1 Introduction

Nile Delta Basin (NDB) is a fan-shaped delta that represents passive-margin basin covering about 250,000 km2, in eastern Mediterranean region (Sestini 1989). It has a 6.0 km thick sedimentary section from the Oligocene to the Quaternary strata (Hussein and Abd-Allah 2001). The NDB has an extended history of gas exploration from the Oligocene to the Quaternary rocks throughout the last 50 years. Messinian Abu Madi reservoir (AMR) is the main gas reservoir in NDB and has numerous published works since 1963 (e.g. Abu El-Ella 1990; Salem et al. 2005; Pigott and Abdel-Fattah 2014; Shebl et al. 2019; Sarhan 2021; Abdel‐Fattah 2014; Abdel-Fattah and Tawfik 2015; Shehata et al. 2023; Abdel-Fattah et al. 2022 & 2023; Sarhan et al. 2022; Sarhan 2022a, b, c; El-Fawal et al. 2016; Ewida and Sarhan 2023; Nabawy et al. 2018; Sarhan and Safa 2019).

The Messinian AMR is composed of sandstones with shale interbeds (Rizzini et al. 1978) and was deposited as incised valley fills (IVFs). It represents the proto-Nile River sensu Barbar (1981), which eroded the pre-existing continental shelf/slope areas at the northern NDB. This incision was a response to the global reduction in the sea level through the Messinian age (Sestini 1989; EGPC 1994; El-Heiny and Morsi 1992; Barber 1981; Harms and Wary 1990). This sea level fall also caused a deposition of thick section of evaporites called the "Rosetta Formation" which is the equivalent of the Abu Madi Formation in the NDB. Moreover, Abu Madi Formation was formed at the final stage of the Messinian syn-rift megasequence, which was related to the opening of the Red Sea-Gulf of Suez. The resulted rift propagated to the northeastward and affected the depositional regime beneath NDB (Sarhan et al. 2014; Sarhan 2015, 2022a).

The main aims of this study are: (1) to present detailed facies descriptions and sedimentological analysis of studied wells that allow determining the depositional environments; (2) to construct a depositional model, interpreting the spatial and temporal variations of the identified facies; (3) to evaluate the key petrophysical parameters of the AMR sandstones; and (4) to determine the effect of facies heterogeneities on petrophysical characteristics and reservoir quality. The findings of this study have important implications for the future exploration of new potential Abu Madi targets in Nile Delta Basin.

2 Geologic setting

The NDB was formed by thermal subsidence that followed the Triassic–Cretaceous rift that detached African–Arabian plate and Eurasian plate (e.g. May 1991; Dolson et al. 2001). Southern borderline of this rift was referred to as "Hinge Zone" (Harms and Wray 1990). The NW offshore part of the NDB is controlled by the NE–SW fault of the Rosetta trend, whereas the NE portion is affected by NW–SE fault of the Bardawil trend (Fig. 1).

Fig. 1
figure 1

A Location Map (black square) displays the study area in the offshore Nile Delta and the geologic setting in the Messinian age (modified after Abdel Aal et al. 1994). B Locations of the inspected wells within the study area

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The NDB was formed by the subsequent thermal cooling of a major rift initiated throughout the Jurassic–Early Cretaceous along northern Egypt. Its southern border called the "Hinge Zone" (Fig. 1) which divides northern NDB on an E-W trend and affected the tectonic and stratigraphic evolution of northern central part of the NDB (Said 1962; Said 1981; Arisi Rota et al. 1994; Sestini 1989; Bertello et al. 1996; Harms and  Wray 1990; Mosconi et al. 1996). Abundant extensional faults were originated along this Hinge Zone and throw toward the north direction. As a result, the sedimentary section became thicker, with a thickness of 5000 up to 7000 m northern this line, while southern this Hinge Zone, the thickness varies from 500 to 1500 m (Harms and Wray 1990; EGPC 1994; Sarhan and Hemdan 1994).

Through the Messinian time, the Mediterranean Sea was desiccated and isolated from the ocean. Afterward, the Eonile was developed as north-trending streams at the latitude of Cairo (Said 1990; Harms and Wray 1990). Eonile cut deep canyon (i.e. Abu Madi incision) (Fig. 2) when drainage systems transformed northward in the Late Miocene from the northwestward throughout the Early Miocene (Ross and Uchupi 1977). During the Pliocene–Pleistocene time, a compression belt emerged in the NDB, associated with a sinistral-wrench movement. This resulted in forming the Pelusium fault which runs NE-SW over the northwest offshore areas of Sinai, restricts the southern sector of the NDB and extends westward of the African Plate (Zaghloul et al. 2001).

Fig. 2
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E-W seismic sections show the geometry of the Abu Madi incision within the study area (modified after Sarhan et al. 2022)

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3 Data and methods

The provided geophysical data for the current study encompasses the composite logs as well as the conventional wireline-log data (gamma ray, density, neutron, and resistivity) for four wells; BE-3, BN-2, BN-1 and Mina-1 well located within the Abu Madi incision (Fig. 1B). Detailed-core descriptions together with the petrographic identification correspond the AMR of the Mina-1 well and BE-1 well are also available.

The methods used to evaluate the AMR in the studied wells began with the sedimentary facies were recognized based on the accessible core data of Mina-1 well (from the depth interval 3592–3602.70 m) and the conventional core intervals of 150 m from BE-1 well (which is located within Baltim field close to the examined wells). The interpreted facies were calibrated with their wireline-log responses to define a series of well log-characteristics which led to the interpretation of probable sedimentary facies over the un-cored intervals.

The estimation of the primary petrophysical parameters to outline the most promising gas-bearing zones inside the AMR in the inspected wells were done by applying the following equations:

3.1 Shale volume (Vsh)

Based on the gamma ray logs, quantity of clay in the investigated zones was calculated using Asquith and Gibson’s Eq. (1982):

$${\text{V}}_{{{\text{Sh}}}} = \, ({\text{GR}} - {\text{GR}}_{{{\text{min}}}} )/({\text{GR}}_{{{\text{max}}}} - {\text{GR}}_{{{\text{min}}}} )$$
(1)

where GR = gamma ray value, GRmin = the smallest gamma ray value, GRmax = the highest gamma ray value.

3.2 Total porosity (ϕT)

It was computed from density- neutron logs by using the subsequent equation after Asquith and Gibson (1982):

$$\Phi_{{\text{T}}} = \, (\Phi_{{\text{N}}} + \Phi_{{\text{D}}} )/{2}$$
(2)

where ΦN = porosity value from neutron log; ΦD = density porosity and can be computed as:

$$\Phi_{{\text{D}}} = \, (\rho_{{{\text{ma}}}} - \rho_{{\text{b}}} )/(\rho_{{{\text{ma}}}} - \rho_{{{\text{fl}}}} )$$
(3)

where ρma = matrix density, ρb = bulk density of the studied formation, ρfl = fluid density.

3.3 Effective porosity (ϕe)

The following equation was applied to calculate the effective porosity (after Asquith and Gibson 1982):

$$\phi_{{{\text{e }} = }} \phi_{{\text{T}}} * \left( {{1 } - {\text{ V}}_{{{\text{Sh}}}} } \right)$$
(4)

3.4 Water saturation (Sw)

It was determined using Indonesia equation (Poupon and Leveaux 1971) as follow:

$$S_{w} = \left\{ {\left[ {\left( {\frac{{V_{{{\text{Sh}}}}^{{2 - V_{{{\text{Sh}}}} }} }}{{R_{{{\text{Sh}}}} }}} \right)^{\frac{1}{2}} + \left( {\frac{{\phi_{e}^{m} }}{{R_{{\text{w}}} }}} \right)^{\frac{1}{2}} } \right]^{2} R_{t} } \right\}^{{\frac{ - 1}{n}}}$$
(5)

where RSh = shale resistivity, Rt = true resistivity, Rw = connate water resistivity, m = cementation exponent, n = saturation exponent, a = tortuosity factor.

4 Results and interpretations

4.1 Facies analysis

Seven main clastic facies types are identified in the Abu Madi Formation based on the studied cored intervals from Well BE-1 and Well Mina-1 (Figs. 3 and 4). These facies were categorized based on grain size and physical sedimentary structures according to facies classification schemes presented by Miall (1996) and Colombera et al. (2013). Detailed descriptions and interpretations of the identified facies (Fig. 4) are given below. Table 1 summarizes the characteristics of the recognized facies. The clastic facies consist of cyclic alternations of upward-fining sandstones and mudstones. The very fine to coarse-grained sandstones prevail at the base of the Abu Madi Formation (Fig. 3). The mudstones are generally occur upwards at the top of upward-fining packages (Figs. 3 and 4).

Fig. 3
figure 3figure 3

Detailed sedimentary logs of the reservoir cored intervals of A Mina-1 well and B BE-1 well, offshore Nile Delta, Egypt. Location of the cores in Fig. 1

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Fig. 4
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Detailed core photographs A–M and thin sections (‘1’ to ‘6’) for the identified facies. A Facies St characterized by upward decrease of sand size and fine to coarse grained monocrystalline quartz with concavo-convex contact (section ‘1’) overlain by Facies Sr and Facies Sl; B Facies Sp with millimeter-thick organic-rich laminae and dominated by monocrystalline quartz, feldspars, detrital clays, iron oxides and foraminifera (section ‘2’); C Facies Sm overlain by Facies He and Facies Sr with lenticular and rippled cross laminations (section ‘3’), and increase of sand content towards its top; D Massive sandstone (Facies Sm) overlain by alternation of Facies He and Facies Fl; E Heterolithic sandstone and mudstone of Facies He overlain by Facies Sp with horizontal burrows (Hb); F Heterolithic sandstone and mudstone of Facies He overlain by massive sandstone of Facies Sm with gypsum veins and upward increase of grain size; G Parallel laminated sandstones Facies Sl with drifted carbonaceous material dominated by subarkose (section ‘4’); H Planar cross-bedding characteristic of Facies-Sp with horizontal burrows (Hb); I Facies Sm overlain by Facies Fl with desiccation cracks and gypsum veinlets; J Heterolithic sandstone and mudstone of Facies He dominated by very fine grained sandstone to siltstone (section ‘5’) and overlain by Facies Sm and Sl; K Parallel-laminations of Facies Sl alternating with Facies Fl through discordant surfaces; L Massive sandstone Facies Sm overlain by parallel lamination sandstone Facies Sl across transitional subfacies with faint lamination; and M Massive sandstone overlain by alternating parallel laminated sandstone Facies Sl (section ‘6’) with syneresis cracks and laminated/massive mudstone of Facies Fl

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Table 1 Description and interpretation of the sedimentary facies identified in the AMR, offshore Nile Delta Basin
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4.1.1 Trough cross-bedded sandstone (Facies St)

4.1.1.1 Description

Facies St forms the basis of the fining-upward succession. Cross-beds sets ranges from 5 to 10 cm with cosets thickness up to 50 cm. This facies is gradationally overlies facies Sm and underlies facies Sl and Sr. It consists of gray, coarse- to fine-grained, subrounded to subangular and poorly to moderately sorted sandstone with sharp-based, subtle trough cross-bedding (Figs. 4A and 5). Petrographically this sandstone ranges from quartz arenite to subarkose (Fig. 6). The sandstones show relatively mature compositions and are composed of monocrystalline and polycrystalline quartz, as well as low amounts of plagioclase and K-feldspars. Other detrital minerals include iron oxides, glauconite, micas and detrital clays. The sandstones are commonly cemented by sparry calcite with scattered patches of pore-filling clays. The grain contacts exhibit concavo-convex and long contacts (Fig. 4 section ‘1’). Porosity ranges between 13 and 18% and is mostly moderately to poorly connected primary intergranular porosity with some secondary porosity.

Fig. 5
figure 5

Summary of the seven identified sedimentary facies (see legend) in this study, complemented by core photographs, core logs and grain-size parameters. The grain-size distribution plots and statistical grain-size data represent the average measurements for each facies

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Fig. 6
figure 6

QFL ternary diagrams for compositional classification of the Abu Madi Formation from Mina-1 Well after McBride (1963) as modified by Pettijohn et al. (1987)

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4.1.1.2 Interpretation

Facies St sandstones are interpreted to be deposited as 3D dune bedforms in a braided fluvial channel resulting from migration of sinuous-crested dunes (Miall 1996). The large-scale trough cross sets indicate sedimentation of migrating channel sandy bars under a lower flow regime (Miall 1996; Hjellbakk 1997). The small-scale trough cross-bedding suggest deposition by migrating dunes on the channel bar lee slope (Collinson 1996). The fining upward facies trend indicates decelerating flow conditions. The poor sorting suggests high sediment concentrations.

4.1.2 Planar cross-bedded sandstone (Facies Sp)

4.1.2.1 Description

Facies Sp consists of stacked fining-upward sets of planar cross-strata. The set thickness occurs at the decimeter scale with cosets thickness up to 1.5 m. Individual cross-beds frequently graded and fine upward. The cross-bed sets are generally show flat truncation surfaces. These sandstones are associated with millimeter-thick organic-rich laminae along cross strata (Fig. 4B). This facies is gradationally overlies facies Sl and Sm, and underlies facies Sl and He. Facies Sp comprises gray, medium- to fine-grained, subrounded to subangular, poorly to moderately sorted sandstones with small scale planar cross-bedding (Fig. 5). Petrographically, these sandstones are nearly all quartz arenites with some subarkose (Fig. 6). Detrital mineralogy is predominated by monocrystalline besides subordinate polycrystalline quartz (c. 85%) with considerable plagioclase and K-feldspars (c. 10%). Other constituents include detrital clays, iron oxides, glauconite, carbonate bioclasts, micas and some opaque minerals. The abundance of monocrystalline quartz grains reflects higher compositional maturity. These sandstones are generally cemented by calcite. The grain contacts include point and concavo-convex contacts. Porosity ranges from 7 to 25% and is dominated by moderately to poorly connected intergranular porosity with some secondary porosity.

4.1.2.2 Interpretation

Facies Sp characterizes the deposits of 2D dune bedforms inside a braided fluvial channel. The planar geometry of fine to medium grain size indicates deposition by migration of straight crested (2D) subaqueous dunes due to deposition in lower flow regime currents (Collinson 1996). The large-scale sets of facies Sp with low-angel cross-bedding were created due to bar migration (Cant and Walker 1976). The thickness of individual cross bed sets of 5–15 cm suggests dune height of 15–50 cm and approximate channel water depths ranging from 1.5 to 5 m (cf. Leclair and Bridge 2001). The upward changes from parallel laminated sandstones Sl to planar cross-bedded sandstones Sp indicate periodic fluctuations in flow regime. The grain sorting in cross-strata is the primary factor influencing grainfall and associated grainflow processes on the dune lee slope (Reesink and Bridge 2007).

4.1.3 Parallel laminated sandstone (Facies Sl)

4.1.3.1 Description

Facies Sl is the most common facies types of the AMR interval. It is characterized by sheet-like geometry and low angle (< 10°) to parallel planar laminations (Figs. 3 and 4). The lower and upper bed contacts are gradational with facies Sm, St, Sp and Fl. It is occasionally truncated by erosion and overlain by facies Fl of the overbank floodplain deposits. These sandstones commonly have millimeter-thick mudstone with carbonaceous matter along lamination that defines the preserved structure (Fig. 4J, section ‘6’). Facies Sl is dominantly composed of gray, moderately to well-sorted, angular to subrounded, fine- to medium-grained argillaceous sandstones with patchy calcite cement (Figs. 4 section ‘4’ and 5). Facies Sl sandstones are classified as subarkose to quartz arenite (Fig. 6). Detrital minerals are predominated by microcrystalline quartz (constitutes 80 upto 90% of the rock) with subordinate plagioclase and K-feldspars (Fig. 4 section ‘4’). Other components include detrital clay, iron oxides, glauconite, opaque minerals, pyrite, and micas. These sandstones are slightly physically compacted with concavo-convex contact. Porosity ranges from 8 to 19% and dominated by primary intergranular porosity with some secondary porosity.

4.1.3.2 Interpretation

The sheet-like, planar lamination associated with fine grained sandstones indicates deposition under upper flow regime flat bed conditions (Collinson et al. 2006). These facies are most likely deposited by episodic, unconfined sheet flow sandy currents. It is interpreted as having been formed by intermittent non-channelized traction currents (Martinsen 1990). The organic-rich laminae within these deposits indicate deposition in close proximity to fluvial overbank-floodplain setting. These sandstones could also represent large, flat bars deposits formed by high velocity currents followed by waning flow and finer-grained sediments settling from suspension. The thin minor intercalations of fine-grained lithologies with facies Sl are attributed to rapid fluctuations in flow regime.

Locally, the faintly laminated sandstone forms the transition from massive sandstones (Sm) to laminated sandstones (Sl). The transitional facies are interpreted to result from hyperconcentrated flood-flows (Hjellbakk 1997) in which the bedload tractional deposits form under a high concentration of suspended loads that quickly settle down from suspension (Lowe 1988). The faint lamination formed as the tractional currents are powerful enough to maintain bedload sediment concentration and the sediment fallout rate is high enough to prevent the grain sorting process.

4.1.4 Massive sandstone (Facies Sm)

4.1.4.1 Description

Facies Sm forms tabular, centimeter to decimeter-scale sandstones that have sharp bases and tops. The sandstones are massive and featureless, lacking any sedimentary structures (Fig. 4C, F and I). This facies is gradationally overlies facies Sp and Sl, and underlies facies St, Sr and Fl. These sandstones consist of gray, moderately to well sorted, subrounded to subangular, fine- to coarse-grained quartz arenite to arkose with calcite cement (Figs. 5 and 6). Petrographically, monocrystalline quartz is the dominant detrital fraction with considerable amounts of plagioclase, K-feldspar, detrital clays. Other components include carbonate bioclasts, glauconite, some opaques, micas and iron oxides. Porosity ranges from 8 to 23% and is dominated by primary intergranular porosity.

4.1.4.2 Interpretation

The tabular, structureless massive sandstone of Facies Sm is interpreted as a product of fluidized sediment flow deposits in a braided fluvial system. However, it is also deposited at the base of fluvial-channel point-bar deposits. Massive sandstone is formed by the deposition of “hyperconcentrated flood flows” (sensu Smith 1986). They are commonly structureless due to their rapid deposition due to deceleration of currents, and their lack of subsequent homogenization by sorting mechanisms. The structureless massive sandstones are documented in a fluvial setting (e.g. Jones and Rust 1983; Webb et al. 2015). These massive sandstones represent the deposits of dominantly perennial braided fluvial channels (Martin and Turner 1998).

4.1.5 Ripple cross-laminated sandstone/siltstone (Facies Sr)

4.1.5.1 Description

Facies Sr forms the top of the upward-fining package and consist of several centimeters to a few decimeters in thickness ranges from (Figs. 3 and 4). Facies Sr is very commonly underlain by Facies Sm with sharp lower contact and gradationally overlain by Facies Sl and Facies Fl. This facies consists of alternating unidirectional current rippled cross lamination (< 2 cm in height) and millimeter-thick mud drapes with carbonaceous mudstone (Fig. 4C). However, bidirectional current ripples are locally present. The ripple cross-laminations faint upwards due to the increase in sand content and the sandstone homogeneity. Facies Sr is dominantly composed of gray color, moderately to well-sorted, angular to subrounded, very fine- to fine-grained argillaceous sandstones, with no observed bioturbation (Fig. 4C). Theses sandstones are classified as quartz wackes with patchy calcite cement (Figs. 4 section ‘3’ and 6). The sediments are primarily formed of monocrystalline quartz, detrital clays, feldspars, micas, some opaques, pyrite and iron oxide. The porosity is commonly in the range of 5–9.87%. The effective porosity is mostly secondary as fracture porosity after leaching, compaction and cementation has reduced the primary intergranular porosity. The pore spaces are partly filled with pore-occluding calcite as well as detrital clay matrix.

4.1.5.2 Interpretation

The dominance of downstream mutually erosive current ripples reflects a combination of low rate of sedimentation and decreasing current velocities (Miall 1996). The documented sedimentary structures indicate deposition within floodplain environment or in channel during waning conditions (Hjellbakk 1997). Facies Sr is interpreted to represent abandoned pool on top of fluvial bar, between bars or in floodplain (Kirk 1983). Current ripple-laminated sandstones indicate unidirectional traction bedload dominated sedimentation under lower-flow regime, while alternating mud drapes reflect settling from suspension as flow decelerates (Miall 1985, 1996). The unidirectional current ripples indicate a relatively low-energy environment under low current flow velocities (20–60 cm/s) (Rubin and McCulloch 1980). The presence of ripples may be a sign of a reduction in fluvial sediments input or a change in the seasonal flow within the fluvial system (Webb et al. 2015). The bidirectional current ripple cross laminations reveal reversing current directions that might suggest tidal influence on deposition.

4.1.6 Heterolithic sandstone and mudstone (Facies He)

4.1.6.1 Description

Facies He forms few centimeter-thick of sheet sandstone alternating with millimeter-thick laminae of mudstones (Figs. 3 and 4). This facies overlies the massive sandstones Facies Sm, and underlies Facies Sr, Facies Sp and Facies Fl (Fig. 4C,D). It displays upper gradational and lower sharp/erosive bed boundaries (Fig. 4). Facies He is dominantly composed of gray, moderately sorted, angular to subrounded, siltstone to fine-grained sandstones with minor mudstone intercalations. The dominant sedimentary structures include current ripples as well as parallel and lenticular laminations (Fig. 4). Facies He sandstones are classified as subarkose to quartz wackes (Fig. 6). Detrital mineralogy is dominated by microcrystalline quartz, some plagioclase, K-feldspars and detrital clays. Other detrital components include iron oxides, pyrite, opaque minerals, and micas. Mudstones are dark gray, well-sorted and dominantly parallel laminated. Porosity ranges between 6 and 11% and is dominated by intergranular porosity with some developed secondary porosity. The pore spaces is occluded by calcite and iron oxides cements.

4.1.6.2 Interpretation

The sheet-like heterolithic sandstone and mudstones (Facies He) indicates deposition in floodplain setting during waning flow conditions (Miall 1996; Hjellbakk 1997). The sheet-like alternation of siltstone and claystone laminae indicates settling from suspension in the uppermost part of sandy bars and/or abandoned floodplain environments (Allen 1963; Collinson 1996). Facies Fl could alternatively representing the upper portion of the point-bar, which is evolved by the lateral accretion during submersion (Carling et al. 2015). The thin current rippled sandstone and mudstone couplets were formed by alternation of traction bedload and suspension fallout and may suggest a tidally influenced fluvial sediments (Reineck and Wunderlich 1968).

4.1.7 Parallel laminated/massive mudstone (Facies Fl)

4.1.7.1 Description

Parallel laminated/massive mudstone Facies (Fl) consist of a few centimeters to several decimeters thick successions occurs particularly in the upper part of the AMR (Fig. 3). It overlies the massive sandstones of Facies Sm commonly with gradational lower boundaries, and underlies Facies Sl and Sp (Fig. 4I, K and M). The laminated mudstones are mainly medium gray, blocky, carbonaceous, pyritic, massive, and most commonly laminated. Internally, facies Fl contains thin, siltstone laminae. The dominant sedimentary structures include planar to lenticular, millimeter-scale laminations, and mud drapes. Facies Fl deposits are commonly associated with rootlets, desiccation and syneresis cracks with no bioturbation. Lamination is defined by concentrations of preserved carbonaceous plant material and micas.

4.1.7.2 Interpretation

Facies Fl is interpreted as the product of slack-water suspension fallout in fluvial mud dominated floodplain environments (Collinson et al. 2006). The sheet-like, thin bed and proximity to channel sand bodies suggest deposition in distal floodplains. The braided fluvial sandstone is truncated by floodplain mudstone that represents flooding event as indicated by the erosional contact between Facies Fl and the underlying Facies Sm. The dominance of massive and parallel lamination and lack of cross-lamination in the fine-grained sediments indicate that currents did not operate during deposition. However, the presence of siltstone and sandstone laminae represent periods of increased current flows or sediment influx resulting in the development of delicate lenticular lamination (Webb et al. 2015). Laminated and massive mudstones were deposited in low energy environments by suspension mud settling (McCormick and Grotzinger 1993). The desiccation/syneresis cracks, and the lack of bioturbation suggest deposition in a freshwater overbank (floodplain) environment. These mudstones are commonly display desiccation mud cracks, carbonaceous material, and root traces, suggesting subaerial depositional conditions. The fine-grained sandstone beds alternating with mudstones contain mud cracks could suggest channel abandonment (Prescott et al. 2014).

4.2 Facies associations

4.2.1 Facies association 1: braided fluvial channel

The fluvial-channel deposits of FA1 comprise the deposits of axial fluvial systems and consist of 3 to 65 m thick successions composed of medium-grained, massive to cross-bedded sandstone bodies. It is composed of multiple channels in channel bar, which mainly migrates downstream. The braided fluvial systems contain thin overbank deposits. The lack of coarse-grained sandstone and basal conglomerates, including mudstone rip-up clasts suggest more distal fluvial settings. The vertically stacked channel fill sandstone bodies forming amalgamated, perennial multistory fluvial channel fill sands that display normal grading indicating decelerating flow conditions. Mudstones of dark gray color with desiccation/syneresis cracks may represent a subaerial, poor-drained floodplain environment. Cross-bedded sandstones with basal channel-lag pebbly sandstones to conglomerates with mud rip-up clasts in more proximal braided fluvial channels (e.g. Leila et al. 2020; Selim et al. 2022).

4.2.2 Facies association 2: point bar

FA2 comprises fining-upward up to 20 m thick and made up of coarse-grained massive and laminated sandstones, and heterolithic sandstone and mudstone (Facies Sm, Facies Sl and Facies He, respectively) with basal erosion surface (Fig. 4B). These are mainly overlain by floodplain mudstones. The massive structure, grain size and associated low-angle cross-lamination suggest that these sandstones were deposited rapidly by unidirectional currents. The vertical facies model, upward-fining sequence overlain by channel abandonment deposits suggest deposition in upper part of point bar (e.g. Plint 1983).

4.2.3 Facies association 3: floodplain

Floodplain deposits of FA3 comprise tabular, thick interval of dark gray parallel laminated/massive mudstones (Facies He) interbedded with ripple cross-laminated sandstones (Facies Sr) (Fig. 3A and B). Thin lenses of structureless carbonate wackestone (5–10 cm thick) are preserved locally. These carbonate lenses were most likely formed by water entrapped in small depressions after fluvial flow deceleration (Allen 1974). The sharp-based, fine grain size with plant debris and rhizoliths overlying fining-upward channel sand bar suggest floodplain deposits.

4.2.4 Facies association 4: abandoned channel

FA4 sediments comprise sharp based, upward-fining succession of cross-bedded as well as laminated sandstones intercalated by laminated mudstones (Fig. 3A). The upward-fining log sequence alternating with upward thinning mudstones, and associated desiccation cracks indicate deposition in abandoned channel (e.g. Allen 1965; Miall 1996; Hjellbakk 1997; Prescott et al. 2014).

4.3 Depositional model

The Messinian stage encompasses a complex interaction between tectonic activity at the Gibraltar Strait, climate and sea level changes caused progressive restriction and isolation of the Mediterranean Sea followed by complete basin desiccation (Krijgsman et al. 1999, 2018; Kontakiotis et al. 2002). This time marks the start of Messinian Salinity Crisis and substantial evaporite deposition in the Mediterranean Basin (Hsü et al. 1972, 1973; Roveri et al. 2014). Base level fall, induced by glacio-eustatic sea level oscillations, together with regional uplift and widespread erosion resulted in deep incised valleys (e.g. Madof et al. 2019; Rubino et al. 2015; Dalla et al. 1997; Clauzon 1982; Barber 1981), e.g. Abu Madi incision, resulting in the generation of excess accommodation space and the influx of clastic sedimentation into the Mediterranean margin. A regional unconformity is widely developed within the Messenian succession and is easy to identify at the base of the nonmarine Abu Madi sequence and the fully marine mudstones of underlying Qawasim Formation.

Abu Madi incision during sea-level lowstand was succeeded by sediment bypassing and a significant period may have elapsed before any sedimentation took place (Fig. 7A). Sedimentation patterns were dominated by progradation of axial fluvial systems with subordinate amounts of the marginally sourced deposits. The sand bodies have markedly differing architectures and stratal geometries. Individual channels are characterized by sharp or scoured basal contacts with conglomerates, including mudstone rip-up clasts. The amalgamated and vertically stacked fluvial-channel fills comprise the deposits of composite multistory braided fluvial system (Fig. 7B). The dominance of unidirectional flow, vertical stacking pattern, coarser-grained sandstone with minor floodplain mudstones indicates a high degree of braided-channel amalgamation that are particularly characteristic of sea-level lowstands (cf. Magalhães et al. 2014). Continued sedimentation can lead to reduction of channels gradient at these points such that the next flows would have decelerated and deposited sediment even more rapidly. Thus, the loci of sedimentation would have drifted up-channel. As this back-filling process continued, an upward-fining succession would have been deposited. These channel belts are separated by a relatively thin succession of overbank mudstone sediments, which corresponds to periods of floodplain development. Variations in the fluvial style from coarse-grained, braided-fluvial channel into fine-grained, meandering fluvial channels corresponding to changes in development of accommodation space and sediment supply (Fig. 7C). These sand bodies appear to have a lesser lateral extent and are interpreted to represent more-sinuous point-bar deposits (Fig. 7C). The active channel-fill sandstones are usually fine upwards and pass into mudstones deposited in passive channel-fill/floodplain associations. The fluvial facies changes in the Abu Madi Formation explain how temporal changes in sediment supply and accommodation space affected fluvial style (see Fig. 7).

Fig. 7
figure 7

Schematic diagram showing the fluvial style variations with time in the Abu Madi incised valley due to changes in the development of accommodation space and sediment supply

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Ultimately, the meandering fluvial deposits are commonly pass upward into tidally influenced fluvial deposits/estuary system that was formed during transgressive event (Fig. 7D). The estuarine sediments show aggradational and retrogradational stacking patterns and mainly composed of heterolithic facies (Facies Sr and Facies He). The heterolithic sandstone and mudstone indicate cyclic alternations of current and slack water deposits, and hence a transition from predominantly fluvial to tidal influenced fluvial deposits. The presence of bidirectional ripples, alternation of thin rippled sand and mud, and lenticular and flaser laminations suggest tidal environment (e.g. Reineck and Wunderlich, 1968).

4.4 Petrophysical appraisal

Visual and quantitative investigations of the AMR in the accessible wells (Figs. 8, 9, 10 and 11) exposed that the potential gas-bearing intervals are located at depths between 3633 and 3673m (i.e. 40 m thick) in the BE-3 well, while, in the BN-2 well, the productive zone is located among 3705m and 3730m (25m thick). In BN-1 and Mina-1 wells, zones demonstrating positive signs of gas exist at depths from 3738 to 3786 m (48 m thick) and 3597 to 3625 m (28 m thick), respectively. The calculated petrophysical parameters (average values) for these favorable zones are summerized in Table 2. Total porosity ranges between 20% (BE-3 well) and 26% (Mina-1 well); the effective porosity fluctuates between 18% (BE-3 well) and 25% (Mina-1 well); the shale content varies between 9% (BE-3 well) and 4% (Mina-1 well); the hydrocarbon saturation oscillates between 55% (BE-3 well) and 85% (Mina-1 well).

Fig. 8
figure 8

Petrophysical analysis of the well Baltim East-3 indicating well log-based key petrophysical properties of the reservoir interval, Abu Madi sandstones (modified after Sarhan 2023)

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Fig. 9
figure 9

Petrophysical assessment of the well Baltim North-2 indicating well log-based key petrophysical properties of the reservoir interval, Abu Madi sandstones (modified after Sarhan 2023)

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Fig. 10
figure 10

Petrophysical analysis of the well Baltim North-1 indicating well log-based key petrophysical properties of the reservoir interval, Abu Madi sandstones (modified after Sarhan 2023)

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Fig. 11
figure 11

Petrophysical appraisal of the well Mina-1 indicating well log-based key petrophysical properties of the reservoir interval, Abu Madi sandstones

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Table 2 Summery table displays the average values of the calculated petrophysical parameters for the AMR in the inspected wells
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The calculated petrophysical parameters revealed that the petrophysical characteristics and the quality of AMR increase northwestward of the studied area by displaying; a decrease in shale volume, an increase in total porosity, an increase in effective porosity, decreasing in water saturation and increasing in hydrocarbon saturation (Fig. 12). Although Abu Madi Formation deposited as the topmost part of the Messinian syn-rift megasequence in the NDB (Sarhan et al. 2014); the obtained results indicate that the tectonic effect on the AMR quality decreases in northern portion of Abu Madi incised-valley rather than its southern part, which displays noticeable differences in the petrophysical properties (Sarhan 2022a).

Fig. 12
figure 12

Bar graph summarizing the calculated petrophysical parameters for the gas zones in the inspected wells

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4.5 Reservoir rock typing

Four reservoir rock types (RRTs) have been distinguished in Abu Madi Formation, offshore Nile Delta Basin based on their texture, porosity, permeability, and reservoir quality (Table 3).

Table 3 Reservoir rock types and their petrophysical properties and quality
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4.5.1 Reservoir rock type 1 (RRT1)

Rocks belonging to the RRT1 are represented by trough cross-bedded sandstones (St) and planar cross-bedded sandstones (Sp). Porosity is mostly primary intergranular porosity with some secondary intragranular pore spaces. The reservoir quality is good with porosity varies from 14.43% to 24.87%, and permeability in the range of 25–555 mD (Table 3).

4.5.2 Reservoir rock type 2 (RRT2)

The RRT2 distinguishes the planar laminated sandstone facies (Sl) with connected primary intergranular, secondary intragranular, with low intercrystalline microporosity. The porosity ranges between 20.1 and 24.6%, whereas the permeability varies from 97 to 212 mD indicating moderate to good reservoir quality (Table 3).

4.5.3 Reservoir rock type 3 (RRT3)

The RRT3 is usually composed of massive sandstones (Sm). The porosity is composed mainly of well-connected intergranular porosity indicating very good reservoir quality. Porosity value is up to 26% and permeability ranges between 460 and 1440 mD (Table 3). The poor cementation, open grain packing along with moderate intergranular porosity are responsible for the very good reservoir quality.

4.5.4 Reservoir rock type 4 (RRT4)

The RRT4 characterizes the ripple cross-laminated sandstone (Sr). The reservoir quality is poor due to calcite cementation and detrital clays fill the intergranular pore spaces. RRT4 shows a porosity of about 5.4% and permeability as low as 0.6 mD (Table 3).

5 Discussion

The occurrence of reservoir-quality porosity in Abu Madi sandstones is mainly a function of initial depositional facies; the cleaner and mature the sands, the higher the initial and best porosity. The Upper Miocene Abu Madi Formation commonly comprises a mixture of sediment facies types that are interpreted as being formed in a fluvial-braidplain, point bar, floodplain and abandoned channel environments. The core data reveal that the gas-bearing sandstone facies (St, Sp, Sl, Sm and Sr) are separated by heterolithic sandstones and mudstones (He) and laminated/massive mudstone facies (Fl) (Fig. 3). Other tidal influenced predominantly heterolithic facies are encountered in the upper part of Abu Madi Formation and show poor reservoir quality. The mud-dominated facies can constitute significant intra-reservoir permeability barriers to fluid flow. Therefore, the recognition of these intervals and their areal extent is crucial in the reservoir characterization process and significantly influences hydrocarbon production.

The amalgamated braided fluvial channel and point bar sandstones form good reservoirs as they were deposited with high initial porosities and low detrital clay contents. These sandstones have average porosities ranging from 9.6 to 26% and permeabilities in the 25 to 1440 mD range. These sandstones are compositionally mature, texturally less mature and composed of monocrystalline quartz with some polycrystalline quartz, low amounts of feldspars and minor lithic fragments, and range from subarkose to quartz arenite. However, with increasing initial detrital clay content there is an increasing susceptibility to pore-occluding diagenesis. Much of the pore-reducing diagenesis occurred immediately after deposition during early diagenesis including calcite cementation, pore-filling authigenic clays and grain-coating chlorite (Abdel Fattah et al. 2022). The reservoir properties of the fluvial sand bodies are mainly controlled by sedimentary facies (grain size, sorting and the clay content) rather than by diagenesis. However, locally variable diagenesis created a subsequent, secondary imprint on these reservoir parameters. Secondary porosity was mostly produced by: (1) dissolution of K-feldspar during burial by low pH meteoric water, resulting in the formation of intragranular porosity (Abdel Fattah et al. 2022); and (2) sediment compaction and tectonic deformation, which produced an open-fracture system that significantly influences reservoir quality.

The multistory amalgamated braided fluvial channels mainly consist of vertically and laterally stacked, progradational to aggradational, upward-fining vertical facies successions. These sand bodies comprise coarse- to medium-grained channel sand bars that are overlain by floodplain mudstone deposits. The sand bars are composed of trough plus planar cross-bedded sandstone (St and Sp; RRT1) that grade upwards into planar laminated sandstones (Sl, RRT2) and massive sandstones (Sm; RRT3), which have good reservoir characteristics and attain good reservoir quality. Also, the fluvial channel point bar deposits display upward-fining trends composed of coarse-grained massive sandstones (Sm; RRT3) grade upward into thin planar laminated sandstones (Sl; RRT2) intercalated with rippled cross-laminated sandstone/siltstone (Sr; RRT4) and capped by fine-grained mudstones of floodplains and abandoned channels. Overall, the point bars have good reservoir properties and reservoir quality. Sandstone bodies encompass rippled cross-laminated sandstone/siltstone (Sr; RRT4) in their upper parts and therefore they have low reservoir properties and poor reservoir quality.

Modifications in fluvial architecture and sand-body geometries within the IVF systems have resulted from the complex interplay of the relative sea-level variations, sediment supply and accommodation space development (Posamentier and Allen 1999; Shanley and McCabe 1994). The principal control on porosity and permeability characteristics is primary depositional lithology, even with deep burial. Therefore, the highest quality sandstones usually occur in the lowstand systems tract (LST1 and LST2; Abdel-Fattah et al. 2022; Shehata et al. 2023), reflecting deposition in a high-energy, fluvial sand-bar environment. Fluvial facies changes illustrate how temporal changes in sediment supply and accommodation space affected fluvial style. An upward change in the fluvial style is recognized from medium-grained, braided-river in lower portion of Abu Madi Formation towards more-sinuous point-bar deposits (Fig. 7). Therefore, facies analysis, sequence stratigraphy, and petrophysical evaluation are the main controlling parameters that enable improving the development of the Abu Madi IVF reservoir system. A better understanding of these factors and the associated reservoir quality allows accurate prediction for potential reservoir facies and improves the offshore Nile Delta fields development plans.

6 Conclusions

  1. (1)

    The Abu Madi reservoir has been studied in four wells located offshore of the Nile Delta Basin. A total of seven clastic facies representing cyclic alternations of sandstone and mudstone were identified based on core data and conventional wireline logs. These are grouped into four depositional facies assemblages during the Messinian period; braided fluvial channel, point bar, floodplain, and abandoned channel. These successive fluvial facies explain how temporal changes in sediment supply together with the accommodation space development controlled the IVF fluvial system during the Messinian period.

  2. (2)

    The fluvial deposits of the Abu Madi reservoir grade into tidally influenced fluvial channel/estuarine deposits that were formed during transgressive system tract. These estuarine sediments show aggradational and retrogradational stacking patterns and principally composed of heterolithic facies.

  3. (3)

    The petrophysical analysis shows that the gas accumulations within the AMR sandstones have higher porosities (total porosity of 0.20–0.26 v/v and effective porosity ranges from 0.18 to 0.25 v/v), low shale content (0.04–0.09 v/v) and high hydrocarbon saturation (0.55–0.85 v/v).

  4. (4)

    Four RRT have been recognized in the AMR sandstones. The trough and planar cross-bedded sandstones are classified as RRT1 has good porosity and horizontal permeability, and good reservoir quality. RRT2 shows good reservoir quality and is represented by parallel laminated sandstone with good porosity and moderate permeability. RRT3 characterizes the massive sandstone and has the highest reservoir properties and the best reservoir quality. RRT4 describes the ripple cross-laminated sandstone/siltstone and show low porosity and permeability.

  5. (5)

    The calculated petrophysical parameters reflect a significant increase in the reservoir quality northwards confirming that the prevailed tectonic regime during the deposition of the AMR has a low effect on the northern part of the Abu Madi incision.