Biostratigraphy of Yamama Formation in Faihaa Oil Field, Southern Iraq

The Yamama Formation was studied in three wells (Fh-1, Fh-2, and Fh-3) within Faihaa oil field, south Iraq. Thin sections were studied by using the polarizing microscope examination in order to determine microfossils and biozone. Thirty-five species of benthic foraminifera were recognized, including four index species. In addition, twelve species of calcareous green algae were recognized, including two index species. Other fossils that were recognized in Yamama Formation include Gastropoda, Bryozoa, Coral, Rudist, and Pelecypoda. Six biozones were observed, which are Charentia cuvillieri sp. (Range Zone of Berriasian age), Psudochryalidina infracretacea sp. (Range Zone of Berriasian age), Pseudocyclammina Lituus sp. (Range Zone of Valanginian age), Nezzazata Perforate sp.andChoffatella sp.(Assemblage Zoneof Berriasian-Valanginian age), Desycladales Green AlgaeCylindroporella sp. (Range Zone of Early CretaceousBerriasian age), and Desycladales Green AlgaeSalpingoporella cf. circassa sp. (Range Zone of Valanginian age). According to these biozones, the age of Yamama Formation was distinguished to be the Berriasian-Valanginian.


Introduction
Yamama Formation is one of the most important oil production reservoirs in the southern Mesopotamian Zone, which belongs to the Early Cretaceous (Berriasian-Valanginian) sequence. The Early Cretaceous succession (late Berriasian-Aptian) extends, from the shore to deep basin, by the Zubair, Ratawi, Yamama, Shuiaba, and Sulaiy formations. It includes important carbonate reservoirs in southern Iraq, namely in the West Qurna, North Rumaila, and Majnoon fields [1]. The lower and upper contacts of Yamama Formation are conformable with Sulaiy and Ratawi formations,
The previous studies of Yamama Formation subdivided the formation into different units based on lithological variations and cyclicity. An earlier study [3] divided the Yamama Formation into six units and combined Yamama/Sulaiy Formation as a peloidal limestone overlain by the Ratawi Formation. Another work [4] divided the Yamama Formation into six depositional cycles representing a general regressive sequence. In the southern part of Iraq, Yamama Formation was subdivided into two upper and lower members or reservoir units separated by barriers [5]. Biostratigraphy and microfacies of Yamama Formation in the selected wells, southern Iraq, were also investigated [6]. The biostratigraphic studies of Yamama Formation were mainly based on benthic foraminifera fossils. The current study used both benthic foraminifera and green calcareous algae in the biostratigraphic zonation and age determination.

Location of the study area
The Faihaa oil field is located in Basrah Governorate, 20km south-east Basrah city, and extends across the border with Iran (Figure-1). The distance between wells Fh-1 and Fh-2 is 6.606 Km, while that between wells Fh-1-and Fh-3 is 6.075km ( Figure.2).

Al-Hassani and Al-Dulaimi
Iraqi Journal of Science, 2021, Vol. 62, No. 5, pp: 1570-1586 1573 Faihaa oilfield is located in the east of the Mesopotamian Basin in the Zubair subzone that is characterized by a subsurface geologic structure covered by sediments from the Quaternary. This zone is bounded by two faults, the first belongs to Al-Batin fault zone, whereas the second belongs to the Najd fault system between Ramadi-Musaiyib fault zone in the SW and Tikrit-Amara fault zone in the NE [7]. The collision of the Arabian-Iranian plates generated regional compression effect that produced NW-SE trending fold structures. The effect of folding extends along the NW-SE regional trend of the Zagros fold -Thrust Belt. The Figure.2 Structural contour map at the top of Yamama Formation in Faihaa oil field with the locations of the selected wells [8]. structure of Faihaa oil field belongs to this trend, which represents an approximately N-S anticline with three structural closures ( Figure.2).

Al-Hassani and Al-Dulaimi
Iraqi Journal of Science, 2021, Vol. 62, No. 5, pp: 1570-1586 1574 The Yamama Formation was deposited during the Lower Cretaceous period within the main retrogressive depositional cycle [1] The shallow water carbonates of Yamama Formation covered large areas in the eastern shelf platform of the Arabian plate, and their depositions were affected by a moderately high, but falling, eustatic sea level [9]. The formation is assigned to the Berriasian-Valanginian age [3]. The Thickness of the Formation is up to 400m [2], while its maximum thickness in the study area reaches 324m at well Fh-1 ( Figure.3).

Biostratigraphy of Yamama Formation at Well Fh-2
The following microfossils are identified in the core samples of Yamama Formation at Fh-1 (Figure.

Biozones of Yamama Formation at Well Fh-1
The Biostratigraphy zones of the current study depend on benthic foraminifera and calcareous green algae. The definitions of these biozones were achieved based on the stratigraphic distribution of these many types of fauna (Figs. 4, 5, and 6), with the following six biozones being distinguished.

Charentia cuvillieri sp. Range Zone
This biozone was identified depending on the range of extension of the species. The zone was determined based on the first and last occurrence of the species Charentia Cuvillieri in well Fh-1. This species is found in Yamama Formation (Figure. 4) and the thickness of this Biozone is 72m at Fh-1.

Psudochryalidina infracretacea sp. Range Zone
This biozone was identified depending on the range of extension of the species. The zone was determined based on the first and last occurrence of the species Psudochryalidina infracretacea in well Fh-2. This species is found in Yamama Formation (Figure.5), with the thickness of this Biozone being 197.2m at Fh-2.

Nezzazata Perforate sp.-Choffatella sp. Assemblage Zone
This biozone is defined by three or more different taxa, which may, or may not, be related to each other. The boundaries of the assemblage zone were defined by the occurrence of the typical specified fossil assemblage. This can include the appearance, but also the disappearance, of certain taxa [16]. The occurrence of Nezzazata perforate sp. and Choffatella sp. was recorded. The assemblage zone have the same trend of appearing and disappearing taxa in almost the same depth in well Fh-1 ( Figure.4).