Large Benthonic Foraminifera Biozonation of Mishrif Formation at Tuba and Zubair Oilfield, Southern Iraq

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Introduction
The interest in studying the Cretaceous period in Iraq is of great importance because of not only the large number of formations deposited during it, nor the phases or structural movements that occurred, but also because most of the essential oil reservoirs are present in the formations of this period (Buday, 1980).As a component of the Wasia Group, The Mishrif Formation, spanning from the Cenomanian to the Early Turonian period, is a prominent and geographically extensive carbonate reservoir formation in central and southern Iraq, as well as in the Arabian Gulf.This has been noted in various scholarly works, including those by Al-Khersan (1975), Reulet (1982), Harris and Frost (1984), Videtich et al. (1988), Alsharhan and Narin (1988), Burchette (1993), AlSharhan (1995), Aqrawi et al. (2010), and Mahdi and Aqrawi (2013).It is worth noting that the Mishrif Formation accounts for 30% of Iraq's overall oil reserves, as reported by Al-Sakini (1992).
The Cenomanian Mahlban, Moatsi, and Fahad formations are considered to be the comparable counterparts of the Mishrif Formation in the central region of Iraq.The formations in question are representative in nature and are analogous to the Mishrif and Rumaila formations located in the southern region of Iraq.The formations identified in the Mesopotamian basin are considered to be chronostratigraphic units that belong to the Middle Cretaceous period.
The Mishrif Formation is situated within the secondary sedimentary cycle, specifically the Cenomanian-Early Turonian period.The formation is widely regarded as a highly productive oil reservoir in both Iraq and the Arabian Gulf, due to its different petrophysical properties and widespread.Therefore, the formation has received many local and regional geological studies that dealt with the fossils content or stratigraphic features within specific fields or areas of several oilfields (Al-Jumaily, 2001).
There have been many studies of the Mishrif Formation, and the majority of these studies have concentrated on the biological content and stratigraphic nature of the deposits, Rabanit (1952) referred to the Mishrif Formation and, for the first time in Zb-3 well inside the upper section, described the Khutiah Formation within the Wasia group.Gaddo (1971) studied the microfacies, paleoenvironments, and petrography of the Mishrif Formation.Alsiddiki (1978) Studied a wide range of Mishrif Formation related topics, including lithostratigraphy, biostratigraphy, thickness variations, lateral expansions, and age determination.Al-Nuaimy (1990) studied g species of large Foraminifera, including Alveolinidin the Mishrif Formation.Abdul-Kareem and Yosif (1990) divided the formation into five basic zones from bottom to top. 1. Oligostegina sp., Hedbergella washitensis zone.2. Praealveolina cretacea-Ovalveolina ovum zone, which is divided into two sub-zones: a. Ovalveolina ovum sub zone, b.Cisalveolina fallax, Tabrina bingstani sub-zone, 3. Dicyclina schlumbergeri -Qataria dukhani zone, 4. Rotalina sp.Zone. 5. Miliolide indet.
Al-Dulaimy and Al-Sheikhly (2013) studied the Mishrif Formation's biostratigraphy from the oil wells Halfaya-1, Amara-1, and Majnoon-1 in southeast Iraq.Saqer (2014) created a reservoir geological model by using mirofacies and well logs data of Tuba oilfield.Al-Marsumi (2014) conducted a seismic reflection interpretation of Tuba Oilfield over 261 km 2 area.Mohammed and Mahdi (2019) constructed a 3D geological model of the Mishrif Formation in Tuba field, and deduced that the facies model indicates the rudist biostrome and shoal facies associations form the essential oil-bearing units in the formation.Abbas and Mahdi, (2020)  The purpose of biostratigraphy studies is to determine the relative arrangement of strata based on their fossils content, and descriptive biostratigraphy is used in determination zones for regional or local stratigraphic correlation (Keller, 2004).Nineteen species belonging to fourteen genera recognized of benthonic foraminifera in this study.The content of fossils for Mishrif Formation is described, and the formation is divided into several biozones depending on index fossils present in Tuba and Zubair oilfields.

Location of the Study Area
The investigation area is situated in the Mesopotamian Zone, in the southern Iraqi oilfields of Tuba and Zubair.The Tuba Oilfield is situated approximately 35 km south-west of Basra City, flanked by Zubair to the east at a distance of 5 km and South Rumaila to the west at a distance of 2 km.The Tuba anticline goes roughly N-S, measures 29 km in length, and has a maximum width of 9 km.The present study involved the investigation of the TU-5, TU-24, and TU-40 wells located within the oilfield.
The Zubair Oilfield, which occupies an area of 1,170 km2, is situated 20 kilometers to the West of Basra City in southern Iraq.It is located between latitudes 47° 32' and 47° 45' and longitudes 30° 42' and  30° 05'.The field is located in the northeastern region of the Arabian Peninsula, close to the Kuwaiti border, and it forms a 60 km long North-South anticline.From South to North, it is made up of four domes Safwan, Rafdhyia, Shuaiba, and Hammar.This oilfield's ZB-41, ZB-42, and ZB-46 wells were studied.(Fig. 1 and Table 1).

Materials and Methods
The petrographic description of core and cutting samples was done by preparing it as a 300-thin section that had been examined using a polarized microscope (Leica, pro-Las core 4.13).These samples were taken within intervals of 1 m, the examination describes paleontology, lithology, and the microfacies characters of the formation.

Stratigraphy of Mishrif Formation
The Mishrif Formation, ranging from the Cenomanian to the Early Turonian epoch, is a heterogeneous formation that was originally identified as comprising organic detrital limestones superimposed by limonitic freshwater limestones (Bellen et al., 1959in Aqrawi et al., 2010).As per the findings of Aqrawi et al. (2010), the Mishrif Formation is characterized as a marine shelf sequence that displays progradation.The formation under consideration comprises two primary sedimentary cycles that are abruptly terminated by an unconformity, which corresponds to the boundary between the Mishrif and the Khasib formations.The lower boundary of the Mishrif Formation displays a state of conformity with the underlying Rumaila Formation.
Based on Sharland et al. (2001) conclusions, the Mishrif Formation deposited during the Megasequence AP8 of the Late Tithonian-Early Turonian period, which falls under the Albian-Early Turonian Sequence (Wasi'a Group) (Fig. 2).The Lithology of the Mishrif Formation in the Tuba and Zubair Oilfields consists of limestone units with marker shale units in the middle part of the formation; the limestone can be vuggy, fossiliferous, argillaceous, and stylolite (Jassim and Goff, 2006).

Results (Biostratigraphy of Mishrif Formation in Tuba and Zubair Oilfields)
Mishrif Formation has been determined by Smout (1956) and Owen and Naser (1958); they indicated the formation is located within the Turonian because it contains Nezzazata; other studies by (Bellen et al., 1959) indicated the formation in the Cenomanian age depending on the vertical distribution of some types of large benthonic foraminifera that characteristic in the formation, there are previous studies that divided Mishrif Formation into biostratigraphic zones, including Al-Khersan (1975); Brun et al. (1975); Abdul-Kareem and Yosif (1990); Philip et al. (1995), Al-Tharb (1996), Sharbazhery (1999), Al-Badri (2005), etc.
A detailed biostratigraphic study of the Mishrif Formation of Tuba and Zubair oilfields uses foraminifera and their distribution (Figs. 4,5,6,7,8,9,and 10) and the correlation of the Mishrif Formation biozones.The specifications of the four biozones are below:

Miliolids Biozone
Definition: From the Mishrif Formation's base, this Biozone is being studied in the Tuba and Zubair Oilfields.
Discussion: Miliolid wackestone-packstone containing fragments of rudist, gastropoda, pelecypoda, and algal remnants are its defining characteristics.Nezzazata conica, N. concave, and other benthonic foraminifera make their debuts at exactly the same time as miliolids, marking the beginning of the miliolids biozone.

Nezzazata simplex -Nezzazata conica Concurrent Biozone
Definition: In the middle of these sections, the biozone Nezzazata simplex-Nezzazata conica is distinguishable since this is where these two species originally appeared, together with Praealveolina cretacea and Pseudorhapydionia laurinensis.
According to Tasli et al., (2005), the Middle and Upper Cenomanian strata of Southern Turkey fall into the biozones of Biconcava bentori and Pseudorhapydoinina dubia.Similar biostratigraphic information was gathered by Velić (2007) from the Karst Dinaridos in Southeast Europe.Pseudorhipidionia casertana has been identified in the Middle Cenomanian Chrysalidina gradata partial range zone of Northwest Syria by Ghanem and Kuss (2013) (Orabi and Hamad, 2018) .
The Nezzazata conica was identified by Sari et al. (2009) as a Middle to Upper Cenomanian sequence in Turkey's southwest.Southwest Turkey's-Bernaus Middle to Late Cenomanian strata have a number of Nezzazata simplex taxon range zone recordings.
As a marker for the Middle Cenomanian in the South of Iraq, Bernaus and Masse (2006) identified the novel species Carinoconus iraqiensis sp. in association with Praealveolina cretacea .
Nezzazata simplex in southern Iraq is thought to be an indicator of Early Cenomanian-Late Cenomanian deposits, according to Al-Naqib (1967); Gaddo (1971);and Brun (. 1975 ) Based on Praealveolina tenuis, El-Naggar and Al-Rifaiy (1973)  The biostratigraphic data regarding Nezzazata simplex and Nezzazata conica is also adequate support for the Middle Cenomanian age determination of this Zone.

Pseudorhapydionian laurinensis, Praealveoilina cretacea Concurrent Biozone
Definition: In this zone, Nezzazata simplex is last seen along with Psudorhapydionian laurinensis and Praealveoilina cretacea, which are first and last seen, respectively.
Discussion: Orabi and Hamad (2018) use the Praealveoilina cretacea as an index for the Late Cenomanian of Egypt.In Northwest Syria's Upper Cenomanian, Ghanem and Kuss (2013) recorded the first appearance of Praealveolina cretacea as an indication.
Pseudorhapydionia casertana, Cisalveolina fraasi, and Pseudorhapydionia laurinensis were described by Velić (2007) from the Upper Cenomanian sediments in southeast Europe.The Late Cenomanian age of the Nezzazata concave and Praealveolina cretacea Assemblage zone in south Zagros, Iran, was determined by Afghah and Fadaei (2014).The Praealveolian Cretacea was differentiated from Jordan's Late Cenomanian series by Bender (1974) and Dilley (1985).According to Schroeder and Neumann (1985), the Mediterranean region dates to the Middle-Late Cenomanian, as shown by Praealveolian cretacea and Chrysalidina gradata.

Pseudolituonella reicheli, and Chrysalidina gradate Concurrent Biozone
Definition: Pseudolituonella reicheli and Chrysalidina gradata are found just once and last in this biozone, which is one of the three sections.
Chrysalidina gradata from the Middle and Upper Cenomanian deposit in Southeast Europe was identified by Velić (2007).Chrysalidina gradata and Pseudolituonella recheli, which were last seen in Turkey, mark the Cenomanian-Turonian border (Sari et al., 2004).
Chrysalidina gradata and Pseudolituonella reicheli were identified as Upper Cenomanian species from southern Iraq by Al-Jumaily (2001).According to the foraminifera included in the study, this zone is Late Cenomanian in age.Al-Dulaimy et al., (2022) used Pseudolituonella reicheli and Chrysalidina gradate to date the Mishrif Formation as Turonian, however foraminifera components point to a Late Cenomanian age.

Conclusions
The present investigation refers to the biostratigraphic zonation of the Mishrif Formation in the wells of TU-5, TU-24, TU-40, ZB-41, ZB-42, and ZB-46.The findings reveal that the skeletal grains predominantly comprise benthonic foraminifera.The biostratigraphic investigation of the studied wells of Tuba and Zubair Oilfields indicates four biozones concerned with benthonic foraminifera as follows: Miliolids Abundance Biozone.
. The foraminiferal component of this biozone indicates that it is Early Late Cenomanian in age.Based on Pseudorhapydionian laurinensis, Praealveoilina cretacea, Al-Dulaimy et al., (2022) calculated the age of the Mishrif Formation as Turonian; nevertheless, the foraminiferal component of this biozone places its age as Early Late Cenomanian .