BIOSTRATIGRAPHY OF KHASIB FORMATION BY USING PLANKTONIC FORAMINIFERA AT SELECTED WELLS IN RUMILA OIL FIELD, SOUTHERN IRAQ

Khasib Formation is one of the important formations in Iraq due to the wide spread and varying in thicknesses. In some oilfields like West Qurna, Khasib Formation represents a production reservoir.  In this study, the selected fossils at this formation are studied precisely and ignored some species. In Rumaila Oilfield, eight oil wells were taken and fifty-five samples from planktonic foraminifera were collected, analyzed and identified. Twelve species belong to seven genera were chosen and diagnosed accurately, these are Marginotruncana scheegansi, Marginotruncana renzi, Clavihedbergella simplex, Whiteinella archeocretacea, Praeglobotruncana algeriana, Globotruncana imbricate, Globotruncana sigali, Heterohelix Russi, Heterohelix globulosa, Planoheterohelix globulosa, Heterohelix navarroensis, Heterohelix moreman. Marginotruncana scheegansi Assemblages Zone was suggested to the studied section. It corresponds with the regional zone of sigal (1977). The results of the present study showed that the formation age is Turonian-Coniacian, and there is no evidence of the Campanian age of Khasib Formation as mentioned in the previous studies.


INTRODUCTION
Khasib Formation is located within L. Turonian-E. Campanian sequence. The formation represents a few studies because it does not contain hydrocarbons, most of the studies focus on oil-formations like Mishrif and Zubair. The purpose of this study is to extract fossils from Khasib Formation by picking methods, while the other studied identified the fossils by thin sections, the latest studies conducted for the region by Darmoian (1975). The previous studies of the formation are related to DOI:10.46717/igj.53.1D.5Rw-2020-05-04 petroleum fields such as: (Al-Khayat and Razoyan, 1979;Al-Hamdani, 1986;Aqrawi, 1996;Al-Fatlawi, 2000;Al-Shaoosh, 2002;Al-Ani, 2004;Al Shujairi, 2017). Khasib Formation has extensive spreading in the middle and south of Iraq. The lower boundary with Mishrif Formation is disconformable, whereas in contrast to the top it is conformable with Tanuma Formation in southern Iraq with the first appearance of shale beds (Jassim and buday, 2006). The corresponding formation to the Khasib Formation in north Iraq was the lower part of Kometan Formation, while Al-Ghadeer Formation in Kuwait and Aroma limestone in Saudi Arabia (Al Shujairi, 2017). The aim of the study is to determine the age of the formation accurately, after extract the index fossils that belong to planktonic foraminifera.

LOCATION OF THE STUDY AREA
The Rumaila oilfield considers a huge oilfield that placed in the south of Iraq, the first drilling started in the middle of fifties with cooperation between Basrah Petroleum Company, and Iraq Petroleum Company. Seventeen billion barrels have been expected to the field production, therefore, Rumaila oilfield could be the 9th biggest oilfield in the world (Alsharan and Narin 1997, Handhal and Mahdi, 2016). The field situated about thirty-two km from Kuwaiti border and about fifty km west of the city of Basrah is principle for production in Basrah city. Eight wells were selected for study based on their distribution in the field and on the availability of possible information that would benefit the study, as well as the availability of rock samples ( Fig.1) and (Table 1).

STRATIGRAPHIC SETTING
Khasib Formation (Late Turonian-Early Campanian) was first described by Rabanit, (1952) in (Bellen et al., 1959), where the type section of the formation is well Zb-3, located within the Mesopotamian region, which described by Owen and Nasr (1958). They mentioned that the lithology of Khasib is comprises double lithology, about 20 m of the formation consists of grey shale at the bottom, while the other part about 30 m comprises of grey argillaceous limestone (Bellen et al., 1959).   Khasib Formation is the lower part of Late Turonian-Danian Megasequence AP9 which is split into two second-order sequence by Sharland et al., (2001). Mishrif Formation represents the unconformable regional surface with Khasib Formation resulting from the uplift erosion of Early Turonian. The top border is the conformable surface with the upper Cretaceous Tanuma Formation.
From the shallow marine limestone to deep shelf of layering between limestone and shales of Khasib and Tanuma formations, there is a long period of depositional takeoff during the early Turonian.
Fatlawi (2000) through her study on the fossils in the Khasib Formation was attributed to the Upper Turonian, and was considered the first formation of the secondary sedimentary cycle (Turonian-Lower Campanian), which includes the Khasib, Tanuma and Saadi formations which are affected by single, high-repetitive post-hydrostatic movements at regular intervals, resulting in a change in the distribution and irregularity of the movement. This secondary cycle returns to the main cycle (Cenomanian-Lower Campanian) (Buday, 1980) and (Al Shujairi, 2017).

METHODOLOGY
Rock cutting samples were taken from the studied sections every 10 m in case of rock homogeneity, but in case of change in the rock lithology, the samples are taken for each 5 m, 56 samples are the total samples from South and North Rumaila Oilfield. Due to the lack of Khasib Formation to the production of the oil at this field, the formation was not studied in detail and were not taken core samples only cutting samples. 100 gm from cutting samples were taken and then added the distilled water 400 ml and then placed on the hot plate with the addition of the 5 gm sodium hydroxide for dispersion, then wash the sample and sieved by 63 micron sieve after drying of the sample.
Subsequently, the fossils are packing by brush (000) under the normal microscope and then moved to the slide container for saving the fossils.

SYSTEMATIC DESCRIPTION
The systemic study is depended on Loeblich and Tappan's classification (1964) and Debenay  chamber globular to sub globular increasing slowly in size, aperture is an interiomarginal, Sutures are depressed and straight to slightly crooked, Plate 2 (D).

Genus Heterohelix navarroensis loeblich, 1966
Description: Hyaline calcareous test wall, pores randomly scattered. Test small subtriangular in side view, chamber arrangement biserial, the number of chambers are eight to ten, the chambers coil are low and not much inflated, shape of chamber globular and sub globular, spiral sutures moderately scattered, the main pore represent a short arch at the interior margin of the last chamber, Plate 2 (E).

Genus Heterohelix moreman, Cushman, 1938
Description: Test walI is porcelain, pores randomly scattered, chamber arrangement biserial, the number of chamber is ten to twelve, shape of chamber subglobular to slightly reniform, suture line moderately depressed, aperture a high arched at the internal margin of the last chamber, Plate 2 (F).

BIOZONATION
The results of the identified fossils at the selected wells are approximate, therefore they collected data in one figure (Fig. 3), the assemblage zone is a suitable zone for Khasib Formation because of existence three or more fossil taxa that, taken together, distinguishes it in biostratigraphic character from adjacent strata, such as Marginotruncana scheegansi, Marginotruncana renzi and Whiteinella archeocretacea. Therefore Marginotruncana scheegansi Assemblages Zone represents the recommended biozone for Khasib Formation, and this biozone corresponds partially with regional biozone of Sigal (1977), While the suggested age of Khasib Formation is Turonian -Coniacian, due to including the overlapping parts of the range zones of two specified taxa. The first appearance of Marginotruncana scheegansi represents the beginning of deposition of Khasib Formation at the Turonian age, as well as other fossils like Marginotruncana renzi and Globotruncana sigali, represent the end of the deposition of formation with last appearance of Praeglobotruncana algeriana in addition to Clavihedbergella simplex, with ending of Coniacian. This zone is closing to many regional studies that depending on index fossils for each period which illustrated in Figure (4)

DISCUSSION
The current study aims to determine the age of Khasib Formation precisely, therefore after the collecting of fossils and classified them to determine the age for each of the picked species, they indicated that Khasib Formation is deposited at Turonian -Coniacian age. The stratigraphic position with sedimentary column for studied formation point to that the Khasib Formation was deposited at the Early Turonian age and could not be older than this age, there is important evidence for this age, the most significant of which is the presence of the following fossils: Marginotruncana scheegansi, Marginotruncana renzi and Globotruncana sigali, but these species could continue to appearance into Santonian age, while there are some of species ended at the contact of Khasib and Tanuma formations with Coniacian age, such as Globotruncana sigali, Praeglobotruncana algeriana, Globotruncana imbricata . The results of present study did not determine the continuation of the Khasib Formation until to the age of Companion that were identified by previous studies such as (Owen and Nasr, 1958, Bellen et al., 1959, Chatton and Hart, 1961  Formation. It equivalent to partially regional zones of Sigal's (1977) and Bolli (1966