Sedimentary and Tectonic, Implications on Late Cretaceous Reservoir Succession Along the Oil Fields, Mosul Block, Northwestern Iraq

Abstract


Introduction
The Mosul Block, as proposed by Dunnington (1958), Lovelock (1969), and Jassim & Goff (2006), is a region of tectonic interest in Iraq's Foot Hill Zone, a part of the Low Folded Zone according to Buday and Jassim (1987).Sufayia and Ain Zalah oil fields are located approximately 120 km and 80 km, to the northwest of Mosul, respectively near the Syrian border and well Tel-Hajar-1 in the South of Mosul Block and Jabal Sinjar (Fig. 1).Iraq's largest hydrocarbon reservoirs from the Cretaceous, which are part of mega-sequences AP8 and AP9 on the Arabian Plate, make it one of the world's most abundant hydrocarbon provinces Beydoun (1991).The Cretaceous succession in Iraq boasts a thickness of up to 3,000 m (Sharland et al., 2001;Aqrawi et al., 2010).
The Mosul Block endured extreme uplift until the Oligocene period (Al-Azzawi, 2013).The hydrocarbon potential during the Upper Cretaceous has resulted in the thinning of strata.Historic exploration and development for petroleum first well Ain-Zalah-1(Az-1) was drilled in 1930 in the Mosul Block, Later, in 1973, Tel-Hajer (Th-1) was drilled South of Jabal Sinjar, and Sufayia-1(Sf-1), was drilled in 1970, This study intends to investigate the Late Cretaceous section's sedimentary facies and stratigraphy as well as the effects on the reservoir and the scenario of tectonic evolution through the Cretaceous reservoir three fields Tel-Hajar, Ain-Zalah, and Sufayia.

Geological Setting
Iraq's largest hydrocarbon reserves are hosted in the Cretaceous sediments, particularly in the Mesopotamian and Foreland Basin.Because warm equatorial climates are common, carbonate rocks make up the majority of the lithologies in the Cretaceous succession found in Iraq and most of the Middle East Al-Hajj et al. (2006).
The depositional history of Late Cretaceous rocks and simplified tectonic models are established in Mosul block NW Iraq.The location is situated around 100 km northwest of Mosul.The W-E axis is oriented parallel to the Taurus suture in the analyzed fields.Mosul Blocks have folds that encircle Cretaceous and Triassic reservoirs, with an asymmetric dissociation (Kaddouri,1979).The Sufayia field and Ain Zalah fields are located in the foothill zone of the Butmah-Mosul belt and on an Unstable Shelf as mentioned in Jassim and Goff (2006), The geological context is extensively documented by Ameen (1979), Numan & Smith (1984), and Al-Azzawi (2013).In the Zagros fold-thrust belt of Iraq, three main blocks, namely Sinjar, Mosul, and Kirkuk, are the dominant features.The formations are organized in a west-to-east direction and are divided by the Mosul Fault and the Greater Zab Fault, which have a northeast-to-southwest orientation (Al-Azzawi, 2013).The Tel-Hajar well is situated at a distance of approximately 30 km in the southwestern direction from Jabal Sinjar.The Sufayia structure field is located in northeastern Iraq, near the Iraq-Syrian border.It is separated from the Syrian Suaday structure by a tiny saddle, which is an extension of the southeast plunge of the Suaday structure, as mentioned by Doski and McClay (2022).

Materials and Methods
• Sampling for the selected wells included core and cutting samples from the cretaceous section (Plate 1A).• The laboratory stage involved the selection of samples for thin section preparation.Petrographic study and microfacies analysis were conducted based on more than 80 thin sections of cuttings from Wells Sufayia-1, Ain Zalah-29, and Tal-Hajar-1.• Petrographic analysis was performed using a polarizing microscope, and the microfacies technique was employed for identification, following the classification of Dunham (1962) modified by Wilson (1975) andFlugel (2004), to determine the deposition environment.• The study also included facies and microfacies analysis, along with the development of a stratigraphic framework.• An examination of available well logs was conducted to establish correlations between log responses, facies and diagenetic variations, and stratigraphic implications on the reservoir.

Lithostratigraphy
The Upper Cretaceous thick carbonate sequences in the studied well in oil fields (Sufayia, Ain-Zalah, and Tal-Hajar) fields in the Mosul Block with variations in thickness and facies, which are closely related to deposition and tectonic phase have impacted the reservoir and stratigraphy, as described by Numan (1997).However, NW Iraq.Stratigraphic are diachronous and erosional unconformities in the studied wells Gir Bir (Cenomanian-early Turonian), Wajnah (Late Santonian), Mushorah (early Campanian), and Shiranish (Late Campanian-Mastetchtian) formations are studied of these successions (Fig. 2).Marly limestone, fractured dolomite, and limestones (mostly recrystallized) are examples of the carbonate rocks of the Gir Bir Formation (Al-Hadidy, 2010).As reservoir sections (Turonian to Santonian) are missing in Th-1 well south Mosul Blocks, all reservoir sections are present (Gir Bir Formation, Cenomanian-Early Turonian) thinning deep-water Mushorah and Shiranish Formations (Santonian to Maastrichtian) illustrated in Fig. 2. Stratigraphic column for the Sufayia field, representing the Cretaceous period from the oldest to younger Gir Bir Formation the main reservoir (Fig. 3).

Gir Bir Formation (Cenomanian-early Turonian)
The Gir Bir Formation is a main production reservoir formation in NW Iraq, displaying varying thicknesses ranging from 0 to 180 m.It is absent in the TH-1 well and increases in thickness toward Sufayia and Ain Zalah, where it reaches a maximum thickness of 150 m, as observed in Sf-1 (Figs. 3  and 4).The Gir Bir carbonates were deposited in protected lagoons, rudist build-ups, outer shelf, slope/shoal, and back-reef/shoal in a regressive shallowing upward sequence.equivalent to the Mishrif Formation in southern and central Iraq.

Basal Mushorah Unconformity (Turonian-Santonian)
This unit is not present in the Tel-Hajar structure inside the South Jabal Sinjar field.The unconformity has variations in thickness, with a more noticeable increase towards the Ain Zalah field.In this area, it transforms into lagoon facies referred to as the Wajnah Formation, as documented by Chatton (1962).
The unconformity in the Sufayia wells is characterized by alternating layers of conglomerates and fine marl sediments.This arrangement serves as evidence of a parallel unconformity.The deposits come from the Gir Bir Formation, which is the oldest formation.The conglomerates in question extend towards the shore region Krumbein (1942), indicating the slope of the sedimentary basin towards the Ain Zalah area.The thickness of varies between 15 and 18 m (Fig. 4).

Mushorah Formation (Late Turonian-Early Campanian)
This formation consists of Calcisphere Recrystallized Limestone secondary porosity containing chert nodules, with limestone units in the field wells measuring 20 to 40 m in thickness.The thickness increases toward the east and in the direction of the Ain Zalah and Tel-Hajar fields (Fig. 4).
The upper boundary is characterized by an unconformity with the Shiranish Formation, marked by a change in facies from crystallized limestone to limestone containing a high percentage of Calcisphere.

Tel-Hajar formation (Cenomanian-Late Campanian)
The Tel-Hajar Formation, first described by Kaddouri (1979), was informally named after the Tel-Hajar well-1.This formation is located thirty kilometers southwest of Sinjar town.It comprises conglomerate, Shell fragment dolomite facies, and coarse dolomite crystal facies in the upper part of the formation as well as sandy grains.See Fig. 4 for more details.limestone, and dolostone pebbles with reworked fossils according to Kaddouri (1979).This formation overlays the Qamchuqa Formation (Albian).

Shiranish Formation (Late Campanian-Maastrichtian)
The Shiranish Formation is observed in all portions of the field wells depicted in Fig. 4. The facies play a crucial role in the production of basinal marly limestone and fractured reservoirs, as initially documented by Henson (1950) and Bellen et al. (1959).Shiranish is composed of blue marl and thinbedded marly and globigerina limestones, with a thickness that varies between 250 and 361 meters.
The thickness varies between 250 and 361 meters (Mousa and Shakir, 2023).The basinal facies of the Shiranish Formation coexist with shallow facies of the Aqrah Formation as lens-shaped deposits.The Aqrah Formation consists of fine-crystalline limestone that is non-dolomitic in certain areas and has well-developed primary porosity.The Aqrah Formation is commonly observed and is restricted to the center region of the Shiranish Formation.

Microfacies Analysis
Microfacies analysis for reservoir section formation is based on Dunham (1962).The identified facies were compared with standard facies, as developed by Wilson (1975) and elaborated by Flugel (2004), to determine the standard environment for all facies.

Gir Bir Formation
• Skeletal Lime Mudstone-Wackestone Microfacies These facies consist mainly of micrite with a small percentage of grains, approximately 10%.They are composed of limestone and dolomitic limestone.These facies are sequentially distributed in the upper and lower parts of the formation in wells plate 1(B) Sf-1 and Az-29.This Facies compared with standard facies SMF-17 were deposited in facies zone FZ-7 within a lagoon environment.As well as being impacted by the diagenesis processes of dissolution, dolomitization, and recrystallization (Plates 1C, D, and E).

• Miliolid Lime Wackestone Microfacies
These facies are skeletal of miliolids, constituting up to 60% of the facies components, The ground is affected by recrystallization, along with the presence of various types of benthic foraminifera.These facies are present in the Sf-1 well Plate 1(F).These facies could be compared with standard facies SMF-18 and were deposited in facies zone FZ-8 according to Flugel ( 2004) & Wilson (1975) within an isolated lagoon environment.

• Orbitolina Lime Wackestone Microfacies
The middle and lower parts of the formation in the Sf-1 and Az-29 wells Composed of Skeletal Orbitolina comprise the largest percentage of grains in these facies, exceeding 70% of the components, with a small percentage of benthic foraminifera present (Plate 2A).The abundance of Orbitolina indicates the deposition in areas far from the reef in the fore reef (BouDagher-Fadel, 2008).

• Echinoderm Lime Grain Stone Microfacies
The upper and middle sections of the Sf-1 well exhibit Bioclastic facies dominated by Rudists, bioclasts, and echinoderms, which make up 70% of the components.Additionally, there are traces of ostracods.The porosity in this section includes both mold porosity and intergranular porosity.This facies corresponds to the SMF-12 deposited in FZ-5, as described by Wilson's models in 1975.The presence of these places indicates their susceptibility to currents and strong waves.

Basal Mushorah
The basal Mushorah is the lower part of thickness ranging from 1 to 4 m.well Sf-1 well, variation They consist of pieces of calcareous grains with sizes ranging from 2 to 10 cm.erosional beds within the submarine environment of peloids near the shoreline (Plate 2B).

• Skeletal Lime Mudstone Microfacies
These facies consist of bioclasts, Planktonic foraminifera, to 10% of their constituents.They are found in the upper portions of the formation in the Sf-1, Az-29, and Th-1 wells, as well impacted by the diagenesis processes of dissolution and intercrystalline (Plate 2C).These facies are compared with standard facies SMF-3 deposited within facies zone FZ-3 according to Wilson (1975) andFlugel (2004) within an open marine environment, located far from the reef slope.

• Calcispheres Lime Wackestone Microfacies
These facies are composed of calcareous balls, representing 65% of the facies components, along with a small percentage of benthic foraminifera and bioclasts, this facies compared with standard facies SMF-3 deposited within facies zone FZ-3 according to Wilson (1975) andFlugel (2004).They are distributed in various locations throughout the open shelf in the Sf-1, Az-29, and Th-1 wells.
• Planktonic Foraminifera Lime Wackestone Microfacies These facies are most prevalent in the upper parts of the formation in the Sf-1 and Az-29 wells and in the middle parts of the Th-1 well.They consist of benthic foraminifera in addition to the presence of planktonic foraminifera (Plate 2D).This facies coincide with SMF-2 deposited in FZ-4 according to models of Wilson (1975) within the open marine environment.
• Benthonic Foraminifera Lime Packestone Microfacies These facies mainly consist of benthic foraminifera, representing 75% of the components (Plate 2(E)).These Facies are compared with standard facies SMF-3 deposited within facies zone FZ-2 according to moduls of Wilson (1975).They are found in the open marine environment and are situated in the middle of the formation from the Az-29 well.

• Lime Mudstone Microfacies
These facies are found in the upper and middle parts of the formation in the Th-1, AZ-29, and Sf-1 wells.It includes planktonic bioclasts.These facies is compared with standard facies SMF-3 deposited within facies zone FZ-2,3 according to Wilson,1975.They are indicative of a Bathyal upper environment.

• Planktonic Foraminifera Lime Wacke Stone Microfacies
Upper and lower parts of the formation.These facies were identified in the Az-29 and Sf-1 wells, Planktonic foraminifera and echinoderms., this facies compared with standard facies SMF-3 deposited within facies zone FZ-2 according to Wilson (1975) andFlugel (2004).They are characteristic of an open-shelf environment.

• Benthonic Foraminifera Lime Wackestone Microfacies
The Sf-1, Az-29, and Th-1 wells contain these lithofacies, primarily composed of benthic foraminifera, constituting 75% of the components.This facies coincide with SMF-9 deposited in FZ-2 according to models of Wilson (1975), represented in the outer shelf setting.

• Echinoderm-Bioclast Lime Packestone Microfacies
These facies are present in the upper and lower parts of the formation in the Sf-1, Az-29, and Th-1 wells.They consist of echinoderms, bioclasts, and planktonic foraminifera, as illustrated in Plate 2 F. This facies is compared with standard facies SMF-11 deposited within facies zone FZ-5.They are indicative of a Platform-Margin Reefs environment.

• Lime Mudstone Microfacies
These facies can be found in the lower part of the Th-1 well.They are composed of sand-filled dolomite layers interspersed with thin conglomerate layers.Within these facies, planktonic foraminifera make up around 10% of the components (Plate 2G).These facies are compared with standard facies SMF-6 zone FZ-3.This facies are associated with an open-shelf environment.

• Lime Wackestone Microfacies
The facies are located in the middle part of the formation in the Th-1 well.Comprising 30%-60% of Ostracods and echinoderms, Recrystallization, with a diagenesis process, has affected these facies (Plate 2H).This facies equivalent to standard facies SMF-19 zone FZ-5.indicating a lagoon depositional system.
• Planktonic Foraminifera Lime Packestone Microfacies Represent all parts of the formation in the Th-1 well and consist of dolomite with coarse crystals.They have been influenced by dissolution processes, as depicted in Plate 2(G).This facies represents standard facies SMF-24 zone FZ-6, they are associated with a mid-tide coastal environment.

Tectono-Stratigraphic Synthesis
During the Late Cretaceous period, the Mosul block in northwest Iraq was believed to have experienced violence due to basin isolation, uplift, and erosion.The wells in Table 1 were studied along the reservoir section.The elevation of the Khleisia and Mosul Blocks from the ancient and current Tethys main basin (Jassim and Goff, 2006).The Mosul block uplift is causing significant erosion and alteration of sedimentary strata (Ameen, 1991).The transition from extensional tectonism to compression tectonism occurred throughout the Cretaceous epoch, as described by Al-Muhamed et al. (2023).This period included the deposition of the Gir Bir Formation (Cenomanian) the sub-basin environments deposited as a form of rudeist limestone in the Northwestern (Mosul Block) region main reservoir shallow-water carbonates platform was deposited in this phase and discontinuity was the end of the second Austrian tectonic movement in Sufayia-1 and Ain Zalah-29 and missing intel Hajar-1, Table -1.During the Second Austrian Alpine movement (Buday and Jassim, 1984) the upper Cretaceous period of the study area was affected by the compression, which leads the uplift (Mosul Blocks) of parts of the passive margin that led to confining sediments towards the stable shelf.The deposition during this stage was influenced by local differential subsidence and eustatic sea-level fluctuations deposition in the Tel-Hajar structure was uplifting during this phase, controlled by the tectonic factor more than the marine factor (Fig. 5A).
• Phase II (Turonian to Late Campanian, obduction phase) The Wajnah, Mushorah, and Tel-Hajar formations represent the successive motions observed in the Tel Hajar well-1.The sedimentary basin in the south side of the study area, specifically the Tel-Hajar field south of Jabal Sinjar, experienced elevating activities.This epoch witnessed the removal of Wajnah, Mushorah, and Tel-Hajer formations, as depicted in Fig. 5B.The onset of the lrmidain tectonic movement activity, characterized by compressive forces, facilitated the final uplift of the passive margin and Northwestern Mosul Blocks in Iraq.
After that, the Armada tectonic movement became active and its effect appeared in Tel Hajar well missing all formations (Wajnah and Mushorah which are equivalent to Sadi, Tunuma and Khasib formations).The previous sediments as source rocks (Sargelu and Naokelekan formations), probably representing immature sediments deposited in a tectonically active setting in North Iraq (Mina and Abdulla, 2023).
• Phase III (Late Campanian to Maastrichtian) During this period, three Formations were deposited under the effect of the Laramide tectonic movement, these Formations are reservoir Shiranish (basinal) and Aqra (Shoal facies) or Harth Formation (Fig. 6) Northeastern Iraq is linked to subduction during the early stages of the Middle Alpine Orogeny, the second episode of the Austrian Orogeny, and the uplift of the Mosul Block Shiranish Formation.This phase is characterized by the beginning to rapidly drown.with the presence of basinal facies with varying thicknesses covering the entire Mosul block region after obduction (Al-Dulaimi, et al., 2023).It shows cases of the development of the basinal fractured carbonate as a reservoir in the Ain Zalah-29 and Sufayia -1 wells in the Tel Hajer -1 well.The Shiranish Formation in three wells, with a tongue of skeletal shoal Aqra Formation-dominated carbonates in the northeast prograding onto the deep basinal facies in the central part of the study area (Fig. 5C).

Conclusions
This work highlights the significance of studying the Upper Cretaceous reservoirs in the Mosul block of NW Iraq, particularly in the Sufayia, Ain Zalah, and south Sinjar areas.These regions show promising potential as productive reservoirs, especially in terms of their water-holding capacity in the Tel-Hajar fields.The study examines the stratigraphic framework and the impact of different phases from wells in Tel Hajer well-1.A significant unconformity was discovered in the Tel-Hajer Formation.The formation extends from the Cenomanian to the Late Campanian in the Tel-Hajer well-1, elevating locally in the platform in other explored areas north of Sinjar, and an open-marine deep shelf environment in eastern Iraq.The Gir Bir Formation exhibits diagenesis that is associated with unconformities.The LimeWackestone-Packestone Orbitolina-Rudust Lime Grainstone Microfacies (FZ-5, 6, and 7) is a specific type of microfacies characterized by the presence of lime wackestone and packstone, as well as orbitolina and rudest lime grainstone.The Lime Wackestone facies consistently dominate the top areas around the unconformity with the Mushorah Formation in all well sections 4).The formation of reservoir rock occurs during the Cenomanian to Early Campanian period.The names of the locations are Ain Zalah-29 and Sufayia-1.The skeletal grains of benthic foraminifera are absent in the Tel-Hajar-1 Formation.Shiranish Formation: Lime planktonic mudstone microfacies with deep basinal FZ -2, 3, and 4. The carbonate platform comprised four distinct types of facies and subenvironments.These were found in the Gir Bir and Mushorah formations, which are shelf and slope platforms.They were basinal, similar to the broken basinal Shiranish Formation.Diagenesis processes influenced the Mushorah Formation.The Gir Bir and upper Wajnah formations were dominated by the Tel-Hajar well-1 Neomorphism, which is positive or uplifted.Silicification resulted in chert, which had little impact on Mushorah carbonates.Dolomitization has slightly improved the reservoir quality.Dissolution, Gir Bir, Mushorah, and Shrianish are a few instances.The formation is a more effective process than cementation.It works in most reservoir beds, increasing the porosity of carbonates.

Fig. 1 .
Fig.1.Location map of the studied wells in the Mosul Block, Northwest Iraq

Fig. 5 .
Fig.5.3D schematic illustrations representing the Cretaceous facies and depositional model evolution in Mosul Block northwest Iraq affected by the uplift phases and stratigraphy evolution.A-Cenomanian-Early Turonian, B. Turonian to Late Campanian, and C. Late Campanian to Maastrichtian