New records of planktonic foraminifera in the Shuaiba Formation (Aptian Age), Mesopotamian plain, South of Iraq

Shuaiba Formation is an important formation in Iraq, because of their deposition in the important period during the geological history of Arabian plate. The study is focused on a number of selected wells from several fields in southern Iraq, despite the many of oil studies to Shuaiba Formation but it lacks to paleontological studies. Four selected wells are chosen for the current study, Zb-290, Ru-358, R-624, WQ1353, the selected wells are located within different fields, these are Zubair, Rumaila and West Qurna Oil Fields. In this study fourteen species followed to genus Hedbergella were discovered for first time as well as three genera followed to genus Heterohelix in the Shuaiba Formation at the different oil fields, Hedbergella tunisiensis Range Zone is suggested biozone to the current study, the age of this biozone is Aptian, most of the other genera located within this zone.


Introduction
The study was interested in an important age of geological history of Iraq, because of most of the Iraqi oils formed in that period. Although the Shuaiba Formation is important in the oil studies, the paleontological studies are rare. The aims of the current study include the identification of the microfossils of Shuaiba Formation, and determination the age of the formation. Due to the lack of Shuaiba Formation to the production of the oil, the formation was not studied in detail and were not taken core samples only cutting samples. Therefore, the field contains only a few identified genera. Therefore, this study focused in detail to the planktonic foraminifera. The identified species in the
Iraqi Journal of Science, 2019, Vol. 60, No.6, pp: 1322-1335 1323 current study added new information about the Aptian age and added new biozone depending on planktonic genera. Three oil fields were selected to cover the largest area of southern Iraq.

Study Area
Four selected wells are chosen for the current study, Zb-290, Ru-358, R-624, WQ1-353 (Table-1) ( Figure-1), the selected wells are located within different fields, these are Zubair, Rumaila and West Qurna Oil Fields. The structural trap of Zubair oil field is a large gentle anticline oriented north/northwest to south/southeast approximately 60 km long and 10-15 km wide. Zubair Field consists of four domes divided by saddles. Rumaila Structure is longitudinal semi symmetrical anticline, the West flank angle is 3.50 and the East flank angle is 2.50. The length of the structure is 41 Km and the width 12 Km. the direction of the structure axis is North West-South East. The importance of producing formations are Zubair, Nahr Umr and Mishrif. West Qurna Field was discovered in the early 70s [1]. The seismic survey confirmed the existence of a third dome on the Rumaila anticlinal axis, separated from North Rumaila by a shallow saddle. This northernmost dome is called a West Qurna, it was drilled by INOC in October 1973.

Methodology
Based on [2], the extract of the microfossils with simple modified for studied case. Steps for planktonic foraminifers' extraction from carbonate rock by preparation Acetic acid (CH 3 COOH), Hydrochloric acid (HCl) and Hydrogen peroxide (H 2 O 2 ), the steps are: a. Take 100 g of drilling cutting and place it in the baker. b. Sunk the sample in diluted Acetic acid (75% CH3COOH, 25% H2O) for 8 hours within a beaker. After 8 hours, wash the sample with distilled water. c. Add diluted hydrochloric acid (10% HCl, 90% H2O) to the sample for 2 hours within a beaker. After 2 hours, wash the sample with distilled water.  f. Dry the sample and put it in an oven at 80 ° C, and prepared it for picking. g. The microfossils were picked by the binocular microscope, and then photting them with digital camera type in the geology department, University of Basrah. .

Stratigraphy of Shuaiba Formation
The Shuaiba Formation was first defined by Owen and Nasr in 1958, The Shuaiba formation was deposit during the lower Cretaceous (Aptian). The thickness of Formation is 50-100m. (Figure-2). The formation is comprised of massive limestone and it is grading into chalky limestone with shale in the top, it was deposit in an open marine environment [3]. Shuaiba Formation overlies of Zubair formation with conformable and gradational contact, while the upper contact is unconformable with Nahr Umr Formation. The lithology and fossils of formation contain pseudo-oolitic limestone and Orbitolina cf. discoidea Gras, Choffatella decipiens Schlumberger and globogerinids [4].

Classification
The present study introduces the classification of many genera for first time, 14 genera belong to Hedbergella, while 3 genera belong to Heterohelix. [5] classification was used for classifying the studied fossils.