Nukhul Formation Research Articles

Reservoir Rock Discrimination Based on Integrated Image Logs and Petrographic Analysis: A Case Study from the Early Miocene Nukhul Carbonate, Southern Gulf of Suez, Egypt

AbstractThe discrimination of rock types within the limestones and dolostones of the Nukhul Formation in the West Younis Field (Gulf of Suez Basin, Egypt) presents significant challenges due to their multi-scale compositional and diagenetic heterogeneity, diverse pore types, complex microstructures, and limited core data. This study aims to characterize the carbonate reservoir of the Early Miocene sediments and establish distinct reservoir rock types by employing textural analysis, geological interpretations (i.e., structural interpretation, fracture analysis, reservoir characteristics) using advanced imaging tools, and petrophysical measurements to model porosity/permeability profiles across the reservoir. A new dataset was obtained from the latest exploratory well in the West Younis Field, incorporating microresistivity and acoustic image logs, well logs, nuclear magnetic resonance (NMR) tools, and drill cutting petrographic analysis. The integration of these datasets provided a comprehensive understanding of the properties of the Early Miocene carbonate reservoir. Based on image logs, the carbonate facies were divided into four reservoir units. Petrographic evaluation further classified two facies (A and B) based on diagenetic factors controlling reservoir quality. The results revealed the occurrence of multiple phases of dolomitization, which influenced the reservoir quality. Early-stage dolomitization enhanced reservoir quality, while late-stage idiotopic dolomite crystal growth diminished it. The study also provided comprehensive information on the original rock fabric/texture, diagenetic processes, porosity types and origins, as well as the spatial distribution of pores (permeability index) within this complex carbonate reservoir. By employing an integrated technique, this study successfully differentiated the carbonate reservoir into distinct rock types, leading to improved reservoir characterization and field development. Additionally, the findings contribute valuable insights for the development and exploration of the Early Miocene carbonate section in the southern Gulf of Suez.

Biostratigraphy of the Lower Miocene Nukhul Formation, Gulf of Suez, Egypt

Biostratigraphy of the Lower Miocene Nukhul Formation, Gulf of Suez, Egypt

Sequence stratigraphy and reservoir quality of the Gulf of Suez syn-rift deposits of the Nukhul formation: Implications of rift initiation and the impact of eustacy and tectonic on deposition

The Nukhul Formation represents the sediments deposited during the earliest phase of the syn-rift period in the Gulf of Suez rift. It is the primary hydrocarbon reservoir for fifteen fields in the region. However, the timing of rifting needs to be better constrained, and the influence of local tectonics and global eustatic sea-level fluctuations on reservoir quality and deposition is still being determined. This research aims to establish a stratigraphic sequence framework, determine the age of the Suez Rift commencement, use changes in paleobathymetry, and assess reservoir quality. The methods employed involve the analysis of four boreholes in the Abu Rudeis-Sidri field, incorporating biostratigraphy, well-logs, and 3D seismic data. The integrated paleo-structure and biostratigraphic analyses indicate that the beginning of rifting in the Gulf of Suez occurred at 23.53 Ma at the first common occurrence (FCO) of Trilobatus primordius. The paleobathymetric analysis of the Nukhul Formation reveals deeper paleo-water depths during certain intervals, indicating deposition during a phase of high subsidence and rift-climax. The Nukhul stratigraphic sequence reflects sea-level changes leading to shifts in accommodation, and facies changes, erosion, and sequence formation during the late Chattian and early Burdigalian stages. However, the reservoir characterization of the Nukhul Formation is significantly affected by various factors, including cementation, pore system properties, sediment sorting, rift system geometry, and relative sea-level fluctuations. These factors must be carefully considered in any regional exploration or development efforts. The study also reveals that tectonic activity played a role in forming fault-bounded basins, and eustatic sea-level changes influenced the rate and timing of sediment accumulation. These findings can aid in predicting the distribution of the Nukhul sedimentary deposits and improve the ability to explore and develop their resources.

Age Assignment, Palaeoecology and Sequence Stratigraphic Framework of the Nukhul Formation at Wadi Baba, West-Central Sinai (Egypt)

The Nukhul Formation has been a point of controversy until now due to a lack of index fossils, varied facies modifications, and different time gaps. As a result, the calcareous nannofossils of the exposed Miocene rocks at Wadi Baba, west-central Sinai have been studied quantitatively for the first time. An important sequence of calcareous nannofossil bioevents has been recorded in the Nukhul Formation, including the FOs of Discoaster druggii, Sphenolithus dissimilis, Helicosphaera ampilaperta, and Sphenolithus disbelemnos, which correspond to the Aquitanian age of upper NN1 and NN2 zones. The Nukhul Formation contains four intervals that reflect eutrophic conditions in the nearshore habitats with substantial nutrient input from the terrigenous influx, as shown by the dominance of cool-water taxa, Coccoliths pelagicus and Reticulofenestra minuta. The lithofacies, microfacies types, abundance, dominance, and diversity of nannofossil assemblages classified the studied interval into two depositional sequences with four system tracts. These tracts refer to fluctuations in relative sea level changes in response to the eustatic sea-level curve.

Assessment of the petrophysical properties and hydrocarbon potential of the Lower Miocene Nukhul Formation in the Abu Rudeis-Sidri Field, Gulf of Suez Basin, Egypt

Lower Miocene rift sediments of the Nukhul Formation are one of the prominent hydrocarbon producers in the Gulf of Suez basin. In this study, we focused on the oil producing Nukhul sandstones of the Abu Rudeis-Sidri Field, located in the east central Gulf of Suez. Nukhul Formation is characterized by the prominent low amplitude seismic reflectors and represents the youngest identified reflector (Lower Miocene) in the study area. Petrophysical assessment was carried out using wireline logs to infer the reservoir characteristics. The Nukhul sandstone reservoir exhibits lower shale volume (< 0.1 dec dominantly), 0.07–0.16 dec total porosity and effective porosity up to 0.13 dec within the Nukhul sandstone interval. Bulk density-neutron porosity cross plot infers primarily sandstone matrix with the influence of carbonates, which characterizes the studied reservoir as calcareous sandstones. Spectral gamma ray data indicates montmorillonite as the principal clay phase along with minor kaolinite and illite. The calculated water saturation of the reservoir zone in the three productive wells ranges between 0.17 and 0.34 dec (i.e., the hydrocarbon saturation equals 0.66–0.87 dec). A small range of bulk volume of water (0.011–0.03 dec) indicates superior quality of the hydrocarbon-bearing sandstone intervals. Absence of productive sands in one of the studied wells, drilled in the hanging wall implied structural control on hydrocarbon accumulation in the study area. This study provides crucial insights regarding the quantitative petrophysical characteristics, reservoir quality distribution and hydrocarbon potential of the Lower Miocene Nukhul clastic reservoir.

Sandstone Reservoir Assessment of Nukhul Formation Using Well logging Analysis, Eastern Gulf of Suez, Egypt

Sandstone Reservoir Assessment of Nukhul Formation Using Well logging Analysis, Eastern Gulf of Suez, Egypt

Geophysical and hydrocarbon prospect evaluation of Nukhul Formation at Rabeh East oil field, Southern Gulf of Suez Basin, Egypt

Nukhul Formation is one of the primary oil reservoirs in the Gulf of Suez Basin. Rabeh East is an oil producer field located at the southern border of the Gulf of Suez. The present work deals with the geophysical investigation of Nukhul Formation in Rabeh East field using seismic lines and well log data of four wells, namely RE-8, RE-22, RE-25 and Nageh-1. The interpreted seismic profiles display that the RE-8 Well is the only well drilled within the up-thrown side of a significant horst fault block bounded by two normal faults. However, the other wells penetrated the downthrown side. The qualitative interpretation of the well logging data for RE-8 Well delineated two intervals have good petrophysical parameters and ability to store and produce oil. These zones locate between depths 5411.5 and 5424 ft (zone I) and between 5451 and 5459.5 ft (zone II). The calculated petrophysical parameters for zone I display water saturation (22–44%), shale volume (10–23%), total porosity (18–23%), effective porosity (12–20%) and bulk volume of water (0.04–0.06). Zone II exhibits water saturation (13–45%), shale volume (10–30%), total porosity (18–24%), effective porosity (11–20%) and bulk volume of water (0.03–0.05). This analysis reflects excellent petrophysical characteristics for the sandstones of Nukhul Formation in Rabeh East oil field for producing oil if the wells drilled in a suitable structural closure.

Geochemical studies of carbonates of the Tayiba Formation (Upper Eocene), Abu Zenima area, West Central Sinai

BackgroundTayiba Formation situated at the entrance of Wadi El-Tayiba. It unconformably overlies the Late Eocene Tanka Formation and unconformably underlies the Early Miocene Nukhul Formation. Severely authors studied this formation geologically, stratigraphically, and petrographically. But, geochemical studies on the rare earth elements to illustrate the depositional environments are seldom.ResultsResults were revealed that, mineralogically, the carbonate rocks in the Tayiba Formation are mainly composed of calcite, dolomite, quartz and clay minerals. The terminal parts of the studied Tayiba Formation are more dolomitized relative to the middle beds.Geochemical analysis revealed that the carbonates of Tayiba Formation were deposited under shallow, oxidizing, marine environment. The dolomitization process has taken place in marine environment. Carbonate rocks of the Tayiba Formation are highly enriched in trace elements and highly to moderately rare earth elements. The studied carbonate rocks characterized by light REE (LREE) enrichment than heavy REE (HREE).ConclusionsThe redox-sensitive and rare earth elements study indicates that the carbonate rocks in the Tayiba Formation were deposited under anoxic to oxic marine environment. Also, the trace and rare earth elements values indicated that the studied carbonate rocks are mainly hydrogenous sources. These elements may be derived from more mafic sources.

Dinoflagellate cysts stratigraphy and paleoecology from some Lower Miocene rocks, GS9-1X well, northern Gulf of Suez, Egypt

Palynological analysis of a Lower Miocene section from the GS9-1 well, northern Gulf of Suez, Egypt, has yielded significant dinoflagellate cysts of stratigraphic significance such as Exochosphaeridium insigne, Sumatradinium soucouyantiae and Hytrichokolpoma rigaudiae, which enabled age dating of the well section as Lower Miocene. Two distinct palynofacies are recognised. Palynofacies “A” was recovered from the basal part of the section from the Nukhul Formation. It is dominated by amorphous organic matter (AOM) and contains frequent translucent and opaque phytoclasts indicating a kerogen type II. Palynofacies “B” was recovered from the overlying interval of the well from the Rudeis Formation. It shows an inverse pattern, where phytoclasts are the main components and AOM declines markedly indicating a kerogen type III. Palynomorphs (mostly dinoflagellates) occur in both facies but with lesser percentages than palynodebris. Paleoenvironmental implications are inferred in the light of the recently established information from the distribution of modern-day dinoflagellate cysts (e.g. Zonneveld et al. 2013) since many of the recovered dinoflagellate cysts from the investigated well section still thriving today. A normal (shallow) marine environment is generally inferred for the studied Miocene section, with more peridinioid dinoflagellate cysts than gouyaulacoids. Higher up in the section, terrestrial palynomorphs dominate the dinoflagellate cysts; the latter are rare or missing. This suggests development of a marginal basin that is probably connected to local uplift or sea regression.

Cyclo-sequence stratigraphy of the Miocene syn-rift succession, Al Amir and Geyad fields, southwestern part of the Gulf of Suez rift

A cyclostratigraphic analysis is performed using high-resolution well data to study the facies development, characteristics, and patterns of the Miocene syn-rift succession in the Gulf of Suez. The impact of syn-depositional tectonics on the distribution of reservoir units is also highlighted. The study area is in Al Amir and Geyad fields, which are located in the southwestern onshore part of the Gulf of Suez rift. The Cyclolog® software workflow has been applied on nine wells from these two fields using the Gamma Ray log of the Miocene sequences. The Predictive Error Filter Analysis (PEFA) patterns and Integrated Predictive Error Filter Analysis (INPEFA) curves support the definition and distinction of seven 3rd order cycles. These cycles are numbered as 1.4, and 1.5 for Nukhul Formation, 2.1, 2.2, and 2.3 for Rudeis Formation, 2.4 for Kareem Formation, and 2.5 for Belayim Formation. The Kareem Formation is represented by cycle 2.4 and could be divided into higher order subcycles 2.4 (a, b (u and l), c, d and e). The oil reservoir units occur in subcycle 2.4.b (u and l), whereas subcycle 2.4.b (l) is missing in Geyad field and subcycle 2.4.b (u) is missing in Al Amir SE-1X St well. Correlation profiles and isochore maps of the Upper Rudeis and Kareem formations (cycles 2.3 and 2.4 respectively) indicate the relationship between sediment supply and accommodation space which are related to tectonic subsidence of the West Zeit Basin and sediment supply from the Esh El Mellaha range.

The Lower Miocene Nukhul Formation (Gulf of Suez, Egypt): microfacies and reservoir characteristics

The Lower Miocene Nukhul Formation was deposited during early stages of Gulf of Suez rifting. Outcrop of this formation at Gebel el Zeit, Egypt is up to 100 m (328 ft) in thickness and consists of lower sandstone-dominated and upper carbonate-dominated units. Samples were collected from the Nukhul Formation outcrop in order to describe the lithofacies, mineralogy, and petrographic characteristics to better understand the hydrocarbon reservoir potential of the unit. The investigations showed that the lower part of Nukhul Formation at North Gebel el Zeit consists of poorly sorted, dolomitic sandstone and sandy dolomite which were deposited in a shallow-marine setting. The presence of polycrystalline and crushed quartz grains in Nukhul strata indicates that these strata were sourced by recycled sediments associated with local uplift tied to rifting. The upper Nukhul consists of dolomitized carbonates with preservation of original textures; primary limestone depositional textures include intraclast packstone, rudstone, and coral-algal boundstone, wackestone, and grainstone with minor floatstone. Carbonate rock strata were deposited in shallow marine, peritidal to subtidal environments that developed on fault-block highs. The presence of zoned saddle dolomite suggests some component of fault-controlled fluid flow. Reservoir characteristics of the Lower Miocene Nukhul Formation at the Gebel el Zeit indicates favorable properties for reservoir development in both the lower siliciclastic-dominated dolomite and the upper dolomitized carbonate portions of the formation.

STRUCTURAL ARCHITECTURE OF TARBOUL BASIN, SOUTHERN GULF OF SUEZ, EGYPT

The surface structural architecture basin configuration and petroleum system are important steps in studying and evaluating frontier sedimentary basins. The Tarboul basin (TH) is a typical marginal half garben at the onshore part of the southern Gulf of Suez rift. It is bounded from the west by the NW-SE striking Oligo-Miocene rift boundary fault (RBF) and from the East by the NW-SE striking Gemsa basin major bounding fault. The TH has a tripartite geometry bordered by the RBF, rift onset unconformity and the post rift unconformity. The Pre-rift strata include undifferentiated Nubia sandstones of Paleozoic-Lower Cretaceous age, Raha Formation of Cenomanian age, Wata Formation of Turonian age, Matulla Formation of Coniacian - Santonian age, Duwi Formation of Campanian age, Sudr Formation of Masstrachtian age and Thebes Formation of lower Eocene age. The synrift strata are represented by Lower Miocene Nukhul Formation and Lower-Middle Miocene Rudeis Formation. The Syn-rift packages roll into and stratigraphically thicken toward the RBF. The up dip northeastern part of the TH is characterized by a flexural faulted margin where there is an exposed monoclinal structure. Integrating surface and subsurface mapping of the monocline facilitated the prediction of the upward propagated fault that formed this structure. The TH has a SE plunging NW-SE oriented synclinal structure representing a hanging wall syncline related to the upward propagation of the RBF. The exposed Pre-Miocene rock units at the northwest part of this structure indicate the decrease in the throw of the RBF toward the onshore part of the adjoining Morgan Transfer Zone.

SUBSURFACE STRUCTURAL SETTING OF AL AMIR AND GEYAD FIELDS, SOUTHWESTERN PART OF THE GULF OF SUEZ RIFT, EGYPT

The study area is located within the southern dip province of the Gulf of Suez rift and includes two oil fields, Al Amir and Geyad. This area is located to the west of Gebel El Zeit and covers an area of about 321 km2. The study is based on the analysis of seismic and borehole data. Six Miocene seismic horizons are mapped from Top Zeit down to top Nukhul Formation. The study area is characterized by SW dip direction and is dissected by major NW-SE oriented, rift-parallel faults and some NE-SW oriented cross faults creating the two blocks of Al Amir and Geyad oil fields. The study shows the effect of these faults on the distribution of the clastic reservoir of the Kareem Formation.

Fault block rotation and footwall erosion in the southern Suez rift: Implications for hydrocarbon exploration

A dataset in the southern Suez rift including 400 km2 of 3D depth-migrated seismic data, thirty six boreholes with geological information and outcrop data from several exposures, was examined and interpreted in order to understand the controls on the erosion of the pre-rift reservoirs at the updip edges of tilted fault blocks and its impact on hydrocarbon exploration. Surface and subsurface mapping demonstrated the existence of five regional major tilted fault blocks bounded by major down to the northeast, rift-parallel normal faults. These blocks are characterized by steep stratal dip (30–40°) and their bounding faults are dipping at 20–30° toward the NE. The major faults in the area have been subdivided into three phases based on their age. The first-phase faults were formed at the early rift opening phase and were contemporaneous with the deposition of the lowermost syn-rift unit (Nukhul Formation). The second-phase faults are contemporaneous with deposition of the Rudeis Formation (main syn-rift unit) while the third-phase faults are the youngest. Detailed study indicates that the first-phase faults controlled the rotation of the tilted fault blocks and played a major role in the erosion of the pre-rift reservoirs and top seal units at the updip edges of these blocks. The lowermost syn-rift unit (Nukhul Formation) was deposited in the downdip areas of the tilted faults blocks and did not cover the updip eroded areas. A proposed model to demonstrate the risk of drilling the crests of tilted fault blocks close to the main block-bounding faults is described in this paper and can be applied in other highly extended rift basins.

Chattian-Aquitanian calcareous nannofossil biostratigraphy of the Nukhul Formation in Wadi Baba Section, West Central Sinai, Egypt

The Nukhul Formation crops out in west-central Sinai (Egypt), but its age is inadequately resolved due to the scarcity of diagnostic planktonic foraminifera and calcareous nannofossil taxa, and the common occurrence of reworked specimens. The detailed analysis of the calcareous nannofossil content of one section measured in Wadi Baba, located in west-central Sinai, allows identification of 41 calcareous nannofossil species belonging to 18 different genera. Our results indicate that this section spans from Zone NP25 (Sphenolithus ciperoensis) to Zone NN2 (Discoaster druggii), suggesting a Chattian and Aquitanian ages for the Nukhul Formation in the studied section. Eleven bioevents are recognized throughout the studied section which have improved the biostratigraphic resolution attainable using standard zonation. In particular, the following biohorizons are found in the lower part of the succession, within Zone NP25 (Sphenolithus ciperoensis): LCO of Sphenolithus ciperoensis, LO of Chiasmolithus altus, LCO of Cyclicargolithus abisectus, LO of Reticulofenestra hesslandii, LO of Reticulofenestra bisecta, LO of Zygrhablithus bijugatus, FO of Triquetrorhabdulus carinatus, FO of Helicosphaera truempyi, FO of Helicosphaera granulata, LO of Pontosphaera enormis and LO of Calcidiscus gallagheri). Two additional bioevents were detected in the upper part of the Nukhul Formation within Zone NN2: FO of Discoaster druggii and FO of Helicosphaera ampliaperta). These data are consistent with the hypothesis that the Suez Rift System was started in the late Oligocene, as the deposition of the Nukhul Formation was started due to the opening of the Gulf basin during the Chattian. A prominent unconformity surface is traceable near the top of the Nukhul Formation at Wadi Baba section that indicates a hiatus of ca. 2.7 Myr based on the biostratigraphic data.

Integrated biostratigraphy of the upper Oligocene–middle Miocene successions in west central Sinai, Egypt

The nannofossil and planktonic foraminiferal biostratigraphy in four upper Oligocene–middle Miocene sections are examined in Nukhul–Sudr area of west central Sinai, Egypt. The integration of calcareous nannofossils and planktonic foraminifera is used to verify the ages and determine the biozones of the upper Oligocene–middle Miocene units in the studied area. This target is important in the light of the great lithofacies changes during the time interval. The detailed examination of the nannofossil and planktonic foraminiferal contents in these sections led to identification of 86 calcareous nannofossil species belonging to 22 genera, 10 families and 3 orders, in addition to 64 planktonic foraminiferal species belonging to 11 genera, 4 families and 2 superfamilies. The identified nannofossil and planktonic foraminiferal assemblages allow to distinguish five calcareous nannofossil biozones and six planktonic foraminiferal biozones. The biostratigraphic integration suggested the Chattian–Aquitanian age for the Nukhul Formation where the Globigerina ciperoensis Zone (P22) and Globigerinoides primordius Zone (M1a) correspond to calcareous nannofossil Sphenolithus ciperoensis Zone (NP25) and Discoaster druggii Zone (NN2), respectively. The Rudeis Formation is assigned to the Burdigalian–Langhian age depending on correspondence of Catpsydrax dissimilis Zone (M2), Globigerinoides bisphericus Zone (M4b) and Praeorbulina sicana Zone (M5) with Discoaster druggii zone (NN2), Sphenolithus belemnos Zone (NN3) and Helicosphaera ampliaperta Zone (NN4). The Somar Formation is found barren ofany microfossils, but it contains index pectens and oysters of Burdigalian age which may be equivalent to the lower part of the Rudeis Formation. The Kareem and Sarbut El-Gamal formations are represented by evaporitic and conglomeratic succession, where no foraminifera or nannofossils are recorded and assigned to the Langhian age according to their stratigraphic position. The Belayim Formation is assigned to the Serravallian age, due to the presence of Globorotalia fohsi Zone (M8) which equivalent to Discoaster exilis Zone (NN6).

Palaeoenvironment of the lower and middle Miocene successions in the Gulf of Suez region based on both planktonic and benthonic foraminifera

Based on the planktonic and benthonic foraminiferal content the lower and middle Miocene successions at the Gulf of Suez in Egypt have been identified (30 planktonic species and 101 benthonic species), seven paleoecologic parameters are studied, which include; foraminiferal frequency; predominance of the planktonic foraminiferal groups; predominance of benthonic foraminiferal groups, diversity of the foraminiferal species, P/B foraminiferal ratio, A/C foraminiferal ratio and benthonic genus/genera predominance. According to this study, the studied successions are divided into five ecozones. The paleoenvironment of each ecozone is deduced. The deduced paleoenvironments point to outer neritic for the Nukhul formation, middle neritic to inner neritic for the Rudeis formation, inner neritic to littoral for the Kareem formation and littoral neritic for the Belayim formation. A proposed paleobathymetric curve is presented. The interpretation of this curve helped to deduce the tectonic history of the early and middle Miocene in the studied areas.

Age and paleoenvironment of the Nukhul Formation, Gulf of Suez, Egypt: Insights from palynology, palynofacies and organic geochemistry

ABSTRACTPalynological results of a detailed study carried out on 56 samples retrieved from two selected wells (GH 404-2A and SA-E6A) of the Hilal and Shoab Ali fields within the southern part of the Gulf of Suez, Egypt, are presented. This study is mainly focused on the poorly dated Nukhul Formation, for which very little information from palynology is available despite its importance from a petroleum viewpoint. The assemblages discovered in our study are moderately preserved and reveal a sparse but significant record of spores and pollen and dinoflagellates together with highly diverse fungi and algal taxa, e.g. Botryococcus and Pediastrum.A latest Oligocene–Early Miocene (Chattian–Aquitanian) age has been suggested for the Nukhul Formation, based on compiling palynostratigraphic and ecologic data obtained from palynomorphs that have previously been assumed to be representatives for this period on a regional scale. In addition, the Oligocene/Miocene Boundary (OMB) could be lithostratigraphically defined within the studied formation, most likely at the boundary between the lower Shoab Ali Member and upper Ghara Member. A fungal/algal ‘event’ within the interval from 11,370–11,430 ft in the GH 404-2A Well may be associated with a strong regressive phase. Such a regression was previously observed in the Nile Delta and other locations around the Red Sea province, and may be assigned to the global Mi-1 glaciation event at the OMB. However, not only glacial-driven eustacy but also tectonic activity related to the Gulf of Suez rifting may have contributed in forming such an event.Palynofacies investigations were carried out under both transmitted and fluorescence microscopy and the results were partly supplemented by existing organic geochemical analyses (GH 404-2A Well) involving Rock-Eval pyrolysis and total organic carbon (TOC) measurements. The analysis was used to interpret the depositional regime, paleoenvironment and thermal maturation history of the studied succession. These results support the temporary existence of shallow, pond- or lake-like aquatic habitats during deposition of the lower Shoab Ali Member that evolved into a shallow-marine environment with the onset of the deposition of upper Ghara Member of the Nukhul Formation.

A fungal proliferation near the probable Oligocene/Miocene boundary, Nukhul Formation, Gulf of Suez, Egypt

Abstract. Moderately to well-preserved palynomorph assemblages were recorded from thirty samples of the Nukhul Formation (GH 404-2A Well), southern Gulf of Suez, Egypt. The taxa are dominated by highly diverse fungi, freshwater algae (e.g. Botryococcus, Pediastrum) beside a sparse record of spores and pollen. Marine palynomorphs, such as dinoflagellate cysts (dinocysts), are very rare. The stratigraphy and age of the Nukhul Formation is highly debated due to lack of diagnostic fossils (e.g. foraminifera, nannoplankton). It has been referred mostly to the Early Miocene; however, some recent publications interpret it as being of latest Oligocene–Early Miocene age. A prominent fungal proliferation composed of diverse and moderately well-preserved fungal spores, fungal fragments, fructifications and hyphae is recorded. This fungi-rich interval occurs mainly from 11370 to 11430 ft in the GH 404-2A Well. Such an observation has not been noted previously within the Nukhul Formation or its stratigraphic equivalents in Egypt. This putative ‘eco-event’ is probably associated with the well-known eustatic sea-level fall in the latest Chattian to early Aquitanian or at the Oligocene/Miocene boundary (OMB). It also seems possible that it may represent a more local event related to the rifting of the Gulf of Suez during this period. The high diversity of fossil fungi is interpreted herein as an indication of an episodic prevalence of humid climate at the end of a regressive phase, as also indicated by a lithological change near the top of the Shoab Ali Member of the Nukhul Formation. In addition, the co-occurrence of freshwater algae, mainly Botryococcus and Pediastrum, together with some aquatic fungal genera, such as Involutisporonites, Paragrantisporites, Quilonia, Striadiporites and Reduviasporonites, suggests the temporary existence of shallow, pond- or lake-like aquatic habitats, possibly related to tectonic activity.

Late Oligocene–Early Miocene Nukhul Sequence, Gulf of Suez and Red Sea

ABSTRACT Egypt’s Late Oligocene–Early Miocene Nukhul Formation was deposited during the earliest geological evolution of the Gulf of Suez and Red Sea Rift System. In this paper the formation is cast as a depositional sequence based on published sections, and correlated across the Gulf of Suez and northern Red Sea. The resulting correlations indicate that deposition was initiated in local grabens by the oldest continental clastics of the lower member of the Nukhul Formation, the Shoab Ali Member. The member overlies the Suez Rift Unconformity, a term proposed for the entire Red Sea. Although this member can attain a thickness of ca. 1,000 ft (305 m) locally in grabens, it is generally absent over horsts. Sedimentary facies of the member are interpreted as indicating an initial alluvial-fluvial setting that evolved to an estuarine and coastal setting. The upper part of the Nukhul Formation records a regional shallow-marine transgression, which can be subdivided into three correlative Upper Nukhul members. These sediments are absent over the highest paleo-horsts, but reach up to 900 ft (275 m) in thickness in grabens. In the southern Gulf of Suez the Ghara Member represents the Upper Nukhul members. In places it consists of four cycles, each of which starts with an anhydrite bed and is overlain by deposits of mixed lithology (sandstone, marl, and limestone). The four cycles are interpreted as transgressive-regressive subsequences that can be correlated across ca. 60 km in the Gulf of Suez. The Ghara Member correlates to Saudi Arabia’s Yanbu Formation, which consists of massive salt in wells drilled on the Red Sea coastal plains. The Yanbu Salt is dated by strontium-isotope analysis at ca. 23.1–21.6 Ma (earliest Aquitanian). The Nukhul Formation is capped by the Sub-Rudeis Unconformity or correlative Rudeis Sequence Boundary, and overlain by the Rudeis Formation. The Nukhul Formation is here proposed as the Nukhul Sequence and defined in the Wadi Dib-1 Well, wherein it consists of Nukhul subsequences 1 to 10 (in descending order, ranging in thickness between 33–84 m). The lower six Nukhul subsequences 10 to 5 are characterized by shale-to-sandstone cycles of the Shoab Ali Member, and the upper four are represented by the cycles of the Ghara Member. The 10 subsequences are interpreted as tracking the 405,000 year eccentricity signal of the Earth’s orbit and to span ca. 4.0 million years between ca. 25.0 and 21.0 Ma.