Rapid increase in the industrial activities soars the demand for huge additional energy sources especially the fossil fuel. As a result, the exploration started in frontier areas such as deep marine, structurally and stratigraphically complex and unconventional areas looking for additional recoverable reserves of both oil and gas to satisfy the growing demand of energy.

An increase in the hydrocarbon exploration of deep-water Tertiary Basins, coupled with the acquisition of large, high-quality 3-D seismic data sets, has led to a considerable focus on deep-water depositional systems. Well documented seismically-based examples include offshore Angola [1] ; offshore Nigeria [2] ; the Gulf of Mexico [3] [4] .

Models for deep-water slope settings and classifications for the associated channel elements are provided by [5] . The role of these slope turbidites as hydrocarbon reservoirs has been emphasized by [1] [6] [7] .

Despite the numerous seismically-based examples of deep-water systems in the literature, there is a relative scarcity of published examples about detailed 3-D architecture of composite canyon fills that have been described using high-quality seismic data. Few examples are derived from Pleistocene sections offshore Nile Delta of Egypt where it is possible to integrate several data types (e.g., cores, logs, pressure data, etc.). The seismic-stratigraphy of the Pliocene deep-water canyon-filling reservoirs of the offshore west Nile Delta was first described by [8] .

The Nile Delta is one of the classical deltas in the world, with a great history created by the different civilizations. Beside its great history, the Nile Delta area is one of the major gas provinces and one of the most promising areas for future petroleum exploration in north eastern Africa. Offshore exploration, using high quality 3-D seismic data, has resulted in the discovery of significant reserves up to 42 TCF with approximately 50 TCF (Trillion Cubic feet) yet to find [9] .

The Nile Delta is located in northern Egypt (Figure 1), where the Nile River spreads out and drains into the Mediterranean Sea. It has an area of about 12,500 km², very flat at the north and reaches up to 18 m above sea level at Cairo. The Nile Delta is considered one of the world’s largest river deltas. It covers approximately 230 km of Mediterranean coastline from Alexandria in the west to Port Said in the east. The outer edges of the delta are eroding, and some coastal lakes such as El Manzala and Burulls have experienced an increasing salinity levels as their connection to the Mediterranean Sea increases.

The study area is part of the West Delta Deep Marine (WDDM) license which extends from 90 to 100 km offshore (250 - 1500 m water depth) of the present Nile Delta.

The WDDM license covers 8200 km² on the north-western margin of the Nile delta cone. Exploration activities at WDDM started in 1997. A series of successive successful exploration and appraisal wells were drilled by British Gas (BG)-Egypt and Rashpetco. The main drilling target was the Pliocene gas-bearing sands in slope canyon settings on the concession. Several wells were drilled along these canyons namely, from west to east― Saurus, Sequoia, Saffron, Scarab, Sapphire, Serpent, Simian and Sienna (Figure 1). Sequoia, Saffron, Scarab, Serpent and Sienna are of Early Pliocene. Saurus and Sienna canyons were drilled in the Middle Pliocene age. Sapphire reservoir is another relatively deeper reservoir of the Late Pliocene age.

The studied area contains Simian and Sienna fields, located in WDDM in Simian/Sienna development lease. The fields are approximately 120 kilometers offshore Idku, near Alexandria (Figure 1).

The Simian field was discovered by BG-Egypt. The first well, the Simian-l was drilled in 1999. Simian is a combined stratigraphic-structural trap with dip-closure along the northern and southern margins. The Simian field consists of a number of deep marine channels constrained within a NNE-SSW trending initial channel valley cut. Simian channel system consists of two main branches which merge to the north where the maximum width of the field is over 4.5 km [10] . The SimSat-P1 reservoir is a thin, sheet-like channel system. The reservoir is located at the eastern side of the study area; it is shallower than the main Simian Field (Figure 2). The first discovery well is SimSat-P1 which is located approximately 125 km NE of Alexandria in the Eastern portion of the WDDM concession. Water depth at the well location is approximately 891 m.

The Sienna field is located to the east of Simian. The Sienna field trends NNW-SSE and directly overlies the northernmost part of Simian. Sienna is also a combined structural-stratigraphic trap and has two main branches. The western branch is a typical WDDM channel trend (NNE-SSW) and is confined within a canyon cut. The eastern branch is a combination of channel and sheet sand, which have spilled from the main western channel.

The importance of this study is that it provides an example of integrating different pieces of subsurface data to build a comprehensive geologic understanding of one of the most promising gas potentials in Egypt. The objectives of this paper are to: interpret the main depositional environments of the Plio-Pleistocene reservoirs at the studied area based on seismic, logs and core data; define the depositional architectures and evolution through time; map the regional extension of the reservoir channel bodies by using detailed seismic interpretation; and evaluate the reservoir quality based on the available petrophysical analyses.

The geologic evolution of the Nile Delta (Figure 3) can be traced back to the Late Eocene-Oligocene time [11] - [13] . Recent exploration activities focused on Plio-Pleistocene sequences where the main gas reservoirs are encountered.

The structural configuration of the western side of the offshore Nile Delta is dominated by a major NE-SW striking/NW down throwing normal fault. Seismic interpretation indicates that the maximum throw reaches around 900 ms. This fault throw decreases northward and eventually dies out against a series of NW-SE striking faults. Thickness of the Pliocene-Pleistocene succession remains considerable even in the northern part of the concession. The present slope has a gradient of around 1:25. Regional seismic profiles reveal presence of large scale clinoforms and shelf-slope break. Seismic profiles show that the shelf-slope break was migrated northward with time and evidence of back stepping phases.

The major structural feature within the concession is the SW-NE-trending Nile Delta Offshore Anticline (NDOA). It has been active at various periods during the Pliocene and Pleistocene (Figure 4). Absence of significant thickening of Plio-Pleistocene sequences in the seismic sections indicates that the NDOA did not have a major impact on the depositional geometries [8] .

Early to Late Pliocene is represented by the Kafr El Sheikh Formation. It started with the deposition of the Early Pliocene sediment during marine flooding. This Formation is incised at the top by low-stand and progradingclastics of the Plio-Pleistocene age (El Wastani Formation). The old paleogeography and tectonic elements play an important role in the thinning of the formation onshore. The paleobathymetry of the Kafr El Sheikh indicates an inner to middle tides and it goes outer and upper bathyal northward.

Late Pliocene to Early Pleistocene is represented by El Wastani Formation. It forms a regressive sequence that unconformably overlies the Kafr El Sheikh Formation. The depth of El Wastani Formation ranges from 900 m

to 1000 m. The thickness appears to be controlled by the Rosetta fault that was active in the top of the Kafr El- Sheikh Formation due to the dipping of the formation to the NW and SW.

El Wastani Formation was deposited as low-stands channels extending towards the north in Scarab and Saffron areas. It reflects fan deposits and overbanks on some areas. The paleobathymetry of El Wastani Formation shows a shallow marine, inner to middle neritic southward that changes to outer to upper bathyal in the north.

Pleistocene to Holocene is represented by the MitGhamr and the Bilqas Formations. The MitGhamr Formation was recorded as 464 m thick in the MitGhamr well#1 [13] . It consists of thick layers of sand and pebbles at its base with clay and carbonaceous interbeds of limited thickness. This Formation grades in facies to interbedded sand with clay. The sand is rich in peat, coastal and lagoon fauna. It constitutes a fill-up of the top basin with shelly sands, coquina beds, clays and peat and deposits from the Nile flood. The depositional environment of this formation is probably shallow marine to fluvial environment.

The Bilqas Formation covers the whole Nile Delta area with type section of 25 m at the Bilqas well#1. It is composed of interbedded sand with clay and is rich in molluscan fragments. The clay beds contain plant remains and carbonaceous debris mixed with windblown sand. The faunal association reflects a lagoon, brackish swamp, environment interrupted by beach sand [15] .

The studied wells penetrated a stratigraphic sequence extends from Pleistocene to Pliocene. The rock unit boundaries are identified based on foraminiferal and Nannoplanktonic analyses. The stratigraphic analysis of the studied wells is illustrated in Table 1.

The SimSat-P1, SimSat-P2, Sienna-1 & Sienna-2 reservoirs were covered by 3D surveys in 1998, 2003 and 2006. Seismic features and attributes were identified and subsequently worked up by using the 2006 survey. For

all of the seismic surveys, gas-bearing reservoirs in the Pliocene have soft kicks corresponding to peaks as tops. The bases are marked by hard kicks or troughs. The standard color convention which isused to present all seismic data is a cyan marker on blue to represent a peak (soft kick) and a yellow marker on red to represent a trough (hard kick).

The interpretation of the reprocessed seismic data indicates that the upper most Plio-Pleistocene sequence consists of broadly parallel and continuous reflector that was produced by prograding oblique/complex clinoforms of deep marine shale. This shale represents the major delta outbuilding phase. Formation micro imaging (FMI) was used as a main tool for lithology and sedimentary structure interpretation. The interpreted lithologies were grouped into lithofacies associations that are directly related to certain depositional environments. The FMI have added fine scale details such as vertical facies variations and stratigraphic dip changes, which are both crucial for defining the main lithofacies paleo-slope and paleo-current orientations. FMI tool also provided strong clues for identification of key depositional surfaces.

Detailed sedimentological interpretation was performed over the study area by using FMI along the drilled wells, core data and seismic attributes. The main objectives are to describe lithofacies and determine the depositional environment based on the interpreted sedimentary structures.

The following is a description of each field and the type of channel features existed:

The depositional setting of the Plio-Pliestocene, Wastani Formation, is largely controlled by both relative sea level changes and slope generated by major structural trends (Rosetta and NDOA). The channel evolution through time is not very clearly defined due to the lack of drilled wells, cores and stratigraphic heterogeneities of the reservoirs. High resolution seismic coverage of the Simian-Sienna lease along with FMI data of the available wells provide an excellent opportunity to understand the depositional evolution, reservoir architectures and sand body geometries. The channel architectures and evolution stages are not different from those described nearby fields within WDDM concession such as Scarab [8] and Sequoia fields [19] .

Seismic cross sections across the study area indicate that the sandstone units are characterized by unconstrained sheet-like sandstone and siltstone beds referring most probably to a fan (lobe) deposition. The flat- bedded sand facies is most probably supportive of a more distal, possibly mid-fan setting. Seismic cross sections and FMI show variable types of unconfined and confined channels complexes across the study area. The channels are either single story that refer to a short lived progradational event or multi-story that refer to successive aggradational process. Seismic maximum attribute maps show that laterally amalgamated sinusoid channels are the main channel feature within the unconfined type.

Sienna channel complex is stratigraphically the oldest channel complex in Simian/Sienna development lease. Seismic time slices indicate that Sienna channel complex consists of unconfined channel system with clearly defined evolution stages (Figure 13).

The depositional scenario started with an abandonment stage associated with maximum flooding. This early stage was followed by a fall in relative sea level that resulted in channel progradation particularly towards the west side of the field. The channels fan out towards the west and northwest forming sand bodies with small scale at the west (inner to mid fan) to wide scale, sheet like, at the northwest (outer fan). The east side was only dominated by mud-fill channels and flooding shale due to local changes in seas floor slope at that time (Figure 13).

Progradtional phase widened with time and resulted in accumulation of laterally amalgamated and vertically stacked sinusoidal channels. The final stage in Sienna channel evolution is represented by abandonment phase associated with another local flooding event.

SimSat-P1 and SimSat-P2 reservoirs form isolated sheet-like sand bodies as indicated from the horizontal time slices (Figure 14 and Figure 15). These sheet-like sand bodies form most probably relics of fan deposi-

tional setting. The depositional setting started with flooding event resulted in pre-fan background mudstone and shale deposition followed by progradational phase and deposition of proximal, inner and mid fan deposits. Another flooding stage was followed and resulted in deposition of finer grained, sandstone and mudstone beds of outer fan setting followed by post fan shale beds that indicate the end of the final flooding event.

High-resolution seismic coverage of the SimSat-Sienna fields (West Delta Deep Marine Concession) provides a unique 3-D insight into the sand-body makeup, depositional evolution, and overall reservoir architecture of the submarine slope channel systems. These upper Pliocene channel systems are characterized by a history of erosional and depositional stages probably related to sea level changes. These stages are represented by laterally amalgamated, vertically stacked and abandoned channel bodies. The results concluded can be used as a good analogue to other areas of the world where seismic data is either missing or not of the same resolution.

Integrating high resolution seismic imaging with well data, formation micro images, and core data have significantly contributed in understanding the characteristics and evolution of the studied reservoirs.

Sienna channel complex is represented by unconfined channel system with clearly defined evolution stages, most probably influenced by sea level oscillation. The reservoir is characterized by fine grain siltstones and sandstone interbedded with mudstones and shale that were accumulated in relatively low energy, slope environment. SimSat-P1 and SimSat-P2 reservoirs are characterized by sheet-like sand bodies that also reflect progradational, retro-gradational and flooding events. SimSat-P1 and SimSatP-2 reservoirs represent low energy, less stacked and smaller sand body geometry compared to deeper (older) Pliocene Simian channel system which may indicate decline of energy regime and/or intensity of clastic sediment that the basin received with time.

The authors express their thanks to Petronas and Egyptian General Petroleum Corporation (EGPC) for the permission to publish this paper. We also thank Prof. Adam El Shahat, Geology Department at Mansoura University in Egypt for his revision of this paper. We express our gratitude to Prof. Mahmoud Kora, Geology Department at Mansoura University in Egypt for his constructive recommendations.