ABSTRACT Following recent hydrocarbon exploration activity, a new study has been carried out with the goal of defining the high-resolution biochronologic sequence stratigraphy of the Oligocene section in the offshore Nile Delta. A total of 98 samples were selected from 2 wells: Burullus 1 Memphis and Petrobel 1 Habbar. The study is based on a high-resolution microfossil biochronology including benthic and planktonic foraminifera and nannofossils as well as facies analysis and wire-line log calibration to investigate the evolution of Oligocene gas-bearing sequences in the southeastern Mediterranean region. The planktonic–benthic ratio, foraminiferal diversity, and upper depth limit of benthic foraminiferal taxa as well as the abundance of calcareous nannoplankton help in interpreting the depositional paleoenvironments of the Oligocene succession as fluvial, shallow-marine, middle- to outer-shelf, and bathyal domains. Six sequences were identified, three of which are lower Oligocene (Rupelian) (RuSeq1, RuSeq2, and RuSeq3), and the others are upper Oligocene (Chattian) (ChSeq4, ChSeq5, and ChSeq6). The sequence boundaries and flooding surfaces were correlated with the global eustatic sea-level models. Three major breaks—Eocene–Oligocene, lower–upper Oligocene, and upper Oligocene–lower Miocene boundaries—with intraformational breaks were well defined. INTRODUCTION The upper Paleogene–Neogene successions in the offshore Nile Delta contain significant giant gas exploration targets that provide indispensable future reserves (Dolson et al., 2000). The Oligocene strata, in particular, are some of the most important hydrocarbon reservoirs in the Nile Delta province and the Mediterranean region (Dolson et al., 2001, 2014). Most studies have dealt with the Miocene and Pliocene reservoirs, which were considered as the most important gas reservoirs in Egypt. The Oligocene successions in northern Egypt have been studied with regard to regional stratigraphy and sedimentary architecture (Marzouk, 1970; Omara and Ouda, 1972; Cherif et al., 1993; El Heiny and Enani, 1996; Soliman and Orabi, 2000; Selim, 2018). In the northern Sinai, upper Oligocene marine strata were recorded based on larger benthic foraminifera (Boukhary et al., 2008; Kuss and Boukhary, 2008). A few studies dealing with the benthic foraminifera from the Oligocene strata in the offshore eastern Nile Delta were performed by Sallam (2013). The data on the Oligocene succession are poor because of the lack of deep-water wells. The Burullus 1 Memphis well reaches the Cretaceous section and is one of the deepest wells in the Mediterranean region (4859 m [15,941 ft] total depth). The present work is a high-resolution sequence biostratigraphic analysis based on the investigation of recovered Oligocene cutting samples and supported by available wire-line logs and application of Cyclolog software of Nio et al. (2005) from the Burullus 1 Memphis and Petrobel 1 Habbar wells in the northern offshore Nile Delta (Figure 1). Figure 1. Geologic and location map of the study area: offshore Nile Delta, southeastern Mediterranean region, Egypt. STRATIGRAPHIC AND GEOLOGIC SETTING In Egypt, Oligocene strata represent a period of rifting in the Gulf of Suez and northward tilting of Egypt toward the Mediterranean (Dolson et al., 2014). Three major fault trends were detected in the eastern part of the Nile Delta: a northeast-southwest Jurassic–Lower Cretaceous trend contemporaneous with the Mediterranean rifting, a northwest-southeast Oligocene–Miocene trend contemporaneous with the Gulf of Suez rifting, and an east-northeast–west-southwest fault trend (Abdel Aal et al., 1996, 2001). The northeast–east-northeast trend (hinge zone) is a faulted flexure zone dividing the Nile Delta into two sedimentary subprovinces and resulted in the thickening of the sedimentary section in the northern Nile Delta (El-Heiny and Morsi 1992; Boukhary et al., 2016; Makled et al., 2017). These fault trends separate the Mediterranean basin into five domains (Abdel Aal et al., 2001): platform, rotated fault blocks, basin floor, diapiric salt basin, and inverted salt basin. The study area lies within the platform domain southeast of the northeast-southwest (Rosetta) fault trend (Figure 1). During the Late Cretaceous–Paleogene, Syrian arc deformations with a right lateral transpression and compressional tectonics were developed, resulting in rift basin inversion and a series of northeast-southwest folds (Figure 2) (Sestini, 1995; Ayyad and Darwish 1996; Dolson et al., 2000; Abd El-Motaal and Kusky, 2003; Abd-Allah, 2008). Figure 2. Tectonostratigraphic framework of the Nile Delta and Mediterranean (modified after Dolson et al., 2001, 2014). Ch. cubensis = Chiloguembelina cubensis; H. alabamensis = Hantkenina alabamensis; P. kugleri = Paragloborotalia kugleri; Quat. = Quaternary. A thick succession of nonmarine Oligocene sediments was deposited and is preserved in many outcrops across northern Egypt, particularly in the Fayoum and Whale Valley depressions (Dolson et al., 2014). The Oligocene and Miocene strata of the Nile Delta basin include shales and marls with sufficient quantities of organic carbon to be considered as good sources for gas (Dolson et al., 2001; Shaaban et al., 2006). Oligocene strata are represented in the Nile Delta and Mediterranean regions by the Tineh Formation (Figure 2). It is middle Oligocene to late Miocene (El-Heiny and Morsi, 1992) and is composed of marine to fluviomarine shale and sandstone interbeds (El-Heiny and Enani, 1996). This formation is unconformably overlain by the lower Miocene Qantara Formation and underlain by the middle Eocene Appolonia Formation. In the Burullus 1 Memphis well, the Tineh Formation is approximately 275 m (∼902 ft) thick, is composed of medium to dark gray shale, is silty with disseminated carbonaceous matter in places, and is intercalated with thin, argillaceous limestone beds. In the Petrobel 1 Habbar well, it is approximately 400 m (∼1312 ft) thick and is composed mainly of gray, dark gray, and greenish-gray shale that is silty in parts with sandstone interbeds. However, the lower Oligocene strata were not penetrated in the Petrobel 1 Habbar well. Figure 3. Stratigraphic distribution chart of the recorded planktonic foraminifera in the Burullus 1 Memphis well. MATERIALS, METHODS, AND DEPOSITORY The present analytical study is based on a total of 98 drill cutting samples from 2 wells spanning the Oligocene and lower Miocene strata in the offshore Nile Delta. A total of 53 drill cutting samples from the Burullus 1 Memphis well (32° 06′ 22.31′′N and 30° 51′ 40.64′′E) and 45 drill cutting samples from the Petrobel 1 Habbar well (32° 05' 24.65′′N and 31° 11′ 47.17′′E) were analyzed. Wire-line log data include gamma-ray, bulk density (standard resolution formation density), neutron density (array porosity limestone corrected), and resistivity curves. These logs were processed using the Cyclolog software (Nio et al., 2005). Cyclolog analyzes facies-related frequency patterns on geophysical logs and is used to formulate cyclic and stratigraphic interpretations from the log data. This Cyclolog processing of the data resulted in two patterns of curves, which are prediction error filter analysis (PEFA) and integrated prediction error filter analysis (INPEFA). Of the available drill cutting samples, 60 g were processed for analysis of foraminifera and other microfossils. The samples were washed and soaked in water with hydrogen peroxide (15%) and then sieved over 63 μm and dried over a hot electric plate. The residues were examined and photographed using a research-grade stereobinocular microscope and a scanning electron microscope hosted at the Egyptian Mineral Resources Authority. The authors identified planktonic and benthic foraminifera following the scheme of Bolli and Saunders (1985) and Loeblich and Tappan (1988). The material and photographed specimens were reposited in the collection of micropaleontology in the Museum of the Geology Department, Faculty of Science, Cairo University. Calcareous nannoplankton data were available to support the present results and interpretations. Detailed binocular lithofacies description, wire-line log analysis, and high-resolution biostratigraphic data contributed in the reconstruction of the depositional environment and paleogeographic setting of the studied Oligocene succession. The sedimentological stacking patterns (retrogradational, aggradational, and progradational), maximum flooding surface (MFS), sequence boundary, and systems tracts were also identified and characterized. Figure 4. Stratigraphic distribution chart of the recorded benthic foraminifera in the Burullus 1 Memphis well. Aquit. = Aquitanian. RESULTS AND DISCUSSION Foraminiferal Biostratigraphy The distribution range and frequency of the recovered 19 planktonic and 47 benthic foraminiferal species (Figure 3–5) in the investigated area led to the identification of 5 planktonic foraminiferal biozones. These are the M1 Paragloborotalia kugleri, O7 Paragloborotalia pseudokugleri, O6 Globigerina ciperoensis, O5 Paragloborotalia opima, and O2 Turborotalia ampliapertura Zones. Photographs of the planktonic and benthic foraminiferal taxa are shown in Figure 6–9. The foraminiferal zones and the absolute ages (Figure 10) follow those of Berggren et al. (1995), Berggren and Pearson (2005), Wade et al. (2011), Farouk et al. (2014), Ogg et al. (2016), and the International Chronostratigraphic Chart (Cohen et al., 2017). The characteristics of the identified planktonic zones, from top to bottom, are as follows. Figure 5. Stratigraphic distribution chart of the recorded planktonic and benthic foraminifera in the Petrobel 1 Habbar well. Forams. = foraminifera; TD = total depth. P. kugleri (M1) Zone Definition — Total range of the nominate taxon of P. kugleri. Occurrence — It has been recorded in the interval from 4535 to 4505 m (14,878 to 14,780 ft) in the Burullus 1 Memphis well, whereas in the Petrobel 1 Habbar well, the zone has been recorded in the interval from 4100 to 4050 m (13,451 to 13,287 ft). Correlation — This zone is equivalent to zone M1 of Berggren et al. (1995), Berggren and Pearson (2005), and Wade et al. (2011). In Egypt, it was recorded by Farouk et al. (2014) in the Nile Delta region. Figure 6. (A, B) Globigerina ciperoensis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), lower Oligocene. Ventral view (A); dorsal view (B). (C–E) Paragloborotalia mayeri. Depth: 4535–4540 m (14,878–14,895 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Paragloborotalia kugleri. Depth: 4060–4070 m (13,320–13,353 ft), Petrobel 1 Habbar well, Tineh Fm., lower Miocene. Ventral view (F); dorsal view (G). (H–M). Paragloborotalia pseudokugleri. (H–J) Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K–M) Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); side view (L); dorsal view (M). (N, O) Chiloguembelina cubensis. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (P–S) Globigerina angulisuturalis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm. Assemblage — It is characterized by the occurrence of the assemblage Dentoglobigerina sellii, Globoquadrina altispira, and Catapsydrax dissimilis. The benthic foraminifera throughout this interval are Pullenia bulloides, Sphaeroidina bulloides, Globocassidulina subglobosa, Oridorsalis umbonatus, Cibicides lobatulus, and Cibicidoides cf. ungerianus. Age — early Miocene (Aquitanian). P. pseudokugleri (O7) Zone Definition — Interval between the lowest occurrence (LO) of P. pseudokugleri and the LO of P. kugleri. Occurrence — This zone was recorded only in the Burullus 1 Memphis well from a depth of 4625 to 4535 m (15,173 to 14,878 ft) (90 m [295 ft] thick), which represents the upper part of the Oligocene Tineh Formation. Figure 7. (A, B) Catapsydrax dissimilis. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C–E) Paragloborotalia opima opima. Depth: 4705–4710 m (15,436–15,452 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Dentoglobigerina sellii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (F); dorsal view (G). (H–J) Paragloborotalia opima nana. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K, L) Catapsydrax unicavus. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M–O) Dentoglobigerina tripartita. Depth: 4745–4755 m (15,567–15,600 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (M); side view (N); dorsal view (O). (P, Q) Turborotalia ampliapertura. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (R, S) Turborotalia increbescens. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm. Correlation — This zone is equivalent to the upper part of zone P22 of Berggren et al. (1995), upper part of zone O6 of Berggren and Pearson (2005), and zone O7 of Wade et al. (2011). Remarks — The absence of this zone in the Petrobel 1 Habbar well may be caused by the effect of the Gulf of Suez rift (Oligocene–Miocene hiatus surface). Assemblage — It is mostly composed of D. sellii and C. dissimilis assemblage. The benthic foraminifera are characterized by the occurrence of the assemblage P. bulloides, S. bulloides, Gyroidina soldanii, G. subglobosa, O. umbonatus, Bolivina dilatata, C. cf. ungerianus, Ammodiscus cretaceous, and Planulina cf. marialana. Figure 8. (A, B) Globocassidulina subglobosa. Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C, D) Cassidulina laevigata. Depth 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (C); dorsal view (D). (E) Uvigerina auberiana. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (F) Uvigerina auberiana attenuata. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (G, H) Pullenia bulloides. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (I, J) Pullenia compressiuscula quadriloba. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene. (K, L) Ammonia beccarii. Depth: 4120–4130 m (13,517–13,549 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M, N) Ammodiscus cretaceous. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (O, P) Haplophragmoides sp. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Scale bar = 100 μm. Age — late Oligocene (latest Chattian). G. ciperoensis (O6) Zone Definition — Partial range zone between the highest occurrence (HO) of P. opima and the LO of P. pseudokugleri. Occurrence — This zone has been recorded in the Burullus 1 Memphis well from a depth of 4705 to 4625 m (15,436 to 15,173 ft) (80 m [263 ft] thick). In the Petrobel 1 Habbar well, it is 220 m (722 ft) thick, recorded in the interval from 4320 to 4100 m (14,173 to 13,451 ft). Correlation — This zone is equivalent to the lower part of zone P22 of Berggren et al. (1995), the lower part of zone O6 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011). Figure 9. (A) Praeglobobulimina pupoides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (B–D) Gyroidina soldanii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (B); side view (C); dorsal view (D). (E) Bolivina dilatata. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. (F–H) Cibicidioides kullenbergi. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (F); side view (G); dorsal view (H). (I–L) Anomalinoides granosus. (I, J) Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene (K, L) Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (M, N) Sphaeroidina bulloides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (M); dorsal view (N). (O, P) Oridorsalis umbonatus. Depth: 4595–4600 m (15,075–15,091 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (O); dorsal view (P). Scale bar = 100 μm. Assemblage — The recorded planktonic assemblage is P. opima nana, Paragloborotalia mayeri, and C. dissimilis. Also, this zone is characterized by a remarkable common occurrence of the benthic foraminifera Uvigerin auberiana. Age — late Oligocene (Chattian). P. opima (O5) Zone Definition — Interval between the highest common occurrence of Chiloguembelina cubensis and the HO of P. opima. Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4755 to 4705 m (15,600 to 15,436 ft) (50 m [164 ft] thick). In the Petrobel 1 Habbar well, it was recorded in the interval from 4320 to 4500 m (14,173 to 14,763 ft) (180 m [590 ft] thick). Correlation — This zone is equivalent to zone P21b of Berggren et al. (1995), zone O5 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011). Remarks — The marker species C. cubensis was recorded only at the base of the Oligocene succession within the T. ampliapertura Zone in the Burullus 1 Memphis well. Also, the last downhole occurrence of D. sellii is difficult to pick because of caving problems. The lower–upper Oligocene boundary is tentatively placed at a depth of 4725 m (15,501 ft) based on the nannoplankton NP23. Figure 10. Oligocene biozones in correlation with standard zonation (Martini, 1971; Wade et al., 2011; Farouk et al., 2014; Ogg et al., 2016). Aquit. = Aquitanian; Chron = Chronozone (polarity zone); Ch. cubensis = Chiloguembelina cubensis; G. angulisuturalis = Globigerina angulisuturalis; G. ciperoensis = Globigerina ciperoensis; G. sellii = Globigerina sellii; Gd. primordius = Globigerinoides primordius; Gq. dehiscens = Globoquadrina dehiscens; Gt. kugleri = Globorotalia kugleri; P. kugleri = Paragloborotalia kugleri; P. naguewichiensis = Pseudohastigerina naguewichiensis; P. opima = Paragloborotalia opima; P. pseudokugleri = Paragloborotalia pseudokugleri; Pg. otima s.s. = Paragloborotalia opima; T. ampliapertura = Turborotalia ampliapertura. Assemblage — The planktonic assemblage is mostly composed of P. opima nana, P. opima opima, P. mayeri, D. sellii, Catapsydrax unicavus, Dentoglobigerina tripartita, Globigerina angustiumilicata, G. ciperoensis, and C. dissimilis. The benthic foraminifera are characterized by the occurrence of the following assemblage: Praeglobobulimina pupoides, P. bulloides, G. soldanii, Gyroidinoides girardana, Uvigerina costata, G. subglobosa, Pullenia quadriloba, O. umbonatus, A. cretaceous, Cibicidoides ungerianus, Haplophragmoides sp., Dorothia sp., Anomalinoides sp., and Nonion sp. Age — late Oligocene (Rupelian–Chattian).--> Biostratigraphy and sequence stratigraphy of the Oligocene succession, offshore Nile Delta, southeastern Mediterranean, Egypt, and its paleoenvironmental implications

Biostratigraphy and sequence stratigraphy of the Oligocene succession, offshore Nile Delta, southeastern Mediterranean, Egypt, and its paleoenvironmental implications

ABSTRACT

Following recent hydrocarbon exploration activity, a new study has been carried out with the goal of defining the high-resolution biochronologic sequence stratigraphy of the Oligocene section in the offshore Nile Delta. A total of 98 samples were selected from 2 wells: Burullus 1 Memphis and Petrobel 1 Habbar. The study is based on a high-resolution microfossil biochronology including benthic and planktonic foraminifera and nannofossils as well as facies analysis and wire-line log calibration to investigate the evolution of Oligocene gas-bearing sequences in the southeastern Mediterranean region. The planktonic–benthic ratio, foraminiferal diversity, and upper depth limit of benthic foraminiferal taxa as well as the abundance of calcareous nannoplankton help in interpreting the depositional paleoenvironments of the Oligocene succession as fluvial, shallow-marine, middle- to outer-shelf, and bathyal domains. Six sequences were identified, three of which are lower Oligocene (Rupelian) (RuSeq1, RuSeq2, and RuSeq3), and the others are upper Oligocene (Chattian) (ChSeq4, ChSeq5, and ChSeq6). The sequence boundaries and flooding surfaces were correlated with the global eustatic sea-level models. Three major breaks—Eocene–Oligocene, lower–upper Oligocene, and upper Oligocene–lower Miocene boundaries—with intraformational breaks were well defined.

INTRODUCTION

The upper Paleogene–Neogene successions in the offshore Nile Delta contain significant giant gas exploration targets that provide indispensable future reserves (Dolson et al., 2000). The Oligocene strata, in particular, are some of the most important hydrocarbon reservoirs in the Nile Delta province and the Mediterranean region (Dolson et al., 2001, 2014). Most studies have dealt with the Miocene and Pliocene reservoirs, which were considered as the most important gas reservoirs in Egypt. The Oligocene successions in northern Egypt have been studied with regard to regional stratigraphy and sedimentary architecture (Marzouk, 1970; Omara and Ouda, 1972; Cherif et al., 1993; El Heiny and Enani, 1996; Soliman and Orabi, 2000; Selim, 2018). In the northern Sinai, upper Oligocene marine strata were recorded based on larger benthic foraminifera (Boukhary et al., 2008; Kuss and Boukhary, 2008). A few studies dealing with the benthic foraminifera from the Oligocene strata in the offshore eastern Nile Delta were performed by Sallam (2013). The data on the Oligocene succession are poor because of the lack of deep-water wells. The Burullus 1 Memphis well reaches the Cretaceous section and is one of the deepest wells in the Mediterranean region (4859 m [15,941 ft] total depth).

The present work is a high-resolution sequence biostratigraphic analysis based on the investigation of recovered Oligocene cutting samples and supported by available wire-line logs and application of Cyclolog software of Nio et al. (2005) from the Burullus 1 Memphis and Petrobel 1 Habbar wells in the northern offshore Nile Delta (Figure 1).

rtxyyuabevcd Figure 1. Geologic and location map of the study area: offshore Nile Delta, southeastern Mediterranean region, Egypt.

STRATIGRAPHIC AND GEOLOGIC SETTING

In Egypt, Oligocene strata represent a period of rifting in the Gulf of Suez and northward tilting of Egypt toward the Mediterranean (Dolson et al., 2014). Three major fault trends were detected in the eastern part of the Nile Delta: a northeast-southwest Jurassic–Lower Cretaceous trend contemporaneous with the Mediterranean rifting, a northwest-southeast Oligocene–Miocene trend contemporaneous with the Gulf of Suez rifting, and an east-northeast–west-southwest fault trend (Abdel Aal et al., 1996, 2001). The northeast–east-northeast trend (hinge zone) is a faulted flexure zone dividing the Nile Delta into two sedimentary subprovinces and resulted in the thickening of the sedimentary section in the northern Nile Delta (El-Heiny and Morsi 1992; Boukhary et al., 2016; Makled et al., 2017). These fault trends separate the Mediterranean basin into five domains (Abdel Aal et al., 2001): platform, rotated fault blocks, basin floor, diapiric salt basin, and inverted salt basin. The study area lies within the platform domain southeast of the northeast-southwest (Rosetta) fault trend (Figure 1). During the Late Cretaceous–Paleogene, Syrian arc deformations with a right lateral transpression and compressional tectonics were developed, resulting in rift basin inversion and a series of northeast-southwest folds (Figure 2) (Sestini, 1995; Ayyad and Darwish 1996; Dolson et al., 2000; Abd El-Motaal and Kusky, 2003; Abd-Allah, 2008).

Figure 2. Tectonostratigraphic framework of the Nile Delta and Mediterranean (modified after Dolson et al., 2001, 2014). Ch. cubensis = Chiloguembelina cubensis; H. alabamensis = Hantkenina alabamensis; P. kugleri = Paragloborotalia kugleri; Quat. = Quaternary.

A thick succession of nonmarine Oligocene sediments was deposited and is preserved in many outcrops across northern Egypt, particularly in the Fayoum and Whale Valley depressions (Dolson et al., 2014). The Oligocene and Miocene strata of the Nile Delta basin include shales and marls with sufficient quantities of organic carbon to be considered as good sources for gas (Dolson et al., 2001; Shaaban et al., 2006). Oligocene strata are represented in the Nile Delta and Mediterranean regions by the Tineh Formation (Figure 2). It is middle Oligocene to late Miocene (El-Heiny and Morsi, 1992) and is composed of marine to fluviomarine shale and sandstone interbeds (El-Heiny and Enani, 1996). This formation is unconformably overlain by the lower Miocene Qantara Formation and underlain by the middle Eocene Appolonia Formation. In the Burullus 1 Memphis well, the Tineh Formation is approximately 275 m (∼902 ft) thick, is composed of medium to dark gray shale, is silty with disseminated carbonaceous matter in places, and is intercalated with thin, argillaceous limestone beds. In the Petrobel 1 Habbar well, it is approximately 400 m (∼1312 ft) thick and is composed mainly of gray, dark gray, and greenish-gray shale that is silty in parts with sandstone interbeds. However, the lower Oligocene strata were not penetrated in the Petrobel 1 Habbar well.

Figure 3. Stratigraphic distribution chart of the recorded planktonic foraminifera in the Burullus 1 Memphis well.

MATERIALS, METHODS, AND DEPOSITORY

The present analytical study is based on a total of 98 drill cutting samples from 2 wells spanning the Oligocene and lower Miocene strata in the offshore Nile Delta. A total of 53 drill cutting samples from the Burullus 1 Memphis well (32° 06′ 22.31′′N and 30° 51′ 40.64′′E) and 45 drill cutting samples from the Petrobel 1 Habbar well (32° 05' 24.65′′N and 31° 11′ 47.17′′E) were analyzed. Wire-line log data include gamma-ray, bulk density (standard resolution formation density), neutron density (array porosity limestone corrected), and resistivity curves. These logs were processed using the Cyclolog software (Nio et al., 2005). Cyclolog analyzes facies-related frequency patterns on geophysical logs and is used to formulate cyclic and stratigraphic interpretations from the log data. This Cyclolog processing of the data resulted in two patterns of curves, which are prediction error filter analysis (PEFA) and integrated prediction error filter analysis (INPEFA). Of the available drill cutting samples, 60 g were processed for analysis of foraminifera and other microfossils. The samples were washed and soaked in water with hydrogen peroxide (15%) and then sieved over 63 μm and dried over a hot electric plate. The residues were examined and photographed using a research-grade stereobinocular microscope and a scanning electron microscope hosted at the Egyptian Mineral Resources Authority. The authors identified planktonic and benthic foraminifera following the scheme of Bolli and Saunders (1985) and Loeblich and Tappan (1988). The material and photographed specimens were reposited in the collection of micropaleontology in the Museum of the Geology Department, Faculty of Science, Cairo University. Calcareous nannoplankton data were available to support the present results and interpretations. Detailed binocular lithofacies description, wire-line log analysis, and high-resolution biostratigraphic data contributed in the reconstruction of the depositional environment and paleogeographic setting of the studied Oligocene succession. The sedimentological stacking patterns (retrogradational, aggradational, and progradational), maximum flooding surface (MFS), sequence boundary, and systems tracts were also identified and characterized.

Figure 4. Stratigraphic distribution chart of the recorded benthic foraminifera in the Burullus 1 Memphis well. Aquit. = Aquitanian.

RESULTS AND DISCUSSION

Foraminiferal Biostratigraphy

The distribution range and frequency of the recovered 19 planktonic and 47 benthic foraminiferal species (Figure 3–5) in the investigated area led to the identification of 5 planktonic foraminiferal biozones. These are the M1 Paragloborotalia kugleri, O7 Paragloborotalia pseudokugleri, O6 Globigerina ciperoensis, O5 Paragloborotalia opima, and O2 Turborotalia ampliapertura Zones. Photographs of the planktonic and benthic foraminiferal taxa are shown in Figure 6–9. The foraminiferal zones and the absolute ages (Figure 10) follow those of Berggren et al. (1995), Berggren and Pearson (2005), Wade et al. (2011), Farouk et al. (2014), Ogg et al. (2016), and the International Chronostratigraphic Chart (Cohen et al., 2017). The characteristics of the identified planktonic zones, from top to bottom, are as follows.

Figure 5. Stratigraphic distribution chart of the recorded planktonic and benthic foraminifera in the Petrobel 1 Habbar well. Forams. = foraminifera; TD = total depth.

P. kugleri (M1) Zone

Definition — Total range of the nominate taxon of P. kugleri.

Occurrence — It has been recorded in the interval from 4535 to 4505 m (14,878 to 14,780 ft) in the Burullus 1 Memphis well, whereas in the Petrobel 1 Habbar well, the zone has been recorded in the interval from 4100 to 4050 m (13,451 to 13,287 ft).

Correlation — This zone is equivalent to zone M1 of Berggren et al. (1995), Berggren and Pearson (2005), and Wade et al. (2011). In Egypt, it was recorded by Farouk et al. (2014) in the Nile Delta region.

Figure 6. (A, B) Globigerina ciperoensis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), lower Oligocene. Ventral view (A); dorsal view (B). (C–E) Paragloborotalia mayeri. Depth: 4535–4540 m (14,878–14,895 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Paragloborotalia kugleri. Depth: 4060–4070 m (13,320–13,353 ft), Petrobel 1 Habbar well, Tineh Fm., lower Miocene. Ventral view (F); dorsal view (G). (H–M). Paragloborotalia pseudokugleri. (H–J) Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K–M) Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); side view (L); dorsal view (M). (N, O) Chiloguembelina cubensis. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (P–S) Globigerina angulisuturalis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm.

Assemblage — It is characterized by the occurrence of the assemblage Dentoglobigerina sellii, Globoquadrina altispira, and Catapsydrax dissimilis. The benthic foraminifera throughout this interval are Pullenia bulloides, Sphaeroidina bulloides, Globocassidulina subglobosa, Oridorsalis umbonatus, Cibicides lobatulus, and Cibicidoides cf. ungerianus.

Age — early Miocene (Aquitanian).

P. pseudokugleri (O7) Zone

Definition — Interval between the lowest occurrence (LO) of P. pseudokugleri and the LO of P. kugleri.

Occurrence — This zone was recorded only in the Burullus 1 Memphis well from a depth of 4625 to 4535 m (15,173 to 14,878 ft) (90 m [295 ft] thick), which represents the upper part of the Oligocene Tineh Formation.

Figure 7. (A, B) Catapsydrax dissimilis. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C–E) Paragloborotalia opima opima. Depth: 4705–4710 m (15,436–15,452 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Dentoglobigerina sellii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (F); dorsal view (G). (H–J) Paragloborotalia opima nana. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K, L) Catapsydrax unicavus. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M–O) Dentoglobigerina tripartita. Depth: 4745–4755 m (15,567–15,600 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (M); side view (N); dorsal view (O). (P, Q) Turborotalia ampliapertura. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (R, S) Turborotalia increbescens. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm.

Correlation — This zone is equivalent to the upper part of zone P22 of Berggren et al. (1995), upper part of zone O6 of Berggren and Pearson (2005), and zone O7 of Wade et al. (2011).

Remarks — The absence of this zone in the Petrobel 1 Habbar well may be caused by the effect of the Gulf of Suez rift (Oligocene–Miocene hiatus surface).

Assemblage — It is mostly composed of D. sellii and C. dissimilis assemblage. The benthic foraminifera are characterized by the occurrence of the assemblage P. bulloides, S. bulloides, Gyroidina soldanii, G. subglobosa, O. umbonatus, Bolivina dilatata, C. cf. ungerianus, Ammodiscus cretaceous, and Planulina cf. marialana.

Figure 8. (A, B) Globocassidulina subglobosa. Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C, D) Cassidulina laevigata. Depth 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (C); dorsal view (D). (E) Uvigerina auberiana. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (F) Uvigerina auberiana attenuata. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (G, H) Pullenia bulloides. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (I, J) Pullenia compressiuscula quadriloba. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene. (K, L) Ammonia beccarii. Depth: 4120–4130 m (13,517–13,549 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M, N) Ammodiscus cretaceous. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (O, P) Haplophragmoides sp. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Scale bar = 100 μm.

Age — late Oligocene (latest Chattian).

G. ciperoensis (O6) Zone

Definition — Partial range zone between the highest occurrence (HO) of P. opima and the LO of P. pseudokugleri.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well from a depth of 4705 to 4625 m (15,436 to 15,173 ft) (80 m [263 ft] thick). In the Petrobel 1 Habbar well, it is 220 m (722 ft) thick, recorded in the interval from 4320 to 4100 m (14,173 to 13,451 ft).

Correlation — This zone is equivalent to the lower part of zone P22 of Berggren et al. (1995), the lower part of zone O6 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011).

Figure 9. (A) Praeglobobulimina pupoides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (B–D) Gyroidina soldanii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (B); side view (C); dorsal view (D). (E) Bolivina dilatata. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. (F–H) Cibicidioides kullenbergi. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (F); side view (G); dorsal view (H). (I–L) Anomalinoides granosus. (I, J) Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene (K, L) Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (M, N) Sphaeroidina bulloides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (M); dorsal view (N). (O, P) Oridorsalis umbonatus. Depth: 4595–4600 m (15,075–15,091 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (O); dorsal view (P). Scale bar = 100 μm.

Assemblage — The recorded planktonic assemblage is P. opima nana, Paragloborotalia mayeri, and C. dissimilis. Also, this zone is characterized by a remarkable common occurrence of the benthic foraminifera Uvigerin auberiana.

Age — late Oligocene (Chattian).

P. opima (O5) Zone

Definition — Interval between the highest common occurrence of Chiloguembelina cubensis and the HO of P. opima.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4755 to 4705 m (15,600 to 15,436 ft) (50 m [164 ft] thick). In the Petrobel 1 Habbar well, it was recorded in the interval from 4320 to 4500 m (14,173 to 14,763 ft) (180 m [590 ft] thick).

Correlation — This zone is equivalent to zone P21b of Berggren et al. (1995), zone O5 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011).

Remarks — The marker species C. cubensis was recorded only at the base of the Oligocene succession within the T. ampliapertura Zone in the Burullus 1 Memphis well. Also, the last downhole occurrence of D. sellii is difficult to pick because of caving problems. The lower–upper Oligocene boundary is tentatively placed at a depth of 4725 m (15,501 ft) based on the nannoplankton NP23.

Figure 10. Oligocene biozones in correlation with standard zonation (Martini, 1971; Wade et al., 2011; Farouk et al., 2014; Ogg et al., 2016). Aquit. = Aquitanian; Chron = Chronozone (polarity zone); Ch. cubensis = Chiloguembelina cubensis; G. angulisuturalis = Globigerina angulisuturalis; G. ciperoensis = Globigerina ciperoensis; G. sellii = Globigerina sellii; Gd. primordius = Globigerinoides primordius; Gq. dehiscens = Globoquadrina dehiscens; Gt. kugleri = Globorotalia kugleri; P. kugleri = Paragloborotalia kugleri; P. naguewichiensis = Pseudohastigerina naguewichiensis; P. opima = Paragloborotalia opima; P. pseudokugleri = Paragloborotalia pseudokugleri; Pg. otima s.s. = Paragloborotalia opima; T. ampliapertura = Turborotalia ampliapertura.

Assemblage — The planktonic assemblage is mostly composed of P. opima nana, P. opima opima, P. mayeri, D. sellii, Catapsydrax unicavus, Dentoglobigerina tripartita, Globigerina angustiumilicata, G. ciperoensis, and C. dissimilis. The benthic foraminifera are characterized by the occurrence of the following assemblage: Praeglobobulimina pupoides, P. bulloides, G. soldanii, Gyroidinoides girardana, Uvigerina costata, G. subglobosa, Pullenia quadriloba, O. umbonatus, A. cretaceous, Cibicidoides ungerianus, Haplophragmoides sp., Dorothia sp., Anomalinoides sp., and Nonion sp.

Age — late Oligocene (Rupelian–Chattian).

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ABSTRACT

Following recent hydrocarbon exploration activity, a new study has been carried out with the goal of defining the high-resolution biochronologic sequence stratigraphy of the Oligocene section in the offshore Nile Delta. A total of 98 samples were selected from 2 wells: Burullus 1 Memphis and Petrobel 1 Habbar. The study is based on a high-resolution microfossil biochronology including benthic and planktonic foraminifera and nannofossils as well as facies analysis and wire-line log calibration to investigate the evolution of Oligocene gas-bearing sequences in the southeastern Mediterranean region. The planktonic–benthic ratio, foraminiferal diversity, and upper depth limit of benthic foraminiferal taxa as well as the abundance of calcareous nannoplankton help in interpreting the depositional paleoenvironments of the Oligocene succession as fluvial, shallow-marine, middle- to outer-shelf, and bathyal domains. Six sequences were identified, three of which are lower Oligocene (Rupelian) (RuSeq1, RuSeq2, and RuSeq3), and the others are upper Oligocene (Chattian) (ChSeq4, ChSeq5, and ChSeq6). The sequence boundaries and flooding surfaces were correlated with the global eustatic sea-level models. Three major breaks—Eocene–Oligocene, lower–upper Oligocene, and upper Oligocene–lower Miocene boundaries—with intraformational breaks were well defined.

INTRODUCTION

The upper Paleogene–Neogene successions in the offshore Nile Delta contain significant giant gas exploration targets that provide indispensable future reserves (Dolson et al., 2000). The Oligocene strata, in particular, are some of the most important hydrocarbon reservoirs in the Nile Delta province and the Mediterranean region (Dolson et al., 2001, 2014). Most studies have dealt with the Miocene and Pliocene reservoirs, which were considered as the most important gas reservoirs in Egypt. The Oligocene successions in northern Egypt have been studied with regard to regional stratigraphy and sedimentary architecture (Marzouk, 1970; Omara and Ouda, 1972; Cherif et al., 1993; El Heiny and Enani, 1996; Soliman and Orabi, 2000; Selim, 2018). In the northern Sinai, upper Oligocene marine strata were recorded based on larger benthic foraminifera (Boukhary et al., 2008; Kuss and Boukhary, 2008). A few studies dealing with the benthic foraminifera from the Oligocene strata in the offshore eastern Nile Delta were performed by Sallam (2013). The data on the Oligocene succession are poor because of the lack of deep-water wells. The Burullus 1 Memphis well reaches the Cretaceous section and is one of the deepest wells in the Mediterranean region (4859 m [15,941 ft] total depth).

The present work is a high-resolution sequence biostratigraphic analysis based on the investigation of recovered Oligocene cutting samples and supported by available wire-line logs and application of Cyclolog software of Nio et al. (2005) from the Burullus 1 Memphis and Petrobel 1 Habbar wells in the northern offshore Nile Delta (Figure 1).

Figure 1. Geologic and location map of the study area: offshore Nile Delta, southeastern Mediterranean region, Egypt.

STRATIGRAPHIC AND GEOLOGIC SETTING

In Egypt, Oligocene strata represent a period of rifting in the Gulf of Suez and northward tilting of Egypt toward the Mediterranean (Dolson et al., 2014). Three major fault trends were detected in the eastern part of the Nile Delta: a northeast-southwest Jurassic–Lower Cretaceous trend contemporaneous with the Mediterranean rifting, a northwest-southeast Oligocene–Miocene trend contemporaneous with the Gulf of Suez rifting, and an east-northeast–west-southwest fault trend (Abdel Aal et al., 1996, 2001). The northeast–east-northeast trend (hinge zone) is a faulted flexure zone dividing the Nile Delta into two sedimentary subprovinces and resulted in the thickening of the sedimentary section in the northern Nile Delta (El-Heiny and Morsi 1992; Boukhary et al., 2016; Makled et al., 2017). These fault trends separate the Mediterranean basin into five domains (Abdel Aal et al., 2001): platform, rotated fault blocks, basin floor, diapiric salt basin, and inverted salt basin. The study area lies within the platform domain southeast of the northeast-southwest (Rosetta) fault trend (Figure 1). During the Late Cretaceous–Paleogene, Syrian arc deformations with a right lateral transpression and compressional tectonics were developed, resulting in rift basin inversion and a series of northeast-southwest folds (Figure 2) (Sestini, 1995; Ayyad and Darwish 1996; Dolson et al., 2000; Abd El-Motaal and Kusky, 2003; Abd-Allah, 2008).

Figure 2. Tectonostratigraphic framework of the Nile Delta and Mediterranean (modified after Dolson et al., 2001, 2014). Ch. cubensis = Chiloguembelina cubensis; H. alabamensis = Hantkenina alabamensis; P. kugleri = Paragloborotalia kugleri; Quat. = Quaternary.

A thick succession of nonmarine Oligocene sediments was deposited and is preserved in many outcrops across northern Egypt, particularly in the Fayoum and Whale Valley depressions (Dolson et al., 2014). The Oligocene and Miocene strata of the Nile Delta basin include shales and marls with sufficient quantities of organic carbon to be considered as good sources for gas (Dolson et al., 2001; Shaaban et al., 2006). Oligocene strata are represented in the Nile Delta and Mediterranean regions by the Tineh Formation (Figure 2). It is middle Oligocene to late Miocene (El-Heiny and Morsi, 1992) and is composed of marine to fluviomarine shale and sandstone interbeds (El-Heiny and Enani, 1996). This formation is unconformably overlain by the lower Miocene Qantara Formation and underlain by the middle Eocene Appolonia Formation. In the Burullus 1 Memphis well, the Tineh Formation is approximately 275 m (∼902 ft) thick, is composed of medium to dark gray shale, is silty with disseminated carbonaceous matter in places, and is intercalated with thin, argillaceous limestone beds. In the Petrobel 1 Habbar well, it is approximately 400 m (∼1312 ft) thick and is composed mainly of gray, dark gray, and greenish-gray shale that is silty in parts with sandstone interbeds. However, the lower Oligocene strata were not penetrated in the Petrobel 1 Habbar well.

Figure 3. Stratigraphic distribution chart of the recorded planktonic foraminifera in the Burullus 1 Memphis well.

MATERIALS, METHODS, AND DEPOSITORY

The present analytical study is based on a total of 98 drill cutting samples from 2 wells spanning the Oligocene and lower Miocene strata in the offshore Nile Delta. A total of 53 drill cutting samples from the Burullus 1 Memphis well (32° 06′ 22.31′′N and 30° 51′ 40.64′′E) and 45 drill cutting samples from the Petrobel 1 Habbar well (32° 05' 24.65′′N and 31° 11′ 47.17′′E) were analyzed. Wire-line log data include gamma-ray, bulk density (standard resolution formation density), neutron density (array porosity limestone corrected), and resistivity curves. These logs were processed using the Cyclolog software (Nio et al., 2005). Cyclolog analyzes facies-related frequency patterns on geophysical logs and is used to formulate cyclic and stratigraphic interpretations from the log data. This Cyclolog processing of the data resulted in two patterns of curves, which are prediction error filter analysis (PEFA) and integrated prediction error filter analysis (INPEFA). Of the available drill cutting samples, 60 g were processed for analysis of foraminifera and other microfossils. The samples were washed and soaked in water with hydrogen peroxide (15%) and then sieved over 63 μm and dried over a hot electric plate. The residues were examined and photographed using a research-grade stereobinocular microscope and a scanning electron microscope hosted at the Egyptian Mineral Resources Authority. The authors identified planktonic and benthic foraminifera following the scheme of Bolli and Saunders (1985) and Loeblich and Tappan (1988). The material and photographed specimens were reposited in the collection of micropaleontology in the Museum of the Geology Department, Faculty of Science, Cairo University. Calcareous nannoplankton data were available to support the present results and interpretations. Detailed binocular lithofacies description, wire-line log analysis, and high-resolution biostratigraphic data contributed in the reconstruction of the depositional environment and paleogeographic setting of the studied Oligocene succession. The sedimentological stacking patterns (retrogradational, aggradational, and progradational), maximum flooding surface (MFS), sequence boundary, and systems tracts were also identified and characterized.

Figure 4. Stratigraphic distribution chart of the recorded benthic foraminifera in the Burullus 1 Memphis well. Aquit. = Aquitanian.

RESULTS AND DISCUSSION

Foraminiferal Biostratigraphy

The distribution range and frequency of the recovered 19 planktonic and 47 benthic foraminiferal species (Figure 3–5) in the investigated area led to the identification of 5 planktonic foraminiferal biozones. These are the M1 Paragloborotalia kugleri, O7 Paragloborotalia pseudokugleri, O6 Globigerina ciperoensis, O5 Paragloborotalia opima, and O2 Turborotalia ampliapertura Zones. Photographs of the planktonic and benthic foraminiferal taxa are shown in Figure 6–9. The foraminiferal zones and the absolute ages (Figure 10) follow those of Berggren et al. (1995), Berggren and Pearson (2005), Wade et al. (2011), Farouk et al. (2014), Ogg et al. (2016), and the International Chronostratigraphic Chart (Cohen et al., 2017). The characteristics of the identified planktonic zones, from top to bottom, are as follows.

Figure 5. Stratigraphic distribution chart of the recorded planktonic and benthic foraminifera in the Petrobel 1 Habbar well. Forams. = foraminifera; TD = total depth.

P. kugleri (M1) Zone

Definition — Total range of the nominate taxon of P. kugleri.

Occurrence — It has been recorded in the interval from 4535 to 4505 m (14,878 to 14,780 ft) in the Burullus 1 Memphis well, whereas in the Petrobel 1 Habbar well, the zone has been recorded in the interval from 4100 to 4050 m (13,451 to 13,287 ft).

Correlation — This zone is equivalent to zone M1 of Berggren et al. (1995), Berggren and Pearson (2005), and Wade et al. (2011). In Egypt, it was recorded by Farouk et al. (2014) in the Nile Delta region.

Figure 6. (A, B) Globigerina ciperoensis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), lower Oligocene. Ventral view (A); dorsal view (B). (C–E) Paragloborotalia mayeri. Depth: 4535–4540 m (14,878–14,895 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Paragloborotalia kugleri. Depth: 4060–4070 m (13,320–13,353 ft), Petrobel 1 Habbar well, Tineh Fm., lower Miocene. Ventral view (F); dorsal view (G). (H–M). Paragloborotalia pseudokugleri. (H–J) Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K–M) Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); side view (L); dorsal view (M). (N, O) Chiloguembelina cubensis. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (P–S) Globigerina angulisuturalis. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm.

Assemblage — It is characterized by the occurrence of the assemblage Dentoglobigerina sellii, Globoquadrina altispira, and Catapsydrax dissimilis. The benthic foraminifera throughout this interval are Pullenia bulloides, Sphaeroidina bulloides, Globocassidulina subglobosa, Oridorsalis umbonatus, Cibicides lobatulus, and Cibicidoides cf. ungerianus.

Age — early Miocene (Aquitanian).

P. pseudokugleri (O7) Zone

Definition — Interval between the lowest occurrence (LO) of P. pseudokugleri and the LO of P. kugleri.

Occurrence — This zone was recorded only in the Burullus 1 Memphis well from a depth of 4625 to 4535 m (15,173 to 14,878 ft) (90 m [295 ft] thick), which represents the upper part of the Oligocene Tineh Formation.

Figure 7. (A, B) Catapsydrax dissimilis. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C–E) Paragloborotalia opima opima. Depth: 4705–4710 m (15,436–15,452 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (C); side view (D); dorsal view (E). (F, G) Dentoglobigerina sellii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (F); dorsal view (G). (H–J) Paragloborotalia opima nana. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (H); side view (I); dorsal view (J). (K, L) Catapsydrax unicavus. Depth: 4710–4715 m (15,452–15,469 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M–O) Dentoglobigerina tripartita. Depth: 4745–4755 m (15,567–15,600 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (M); side view (N); dorsal view (O). (P, Q) Turborotalia ampliapertura. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (R, S) Turborotalia increbescens. Depth: 4765–4770 m (15,633–15,649 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Scale bar = 100 μm.

Correlation — This zone is equivalent to the upper part of zone P22 of Berggren et al. (1995), upper part of zone O6 of Berggren and Pearson (2005), and zone O7 of Wade et al. (2011).

Remarks — The absence of this zone in the Petrobel 1 Habbar well may be caused by the effect of the Gulf of Suez rift (Oligocene–Miocene hiatus surface).

Assemblage — It is mostly composed of D. sellii and C. dissimilis assemblage. The benthic foraminifera are characterized by the occurrence of the assemblage P. bulloides, S. bulloides, Gyroidina soldanii, G. subglobosa, O. umbonatus, Bolivina dilatata, C. cf. ungerianus, Ammodiscus cretaceous, and Planulina cf. marialana.

Figure 8. (A, B) Globocassidulina subglobosa. Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (C, D) Cassidulina laevigata. Depth 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. Ventral view (C); dorsal view (D). (E) Uvigerina auberiana. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (F) Uvigerina auberiana attenuata. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (G, H) Pullenia bulloides. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (I, J) Pullenia compressiuscula quadriloba. Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene. (K, L) Ammonia beccarii. Depth: 4120–4130 m (13,517–13,549 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (K); dorsal view (L). (M, N) Ammodiscus cretaceous. Depth: 4735–4740 m (15,534–15,551 ft), Burullus 1 Memphis well, Tineh Fm., lower Oligocene. (O, P) Haplophragmoides sp. Depth: 4660–4665 m (15,288–15,305 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Scale bar = 100 μm.

Age — late Oligocene (latest Chattian).

G. ciperoensis (O6) Zone

Definition — Partial range zone between the highest occurrence (HO) of P. opima and the LO of P. pseudokugleri.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well from a depth of 4705 to 4625 m (15,436 to 15,173 ft) (80 m [263 ft] thick). In the Petrobel 1 Habbar well, it is 220 m (722 ft) thick, recorded in the interval from 4320 to 4100 m (14,173 to 13,451 ft).

Correlation — This zone is equivalent to the lower part of zone P22 of Berggren et al. (1995), the lower part of zone O6 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011).

Figure 9. (A) Praeglobobulimina pupoides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Formation (Fm.), upper Oligocene. (B–D) Gyroidina soldanii. Depth: 4605–4610 m (15,108–15,124 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (B); side view (C); dorsal view (D). (E) Bolivina dilatata. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. (F–H) Cibicidioides kullenbergi. Depth: 4130–4140 m (13,549–13,582 ft), Petrobel 1 Habbar well, Tineh Fm., upper Oligocene. Ventral view (F); side view (G); dorsal view (H). (I–L) Anomalinoides granosus. (I, J) Depth: 4530–4535 m (14,862–14,878 ft), Burullus 1 Memphis well, Tineh Fm., Oligocene–Miocene (K, L) Depth: 4600–4605 m (15,091–15,108 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. (M, N) Sphaeroidina bulloides. Depth: 4620–4625 m (15,157–15,173 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (M); dorsal view (N). (O, P) Oridorsalis umbonatus. Depth: 4595–4600 m (15,075–15,091 ft), Burullus 1 Memphis well, Tineh Fm., upper Oligocene. Ventral view (O); dorsal view (P). Scale bar = 100 μm.

Assemblage — The recorded planktonic assemblage is P. opima nana, Paragloborotalia mayeri, and C. dissimilis. Also, this zone is characterized by a remarkable common occurrence of the benthic foraminifera Uvigerin auberiana.

Age — late Oligocene (Chattian).

P. opima (O5) Zone

Definition — Interval between the highest common occurrence of Chiloguembelina cubensis and the HO of P. opima.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4755 to 4705 m (15,600 to 15,436 ft) (50 m [164 ft] thick). In the Petrobel 1 Habbar well, it was recorded in the interval from 4320 to 4500 m (14,173 to 14,763 ft) (180 m [590 ft] thick).

Correlation — This zone is equivalent to zone P21b of Berggren et al. (1995), zone O5 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011).

Remarks — The marker species C. cubensis was recorded only at the base of the Oligocene succession within the T. ampliapertura Zone in the Burullus 1 Memphis well. Also, the last downhole occurrence of D. sellii is difficult to pick because of caving problems. The lower–upper Oligocene boundary is tentatively placed at a depth of 4725 m (15,501 ft) based on the nannoplankton NP23.

Figure 10. Oligocene biozones in correlation with standard zonation (Martini, 1971; Wade et al., 2011; Farouk et al., 2014; Ogg et al., 2016). Aquit. = Aquitanian; Chron = Chronozone (polarity zone); Ch. cubensis = Chiloguembelina cubensis; G. angulisuturalis = Globigerina angulisuturalis; G. ciperoensis = Globigerina ciperoensis; G. sellii = Globigerina sellii; Gd. primordius = Globigerinoides primordius; Gq. dehiscens = Globoquadrina dehiscens; Gt. kugleri = Globorotalia kugleri; P. kugleri = Paragloborotalia kugleri; P. naguewichiensis = Pseudohastigerina naguewichiensis; P. opima = Paragloborotalia opima; P. pseudokugleri = Paragloborotalia pseudokugleri; Pg. otima s.s. = Paragloborotalia opima; T. ampliapertura = Turborotalia ampliapertura.

Assemblage — The planktonic assemblage is mostly composed of P. opima nana, P. opima opima, P. mayeri, D. sellii, Catapsydrax unicavus, Dentoglobigerina tripartita, Globigerina angustiumilicata, G. ciperoensis, and C. dissimilis. The benthic foraminifera are characterized by the occurrence of the following assemblage: Praeglobobulimina pupoides, P. bulloides, G. soldanii, Gyroidinoides girardana, Uvigerina costata, G. subglobosa, Pullenia quadriloba, O. umbonatus, A. cretaceous, Cibicidoides ungerianus, Haplophragmoides sp., Dorothia sp., Anomalinoides sp., and Nonion sp.

Age — late Oligocene (Rupelian–Chattian).

T. ampliapertura (O2) Zone

Definition — Interval between the HO of Pseudohastigerina naguewichiensis and the HO of T. ampliapertura.

Occurrence — This zone has been recorded only in the Burullus 1 Memphis well in the interval from 4795 to 4755 m (15,731 to 15,600 ft) and is 40 m (131 ft) thick. In the Petrobel 1 Habbar well, lower Oligocene strata were not penetrated.

Correlation — This zone is equivalent to zone P19 of Berggren et al. (1995), zone O2 of Berggren and Pearson (2005), and zone O6 of Wade et al. (2011).

Remarks — The P. naguewichiensis is not recorded and the nannoplankton NP21 helps in the recognition of the basal Oligocene, which is unconformably underlain by the middle Eocene units.

Assemblage — The zone is characterized by the occurrence of the following planktonic foraminifera assemblage: P. opima opima, D. sellii, G. angustiumilicata, G. ciperoensis, Turborotalia increbescens, Globigerina euapertura, C. dissimilis, and C. cubensis. The benthic foraminifera are characterized by the occurrence of the following assemblage: G. soldanii, S. bulloides, and G. subglobosa.

Age — early Oligocene (Rupelian).

Nannoplankton Biostratigraphy

The calcareous nannoplankton zonation and the absolute age are based in the present study on the scheme of Martini (1971) and Ogg et al. (2016) (Figure 10). The study of the nannoplanktons revealed five biozones: NN1 Triquetrorhabdulus carinatus, NP25 Sphenolithus ciperoensis, NP24 Sphenolithus distentus, NP23 Sphenolithus predistentus, and NP21 Ericsonia subdisticha (Figure 3–5). The identified biozones, from top to bottom, are discussed as follows.

T. carinatus (NN1) Zone

Definition — Interval between the HO of Dictyococcites bisectus and the LO of Discoaster druggii.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4525 to 4505 m (14,845 to 14,780 ft) (20 m [65 ft] thick). In the Petrobel 1 Habbar well, it was represented in the interval from 4100 to 4050 m (13,451 to 13,287 ft) (50 m [164 ft] thick).

Correlation — This zone is equivalent to the NN1 Zone of Martini (1971).

Assemblage — The zone is characterized by the occurrence of the following assemblage: small Reticulofenestra spp., Cyclicargolithus floridanus, Coccolithus pelagicus, Reticulofenestra pseudoumbilica, Helicosphaera kamptneri, Coccolithus miopelagicus, Sphenolithus moriformis, Discoaster deflandrei, Helicosphaera ampliaperta, Sphenolithus dissimilis, T. carinatus, Triquetrorhabdulus challengeri, Clausicoccus fenestratus, and Cyclicargolithus abisectus.

Age — early Miocene (Aquitanian).

S. ciperoensis (NP25) Zone

Definition — Interval between the HO of S. distentus and the HO of D. bisectus.

Occurrence — This zone (NP25 a-b) has been recorded in the Burullus 1 Memphis well in the interval from 4525 to 4695 m (14,845 to 15,403 ft) and is 170 m (558 ft) thick. In the Petrobel 1 Habbar well, NP25a subzone was only recorded and covered the interval from 4320 to 4100 m (14,845 to 13,451 ft) (220 m [722 ft] thick).

Correlation — This zone corresponds to the NP25 Zone of Martini (1971). In northern Egypt, it is correlated with the NP25 Zone, which was described by Faris et al. (2016).

Remarks — The extinction of D. bisectus is a reliable event in the late Oligocene. This bioevent was not recorded in the Petrobel 1 Habbar well, and this may be a result of the effect of the development of the Gulf of Suez rift.

Assemblage — The zone is characterized by the occurrence of the following assemblage: small Reticulofenestra spp., C. pelagicus, C. floridanus, S. moriformis, D. deflandrei, S. dissimilis, Helicosphaera euphratis, Helicosphaera obliqua, Sphenolithus belemnos, C. fenestratus, C. abisectus, D. bisectus, Helicosphaera bramlettei, large C. miopelagicus, S. ciperoensis, Dictyococcites scrippsae, Helicosphaera recta, and Pontosphaera enormis.

Age — late Oligocene (Chattian).

S. distentus (NP24) Zone

Definition — Interval between the LO of Spenolithus ciperoensis and HO of S. distentus.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4695 to 4725 m (15,403 to 15,501 ft) (30 m [98 ft] thick). In the Petrobel 1 Habbar well, it covered the interval from 4320 to 4500 m (14,173 to 14,763 ft) (180 m [590 ft] thick).

Correlation — This zone is equivalent to the NP24 Zone of Martini (1971) and was recorded by Faris et al. (2016) in northern Egypt.

Assemblage — The zone is characterized by the occurrence of the following assemblage: small Reticulofenestra spp., C. floridanus, S. moriformis, D. deflandrei, S. dissimilis, C. fenestratus, D. bisectus, H. bramlettei, large C. miopelagicus, S. ciperoensis, D. scrippsae, Thoracosphaera tesserula, Helicosphaera compacta, S. distentus, Sphenolithus pseudoradians, and Chiasmolithus altus.

Age — late Oligocene (Chattian).

S. predistentus (NP23) Zone

Definition — Interval between the HO of Reticulofenestra umbilica and the LO of S. ciperoensis.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4725 to 4785 m (15,501 to 15,698 ft) and is 60 m (197 ft) thick.

Correlation — The zone is equivalent to the NP23 Zone of Martini (1971).

Remarks — The HO of R. umbilica is difficult to pick in the Burullus 1 Memphis well, whereas in the Petrobel 1 Habbar well, lower Oligocene strata were not penetrated.

Assemblage — The zone is characterized by the occurrence of the following assemblage: small Reticulofenestra spp., C. floridanus, S. moriformis, D. deflandrei, S. dissimilis, C. fenestratus, C. abisectus, D. bisectus, H. bramlettei, large C. miopelagicus, S. ciperoensis, Zygrhablithus bijugatus, H. compacta, S. distentus, S. pseudoradians, C. altus, Reticulofenestra hampdenensis, and S. predistentus.

Age — early Oligocene (Rupelian).

E. subdisticha (NP21) Zone

Definition — Interval between the HO of Discoaster saipanensis and the HO of Ericsonia formosus.

Occurrence — This zone has been recorded in the Burullus 1 Memphis well in the interval from 4785 to 4795 m (15,698 to 15,731 ft) (10 m [33 ft] thick).

Correlation — The zone is equivalent to the NP21 Zone of Martini (1971).

Remarks — This zone is only recorded in the Burullus 1 Memphis well and is unconformably underlain by middle Eocene strata.

Assemblage — The zone is characterized by the occurrence of the following assemblage: C. floridanus, S. moriformis, D. deflandrei, D. bisectus, H. bramlettei, large C. miopelagicus, Z. bijugatus, E. formosus, Isthmolithus recurves, and R. umbilica.

Age — early Oligocene (Rupelian).

Gaps in the Stratigraphic Record

Several hiatus surfaces were defined as sequence boundaries forming the main focus of the Oligocene sequences and were correlated with other related geographical regions and with those of the regional sea-level fluctuations of Snedden and Liu (2010) and Gradstein et al. (2012). These surfaces are significant because they mark bypass points for reservoir facies in the offshore Mediterranean and remain future exploration targets (Dolson et al., 2002). Three major gaps were identified: lower Oligocene–Eocene, lower–upper Oligocene, and upper Oligocene–lower Miocene boundaries, with intraformational breaks well defined by the high- resolution biostratigraphic analyses (Figure 11). The global sea-level fluctuations described by Snedden and Liu (2010) support the definitions of such breaks (Figures 12, 13).

Figure 11. Sequence stratigraphic boundaries of the offshore Nile Delta in comparison with other related geographically region. The international chronostratigraphic scale after Ogg et al. (2016). Aquit. = Aquitanian; Chron = Chronozone; HST = highstand systems tract; LST = lowstand systems tract; MFS = maximum flooding surface; SB = sequence boundary; TST = transgressive systems tract.

The lowermost boundary of the Oligocene strata overlying the upper Eocene carbonates was correlated with the Pg30 sequence boundary (33.5 Ma) of Simmons et al. (2007) and with the sequence boundary (36 Ma) described by Dolson et al. (2002) (Figure 11). During this period, there was a northward tilting of Egypt toward the Mediterranean caused by the opening of the Gulf of Suez as well as the evolving Syrian arc fold system (Dolson et al., 2002). This northward tilting permits a large volume of clastics to be available for translation of reservoir facies basinward (Dolson et al., 2002). In the study area, this break (ca. 37.8–33.9 Ma) is well defined in the Burullus 1 Memphis well and exists at the top of the middle Eocene Apollonia Formation. It coincides with the climax of the Syrian arc inversion (Dolson et al., 2002). Also, this break is marked by the absence of the lowstand systems tract (LST) and transgressive systems tract (TST) of the lower Oligocene (Rupelian) sequence (RuSeq1) and coincides with the MFS of this sequence (Figure 12).

Figure 12. Depositional sequences of the Oligocene succession in the Burullus 1 Memphis well in comparison with the global third-order cycles and sea-level curves of Snedden and Liu (2010). Red triangles represent regression (R), and gray triangles represent transgression (T). Present-day sea level is 0 on shown sea-level curve. APLC = array porosity limestone corrected; Aquit. = Aquitanian; Chseq = Chattian sequence; GR = gamma ray; HDRA = high-density correction; HST = highstand systems tract; INPEFA = integrated prediction error filter analysis; LST = lowstand systems tract; MF = maximum flooding; Mioc. = Miocene; RHOZ = standard resolution formation density; Ruseq = Rupelian sequence; TST = transgressive systems tract; Vlimestone = limestone volume; Vshale = shale volume; Vwater = water volume.

The second major lower–upper Oligocene sequence boundary represents a major sequence boundary in the Nile Delta region. The eustatic sea-level fall forced the fluvial feeder channels toward the shelf, and the upper Oligocene succession is represented by the prograding of early-formed shelf-margin deltas (Selim, 2018). This sequence boundary was placed at 28.5 Ma by Kellner et al. (2009) (Figure 11). It is synchronous with the global sea-level drop at the top of NP23 (28.4 Ma) and correlates with that described by Snedden and Liu (2010).

The third major Oligocene–Miocene boundary represents a period of extensive erosion in Egypt related to the development of the Gulf of Suez rift (McClay et al., 1998; Bosworth and McClay 2001). It is correlated with a boundary that was described by Dolson et al. (2002) at 24 Ma and by Kellner et al. (2009) at 23.8 Ma. Also, it was defined by Farouk et al. (2014) at 23.03 Ma (Figure 11). The absence of the planktonic foraminifera Paragloborotalia psuedokugleri (O7) and the upper part of the NP25 Zone is related to the erosion at this sequence boundary.

Figure 13. Depositional sequences of the Oligocene succession in the Petrobel 1 Habbar well in comparison with the global third-order cycles and sea-level curves of Snedden and Liu (2010). Red triangles represent regression (R), and gray triangles represent transgression (T). Present-day sea level is 0 on shown sea-level curve. APLC = array porosity limestone corrected; Aquit. = Aquitanian; Chseq = Chattian sequence; GR = gamma ray; HDRA = high-density correction; HST = highstand systems tract; INPEFA = integrated prediction error filter analysis; LST = lowstand systems tract; MF = maximum flooding; RHOZ = standard resolution formation density; TD = total depth; TST = transgressive systems tract; Vsand = sand volume; Vshale = shale volume; Vsiltstone = siltstone volume; Vwater = water volume.

The intraformational hiatus surfaces are synchronous with the global sea-level drop of Snedden and Liu (2010) and include the (1) intraformational sequence boundary (32 Ma), which was well described in Egypt by Dolson et al. (2002) and Kellner et al. (2009) as well as in the Arabian plate by Haq and Al-Qahtani (2005); (2) intraformational sequence boundary (29.45 Ma), which was defined by Kellner et al. (2009) at 29.4 Ma; (3) intraformational sequence boundary (27.27 Ma), which was placed at 27.4 Ma by Kellner et al. (2009) and at circa 27 Ma by Haq and Al-Qahtani (2005) and Buchbinder et al. (2005); and (4) intraformational sequence boundary (24.82 Ma), which represents the upper part of the Tineh strata and was described by Kellner et al. (2009) at 25.3 Ma in the Nile Delta region (Figure 11).

Sequence Stratigraphy

The depositional sequences and systems tracts of the Oligocene strata were estimated based on a high-resolution biostratigraphic analysis, and the identification of facies types were based on a detailed lithologic description as well as wire-line logs and the generation of PEFA and INPEFA patterns by Cyclolog software (Nio et al., 2005). The faunal parameters (planktonic–benthic [P/P + B] ratio, diversity, and abundance) and benthic biofacies were used to indicate the relative sea-level oscillations. Integrating these parameters with the INPEFA curves allowed us to identify stratigraphic sequences, sequence boundaries, and flooding surfaces in addition to higher-order cycles and reservoir scale patterns. The hierarchy of these sequences (progradation, aggradation, or retrogradation) and the available accommodation space are controlled by tectonic setting, sediment supply, and productivity as well as the global eustatic sea-level fall and rise. The sequence boundaries and flooding surfaces of the Oligocene succession were described in the Nile Delta by Dolson et al. (2002) and Kellner et al. (2009) (Figure 11). These can be correlated with those given by Buchbinder et al. (2005) in the Levant Basin and by Haq and Al-Qahtani (2005) and Simmons et al. (2007) in the Arabian plate. Three systems tracts (late lowstand to early TST, TST, and highstand systems tract [HST]) and the regional flooding surface Pg50 were identified by Kuss and Boukhary (2008), summarizing the sequence of the upper Chattian deposits in Rizan Aneiza of the northern Sinai, Egypt. The sequence hierarchy of the Oligocene units in this contribution shows additional interpretation, which is compared with other neighboring regions (Figure 11). The TST and HST seem to decrease in thickness in the basinward direction (west-northwest), which are interpreted as condensed intervals (Figure 14). These represent thin marine intervals that are accumulated in the basinal parts and are characterized by low sedimentation rates that can be used as paleomarkers of time (Loutit et al., 1988). Six third-order cycles and their boundaries were identified within the sequence stratigraphic architecture in the northern offshore Nile Delta. Three of these are lower Oligocene (RuSeq1, RuSeq2, and RuSeq3), and the others are upper Oligocene (Chattian) sequences (ChSeq4, ChSeq5, and ChSeq6). The studied gas-bearing sequences of Oligocene age could be correlated with the global third-order cycles and sea-level curves of Snedden and Liu (2010) as well as the global transgressive–regressive cycles of Gradstein et al. (2012) and Ogg et al. (2016). These sequences and the identified systems tracts are described here from base to top (Figures 12–14.

Figure 14. Correlation between proposed depositional sequences in the studied area. Red triangles represent regression, and gray triangles represent transgression. Aquit. = Aquitanian; Chseq = Chattian sequence; GR = gamma ray; INPEFA = integrated prediction error filter analysis; Lm = limestone; Mioc. = Miocene; Ruseq = Rupelian sequence; Sd = sand; Sh = shale.

Third-Order Rupelian Sequence 1 (33.9–32.19 Ma)

This cycle is the oldest depositional sequence in the study area and covers the lower part of the Tineh Formation. It is represented only in the Burullus 1 Memphis well in the interval from 4796 to 4788 m (15,734 to 15,708 ft), is composed mainly of shale with limestone interbeds, and unconformably overlies the middle Eocene carbonates (Figures 12, 14). This sequence occurs within the NP21 E. subdisticha nannofossil (ca. 32.9–34.5 Ma). The MFS is tentatively placed at 4475 m (14,681 ft), as indicated by the presence of deep-water benthic foraminifera such as G. soldanii (Murray, 1991) (Figure 15). This horizon correlates with the Pg30 regional flooding surface defined by Simmons et al. (2007). Also, Haq and Al Qahtani (2005) described the MFS at 33 Ma. This cycle is represented mainly by the HST, which was defined and described based on biotaxa abundance and diversity.

Figure 15. Upper depth limit of benthic foraminifera with respect to planktonic–benthic ratio after Van Morkhoven et al. (1986), Van der Zwann et al. (1990), Murray (1991), and Holbourn et al. (2013).

The HST is delineated from a depth of 4796 to 4788 m (15,734 to 15,708 ft) and is characterized by a relative upward decrease in nannofossils as well as abundance and diversity of benthic foraminifera (four species), which confirm a shallowing trend through time (Figure 16). It is represented by a retrogradational pattern (depositional positive trend on the right side), which was observed through the INPEFA curve pattern (Figure 12).

Figure 16. Stratigraphic distribution of benthic, planktonic, and nannoplankton parameters; benthic association; and depositional paleoenvironments of the Burullus 1 Memphis well. Aquit. = Aquitanian.

Third-Order Rupelian Sequence 2 (32.19–29.45 Ma)

This depositional sequence is essentially composed of shale with a few limestone interbeds and shows a retrogradational INPEFA (Figure 12). It is represented mainly in the Burullus 1 Memphis well from a depth of 4788 to 4744 m (15,708 to 15,564 ft) based on the occurrence of the planktonic foraminifer T. ampliapertura that coincides with a rapid relative sea-level rise (first downhole occurrence at ca. 30.3 Ma). This bioevent can be correlated with the MFS defined by Dolson et al. (2002) at circa 30 Ma and by Kellner et al. (2009) at 30.6 Ma. This cycle can be subdivided into two subcycles (RuSeq2a and RuSeq2b) of possible fourth-order rank, as indicated by paleontological and physical criteria.

The lower RuSeq2a subcycle is defined from a depth of 4788 to 4764 m (15,708 to 15,629 ft) and has a retrogradational Cyclolog pattern. It is dominated by light gray shale that is silty and calcareous to highly calcareous in places, intercalated with thin beds of limestone. This facies is interpreted to represent deeper-water deposits (TST deposits) followed by HST deposits of dark gray and in places pyritic shale with thin beds of limestone deposited in an inner neritic depositional environment (decrease in the abundance of both planktonic and benthic foraminifera) (Figure 16). The MFS is detected at 4775 m (15,666 ft) based on the abundance of biotaxa and the log pattern of the INPEFA curve (Figure 12).

The RuSeq2b subcycle is represented by light gray, gray, and rarely brownish-gray, in places silty, calcareous to highly calcareous shale grading to marl, intercalated with thin beds of limestone. This cycle has a retrogradational INPEFA pattern characteristic of an inner to middle neritic depositional environment. The MFS is detected at 4755 m (15,600 ft) and separates the lower transgressive and upper regressive facies. This surface is associated with a peak abundance of foraminifera and P/P + B ratio of approximately 67%. This ratio decreases upward and reaches up to 50%, which indicates a continuing regression.

Third-Order Rupelian Sequence 3 (29.45–28.44 Ma)

This sequence is recognized only in the Burullus 1 Memphis well from 4744 to 4725 m (15,564 to 15,501 ft) and represents the upper part of the Rupelian. It consists of shale with thin intervals of limestone, showing a retrogradational stacking pattern (Figure 12). The MFS is determined based on the inflection, which is clearly obvious on the INPEFA pattern (major gamma ray peak with value decreasing upward). Haq and Al-Qahtani (2005) and Simmons et al. (2007) described this horizon at circa 29.0 Ma (Figure 11).

The TST is defined from 4744 to 4732 m (15,564 to 15,524 ft) and is composed of gray, brownish-gray, and partly greenish-gray shale, in places highly calcareous and intercalated with thin intervals of limestone. The P/P + B ratio is approximately 88%, with the occurrence of deep-water benthic fauna such as G. subglobosa, Cassidulina laevigata, and G. soldanii indicating deposition in an outer neritic environment (Murray, 1991) (Figure 15).

The HST is represented by the interval from 4732 to 4725 m (15,524 to 15,501 ft). The decreasing upward abundance of planktonic and benthic foraminifera and the relatively high P/P + B ratio (up to 69%) indicate an inner-shelf environment (Van der Zwann et al., 1990).

Third-Order Chattian Sequence 4 (28.44–27.27 Ma)

This depositional sequence is recognized in all the studied wells. It characterizes the lower part of the Chattian, which was defined based on the occurrence of P. opima (first downhole occurrence at ca. 27.5 Ma). It is well developed in the Petrobel 1 Habbar well and could be differentiated into three higher-order subcycles (possible fourth order) (Figure 13). Otherwise, the cycle becomes thin and condensed toward the west-northwest of the study area. Two lowstand prograding sandy deposits consisting primarily of fine- to medium-grained and moderately sorted quartz sand can be defined in the proximal part. However, in the distal part, it consists of gray, brownish-gray, and in places greenish and highly calcareous shale with thin beds of limestone, which may be interpreted as being from an LST-TST deposit. The occurrence of P. opima combined with a strong influx of the planktonic foraminifera and calcareous nannofossils suggest a maximum sea level.

The highstand deposits consist of gray shale and sand interbeds becoming shale dominated in the distal part of the cycle. The decrease in the abundance and diversity of foraminifera and nannofossils indicate accumulation of the HST in a shallower depositional setting toward the east-southeast of the study area (Figure 17).

Figure 17. Stratigraphic distribution of benthic, planktonic, and nannoplankton parameters; benthic association; and depositional paleoenvironments of the Petrobel 1 Habbar well. Aquit. = Aquitanian; TD = total depth.

Third-Order Chattian Sequence 5 (27.27–24.84 Ma)

This sequence has been recorded in the studied wells within the G. ciperoensis Zone and the lower part of P. pseudokugleri Zone. It is composed of thick shale and sand interbeds becoming condensed and shale dominated toward the basinal part of the study area.

The LST-TST records a major change in benthic foraminifera compared with the interval below and is characterized by the occurrence of Uvigerina auberiana and Ammonia beccarii as well as rare P. bulloides, S. bulloides, Melonis sp., and Lenticulina sp., indicating deposition in an inner neritic environment (Figure 16). In the Petrobel 1 Habbar well, the LST-TST was deposited in a shallower setting compared with that of the Burullus 1 Memphis well (Figure 17).

The MFS was identified on the basis of the high abundance of planktonic foraminifera and nannofossils as well as the inflection of the INPEFA pattern. It is represented by deeper benthic foraminifera, such as P. bulloides, G. subglobosa, G. soldanii, and A. cretaceous (Figure 16).

The HST is mostly composed of shale lithofacies and shows an aggradational–progradational stacking pattern. The P/P + B ratio ranges from 68% to 73% and decreases toward the top of the cycle, which reflects deposition in an inner neritic environment and a shallowing depositional setting toward the east-southeast of the study area (Figure 16).

Third-Order Chattian Sequence 6 (24.82–23.03 Ma)

This sequence, defined only in the Burullus 1 Memphis well in the interval from 4596 to 4532 m (15,078 to 14,868 ft), represents the upper Chattian deposits and shows a retrogradational stacking pattern (Figure 12). It is composed of dark to medium gray shale that is partly micaceous, in places silty, partly pyritic, and noncalcareous. The interval shows a further decrease in the abundance and diversity of planktonic and benthic foraminifera and is, therefore, assumed to represent LST-TST deposits. These deposits can be correlated with the carbonates of the lower Wadi Arish Member (LST-TST), which was described by Kuss and Boukhary (2008) in North Sinai. This cycle is terminated by the Oligocene–Miocene sequence boundary, which represents a regional hiatus surface in Egypt being correlated with the Gulf of Suez rift opening.

SEQUENCE STRATIGRAPHY AND HYDROCARBON POTENTIAL

Sequence stratigraphic analysis of the Tineh Formation is effective for potential correlation of the Oligocene section in the Mediterranean and Nile Delta. The slope channels of Oligocene lowstand and highstand deposits are important in the development of giant fields in the deep Oligocene in the offshore Nile Delta (Dolson et al., 2014). The LST in particular is one of the most important reservoir intervals across the basin relative to the other systems tracts. The lowstand fluvial deposits (amalgamated channel fills) form the best reservoirs within the fluvial portions of a stratigraphic sequence (Catuneanu, 2006). The probability of the occurrence of hydrocarbons in a lowstand sequence is related to charge, seals, and source rocks. Fluvial, coastal, and shallow-water lowstand reservoirs may be sealed by overlying transgressive shales (Catuneanu, 2006). Two prograding LSTs were recognized within the deposits of the Oligocene succession related to ChSeq4b and ChSeq4c subcycles based on the log pattern and high-resolution biostratigraphic analysis. The log pattern and lithofacies represent sandy deposits consisting primarily of fine- to medium-grained and moderately sorted quartz sand, reflecting the development of submarine canyons as a result of a relative sea-level fall. Also, condensed sections associated with regional transgressions (MFS) may contain significant source rocks (Dolson et al., 2002).

CONCLUSIONS

The present study was based on high-resolution analyses of foraminiferal and calcareous nannofossils calibrated with available wire-line logs and the application of the Cyclolog software (PEFA and INPEFA patterns). This work is an attempt to understand the sequence stratigraphic architecture, depositional settings, paleoenvironments, and evolution of gas-bearing sequences in the Oligocene of the offshore Nile Delta and the Mediterranean region, which has important implications for future hydrocarbon exploration.

The Oligocene succession is represented by the Tineh Formation, which is composed of gray, dark gray, and light gray shale with sandstone interbeds.

The investigation of planktonic foraminifera led to the identification of five planktonic foraminiferal zones. These are the M1 P. kugleri, O7 P. pseudokugleri, O6 G. ciperoensis, O5 P. opima, and O2 T. ampliapertura Zones. The nannoplankton biozones include NN1, NP25, NP24, NP23, and NP21. The established bioevents and biozones were correlated and calibrated chronostratigraphically with the standard worldwide scheme.

The planktonic–benthic ratio, foraminiferal diversity, and the upper depth limit of benthic foraminiferal taxa, including P. bulloides, Sphaeroidina pulloides, G. soldanii, G. subglobosa, C. laevigata, O. umbonatus, Cibicidoides, Lenticulina, Uvigerina, Bulimina, Haplophragmoides, and Quinqueloculina as well as the abundance of calcareous nannofossils help in interpreting the different depositional paleoenvironments. The facies of the Oligocene strata were interpreted as fluvial, shallow-marine, middle- to outer-shelf, and bathyal environments.

A total of six sequences were identified, three of which are lower Oligocene (Rupelian) (RuSeq1, RuSeq2, and RuSeq3) and the others are upper Oligocene (Chattian) (ChSeq4, ChSeq5, and ChSeq6). The gas-bearing sequences can be correlated with the global third-order sea-level curves.

Three major sequence boundaries—lower Oligocene–Eocene, lower–upper Oligocene, and upper Oligocene–lower Miocene—with intraformational breaks were well defined through the application of high-resolution biostratigraphic analyses. Global short-term sea-level drops and missing biozones support the identification of these breaks. These major sequence boundaries are as follows.

• Eocene–Oligocene unconformity (ca. 37.8–33.9 Ma) exists at the top of the middle Eocene Apollonia Formation and coincides with the climax of the Syrian arc inversion.

• Lower–upper Oligocene unconformity is related to the eustatic sea-level fall that forced the fluvial feeder channels toward the shelf.

• Upper Oligocene–lower Miocene unconformity (ca. 24.8–23.03 Ma) represents an extensive erosional event in Egypt that is related to the Gulf of Suez rift.

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ACKNOWLEDGMENTS

The authors are indebted to the Egyptian General Petroleum Corporation for providing well data and cuttings. Special thanks to EREX Petroleum Consultants for providing us with the software used in the present study. We also thank Hanspeter Luterbacher and Sandro Serra (GeoConsulting) for editing and reviewing the manuscript. The valuable discussions with M. Darwish, Cairo University, are highly appreciated. M. El Sharkawi and Amir Said, Cairo University, are acknowledged for reviewing the manuscript. The language revision by Justin Sevannos (language teacher at Pioneer Academy, New Jersey) is highly appreciated. We also thank AAPG Editor Barry J. Katz and the reviewers for their valuable comments that improved the manuscript.

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