Combined Ostracoda and Foraminiferal Biozonation with Environment of Fatha Formation (Middle Miocene) In Different Oilfields, Southern Iraq

Abstract


Introduction
The Fatha Formation is an important stratigraphic unit in the Middle East: it is the caprock to numerous oil reservoirs in Iraq and Iran (Tuker, 1999).Busk and Mayo (1918) are the first to describe the studied formation in Iran, exactly in the province of Fars with the name Lower Fars ( Van Bellen et al., 1959).According to Jassim et al. (1984) the Fatha Formation was proposed to replace the old name in Iraq, and select of Fatah area (Hemrin and Makhul anticlines) as a type locality for it.
The Middle Miocene of Fatha Formation is one of the most widespread formations in Iraq, extending from North to South of Iraq (Al-Juboury et al., 2001;Jassim and Goff, 2006).It is characterized by rhythmic nature (Al-Juboury and McCann, 2008).Each rhythm comprises two to five lithologies namely, greenish-grey marl, limestone, gypsum (and/or anhydrite), halite, and reddish-brown mudstone (with occasional sandstone) (Tamar-Agha et al., 2015).It has uncommercial criteria for petroleum and gas accumulation in some oilfields in central Iraq (Al-Jubury and McCann, 2008).Also, the current formation considers a cup-rocks for many important oilfields in Iraq.However, in northern Iraq, it includes a significant amount of Sulphur (Aqrawi et al., 1989).This study considered is the first detailed paleontological study in the South of Iraq for Fatha Formation, most of the previous studies focused on central and Northern Iraq, especially in the outcrops, but in Basrah oilfields, the studies are rare.The important previous studies are Abdol Rassul et al. (2001), Al-Asadi, (2002), Hawramy, (2013), Hawramy and Ali (2018), Al-Hadithi and Aziz, (2019) and Khalaf and Kharruffa,(2020).They are intensive Ostracoda studies in North of Iraq, while rare studies specifically related to Foraminifera.Mahdi, (2007) and Al-Abbasi et al., (2011) studied the bivalve in Northern Iraq.In addition to several essential stratigraphical studies such as Aqrawi et al. 1989, Al-Jubouri and McCann, 2008, Tamar-Agha et al., 2015and Sissakian et al., 2016., etc.This study mainly aims to identify the important fossils groups (Foraminifera and Ostracods) and then divided them into local biozone and correlated this biozone with other studies.Also, to determine the environment of the formation, this study is first at Southern Iraq that used of core samples, in addition to identifying new species in the formation.

Materials and Methods
The study area is located in selected wells at different oil fields in the south of Iraq.These are three wells in the Nahr Umr oilfield, one well in the Zubair oilfield, one well in the West Qurna I, two wells in the North Rumaila and one well in the South Rumaila oilfield (Table .1), all of them having heavy crude oil in the Fatha Formation.The current study is located between longitude lines 47°14'5" -47°81' 24" and latitude circles 30°15'8"-31° 12' 42" N) (Fig. 1).Preparation of 100 core samples (70 samples of Nahr Umr, 30 samples of West Qurna) and 100 cutting samples (20 of North Rumaila, 20 of South Rumaila and 20 of Zubair, 20 of Nahr Umr and 20 of West Qurna) for picking to identify the critical fossils.First, dried samples, weighing about 50 g.For the picking process, the best procedure was used to extract the fossils according to Karimina et al. (2003) method, but it was modified by Al-Shawi et al. (2019).For hard limestone, hydrogen peroxide (15%) was used for 24 hours, and then wet sieving was used by a 63-micron sieve.Then, the total fossils content of samples was handpicked and counted, using flat black trays, a sable brush and a stereomicroscope.After the extraction of the fossils, they were saved in a micropaleontological slide.Two groups were identified: Foraminifera and Ostracods.Foraminifera were classified depending on Loeblich and Tappan (1988), while Ostracoda was classified according to Moore and Pitart (1961), Morkhoven (1963), andHartman andPuri (1974).

Geological Setting
The Fatha Formation was deposited in the northwest-southeast oriented basin, which extended from Syria, Iraq into Iran.Fatha Formation was associated within the megasequence of Late Eocene-Recent AP11 (Sharland et al., 2001).Megasequence AP11 is associated with the collision of Neo-Tethys terrains along with the N and E sides of the Arabian Plate, also with the opening of the Gulf of Aden and the Red Sea on the S and W sides of the Arabian Plate.The opening of the Red Sea and the Gulf of Aden was associated with thermal uplift, flood basalt, and rifting during Early-Miocene (Makris and Henke, 1992).The N and NE drift of the Arabian Plate and the closure of the Neo-Tethys led to folding and thrusting along the NE margin of the Arabian Plate.The megasequence contains many formations, the Early-Mid Miocene sequence can be divided into two-second order sequences, varied with shallow water carbonates passing up into evaporites.These sequences are the Early Miocene and the Mid Miocene.The formations previously included in the Early-Mid Miocene Sequence include the Asmari, Euphrates, Serikagni, Dhiban, Kalhur Gypsum, Ghar, Jeribe, and Fatha formations (Bellen et al., 1959;Sharland et al., 2001).
The Miocene age has an important regional maximum flooding surface, the third MFS; Ng 30 is present in the middle of the Burdigalian Stage.It presents the contact between the Jeribe and Fatha formations.While the fourth MFS; Ng 40, exists in the middle of the Serravallian Stage and is marked by the contact between the Fatha Formation and the overlying Injana Formation, this stage witnessed a major change in the depositional environments from marine to continental (Sissakian et al., 2016).Tucker and Shawket (1980) stated that the absence of angular unconformities in Iraq's Tertiary sediments (only disconformities) is thought to support the hypothesis of vertical block movements and their participation in the formation of the Fatha depositional basin.The area is flat, which buried longitudinal structures of differing sizes below the Quaternary cover, separated by large synclines.The trending fold structures have an effect on the pre-tertiary layers, which are caused by the basements and faults (Al-Atabi, 2014).
The diapiric salt produced by the Infracambrian Hormoz salt series, which is thought to be underlying areas of Iraq, is most likely responsible for these structures.Negative gravity residuals' relationship with important Zubair and Nahr Umr oilfields confirms these salts (Ditmar, 1971).Structurally, the current study is located within the Zubair Subzone, it is Iraq's greatest oil-producing area and the southernmost subdivision of the Mesopotamian Zone (Al-Kaabi et al., 2023).According to Fouad (2015), the studied area is located in the Outer Platform, which is the main part of the Mesozoic Arabian plate's passive margin of the Foreland basin, exactly at Mesopotamia Foredeep and is greatly influenced by the Alpine orogenic deformation (Fig. 2).The conformable structural arrangement of the oilfields reflects the regional effect of these salts in Iran and the Arabian Gulf countries, and their comparison is very important in hydrocarbon prospecting (Christian, 1997).(Fouad, 2015)

Results
The studied fossils focused on two types of fossils, these are Foraminifera and Ostracoda (Tables 2 and 3).

Foraminifera
Many species of foraminifera have been classified in this study based on the Loeblich and Tappan classifications (1964).42 species belong to 21 genera of Foraminifera have been identified, among which 6 genera belong to 3 families (Hauerinida, Spiroloculinidae, Miliamminidae), 14 genera to 8 families (Rotaliidae, Elphidiidae, Discorboidea, Nonionidae, Rosalinidae, Cibicididae, Cymbaloporidae Gavelinellidae) and one genus to Family Globigerinidae, the detailed data for systematic study was summarized in Table .2, with their photos (Plates 1 to 6).Several of identified species has characterized environments, such as:

Elphidium craticulatum
It is a public species that occur in the low tidal, and also existed in shallow subtidal with depths not exceeding twenty meters, it prefers the pure sand in normal marine salinity (Hayward et al., 1997). .

Ammonia beccaria
It is the most frequent species in the study area after Elphidium, and it is distinguished by its occurrence in a variety of environments, which reflects its capability to live in a wide range of conditions.Ammonia species are found from the subtidal to the outer continental shelves (Schweizer and Nikulina, 2011).Ammonia baccarii (Plate 3-1, 3-1A, 3-2, and 3-2A), A.parkinsoniana (Plate 3-4, and 3-4A) and A. tepida (Plate 3-6, and 3-6A), these species are indicative of the brackish environment, living under conditions with temperatures between 15 -30° C and depths not exceeding 50 m (Murray, 1969).

Ostracoda
Forty species related to thirty genera of Ostracoda have been identified in this study, depending on the classification of Moore andPitart (1961), Morkhoven, (1963);and Hartman and Puri, (1974), two genera belong to family Cytheridea, three genera to family Leptocytheridae, two genera belong to family Cytherellidae, four genera belong to family Cytherideidae, one genus belong to family Cytherettidae, one genus belong to family Krithidae and finally one genus belong to family Trachyleberididae, the detailed information about classification illustrated in Table 3; Plates 7 to 11.On the basis of Ostracoda that recorded from the Fatha Formation in the study areas.Many genera were identified for Ostracoda in this study which are indicator environment such as:

Callistocythere
This genus indicates shallow water environment Khalaf, 1984.Species of this genus are predominantly surface dwellers on sandy mud, sand or algae from the littoral to eulittoral zones (Manh and Tsukagoshi, 2015) (Plate 7-9).

Leptocythere
Some species of this genus typically occur in estuarine (brackish water) environments, while others are mainly found in shallow marine (littoral) environments (Morkhoven, 1963).It is abundant and widely distributed across the shallow seawaters stretching from the tropics to the subarctic zone, including the water (Shurupova and Tesakovab, 2021) (Plate 7-8).

Biozonation
Biostratigraphic zones were conducted based on the results of the identified foraminifera and Ostracoda genera depending on thin sections and picking samples, which covered the Middle Miocene in the study area.In order to summarize the results of the current study, all the biozone trends of the identified fossils have been placed within one table for all the studied wells, because most of the extending fossils are located in each well somewhat similar, so making repeated tables for each well will increase the pages of the article.Therefore, the biozones were illustrated in the Figs. 3 and 4; Table 4.It is worth mentioning, that most of the vertical extensions of some genera are missed because of the deposition of evaporite and clastic beds.After plotting the extensions for each fossil, the study suggested these biozones for foraminifera and Ostracoda.

Geological Age
There is a great deal of controversy regarding the age of the Fatha Formation, and many studies have been presented in order to determine the most appropriate age for the studied formation.The process of determining ages using planktonic foraminifera is more effective and accurate than benthonic foraminifera.Inopportunely, the current study is devoid of any planktonic genera, except one genus, therefore, the benthonic fossils were relied upon Bellen (1957) determine the Borlis melo is index fossil to the Middle Miocene.Also, there are several important index fossils to the lower and middle Miocene, such as Austrotrillina howchini, Peneroplis thomasi, Nummulites fichteli, Rotalia viennoti, Miogypsina sp.

Discussion
The current study was conducted in the Fatha Formation in the south of Iraq.Generally, the paleontological studies in Basra oilfields are rare, especially in the Fatha Formation.Most of the studies are related to formations that have petroleum accumulation.Fortunately, exploration wells were made for the Nahr Umr and West Qurna oilfields, core samples were taken for the Fatha Formation.Therefore, this study is important because it relies on confirmed depths, and those results have been strengthened and compared with cutting samples from the other oil fields.The results of the current study are different from the other, most of the foraminifera fossils are not recorded in the middle or northern Iraq, it missed the index fossils and common existence of Elphidium craticulatum, while this species is not recorded in other basins, it reflects the tidal environment, while the rest of the foraminiferal fossils were indicated on the lagoonal environment but this lagoon not isolated on marine water, it could be partially separated lagoon, because of the abundance of ostracods genera that reflect shallow marine environments.The salinity of the Fatha basin in the southern is less than that of the northern basins.The types of lithology and fossils emphasize that conclusion.

Conclusions
Forty-two species related to twenty-one genera of Foraminifera, forty Ostracoda species belonging to thirty genera were described from the Fatha Formation in the studied wells.Based on the fossil taxa found in this study, it was determined that the Fatha Formation was deposited in the lagoonal environment as the main basin but this basin is affected by tidal, subtidal and shallow marine water at the same time, especially in the lower part of the Fatha Formation, where a thin seaway connection remained open and they were in direct contact with the open sea.All genera recorded in the study area belong to benthic Foraminifera except Globigerina quinquelob belongs to planktonic Foraminifera.
The age of the studied formation is Middle Miocene depending on index fossils of ostracods, therefore, the current study determines one biozone for Foraminifera which is Elphidium craticulatum Total Range Zone.Also, one biozone for Ostracoda which is Schneiderella unispinata Assemblage zone, these biozones with age Langhian-Serravallian.

Fig. 1 .
Fig. 1.The location map for the study area, the green circles represent the studied wells at different oilfields

Fig. 3 .
Fig. 3. Biostratigraphic occurrence of identified Foraminifera for Fatha Formation in the studied wells, Southern Iraq

Fig. 4 .
Fig. 4. Biostratigraphic occurrence of identified Ostracoda for Fatha Formation in the studied wells, Southern Iraq

Table 1 .
The tops, bottoms and thickness of the Fatha Formation in studied wells, Southern Iraq

Table 2 .
The important identified Foraminifera with systematic trends that recorded in studied wells

Table 3 .
The Family, genera and species of Ostracoda that recorded in the present study

Table 4 .
Comparing of different local biozones of foraminifera with current study

Table 5 .
Comparing of different local biozones of Ostracoda with current study