Geochemistry and Paleoredox Conditions of The Carbonate Reservoir Khasib Formation in East Baghdad Oilfield-Central Iraq

The carbonate of Khasib Formation in East Baghdad Oilfield, Central Iraq was geochemically investigated to interpret paleoredox conditions and source of rare earth elements (REEs) based on major, trace, and REEs geochemistry. SiO2 and Al2O3 concentrations are nearly the same in both wells, while CaO content just hardly differs between the EB10 and EB81 cores. The EB10 Well has higher concentrations of Th, Y, and Zr than the EB81 Well. However, both the ∑REE of EB10 (1.969-35.35, n=23) and EB81 (9.59-24.88, n=20) wells have low total REE content. These results show that the Khasib Formation's carbonate sedimentation contains seawater-like marine carbonate and PAAS-normalized REE + Y patterns accompanied by 1-light REE depletion (NdN/YbN= 0.40–0.95, n=23, and 0.54–0.90, n=20, respectively), 2-both positive to negative Ce anomalies (Ce/Ce*= 0.28-1.07, n=23; 0.39-1.76, n=20, respectively), as well as the 3-superchondritic Y/Ho ratio (23.25-57.5, n=23; 18.0-53.0, n=20, respectively). The terrigenous contribution, paleoredox conditions, and scavenging mechanisms were responsible for the observed variations in Ce components and Ce anomalies in the investigated cores. The rising U contents in the limestones (1.62-4.72 ppm) and authigenic U (0.66-4.44 ppm) indicate that dysoxic conditions were present when they were deposited. Further evidence from geochemical data suggests that diagenetic processes may be responsible for the positive Eu anomalies found in limestones. This implies that the Khasib Formation limestones may have kept their original seawater-like REE patterns. Due to trace amounts of detrital materials in certain specimens, there is identified variability in the REE + Y pattern and REE content. The current study shows that the limestones still display their original seawater-like patterns as long as shale contamination was minimal, and they act as a proxy for seawater


Introduction:
Carbonate chemical composition reveals physicochemical conditions all through deposition.The major oxides and trace elements supply information on the overall composition of the carbonate reservoirs and depositional environments, allowing us to deduce the nature of the seawater through which they originated.The significance of geochemistry in defining the source area of sedimentary rocks, paleo-weathering conditions, and tectonic evolution of sedimentary basins has been clearly established in numerous research studies [1].
Many researchers have studied the behavior and mode of distribution of rare earth elements (REEs) in carbonate rocks [2].The research discovered that the most significant factors influencing REE enrichment or depletion through carbonate rocks are: The amounts of terrigenous detrital materials; the variability in seawater oxygen levels; the proximity to the origin area; seawater biogenic deposition; surface productivity variation; diagenesis and lithology; scavenging processes in seawater that are related to oxygen level, salinity, and depth.
The upper Turonian-Lower Coniacian Khasib Formation contains about 14% of the Cretaceous hydrocarbon reserves in Iraq and up to 10% of the entire proven oil reserve [3].The formation is 60-130 m thick in central Iraq and is prevalent by bioturbated chalky limestones with marlstones, marly limestones and subordinate shales.The formation is widespread in central and southern Iraq, and it serves as a reservoir rock at oilfields such as Samarra, Balad, Tikrit, East Baghdad (Fig. 1).
According to Al-Temimi [4], the Khasib Formation has been divided into several units in EB10 (from KB1, 2, 3, 4, 5, and 6), and six units in EB81, that reflects the primary oil-bearing unit in the East Baghdad Oilfield, relying on reservoir characteristics (porosity and saturation).The investigation of sediment cores and log information in Al-Badlawi [5] and Abdel-Fattah et al. [6] studies, based on the detection of micro and electrofacies, revealed that the Khasib Formation deposited in a ramp setting impacted by a transgression event and sea level change.
In this research, we attempted to provide comprehensive information on the source of REEs.In addition, the reasons for the incidence of Eu and Ce anomalies in Khasib Formation limestone using mineralogical and geochemical considerations, as well as determining the depositional environment and forecasting the paleoredox conditions.the Zagros Fold Belt, but also westward toward the Widian Basin of Interior Platform, whereas its southern extension is the Mesopotamian Foredeep Basin [7], which contains Neo-Tethys Ocean deposits from the Jurassic & Cretaceous periods.A certain ocean had mostly dysoxic-anoxic palaeoenvironments all along equator and was tectonically unstable.This allowed for the preservation of elevated organic matter and the development of the world's largest oil and gas reserves with in Arabian Province.The lithostratigraphic section consists of marine and subordinates lagoon beds deposited as carbonates, shales, and anhydrites in the southern Neo-Tethys Ocean over a geologic time span spanning the Jurassic, Cretaceous, and Palaeogene periods.In the Mesopotamian Basin, each transgressive-regressive sequence's lateral lithostratigraphic variants and related carbonate bodies (such as prograding shelf margins and forced regressive wedges) could create significant stratigraphic traps [8].Stratigraphic column of East Baghdad Oilfield is showed in Figure (1 right) [9].

Table (2) A list of concentration of trace elements (ppm) of cores taken from the Khasib Fm. limestone of EB10 and EB81 wells
The EB10 well has large variations in Ba, V, and Rb contents, whereas the EB81 well has the smallest variations.The Cu, Zn, and U contents of the EB10 and EB81 wells vary the least.The Sr contents of the EB10 and EB81 core samples (160-440 ppm and 240-640 ppm, respectively) are slightly lower than the typical value for lithosphere carbonates (Sr= 610 ppm, [10]).
The cores of the Khasib limestone exhibit weak depletion in comparing to marine carbonate [10], as well as significant enrichment for elements like V, Ni, Rb, and Sc; for elements like Co, Ba, Sr, Y, Zr, Pb, and Mn, there is a severe depletion; there has been a notable enrichment for the elements Cr, Cu, and Zn for EB10 (Fig. 4).

The limestone's source of REEs
Since limestone typically have lower REE concentrations than shales, it is likely that marine carbonate phases have significantly lower REE concentrations than terrigenous materials [11].
While terrigenous sediments have relatively high REE concentrations and patterns that are unlike those found in seawater, which contributes a lower concentration of REE.The following aspects are visible in seawater-like REE + Y patterns normalized by PAAS: considerable LREE depletion, negatively Ce anomaly, faint positive La anomalous, as well as superchondritic Y/Ho ratios [12].
Recent studies demonstrated that by taking into account the correlation coefficients between specific trace and major elements and REEs, it is possible to identify the contributing of terrigenous source for REEs in limestone.Si, Al, Ti, K, P, Cr, Sc, V, Rb, Th, Zr, Ni, Y, and Co are frequently positively correlated with detrital-derived REEs, while Ca is typically negatively correlated with these elements [13].Analysis of the correlation coefficients between the elements in the Khasib

Ce anomaly
It has been thought that the Ce anomalies in marine carbonate rocks are an appropriate indicator for comprehending the current paleoredox conditions [14].Comparable Ce anomalies in limestone discover the incorporation of REEs +Y straight from seawater or pore water under oxic conditions, just as marine water exhibits a negative Ce anomaly.The scarcity of Ce in comparison to nearby rare earth elements is a significant characteristic of contemporary seawater.This could be clarified by the oxidation of trivalent cerium to the less soluble tetravalent cerium and also the subsequent removal of the cerium by suspended molecules through the scavenging system [15].A less negative to positive anomaly in seawater results from Ce being remobilized and released into the water column in a suboxic to anoxic environment.However, accurate measurements of the redox conditions at the time and location of deposition may be limited by Ce anomalies throughout marine sediments.
Using Bau and Dulski's [16] plot of Ce/Ce*"equation (1)," vs. Pr/Pr*"equation ( 3),", Ce and La anomalies have been computed.The majority of the data display negative Ce and positive La anomalies, while only a small number of specimens don't show negative Ce anomalies (Figure 8).
Ce/Ce* ratio depends on the ratios of clastic contamination and pure seawater precipitate in these two cores, in addition to their respective REEs + Y concentrations.The Ce/Ce* ratio gets closer to 1 as clastic contamination rises (Table 4).Ce/Ce* values in seawater vary between 0.1 and 0.4 [14].Fewer limestone samples exhibit positive Ce anomalies, which are primarily caused by paleoredox conditions, terrigenous input, diagenesis [14], scavenging process [17], while more limestone samples exhibit negative Ce anomalies.The Ce/Ce* values in the current study have a weak positive correlation with scavenging-type particle reactive elements, such as Fe and Mn (R 2 = 0.0400 and 0.0007, respectively) (Figure 9 a, b).These correlations demonstrate that the limestone under study precipitated inside a shallow marine deposition environment, in which the role of scavenging processes is relatively less significant than in deep marine environments.In some situations, the redox potential of Fe or/and Mn may also be related to Ce distribution.Using other redox sensitive elements (such as uranium content and uranium produced authigenically), it is possible to test whether the variations in Ce anomalies are caused by scavenging processes or paleo-redox changes.support the idea that diagenesis played a significant role in the occurrence of Eu anomalies in the Khasib limestone.Additionally, the positive correlation between both the pair of Sr-Mn can be used to infer the impact of diagenetic alteration in the limestone [2], (Fig. 10d).Limestones may exhibit positive Eu anomalies as a result of detrital feldspar inclusions in sediments [13].Different types of feldspars can be identified in sediments using oxide ratios like Na2O/Al2O3 and K2O/Al2O3.According to Figure 10e   Er/Nd ratios range from 0.09 to 0.2 for EB10 and 0.12-0.17for EB81, further demonstrating the impact of detrital material.LaN/YbN ratios in the limestones exhibit very minor variations (Table

2-
The positive correlations among Ce/Ce* and elements such as Fe2O3, Mn, Al2O3, Th, Zr, and Y as well as the negative correlations between Ce/Ce* and some elements such as CaO and U show that detrital input significantly controlled the variation in values of Ce anomalies in the limestone.
Additionally, the previously stated negative correlations provide a compelling justification for the limestone's deposition in a shallow marine environment.

3-
The strong correlations between Eu/Eu* and elements like Th, Y, and Zr led us to draw the conclusion that diagenetic processes were responsible for the positive Eu anomalies in the Khasib limestone.

Fig. ( 1
Fig. (1): (a) Position map showing the northeast Arabian Peninsula province of Iraq, along with the locations of basins, oil resources, and boreholes used in this study.(b) The EB10 (East Baghdad 10) and EB81 (East Baghdad 81) wells that were studied are highlighted (left).East Baghdad Oilfield stratigraphic column (right) [9].
limestone reveals that the REEs have positive correlations with SiO2 (R2= 0.556; Fig.7a), Al2O3 (R2= 0.762; Fig.7b), TiO2 (R2= 0.561; Fig.7d), P2O5 (R2= 0.668; Fig.7e), Cr (R2= 0.02; Fig.7f), Rb (R2= 0.036; Fig.7g), Th (R2= 0.559; Fig.7h), Zr (R2= 0.407; Fig.7i), and Co (R2= 0.15; Fig.7j), while a negative correlation (R2= -0.876; Fig. 7c) with CaO.These connections imply a few detrital influence on the REE contents of the sample.Because of the greater REE concentration in detrital particles and the generally flat REE pattern typical of common detrital materials.Seawater signatures can be effectively preserved in carbonate rocks while maintaining their original seawater-like REE patterns.One limestone specimen (Kh7) of the EB10 well has a relatively low ∑REE content (1.969 ppm) as well as LREE depletion inside the typical range for modern seawater (NdN/YbN = 0.4; contemporary shallow water = 0.205-0.492for 50 m water depth).If PAAS and the reduced ∑REE limestone sample are mixed cautiously, the extent that the detrital content impacts the REE patterns in the EB10 and EB81 could be assessed (Kh7).Due to its low ∑REE content, important LREE depletion, REE pattern resembling that of modern seawater, and superchondritic Y/Ho ratio, the limestone specimen (Kh7) can be regarded as the least clastic input-contaminated and used as a seawater-like content.Shale has a rising REE concentration, and adding even 1% to 2% of finegrained material could dramatically change the LREE depletion, Ce anomalies, and REE patterns [14].Additionally, the limestones with higher REE concentrations (5.05-35.35ppm) display distinctive REE + Y patterns that resemble seawater.Just a small amount of REE from terrigenous materials (less 5 % of local shale contamination) contributes to the seawater-like REE + Y patterns found within those limestones, which are primarily caused by the absorption of REE from current seawaters.Therefore, the ancient limestones deposited inside the proximal portion of the basin with very little detrital material are appropriate to comprehend the REE patterns of ancient shallow seawater as well as act as an important seawater proxy.
The values presented by Madhavaraju and Gonzalez-Leon[18] (about 1.0) for terrigenous materials, are remarkably near to the LaN/YbN ratios of Khasib limestone.As a result, the studied limestones exhibit a seawater-like REE+Y pattern with an enrichment of HREE relative to LREE and are contaminated with terrigenous materials.Minor variations exist in the concentrations of major, trace, and REE in the Khasib limestones of the EB10 and EB81.Terrigenous input and scavenging process governed both positive and negative Ce anomalies as well as the observed variations in Ce and Ce anomalies in these limestones.The limestones in this study have increasing levels of U and authigenic U, which indicates that dysoxic conditions were present when they were formed.The limestones have low ∑REE components, high Y/Ho ratios, low Er/Nd ratios, LaN/YbN ratios close to 1, and seawaterlike REE+Y patterns, which indicate that the REE concentrations were primarily derived from seawater, with the inclusion of terrigenous input.Strongly positive correlations between REEs and SiO2, Al2O3, TiO2, P2O5, Cr, Rb, Th, Zr, and Co, as well as a negative correlation between REEs and CaO, provide additional evidence for this claim.
Er/Nd values and positive correlations between Eu/Eu* and Ce/Ce* show that diagenetic fluids played a significant role in determining the concentration of REEs in the limestone.

Table (
1) shows the major oxide concentrations in the EB10 and EB81 wells.Both wells have nearly identical SiO2 and Al2O3 contents (5.695% and 1.946% for EB-10; 5.327% and 1.845% for EB81, respectively).TiO2 levels in the EB10 cores are higher (0.080%) than in the EB81 cores (0.058%).CaO content varies little between the EB10 and EB81 core samples (42.00-54.32%and 45.36-51.52%,respectively).The EB10 cores have large variations in MgO and Fe2O3 contents (0.50-2.36% and 0.16-5.60%,respectively), whereas the EB81 cores have small variations (0.60-1.93% and 0.17-1.48%, respectively).The limestone samples contain very little Na2O, K2O, and P2O5.The abundance of CaO among the major elements indicates that CaCO3 has been precipitated straight from seawater in the Khasib carbonate, and the dominance of CaO over MgO indicates that the current carbonate phase is primarily calcite [1].The reduced MgO content in Khasib carbonate indicates a less saline environment and/or the effect of freshwater diagenesis, which causes extensive leaching in the later phases of diagenesis.Khasib carbonate limestone have low Co, Pb, and Sc contents.

Table ( 3) A list of concentration of rare earth elements (ppm) of cores taken from the Khasib Fm. limestone of EB10 and EB81 wells Table (4) lists the anomalies and elemental ratios found in Khasib Fm. limestone.
and 10f, the Khasib limestone exhibits negative correlations between Na2O/Al2O3 and K2O/Al2O3 with Eu/Eu* (R 2 =0.005 and 0.02 respectively).It appears that the occurrence of positive Eu anomalies was not significantly influenced by the presence of feldspars in the studied limestone.