MICROFACIES ANALYSIS OF THE MISHRIF FORMATION (MID-CRETACEOUS) AT THE TUBA AND ZUBAIR OILFIELDS, SOUTHERN IRAQ

The Cenomanian-Early Turonian Mishrif Formation, which has been studied in selected Oilfields of Tuba Well (TU-5, TU-24, TU-40) and Zubair Well (ZB-41, ZB-42, and ZB-46), which are located in the Mesopotamian basin, southern Iraq, and considered major carbonate reservoir in Iraq and the Arabian Gulf. Seven microfacies types are identified showing that Mishrif formation deposited in five sedimentary facies: 1. Basin Facies Associations; 2. Deep Marine Facies Associations: Bioclast pelagic foram mudstone-wackestone; 3. Shallow Open Marine Facies Associations: Foraminiferal bioclastic wackestone-packstones; 4. Rudisted Reef Facies Associations; 5. Shoal Facies Associations: Peloidal packstone-grainstone; 6. aBack-shoal Facies Association: a. Foraminiferal-bioclastic wackestone-packstone, b. Rudisted-foraminiferal wackestone-packstone; 7. Lagoon Facies Associations: a. Fossiliferous mudstone-wackestone, b. Bioclast foraminiferal wackestone, c. Benthic foraminiferal wackestone.


INTRODUCTION
The study of the Cretaceous period in Iraq holds significant academic value due to the presence of numerous formations deposited during this period, as well as the occurrence of phases and structural movements.Additionally, it is noteworthy that the formations of this period contain a substantial proportion of the essential oil reservoirs, as highlighted by Buday (1980).The Mishrif Formation, which extends from the Cenomanian to the Early Turonian period, is a significant carbonate reservoir formation that is widely distributed in central and southern Iraq, as well as in the Arabian Gulf.This has been documented by various sources, including (Al-Khersan, 1975;Alsharhan & Nairn, 1988, 1993;Aqrawi et al., 2010;Burchette, 1993;Harris & Frost, 1984;Mahdi & Aqrawi, 2014;Reulet, 1982;Videtich et al., 1988).Additionally, it has been reported by Al-Sakini (1992) that the Mishrif Formation in Iraq contains 30% of the country's entire oil reserves.
The formations known as Cenomanian Mahlban, Moatsi, and Fahad exhibit similarities to the Mishrif Formation located in the central region of Iraq (Jassim & Goff, 2006a).The aforementioned representational groups are associated with the Mishrif and Rumaila regions in southern Iraq.All geological formations in the Mesopotamian basin are classified as chronostratigraphic units that date back to the middle Cretaceous period.The Mishrif Formation is among the Wasia groupings, as stated by Alsharhan & Nairn (1988).
The Mishrif Formation is present in the Cenomanian-early Turonian, which is a secondary sedimentary cycle.From an economic standpoint, the formation is considered to be among the most valuable oil resources in Iraq and the Arabian Gulf (Jassim & Goff, 2006b).The geological extension and facies characteristics of the rocky features, which contain porous units carrying rudist, render it an ideal hydrocarbon reservoir.The formation in question has been the subject of numerous geological studies at both local and regional levels.These studies have focused on various aspects, such as lithological fossils and stratigraphic features within specific fields or areas of multiple fields, as noted by 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).These three formations (i.e., Mishrif, Ahmady, and Rumaila) were the divisions he made.Owen & Nasr (1958) in Van Bellen, the thickness of the formation has been detected and is equal to the Mugwa Formation in Kuwait in (Zb-3) well.Then the rock content is described, whereas the thickness is renewed with 122 m to consider the 35 m belonging to the Mishrif Formation.Chatton & Hart (1961) included the organic detrital neritic limestone units of sub-cycle (Cenomanian-Early Turonian), such as the M'sad, Gir Bir, and Mergi formations in Mishrif Formation, they placed freshwater limestone through the newly introduced Kifl Formation.James & Wynd (1965) suggested the upper part of the Sarvak Formation as the Mishrif Formation.Al Naqib (1967) researched the formation and explained that the upper part of the Wasia Group is identical to the Mishrif Formation.Gaddo (1971) Studied the microfacies, paleoenvironments, and petrography of the Mishrif Formation.Belarabi (1982and Reulet (1970, 1982) studied Sedimentology, microfacies analysis, and depositional environment models.Al-Khersan (1975) studied the Mishrif Formation and believed the formation was deposited within five marine environments: intertidal, littoral, bank margins, bank, and open sea environments.Al-Siddiki (1978) studies have covered a wide range of Mishrif Formation-related topics, including Lithostratigraphy, Biostratigraphy, thickness variations, lateral expansions, and age determination.Sherwani (1983) divided the Mishrif formation environments into supratidal, tidal, and subtidal Rudist facies, Rudist coral banks, and shelf facies.Al-Sherwani (1988) studied the Cenomanian -Early Turonian sedimentological system in S of Iraq.Van Buchem et al. (1996) based on a transect of outcrops in the Adam Foothills.Aqrawi et al. (1998) in their investigation of the sequence stratigraphy of the Mishrif formation in Iraq, exclusively used the regressive cycle.Hussain et al. (2020) studied the Mishrif Formation vertical and horizontal distribution of porosity and permeability values related to stratigraphy in the West Qurna Oilfield.Sharland, Archer, Casey, Davies, et al. (2001) found out that the AP8/AP9 megasequence boundary is formed at 92 million years by the Mishrif top truncation.Al-Jumaily (2001) The Mishrif Formation's microfacies have been investigated, and the results show that the formation has two regressive cycles.Van Buchem et al. (2002) used the same outcrops with subsurface data to construct a more detailed model.Mahdi (2004) examined the Mishrif Formation's sequence stratigraphy and reservoir characteristics in several South Iraqi wells.Al-Ubaidy (2004) studied the stratigraphic sequence of the Mishrif Formation and suggested four major subenvironments shallow restricted, shallow open marine, shoal, and deep marine environment in Zubair Field.Al-Khalidi (2004) studied the Mishrif Formation in S of Iraq and recognized fourteen microfacies in well HF-1.Sadooni (2005) believed that the Dujail Shoal, a nearby uplift, limited the maximum thickness of the Mishrif Formation in the Dujail area since it exhibits build-up areas some distance from the main platform margin.Farzadi (2006) studied the Middle Cretaceous carbonate platforms of the Gulf and Mishrif Formation using seismic stratigraphy, suggesting that the Mishrif Formation is equivalent to the upper part of the Sarvak Formation and represented by a high concentration of organic matter in an intra-shelf basin associated with shallowing upward succession.Al-Kilaby (2009) 2013) studied the Mishrif Formation's biostratigraphy from the oil wells Halfaya-1, Amara-1, and Majnoon-1 in southeast Iraq.Mahdi et al. (2013) investigated the characterization of the southern Mesopotamian Basin of Iraq's mid-Cretaceous Mishrif reservoir.Nasser & Nabaa (2013) checked the microfacies present in Mishrif rocks and the affected digenesis processes.Mahdi & Aqrawi (2014, 2018) studied the Mid-Cretaceous Mishrif Formation in the Southern Mesopotamian Basin of Iraq using sequence stratigraphic analysis.Saeed (2014) investigated the sequential workflow of geological applications for the Cenomanian -Early Turonain carbonate successions of the Mishrif Formation in the Noor Oilfield, South of Iraq (Petrography and microfacies study, petrophysical evaluation, and modeling; Saqer (2014) .In the Tuba Oilfield, the Mishrif Formation's four primary microfacies were identified.The well logs were then analyzed, and a reservoir geological model was created.Al-Shabender (2014) provides a sequential geoscience workflow (geophysical, petrophysical evaluation, and modeling) for the Cenomanian Early -Turonian Mishrif Formation carbonate succession at the Buzurgan oilfield, southeast of Iraq.Al-Mosawy (2014) includes petrography and microfacies investigation, petrophysical evaluation, and modeling for the Cenomanian -Early Turonian Mishrif Formation carbonate succession in the Halfaya Oilfield, southeast Iraq.Al-Ameri (2015) displays the depositional environment and Mishrif Formation reservoir properties in the Rumaila North field.Al-Marsumi (2014) reflection seismic study interpretation of Tuba Oilfield over 261 Km 2 area.Al-Khafaji (2015) interpreted the petrophysical characteristic of Mishrif to set up the 3D geological static model of the Mishrif reservoir.Alrrawi et al. (2015) four main facies have been diagnosed in the Mishrif Formation (Grainstone, packstone, wackestone, and mudstone), which are spread all over the reservoir units.Al-Rahim & Hashem (2015) enhance the precise information method about subsurface reservoir characterizations by improving the petrophysical properties estimation (porosity, water saturation, and lithology) through the combination of good logs and seismic data.Al-Yasi & Jaed (2016) to create a reservoir model for the Mishrif Formation, facies, and readily available well log data integration were done in the Gharraf Oilfield.AlBahadily & Nasser (2017) divided the Mishrif Formation into seven units.They concluded that it consists of four principal oil-bearing units: (MA, MB11,aMB12, andaMB13), whereas MB21, MC1, and MC2 The purpose of this study is to describe the lithological composition of the Tuba and Zubair Oilfields, determine the age of the Mishrif Formation, and interpret depositional paleoenvironments by microfacies analysis.

MATERIALS AND METHODOLOGY
Sampling and describing selected boreholes from two Oilfields by taking rock samples from the cutting and core for these wells, the interval between each sample is approximately (1 m).The preparation of the thin sections for the cutting and core sample, with the petrographic description of thin sections by transmitted light microscope, by examination of 300 thin sections using a polarized microscope (Leica, pro-Las core 4.13), a thin section description is used to identify the paleontological lithology and Microfacies characters.Also, examining the thin sections is used to describe carbonate microfacies, and diagenesis processes and then delineate the age of the formation.

GEOLOGICAL SETTING
The Mishrif Formation was formed in the Mesopotamian Basin.During the Late Permian era, the Neo-Theys Ocean experienced an opening process (Sharland, Archer, Casey, Hall, et al., 2001).Throughout the Early Triassic, the Neo-Theys Ocean steadily enlarged, resulting in a breakup unconformity along the northern and eastern margins of the Arabian plate.The Mesopotamian Basin and the passive margin megasequence were formed as a result of thermal subsidence (Jassim & Goff, 2006).
During the Late Cenomanian -Early Turonian period, the microcontinent that had broken off from the Arabian plate during the Late Tithonian period moved closer to the trench of the intra-oceanic subduction zone (Sharland, Archer, Casey, Hall, et al., 2001).The formation of N -S trending structures in southern Iraq, Kuwait, and Saudi Arabia is thought to be the result of the diachronous collision of these micro-continents with the fore-arc region above the trench.Buday and Jassim (1987) identified five distinct tectonic-physiographic zones, with the Mesopotamian basin being one of the most notable divisions.The Mishrif Formation is situated tectonically in the Mesopotamian basin, which has been separated into the Zubair, Euphrates, and Tigris subzones from the south to the north, as documented by Buday and Jassim (1984).The basin exhibits asymmetry in its foredeep configuration and displays a regional dip towards the northeast and east.The geographical region known as the Mesopotamian Basin is predominantly situated in the southeastern and central areas of Iraq.
The western and southwestern boundaries of the region are demarcated by the Abu-Jir fault zone, while the eastern boundary is formed by the Zagros Mountains, and the northeastern

Iraqi Bulletin of Geology and Mining
Vol.20, No.1, 2024 p 1 20 5 boundary is marked by the Hamrin Mountains.The Mesopotamian Basin is primarily situated on the Mesopotamian block, as well as on adjacent blocks to the northwest and southeast, as depicted in (Figure 1).
Figure 1: The Mishrif Formation's main depositional areas are depicted on this general palaeogeographic map of the Southern Mesopotamian Basin in South Iraq (which has been adapted from Mahdi et al., 2013).

FACIES ASSOCIATIONS IN THE MISHRIF FORMATION
Description and observation of the fossils content and microfacies led to determining the facies association in this study, which is based on (Dunham, 1962), and rudist-bearing facies were classified according to Embry & Klovan (1971).In contrast to the models of typical microfacies and depositional environment belts of carbonates proposed by Wilson (1975) and Fluegel (1982) and taking into account the interpretive model of rudist palaeoenvironment zones proposed by Kauffman (1973); Burchette & Britton (1985) were used to group facies types as "association" rather than a single type or class.Basin, deep marine, shallow open marine, rudist biostrome, shoal, back shoal, and lagoon were the seven facies relationships identified in this study.Similar facies associations were described in other oilfields within the Mesopotamian basin (Mahdi, 2004;Mahdi et al., 2013;Sherwani, 1998).As shown in (Figures 2,3,4,5,6,and 7).

Basin Facies Association
According to Mahdi (2004) and Mahdi & Aqrawi (2014), this association mainly comprises shale units deposited during maximum flooding intervals.Similar units can correlate with other oilfields in the Mesopotamian Basin.Pelagic mudstone and wackestone microfacies represent the environment, comprising a micrite matrix with planktic foraminifera as the main skeletal component with rare calcisphere and sponge spicules.The pelagic mudstone and wackestone microfacies thickness decreases towards the Zubair and the lower (5 m.) Tuba Oil Fields, equivalent to Wilson's (SMF-3) facies zone (FZ-1) of Flügel (2004).

Deep Marine Facies Associations
Deep marine facies association consists mainly of fine-grain skeletal lime mudstones to wackestones, skeletal grain mainly consists of bioclasts that are mostly fine and unidentifiable and the coarser bioclast which is rudist and few benthic foraminiferas such as Rotalia, Nezzazata (Fig. 8a), and Psedudotexteulariella, with some planktic foraminifera, rudist bioclasts have been originated from the destruction of rudist belts by waves and currents, which is equivalent to Wilson's (SMF-3) facies zone (FZ-2) of Flügel (2004).

4.2.A. Bioclast pelagic foram mudstone-wackestone
Pelagic forms including Hedbergella, Oligosteginid, Echinoid fragments, and sponge spicules are present in this facies zone and suggest a deep open sea environment (Simo & Lehmann, 2000).In the presence of the sparse lime mud in the matrix, which represents a low energy environment in this facies zone, the high frequency of Oligosteginid and Hedbergella suggests a very good nutrient condition (Adachi, 2004;Birkeland, 1987;Brasier, 1995;Luciani & Cobianchi, 1999;Silva, 1995).According to Heckel (1972), Sanders & Höfling (2000), and Flügel (2004), the faunal assemblage in this facies zone may endure typical saline open marine conditions.In conclusion, a large volume of lime mud points to a tranquil environment free of agitation.

Shallow Open Marine Facies Associations
The Mishrif Formation's most common facies are shallow open marine ones; it consists mainly of bioclastic or foraminiferal bioclastic as wackestone and packstones (Figure 8b), with rudist bioclastic (Figure 8c) occur in this facies association, the size of these bioclasts ranges in size from fine to coarse grain characterize the Mishrif carbonates.Benthic foraminifera is the most abundant in this environment and is represented by well-preserved fossils with some bioclasts.The area near the biostrome is represented by an association of benthic foraminifera and Rudist debris, corals, and rarely algae.Benthic foraminifera in this facies is Nezzazata (Figure 8d), Dicyclina, Textularia, Praealveolina, calcareous algae, coral, and sponge spicules (Figure 8e), planktic foraminifera is less common.These facies are found seaward the Formation, equivalent to Wilson's (SMF 10) facies zone (FZ-5) of Fügel (2010).The microfacies in detail:

4.3.A. Foraminiferal bioclastic wackestone-packstones
These microfacies are the most dominant in the Mishrif succession and constitute thick horizons at different levels.It is represented mainly by large benthic foraminifera like Prealviolina and Cisalviolina, and smaller faunas like Miliolide and Nazzazata sp. are also common.Rudist debris is also common and abundant at certain intervals; this may indicate nearness to the biostrome body.Echinoderms and gastropods are two more common components.

Rudist Reef Facies Associations
A very coarse-grained bioclastic rudstone fragment makes up these facies as a main component; the rudist occurs as whole shells or coarse bioclasts with relative preservation of their internal structure; Textures consist of grainestone (Figure 8f), These microfacies suggest that rudist reefs were present on the shelf (Mahdi et al., 2013) considered the rudist reef facies association as equivalent to Wilson's (SMF-5, and 7) facies zone (FZ-5) of Flügel (2004).
The depositional environment of rudists in the Mesopotamian basin was characterized by moderate-high energy conditions (Mahdi et al., 2013).During the Cretaceous period, paleobathymetry of this environment was estimated to be approximately 2 -10 m (Scott, 1995).

Shoal Facies Associations
These sediments are sited on the marginal shelf, a coarse-grained peloidal packstone to grainstone, the benthic foraminifera or concentrations of their skeletal grains with rudist debris (Figure 8g), and the culmination of upward coarsening sequence.Also, pelodial packstone to grainstone consists mainly of peloids, intraclasts of various sizes with echinoderm plates, micritized grains, and various sizes of echinoderm plates.In the top regions of the Formation, these microfacies are extremely prevalent.They are equivalent to Wilson's (SMF-8) facies zone (FZ-8) of Flügel (2004).The microfacies in detail:

Back-shoal facies Association
According to Mahdi et al. (2013), the association of rudist fragments and benthic foraminifera indicates that the back-shoal facies association implies a zone of sediment mixing between the shoal and lagoon (Burchette, 1993).It is composed of floatstone, rudistedforaminiferal wackestone-packstone, and foraminiferal-bioclastic wackestone-packstone.According to Mahdi et al. (2013) and Mahdi & Aqrawi (2014), the characteristics of the backshoal facies association point to a low-moderate energy depositional environment on the lee side of rudist biostromes or shoals.The Sarvak Formation in Iran (Razin et al., 2010) and the Mishrif Formation in the United Arab Emirates (Burchette & Britton, 1985) are two formations that exhibit the same facies relationship.They are equivalent to  facies zone (FZ-8) of Flügel (2004).The microfacies in detail:

4.6.A. Foraminiferal-bioclastic wackestone-packstone
The various benthic foraminifera distinguish these microfacies; the miliolids and Nezzazata are examples of benthic foraminifera.There are also rare to widespread green algae.Echinoids, rudists, sponge spicules, peloids, gastropods, and less important components.These facies' fine grain size implies deposition in a Back-shoal, low-moderate energy depositional environment.The diversity of the fauna indicates that the water column and sediment surface had normal salinity, adequate water circulation, and an oxygen concentration.

Microfacies analysis of the Mishrif Formation (Mid-Cretaceous) at Tuba and Zubair Oilfields, Southern Iraq
Ayhem A. Dawood, and Salam I.M. Al-Dulaimi These facies have floatstone (Embry & Klovan, 1971) classification-based textures, and wackestone-packstone (Dunham, 1962) classification-based textures.This microfacies developed in an environment with low to moderate energy on an upper slope.

Lagoon Facies Associations
An area of relatively shallow, quiet water separated from the open marine conditions by a barrier.The lagoon environment is characterized by abundant benthic foraminiferal wackestone (Figure 8h) and Miliolid mudstone to wackestone (Figure 8i).In the bottom and top regions of the Mishrif Formation, the lagoon facies association creates a thick succession of muddominated facies.The benthic foraminifera are abundant and diverse such as Miliolids, Textularia, Pseudolituonella, Nezzazata (Figure 8j), and many others.Also, sponge spicules, algae, and rudist fragments.It comprises fossiliferous mudstone-wackestone, bioclasticforaminiferal wackestone, and benthic foraminiferal wackestone; these deposits spread in most studied wells.They are equivalent to , the facies zone (FZ-8) of Flügel (2004).The microfacies in detail:

4.7.A. Fossiliferous mudstone-wackestone
According to Lakhdar et al. (2006), this facies zone is primarily made up of different frequent benthic forams that imply a lagoon environment next to a tidal flat.According to Bachmann & Hirsch (2006), a shallow bathy with sufficient saline conditions and water circulation results in a nutrient-rich environment.This is indicated by the high diversity and abundance of the skeleton allochems.A lagoon with few available energy sources is indicated by low species diversity and elevated lime mud in some facies (Masse et al., 2003;Sandulli, 2004).In the bottom and top regions of the Mishrif Formation, the lagoon facies association creates a thick succession of mud-dominated facies.

4.7.B. Bioclast foraminiferal wackestone
The varied benthic foraminifera in mud-supported textures is the main characteristic of this microfacies.Benthic foraminifera include the miliolids, alveolinids, orbitolinids, and Nezzazata.There are also rare to widespread green algae.Echinoids, rudists, sponge spicules, peloids, gastropods, and less important components.These facies' fine grain size (textures) imply deposition in an open lagoon, low-energy environment.The diversity of the fauna indicates that the water column and sediment surface had normal salinity, adequate water circulation, and an oxygen concentration.According to Zhicheng et al. (1997), green algae show that airflow and light penetration are both favorable.

4.7.C. Benthic foraminiferal wackestone
The predominant components of these facies are gastropods, shell fragments, green algae, small benthic foraminifera (miliolids and Nezzazata), and rare peloids.Fine-grained micrite forms comprise the matrix.The stratigraphic position and the minimal variety of the skeleton fauna show the restricted low-energy lagoonal environments where these facies were deposited.The deposits may have originated in a lagoonal setting with inadequate links to the marine environment given the paucity of diversity in the bioclasts and the dominance of micrites.A severely stressed environment in shallow, confined locales, with possibly large salinity and temperature changes, is indicated by the foraminifera's low biotic diversity.

Figure 2 :Figure 3 :Figure 4 :Figure 5 :
Figure 2: Stratigraphic section showing the microfacies description and depositional environments of Mishrif Formation at Tuba Oilfield in well TU-5.

Figure 6 :Figure 7 :
Figure 6: Stratigraphic section showing the microfacies description and depositional environments of Mishrif Formation at Zubair Oilfield in well ZB-42.

Iraq Ayhem A. Dawood, and Salam I.M. Al-Dulaimi units
Al-Dulaimy et al. (2022)(2019)properties set under oil-water contact in the study area.BAR2 and BAR3 have to include intervals of oil-bearing.Altameemi & Alzaidy (2018)Studied were done on the Mishrif Formation's sedimentological and reservoir properties at the Noor Oilfield in Southern Iraq.Al Jawad & Tariq (2019)selected wells from the North Rumaila field that showed the Mishrif Formation's reservoir characteristics.Bareh (2019)a 3D integrated geological model has been carried out of the Mishrif Formation in the Tuba field, south of Iraq, deduced that the Facies model indicates the rudist biostrome and shoal facies associations form the essential oil-bearing units in the formation.Chafeet, Hussein et al. (2019)at the Faiha and Sindibad Fields in South Iraq, researchers looked at the microfacies, depositional environments, and diagenetic processes of the Mishrif and Yamama Formation.Al-Dulaimy et al. (2022)Studied the Biozosnation (benthic foraminifera) of the Mishrif Formation at Majnoon and Zubair Oilfields, Southern Iraq.