Palaeoenvironment Conditions During Deposition of Sargelu, Naokelekan, and Najmah Formations in Zey Gawara Area, Kurdistan Region, Iraq: Implications from Major and Trace Elements Proportions

Abstract


Introduction
In the Kurdistan region, all significant hydrocarbon discoveries have actually occurred within structural traps fashioned throughout Alpine Orogeny (Fig. 1).During the stratigraphical development, subsidence and deposition played a significant role for generating and migration of hydrocarbon from deeper source rocks.The generated hydrocarbons were migrated a comparatively short distance to charge the Zagros anticlines (English et al., 2015).Over the last several years, hydrocarbon discoveries were identified in a variety of Jurassic fractured carbonate layers in northern Iraq, e.g., Atrush, Shaikan, Zey Gawra, Sarsang, Mirawa, and Bakrman oilfields, as cracked and variously dolomitized limestones are the primary targets from Jurassic formations (Sarki, Sehkaniyan, Sargelu, Naokelekan, and Najmah) (English et al., 2015).This current work focuses on the palaeodepositional conditions of Middle -Upper Jurassic sequence that were penetrated in Z1 Well of the Hawler Block Oilfield in Zey Gawra area of Erbil Governorate, Kurdistan region Iraq's northern part (Fig. 1).
Fig. 1.Map shows the location of the main hydrocarbon accumulation within structural traps throughout northern Mesopotamian basin (Mohialdeen et al., 2021).
Sargelu and Naokelekan formations' subordinate shales and deep-water carbonates with abundant organic matter comprise this succession (Abdula et al., 2015;Abdula, 2016a) which overlaid by the oolitic shallow-water Najmah, which is assumed to be approximately the same age as the Naokelekan Formation (Abdula et al., 2015).The inorganic geochemical analysis conducted in the current work aims to evaluate the palaeoredox, palaeoclimate, palaeoweathering, tectonic setting, and provenance of these important sequences in Z1 Well.

Materials and Methods
Three cuttings samples have been taken from the Z1 Well at different depths in Hawler Block Oilfield including Sargelu, Naokelekan, and Najmah which comprise of claystone, shales and claystone, and limestone, respectively.The cuttings were stored at the Directorate of Geological Survey, Cuttings Store, Erbil, Iraq.The cuttings samples first were cleaned from contaminations to eliminate the drilling mud fluid by Benzene polar solvent.After washing, the samples filtered and dried.The powdered product submitted to FullTech Laboratory Erbil office which analyzed in their laboratories in Iran.The samples analyzed by X-ray Fluorescence Spectrometer (XRF) to find the concentration of major oxides.The trace elements were identified using an Inductively Coupled Plasma by both Mass Spectrometry and Optical Emission Spectrometry.

Geological and Tectonic Settings
Geologically Iraq lies in the conversion between the Arabian Shield in the Southwest and the extremely distorted Taurus and Zagros Suture Zone in the North and Northeast.It is strongly affected by eustatic sea-level fluctuations.The development of the region has been influenced by the mobility of the Precambrian basement and by tectonism along the Neo-Tethyan margin (Jassim and Buday, 2006).
The major tectonic event was begun after Triassic when the Gondwana splits, Indian Plate started to separate and in the Late Jurassic isolated from Arabia (Grabowski and Norton, 1995).A new passive margin and new accommodation space were developed along the subsiding shelf at the plate's northwest edge as a consequence of rifting that occurred in the late Early Jurassic (Sharland et al., 2004).Liassic was the time of general steadiness as represented by deposition of evaporite and carbonate units e.g., Alan, Mus, and Adaiyah or their equivalents, Sarki and Sehkaniyan formations (Fig. 2).Due to Toarcian rifting, a new basin formed in the Middle Jurassic.It was characterized by the deposition of the Sargelu and its equivalents throughout the region (Sadooni, 1997).The variety in Najmah Formation thickness as described by Dunnington (1958) reflects the basin geometry after Bathonian onwards.Along the western margin of a basin, the Najmah oolites are developed, which is represented to the east-northeast by the Naokelekan in the Mountainous Zone (Sadooni, 1997).Depending on tectonic subdivision of Iraq, the study area is located in the Foothill Zone in northern Iraq.
The amount of an element that has been enriched or depleted in a sample is often measured in relation to the amount of the same element in a reference material (Fig. 3), which is typically the average crustal rocks or average shale (Wedepohl, 1991).The EFs have been calculated for the used trace elements by the equation: EFelement = (element/Al) sample/(element/Al) average shale (Ross and Bustin 2009).The EFs of V, Ni, and Cu in all three formations are enriched, Zn, Ba, and Mo are strongly enriched, whilst Cr is depleted at Sargelu and Naokelekan formations and enriched in Najmah Formation (Fig. 3).Furthermore, the trace elements are plotted with Al in order to indicate the detrital control on the trace elements of samples.The crossplots of trace elements with Al for Sargelu and Naokelekan samples indicate a weak detrital control on the abundance of the used elements, whilst the crossplots indicate a strong detrital control on the abundance of the elements in Najmah Formation (Fig. 3).signifies the enrichment or depletion of an element.

Palaeoredox Conditions through Geochemical Proxies
The dissolved oxygen concentration varies depending on the depositional setting and the water depth which is supportive to interpret the depositional setting.Environmental redox settings classified as oxic, suboxic, dysoxic and anoxic amongst euxinic conditions using H2S measurements (Wignall, 1994;Wang et al., 2018).To establish the palaeoredox conditions for the Sargelu, Naokelekan, and Najmah formations within Z1 Well of Hawler Block Oilfield in the area of Zey Gawra from northern Iraq, numerous geochemical proxies were measured using various chemical elements and elemental ratios (e.g., Hatch and Leventhal, 1992;Deepulal et al., 2012).These include V, Ni, Cr, Sc, Co, Cu, Mo, and Zn and the rations of V/Ni, V/(V+Ni), V/Cr, V/Sc, Ni/Co, and (Cu+Mo)/Zn (Adegoke et al., 2014).
V is sensitized to changing redox conditions hence it is reliable element especially when compared to Cr hence Cr has higher concentration than V in oxic conditions; therefore, the V/Cr proportion <2 for oxic, >2 to 4.5 for dysoxic, and >4.5 for suboxic to anoxic conditions were proposed by Jones and Manning (1994).The V/Cr value of Sargelu (3.35) would advise that the formation was deposited under dysoxic settings.The V/Cr values for the overlying Naokelekan and Najmah decrease to 1.96, 1.58, respectively (Table 3) specify that both formations were deposited under oxic to suboxic conditions.
The V/Sc is a redox condition proxy inference; the values of 9.1 and >9.1 infer the oxic and suboxic environments of deposition, respectively (Kimura and Watanabe, 2001).The V/Sc value of the Sargelu, Naokelekan, and Najmah are 24.25, 9.81, and 23.75, respectively indicating suboxic conditions for all three formations (Table 3).Krejci-Graf (1966) and Deepulal et al. (2012) have suggested the Ni/Co ratio as a palaeoredox conditions proxy.In comparison to Ni, Co concentration is a complementing palaeoredox indicator that is particularly enhanced in an oxic condition (Adegoke et al., 2014).According to Adegoke et al. (2014), the oxic environments have Ni/Co <5, whilst >5 infers suboxic environments.The Ni/Co value of the Sargelu Formation (8.76) is slightly higher than overlying Naokelekan Formation value (6.84) which suggested they were deposited under suboxic environment.The Co value for Najmah Formation is below detection limit <1 due to this the Ni/Co ratio is not calculated (Table 3).(Lewan, 1984).Vanadium and Nickle are significant redox detectors during deposition (Lewan, 1984;Barwise, 1990).The condition of deposition based on V/Ni ratio can be classified into anoxic environment >3, suboxic condition between 1.9-3, and dysoxic to oxic settings <1.9 (Lewan, 1984).The V/Ni ratios for samples of Sargelu, Naokelekan, and Najmah formations are 1.78, 1.06 and 1.72, respectively; thus, suggest dysoxic to oxic conditions for their depositional environment (Table 3).
According to Hatch and Leventhal (1992), the proportions of V/(V+Ni) 0.46 are occur within oxic settings, >0.46 as dysoxic, >0.54 suboxic-anoxic, and >0.84 signifies an euxinic environment.The V/(V+Ni) values are 0.64, 0.63, and 0.51 and sulphur content 1.8, 1.3, and 0.50 wt.%, respectively for Sargelu, Najmah, and Naokelekan formations (Table 3).Accordingly, Sargelu and Najmah were deposited in suboxic to anoxic environment and Naokelekan Formation was deposited in dysoxic environment (Fig. 4).The proportion of (Cu+Mo)/Zn as a palaeoredox proxy was noticed that increases in reducing settings, and decreases in oxidizing settings (Hallberg, 1976).The calculated (Cu+Mo)/Zn ratios for Sargelu, Naokelekan, and Najmah formations' samples (0.73, 0.54, and 0.22) show low values, which may indicate the increasing oxidation conditions from lower part (Sargelu) toward upper part of succession (Najmah) during the time of their deposition (Table 3).These geochemical proxies would suggest the suboxic to anoxic for Sargelu Formation as suggested by Elyas (2014) and suboxic to dysoxic for Naokelekan and Najmah formations during the deposition of their organic matter which is inferred as mixed marine/terrestrial.These results may differ from another locality throughout northern Iraq due to tectonic activity, eustatic sea-level changes, and climate variations that were controlling the sedimentation.

Palaeoclimate
The weathering resistance SiO2 vs. nonresistance (Al2O3+K2O+Na2O) can be used as a humidity delegation.Although it is unreliable for the Najmah Formation because of its high carbonate content, but it has been used because it also contains clastics.Fig. 6 shows that the Sargelu and Naokelekan were deposited in semiarid condition also shows the humid and warm condition for overlaying Najmah Formation.The ratio of Al2O3 to TiO2 was used due to their comparatively steady under around balanced pH situations (Akul'shina, 1976).Ratio values of 20 interpret as humid, >20-30 semihumid, and >30 as semiarid environments.Accordingly, the Sargelu and Naokelekan samples (25.76 and 21.81) were deposited under a semihumid environment whereas Najmah Formation (8.62) indicates as humid environment (Akul'shina, 1976).A number of authors, including Jia et al. (2013) and Yandoka et al. (2015) have advised utilizing the ratio of strontium to copper to investigate the paleohumidity conditions although Sr and Cu concentration persuaded by water depth and scale of basin.According to these authors, the Sr/Cu ratios >5 indicate a hot and arid environment, whilst ratios between 1.3-5 designate a warm humid environment.The Sr/Cu values of Sargelu and Naokelekan formations (3.50 and 4.36) which suggest warm semiarid climate condition during the deposition of the sediments, whilst Najmah Formation recorded (18.11) suggests hot and arid climate (Table 3).According to Wang et al. (2017), the proportion of Sr/Ba signifies arid or humid climate too depending on the ratio's value.The moderate Sr/Ba ratio of Sargelu and Naokelekan formations indicate semiarid climate and moderate ratio value of Najmah Formation indicates humid climate.The palaeoclimate proxies suggest the semiarid climate for Sargelu and Naokelekan formations and semi humid climate for Najmah Formation.

Palaeoweathering
Chemical weathering has a significant impact on sediment composition and existed minerals by eliminating some cations e.g., calcium, sodium potassium, and strontium in comparison to enduring elements e.g., aluminum and titanium, via altering feldspar to clay minerals.Weathering settings can be assessed in numerous methods e.g., the chemical index of alteration, the plagioclase index of alteration, and the chemical variability index (Nesbitt and Young, 1989).The CIA can be calculated based on Nesbitt and Young's (1982) equation as CIA=[(Al2O3)/(Al2O3 +CaO*+Na2O+K2O)] × 100, whereas proportions of Al 2 O 3 , CaO, Na 2 O and K 2 O are in molar, as well as CaO*, which is restricted to a calcium silicate mineral derivative.The CaO* is acquired which is corrected to apatite [molar CaO (corrected)=molar CaO-(10/3 molar P 2 O 5 )], whereas if the value of CaO >Na 2 O, it is taken as Na 2 O, otherwise the amount of CaO* is presumed to be equal to the CaO (McLennan et al., 1993).The CIA value for the Sargelu Formation (80.92) and Naokelekan Formation (73.37) are suggesting that source rocks of both suffered modest to a forceful weathering degree (Taylor and McLennan, 1995).Furthermore, this consequence supported by scatter plots of CIA versus Al2O3/ Na2O which are useful for determining the intensity of chemical weathering (Fig. 6).As well as the information about the weathering can be provided by the values of the PIA (PIA=[(Al2O3-K2O)/ (Al2O3+CaO*+ Na2O -K2O)] × 100) (Fedo et al., 1995).The increasing of PIA values indicates an increasing degree of weathering which is observed from the calculated PIA values for Sargelu Formation (84.07) and Naokelekan Formation (80.55), implying intense parent plagioclase weathering.The CIA and PIA are not calculated for Najmah Formation because of the low concentration of P2O5 which is below detection limit (Table 3).For assessing the strength of chemical weathering and maturity of deposits, Cox et al. (1995) equation ICV=(CaO+K2O+Na2O+Fe2O3+ MgO+TiO2)/ Al2O3 can be utilized.Depending on this formula compared to clay minerals, nonclay minerals commonly show a higher proportion e.g.plagioclase, Kfeldspar, amphiboles, and pyroxenes, whereas clay minerals e.g.kaolinite and illite as well as muscovite have lower ratio (Cullers and Podkovyrov, 2000).Immature deposits that have ICV >1 are first-cycle sediments originated within active tectonic settings, while mature sediments have ICV values <1 are deposited on cratons or quiescent tectonic settings such as passive margins which indicate active sediment reprocessing and strength of wethering (Cullers and Podkovyrov, 2000).The calculated ICV for Sargelu, Naokelekan, and Najmah formations' samples (7.87, 6.23 and 79.42, respectively) which are >1 and indicate immature sediments with plagioclase, K-feldspar, amphiboles, and pyroxenes rock forming minerals and deposited in tectonically active setting (Table 3).

Tectonic Setting
The plate tectonic settings have great effect on rocks' chemical composition during their precipitation in depositional environment (Makeen et al., 2015).Hence tectonic settings of their provenances are anticipated from elements concentrations and bivariate plot of main oxides (Tao et al., 2013).The K2O/Na2O and SiO2 rise from volcanic-arc via active to passive margin environments.The Najmah sample plot in the oceanic island arc field and Sargelu and Naokelekan samples in the field of active continental margin on the log (K2O/Na2O) vs. SiO2 diagram (Fig. 7).The ICV values >1 furthermore supports the active tectonic settings for Sargelu and Naokelekan formations whereas Najmah Formation overlaid them in an oceanic island arc.This tectonic setting concluded for the studied samples agrees with the tectonic setting in Iraq during the Lower Triassic-Upper Jurassic (Numan, 1997).

Provenance Indices
The provenance characteristics are typically inferred from the component geochemical data of clastic deposits (Akkoca et al., 2019).Due to considering that these oxides are steady during weathering, and erosion, the Al2O3/TiO2 ratios are frequently utilized to identify the composition of source rock (Hayashi et al., 1997).The ratio of Al2O3 to TiO2 3 to 8 reported to be basic, ~8-21 transitional, and ~21-70 acidic eruptive rocks (Akkoca et al., 2019).The proportion of Al2O3 to TiO2 of Sargelu, Naokelekan, and Najmah formations' samples are 25.76, 21.81, and 8.62, respectively (Table 3), thus suggesting felsic igneous rocks for Sargelu and Naokelekan formations' samples and probably felsicintermediate igneous rocks for Najmah Formation.
The plot of TiO2 vs. Zr was applied for provenance investigations.The plot displays that the Sargelu and Naokelekan were derivated from felsic igneous rocks, while the Najmah was derivated from felsicintermediate igneous rocks (Fig. 8).This diagram can not be releabe for Najmah Formation because it is mainly composed of carbonate with a proportion of clastic.

Conclusions
The geochemical proxies would suggest the suboxic to anoxic for Sargelu Formation and suboxic to dysoxic depositional condition for Naokelekan and Najmah formations.The palaeoclimate proxies suggest the semiarid climate for Sargelu and Naokelekan formations and semi humid climate for Najmah Formation.The weathering indices e.g., the CIA and the PIA suggest mainly modest to forceful weathering for Sargelu and Naokelekan.The chemical variability index specifies that the Sargelu, Naokelekan, and Najmah are compositionally immature sediments.The tectonic indices suggest that the deposition of Sargelu and Naokelekan were in active continental margin and of Najmah Formation in the oceanic island arc.The geochemical results from major, trace data and provenance diagrams, revealed the felsic igneous rocks for Sargelu and Naokelekan formations and felsic-intermediate for Najmah Formation.

Fig. 2 .
Fig. 2. Stratigraphic column of Z1 Well showing the lithology in Zey Gawra area, Kurdistan region of Iraq.

Fig. 3 .
Fig. 3. EF diagram of the used trace elements for the studied samples.A horizontal line (EF=1) signifies the enrichment or depletion of an element.

Table 1 .
Element oxides amount (in wt.%) for the samples of the Sargelu, Naokelekan, and Najmah formations in Z1 Well.

Table 2 .
Element concentrations for the samples of the Sargelu, Naokelekan, and Najmah in Z1 Well.

Table 3 .
The various geochemical proxies calculated for the samples of Sargelu, Naokelekan, and Najmah in Z1 Well.