Pre-Stack Imaging for the Messinian Sub Salt Sequences in the Levantine Basin of the East Mediterranean: A Case Study, Offshore Lebanon

Abstract


Introduction
The offshore East Mediterranean region represents a promising area of high interest for hydrocarbon exploration.It is a gas-prone area; thereby during the last decade, exploration activities in the Mediterranean were in a serious progressive development for the offshore gas fields along the Eastern Mediterranean shorelines of Egypt, Cyprus, Israel, Palestine, and Egypt which will completely reform the gas market in all over the world.In Egypt, many giant gas fields have been discovered, e.g., Zohr, Nargas, Shorouk, and Noor.Besides, Cyprus announced another giant gas field including Aphrodite, Israel started production from Leviathan, Karish, Athena, Dalit, and Tamar, while the Marine Gas Field has been discovered in offshore Gaza.So, recently in 2019, the Lebanon government signed two agreements for exploration and production with an international consortium covering offshore blocks 4 and 9 located in the eastern province of the Mediterranean Sea.
It has a complex and long tectonic history, where In the Early Triassic-Late Cretaceous, many phases of rifting between the Eurasia and Africa plates took place (Barakat, 2010;El-Gendy et al., 2022;Sarhan et al., 2022).The Triassic sediments formed the main reason for the highly attenuated continental crust followed by several sedimentation periods till the Pilo-Quaternary sediments (Fig. 1).The East Mediterranean includes Levantine and Herodotus basins which have the same age, and the sediments type and thickness (10-15 km).Both basins are predominated by many stratigraphic and structural traps (Roberts and Peace, 2007;Elia, 2013).The Messinian salt thickness is about 1-2 km in the Levantine basin and about 2-3 km in the Herodotus Basin (El-Bassiony et al., 2018).The Messinian salinity crisis is a dramatic event that developed from the slow isolation of the Mediterranean Sea from the Atlantic Ocean due to the closure of the marine gateway at the Gibraltar straights (Garcia-Castellanos and Villaseñor, 2011).This resulted in a dramatic deposition of a thick evaporite succession up to 3 km in the deepest parts of the Mediterranean basins (Woodside, 1977;Ryan, 2009).Many facies of Messinian sediments are observed offshore and onshore Nile delta cone (Nabawy et al., 2018;Abdel Fattah et al., 2022).
This could suggest that the development of the Messinian saline giant was the result of a favorable combination of tectonic, climatic, and glacio-eustatic conditions at the early stage of the crisis (Barakat and Dominik, 2010;Rouchy and Caruso, 2006).According to Warren (2010), this giant salt accumulation occurred shortly after the large-scale extraction of water from the ocean to form the ice cap of Antarctica and the deposition of the Middle Miocene Red Sea rift evaporites (Fig. 2).These salt regional extensions formed the main cap sealing rocks of many petroleum systems in the East Mediterranean, so during the last decade, many giant natural gas reservoirs have been discovered in the eastern parts of the Mediterranean Basin, e.g., Tamar, Leviathan, Aphrodite, and Zohr fields.The discovery of Zohr Field in 2015 in the offshore Nile Delta changes the scenarios and future plans for hydrocarbon exploration in the East Mediterranean (Esestime et al., 2016;Nikolaou, 2016;El-Gendy et al., 2022).East Mediterranean became the hottest spot in the region and more attractive for many international oil and gas companies.However, for all explorations, the subsurface seismic imaging and the image resolution are the base for reducing the exploration risks.Recently, many 2D/3D seismic acquisition surveys are already acquired in East Mediterranean to delineate its subsurface geology and structure.The application of the updated seismic acquisition in offshore acquisition and seismic processing is needed for a new opportunities.Also, the reprocessing of old acquisition surveys is critically needed to update the seismic data set for the East Mediterranean.Processing seismic data is one of the main exploration steps and development in the oil and gas industry.It depends on many factors: acquisition parameters of the survey, shallow and deep marine data, the subsurface geologic and structural setting, optimum seismic processing, and the flow and support information (from wells, legacy velocity, etc., Haque et al., 2022).(Lugli et al., 2015).Though the regional sub-salt areas in the East Mediterranean play an important role not only in oil and gas exploration, but also they show high exploration and production risks.This can be explained by that the complex evaporites lead to poor image quality when the salt velocity variations are not properly modeled.Thereby, the final interpretation of the areas of interest will not be accurate (Abdeen et al., 2021;Abdullah et al., 2021;Nabawy and El Sharawy, 2018;Nabawy and Shehata, 2015;Barakat et al., 2022;Radwan et al., 2022aRadwan et al., , 2022 b) b).For the offshore Lebanon area, the Messinian evaporites created potential errors in the sub-salt seismic depth conversion due to its unique properties, thickness variations, and vertical changes in seismic property that could result in velocity anomalies.The study area is considered unique for frontier exploration, therefore delineating the Messinian evaporitic succession and its structure setting is essential to reduce the risk in depth for sub-salt layer prospects.Pre-stack time migration is based on the vertical velocity (1D velocity) without considering the horizontal velocity variation of the presented events, while depth migration is based on the lateral variation of the velocity (Yilmaz, 2001).
The salt layer is primarily represented by complex intra-salt reflectivity.Despite that, a constant velocity layer flattens the structure of the Base Of Salt (BOS).In regions, where the BOS shows clear salt velocity variations, a variable salt velocity will be embedded (El-Bassiony et al., 2018;El-Nikhely et al., 2022;Lugli et al., 2015).Feng and Reshef (2016) stated that the intra-salt reflectivity takes the form of thin clay beds with much lower velocities than salt as recorded by well logs (Bell et al., 2018;Barakat et al., 2021).Therefore, the extension of salt layers and structures masks the seismic waves and attenuates them, i.e., highly reduces the resolution of the underlying subsalt events.This masking is due to the salt layers having the following characteristics.
• Salt generally has very fast velocities relative to the surrounding sediments.In addition, the transition from the salt to the sediments is very sharp, i.e., a very large velocity contrast will be generated, and • Salt intrusions are impervious and impermeable forming seal/cap rocks; therefore they very often set up good conditions for oil and gas trapping.So, salt features are always attractive exploration targets.Despite the pre-stack time migration correctly images the complex water bottom, it yields false structures that are associated with the subsalt events due to the very low resolution of the obtained profiles.On the other side, the pre-stack depth migration gives a true subsalt/reflector image when the velocity model is verified (Yilmaz, 2001).It is dependent on the accuracy of the velocity model building to provide a global image for masked structures.However, its accurate interpretation is needed to improve the final depth imaging.

Geologic Setting
Levantine and the Herodotus basins are the major basins of the eastern Mediterranean Sea.The present area of interest is located in the northeast of the Levantine basin, offshore Lebanon (covering all Lebanon blocks).It is bounded by the Latakia Ridge to the North, the Nile Delta cone to the South, the Herodotus Basin and the Eratosthenes Continental Block to the West, and the Levant Margin to the East.It has been originated due to several tectonic rifting phases in the Early Triassic-Late Cretaceous followed by the post-Cretaceous thermal subsidence and the overload of the Nile Delta sediments (Gardosh and Druckman, 2006;Shehata et al., 2023).
The sedimentary thickness is approximately 12 km interval extending from the Jurassic/Cretaceous to the recent age.The Mesozoic succession consists mainly of carbonates.The deposition of the thick Messinian salt (2 km) in Levantine basin is associated with the Messinian salinity crisis event that affected the major evaporite in Late Miocene (5.96-5.33Ma).The salt structures are sometimes represented by extended layers but also by domal structure due to diapiric movements and/or syn-depositional folding (Nabawy and Shehata, 2015;El-Bassiony et al., 2018;Barakat et al., 2019;Elgendy, 2020;Nabawy et al., 2020;Kassem et al., 2022).
Along the northern margins of the Levantine Basin, the Plio-Pleistocene sediments were mostly derived from the drainage systems along the eastern margins of the basin that carried sediments from the uplifted mountain ranges associated with the Dead Sea transform zone (El-Bassiony et al., 2018).
In general, there are two main production plays in the East Mediterranean, 1) Cretaceous carbonate platform play: e.g., the gas play in the isolated carbonate build-up coral reefs of Zohr Field, (Fig. 3), and 2) Cenozoic siliciclastic play: e.g., the gas play in the Oligocene-Miocene Tamar sands, (Fig. 4).The petroleum regimes are dominated by stratigraphic and structural traps, e.g., pinch out and anticline folds.Most of the Levantine plays are located in deep water, so active exploration is needed to fully evaluate the hydrocarbon potential and understand the distribution of the Messinian salt layer and decrease the exploration risk.Based on the schematic model for the petroleum system offshore Lebanon, many plays could be expected in offshore Lebanon (Nader et al., 2011;Fig. 5).Based on the geological history of the recent discoveries in the East Mediterranean, subsalt seismic imaging is still considered a challenge, and thereby geologic constraints are essential to construct velocity models that accurately represent the main geological reflectors.The presence of complex evaporites leads to poor image quality.The complexity of the structure coupled with the strong sharp velocity contrasts associated with the salt means that time processing always fails to resolve the problem.

Materials and Methods
The 2D seismic data from offshore Lebanon were used to demonstrate the result of the research which was acquired by PGS in 2008 and 2011 using Geostreamer Technology.The Seismic data were provided by PGS and approved by LPA (Lebanese Petroleum Administration).The lines covered offshore Lebanon through a 10x10 km grid.The acquisition parameters for the 2008 and 2011 seismic surveys are listed in Table 1.The reprocessing sequence included three main stages 1) Pre Imaging stage, 2) the imaging stage, and 3) post-imaging stage (Fig. 6).
• Pre imaging stage has some processing steps applied to improve the data quality in terms of removing the ghosting, bubbles, and contaminated multiples, • Imaging stage is the most important stage where the event moves to this true position.In the study area, depth imaging flow was used and supported by velocity model building to get the true position for the subsurface layers, and • Finally the post-imaging stage was applied to enhance the final subsurface image and improve the stack response.Pre-stack Time Imaging (PreSTM) has been conducted in the past but due to the existence of the Messinian salt accumulation in the area and its masking impact on the underlying structures, the subsalt interpretation was too complicated and misleading to improper structure architecture.Thereby, recently the seismic data is reprocessed using Pre Stack Depth imaging (PreSDM) to improve the subsurface images and increase the opportunity for exploring the area under investigation.The present case study will show a comparison between the time migration and the depth migration images using the recommended Velocity Model Building flow (VMB) technique in the East Mediterranean.A total of 60 seismic lines trending N-S, E-W, NE-SW, and NE-SE are available, among these permission has been issued for releasing and presenting three seismic profiles comparatively crossing the study area; line A trending N-S, Line B trending E-W, and Line C trending NE-SW (Fig. 7).This is to demonstrate the result and recommending the depth imaging technique.The interpretation horizons used for the VMB are the Seabed, Base Pliocene Top Salt, Base Salt, and the Senonian Unconformity.The Messinian and the pre-Messinian sections are the most complicated region due to the lateral velocity variation in the salt, as well as some irregularities in the top/base of the salt layer interfaces.So, more attention was paid to the velocity processing and structural interpretation in areas dominated by the Messinian and Pre-Messinian sections.The initial velocity model was designed from Legacy velocity after conversion to the interval velocity in the time domain.Also, the global velocity smoothing was done and the model was finally converted to the depth domain.The final velocity model was built through a series of velocity improvements.Several tomographic iterations were run, each including one or a subset of layers defined by the key horizons.This round of improvements increased the gathers' flatness.Seismic velocity in the Messinian salt layer is high around 4300 m/s, while through the Senonian unconformity, the velocity varies between 2400 and 5000 m/s in the deeper carbonate section The final velocity models (Figs. 10,12,14) were obtained using the Kirchhoff Pre-stack depth migration and the output was stretched back to the time domain to run the post-migration imaging enhancement.The obtained image was finally compared with Legacy Kirchhoff Pre-stack time migration.

Imaging Results and Interpretation
Processing and interpreting the N-S seismic profile and based on the acoustic character, the stratigraphic sequence can be divided into 4 units (Fig. 10): 1) Messinian-Pliocene and Quaternary clastics with some convolute structure at its lower surface, 2) Messinian clastics (MSC) sequence which is characterized by a convoluted base and the top surface and disappear to the south indicating deposition after the subsidence of the sea beds, 3) the Pre-MSC clastic sequences with a main fold in the center and a series of local synclines and anticlines in around; it is too thick unit but its thickness diminishes to the south direction, mostly due to contemporaneous subsidence, and 4) a carbonate sequence forming the bedrock.
However, a comparison between the final stacks of the legacy Time migration and the new reprocessing depth migration of the present area of study took place after the application of the full processing sequence.A similar processing sequence was applied to both datasets, but the resultant seismic reprocessing confirmed that the best subsurface image of the sub-salt layers can be achieved by using Pre-Stack Depth migration when supported by an accurate velocity model (Figs. 8,9,11A,B and 13A,B).
Processing and interpreting the E-W and NE-SW seismic profiles indicate the same four units but with many simple structures, where pinch out to the east is noticed with less intense convolute structure (Figs. 10,12).
It can be observed from the comparison that, the depth migration processing showed good improvements in comparison to the legacy data (Time migration) in terms of improved resolution and reflector continuity of the base salt event and in the pre-salt sediments.Attenuation of the apparent distortion of the structure of the base salt and the pre-salt sediment reflectors states a better image for the shallow carbonate platform and refers to that those steep dip events were nicely imaged (Figs. 9, 11B, 13B).Despite the study having been applied using 2D data, better results are expected using 3D data.

Seismic Stratigraphic Interpretation
Considering the seismic stratigraphic sequence, two main reflecting surfaces define the top (TS/TES Top Surface and Top Erosion surface) and the bottom (BS/BES Bottom Surface and Bottom Erosion surface) of the MSC sequence in the East Mediterranean (Lofi et al., 2018).At the continental margins, these surfaces are merged forming the Margin Erosion Surface (MES).The aforementioned stratigraphic units are separated from each other by these reflectors and are characterized by the following seismic features: • Unit-1: It represents the Messinian-Plio Quaternary clastic sequence that is characterized by high amplitude, high-frequency reflection, and relatively good continuity.It is topped by the seabed reflector which varies from 60msec to the east and 2800msec to the west with a gentle slope.A complex canyon structure in the sea reflector is observed at the margin of the Levantine basin to the west and southwest.(Fig. 15A).• Unit-2: It represents the MSC sequence; its top is characterized by high amplitude, and strong continuous reflection events that have been generated by the high acoustic impedance contrast across the boundary between the deep water clastic deposits and the Messinian evaporates.The base of this unit is well distinguished from the Tortonian and old accumulations.The thickness of this unit reaches 1.4 km in the west and pinches out to the east on the margin of the Levantine basin.The time-structure map on the top and the base of the Messinian section shows a relatively uniform gentle slope across the centre with a rapid dipping to the west and wedging out towards the margin of the Levantine basin and to the north towards Latakia Ridge (Fig. 15B).Similarly, is thickness decreases gradually with some dipping to the west.and is dominated by dome structures to the west.Finally, the four stratigraphic units are terminated against Latakia Ridge to the north as well as the Levantine margin to the east due to the complex structure of the thrust belt.As a result of the improved data processing using the Pre Stack Depth Migration, the different stratigraphic units for the offshore Lebanon data is comparable to that of the Zohr Field (Fig. 16).This may support the probability of discovering new plays in the future.But it needs some effort from the oil companies to build a strong database and 3D images for the subsurface to reduce the uncertainty level and drilling risks in this area.

Conclusions
Based on the geologic history of the Levantine basin, many plays can be expected related to structure and lithology or a combination of both.The Messinian salt layer plays an important role in terms of the cap rock for many reservoirs in the East Med.Subsalt layer imaging is always challenging due to the lateral velocity variation of the salt layer, which may lead to incorrect subsurface seismic images.The study area confirmed that Depth migration became the most suitable method for imaging the subsurface layers in the East Mediterranean when supported by accurate velocity model building.Good cooperation between the imaging team and interpretation team leads to the correct identification of the salt layers with high accuracy for the velocity model building.The final sub-salt image shows dramatic improvement and will help in any future exploration and development work.The Messinian section is still the main risk in the area due to the lateral variation of the thickness which reaches 2.4 km and pinched out towards the Levantine margin in the east and Latakia ridge in the north and it shows a uniform thickness to the west.
The final imaging results combined with the Seismic stratigraphic results are integrated and comparable with the recent discovery in the Levantine basin in terms of the lithology, structure, and history of Levantine basin.The interpretation of offshore Lebanon data and Zohr Field looks similar which may lead to more discoveries in the future.

Fig. 1 .
Fig. 1.A location map showing a) The dominant structural setting in the Eastern Mediterranean Basin, and; b) the W-E stratigraphic cross-section through the basin (El Bassiony et al., 2018).The study area is outlined by the white trapezium.

Fig. 5 .
Fig.5.NW-SE cross section in the Lebanon offshore area showing: the petroleum system model with some possible offshore plays(Nader et al., 2011).

Fig. 6 .
Fig. 6.The workflow applied to the seismic reprocessing procedure.

Fig. 7 .
Fig. 7.A location map for the study area indicating the available seismic lines; the presented seismic lines are presented in different colours (lines, A, B, and C).

Fig. 10 .
Fig. 10.Line A (N-S) Interpreted seismic section overlaid with velocity model for the study area.(Seismic courtesy of PGS).

Fig. 11 .
Fig. 11.Line B (W-E) A) Legacy Final stack display (Time migration image), and B) reprocessing the final stack display (Depth migration image stretched to Time).(Seismic courtesy of PGS).

Fig. 12 .
Fig. 12. Line B (W-E) Interpreted seismic section overlaid with velocity model for the study area.(Seismic courtesy of PGS).

Fig. 13 .
Fig. 13.Line C (NE-SW) A) Legacy Final stack display (Time migration image), and B) reprocessing the final stack display (Depth migration image stretched to Time).(Seismic courtesy of PGS).

Fig. 14 .
Fig. 14.Line C (NE-SW) Interpreted seismic section overlaid with velocity model for the study area.(Seismic courtesy of PGS).

Table 1 .
Acquisition parameters of the study area.