole logs with a comprehensive seismic facies analysis. Sequence boundaries indicate a sudden basinal relocation of the deposition, hence it is interpreted as sharp shoaly depositional facies (sea regression) over the unconformities [36]. Sequence stratigraphy analyses are lined by the models established on sequence stratigraphy according to Bouma [37] and Catuneanu et al. [38]. Stratigraphic sequence analysis subdivides the stratigraphy into sequences based on sedimentary and depositional history with boundaries identified by surface erosion (unconformities) or such correlated conformities [39].

3.5 Petrophysical Analysis

Due to the unavailability of core data in WEB-1well, well logs and petrophysical analysis using the gamma-ray log patterns are used instead of the lithological log to provide a means of interpreting the association between the paleo-depositional environment and the facies. The responses of gamma rays are interpreted based on the criteria established by Emery and Myers [40], Rider [41], Kendall and Pomar [42]. Based on these GR patterns, Kafr El-Shaikh Formation reveals a facies-related turbidite deposition system. So, the description of Kafr El-Shaikh Formation turbidite facies depends on Mutti [43] classification, which covers a broad range of facies of cohesive debris flow deposits. Based on borehole log analysis, petrophysical analysis, and the reservoir quality and properties (e.g., shale volume, porosity, permeability, and hydrocarbon saturation) are determined.

3.6 Paleo-geomorphological Modeling

A paleo-geomorphological configuration model is constructed to set the paleo-depositional system dominating the reservoir sediment quality. The depositional systems' evolution is marked by the morphological changes within the neighboring constituents [44]. Paleogeomorphology is a suitable procedure for understanding and portraying patterns of the sediments' distribution within the depositional systems. Paleogeomorphology and sediments through sedimentation also dominate the locative sediments allocation in various sedimentation systems intervened in basins [26, 45,46,47,48]. As consequence, a paleogeomorphology restoration is a pivotal approach for rebuilding the depositional systems with models for defining the distribution of reservoir quality rocks [49].

4 Results

4.1 Seismic Geomorphology

Seismic to well tying is applied and linked the stratigraphic horizons tops to the corresponding seismic reflectors using synthetic seismograms as shown in Fig. 4. The seismic section (Fig. 5a) running across the WEB channel, and the extracted RMS amplitude (Fig. 5b) shows the bright yellowish/reddish spots representing the WEB canyon reservoir. Through this seismic section, the channel is easily identified with its lens geometry (positive relief) as a convex form for the constructive channel levees system, which is in tune with a constructive channel levees complex. The positive relief is protruded by different compaction for the various facies, with the channel axis being more sandy than the levees. The channel fill with its complex seismic character is evidenced through the incised surfaces, the gradational fill, and the lateral accumulation in considerable segments (sedimentation/erosion channel).

Fig. 4
figure 4

Synthetic seismogram of WEB-1 well, displaying the seismic well tie, which reaching a satisfaction percentage of 0.7 correlation coefficient

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Fig. 5
figure 5

WNW-ESE composite Seismic section crossing the WEB canyon representing convex form constructive channels levees system (a), RMS attribute at the time slice − 2220 ms of the top WEB canyon (b). The white line shows the composite seismic section shown above, the blue NE-SW lines shows faults crossing the WEB canyon, and the contour lines shows the two-way time values on the canyon horizon

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Two main horizons are interpreted for representing the WEB canyon top and base. The first horizon is the canyon base two-way time (Time structure) map that shows the WEB channel complex deposited trending in NW–SE structural low (Fig. 6a). The second horizon is the canyon top time structure map that sheds light on the NW–SE trending closure of the structures with the segmentalization of the channel-levees system that is affected by faults of NE–SW trends in accordance to Fig. 6b. The canyon top reflects a four-way dip structure and transverse faults with an antiform shape, which confirms a longitudinal structure closure. The canyon isochrone map (time-thickness map) (Fig. 6c) shows ranges from approximately 250 ms along the channel axis to around 0–10 ms at the edge of the WEB canyon system. It confirms that trapping sediments occur in the WEB canyon channel axis with over-bank deposits on both sides of the channel. Along the channel axis, the WEB canyon shows medium to high amplitude semi-continuous to continuous seismic reflectors (Fig. 7).

Fig. 6
figure 6

Two-way time map on the base of WEB canyon (a), two-way time map on the top WEB canyon (b), isochrone map of WEB canyon (c). (Red dashed line is the maximum WEB extension), shows ranges from approximately 250 ms along the channel axis to around 0–10 ms at the edge of the WEB canyon system

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Fig. 7
figure 7

Composite seismic section directed NW–SE through WEB channel axis (a), RMS attribute at the time slice (− 2220 ms) of the top WEB canyon (b). The white line shows the composite seismic section shown above, the blue NE–SW lines show faults crossing the WEB canyon, and the contour lines show the two-way time values on the canyon horizon

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For the spectral decomposition analysis, the power spectrum of the 3D seismic cube within the interval from 2000 to 2400 ms (where the channel extends laterally and vertically) resulted in a dominant frequency from 12 to 23 Hz (around 20 Hz) (Fig. 8). The seismic cube is specified by different iso-frequencies produced from the seismic cube equals 5, 8, 12, 15, 16, 25, 34, 35, 45, 55, 65, and 75 Hz. After the test, the best three cubes are 5, 20, and 45 Hz, where we used the red color for 5 Hz, which is tuned as the lowest frequency, while 20 Hz is tuned as the best near dominant frequency in the green color and the blue one tuned as the highest frequency is 45 Hz.

Fig. 8
figure 8

Seismic line XL 10745 passed the WEB-1 well shown in the base map (a), base map shows the location of the seismic line XL 10745 (b), the extracted power spectrum of 3D seismic time cube within interval 2000–2400 ms (c)

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In the study area, Kafr El-Sheikh sequence stratigraphic segregation demonstrates the accumulating stratigraphic framework through patterns with the different preserved lithologies and lithofacies. This helps in defining the change of historical stages for the base level that marks Kafr El-Sheikh Formation’s intra-evolution across the changes in the eustatic sea level and the basin slope. Delineating the stratigraphic sequence surfaces such as the maximum flooding surfaces and sequence boundaries based on logs supported by seismic sections, with mapping the WEB incised canyon (Fig. 9). Sequence and depositional facies analyses emphasize changes in prorated sea level, which resulted in detecting a third order of depositional sequences predominating in the WEB canyon in Kafr El-Sheikh Formation and affecting the depositional systems accompanied by its associated facies.

Fig. 9
figure 9

Well log-seismic based sequence and depositional facies analysis, delineating the stratigraphic sequence surfaces such as the maximum flooding surfaces and sequence boundaries, with mapping the WEB incised canyon

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4.2 Geological-Depositional Model and Canyon Architecture

In the study area, there are distinct sedimentological facies interpreted at WEB-1 well showing multi-GR responses. Turbidite sediments consist of siltstones, shale, and sandstone multi-interbed cycles, which are recognized as shaliness on the gamma-ray log. Figure 10 illustrates the log responses from the bottom upward which correspond to channels of mud fill for the incised canyon described as hemipelagic mud and distal shale suspensions. It reflects a permeability barrier between the upmost and basal sandy facies. Other facies are reflected by logs as a fining upward log signature with sharp base and top, clean sands interpreted as splay and lobe that is generated by high-density current classified by Mutti's [43] classification (F7, F8). Intercalations of silty heterogeneous layers are interpreted as silty distal levees with diluted turbidite currents. Serrated log response represents variations altered by exotic minerals, heterolithic, with relatively thinner beds that are generated by low-density current (F9 from Mutti's [43] classification).

Fig. 10
figure 10

Stratigraphic interpretation and depositional elements delineation showing distinct sedimentological facies interpreted at WEB-1 well with a complete log suite proved a very good reservoir quality

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4.3 Petrophysical Parameters

The petrophysical evaluation and lithological computation of WEB-1 well prove that Kafr El-Shaikh Formation in West El-Burullus is mainly shale intercalated with occasional sandstone with an average porosity of around 25% (Fig. 10) that represents a very good reservoir with turbidite facies. The WEB-1 well results indicate the encountered channelized reservoir showing various fining upward sedimentological units. These units can be subdivided depending on their petrophysical characteristics. Four main sedimentological units are identified with their quality, from the bottom to top, incised canyon fill, splays and lobes, silty distal levee, and sandy proximal levee. The channel axis with the incised canyon fill sediments are consisting of a low ratio of net-to-gross fine-grained deposits that may reflect the deposition of intra-channel positions strongly mixed with basin marine sediments. In contrast, splays and lobes consist of high net-to-gross ratio clean, homogeneous fine to medium-grained sand reservoirs. Geometrically, these sediments are sheet-like that is thickening in the northwest direction. The silty distal levees consist of a moderate net-to-gross ratio, heterolithic silt with very fine-grained sand. The sandy proximal levees consist of a high ratio of net-to-gross, homogeneous medium-grained sand. Geometrically, those sediments are downlapping onto the canyon edge to the west and onlapping toward the canyon edge to the east.

The logs interpretation of Kafr El-Sheikh Formation in WEB-1 well showed the interval between 1270 to 1580 m consists mainly of shale interbedded with minor streaks of sandstone and the interval between 1580 to 2442 m consists mainly of shales interbedded with sandstone, probably a thick turbidite system made of an alternate of sand prone proximal levees, shale intervals (marine shale and distal levees) and stacked channels. The depths between 2146.6 m and 2442 m can be divided into nine zones (1, 3, 5, 7, and 9 shale zones while 2, 4, 6, and 8 sandstone zones). From the petrophysical results, Zone-2 has a good hydrocarbon indication in the interval from 2146 to 2185 that is formed of thinly laminated sandstones corresponding to the proximal levees of the ultimately abandoned channel of the turbiditic system. Zone-2 sandstone is a very good quality reservoir and gas-bearing (16 m net pay thickness, 42% water saturation, 35% effective porosity, and 36% of shale volume) so it is the main reservoir in WEB-1 well (Fig. 11a–c). Several good sand reservoirs made of stacked channel facies were found to be deeper, but it was water-bearing.

Fig. 11
figure 11

Petrophysical evaluation and lithological composition of WEB-1 well (a), Zones of Kafr El-Shaikh Formation (b) mainly shale intercalated with occasional sandstone, Reservoir quality and the main WEB canyon reservoir in zone-2 (c)

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The gas–water contact proved by the Modular Formation Dynamics Tester plotting (MDT) for 17 good pressure readings which are shown in Kafr El-Sheikh Formation between 2088.65 and 2321.88 m TVDSS, nearly at depth of 2149 m TVDSS (Table 1) and the intersection between water gradient (0.437 psi/ft) and gas gradient at (0.119 psi/ft) at 2149 m TVDSS. The pressure gradient (0.119 psi/ft) indicates the gas is likely rich and thermogenic gas (maybe Miocene sourced) and migrated upward through the major faults such as the Rosetta fault and its successive faults (Fig. 12a). Cross-plotting relationships have a huge effort in the lithological and gas affect identification. A neutron-density cross-plot also shows the main lithology of Kafr El-Sheik Formation is shale (on the right side of the cross-plot where the high neutron and density values) with sandstone has a gas effect on the left side of the cross-plot where the low neutron and density values (Fig. 12b).

Table 1 MDT plotting for Kafr El-Sheikh Formation, WEB-1 well canyon and its reservoir
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Fig. 12
figure 12

MDT plotting for Kafr El-Sheikh Formation reservoir of WEB-1 well (a), Neutron-density cross-plot of Kafr El-Sheikh Formation for WEB-1 well (b), where the gas effect only appears in zone 2 of web canyon reservoir (see Fig. 11)

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5 Discussion

The West El-Burullus area has sophisticated sediment filling, structural deformations, differential compactions, and erosions that can be combined to establish some synthetic reactions that reflect paleogeomorphology. Adequate geological-depositional models for the distribution of WEB canyon reservoirs could lead to the proper strategies for exploration, and development of the West El-Burullus area. Conceptual geo-depositional models represented throughout the reservoir architecture of Kafr El-Sheikh sediments are built based on many approaches used in this work to provide a clear picture of sediment distribution and depositional history throughout distinct depositional stages.

5.1 WEB Canyon Geomorphology

The principal feature of litho-geomorphologic facies is the time–frequency RGB blended slices that derived from the seismic data in this study. It shows sequences of infill and incisions developing into the Kafr El-Sheikh Formation namely West El-Burullus canyon (WEB canyon). The bottom erosion at the surface dips basinward which displays an asymmetrical canyon about 3 km wide and traced depth between 2040 and 2350 ms. The WEB canyon's lowest part is discerned as the first establishment stage, which reflects the first incision channel (Fig. 13a), while the last stage at the top of WEB canyon reflects the last depositional system of levees, crevasse splay, and lobes (Fig. 13b). All the levels of evolution represent channel morphology filling as unidirectional onlap termination across the canyon side borders. The time–frequency RGB blended slices illustrate litho-morphological facies distribution of incised turbidites channel fills that are deposited in the conjugation of the sea-level rise. The reservoir fairway shows the NW–SE trend, with its levees and lobes geometry. It uncovers zones of higher anomalous through the interpreted paleo-canyon (Fig. 13a). This area of the WEB canyon appears the most superior location for both turbidite hydrocarbon potential and reservoir lithofacies. The turbidite sedimentation is proven by moderate to sinuosity channel geomorphology that is reflected through RGB and RMS horizontal slices.

Fig. 13
figure 13

Spectral decomposition (RGB map) with cartographic interpretation showing WEB depositional system at the base of WEB canyon (a), spectral decomposition (RGB map) with cartographic interpretation showing WEB depositional system at the top of WEB canyon (b)

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5.2 Seismic Facies

The construction of seismic facies mapping is based on seismic data tied to the WEB-1 well and logs. Based on amplitudes, five main seismic facies are interpreted and identified from the view of depositional facies (Fig. 14), listed as follows:

  1. (1)

    Medium to high amplitude semi-continuous reflectors point to channel complexes,

  2. (2)

    Medium to high amplitudes with continuous reflectors corresponding to proximal facies levees, or splays lobes,

  3. (3)

    Low to medium amplitude with semi-continuous reflectors corresponds to distal facies levees,

  4. (4)

    Medium to high amplitudes with discontinuous reflectors corresponding to small or non-amalgamated channels, and

  5. (5)

    Semi-transparent seismic facies are interpreted as plugged channels.

Fig. 14
figure 14

WEB seismic facies types and their interpretation in-terms of depositional facies

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Figure 15 illustrates a nearly E–W-oriented vertical seismic section, through WEB canyon in which five seismic facies are observed, and by using seismic attribute analysis (RMS amplitude and spectral decomposition approaches), these facies are represented as follows:

Fig. 15
figure 15

Illustrates WEB canyon shown in the spectral decomposition (RGB) slice (a), RMS attribute slice (b), cartographic sketch (c), and interpreted seismic section (d)

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  1. (1)

    Medium amplitude continuous reflector in pink interpreted as basal lobe,

  2. (2)

    Channel axis, high to medium amplitude, discontinuous, and the shingled reflectors (in orange) could be interpreted as amalgamated channels (channel complexes),

    High to medium continuous reflectors with channel axis side banks (in yellow) are interpreted as proximal levees constituted due to excessive spillage of directed channelized turbidity currents. The amplitude reflection becomes less intense away from the channel axis,

  3. (3)

    Low amplitude continuous reflectors that dimmed in the spectral decomposition, RMS, and coherency are interpreted as distal levees located at the extension of proximal levees, and

  4. (4)

    The transparent seismic facies (red) on-axis of the last channels are interpreted as shaly plugged channels.

Otherwise, RMS visualizes the channel features and other turbidite geometry lithofacies with higher values through all WEB canyon evolution stages with time, which conforms and matches with the spectral decomposition outputs in Fig. 13a, b.

5.3 Depositional Stages

The canyon proximal and distal facies evolution shows an isochoric distribution of channel levees complex, which develops a lens geometry in northern parts. This change in channel morphology is expounded as a change in the topography with a slope gradient steeper to the south than to the north (Fig. 6c). RMS, coherency, and spectral decomposition attribute horizon slices helped us to predict the canyon channels' complexity during all the depositional stages. The depositional process reflected four depositional stages.

During the first depositional stage of our depositional model, a basal lobe formed and then eroded by a single erosive channel. It is a uniform medium amplitude zone, covering about 25.5 km2 illustrated through the RMS attribute slice (pink color at the seismic section), and the vertical seismic section crossing the canyon shows synoptic tabular geometry (Fig. 16a). Nevertheless, the first sinuous erosion channel appears very clearly with its lobes at the WEB canyon base by spectral decomposition through the horizon slice in which only the erosive part is preserved (Fig. 13a).

Fig. 16
figure 16

RMS attribute time slices showing the WEB canyon depositional system between − 2216 and − 2260 ms (a), between − 2184 and − 2208 ms (b), between − 2160 and − 2170 ms (c), between − 2030 and − 2150 ms (d). The black straight line across the canyon represent the vertical seismic section location

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The second depositional stage accentuated an invasion of amalgamated channels into the area with muddy and silty sediments within its incision, with sandy proximal levees, silty distal facies, and lobes. Figure 16b shows high amplitudes elongated objects on the RMS attribute slices that are interpreted as amalgamated channels. In the crossing seismic line, amalgamated channels harmonize with discontinuous high-amplitude reflectors.

A medium to high amplitude lobate appears alongside the channel belts through the third depositional stage. It is a medium to high amplitude continuous reflector as reflected on the seismic cross section and could be interpreted as a crevasse splay up-growth (Fig. 16c).

The fourth and last depositional stage reveals the last elementary two channels, which are plugged and accompany by shaly material. Accordingly, in this stage, it may be congruous to halt the dynamics of the sediment source or the system migration. RMS clearly shows the deposit segments such as levees, splays, and lobes that are confirmed by the WEB-1 well-containing hydrocarbons (Fig. 16d), while the last two muddy plugged channels appeared dim after the spectral decomposition study (Fig. 13b). Coherency completes the picture in the last stage and shows the transverse faults affecting the stratigraphic system and the dimming of the last plugged channels (Fig. 17).

Fig. 17
figure 17

Coherency attribute slice at time − 2220 ms that illustrates the last depositional stage showing the transverse faults affecting the stratigraphic system and the dimming of the last plugged muddy channels

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5.4 WEB Depositional Model

As revealed from all principles and their applications that are used in this study, the WEB canyon segmentation can represent from bottom to top: (1) basal segment lobe, which is marine mud, (2) then the first incision channel followed by amalgamated segregated channels fill, and (3) the last stage with plugged muddy channels that may due to stopping the sediments feeder. All these stages are interspersed with the sedimentation of splays, lobes, and levees with their proximal and distal facies. The turbidity channel facies such as levees, lobes, and crevasse splays act as reservoirs incise into Kafr El-Sheikh shales where it is considered as vertical and lateral stratigraphic sealing for the intervals of the WEB reservoir (Fig. 18).

Fig. 18
figure 18

Log analysis of the reservoir in WEB canyon and its depositional model

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The WEB-1 well was drilled to test the reservoir presence, quality, and the Pliocene hydrocarbon potentiality of the NW–SE trending incised canyon system (WEB canyon) within the Kafr El-Sheikh Formation that was identified by the 3D seismic survey. The canyon has been filled by back-stepping confined channels and hemiplegic shale. Then, by the overflowing of channels with the pond turbidite system, a clean coarse-grained reservoir has been deposited by high-density turbidity currents (lower gas-bearing reservoir). After that, the low-density turbidity current overflow yielded deposition of the channel-levee complex (upper gas-bearing reservoir). Finally, owing to the sea level rising and the stopping or migrating of the feeder system, depositions of the mud-fill channel and the hemipelagic shale were deposited.

The Pliocene WEB canyon infills represent turbidity depositional systems, through which the geological-depositional model could be expressed from the bottom upward as a gradation changing the first channel incisions to upper amalgamated channels. These amalgamated channels reflected depositional facies from the incision muddy channels axis to the over-banks filled with proximal to distal levees, lobes, and crevasses splay sealed by the surrounding hemiplegic shale of Kafr El-Sheikh Formation. Generally, hydrocarbon exploration may depend on the channel axis where thick deposits are present. This study showed that the spots of the turbidity facies are associated with coarse-grained sandstones, which are deposited over the banks of the distributary channels, like proximal levees and crevasse splay. These proximal levees and crevasse splay are more promising for hydrocarbon accumulation than commonly thought, especially in turbidites channel. Whereby the reservoir petrophysical characteristics, e.g., porosity, the WEB reservoir reflected a significantly high reservoir quality, which is diminished with a sequential intention in marine action. This is skillfully harmonious with the extractor's relative attributes and the results from RGB maps, and the borehole logs.

The heterogeneity of depositional facies is considered the main factor that controls the reservoir porosity and permeability. Figure 19 represents a complete conception of the West El-Burullus Pliocene canyon depositional model with its evolution stages as disclosed from the previously mentioned disciplines through our work which is the structural interpretation, seismic facies analysis, seismic geomorphology, sequence stratigraphy, and finally, depositional facies analyses.

Fig. 19
figure 19

Depositional model (paleo-geomorphological model) of Kafr El Sheikh WEB canyon, illustrating the evolution of depositional systems and its facies associations

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6 Conclusion

Seismic geomorphology becomes more rigid with the incorporation of spectral decomposition with horizons attributes (RMS and Coherency) that help a depiction of the litho-morphological and facies distribution of concealed Kafr El-Sheikh Formation turbidity incisions (WEB canyon), recognizing reservoir trending and the canyon fairway which is trending NW–SE with a sinuous shape and channelized geometry. The turbidite facies association enriches grains finer to coarse from shaly sandstones that are deposited in a marine environment.

Kafr El-Sheik Formation in the West El-Burullus area is mainly shale intercalated with turbidite sand bodies that are excellent reservoirs. Kafr El-Sheik Formation comprises a third-order sequence of turbidite units infilling the WEB canyon. The WEB canyon has five main sedimentary facies assembly from the undermost up; the basal lobe at the bottom of the WEB incised canyon, amalgamated channels fill, followed by splays, lobes, levees either proximal or distal facies, then the last plugged channels. All facies have different reservoir property that helps to delineate the best locations to develop the field and a new concept for analyzing the turbidite importance in exploration studies.

The turbidity quality and its facies association with fructified medium to coarse-grained sandstones that are deposited in a submarine setting, with conspicuously high quality (either porosity or permeability) of the reservoir, representing the best facies for the area, should be considered in the adjacent concession areas of the West El-Burullus area. A complete paleo-geomorphological depositional model and canyon architecture are revealed and depicted from seismic geomorphology, seismic facies analysis, and sequence stratigraphy. Integrating these aspects with well logging accompanied by a petrophysical evaluation, made it clear to depict the heterogeneity/architecture, of the Kafr El-Sheikh reservoir. The revealed sedimentary facies (turbidite channels) provide a vigorous insight into considerable criteria for future exploration and development strategies. Alongside Kafr El-Sheikh reservoir sediments, points to good sedimentary facies (turbidite channels) that provide a vigorous insight into considerable criteria for future exploration and development strategies. Kafr El-Sheikh shales are considered vertical and lateral stratigraphic sealing for the intervals of the WEB reservoir.

Additionally, a petroleum system with seal analysis research should be conducted to detect trapping and source/charge time coincident with identifying migratory paths to anticipate the greatest prospective places for hydrocarbon occurrence. Furthermore, by combining geophysical and geological data, channels and their structural settings can be identified and mapped, allowing new well locations to be optimized for future reservoir development. This solves one of the key challenges in the West El-Burullus area: "channel architecture and fairway delineation".

Although hydrocarbon exploration mainly relies on channel-axis sedimentary sediments, our study also showed that the turbidity facies sweet spot was also found in overbank sedimentary distribution channels containing coarse-grained sandstones such as proximal levees and splays. It has been proven to be more promising than commonly thought.