Iraqi Geological Journal

Abstract


Introduction
Important hydrocarbon reservoirs are found in the Cretaceous carbonate succession in many areas of the Arabian Plate, including Central and Southern Iraq.Some of these reservoirs are found in the Mauddud Formation, which is dispersed throughout many oil fields (Sadooni and Al Sharhan, 2003).Owen and Nasr (1958) characterized the Mauddud Formation from Zubair well No. 3 in Southern Iraq, where it is composed of pseudo-oolitic creamy layers of shale ranging in color from green to blue, they identified the following fossils.Boggia sp., Trocholina sp., Praealveolina sp., Iraqia simplex sp., and Archaeolithothamnium sp.The Mauddud reservoir, part of the Khabaz oil field, is regarded as one of the principal carbonate reservoirs in northern Iraq (Ahmed and Hamd-Allah, 2021).Bellen et al. (1959) characterized this formation in Central Iraq from the Falluja-1, Awasil-5, Makhul-1, and Makhul-2 wells.More recently, Mauddud described it from the Ahdab, and East Baghdad fields in these regions.Formation is made up of organic detrital limestone with a marly matrix and recognized the following foraminifera Trocholina altispira,Trocholina arbica, Trocholina lenticularis, Orbitolina cf.concava, Rabanitina basranesis.The Mauddud Formation is made up of orbitolina-bearing limestone and dolomites that were formed in the Albian-Early Cenomanian sequence age.According to Sadooni and Al sharhan (2003), the facies in the formation vary in thickness due to eroisional truncation.The depositional environment, according to Cross et al., (2010) was a shallow marine ramp setting with low to high energy.And according to Faisal and Mahdi, (2020) the facies model of Mauddud Formation shows the dominance of open marine facies in the upper and middle parts of the formation, whereas mid-ramp facies occupies the lower part.The shoal facies represents approximately continuous units in their wells which are studied in Badra oil field.The lower contact of the Mauddud Formation is conformable and gradational with the Nahr Umar Formation, whereas the upper contact of the formation is unconformable with the Ahmadi Formation.It is composed of neomorphosed and dolomitized limestone (Shubber,1986) While in northern Iraq, the Qamchuga Formation, an equivalent of the Mauddud Formation, has been given a distinct stratigraphic description (ponikarov et al.,1967) (Fig. 1).Mauddud Formation produces oil from limestone units; it represents the main reservoir in the Badra oil field (Faisal and Mahdi, 2020).Hameed and Faisal, (2023) studied Petrophysical Properties of Mauddud Formation in Selected Wells in Al-Ahdab Oil Field , Middle Iraq and founded that the formation was divided into five units, two of them were considered good reservoirs having good petrophysical properties (high porosity, Low water saturation, and low shale volume) and the other three are not reservoirs because of poor petrophysical properties.Ahmed and Hamd-Allah (2021) researched the geological model for Mauddud Reservoir in Kirkuk, northern Iraq's Khabaz Oil Field.The reservoir in their study is oil-bearing with the original gas cap, and they are concerned with its geological modeling.By defining the facies, evaluating the petrophysical characteristics of this complicated reservoir, and estimating the amount of hydrocarbon.Depending on the variations in petrophysical properties (porosity and permeability), the reservoirs were divided into four zones.By analyzing the formation's biostratigraphy in the Badra oil field in Eastern Iraq by Al-Yassery (2015) and Norri et al., (2016) and in the Ratawi oil field in Southern Iraq by Ezzulddin and Ibrahim (2022), they identified three biozones of benthonic foraminifera, and the age of the formation is determined according to these biozones to be Late Albian-Early Cenomanian.This study focuses on the biostratigraphy of the Mauddud Formation in the East Baghdad oil field.It identifies the foraminifers' zonation and determines the formation's age in the examined section using assemblages of microfossils (foraminifera and algae).

Stratigraphy and Tectonic setting
The study area is located in the Mesopotamian zone, the eastern most unit of the stable shelf; it was likely uplifted during the Hercynian deformation but began to subside from the late Permian period onward.The zone contains buried faulted structures beneath the Quaternary cover that are separated by broad synclines.The Mesopotamian zone is divided into three subzones: the Zubair, Tigris, and Euphrates subzones, which are then acknowledged in the northern Gulf of Iraq and Kuwait, the Eastern Arabian Peninsula of Saudi Arabia and Bahrain, as well as the Southern Gulf of the United Arab Emirates and Oman (Fig. 1), (Jassim and Goff, 2006).The Mauddud Formation was first described by Henson in 1947 from the subsurface section of Dukhan well No. 1 in Qatar, where it gets its name from the town of Ain Mauddud, which is close to Dukhan, Qatar, the definition was revised and amended by (Sugden ,1958;Buday. 1980).The Nahr Umr, Jawan, Rim Siltstone, Qamchuqa, and Balambo Formations, as well as the Mauddud Formation carbonate succession, were all assigned to the Albian-Early Turonian sequence (Wasia Group) (Wasia Group) during the Cretaceous period (Jassim and Goff, 2006).Southeast Iraq's Mauddud Formation was the subject of an investigation by Al Siddiki in 1978, which concentrated on the most important microfossils, lithology, and depositional environment.He assigned the age of formation as Cenomanian and used an isopach map to describe the distribution and direction of the increase in thickness and the depositional basin.Although the upper contact of the studied Formation is marked by a break and is either nonsequential or unconformable, it is an unconformity in the north-central, north, and northeast parts of Iraq, while the formation may be lost locally in northwest Iraq as a result of Cretaceous erosion.The lower contact this formation is conformable and gradational with Nahr Umr, Lower Balambo, or Lower Sarmord Formation (Jassim and Goff, 2006).The formation was originally believed to extend into the Cenomanian because of the frequent occurrence of some species of Orbitolina concava group (Bellen et al., 1959).During his research of Cretaceous rocks in numerous wells in South Iraq, (Brun, 1970) listed various microfossil species in the Mauddud Formation, but the fauna of theformation in West Iraq is only of Albian age (Catroky andKarim, 1981in Jassim andGoff,2006;Jassim et al., 1984).Mohammed (2012) discovered and described a new species of Orbitolinid foraminifera in the Mauddud Formation related to the Late Albian period.This formation is Albian in age, according to Ibrahim, (1981).The Mauddud Formation's age was revised and amended by (Sayyab and Mohammed, 1984;1985) to Albian-Early Cenomanian due to the existence of a group of fossils with a narrow stratigraphic range.(Mohammed, 1996) established the biostratigraphy of the Orbitolinids in the Mauddud Formation and dated the formation by studying Orbitolinids (foraminifera) from the Lower Cretaceous (Barremian-Turonian) of Iraq, and gave the age of the formation Early Albian-Early change, and it is composed of limestone, dolostone, and dolomitic limestone with unique marl interaction (Al-Qayim et al., 2010).Al-Khersan, (1973) studied the fossils present in this formation and discussed the biostratigraphy and paleoecology of the formation.This formation was deposited over the Nahr Umr Formation during transgressive cycles, and the final part of regressive cycles, which are separated by transitional cycles (Al-Khayat and Razoian, 1978).A ramp bordered by shoals and rudist biostromes depicts the depositional setting of the Mauddud Formation in the Arabian plate (Cross et al., 2010).Albian carbonates in northern Iraq have a thickness of 400 meters, and the formation is 50 to 250 meters deep below the surface (Jassim and Goff, 2006).In their study of the depositional environment of the Mauddud formation done by Al-Dabbas et al. (2012), they note that the sedimentary microfacies of the formation include dolostone lithofacies and green shale lithofacies in addition to lime mudstone, wackestone, and packstone.In their study of the microfacies interpretation and deposition environment for the Mauddud Formation in the Rataw oil field, Hameed andSaleh, (2021, 2022) concluded that the formation was deposited in five different types of environments: outer ramp, mid ramp, inner ramp, restricted, and shoal.In his investigation of the microfacies of the Mauddud Formation from the subsurfaces of southern and southeastern Iraq, (Mohammed, 1981) divided the formation into six units based on the fossils that made up the formation.At the Badra oil field in Central Iraq, Fasial and Mahdi (2020) examine the diagenetic processes over the print and pore types of the Mauddud Formation.The Albian portion of the Sarvak Formation is equivalent to the Mauddud Formation in southwest Iran, The neritic dolomitic limestone of the upper portion of the Aasafir Formation in central Syria and the Palmyrides is equivalent to the Mauddud Formation (Jassim and Goff, 2006).The Mauddud Formation declines in Southeast across the Anah Qalat Dizeh fault as the formation changes into the basinal limestones of the Balambo Formation,the Mauddud Formation outcrops in western Iraq on the southeast side of the Hauran Anticlinorium (Bellen et al., 1959).

Material and Methods
The study area is located in the East Baghdad oil field, which is about 20 kilometers from Baghdad's city center and is over 120 kilometers long and between 20 and 30 kilometers wide.Table.1 Fig. 2; Oapec, 1987).Three wells have been chosen for the study that are distributed along the anticline structure and oriented NE-SE of the East Baghdad oil field.In this study, a subsurface section of the Mauddud Formation was selected for investigation.290 samples of core and cutting were collected from the studied area, with typical sampling intervals of 2 to 5 meters.Thin sections were prepared from each sample for fossils identification.Benthic foraminifera are often well preserved and abundant and it is used to classify the studied interval into three biozones based on Brun (1970); Al-Khersan, (1973); Al-Siddiki, (1978); Mohammed (1981Mohammed ( ,1992Mohammed ( , 1996Mohammed ( , 2017)); Sampo, (1969); Sugden and Standring, 1975;Peybernes,1976;Sayyab and Mohammed, 1984;Bozargani,1964;Leoblich and Tappan, (1988); Satorio and Venturina,(1988)

Biostratigraphy and Biozone
Biostratigraphy is the study of the relative ordering of strata based on their fossil content.Descriptive biostratigraphy is used in determination zones for regional or local stratigraphic correlation (Keller, 2004).The Mauddud Formation, which consists of limestone and dolomitic limestone, has a trend to be marly in some areas.The Mauddud biostratigraphy Based on the distribution of benthic foraminifera and associated fossils, the formation of three subsurface wells, whose thickness varied from Occurrence: This zone is located in the Mauddud Formation's bottom portion and measures approximately 60 meters in thickness in EB.55, 71 meters in thickness in EB.58, and 64 meters in thickness in EB.59 (Figs. 3, 4, and 5).

Remarks and correlation:
Index fossils identify this zone as Orbitolina qatarica.The taxonomic genus related to this taxon includes: Mesorbitolina aculata, Trocholina alpina, Nezzazata conica, and Nezzazata simplex, Textularia sp., Valvulina sp., Quinquiloculina sp., Other specimens include Pseudochrysalidina conica, Praealveolina sp., Trocholina arabica, Chrysaldina qradata, Cuneolina pavonia, Acicularia sp., Acicularia antiqua, Permocalculus sp., Permocalculus irenae.etc..According to several researchers, some of these occurrences have been reported (Table 2): (Sampo, 1969) found that this biozone refers to the Albian age in Iran.(Loutfi and Jaber, 1970) mentioned Orbitolina qatarica designated Albian age from Saudi Arabia and Kuwait in the off-shore region, and (Sugden and Standring, 1975) described the zone as the origin of Qatar's Albian rocks.The index species Orbitolina qatarica designated Late Albian age in many parts of the world: Lawa and Gharib (2014) characterize this biozone as belonging to the Late Albian period and are found in the upper Qamchuqa Formation of North Iraq's Zagros fold-thrust belt.This biozone is Late Albian, according to Schröder (1975) and Mohammed (1996), Al-Yassery (2015), Norri et al., (2016) in eastern Iraq, and Ezzulddin and Ibrahim (2022) in south Iraq.Berthou, 1984 dated the Orbitolina qatarica from Portugal as Late Albian-Early Cenomanian.Henson (1947) first discovered Orbitolina qatarica in the Early Cenomanian of Qatar, which was identified in German as Early Cenomanian by Schröder (1962).According to Scott (2010), this zone was discovered in the Early Cenomanian of China.Simmons and Williams (1992) describe the biozone of the Middle Cenomanian Zone in the Middle East.This research uses the Orbitlina qatarica biozone, which dates from the Late Albian.

Orbitolina Sefini Range Zone
Definition: This zone is described as beginning with Orbitolina sefini and ending with the first appearance of Orbitolina concava.

Remark and correlation:
The faunal characterized this zone, Nezzazate simplex, Nezzazata conica, Leoblich and Tappan (1988) stated that Orbitolina concava was found in the Albian-Cenomanian rocks of France, Spain, and Qatar.While (Schröder, 1975) and (Mohammed, 1996) determined this zone is of Early Cenomanian age.The species Orbitolina concava was discovered in strata dating to the Cenomanian age in all of the following nations: Iran (Sampo, 1969), Iraq and Qatar (Henson, 1947), Southern France (Dufaure, 1959), Germany (Schröder, 1962), Italy, Switzerland, and Spain (Ellis and Messina, 1966), Zagros (Satorio and Venturina, 1988), Chia (Zhang, 2000), Qamchuqu Formation in the north of Iraq (Lawa andGharib, 2014), Eastern Iraq (Al-Yassery, 2015) In this study this Orbitolina concava biozone refers to the Early Cenomanian age.The presence of an abundance of fauna, particularly benthic foraminifera and algae, suggested a shallow marine environment, represented by a shoal environment, which indicated the ideal environment for Orbitolina.It also suggested that the decreasing distribution of orbitolina was a result of rising salinity (Lozo, 1944;Cobban, 1957in Douglass, 1960).Orbitolina's environment can be characterized as clear, warm, shallow water with normal salinity that thrives in a shoal environment.The depositional environment of the Mauddud formation in the examined wells occurred in a shallow marine environment represented by a shoal environment, which designated the optimum condition for Orbitolina, and the reasons for the poor distribution of Orbitolina as a result of increasing ocean acidification.Comminuted rudist clasts, echinoderm grains, benthic foraminifera (such as Orbitolina sp., Textularia sp., Nezzazata sp., and miliolids), and other findings point to re-sedimentation that was likely caused by turbidity currents on the slope.Foraminifera and mixed algae both support this.The fact that they were moved from a shallow-water environment to the outer shelf in front of shoals is further supported by the abundance of conical Orbitolina (Esfandyari et al., 2023).Large rudist grains that exhibit well-preserved interior structure and are occasionally bored, along with tiny rudist grains.Rudists (average 15%) occasionally coexist with small bioclastic grains in a mud-supported matrix and are encircled by a lime-mud matrix.More frequently found are large benthic foraminifers (such as Orbitolina), oysters, and fauna from lagoons such as miliolids, Textularia, and Nezzazata (Esfandyari et al., 2023).The rudist fragments are poorly sorted and comminuted with evidence of burial compaction.Large rudist grains imply a high energy level above the Fair Weather Wave Base (FWWB) in an open marine setting (Wilson, 1975).Morphological variation of Orbitolina is an indicator of environmental condition, particularly, the water depth (Simmons et al., 2000).Elongate and smooth shells indicate a middle shelf setting (Reiss and Hottinger, 1984;Grafe, 2005).

Conclusions
The Mauddud Formation carbonate succession is a significant formation made up of limestone, limestone that has undergone dolomitization, and marly limestone in some areas.The formation's lower contact is conformable with the Nahr Umr Formation, while its upper contact is unconformable with Ahmadi Formation.Investigations have been made into the creation of three subsurface wells, whose thickness ranged from 184.4 to 215 meters.The foraminiferal and algae fauna from these wells contained 20 species that belonged to 15 groups.Based on benthic foraminifera, the Mauddud Formation was divided into three biozones: the Orbitolina concava range zone, the Orbitolina sefini range zone, and the Orbitolina qatarica range zone.These foraminiferal biozones provide information on the formation's age (Late Albian-Early Cenomanian).The depositional environment of the Mauddud formation in the examined wells occurred in a shallow marine environment represented by a shoal and lagoonal environment.

Fig. 3 .
Fig.3.Biozone and Biostratigraphic distribution of Foraminifers and Skeletal grain of Mauddud Formation in well EB-55.

Table 1 .
Coordinates of three selected sections of the research region and thickness with the top and bottom of the formation