Biozonation (benthic foraminifera) of Mishrif Formation at Majnoon and Zubair oil fields, southern Iraq

: The Mishrif Formation represents a carbonate succession that deposited in major parts of the Arabian Plate during the Cenomanian stage. The formation is rich in fossils including foraminifera, rudists, and algae. This study includes determination of the biozones and biostratigraphic limits of three boreholes sections of Mishrif Formation (Cenomanian) from Majnoon and Zubair oil fields in southern Iraq within the Mesopotamian Basin. A new biostratigraphic scheme is introduced based on the study of benthic foraminifera that occur in the complete succession of Mishrif Formation. Depending on the vertical distribution of benthic foraminifera, four biozones in the studied sections have been established, they include: 1 - Miliolids Abundance Biozone that is characterized by the first appearance of miliolids to first appearance of Nezzazata conica , and include rudist, gastropoda, pelecypoda and algae, 2 - Nezzazata simplex – Nezzazata conica Concurrent Biozone ( Middle Cenomanian) is distinguished by the first appearance of Nezzazata simplex, Nezzazata conica , Praealveolina cretacea , and Pseudorhapydionia laurinensis . Other associated benthic foraminifera include Nezzazata concave , Praealveolina tenuis, Chrysalidina sp ., Cuneolina pavonia , Multispirina iranica , Biconcava bentori, Qataria dukhani , Dicyclina schlumbergari , Tabrina beingstani, Cisalveolina sp., and Carinoconus iraqiensis. 3 - Pseudorhapydionian laurinensis – Praealveolina cretacea Concurrent Biozone (Early Late Cenomanian) is identified by the last occurrence of Nezzazata simplex with the first occurrence of Psudorhapydionian laurinensis , and Praealveoilina cretacea, and the last occurrence of both taxa. This biozone also shows the occurrence of Cisalveolina fallax, Tabarian bingstani, Carinoconus iraqiensis, Spiroloculina sp., Chrysalidina gradata, Biconcava bentori, Qataria dukhani, Pseudotexularella sp., and Dicyclina schlumbergari. 4 - Pseudolituonella reicheli - Chrysalidina gradata Concurrent Biozone (Latest Cenomanian) is marked by the first and last occurrence of Pseudolituonella reicheli and Chrysalidina gradate. Other diagnosed foraminifera include: Spiroloculina sp., Rotalia sp., Pseudorhapydionia sp., Quinquelaculina sp., Nummulculina heimi , and Discorbis sp. Based on the stratigraphic ranges of the available fossils of the studied sections, the age of Mishrif Formation is estimated to be of Middle-latest Cenomanian.

The biostratigraphic scheme includes three major zones as follow (from bottom to top): 1 -Hedbergella gr.washitensis zone: The upper limit of this zone represents the last appearance of Hedbergella gr.washitensis.The zone occupies the bottom of Mishrif Formation.Therefore, the lower limit of this zone extends towards Rumaila Formation.Other associated fauna include Oligostegina, Hedbergella sp. and Asterohedbergella asterospinosa.According to Brun et al. (1975), a Middle Cenomanian age is assigned to this zone.
2 -Praealveolina gr.cretacea zone: This zone represents a large and important biostratigraphic unit in the Mishrif Formation.The lower limit falls within the first occurrence of Praealveolina cretacea.It coincides with the lower boundary of the Mishrif Formation where the Hedbergella gr.washitensis is absent in some wells.This large zone is divided into two subzones.The lower one is characterized by the occurrence of Ovalveolina sp. and Neoiraqia convexa.The upper subzone is mainly made up of the following assemblages: Praealveolina gr.cretacea, Pseudotextulariella IRK sp2, Cyclodomia iranica, Pseudocyclammina rugosa, Discorbis IRK sp2.Brun et al. (1975) Brun et al. (1975).
The lithology of Mishrif Formation at the studied sections is characterized by gray-brown limestone with pelecypod and gastropod shells, with high occurrence of foraminiferal limestone and rudist debris.The formation is variably dolomitized.
The contacts of the formation are conformable both with the underlying Rumaila Formation and overlying Khasib Formation.
The formation is equivalent to the upper part of the Magwa Formation (the Mishrif Fm.) in Kuwait, to the Sarvak Formation in the Zagros Iran, to the lower part of Judea Formation in the central and northeast Syria, and to Mardin Formation in southeast Turkey (Jassim & Goff, 2006).

STUDY AREA
The study area is located in the south of Iraq, and it includes two oil fields, Majnoon and Zubair (Figure 1), the Majnoon oil field is located about 60 km northwest of Basra city and the Zubair oil field is located about 20 km south of Basra city.
Different cores were sampled for micropaleontology and petrographic study.A total of 90 samples were collected from the three sections and 150 thin sections were prepared from the core samples in the laboratory.Microscopic study was provided in Baghdad University, College of Science, Department of Geology.The biozones of Mishrif Formation is determined according to the vertical distribution of benthic foraminifera.

GEOLOGICAL SETTING
Majnoon and Zubair oilfields are located in the southern Mesopotamian Basin, where more than 3000 m of Cretaceous sediments were deposited under the influence of tectonic, eustatic and climatic controls (Sadooni & Aqrawi, 2000).The Mesopotamian basin is subdivided into tectonic subzones, which are characterized by structural highs and lows with different trends.The structures are formed by the deformation of the northeastern Tethyan margin of the Arabian Plate during Cenomanian-Early Turonian (Jassim & Goff, 2006).
During the Cretaceous, the basin was part of a widespread carbonate platform located on the NE passive margin of the Arabian Plate (Murris, 1980).This margin was characterized by shallow, warm waters of Neo-Tethys (Sharland et al., 2001).During the Middle Cenomanian-Turonian period sea-level changes together with regional tectonic deformation of the Arabian Platform controlled the availability of accommodation space and therefore the depositional profile during development of Late Cretaceous sequence.These factors planned the maximum flooding surfaces and sequence boundaries which have been identified (Sharland et al., 2001;Farzadi, 2006).
The sediments were deposited on platforms within an inner shelf basin on the passive margin of the Arabian Plate (Ziegler, 2001).High organic accumulations with rich carbonate deposits controlling the growth and development of many positive elements and structures such as Mishrif, Ahmadi, and Rumaila formations were built-up as result of these changes (Van Buchem et al., 2002).
The shallow depositional conditions are dominant in Majnoon oil field, where thick lagoonal successions were deposited indicating higher accommodation space in the eastern part of the Mesopotamian Basin.At Zubair oil field, the deposition of Mishrif Formation was affected by tectonic activity as demonstrated by growth of rudist biostromes and shoal facies along crestal parts of Zubair structure, in addition to the presence of forced regressive succession at the uppermost part of Mishrif Formation (Mahdi et al., 2013;Mahdi & Aqrawi, 2014).
The carbonate production in the Mesopotamian basin resulted in thick succession of pelagic basinal facies to reefal and foraminiferal-rich shelf facies.The Mishrif Formation (Middle Cenomanian-Early Turonian) has carbonate sequence sediments rich in rudists, algal, coral reef, and benthonic foraminifera formed above structure within the Mesopotamian basin (Ziegler, 2001).The type section of Mishrif Formation occurs in well Zubair-3 (at Zubair oil field), which was selected by Rabanit (1952) The depositional environments for the Mishrif have wide range starting with fresh water in the upper Mishrif to the deep marine environment at the lower Mishrif (Aqrawi et al., 2010).The carbonate facies of the formation reflect various depositional environments including deep marine, shallow open marine, rudist biostrome, shoal, back-shoal, lagoon, and tidal flat (Mahdi et al., 2013;Mahdi & Aqrawi, 2014).The benthic foraminifera in lagoonal facies are diverse, including miliolids, alveolinids, textularids, and Nezzazata.The abundance of these fauna and their association with mud-supported facies indicate low-energy conditions below wave base in the subtidal zone (Mahdi et al., 2013).
The rudist facies passes into deeper water open-marine facies, near Luhais, Ratawi and Afaq (Al-Khersan, 1975) and also from Majnoon and Buzurgan fields.Similar facies are present all around the northern end of the Gulf (Ziegler, 2001).The beginning of sea withdrawal is represented by an extensive evaporate pan, Kifl Formation, in response to the epiorogenic movements which acted through Cenomanian-Turonian time span producing a subaerial unconformity (Melhi & Diah, 1984).
The lower boundary of the Mishrif Formation is represented by the change from the basinal Rumaila Formation to the shallow open marine.The upper boundary with the Khasib Formation is truncated by an unconformity surface separating the Middle from Late Cretaceous.Chatton & Hart (1961), Al-Sayyab (1984) and Ziegler (2001) suggested that the upper unconformable boundary of the formation represents the end of the Cenomanian-Turonian cycle.The overlying Khasib Formation represents transgressive, basinal Upper Turonian-Coniacian succession in Iraq (Dunnington, in van Bellen et al., 1959-2005;Darmoian, 1975).Evidence of exposure and erosion along the Mishrif-Khasib boundary include the occurrence of karstic conglomerates and breccias, extensive dissolution and cement zones, and fresh-water algal limestones (e.g.Al-Khersan, 1973;Al-Siddiki, 1978;Sadooni, 2005;Mahdi et al., 2013).The same contact represents the upper boundary of the middle Cretaceous Wasia Group, which has a regional extent and reported throughout the Arabian Plate, and resulted from tectonic uplift and Mid-Turonian eustatic sea-level fall (Sharland et al., 2001).In central parts of the Mesopotamian Basin, this boundary of is conformable with the overlying Kifl Evaporite Formation (Turonian) (Al-Naqib, 1967).
The Austrian orogeny caused the beginning of sea regression represent by an extensive of evaporate sediments pan (Kifl Formation) which is response to the epiorogenic movements acted through the Cenomanian-Turonian period leading to a subaerial unconformity (Melhi & Diah, 1984).This lead to decreasing the rate of subsidence and developed the maximum sea level fall.
During the deposition of Mishrif Formation, the palaeogeography of the Mesopotamian Basin was characterized by two high energy margins, where rudist biostromes were developed (Aqrawi et al., 2010).These margins rimmed an intrashelf basin, which was dominated by mud-rich facies, and has been referred to as the Najaf Basin (Aqrawi et al., 2010).

BIOSTRATIGRAPHY
The vertical distribution of identified benthonic foraminifera of the stratigraphic study of the Mishrif Formation supports four biozones for the studied sections.Detailed biostratigraphical study of these biozones is shown as follow:

Biozone no. 1 (miliolids abundance biozone)
This biozone is investigated from the base of the Mishrif Formation which ranged 5-11 meters in thickness (Figures 2, 3 and 4).It is characterized by high occurrence of miliolids wackestone-packestone with fragments of rudist, gastropoda, pelecypoda and algal remains.Miliolids biozone is marked by the first appearance of miliolids to first appearance of Nezzazata conica, N. concava (Plate 3, A) and other benthic foraminifera.

Biozone no. 3 (Pseudorhapydionian laurinensis, Praealveoilina cretacea concurrent biozone)
This zone is characterized by grey medium bedded of thick bedded rudist limestone with fragments of bivalve, gastropod and ostracod.The thickness of this biozone is illustrated in Figures 2, 3 and 4.
This zone shows the last occurrence of Nezzazata simplex corresponding with first occurrence of Psudorhapydionian  The Praealveoilina cretacea is an index for Late Cenomanian of Egypt (Orabi & Hamad, 2018).Ghanem & Kuss (2013)  According to the foraminiferal constituent of this biozone, its age is Early Late Cenomanian.

CONCLUSIONS
Previous studies assigned a Cenomanian-Early Turonian age for Mishrif Formation, while the current study shows that the age of the formation is Middle-late late Cenomanian, which indicates the absence of Turonian succession in the study area as evidenced by the absence of benthic foraminifera and occurrence of planktonic foraminifera in their place that belong to Khasib Formation.This is due to the truncation of Turonian biostratigraphic zones by erosion, and therefore most of Turonian successions are absent in the eastern Arabian Plate region (Harris et al.,1984).During Turonian, the erosion process lasted several million years in the Arabian Plate (Scott, 1990), and it is related to tectonic activity and a global eustatic fall in sea level (Sharland et al., 2001).The effect of these events is recorder in the uppermost part of Mishrif Formation in Majnoon and Zubair oil fields (Mahdi et al., 2013;Mahdi & Aqrawi, 2014).
illustrated the last occurrence of Thomasinella recorded in Middle Cenomanian age from Israel deposit.Weidich & Al-Harithi (1990) recorded the presence of Thomasinella punica in Jordan sediments indicating Middle Cenomanian (Orabi & Hamad, 2018).Afghah & Fadaei (2014) determined the Nezzazata conica, Chrysalidina Assemblage zone as Middle Cenomanian age in south west Iran.Afghah & Fadaei (2014) confirmed Nezzazata conica -Nezzazata simplex Assemblage Zone Middle Cenomanian in Zagros area from Iran.Aguilera-Franco (2003) documented Nezzazata conica from Cenomanian sequence of southern México.Sari et al. (2009) recorded the Nezzazata conica as Middle to Upper Cenomanian sequence in the southwestern area of Turkey.Various recording of Nezzazata simplex taxon range zone have been documented in the Middle to Late Cenomanian strata of south west Turkey.Bernaus & Masse (2006) recorded the new species Carinoconus iraqiensis sp.associated with Praealveolina cretacea as indicator for Middle Cenomanian in southern of Iraq.
the occurrence of Nezzazata simplex is an indicator for Early Cenomanian -Late Cenomanian deposits at southern Iraq.El-Naggar & Al-Rifaiy (1973) suggested the age of the Mishrif Formation of southern Iraq as Late Cenomanian depending on Praealveolina tenuis and documented the Biconcava bentori as an indicator for Cenomanian -Turonian in southern Iraq.Al-Dulaimy & Al-Sheikhly (2013) determined the age of the Mishrif Formation in southern Iraq as Cenomanian -Early Turonian depending on Nezzazata simplex, Parealveolina tenuis, Biconcava bentori and Cisalveoina fallax.

Figure 3 :
Figure 3: Biostratigraphy of the Cenomanian succession of Mishrif Formation at Zubair well 43.

Figure 4 :
Figure 4: Biostratigraphy of the Cenomanian succession of Mishrif Formation at Zubair well 47.
documented the first occurrence of Praealveolina cretacea as indicator for Upper Cenomanian of the Northwest Syria.Velić (2007) described Psudorhapydionian laurinensis, Cisalveolina fraasi and Pseudorhapydionia casertana from the Upper Cenomanian deposits in southeastern Europe.Afghah & Fadaei (2014) recorded the Nezzazata concave and Praealveolina cretacea Assemblage zone as a Late Cenomanian age in south Zagros Iran.Bender (1974) and Dilley (1985) distinguished the Praealveolian cretacea from the Late Cenomanian sequence of Jordan.Schroeder & Neumann (1985) mentioned that the presence of Praealveolian cretacea and Chrysalidina gradata indicate the middle -Late Cenomanian age in Mediterranean area.Al-Dulaimy & Al-Sheikhly (2013) recorded the occurrence of Praealveolian cretacea from the Cenomanian -Early Turonian deposit in southern Iraq.
determined the middle to late Cenomanian age of Milna Formation by the presence of benthic foraminifera that include Broekina (Pastrikella) balcanica Cherchi et al., Chrysalidina gradata D'orbigny, Pseudorpydionin dubia, Vidalina radoicicae.Al-Jumaily (2001) recorded the Chrysalidina gradata and Pseudolituonella reicheli as Upper Cenomanian from southern Iraq.In present study this zone indicates a Late Cenomanian age according to foraminifera constituents.