Investigation of the Geochemical Properties and Origin of the Crude Oils Accumulated in the Mishrif Reservoirs in the Zubair, Halfaya, and Buzurgan Oilfields, Southern Iraq

The Mishrif Formation is among southern Iraq's most important reservoirs, which contains a third of the oil in the Cretaceous reservoirs, which is a broad carbonaceous succession in Iraq and the surrounding area. For detecting differences in the geochemical characteristics of crude oil, three crude oil samples were obtained from the Mishrif carbonate reservoir intervals in the Cretaceous at the Zubair, Halfaya and Buzurgan oilfields in southern Iraq. Analyses utilize API gravity, sulfur concentration, Gc, Gc/MS, and bulk carbon isotope compositions. The low API (23 to 28) and high sulfur content (4.45 to 5.36 wt%) of the oils studied can be linked to the deposition of a marine carbonate environment under sulfate-reducing environments. The anoxic, non-biodegradation, organic matter type II-S, marine carbonate depositional was indicated by the pristane/ nC 17 (0.16 to 0.26), phytane/ nC 18 (0.29 to 0.31), narrow Pr/Ph ratio range from 0.76 to 0.78, high C 29 /C 30 hopane ratios of 1.01–1.71, and low C 26 /C 25 tricyclic terpene ratios in the related source-rock. TAS3 (CR) ratios of 0.33 to 0.36, C 27 Ts of 0.18 to 0.22, and 29 sterane 20S/20R sterane ratios all refer to early maturity. Biomarker parameters and bulk carbon isotope values in the oil analysis match those found in the Sulaiy and Yamama sources of the Late Jurassic-Early Cretaceous.


Introduction
The Cenomanian -Turonian Mishrif reservoir (Chatton and Hart, 1991) is composed of carbonate deposits from rudist-bearing units and extends throughout southern and central Iraq within the Mesopotamian foredeep and Zagros fold belt (Aqrawi et al., 2010). With its excellent reservoir and petrophysical characteristics, it is regarded as one of Iraqi most important reservoirs . It contains oil in various oilfields, including Amara, Gharraf, Majnoon, Buzurgan, Jabal Fauqi, Halfaya, East Baghdad, Ahdab, North and South Rumaila, West Qurna, Noor, Tuba, Ratawi and Nasiriyah ( Fig. 1) (Aqrawi et al., 2010), and its oil reserves account for almost 40% of Iraqi total oil reserves (Al-Sakini, 1992). The Mishrif Formation is a widespread carbonate succession including regions throughout the Arabian Gulf. It is stratigraphically believed to be part of the Wasia Group (Mahdi andAqrawi, 2014, Al-Mimar andAwadh, 2019). The Yamama, Zubair, Mauddud, Rumaila, and Mishrif formations are all part of Megasequence Arabin Plate 8 (AP8). These carbonate formations that the oilfield's major oil-bearing strata, represent more than half of the oilfield's reserves and production in the Arabian Plate (Sharland et al., 2001). Although a multitude of deep-water basins occurred inside the shelf during the Cretaceous Period, these successions were deposited on the passive margin, which was later covered by shallow water (Murris, 1980) Porous Cenomanian and early Turonian shelfal and shelf-margin limestones are found across Iraq (Fig. 2). For the first time in the Zubair Oilfield, Rabanit (1952) reported the Mishrif Formation type section at well Zb-3. Numerous researchers and petroleum companies have studied the Mishrif Formation in many Iraqi oilfields over the past decades (Al-Khersan, 1973;Buday and Jassim, 1987;Aqrawi et al., 1998;Aqrawi et al., 2010 andMahdi andAqrawi, 2014). In comparison, there have been limited organic geochemical studies of the oil reserves in the Mishrif Formation. The first geochemical study published was (Al-Ameri et al., 2009), which was followed by subsequent studies such as: Al-Khafaji (2015); Al-Ameri and Al-Zaidi (2014); Awadh et al. (2018); Al-Khafaji et al. (2018;2019a;2020 and2021a);Al-Mimar et al., 2018;Abdula, (2020); Boschetti et al., 2020;and Alsultan et al. (2021).
The most of the oil and gas discovered in Iraq's Middle and South originated from organic-rich, oilprone carbonates in the Middle Jurassic Sargelu and Naokelekan formations. These source rocks are widely scattered and mature for oil and gas generation over the Zagros fold-belt and Mesopotamian foredeep. The Upper Jurassic-Lower Cretaceous Sulaiy and Lower Cretaceous Yamama oil-prone source rocks also had a key role in the generation of considerable amounts of hydrocarbons (Al-Ameri et al., 2009;Pitman et al., 2004;Abeed et al., 2011;Chafeet et al., 2020;Al-Khafaji et al., 2019b, and2021b).
The study's goal is to detect any differences in the geochemical characteristics of crude oil in the Mishrif Formation, in three oilfields in Maysan and Basra, as well as to infer sedimentary environment of the corresponding source rocks.

Geologic Setting
The Zubair Oilfield is located in the Mesopotamian Basin's Zubair Subzone in Basra, while the Halfaya and Buzurgan oilfields are located in the Mesopotamian Basin's Euphrates Subzone in Maysan, southern Iraq (Fig.3). Over 3000 m of sediments of the Cretaceous Period were accumulated in this basin and throughout most of the Cretaceous because the Mesopotamian basin of southern Iraq was part of the wider carbonate platform on the Arabian Plate's NE passive border due to tectonic, eustatic, and climatic conditions. The warm shallow waters of Neo-Tethys encountered this margin. A deep carbonate accumulation is deposited results of the transition environment from pelagic facies to foraminiferal-rich lagoonal and reefal. The limestones of the Mishrif Formation are significant oil reserves as mentioned in Sadooni (2005). The porosity of the Mishrif reservoir is mostly developed in the highstand parts of sequences, which are separated by argillaceous limestones deposited as transgression strata above sequence borders (Sadooni, 2005). According to Sadooni and Aqrawi (2000), the Mishrif Formation was deposited during the Late Cenomanian, during the period of generally high sea levels (Murris, 1980). The Mishrif Formation, which is overlain by a sharp contact of Khasib Formation that corresponds to an early-middle Turonian unconformity. The Mishrif Formation, however, penetrates gradationally into the Rumaila Formation ( Fig. 2) near its lower limit, and the two formations are difficult be distinguished from each other in many wells. According to Buday and Jassim (1987), the Buzurgan Oilfield is located in the Folded Zone, the Halfayia Oilfield is located in the Mesopotamian Basin's Euphrates Subzone, and the Zubair Oilfield is located in the Mesopotamian Basin's Zubair Subzone (Fig. 4).  Buday and Jassim, 1987)

Methods and Materials
Three crude oil samples were obtained from the Mishrif carbonate reservoir intervals in Zubair Oilfield in Basra, Halfaya, and Buzurgan oilfields in the Maysan (Table A and B) and applied to detailed analytical investigations, including, sulfur content, API gravity and carbon isotope analyses, as well as saturated hydrocarbon fraction. The GeoMark Research Institute in Houston, Texas, USA, analyzed the studied samples.
The crude oil gravity analysis (API) was carried out on 1-2 ml of the studied oil samples and evaluated at 60 °C using the Anton Par DMA of the 500-density m. The sulfur content (weight percent) in the crude oil samples was determined using veriouse isotope selective elemental instruments. The asphaltene concentration in the oil samples was caused by using the pentane solvents, and the residual maltenes were fractionated into polar fractions, saturated, aromatic, and in alumina high-performance liquid chromatography using hexane, methylene chloride, and 50:50 mix of methylene chloride and methanol, respectively.
An Agilent mass spectrometer was used to analyze the saturated fraction utilizing gas chromatography-mass spectrometry (GC-MS). For the GC experimental analysis, a flame ionization detector (FID) that used in a length of 50 m of an HP-column, a diameter of 0.2 mm, and a film thickness of 0.11 m.  The stable carbon isotope concentration (δ 13 C) of the saturate and aromatic fractions was determined using Isoprimevario's Vision isotope and isotope ratio mass spectrometers (IRMS). The values are expressed regarding Pee Dee Belemnite using the traditional delta-notation (PDB). The detected carbon isotope levels were used to identify the cured oils and determine where the likely source rocks came from.

Carbon Isotope and Physical Characteristics
The examined oils had greater API gravities ranging from 23.31 to 28.92 than the average of the other Mishrif samples from the southern Iraq oilfields (Table 1A), but they also have a high sulfur (S) concentration of up to 5.36 wt%. The low API and maturity of the crude oil samples is usually attributed to sulfur-rich oils or biodegradation (Fig. 5) (Peters et al., 2005). Also, observe that the sample from Buzurgan well-3 had the lowest value of sulfur content and the highest API gravity. The high sulfur content (4.45 to 5.36 wt percent) of the oils investigated can be attributed to the deposition of a marine carbonate environment during sulfate-reducing settings.The saturated (-27.42 to -27.66) and aromatic (-27.49 to -27.78) hydrocarbon fractions (δ 13 C) of the analyzed Mishrif oil samples (Fig. 6) indicate Upper Jurassic to Lower Cretaceous marine source rock deposition (Andrusevich et al., 1984). Fig.6. The Sofer plot of the stable carbon isotope ratios indicates marine carbonate organic matter (Sofer, 1984). The age of source rocks mention in figure based on (Al-Khafaji et, al. 2019) Fig. 7 depicts the distribution of C4-C35 normal alkanes and isoprenoids, which displays C4-C15 nalkanes dominating over C20-C35 n-alkanes. As shown in Table 1B, the carbon preference index (CPI) values were low, ranging from 0.91 to 0.96, and the phytane (Ph) greater than pristane (Pr), resulting in a rather narrow Pr/Ph ratio range of 0.76 to 0.78. The Pr/Ph ratio easily differentiates crude oils from various source rocks. Pr/Ph˂ 1 in Zubair and Halfaya crude oil implies anoxic source-rock deposition, especially when substantial porphyrin and sulfur concentrations are present, but Pr/Ph>1 in the Buzurgan sample indicates more oxic deposition (Peters et al., 2005). In the related source-rock depositional environment, the pristane/nC17 (0.16 to 0.26) and phytane/nC18 (0.29 to 0.31) ratios indicated an anoxic, non-biodegradation, organic matter type II-S, as illustrated in Fig. 8. Even though the Pr/Ph of petroleum reflects the composition of the organic matter that contributes to it, it normally rises with thermal maturation (Peters et al., 2005). As a result of its higher maturity than other samples, the Buzurgan sample has a high Pr/Ph. All the tested samples are within the range of biologic non-biodegraded oil at the beginning of its thermal maturation and are derived from the II-S kerogen type, as shown in Fig. 8. Low concentrations of tricyclic and tetracyclic terpenes, as well as pentacyclic terpenes, appear in the m/z 191 ions of mass fragmentograms (Fig. 9). As illustrated in Fig. 9, abundant C29-norhopane leads to high C29/C30 ratios of 1.62-1.74 (Table 1A), which is common in crude oils derived from marine carbonate sources (Fig. 10) (Peters et al., 2005). The homohopane distributions were controlled by C31, which rapidly declined in content as carbon levels increased in all samples.

Biomarker and Non-Biomarker Distributions
The C31/C30-hopane ratio of the oil samples studied ranges from 0.33 to 0.35, indicating that they are derived from marine carbonate sources (Petres et al., 2005). Marine oils typically have C26/C25 tricyclic terpene ratios of 0.72-0.76 confirmed the previous finding. The C35 homohopane index (C35S/C34S hopanes) of the tested oil samples was found to be in the range of 1.06-1.11, while the gammacerane index (Ga/C31R hopane) was found to be in the range of 0.20-0.23 ( Fig. 11; Table 1B), indicating higher salinity and marine carbonate source rocks.   Fig.11. Plots of the C35S/C34S homohopane, C29/C30, and Ga/C31R hopane, for the studied oil samples, indicated anoxic environments (modified from Peters et al. (2005)) Gammacerane can be used to differentiate between the various petroleum families. The presence of gammacerane biomarkers in the oil samples, as well as relatively high C35S/C34S homohopane indicators, imply anoxic marine carbonate source rock deposition (Peters et al., 2005). According to prior biomarker findings, the oils originated from sulfur-rich kerogen Type II-S and marine carbonate source rocks were deposited in an anoxic environment.

Thermal Maturity
Many particular maturity biomarker ratios are determined using m/z 191 and m/z 217 mass analysis results of the saturated hydrocarbon fraction as maturity indicators. TAS3 (CR) ratio is considered a more fundamental indicator of maturity parameter than Ts/(Ts+Tm) since it is less reliant on source organofacies (Fig. 12A). The samples showed a ratio of 0.33 to 0.36, indicating that the oil had been early maturation. Because of its so resistance to biodegradation, the C27 Ts/Tm is indeed a thermally sensitive terpane parameter (Fig. 12B). The ratio in the investigated samples range from 0.18 to 0.22, reflecting early maturation. The C29 sterane 20S/ (20S+20R) sterane ratios, which increase with maturation, are also used to determine the maturity level of oils (Fig. 12C). The studied oil samples exhibit low ratio of the C29 sterane 20S/ (20S+ 20R) in the range of 0.38-0.41 and 0.43-0.45, showing that they are usually consistent early-mature source rock origin (Zumberge et al., 2005) (Fig. 12D).

Age of Source Rocks
In the Mesopotamian Basin, the Sulaiy and Yamama formations from the Late Jurassic to Early Cretaceous as well as Middle Jurassic Sargelue are the most well-known likely source rocks, which formed in an anoxic marine carbonate environment (Abeed et al., 2011;Al-Khafaji et al., 2021a and2021b). The biomarkers distribution of the Yamama and Sulaiy source rock in e.g. Al-Khafaji et al. ( 2019a) is similar to those of the oils investigated in this study. The calculated C28/C29 ratio of 0.8 indicates that the Mishrif-reservoired oil was originated from carbonate source rocks from Upper Jurassic to Lower Cretaceous, confirmed by e.g. Al-Ameri et al. (2009) (Fig. 13). Furthermore, biomarker parameter ratios and carbon isotope values from both probable source rocks and crude oils were used to set similarity. As a result, the Sulaiy and Yamama formations of the L. Jurassic to E. Cretaceous are potential sources for the oils studied in this work. This result is supported by previous studies such as Al-Khafaji et al. (2019a and2019b).

Conclusion
The geochemical properties of crude oils in the southern Iraqi oilfields of Buzurgan, Halfaya, and Zubair have been investigated. The oils studied have a high sulfur content, aromatic hydrocarbon components, and low API gravity. The oils are derived from a source rock which has a high-S kerogen content (Type II-S). Low Pr/Ph ratios, Ga/C31R hopane, and C35 homohopane index, all indicate sulphate-reducing conditions during source rock deposition. The maturity biomarkers ratio such as TAS3 (CR), C27 Ts, C29 sterane 20S/ (20S+ 20R) sterane, API gravity in the oils all point to early maturation. The biomarker ratios and bulk carbon isotope values of the oils match those of the L. Jurassic to E. Cretaceous source rocks.