Depositional and Sratigraphic Evolution of the Early-Middle Miocene Succession in Hamrin, Ajeel and Mansuriyha Oil Fields, Central Iraq

Abstract


Introduction
The Miocene succession in Iraq has large diversity in lithology, fossil content, depositional environments, tectonic effects, and the geographical distribution ( Sissakian et al., 2016).Carbonateevaporite deposition occurred in Early Miocene time.Basin wide evaporites and marls were deposited in mid Miocene time (Aqrawi et al., 2010).The Early-Middle Miocene succession in Iraq are represented by the Serikagni, Euphrates and Dhiban formations which deposited during the Early Miocene, and the Jeribe and Fatha succession was deposited during Middle Miocene age.
This study includes microfacies analysis, depositional environments, sequence stratigraphy and basin development of Early-middle Miocene in Hamrin and Ajeel oil fields and Mansuriyha Gas Field.The studied area includes four boreholes in three oil fields located in central Iraq, they are Hamrin (Hr-2) and Ajeel (Aj-13, and 19) oil feilds, and Mansuriyha (Ms-2) Gas Field (Fig 1).Hamrin Oil Field is one of the important fields in the northern part of Iraq.This field represents an asymmetrical anticline which extend from northwest to southeast for more than 100 km, with width from 4 to 7 km.Hamrin Oil Field is sited at the south western part of the Low Folded Zone within the Unstable Shelf (Al-Sulaiman and Ahmed, 2021).
Ajeel oil field is sited about 30 km northeast of Tikrit City, northern Iraq.The elevations of the oil field area is ranging from 150 to 170 m above sea level (Al-Jawad et al., 2014).The Mansuriya Gas Field is located in Diyala Governorate about 45 km to the northeast of Baquba City.It is also sited about 100 km northeast of Baghdad (Azez et al., 2020).
Al-Zaidy (2013) studied the backstripping analysis and basin evolution of the Neogene sequence in northeastern Iraq.He suggested that the Sirekagni and Euphrates formations were deposited at the transgressive stage with high values of subsidence and accumulation rates in the Hamrin-Makhul, and northern part of Tigris subzones.While the area within Chemchemal-Arbil and Butmah-Mosul subzones were represented uplifting areas during Early Miocene (Aquitanian age).This stage ended with a regression of the sea to the southeast with deposition of Dhiban Formation with low values of accumulation and subsidence rates.During the Early Burdigalian, the Jeribe Formation was deposited during another transgressive stage, which covered the area except the uplift positive area in Chemchemal-Arbil and Butmah-Mosul subzones.Al-Dabbas et al. (2013) studied the sedimentological and diagenetic features of Jeribe succession in the northern oilfields.According to this study, the Jeribe Formation was deposited respectively in the open marine, restricted shallow marine, and edge-platform which represent the lagoon depositional environment.While the succession of the lower parts of the Jeribe Formation, especially in well Hm-2, which reflect a deposition in distal fore-slope environment.
Al-Khaykanee and Al-Dulaimi (2016) studied the Miocene succession in Ajil Oil Field and they suggested two biozones to describe the paleoenvironments.The first, in Euphrates succession "Ammonia beccarii-Miogypsina globulina" (Assemblage zone), and the second in Jeribe succession "Borelis melo curdica" (Range zone).Dhiban succession was including only small pieces of distorted bioclasts and dominated by anhydrites, therefore, it lacks the biostratigraphic zonation.Omar et al. (2019) studied the sedimentology and microfacies analysis of the Early Miocene sequence in the Zurbatiya area.They suggested that the depositional environment was deep open marine, outer shelf and the slope/toe slope for Serikagni Formation, and lagoon, restricted and semi-restricted for Dhiban Formation.Al-Zaidy et al. (2013) and Homadi and Al-Zaidy (2020) studied the carbonate microfacies and stratigraphic evolution for the late Cretaceous succession in central Iraq near the studied area.Aoudah et al. (2022) suggested Missan Group to define the sequence of Oligocene-Early Miocene.This sequence was divided into two sedimentary cycles, where each one of these cycles were endded with sequence boundary, equivalent to the siliciclastic residues lowstand in the basin depocenter.The second cycle was represented the Early Miocene Epoch (Aquitanian), this including Euphrates Formation which deposited during the transgression and still stand sea level conditions and ended with lowstand sediments in the basin depocenter which represented by Upper Missan Sandstone Member.These two sedimentary cycles are bounded conformably by Lower Miocene (Jeribe Formation) and Oligocene (Tarjil Formation).

Materials and Methods
The present study depends on data from four wells that include 310 cutting and core samples from Aj-13, Aj-19, Hr-10, and Ms-2 wells.From these samples thin sections are prepared and studied by the aid of polarizer microscope, in addition to available well logs (gamma and sonic).
Table 1 shows geographic coordinate of the studied wells and the thickness of the formations that their samples used in microfacies analysis.

Geological and Tectonic Settings
The study area is located in the Low Folded Zone represented by Hamrin and Ajeel oil fields and extend to Mesopotamian basin including Mansuriya Gas Field (Fig 1).The Low Folded Zone contains outcropping Neogene sedimentary rocks.The Low Folded Zone comprises the Kirkuk-Hamrin and Mosul-Butmah subzones.The Kirkuk-Hamrin Subzone to the southeast of Greater Zab River which includes long unbroken anticlines.Folds are generally flattened towards the Mesopotamian Zone where relatively characterized by narrow anticlines and separated by wide synclines.The Kirkuk-Hamrin Subzone is further divided into two smaller units, Hamrin-Makhul and Kirkuk-Chemchemal (Aqrawi et al., 2010).
The Hamrin Makhul Unit was located in the southern part of the Low Folded Zone of the Unstable Shelf formed during Tertiary (Neogen Period), and it represents the southwestern borders of the Low Folded Zone (Abdulzahraa, 2013).The Mesopotamian Zone contains the largest and richest hydrocarbone provinces in Iraq and is dominated by having the Cretaceous resevoirs.At present-day, this zone is characterizing by a flat-lying area located between northern part of Central Iraq and Kuwait.The Mesopotamian basin is covered by loess sediments, quaternary fluvial and flooding plain deposits of the Tigris and Euphrates rivers.The Mesopotamian Zone was divided tectonically from south to north into the Zubair, Euphrates and Tigris subzones (Aqrawi et al, 2010).Buday andJassim (1984 and1987) have defined the Mesopotamian Zone and considered it a separate structurally unit within Unstable Shelf.It is boarded from the northeastern part by the first superficial and topographical prominent anticlinal ranges which are represented by Makhoul, Himreen, Badra and Buzurgan.On the other hand, the southwestern side of the zone coincides with the Euphrates Fault System (Fouad and Sissakian, 2011).
The Serikagni succession represents basinal association facies.The formation was first identified by Bellen et al. (1959).The type locality lies at Bara Village in Jebel Sinjar area within Foothill Zone.The formation consists in the type locality of globigerinal, chalky limestones with a few more calcareous units.The thickness of the succession in the type area reach 150 m.Fossils are abundant.The assemblages consist mainly of foraminifera.The fossils prove the late Lower Miocene ) Buday, 1980).
Euphrates succession is the most widespread in the central part of Iraq.It was identified by De Boeckh in 1929 and then studied by Bellen in 1957(Bellen et al., 1959).The type area near Wadi Fuhami near Anah City within Stable Shelf which comprising of 8 meters of marly, chalky and wellbedded recrystallized limestones.However, it represents small parts of the succession and doesn't include the basal-conglomerates.Sandstone and anhydrite also occurring in some subsurface sections (Bellen et al., 1959), and are possibly inter-tongues of the Ghar and Dhiban successions, respectively.The formation composed of limestone with oolitic and chalky, which locally include coral and bioclasts of coquina; they are often recrystallized and siliceous sediments.Rock units of green marl, argillaceous sandstone, breccia, conglomerate, and conglomeratic limestone also occur (Jassim and Goff, 2006).
The Dhiban succession was first defined by Henson (1940) and then studied by Bellen et al. (1959) from the type locality near Dhiban Village in the Sinjar area within Foothill Zone.The succession consists of 72 meters of evaporites, thin-bedded marls and brecciated recrystallized limestones.The thickness of the succession is more than 150 meters but it averages about 70 meters (Jassim and Goff, 2006).The Dhiban succession age has been identified according to stratigraphic position with other formations.The Dhiban Formation is overlain the Serikagni succession, interfingering with the Euphrates Formation and is overlying by the Jeribe succession.Therefore, its age has been recorded as Early Miocene (Jassim and Goff, 2006).
The Jeribe Formation was identified by Bellen et al. (1959( from the type area in Jaddala Village within the Sinjar anticline, and assumed to be of Early-Miocene age.However, the formation was later included within the Mid-Miocene succession (Jassim and Goff, 2006).The thickness of the Jeribe Formation in the type locality reaches 70 meters with fossils are sometimes abundant.The index fossil for the formation is Borelis melo curdica., and depending on the index fossil, the age of the formation was determined to be Middle Miocene (Buday, 1980).

Paleoenvironment
Paleoenvironment analysis refering to study or use of the ancient geologic materials (rocks) to infer the depositional environments or settings within which they were deposited (Kennedy, 1998;Okon and Ojong, 2019).
Facies association is defined as collection of commonly associated sedimentary features such as rock types, sedimentary structures and fauna (Miall, 2000in Ajaegwu et al., 2015), and is compared with the standard microfacies types and depositional environment belts of carbonates proposed by Wilson (1975).Five facies associations were distinguished within the study fields: deep marine, slope, platform-margin, open marine, restricted interior platform, and evaporitic interior platform.The facies associations interpreted were based on texture and carbonate grains.Figs. 2, 3, 4 and 5 show the facies distribution and their legend in studied wells.

Deep marine environment
This association is observed within Sarikagni Formation, it is represented by Planktonic wackestone.This microfacies consists of planktonic foraminifera (Plt.1.A), it was observed in Ms-2, and charactrized by micritic matrix with common planktonic foraminifera.The microfacies types indicate deep marine and quite water sedimentation (Flugel, 2010).The calcareous and siliceous shelled planktonic organisms settle through the water-column to the sea-floor after death.The geographical distributions of these organisms in surfaces-waters is affected by nutrients and prevailing ocean currents.After the tiny-shells settle onto the ocean-floor, they may be transported by the turbidity currents (Boggs, 2006).

Slope environment
This association is observed in Euphrates Formation in well Hamreen-10, and it is represented by bioclastic-planktonic mudstone-wackestone (Plt.1.B).The debris of algae and planktonic foraminifera are the main constituents of this facies in addition to shell fragments.
Slope sediments adjacent to steeply sloping carbonate platforms have abundant breccias and turbidite sediments which interbedded with periplatform lime-muds and terrigenous-muds.The slope can be anoxic and lacking benthonic organisms.These sediments are laminated and non-burrowed.Oxic-slope sediments are burrowed and fossiliferous.(Flugel, 2010).

Open marine
This association was observed in all wells in particular in Jeribe and Euphrates formations.It include bioclastic mudstone-wackestone (Plt.1.C), bioclastic peloidal wackestone-packestone (Plt.1.D) and bioclastic foraminiferal wackestone -packstone (Plt.1.E).Open marine is a flat carbonate platform top within euphotic zone, normally above fair weather base wave (Flugel, 2010).This explains the reason of abundance of organisms in this environment.

Restricted interior platform
This association is the dominance in study wells,it consist of ooidal packstone, Foraminiferal peloidal packstone, dolomitic lime mudstone (Plt.1.F), Dolomite (Plt.2.A) and evaporitic peloidal wackestone-packstone(Plt.2.B).These microfacies are not well-connected with the open marine, causing high variations in salinity and temperature.Water-depths below one meter and a few meters to a few tens of meters (Flugel, 2010).

Evaporitic interior platform
In this facies association gypsum, anhydrites or halites may be deposited with carbonates.It includes Evaporitic lime mudstone to wackestone (Plt.2.C), evaporite (Plt.2.D) and evaporitic lime mudstone (Plt.2.E).This microfacies is common in humid and tropical conditions where is only episodic influx of normal marine waters and arid climate so that gypsum may be deposited beside carbonates (Flugel, 2010).

Stratigraphic development
The stratigraphic sequence is a methodology that uses the sedimentary strata order which accumulated within the framework of major deposition and erosion surfaces to interpret the depositional settings of clastics and carbonates sediments from continent, marginal marine, basin margins, and downslope conditions.The surfaces that bounding and subdividing the strata are often interpreted to be formed during the relative sea-level changes which causing associated deposition and erosion (Kendall, 2014).The basic assumption for stratigraphic sequence is that sequences and their systems are essentially controlled by: (1) tectonic subsidence, (2) eustacy, (3) volume of accumulated sediments, and (4) climate.

•
Well Aj -13 In this succession two sequences are observed (C1 and C2) (Fig. 6).The first sequence (C1) was started with transgressive system tract (TST) reflected by evaporitic interior platform, restricted interior platform and open marine occupies the lower part of Euphrates Formation.The TST bounded below by SB and above by MFS.The highstand system tract (HST) divided to three cycles (CE11, CE21, and CDh).The first two cycles (CE11 and CE21) occupy the middle and upper parts of Euphrates Formation reflected by restricted and brackish interior platform environments, while cycle (CDh) is occupies Dhiban Formation and reflected by evaporitic interior platform environment, this sub cycle end at unconformable boundary (SB2) which separated between Dhiban and Jeribe formations.
Sequence C2 started with TST which is observed in the lower part of Jeribe Formation, represented by evaporitic and restricted interior platform.The highstand system tract (HST) occupies the middle and upper parts of the formation and divided to two cycles (CJ12 and CJ22) which represented by evaporitic interior platform and open marine environments.The high system tract (HST) stared from MFS and ended at conformable boundary that separated between Jeribe Formation and Fatha Formation.Fig. 6 shows the sequence stratigraphy of well Aj-13.

• Well Aj -19
In this well the first sequence (C1) started with thick transgressive system tract (TST) occupies the lower and middle parts of Euphraties Formation reflected by restricted interior platform and open marine environments, while the upper part of formation occupied by highstand system tract (HST) which represented by restricted interior platform, open marine and evaporitic interior platform (Fig. 7).
Cycle C2 also represents by relatively thick transgressive system tract (TST) occupies the lower part, middle part and a bit of upper part of Jeribe formation, and divided in to two cycles (CJ12 and CJ22) represented by open marine and restricted interior platform environments, while the upper part of formation occupied by thin highstand system tract (HST) represented by open marine environment.Fig. 7 shows the sequence stratigraphy of well Aj-19  • Well Hr-10 In this well, the sequence C1 started with thin transgressive system tract (TST) reflected by toe of slope and it occupied a bit of lower part of Euphrates Formation represented by Toe of slope.The highstand system tract (HST) is divided to five cycles (CE1, CE2, CE3, CE/Dh and CDh) (Fig. 8).In this well, sequence C2 started with relatively thin transgressive system tract (TST) occupies the lower part of Jeribe Formation represented by open marine environment.Middle and upper part of formation occupied by highstand system tract (HST) which divided in to two cycles (CJ12 and CJ22) reflected by evaporitic interior platform, restricted interior platform, and open marine environments.Figure 8 shows the sequence stratigraphy of well Hr-10.

•
Ms-2 Two sequences are observed in this well (C1 and C2) (Fig. 9).Sequence C1 started with thin transgressive system tract (TST) bounded below by SB1 and above by MFS which corresponding with conformable boundary that separated between Serikagni Formation and Dhiban.Sequence C2 started with relatively thin transgressive system tract (TST) occupies the lower and middle parts of Jeribe Formation, and represented by restricted interior platform environment.The highstand system tract (HST) occupied the upper part of Jeribe Formation started from MFS and ended at conformable boundary which separated between Jeribe and Fatha formations, and represented by open marine and restricted interior platform environments.Fig. 9 shows the sequence stratigraphy of well Ms-2.

Fig. 1 .
Fig. 1.Location and tectonic map of the study area according toSissakian and Fouad (2012)

Fig. 8 .
Fig. 8. Sequence stratigraphic section showing facies associations and depositional cycles for studied succession in Hr -10.
The highstand system tract (HST) is divided in to three cycles (CDh11, CDh21, and CDh31).The cycle CDh11 occupied the lower part of Dhiban Formation represented by open marine, evaporitic interior platform, and restricted interior platform environments, while CDh21 sited in the middle part of Dhiban Formation represented by restricted interior platform environment.Finally the cycle CDh31 sited in the upper parte of Dhiban Formation represented by open marine facies.

Fig. 9 .
Fig. 9. Sequence stratigraphic section showing facies associations and depositional cycles for studied succession in Ms -2.

Table 1 .
Geographic coordinate of the studied wells and the thickness of the formations

•
The Early-Middle Miocene succession in Iraq is represented by the Serikagni, Euphrates and Dhiban formations, which deposited during the Early Miocene, and the Jeribe and Fatha formations were deposited during Middle Miocene age.•Five facies associations were distinguished within the studied fields: deep marine, slop, platformmargin, open marine, restricted interior platform, and evaporitic interior platform.The facies associations interpreted were based on texture with skeletal and non-skeletal grains.• Deep marine environment association facies consists of planktonic foraminifera wackestone, it was observed in well Ms-2.• Slope facies association is observed in Euphrates Formation in well Hamreen-10, and it represented by bioclastic-planktonic mudstone-wackestone. • Open marine association is observed in all studied wells in particular in Jeribe and Euphrates formations, and it includes bioclastic mudstone-wackestone, bioclastic peloidal wackestonepackstone and bioclastic foraminiferal wackestone-packestone.• Restricted interior platform association is dominated in all studied wells, it consists of Ooidal packstone, foraminiferal peloidal packstone, dolomitic lime mudstone, dolomititic and Evaporitic peloidal wackestone-packstone.