Hydrocarbon Potential and Depositional Environment of the Shiranish Formation in Northern Iraq

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Introduction
Upper Campanian-Maastrichtian Shiranish Formation, which is the first described in North eastern Zako Town in High Folded Zone North of Iraq, consists of argillaceous limestone in the lower part and pelagic marl in the upper part; with variable thickness between 227 m in type section and more than 2000 m in Anah Graben in West Iraq. Many depositional environments were suggested for Shiranish Formation in different areas of north Iraq. Mainly in marine, middle shelf, outer shelf, and upper bathyal environmets were suggested for deposition of Shiranish Formation in Jabal Sinjar area in (Al-Banna, 2010); outer shelf-upper bathyal in Khabaz oilfield at Kirkuk area (Al-Juboury, 2011); outer shelfmiddle bathyal between depth sea (150-1500) m in Qaiyarah oilfield (Ahmed, 2011); turbidites and submarine fans depositional environment at Bekhme area (Al-Rashedi, 2013), and middle shelf to middle bathyal at Dokan area (Malak, 2011). However, the latter suggested that depositional environment of lower part of the formation is deeper than that of the upper part.
The Shiranish Formation considers the main reservoir units in several oilfields of central Iraq (East Baghdad and Ahdab oilfields) and north Iraq (such as Ain Zala, Jawan, Najmah, Qasab, Jawan, and Pulkana oilfields). In addition, recrystallized and fractured limestone of Shiranish Formation forms reservoir in Butmah, Ain Zala and parts of Kirkuk oilfields in north Iraq. 30000 b/d were produced from fractured limestone of Shiranish Formation in Ain Zala oilfield; matrix porosity is up to 11% but the permeability is low (0.01 mD). In Sufaya oilfield in northwest Iraq, permeability is higher (13 mD) (Aqrawi et al., 2010). Non-commercial oils were discovered in Shiranish Formation in Shoresh-1 Well, near Erbil (Sachsenhofer et al., 2015).
Shiranish Formation was not considered as source rocks where no proven intervals were identified; however, sometimes it is bituminous, but this bitumen may be migrated from deeper intervals (Aqrawi et al., 2010). Its equivalents in west Iraq contain phosphatic units and they are organic-rich (Jassim and Al-Gailani, 2006). considered additional source rocks for oil in Upper Cretaceous and possibly Tertiary reservoirs in NE Syria (Abboud et al., 2005); and it has source rock characteristic in Euphrates Graben of Syria, with TOC values in the range of 1.37 -2.73 wt.%, hydrogen index (HI) values 212-275 mg HC/g TOC, and it contains type II-III kerogen in the lower part; whereas the upper part has TOC values between 2.27-3.58%, HI values between 263-469 mg HC/g TOC, and has mainly type II kerogen (Ismail et al., 2010), Gurpi Formation in Iran considered local source rock along Khuzestan-Laurestan basin margin, with TOC values between 1-1.5% (Bordenave and Huc, 1995); but TOC decrease to 0.5-0.8% in central Iran (Baniasad et al., 2017). In Iraq, few studies concerned with petroleum generation potential of Shiranish Formation; therefore; this study aims to investigate the source rock potential of Shiranish Formation in three different areas of north Iraq.

Geological Setting
Jassim and Buday (2006 a) divided Iraq into two tectonic zones: Stable Shelf and Unstable Shelf. Unstable Shelf, which is highly affected by Cretaceous and Tertiary deformation, subdivided into four zones: Low Folded (Foothill); High Folded; Zagros Suture; and Imbricated Zone. Stable Shelf, which less, or not affected by deformation; subdivided into three zones; Rutba-Jezira, Salman, and Mesopotamian zones. The study area is located in Low and High Folded zones of the Unstable Shelf ( Fig. 1).
During the Late Campanian-Maastrichtian time the climax of obduction and closure of the New-Tethys were occurred, which contributed to the major marine transgreesion across Iraq. The stress regime in the northeast the Arabian Plate led to formation intraplate basins with NW-SE and E-W trend, in which, several facies were deposited during Campanian-Maastrichtian. Lagoonal to inner shelf facies of Hartha Formation was deposited in Rutba-Jezira zone of west and south Iraq. On Rutba subzone, Tayarat Formation was deposited on broad carbonate shelf. Semi-isolated basin was developed in Anah Graben, in which, phosphorite and silicified carbonates of Digma Formation was deposited. Carbonate and marls of Shiranish Formation were deposited in outer shelf to basinal environments. Toward the thrust front, the Shiranish Formation passes laterally into the Hadiena/Bekhme/Aqra formations, which, in turn, pass northeast into the Tanjero Formation which is flysh facies deposited in fore deep basin created by strong subsidence of Balambo-Tanjero in northeast of Iraq.  ( Jassim and Goff, 2006) modified after (Jassim and Buday, 1984)

Materials and Methods
For this study, 20 core and cutting rock samples of Shiranish Formation were used from three different areas of northern Iraq. 15 samples are subsurface samples from Sufaya (40 and 41 wells) and Pulkana oilfields (well 7); whereas the other 5 samples were from surface outcrop from Dokan area (Table 1). Total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC) and Rock-Eval pyrolysis analyses were performed for all samples. For TC, TOC measurements were done by the use of Solids Module 1000 0 C. The carrier gas for combustion is synthetic air (carbon hydrogen <0.1ppm, CO2 <0.1ppm). The liberated CO2 was measured in the IR, and then it would appear as two peaks, one for TOC and the other for TC. The carbonate content calculated according to the following equation: CaCO3%= TIC*8.34 (1) For pyrolysis analysis, the powder samples were pyrolyzed at 300 o C (hold time 3 min) followed by heating at a rate of 25 o C/min up to 600 o C, held for 5 min. Parameters determined by Rock-Eval pyrolysis include hydrogen index (HI= S2*100/TOC mg HC /g TOC), oxygen index (OI = S3*100/TOC; mg/g TOC, Tmax (temperature of maximum pyrolysis yield) and genetic potential it was calculated from the sum S1+S2. These parameters were used for kerogen classification, potentiality and estimate thermal maturity level.
Most of samples have low TOC values; therefore, one sample with the highest TOC value (sample 1 in Table 1) was selected for molecular geochemistry. For this analysis, the powder sample was extracted using dichloromethane (DCM) as solvent. After precipitation of asphaltene by adding excess of hexane, the extract was fractionated into three fractions: saturated hydrocarbons, aromatic hydrocarbons, and resin (NSO-compounds) using column chromatography on silica gel by successive elution by n-pentane, n-pentane-dichloromethane (40/60 v/v), and methanol.
The Gas Chromatography (GC) analysis was performed on a 30 m × 0.32 mm J&W DB-5 column (0.25 μm film thickness) and temperature programmed from 60 °C to 350 °C at 12 °C/min using an Agilent 7890A gas chromatograph with a FID detector. Helium is used as the carrier gas. Gas Chromatography-Mass Spectrometry (GC-MS) analysis of saturate and aromatic fractions was performed using an Agilent 7890A or 7890B GC interfaced to a 5975C or 5977A mass spectrometer. The J&W HP-5 column (50 m x 0.2 mm; 0.11 µm film thickness) is temperature programmed from 150°C to 325°C at 2°C/min.

Organic Matter Richness
Total Organic Carbon (TOC), or organic carbon (Corg), is the one of the most important factors in assessing source rock potential. It measures the quantity organic carbon expressed as a percentage weight of the rock . Therefore, attention to it is one of the priorities of rock assessment. TOC values of the studied samples are shown in Table 1. Generally, TOC values are low in Sufaya (0.16-0.21% and 0.19-0.31 in Sufaya Wells 40 and 41, respectively) and Dokan (0.12-0.40%) and relatively moderate in Pulkana (0.6-1.54 wt.%). S2 and genetic potential values are consistent with TOC values, they are low in Sufaya (0.07-0.30 and 0.12-0.45, respectively) and Dokan (0.04-0.69 and 0.04-0.69; respectively); and relatively high in Pulkana (0.76-4.47 mg HC/g rock and 2.04-6.51, respectively) (Fig. 2). No significant changes in TOC and S2 values with depth in Sufaya and Pulkana. According to these results, the studied samples of Shiranish Formation in Sufaya and Dokan can be classified as poor source rocks; while in Pulkana it can be considered fair source rocks (Peters and Cassa, 1994). However, in Euphrates Graben of Syria, TOC values are higher and they show changes with depth; they increase from 1.37 to 2.73% in the lower part to 2.27-3.58% in the upper part (Ismail et al., 2010).
The relative low TOC values of the Shiranish Formation at Sufaya and Dokan (at least the studied intervals) is probably due to the dilution effect caused by the high sedimentation rate and/ or the poorer preservation of organic matter when the basin came under the influence of a stronger clastic input with the formation of organic-lean sediments, or these organic-lean sediments could have been deposited under more oxic environment.

Organic Matter Type
Kerogen type is of important criterion for source rock assessment, where different organic matters, or kerogens, have different hydrocarbon-generation potential (Tissot and Welte, 1984). Rock -Eval pyrolysis parameters, including oxygen index (OI) and hydrogen index (HI), are commonly used to describe the organic matter type (Espitalie et al., 1977;Tissot and Welte, 1984;Peters et al., 2005). From OI versus HI and TOC versus S2 diagrams (Figs. 3 and 4), it is clear that Pulkana samples fall within type II, mixed type II/III and type III; Dokan samples mainly within type III; whereas those of Sufaya are fall mainly within type IV, which is composed mainly of terrigenous, or oxidized, organic matter. The dominance of terrigenous organic matter over the marine ones is supported by the terrigenous to aquatic ratio (TAR) and n-C27/n-C17 ratio, which they are 1.12 and 2.61, respectively ( Table 2). The variation in organic matter type is consistent with variation in TOC values , which they are relatively high in Pulkana and low in Dokan and Sufaya. This varation in quantity and quality of organic matter can be due to lateral facies changes caused by variation in paleodepositional environment as discuused later.

Thermal Maturity
Tmax values of the studied samples of Shiranish Formation are between 420-440 o C; indicating the organic matter is immature to early mature. However, one sample has high Tmax value (446 o C). This high value is associated with very low S2 value ( 0.04). With this low S2 value, Tmax value is unreliable for maturity assessment (Hunt, 1996). The low maturity level of the Pulkana samples is supported by high percent of polar fractions (resin+asphaltenes) of rock extract (51.7%). Moreover, carbon preference index (CPI) (Bray and Evans, 1961) is less than one (0.93); indicating low thermal maturity level. The low thermal maturity level is also supported by low moretane/hopane ratio (0.07), cracking ratio of the triaromatic steroids (0.31), Ts/Tm ratio (0.23) (Fig. 5), and C27 diasteranes/regular steranes (0.08); however, the last two ratios are affected by lithology (Peters et al., 2005). In addition, the 20S/20S+20R and ββS/ ββS + ααR of C29 steranes were not reached the equilibrium values (0.31 and 0.43, respectively) ( Table 2); indicating that organic matter is immature-early mature (Peters and Moldowan, 1991;Peters et al., 2005;Burton et al., 2018).  However, hopane and sterane stereoisomer maturity ratios should not be applied to infer maturity level in organic sulfur-rich petroleum systems because they are anomalously elevated at low maturities (French et al., 2020). In northeast Syria (which is very close to Sufaya of northwest Iraq), Abboud et al. (2005) observed that terpanes and steranes fingerprints of Shiranish rock extracts are similar to those of crude oils of Upper Cretaceous and Tertiary reservoirs; therefore, they concluded that Shiranish Formation in NE Syria is early mature (Tmax is 435 o C) and considered as additional source rocks for hydrocarbons reservoired in Upper Cretaceous and Tertiary successions.

Depositional Environment
Carbonate Content (CC) of the studied samples of Shiranish Formation is variable in the three locations (Table 1). The average CC values are medium in Sufaya and Dokan (54 and 59 %, respectively) and relatively high in Pulkana (average 83%). The high CC of the later suggests strong marine influence; this assumption is supported by relatively high sulfur content of the extracted bitumen (3.21 wt. % in sample 1); whereas the medium CC of Sufaya and Dokan(or at least the investigated intervals) suggest marine depositional environment with high influence of terrestrial input. The presence of type III and IV kerogen in these two locations associated with medium CC, low hydrogen index (HI) values, and high oxygen index (OI) values suggests more oxic depositional environment which led to poor preservation of organic matter and/or presence of reworked organic matter. In addition, thickness of Shiranish Formation at Dokan and Sufaya areas is thinner than that of Pulkana area (Al-Jubouri, 2011). Moreover, Shiranish Formation at Sufaya area represented mainly by limestone and sometimes sandy limestone, especially at the upper part; whereas at Pulkana and Doka areas it is marly limestone and marl (Al-Juboury, 2011). This may suggest that Pulkana area is close to basin center (i.e deep area).
The simultaneous increase of organic matter and carbonate content in the Pulkana and, to less extent, Dokan may reflect two different scenarios (Ebli et al., 1998), either the rate of terrigenous material deposition remained constant while the deposition of carbonate and organic matter were accelerated; or the rate of deposition of terrigenous material decreased and that of organic matter and carbonate remained constant. The transition of these two pure scenarios can be envisaged.

Conclusions
The main conclusions of this research are: • Shiranish Formation at Dokan and Sufaya area is poor hydrocarbon source rocks; while at Pulkana it is fair source rocks. • Shiranish Formation contains mainly type II/III and II at Pulkana; and III and IV at Dokan and Sufaya. • The Organic matter maturity level of Shiranish formation at these locations is immature-early mature. • Depositional environment was oxic at time of deposition of Shiranish Formation at Dokan and Sufaya; and suboxic-anoxic at Pulkana area.