Formation evaluation and reservoir characteristics of the Messinian Abu Madi sandstones in Faraskour Gas Field, onshore Nile Delta, Egypt

The Messinian Abu Madi Formation represents the most prospective reservoir target in the Nile Delta. Hydrocarbon exploration endeavors in Nile Delta over the last few decades highlighted some uncertainties related to the predictability and distribution of the Abu Madi best reservoir quality facies. Therefore, this study aims at delineating the factors controlling the petrophysical heterogeneity of the Abu Madi reservoir facies in Faraskour Field, northeastern onshore part of the Nile Delta. This work provides the very first investigation on the reservoir properties of Abu Madi succession outside the main canyon system. In the study area, Abu Madi reservoir is subdivided into two sandstone units (lower fluvial and upper estuarine). Compositionally, quartzose sandstones (quartz > 65%) are more common in the fluvial unit, whereas the estuarine sandstones are often argillaceous (clays > 15%) and glauconitic (glauconite > 10%). The sandstones were classified into four reservoir rock types (RRTI, RRTII, RRTIII, and RRTIV) having different petrophysical characteristics and fluid flow properties. RRTI hosts the quartzose sandstones characterized by mega pore spaces (R35 > 45 µm) and a very well-connected, isotropic pore system. On the other side, RRTIV constitutes the lowest reservoir quality argillaceous sandstones containing meso- and micro-sized pores (R35 > 5 µm) and a pore system dominated by dead ends. Irreducible water saturation increases steadily from RRTI (Swir ~ 5%) to RRTIV (Swir > 20%). Additionally, the gas–water two-phase co-flowing characteristics decrease significantly from RRTI to RRTIV facies. The gaseous hydrocarbons will be able to flow in RRTI facies even at water saturation values exceeding 90%. On the other side, the gas will not be able to displace water in RRTIV sandstones even at water saturation values as low as 40%. Similarly, the influence of confining pressure on porosity and permeability destruction significantly increases from RRTI to RRTIV. Accordingly, RRTI facies are the best reservoir targets and have high potentiality for primary porosity preservation.


Introduction
The Nile Delta with its recent giant onshore and offshore gas discoveries (e.g., Zohr, Noras) is currently proven as the most prolific gas province in North Africa (e.g., Esestime et al. 2016). The main reservoir targets in the Nile Delta range in age from Oligocene to Late Pliocene. The occurrence of many mature source rocks (Jurassic to Early Miocene), as well as the availability of structural and stratigraphic traps, enhanced the potentiality for prospective hydrocarbon accumulations in the Nile Delta (EGPC 1994;Abdel Aal et al. 1994;Dolson et al. 2005). The thermogenic gas, condensate and light oil accumulations in the Nile Delta were mainly sourced from the Jurassic and upper Cretaceous-lower Paleogene source rock intervals (e.g., Vandre et al. 2007; Leila and Moscariello 2017;El-Diasty et al. 2020). On the other hand, the clay-rich successions of the Kafr El-Sheikh Formation constitute the main source of biogenic gases in the Nile Delta onshore region (Vandre et al. 2007). Offshore Nile Delta, the Miocene (Tineh, Qantara, Sidi Salem and Wakar) and Pliocene (Kafr El-Sheikh) shales are proven sources for the biogenic gases (Vandre et al. 2007;Esestime et al. 2016).
The Upper Messinian succession of the Nile Delta (Abu Madi Formation) constitutes a variety of depositional facies (fluvial and estuarine) which constitutes the main hydrocarbon exploration targets, particularly in the 1 3 onshore region (Dolson et al. 2001;Salem et al. 2005;Leila et al. 2016Leila et al. , 2020. The quality of the Abu Madi reservoir facies is largely controlled by their initial depositional conditions, whereas the post-depositional attributes play a relatively minor role (Salem et al. 2005;. Despite their importance hosting the majority of gas reserves in the onshore Nile delta, a detailed rock typing of the Abu Madi sediments as well as accurate identification of their flow properties and the predictability of their spatial distribution and architecture have never been performed. Therefore, the present study will tackle these issues through comprehensive integration between seismic, petrography, as well as routine and special core analyses. Moreover, this study will focus for the first time on Abu Madi system in the extreme northeastern margins of the Nile Delta outside the Abu Madi main canyon play (Fig. 1). This study will enhance the predictability of the Abu Madi reservoir performance which will direct the development strategies in the study area.

Geologic setting
The Nile Delta was initially developed during the Late Eocene-Early Miocene times in an area very close to the Qattara Depression and suddenly moved to its present position during Late Miocene (Salem 1976;Said 1990;Sestini 1995). The tectonic history of the Nile Delta comprised five main stages from the cratonic phase after the Caledonian-Hercynian Orogeny, rift phase (Triassic), passive margin phase (Jurassic-Early Cretaceous), Alpine compressional phase (Late Cretaceous-Eocene), and then a foreland phase during Late Eocene and Oligocene (EGPC 1994). These phases controlled not only the structural and depositional evolution but also the hydrocarbon prospectivity of the Nile Delta (Meshref 1990;Sarhan and Hemdan 1994;Abdel Aal et al. 1994;Kamel et al. 1998). Currently, the Nile Delta presents a passive margin basin in northeastern part of the African continental margin (Dixon and Robertson 1984;Sarhan and Hemdan 1994). The rifting phase of the Tethyan margin and the opening of the Mediterranean resulted in an E-W flexure zone "Hinge Zone" which controlled the Paleogene-Neogene tectono-stratigraphic evolution of the Nile Delta (Fig. 1;Orwig 1982;Harms and Wray 1990;Kamel et al. 1998). The Hinge Zone represents the boundary between the platformal carbonates of the south delta block and the north Nile Delta basinal facies (Sarhan and Hemdan 1994;Kamel et al. 1998).
Shallow-water carbonate sedimentation predominates the Nile Delta during the Jurassic up to Early Cretaceous (Bayoumi and Lotfy 1989;Guiraud and Bosworth 1999). Open marine conditions prevailed during Early Cretaceous, and by the end of the Aptian alternating shallowwater and alluvial sedimentation prevailed in the Nile Delta (Guiraud and Bosworth 1999). During Paleogene, continental sedimentation prevailed subsequent to the onset of the Gulf of Suez rifting phase and the northward tilting of the Nile Delta toward the Mediterranean (Rizzini et al. 1978;Sarhan and Hemdan 1994;Hussein and Abd-Allah 2001;Dolson et al. 2005). Early Miocene was marked by major marine transgression and deposition of thick marine shales and basinal marls (Burdigalian, Qantara Formation). Shallow marine sedimentation predominates the Nile Delta during Serravallian; the high subsidence rates and elevated sediment influx resulted in deposition of thick shallow marine shales interbedded with dolomitic marls and lenticular sandstones (Sidi Salem Formation) ( Fig. 2; Rizzini et al. 1978;Kamel et al. 1998;Dolson et al. 2005). Regressive phase prevailed in the Nile Delta during Tortonian-Early Messinian, and prograding deltaic sedimentation predominates in the onshore regions (Qawasim Formation). Synchronously, deposition of turbiditic complexes predominates in the Nile Delta offshore parts (Wakar Formation). By the end of Miocene, the Mediterranean was partially or completely desiccated during the Messinian salinity crisis (MSC) (Hsü et al. 1973). Subsequently, deep subaerial canyons were incised along the Mediterranean margins and thick marginal and abyssal evaporites were accumulated in the basins. These canyons are poorly documented along the North African margin except in the Nile Delta where the large Eonile canyon is well known from both onshore and offshore regions (e.g., Barber 1981;Palmieri et al. 1996;Dalla et al. 1997;Dolson et al. 2001Dolson et al. , 2005Leila et al. 2020). An incipient sea-level rise during the late stage of the MSC results in a backstepping infill of the Eonile canyon by fluvial and estuarine sandstones (Abu Madi Formation) (Palmieri et al. 1996;Dalla et al. 1997;Dolson et al. 2001Dolson et al. , 2005.

Data and methods
The present study is based on a complete subsurface dataset (seismic, logs and cores) from Faraskour field. The dataset was analyzed and interpreted following a stepwise procedure in order to achieve the study objectives (Fig. 3).

Seismic interpretation
Seismic interpretation comprises detailed analyses of the available 2D reflection seismic profiles covering the study area ( Fig. 1). This constitutes a proper identification of the seismic sequence boundaries bounding the Messinian Abu Madi succession. Moreover, the internal seismic facies encountered within the Abu Madi succession were described by tracking the different reflection configurations and stratal terminations (e.g., Vail et al. 1977;Pigott and Radivojevic 2010).

Sandstone petrography
Petrographic analyses involved the examination of 18 thin sections from the Abu Madi sandstones in Faraskour-4 well using the petrographic microscope under plane polarized light. Thin sections preparation was conducted at Corex Laboratories, Egypt. The thin sections were stained with blue dye for porosity visualization as well as Alizarin Red-S and K-ferricyanide to identify the different carbonate phases. Sodium cobalt nitrate dye was utilized to facilitate the identification of feldspar (e.g., Houghton 1980).

Petrophysical analyses
The different petrophysical parameters (porosity, permeability and fluid saturations) were calculated for the entire Abu Madi Formation using a complete conventional wireline log data retrieved from Faraskour-4 well. Effective porosity was determined from porosity logs and was corrected for the gas effect within the gas-bearing zones following the procedure described by Schlumberger (1974). Water saturation and subsequently hydrocarbon saturation in the uninvaded zone were estimated using Archie's equation (1942). Moreover, the permeability values were calculated as a function of the irreducible water saturation and effective porosity using Timur equation (Timur 1968).
Conventional core analyses (CCA) include porosity, permeability and grain density measurements. These were performed at the Egyptian Petroleum Research Institute (EPRI), Egypt, on a total of 18 conventional core plugs selected from the Messinian Abu Madi succession in Faraskour-4 well (Fig. 2).
The porosity and permeability values were measured using helium gas expansion porosimeter and nitrogen permeameter, respectively, under ambient pressure conditions. Distribution of the measured porosity and permeability values is presented in Fig. 4. In this study, the concept of the hydraulic flow unit proposed by Amaefule et al. (1993) was applied to evaluate the hydraulic flow properties of the Abu Madi Formation. The proposed hydraulic flow parameters which include the reservoir quality index (RQI), normalized porosity index (NPI) and flow zone indicator (FZI) are used in the classification of the Abu Madi Formation into reservoir rock types (RRT). The applied parameters were calculated as follows: (1) RQI = 0.0314 √ K∕ (2) NPI = 1 − Moreover, the pore throat radius was calculated using the equation proposed by Kolodzie (1980) as follows: The special core analyses (SCA) involve porosity and permeability measurements under confining pressure conditions by applying an effective overburden pressure of 4700, 5700 and 6700 psi. This was carried out on four core plug samples by mounting the samples in a hydraulic core holder after saturation with synthetic formation water. Each sample was then loaded into triaxial core holder. A pipette was connected to the end stem of each core holder. The confining stress was raised incrementally, and the reduction in the pore volume was monitored as an increase in the pipette readings.

Abu Madi seismic and sedimentary facies
The Messinian succession is bounded by two seismic sequence boundaries (Figs. 5a, 6a). The lower is distinguished by the erosional truncation terminations in which the pre-MSC seismic reflections were abruptly terminated against the overlying unconformity surface. This seismic unconformity is represented by high-amplitude, irregular reflection displaying highly erosive, wide U-and V-shaped Paving the canyon floor, low-amplitude, semitransparent and semi-chaotic seismic reflections (Fig. 3a) occur and likely correspond to the high-energy subaerial gravity flow deposits synchronously accumulated during the canyon incision (e.g., Leila and Moscariello 2019). These lowamplitude, transparent facies are followed by two parallel, continuous and high-amplitude reflectors (Figs. 5b, 6b). The extracted root-mean-square (RMS) amplitude map along the lower reflector (@ 2350 ms ms) highlights high-amplitude anomalies distributed in NW-SE oriented channels Illustrative flowchart shows the stepwise procedure followed in this study for data analysis and interpretation ( Fig. 7a). Additionally, the RMS amplitude map along the upper reflector (@ 2320 ms) illustrates NW-SE oriented small high-amplitude bars (Fig. 7b). The gamma ray counts corresponding the NW-SE oriented channels are generally low (< 55 API) reflecting a clean sandy composition. Moreover, the blocky-shaped gamma ray motif having a sharp base suggests in-channel deposition (Fig. 7c). The basal part of this interval is characterized by an upward decrease in the gamma ray values, revealing a coarsening upward succession likely reflecting the amalgamation of the channel sand bars. This is followed by a bell-shaped fining-upward gamma ray pattern having comparatively elevated gamma ray counts (> 80 API) suggesting an upward increase in the argillaceous materials. This is consistent with the core observations as the channel bars sandstones consist of coarse-grained massive sandstone and planar cross-stratified sandstones grade upward into fine-grained, parallel laminated argillaceous sandstone (Fig. 7c). The occurrence of dewatering features and dish structures suggest deposition under high-energy conditions (e.g., Miall 1996;Tucker 2001). This depositional pattern corresponds to the backstepping fluvial channel-fill facies that were accumulated during the late stage of the MSC (e.g., Palmieri et al. 1996;Dolson et al. 2001;. The upward decrease in grain size, increase in mud content and appearance of lamination suggest a gradual decrease in the flow energy and possibly the transition from pure fluvial to mixed energy depositional conditions consistent with the upward reversal of the gamma ray log motif (e.g., Terwindt 1971; Van den berg et al. 2007). The sedimentary facies of the Abu Madi upper reflector consist of mudstone/siltstone facies (~ 35 m) which cap the sandstone interval and consist of massive and laminated mudstones interbedded with siltstones. Planar pinstripe laminae, tidal bundles and mud drapes are the main observed sedimentary structures (Fig. 7c). These muddy-rich facies were mostly deposited in a tidal flat environment and record the complete transition from fluvial to tidal-dominated conditions (e.g., Klein 1971;Buatois et al. 1999;. The mudstone facies are followed by finingupward sandstones (11 m thick) which likely represent the tidal channel-fill facies overlain by 13-m-thick argillaceous sandstone interval corresponding to the tidal sand bar facies consistent with seismic observations (Fig. 8b). These facies represent the transgressive estuarine sediments deposited during the late MSC incipient sea-level rise (Lago Mare phase, Dabrio and Polo 1995;Krijgsman et al. 2001).

Abu Madi sandstones' composition
Petrographic investigation reveals that the Abu Madi sandstones vary in composition from clean quartzose to glauconitic and argillaceous sandstones. The quartzose sandstones are dominated by quartz (average (Av.) ~ 65%) and comprise approximately 60% of the Abu Madi fluvial facies (Fig. 8). They consist mainly of detrital quartz grains with subordinate quantities of K-feldspars and lithic fragments (Fig. 9a) and therefore range in composition from quartz arenite and subarkose arenite microfacies. The quartz framework grains are rounded to sub-rounded, ranging from illsorted to moderately sorted and very fine-to coarse-grained. The lithic fragments are mainly granitic; sedimentary lithics (e.g., carbonate, claystone) are also present (Fig. 9b).
Glauconitic sandstones comprise the glauconite-rich (Av. > 10%) samples which constitute approximately 21% of the Abu Madi fluvial interval (Fig. 8). It is made of fineto medium-grained, sub-angular, moderately sorted detrital quartz grains with abundant pore-filling and detrital  (Fig. 9c, d). The glauconitic sandstones also contain subordinate quantities of bioclasts, K-feldspars as well as sedimentary lithic fragments (claystones) and are mostly classified as glauconitic subarkose arenites. Detrital clays are present as pore-filling matrix materials (Fig. 9c). Well-rounded, fine-grained glauconite pellets are very common. Pore-filling patchy glauconite is also observed in the intergranular pores. Replacive glauconite is occasionally present due to replacement of biogenic fragments and bioclasts (Fig. 9c). The abundance of wellrounded pellets as well as the common occurrence of porefilling and replacive components suggests both detrital and authigenic origin of the glauconite (e.g., Loutit et al. 1988;Amorosi and Centineo 1997;Amorosi et al. 2007). Authigenic calcite is common and is often represented by patchy pore-filling spray (Fig. 9c) and micro-spray calcite crystals around the detrital and in the dissolved framework components (Fig. 9d). On wireline logs, the glauconitic sandstones are recognized by comparatively elevated gamma ray (85-95 API) and RHOMA values (2.68-2.70 g/cm 3 ) (Fig. 8).
Argillaceous sandstones constitute the samples containing abundant argillaceous detrital and authigenic materials (total clays > 15%). They comprise approximately 15% of the Abu Madi fluvial facies (Fig. 8). They are often associated with the glauconitic sandstones and consist of fine-to mediumgrained, sub-angular to rounded, moderately sorted quartz grains (Fig. 9e, f). The detrital framework components of the argillaceous sandstones also comprise fine-grained detrital glauconite pellets, bioclasts and claystone rock fragments. They are mostly classified as argillaceous quartz arenite. Detrital clay matrix is common, and clay cement comprises the most abundant authigenic phases (Fig. 9e, f). The argillaceous sandstones have distinctive gamma ray signature characterized by very high counts (> 95 API) (Fig. 8).
The distinctive wireline log signatures of the studied sandstones and their robust correlation with core and petrographic observations provide a good qualitative tool to subdivide the uncored Abu Madi estuarine unit. Glauconitic sandstones constitute approximately 42% of Abu Madi estuarine sandstones (Fig. 10). The quartzose sandstones are also common and comprise about 26%. They are more frequent in the upper part of the estuarine unit. Thin interbeds of argillaceous sandstones are mostly associated with the quartzose facies and constitute about 16% of the sandstone microfacies in the Abu Madi estuarine unit. Another sandstone microfacies is recorded at the base of the Abu Madi estuarine unit and is characterized by a distinctive log signature that was not observed in the underlying fluvial interval. This microfacies overlies the tidal mud flat deposits and is characterized by very low gamma ray (< 35 API) and high matrix density values (> 2.9 g/cm 3 ) (Fig. 10). These characteristics reveal a carbonate-rich facies and mostly coincide with the calcareous sandstones recorded at the base of the Abu Madi estuarine interval (Salem et al. 2005

Petrophysical characterization of the Abu Madi sandstones
The measured core plug petrophysical parameters vary significantly between the studied sandstones microfacies  (Table 1). Abu Madi quartzose sandstones constitute the highest permeability values (Av. = 2958 md, Table 1); however, the values vary greatly (standard deviation (SD) = 1468), typifying heterogeneous flow characteristics. The lowest permeability values are observed in the argillaceous sandstones (Av. = 56.6; SD = 51.4). Porosity values are quite similar in the quartzose and glauconitic sandstones (Av. = 27.1% and 28%, SD = 1.4 and 0.89, respectively). The lowest porosity values occur in the argillaceous sandstones (Av. = 25.9%) and therefore characterized by the lowest storage capacity properties. The grain density also varies greatly in the studied sandstones (Table 1); the quartzose and glauconitic sandstones have similar values (Av. = 2.64 g/cm 3 and 2.65 g/cm 3 ; SD = 0.008 and 0.03, respectively) suggesting a dominant quartz-rich composition. On the other hand, the argillaceous sandstones have a comparatively higher values (Av. = 2.71 g/cm 3 ) reflecting the impact of the clay matrix on the grain density readings.

Pore system characteristics
The pore system of the Abu Madi sandstones constitutes mainly primary intergranular pores, whereas the secondary pore spaces are comparatively less common (Fig. 9). The secondary pores were mainly formed due to diagenetic dissolution of bioclasts; however, they were later filled by either authigenic calcite or glauconite (Fig. 9c). In some particular cases, authigenic microcrystalline calcite coatings preserve the secondary pore spaces. It is evident that the pore system of the quartzose sandstones is characterized by lowest contents of detrital clay matrix and ductile pseudomatrix materials (Fig. 9a). Similarly, authigenic minerals present minor phases except the presence of some pore-lining microcrystalline euhedral quartz overgrowth. Thereby, the primary intergranular pores are very well connected, resulting in the formation of large pore-channel network with widest pore throats (Fig. 9a).
The pore system of the glauconitic sandstones is characterized by small-to moderate-sized intergranular pore spaces. The ductile detrital glauconite pellets are commonly squeezed in the pore spaces between the ridged quartz grains forming pseudomatrix components. The pore throat of the glauconitic sandstone is mostly obstructed by both authigenic pore-filling and glauconite in the pseudomatrix (Fig. 10c, d). Consequently, the glauconitic sandstones have a comparatively narrower pore throats than the quartzose facies. However, the glauconitic sandstone intervals having low quantities of clay matrix are mostly characterized by well-connected pore system. The argillaceous sandstones contain abundant primary intergranular pore spaces. However, the pore-filling clay matrix reduces the pore throat radius, resulting in a tortoise pore system with poor pore connectivity (Fig. 10e, f). Therefore, the pore system of the argillaceous microfacies is mostly characterized by abundant isolated dead ends (Fig. 10e, f).

Petrophysical rock typing
Four different reservoir rock types (RRT) are observed in the studied Abu Madi sandstones. These rock types have widely variable hydraulic storage (porosity) and flow (permeability) characteristics (Fig. 11a) and therefore represent different hydraulic flow units (HFU) (e.g., Bear 1972;Ebanks 1987;Amaefule et al. 1993; Table 2). RRTI has the best hydraulic flow characteristics with the highest RQI and FZI values (Table 2). RRTI hosts only quartzose sandstone samples characterized by the highest porosity and permeability values with well-connected pore-channel network (Fig. 11a). RRTI sandstones often have large and elongate pores which are mainly distributed in connected pore channels with large pore throats boosting their reservoir flow characteristics. Similarly, RRTII is also dominated by the quartzose sandstones and therefore has grain density values similar to RRTI (Fig. 11b). Both RRTI and RRTII have common mega pore spaces (> 10 µm) (Fig. 11c, Table 2) and are considered the main fluid flow conduits within the Abu Madi Formation in the study area. However, the wide variation in pore throat radius of RRTI and RRTII despite their similar grain density values (Fig. 11c, d) is mostly attributed to the variation in grain size. RRTII sandstones are often dominated by the fine-grained sediments characterized by high porosity but comparatively smaller pore throats. RRTIII and RRTIV comprise the fluid flow baffles and barriers with moderate to low storage and flow capacities (Table 2). They constitute the glauconitic and argillaceous sandstones with pore systems dominated by pore-lining glauconite and clay matrix which effectively reduced the radius of the pore throats (< 10 µm).
In terms of hydraulic flow properties, the RRTI and RRTII quartzose sandstones have excellent reservoir properties and flow characteristics and are considered as the main hydraulic flow conduits within the Abu Madi Formation in the study area (Fig. 12a). On the other hand, the argillaceous and glauconitic sandstones of RRTIII and RRTIV present the flow baffles and barriers. The log-log plot of RQI versus NPI typifies a set of straight lines corresponding to variable FZI ranks which reflect different classification of the hydraulic flow units (HFUs). Five HFUs have been recognized; each HFU hosts a cluster of samples having homogeneous hydraulic flow properties (Fig. 12b). HFU4 and HFU5 have the best flow properties and thereby comprise the flow conduits intervals in RRTI and RRTII sandstones. In contrast, Table 1 Values of the petrophysical characteristics obtained from the conventional core analysis (CCA) for the studied Abu Madi sandstones Reservoir quality index (RQI), normalized porosity index (NPI), flow zone indicator (FZI), pore throat radius (R 35 ), minimum (min), maximum (max.), average (Av.) and standard deviation (SD) and number of analyzed samples (n)

Petrophysical parameters
Quartzose sandstone (n = 9) Glauconitic sandstone (n = 5) Argillaceous sandstone (n = 4) the flow baffles and barriers are classified as HFU3, HFU-2 and HFU-1 and therefore host the argillaceous and glauconitic sandstones of RRTIII and RRTIV. The variation in flow properties is largely contributed to the pore throat characteristics. Notably, the flow conduits (RRTI, RRTII) have a pore system dominated by megapores, whereas the pore system of the flow baffles and barriers (RRTIII, RRTIV) is dominated by meso-and micropores (Fig. 12c, d).
The same reservoir rock types are also retrieved from the wire line log responses for the Abu Madi fluvial interval (Fig. 13a, b). The four different rock types are clearly observed on the log-derived porosity and permeability cross-plot. RRTI sandstones have porosity values above 20% and permeability values exceeding 1000 md; RRTII sandstones display lower poro-perm values and, however, display comparatively elevated V Sh (shale volume) and S w (water saturation) values (Table 3) and mostly constitute the fine-grained quartzose sand intervals. Additionally, RRTIII and RRTIV sandstones have the lowest porosity and permeability and, however, display the highest V sh and S w values (Table 3). RRTI and RRTII constitute, respectively, 26% and 39% of the Abu Madi fluvial unit. On the other hand, RRTIII comprises 19% and RRTIV constitutes 15% of the Abu Madi fluvial succession. Accordingly, the Abu Madi fluvial unit being dominated by RRTI and RRTII sandstones hosts the main fluid flow conduits and is therefore considered as the most prospective reservoir target in the study area. On the other side, RRTI sandstones are not recorded in the Abu Madi estuarine interval which contains only three reservoir rock types RRTII, RRTIII  and RRTIV (Fig. 14a, b). The quartzose sandstones in the estuarine unit are mostly classified as RRTII characterized by permeability values lower than 1000 md and wide range of grain density values (2.55-2.95 g/cm 3 ) suggesting their heterogeneous pore system with abundant fine-grained materials. Similar to the fluvial succession, the glauconitic and argillaceous sandstones have the characteristics of RRTIII and RRTIV rocks. In addition, the calcareous sandstones were plotted with the RRTIV rocks, indicating their poor fluid storage and flow characteristics due to the abundance of pore-filling carbonate cements which completely block the pore network (Fig. 14a, b). Quantitatively, the fluid flow baffles and barriers (RRTIII and RRTIV) constitute approximately 74% of the Abu Madi estuarine sandstone unit revealing its poor potentiality as a target for future exploration in the Nile Delta.

Capillary characteristics of the Abu Madi reservoir rock types
Capillary pressure results show that the RRTI sandstones have the lowest irreducible water saturation values (S wir < 10%) which increase steadily from RRTI to RRTIV (Fig. 15a). The high S wir values of RRTIV sandstones are mostly attributed to their elevated clay matrix and cement contents, resulting in a poor connected pore system dominated by dead end pores (Fig. 9e, f). Elevated S wir values in RRTIV sandstones result in increasing their initial displacement pressure, thus decreasing the hydrocarbon flow efficiency within the pore system, and thereby the hydrocarbons cannot displace the pore-water at low pressure values (e.g., Jennings 1987). Additionally, the characteristics of the "gas-water two-phase co-flowing zone" in the   (Fig. 15b). The relative permeability results show that the RRTI sandstones have the lowest water wetting characteristics; and the intensity of water wetting steadily increases from RRTI to RRTIV. This behavior is mostly due to the abundance of dead end pores in the RRTIII and RRTIV rocks (Fig. 9c, e). It is therefore obvious that the pore structure heterogeneity results in an increase in the irreducible water saturation and decrease in the movable fluid saturation (e.g., Wang et al. 2019). It is clear that RRTI sandstones have the best gas flow characteristics and its Krw at the critical gas saturation is approximately 0.9, which means that the gas will still be able to flow in RRTI sandstones even at high water saturation values exceeding 90% (Fig. 15b). On the other hand, Krw at the critical gas saturation is 0.4 for RRTIV Fig. 15 Water saturation versus capillary pressure (a) and gas, water relative permeability (b) illustrating the variations in fluid flow characteristics and pore system within the different reservoir rock types sandstones, indicating that the gas will lose its flow capacity at 40% water saturation.

Impact of confining pressure on poro-perm properties
The impact of confining pressure on porosity increases from RRTI to RRTIV facies (Fig. 16a). RRTI sandstones are the most resistant to porosity and permeability destruction up on compaction (Fig. 16b). The impact on permeability of confining pressure is very small in RRTI facies, indicating that the connectivity of the pore system remained unaffected under elevated confining pressure values. The change in pore volume under the effect of confining pressure is the highest for RRTIV sandstones which lose approximately 7% of its original porosity under 4700 psi confining stress (Fig. 17a). On the other side, the RRTI sandstones only lose 3% of its porosity under the same confining stress. This is mainly attributed to the composition of RRTI sandstones dominated by rigid quartz grains as well as microcrystalline quartz coatings which support the facies framework strength against compaction (e.g., Jonas and McBride 1977;Leila et al. 2018; Fig. 17b). In contrast, the ductile fragments (e.g., claystone) in RRTII sandstones (Fig. 9b) are squeezed in the pore spaces, resulting in a significant decrease in porosity (~ 4.5%) upon compaction (Fig. 17b). Similarly, the brittle platy materials (e.g., clays), ductile clay matrix and glauconite pellets in RRTIII and RRTIV facies are squeezed and broken between the rigid framework grains upon compaction, resulting in a wide porosity reduction (~ 5.5-6.5%) (Fig. 17b). Accordingly, RRTI sandstones have a good potentiality for primary porosity preservation and therefore will not lose their excellent flow characteristics even at great burial depths.

Conclusions
• The Abu Madi Formation in the study area constitutes two main units. The lower is dominated by fluvial sandstones deposited as amalgamated channel sand bars, whereas the upper interval mostly contains finergrained, muddy-rich estuarine sandstones. • The Abu Madi fluvial unit consists mainly of quartzose sandstones, whereas the estuarine interval hosts abundant glauconitic and argillaceous sandstones. • The quartzose sandstones have the highest porosity and permeability values with an isotropic pore system dominated by well-connected pore channels. On the other hand, the glauconitic and argillaceous samples have a Fig. 17 a Impact of confining pressure on the pore volume of the studied RRT, b illustrative drawings and thin sections illustrating the different responses of each RRT to the confining stress highly tortoise pore system dominated by micro-and meso-sized dead ends. • Four reservoir rock types (RRTI, RRTII, RRTIII, and RRTIV) were identified in the studied intervals. Quartzose sandstones are mainly classified as RRTI and RRTII rocks; and RRTIII and RRTIV mostly host the glauconitic and argillaceous sandstone microfacies. • The Abu Madi fluvial unit hosts the highest thickness of RRTI rocks; in contrast, RRTIII and RRTIV predominate in the estuarine interval. • RRTI and RRTII rocks have the best fluid flow properties (Phi, K, RQI, FZI, and R 35 ) and are characterized by well-connected pore system with very low irreducible water saturation. • RRTIII and RRTIV have the worst fluid flow properties with pore system dominated by dead end pores and therefore contain high percentage of irreducible water. • Confining pressure has a greater impact on the RRTIII and RRTIV sandstones, whereas the RRTI sandstones have a larger potentiality for primary porosity preservation and therefore remain the most potential reservoir targets even at elevated burial depths.