1 Introduction

The Gulf of Suez region is the most productive oil rift basin in Africa and the Middle East, and it contains more than 80 active oil fields (Schlumberger 1995; Alsharhan 2003; El Nady et al. 2015; Moustafa and Khalil 2020; Kassem et al. 2021; Radwan and Sen 2021a, b, c, d; Radwan 2021a, b). Many studies on the evolution of the Gulf of Suez rift have been conducted, including structural and sedimentological studies due to the well-exposed syn-rift formations and the large amounts of exploration data on this region (Winn et al. 2001; Radwan et al. 2020, 2021a, b, c). The fault blocks that started at the rifting period represent the main hydrocarbon traps and rule the petroleum accumulation in the Gulf of Suez oil fields (Chowdhary and Taha 1987; Sultan 2002). The Miocene successions in the Gulf of Suez have attracted the attention of many geologists due to their economic importance for oil and gas. The Lower and Middle Miocene Syn-rift sediments are the most prolific petroleum reservoirs in the Suez province. During the early evolution of the Gulf of Suez and Red Sea Syn-Rift System, the Lower Miocene of Egypt (Nukhul Formation) was deposited and it’s the oldest and most overall Syn-rift sediments in the Gulf of Suez (Al-Husseini 2012). In more than 15 fields, the Nukhul Formation is a high-quality petroleum reservoir (Saoudi and Khalil 1986). Nukhul is oil producing in Rudeis-Sidri, Zeit Bay, Hilal, GS173, Darag and Ashrafi fields, while it produces both oil and gas in the Hareed field (Alsharhan 2003). Overall, it contributes to roughly 11.5% of production potential of Gulf of Suez (Alsharhan 2003).

This study focuses on the Abu Rudeis-Sidri Field from Gulf of Suez (Fig. 1). The field produces an average of 2700 BOPD, mostly from Nukhul Formation, with least amounts from the Thebes, Matulla and Nubia intervals (Fahmi et al. 2015). Zahra and Nakhla (2016a, b) inferred the structural settings of the northeastern Gulf of Suez area including Rudeis-Sidri field, using aeromagnetic and seismic data. Burial history and thermal evolution of the field was inferred by Awadalla et al. (2018). Organic-rich pre-Miocene Thebes Formation is the prominent source rock in the Abu Rudeis-Sidri field (Mostafa and Ganz 1990; Mostafa et al. 1993; Awadalla et al. 2022). Elmaadawy et al. (2021) characterized the source rock characteristics of the Duwi and Thebes intervals in the study area. A few researchers worked on the new unconventional reservoir characteristics and potential of the Oligocene–Miocene fractured igneous intrusions (Fahmi et al. 2015; Abd-El Gawad et al. 2022). Till date, the petrophysical properties of the Nukhul producers have not been published, which sets the premise of this work. Wireline log-based quantitative petrophysical interpretation is critical for inferring reservoir pay zone and its production potential which has direct implications for field development planning and resource optimization (Haque et al. 2022). Reservoir characterization quantifies physical and fluid properties of rocks such as porosity, shale volume, fluid saturation (Islam et al. 2021; Qadri et al. 2022; Zhang et al. 2022) and to be integrated with geological, geophysical, structural inferences and basin analysis approaches to get a clear subsurface understanding of the subsurface reservoir distribution (Cheng et al. 2014; Adelu et al. 2019; Jenkins and Torvela 2020). The main aim of this study was to assess the key petrophysical properties of the Nukhul Formation in the Abu Rudeis-Sidri Field, which includes volume of shale, total and effective porosities, water saturation and bulk volume of water. Wireline logs have been the primary input parameter for the petrophysical analysis. Inferences have been drawn on lithological characteristics and clay mineralogy. Focused characterization and reevaluations of the already discovered petroleum systems and producing reservoirs are required to assess the reservoir’s ability and potential to meet the economic and field development requirements (Zhang et al. 2022). Our study is focused on that aspect of the Nukhul Formation from the Abu Rudeis-Sidri field. The quantitative petrophysical assessment presented in this study sheds critical insight about the reservoir quality of the Nukhul sandstones, provides high reliability about its hydrocarbon potential and thereby reducing the risks associated with hydrocarbon exploration.

Fig. 1
figure 1

A Location map of the study area in Egypt; B location of the studied Abu Rudeis-Sidri field in the Eastern part of the Gulf of Suez, modified after Youssef (2011); C location of the four studied wells

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2 Geological setting

The rifting of the Gulf of Suez started during the Lower Miocene due to the divergent movement between the African plate and the Arabian plate which created a series of NW–SE to NNW–SSE trending normal faults (Patton et al. 1994; Omran 1995; Abul Karamat and Meshref 2002; Bosworth et al. 2005). The faulted blocks of the Gulf of Suez are bounded mainly by NW normal faults and connected to each other by WNW-, NNE- and NE-oriented faults (Abd-Allah et al. 2014). The syn-rift Miocene deposit in the Gulf of Suez province changes laterally from clastic sediments to more deeply buried sediments towards the rift axis (Bosworth and McClay 2001). Due to the tilt of the pre-rift blocks, the Miocene rocks overlying the different pre-rift units vary in lithologies and thicknesses (Abd-Allah et al. 2014). The structure of Abu Rudeis-Sidri area is an asymmetrical NW–SE trending anticlinal feature separated by a number of NW–SE fault systems and formed during the late Oligocene-Early Miocene time (rifting phase) (Zahra and Nakhla 2016b). A stratigraphic chart is presented in (Fig. 2) which reflects the change in depositional environment (continental to shallow/deep marine) with geological time (refer to the arrows in Fig. 2), in relation to sea level fluctuations (El Diasty et al. 2020). The lower-middle Miocene succession in the Abu Rudeis-Sidri Field includes the Kareem, Rudeis, and Nukhul formations from top to bottom. Nukhul Formation unconformably lies above the Oligocene Abu Zenima Formation (mostly conglomerate) and conformably overlain by Rudeis Formation. The Nukhul Formation was first described by (Waite and Pooley 1953) as the basal marine Miocene beds in Sinai and Gulf of Suez regions and first introduced by (Ghorab 1964) in a type-section, south of Wadi Nukhul area (29o 01ʹ N, 33o 11ʹ E). In the subsurface succession, The Nukhul Formation was introduced by the National Stratigraphic Subcommittee of the Geological Sciences of Egypt (NCGS 1974). According to (Schutz 1994; Abd El Gawad et al. 2016), the Nukhul interval was deposited in a shallow marine condition. Generally, the Nukhul Formation in Abu Rudeis-Sidri Field consists of shale interbedded with few limestones and sandstones. It has a relatively small thickness in outcrops, but borehole data indicates a large thickness up to 500 m in the West Zeit trough (Peijs et al. 2012). Saoudi and Khalil (1986) reported that Nukhul Formation in Wadi Nukhul consists only of 60 m thick.

Fig. 2
figure 2

General stratigraphic chart of the Gulf of Suez Basin, after El Diasty et al. (2020)

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3 Materials and methods

In this study, we have analyzed four drilled wells: ARS-6 (latitude 28° 51′ 20.23ʺ N, longitude 33° 10′ 33.52ʺ E), ARM-7 (latitude 28° 51′ 43.05ʺ N, longitude 33° 10′ 30.36ʺ E), Sidri-9 (latitude 28° 51′ 0.7ʺ N, longitude 33° 11′ 15.23ʺ E), and Sidri-20 (latitude 28° 50′ 53.42ʺ N, longitude 33° 10′ 28.24ʺ E) (Fig. 3).

Fig. 3
figure 3

Seismic lines and location of the four studied wells within Abu Rudeis-Sidri Field (Figs. 5, 6 marked in red)

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3.1 Well logging

Wireline logs were available from all the four wells which include gamma ray, formation resistivity, neutron porosity and bulk-density. Spectral gamma ray log was available from the well Sidri-9 which provided Uranium, Thorium and Potassium concentrations. All the wells had log data coverage within the Nukhul Formation which is the main target interval for this work. Density log was checked for possible low value responses due to wash out conditions, but caliper log indicated gauged wellbore confirming data quality unaffected by borehole conditions. Mudlogs were also available from all the wells which provided drilling cutting lithological descriptions and used as primary lithology indicators.

3.2 Seismic data interpretation

In this study, we briefly looked into the seismic dataset to understand the lateral distribution of the stratigraphic units across various fault blocks. Seismic reflection profiles provide different fault dips and subsurface structural trends (Khan et al. 2021). Seismic interpretation involves the correlation between seismic data and available well logs (i.e., well tie) to identify consistent stratigraphic sequences and therefore establish the relationship between the stratigraphy and their corresponding seismic reflections (Qadri et al. 2017). Since the primary objective of this study was to assess the key reservoir petrophysical properties, a detailed seismic interpretation was out of scope. Eighteen seismic sections were available from the study area which was tied to four drilled wells. We presented the interpretation of three seismic lines (seismic line No. 89-19-GL-83-05-N, line No. 98-19-GJ-83-07-N and line No. RS-14-83-mig 160001) in this study. The horizons were marked on each seismic dip/strike cross-section by differentiating marker strata and other corresponding points assigned to well logs with primary reflections on the seismic cross-section. The various formation reflectors were identified according to their seismic characters. The continuity, geometry and reflector spacing were used to identify these reflectors. Seismic reflector on the top of the target formation, i.e., Nukhul Formation was picked and marked on the seismic sections along and faults were picked.

3.3 Petrophysical analysis

We analyzed these well logs to identify the hydrocarbon bearing zones within Nukhul Formation to perform the log-based petrophysical analysis. Lithology was interpreted from neutron porosity-bulk density cross plot and clay mineralogy was inferred from Thorium and Potassium concentration available from the spectral gamma ray data (Qadri et al. 2019). Five key petrophysical parameters were estimated from well logs: shale volume, (Vsh) total porosity (Φt), effective porosity (Φe), water saturation (Sw) and bulk volume of water (BVW).

Shale volume was estimated using gamma ray (GR) log (Asquith and Gibson 1982):

$${\text{Vsh}} = \frac{{{\text{GR}} - {\text{GRmin}}}}{{{\text{GRmax}} - {\text{GRmin}}}}$$
(1)

where GRmin and GRmax denote the minimum and maximum GR values of the interval of interest, respectively. Total porosity was estimated using bulk-density and neutron porosity logs (Asquith and Gibson 1982):

$$\Phi {\text{t}} = \frac{{\Phi {\text{nphi}} + \Phi\uprho {\text{b}}}}{2}$$
(2)

where Φnphi and Φρb represent neutron porosity and density porosity, respectively. Φρb is dependent on matrix density and formation fluid density (Qadri et al. 2019). Effective porosity is influenced by shale volume, and it was estimated as (Asquith and Gibson 1982):

$$\Phi {\text{e}} = \left( {1 - {\text{Vsh}}} \right)*\Phi {\text{t}}$$
(3)

Indonesian water saturation model was utilized to estimate Sw (Poupon and Leveaux 1971):

$$Sw = \left\{ {\frac{{\sqrt{\frac{1}{Rt}} }}{{\left\{ {\left( {\frac{{Vsh^{1 - 0.5 Vsh} }}{{\sqrt {Rsh} }}} \right) + \sqrt {\frac{{\Phi e^{m} }}{a Rw}} } \right\}}}} \right\}^{\frac{2}{n}}$$
(4)

where Rsh = shale resistivity, Rt = deep resistivity log, Rw = resistivity of the connate water, a = tortuosity factor, m = cementation exponent, n = saturation exponents, respectively. For the studied Nukhul Formation, we have considered Rw as 0.03 Ωm, a = 1, m = n = 2. A 50% water saturation cut off was utilized to distinguish between water wet and hydrocarbon bearing intervals (Qadri et al. 2019). Bulk volume of water (BVW) in uninvaded zone was estimated from Φe and Sw, following Buckles (1965):

$${\text{BVW}} = \Phi {\text{e}}*{\text{Sw}}$$
(5)

Based on the petrophysical properties, productive reservoir zones are interpreted within the Nukhul Formation. Results are discussed and compared with the other producing Nukhul intervals from offset fields. Inferences are drawn based on the regional geological understanding.

4 Results

4.1 Seismic data interpretation

On seismic sections, three formation intervals are interpreted, Kareem, Rudeis and Nukhul formations from top to bottom (Fig. 4). Kareem Formation, above the Rudeis interval, is characterized by very high amplitude reflectors with parallel to sub parallel configurations and medium continuity. It is considered as a marker reflector in the study area. It is very easy to pick this horizon due to its strong appearance and its distinguished high amplitude characters (Fig. 4). Rudeis Formation lies between the Rudeis and Nukhul Formations. It is characterized by medium amplitude reflectors that shows sub-parallel configurations at its base and high amplitude reflectors with parallel configurations at its top.

Fig. 4
figure 4

Seismic cross section with well tops marked on Sidri-9 profile (seismic line No. 89-19-GL-83-05-N)

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Nukhul Formation is the most prominent reflector in the Gulf of Suez. It represents the youngest identified reflector (Lower Miocene) characterized by low amplitude reflectors that exhibit sub-parallel configurations with low continuity due to the compressional forces affecting the area. Its low amplitude characteristics make it hard to pick the horizon. On seismic sections, the top of the Nukhul Formation is represented by a medium amplitude crinkled reflector intersected and cut by numerous steeply dipping normal faults, forming an extensional fault block (Figs. 5, 6). The occurrence of these normal faults is related to the opening of the Gulf of Suez during the Miocene reflecting the extensional regime.

Fig. 5
figure 5

Interpreted seismic line No. 98-19-GJ-83-07-N (please refer to the Fig. 3 for the line position on the map)

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Fig. 6
figure 6

Interpreted seismic line No. RS-14-83-mig 160001 (please refer to the Fig. 3 for the line position on the map)

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4.2 Petrophysical interpretation

The Nukhul Formation has been characterized based on the wireline logs. We also integrated cuttings lithology from the available mud logs data. The Nukhul Formation consists of interbedded sandstone, shale, and limestone units. The top of Nukhul Formation is marked by a limestone interval. Quantitative petrophysical assessments of the Nukhul Formation in the studied wells are presented in Figs. 7, 10, 11 and 12. In the well ARS-6, the sandstone interval between 2809 and 2824 m exhibits low gamma ray, high resistivity, and density-neutron cross-over (Fig. 7). Cross plot between bulk-density and neutron porosity in the ARS-6 indicates sandstone matrix (Fig. 8), as also confirmed by the cutting lithology information (Fig. 8). The mentioned interval is characterized by Vsh < 0.03 dec (average value of 0.008 dec), 0.05 < Φt < 0.16 (average value of 0.097 dec), 0.03 < Φe < 0.15 (average value of 0.095 dec). Sw varies between 0.20 and 0.66 dec, while average Sw is observed as 0.33. In the Pickett plot (Pickett, 1972), the ARS-6 pay sand exhibits cluster around 25% Sw line which indicates good hydrocarbon potentiality (Fig. 9). Based on the estimated porosity and water saturation BVW is found to be 0.014–0.049 (average value of 0.03). In the well ARM-7, the equivalent sandstone interval occurs around 3383–3388 m, which lies just below the limestone intervals defining the top of Nukhul Formation. This sandstone interval has around 0.22 dec Vsh, Φt of 0.07 dec, Φe of 0.02 and Sw around 0.9, and inferred to be water-bearing and hence unproductive (Fig. 10).

Fig. 7
figure 7

Petrophysical analysis of the Nukhul Formation in the well ARS-6. The reservoir pay zone is highlighted by orange color

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Fig. 8
figure 8

Density-neutron porosity cross plot (Schlumberger 1972) exhibiting lithological distribution in the studied Nukhul reservoir zone from the well ARS-6

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Fig. 9
figure 9

Pickett plots to infer water saturation in the studied Nukhul Formation from the wells ARS-6 and Sidri-9

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Fig. 10
figure 10

Petrophysical analysis of the Nukhul Formation in the well ARM-7

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In the Sidri-20 well, the productive sandstone interval is inferred between 2631 and 2646 m. The zone is characterized by low gamma ray high resistivity and density-neutron cross-over (Fig. 11). Estimated Vsh varies between 0.07 and 0.41 dec with an average value of 0.25. Average porosities are observed to be less than 10% (Φt ~ 0.07 dec, Φe ~ 0.04 dec). Sw varies between 0.13 and 0.54 dec (average 0.33 dec) and average BVW is 0.011. Petrophysical analysis of the well Sidri-9 well is presented in Fig. 12. Like the other studied wells, Nukhul Formation top in Sidri-9 is also marked by a limestone interval. We inferred two pay sands in the Nukhul Formation between 2593–2612 and 2620–2628 m. Both the sands are characterized by low gamma ray (serrated signature), high resistivity and exhibit density-neutron cross-over. Both these sand intervals are separated by limestone parting (with minor shale). The upper sand is thicker and exhibits more porosity (Φt ~ 0.16 dec, Φe ~ 0.13 dec). Average Vsh is around 0.09, with ~ 0.30 dec Sw and 0.036 dec BVW (Fig. 12). In comparison with the upper reservoir, the lower one is thinner but observed to be cleaner with average Vsh of 0.03. Based on the neutron and bulk-density data, the lower pay zone exhibits 0.07 < Φt < 0.25 (average value of 0.135 dec), 0.05 < Φe < 0.23 (average value of 0.126 dec). Based on the higher resistivity signature, the lower pay zone exhibits a lower average Sw of 0.167 and average BVW of 0.02. Pickett plot of both these zones from Sidri-9 indicates data cluster between Sw-25% and 50%, however majority of the data is around Sw 25% line (Fig. 13). This indicates excellent hydrocarbon potential in the Nukhul sandstone reservoirs.

Fig. 11
figure 11

Petrophysical analysis of the Nukhul Formation in the well Sidri-20. The reservoir pay zone is highlighted by orange color

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Fig. 12
figure 12

Petrophysical analysis of the Nukhul Formation in the well Sidri-9. The reservoir pay zones are highlighted by orange color

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Fig. 13
figure 13

Pickett plots to infer water saturation in the studied Nukhul Formation from the wells ARS-6 and Sidri-9

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Cross plot between bulk-density and neutron porosity in both the Sidri wells (Sidri-20 and Sidri-9) exhibits many data points lying close to the limestone line indicating the presence or influence of carbonates in the sandstones (Fig. 14). This behavior can be a result of calcareous cement or matrix in the Nukhul sandstones. The lithological observation matches with the cutting lithology description from the mud logs as well, which reports calcareous sandstones from the Nukhul interval in these Sidri wells. Based on the Potassium (K) and Thorium (Th) concentration from the spectral gamma ray data of the well Sidri-9, we infer montmorillonite to be the dominant clay phase in the studied Nukhul sandstone along with minor presence of kaolinite and illite (Fig. 15). A comparison of the inferred wireline log-based petrophysical properties in the three wells (ARS-6, Sidri-20, and two pay zones in Sideri-9) is presented in Fig. 16.

Fig. 14
figure 14

Density-neutron porosity cross plot (Schlumberger 1972) exhibiting lithological distribution in the studied Nukhul reservoir zones from the wells Sidri-20 and Sidri-9

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Fig. 15
figure 15

Potassium–Thorium cross plot indicating montmorillonite as the dominant clay mineral within the Nukhul sandstone reservoir intervals in the well Sidri-9

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Fig. 16
figure 16

Distribution of the estimated petrophysical parameters of the Nukhul reservoir intervals in the studied wells, Abu Rudeis—Sidri Field

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5 Discussions

The initial rifting phase between Upper Oligocene to Lower Miocene corresponds to the Abu Zenima conglomerate beds transitioning into fluvio-deltaic to marginal marine mixed carbonate-siliciclastic deposits of the Nukhul Formation (Patton et al. 1994). El Atfy et al. (2013) deciphered an algal and fungal proliferation within Nukhul interval which is indicative of shift from regressive environment to a transgressive one. Petrographic study was unavailable from the studied field; however literature survey provided critical insights. Winn et al. (2001) reported that the Nukhul sandstones consist of carbonate matrix. Zaid (2013) studied the petrographic characteristics of Nukhul clastics from West central Sinai and interpreted that the quartz arenites are cemented with ferroan dolomite which also exhibited patchy dissolution contributing to the secondary porosity. El-Soughier and Mahmoud (2019) studied drill cutting samples from an offshore well GS9-1X drilled in the northern Gulf of Suez and inferred that the sandstones of the Nukhul interval are calcareous in nature, intercalated with slightly calcareous shale and crystalline hard limestone. El-Hafez et al. (2019) reported dolomite crystals from the SEM photographs of Nukhul sandstone samples from southwestern Sinai. Our observations from the density-neutron porosity also corroborates with these reporting, as we observed many data points lying close to the limestone and dolomite lines indicating the presence or influence of carbonates in the sandstones, especially in the Sidri wells (Fig. 14). Spectral gamma ray data indicated dominance of montmorillonite along with minor kaolinite and illite within the Nukhul sandstones of the well Sidri-9 (Fig. 15). Similar observation was also made by El-Hafez et al. (2019) who reported montmorillonite and kaolinite as the principal clay phases based on the XRD and SEM results from southwestern Sinai area.

The seismic reflector corresponds to the Nukhul Formation in the Abu Rudeis-Sidri Field is discontinuous and intersected by several normal faults. Out of the four studied wells, ARM-7 was drilled in the hanging wall bounded by steeply dipping normal faults with a throw of more than 500 m. This well did not encounter any productive horizon, which implies structural control on hydrocarbon accumulation. The evaluation of the hydrocarbon potential of the Nukhul Formation in the Abu Rudies-Sidri Field based on well log interpretation shows that this formation is a good oil reservoir, as also observed by Temraz and Dypvik (2018). According to Sarhan (2021), the Nukhul Formation has excellent petrophysical parameters in other fields too, particularly on the southern side of the Gulf of Suez (i.e., Rabeh East oil field), and its success depends on whether the drilled well penetrates a suitable structural closure. The identified productive reflector of Nukhul Formation has been correlated using well log data. The studied sandstone interval has good porosity values, hydrocarbon saturations, lower shale volume, and low water saturations, which are typical characteristics of a conventional productive clastic reservoir. Total porosity ranges between 7 and 16%. A similar porosity range was also reported by Abd El-Hafez et al. (2015) who inferred 3.8–19% porosity based on the routine core analysis of the Nukhul sandstones from southwestern Sinai. Inferred average porosity values in the Sidri-9 (~ 13.5–15.8%) are much higher than the Sidri-20 where Nukhul sandstone exhibits lowest porosities, i.e., 6.76%. This variation indicates higher lateral variability. These two wells also exhibit high variability in the Vsh (sands in Sidri-9 are much cleaner). High lateral variability within Nukhul sandstones was also reported from the onshore Arta field, West Gharib concession in the Eastern Desert (Dongas and Crane 2021). The calculated water saturation of the reservoir zone in the three productive wells ranges between 17 and 34% (i.e., the hydrocarbon saturation equals 66–87%). A small range of BVW indicates superior quality of the hydrocarbon-bearing sandstone intervals.

The petrophysical assessment presented in this study is based on the wireline logs. The inferred rock properties could not be calibrated with the direct core-based measurements, which possess as the primary limitation. Unavailability of routine/special core analysis is a very common challenge in petrophysical evaluation projects, which has been the case for this study as well. Also clay mineralogy can be confirmed based on the X-ray diffraction (XRD) and scanning electron microscopy (SEM) dataset, which were unfortunately unavailable for this study.

6 Summary and conclusions

This study presents the key petrophysical properties of the oil-producing Nukhul sandstones in the Abu Rudeis-Sidri field. In absence of core-based measurements, this study utilized wireline logs to infer the reservoir characteristics. The conclusions of this study are as below:

  • Nukhul sandstones exhibit ~ 10–16% total porosity with less than 34% water saturation indicating the excellent conventional reservoir quality.

  • Lowest inferred Sw is around 17% from the Zone-2 in the well Sidri-9. BVW ranges between 0.011 and 0.036.

  • Petrophysical cross plot indicates the calcareous nature of the Nukhul sandstones, with montmorillonite as dominant clay phase.

  • Highest reservoir pay thickness is inferred in the well Sidri-9 where the reservoir is distributed in two zones, separated by a 5 m thick barrier.

  • The Nukhul sandstones ARM-7 exhibit very high water saturation (~ 90%), therefore inferred unproductive.

  • High lateral variability in shale volume and porosity distribution has been observed within the studied sandstones of nearby wells (i.e., Sidri-20 and Sidri-9).

  • Preliminary seismic interpretation indicates steeply dipping normal faults with high throws. One of the studied wells in such deeper hanging wall did not encounter any productive zone indicating the critical structural implications for reservoir access across fault blocks.

A detailed structural interpretation (with 3D seismic dataset) clubbed with such reservoir petrophysical assessment will be immensely beneficial to understand the lateral variability and continuity of the Nukhul pay sands across the fault compartments of the Abu Rudeis-Sidri field and contribute to identifying infill drilling locations for further field development.