Tectonostratigraphic Framework and Depositional History Pattern of the Cretaceous Successions Period in Southern Iraq

Abstract


Introduction
Most previous geological studies focused on the lithology of subsurface formations, especially formations that produce oil (e.g.Mishrif, Zubair, and Nahr Umar formations).Most of these studies (for instance; Al-Ali et al., 2019;Alsultan, 2021;Al-Garbawi and Al-Shahwan, 2019;Menshed and Al-Mozan. 2021) include sedimentary and stratigraphy research, but these studies were isolated and had an absence of connection with tectonic and structural data.These studies adhered to the old tectonic concepts since Buday (1980).Lately, studies aimed to connect the available geological information with a single framework that can help to predict the oil production process, which is known as the petroleum system.This research aims to study the Cretaceous period in southern Iraq according to the petroleum system concept descriptively and focused.The Arabian passive margin is considered an important continental margin globally due to its natural resources, especially oil and gas.The Cretaceous sediments are among the important geological deposits.The current research focused on the sequences of the Cretaceous period to complete an image of this important period, by selecting 20 oil wells (Gn-1, Dn-1, Ak-1, Ks-1, Wk-1, UmQ-1, R-5, Rt-3, Rt-4, NNU-1, NNU-3, WQ-13, WQ-115, Mj-3, Mj-4, Snd-1, Rf-1, Am-2, Hf-1, and Noor-1) distributed in three oil governorates in southern Iraq (Basra, Maysan, and Thi Qar), which are among the provinces that contain the largest giant oil-producing fields in Iraq, such as Rumaila, West Quran, Majnoon, Nahr Umar, Halfaya, and Ratawi (Fig. 1).Sedimentary and stratigraphy information was collected from previous studies related to the proposed scenario of tectonic accidents in this period, based on studies of Numan (1997 and2000), in order to reach a clear and comprehensive knowledge of the mechanism of sediment distribution and to identify the effects of tectonic movements on the physiology of sedimentary basin.The Lithology bonding method has been used as a philosophy for oil production for past years in Southern Iraq regions, which was based on the production of similar rock layers with neglect of the affective of lateral rocks microfacies variation.The concept of sequence stratigraphy was adopted to solve the problems of oil production, which used the environmental structural link method for genetic package rocks without taking its lithological variation (Homewood, 2000;Emery and Myers, 1996).So, the importance of the current study trying to draw an image of the vertical and lateral extension of the formations under study by dividing them into specific environmental zones that illustrate this idea and give an insight into the status of the sedimentary column of the Cretaceous period in southern Iraq.This study is based on the concepts of plate tectonics theory, instead of the geosynclines theory that was adopted by most of the previous studies.The geosyncline theory provides detailed information on the sedimentary, stratigraphy, and environmental characteristics of a region, but it is unable to give appropriate tectonic explanations.Davies et al. (2002) described the nature of sediments in the Arabian margin during the Early Cretaceous, and concluded that the sediments of this period extend into three longitudinal zones; these are the carbonates zone, the mixed of clastic and carbonates zone, and the clastic zone (Fig. 2).

The Tectonic Setting
Iraq represents the northern and northeastern margin of the Arabian plate.The Arabian plate represented the northeastern part of the African plate, It extended north and northeastwards over the central, southern and southeastern parts of Turkey (Kummel, 1970, Numan, 1997).The tectonic map of Iraq was updated many times during the period from 1984 to 2015 by many authors.Tectonically, many terminologies have been used by many authors, Buday and Jassim (1984) complied the first tectonic map of Iraq relied basically on the old principles of the geosynclinal theory, they used stable shelf and unstable shelf terminology, they have divided the Mesopotamian zone who forms aboard zone in Iraq into three subzones: the Tigris subzone in the north who is the most mobile unite of the Mesopotamian zone, the Euphrates subzone in the west and the Zubire subzone in the south.Based on plate tectonic theory, Numan (1997) put the tectonic division of Iraq.Jassim and Goff (2006) also compiled a tectonic map of Iraq using almost the same terminology but with slight differences, which did not depend on the Eugeosynclinal theory, but considered the Mesopotamian zone is part of the stable shelf.The last tectonic map updated was putting by Fouad (2015).The main part of Iraq is divided into two main parts, the first one is the Inner plateform (stable shelf) and the second part is the outer plate form (Unstable shelf), which is the Abu Jir-Euphrates active fault has represented the contact between the tow part Fouad (2007;2015).
This study depended on the tectonic divisions of Iraq by Fouad (2015).According to this divisions, the study area extends within tow tectonic zones of the Arabian platform , the first area of the studied oil fields is located in the Inner platform and the second one in the outer plateform within the Mesopotamian Foredeep subzone (Fig. 3).

Materials and Methods
During the Cretaceous period, the Arabian plate was affected by two tectonic mega sequence phases (AP8, AP9).These two phases lasted for 86 million years.The (AP8) phase continued for 57 million years and was characterized by mixed carbonate siliciclastic sediments of the Lower Cretaceous period.The location of the Arabian plate during this period was the tropical position, while The (AP9) phase has lasted to 29 million years and dominated by carbonate sediments Sharland et al. (2000) (Fig. 4).The subduction of the oceanic crust of the Neo-Tethys appeared under the Turkish and Iranian plates at the end of the Jurassic period, which lead to a geodynamic inversion of the tectonic system from elongating to compression during the Cretaceous period Numan (2000).The tectonic movement is devided into two episodes as shown in Table 1.The tectonostratigraphic evidence indicates that the extensional tectonism responsible for the separation of the Iranian and Turkish plates from the Arabian plate and the opening of the Neo-Tethys were generated in the Triassic period.The elongation conditions were affected on the two edges of the inactive plates on both sides of the new Tethys ocean during the Jurassic period.The signs of the subduction of the oceanic crust of the new Tethys under the Turkish and Iranian plats, they appeared at the end of the Jurassic period, and the result of this subduction was the occurrence of a geodynamic coup of the tectonic system from extensional tectonism to compression tectonism during the Cretaceous period (Numan, 2007).The compression forces had changed the preexisting listric normal fault into reverse faults, this mechanism exists in the foreland belt of northern Iraq (Numan and Al-Azzawi, 1993).
There are two basic types of continental margins, active and passive margins.Active margins are continental margins that coincide with either transform or convergent plate boundaries, and thus are seismically active.In contrast, passive margins are not seismically active and develop over the edge of a rift after the rift-drift transition (Condi, 1989;Van der Pluijim and Marshak, 2004).
According to Almutury and Alasadi (2008), the passive margin of Mesopotamian had been divided into two phases: First, is the opening phase, which is represented by divergent plate boundaries formed where the plates moved apart from one another.The second is the closing phase, characterized by convergent plate boundaries that formed where plates moved toward each other.
According to the Wilson cycle, the tectonic position of the Arabian plate during the Cretaceous period was part of the closed phase, specifically the subduction Set-up or Pre-collision Set-up mode.This situation is tectonically characterized by the zoon of the subduction oceanic crust of the new Tethys under the Iranian and Turkish plates, which made their edges active margins, while the edge of the Arabian plate remained passive margin (Numan, 2000) (Fig. 5).
A passive margin is defined as a continental margin within a single lithospheric plate and fused to adjacent oceanic crust.It includes continental shelf, continental slope, and continental rise Plummer et al. (2003).The passive margin was generally covered by shallow water, however, a number of deeper water intra-shelf basins had been formed during the Cretaceous (Murris, 1980).
On the tectonic side, the southern Iraq region in the lower cretaceous was part of the passive margin of the Arabian Plate, which represents the confluence of the continental crust with the oceanic crust.Southern Iraq occupies the largest part of the continental shelf area, which is structurally distinguished by containing half-graben basins, which are formed as a result of the presence of a number of Listric Normal Faults formed during the Triassic and Jurassic periods because of the tensile forces, However, during the lower Cretaceous, and as a result of the pressure forces, the movement on its levels changed to the reverse movement (Numan and Al-Azzawi, 1993).
The parts of the passive margin affected by Listerian faults are called Quasiplatform Foreland, and during the Cretaceous in Iraq, they were Submergence and generally uneventful from discontinuity.While the areas not affected by these faults, which currently occupy the Western Sahara region, are called the Stable Platform, during most of the Lower Cretaceous period was a positive region (Numan, 1997 and2000).
The stable platform area was a source of the continental sediment in which the Sub-basins at the passive margin were filled.These basins played a prominent role in determining the quality of sediments and the nature of their distribution in southern Iraq, they worked to complicate the topographical shape of the bottom of the sedimentary basin represented by the passive margin of the Arabian plate.Perhaps this explains the nature of intense rock variation from one region to another within southern Iraq, which is evident in several oil wells in the study area.
During the Second Austrian Alpine movement, the upper Cretaceous period of the study area in southern Iraq was affected by the compression, which leads the uplift parts of the passive margin that led to confining sediments towards the stable shelf, and bringing marine sediments from high marine areas (Fig. 6).With the continuation of the second Austrian movement and the beginning of the Laramid movement, general parts of the passive margin were raised, forming the Foreland basin which was leaning towards the stable shelf area.The sedimentary basin was divided into Intra shelf basins consisting of Lagoonal environments confined in the western parts of the study area and open marine environments in the eastern parts.The sediments of this basin were diverse both horizontally and laterally, depending on the physiography of the sedimentary basin resulting from the intensity of the influence of both movements.

Sequence Stratigraphy
Several previous studies have been conducted on this field, where they concluded a detailed vision of facies, environmental sedimentation, and reservoir properties (e.g.Razoyan, 1998;Al Bayati, 2001;Zaibel, 2001;Razoyan, 2002;Shaawash, 2002;Al Mohammad, 2002;Mahawi, 2003;Al Ali, 2004;Handhal, 2006;Al Bayati et al., 2010;Al Bayati et al., 2011).The Cretaceous period in southern Iraq includes eleven sequence stages, in which seventeen formations were deposited (Suliy, Yamama, Ratawi, Zubair, Shuaiba, Nahr Umar, Mauddud, Ahmadi, Rumaila, Mishrif, Khasib, Tanuma, Saadi, Hartha, Shiranish, Tayarat) (Fig. 7 and Table 2).Generally, the Cretaceous period is characterized by rising eustatic sea level (Haq et al. 1988a).The contact between the lower and upper Cretaceous sequences is conformable except for the sequences of the far southwestern parts of Iraq where the unconformable contact is represented by the deposition of the Wara Formation in the form of a tongue (Razoyan, 1995).Whereas the contact is unconformable between the lower and early Tertiary due to the loss of sediments of this period towards the wells of the Amara region.This sequence ends with the deposition of the Tayarat Formation and then Aalijy Formation, which is the conformable contact between the Lower Cretaceous and the late Jurassic.The Cretaceous period has three unconformity surfaces.The first separates Mauddud Formation from Ahmadi Formation.The second separates Mishrif Formation from Khasib Formation.The third separates the Tayarat Formation from Umm Erduma Formation (Figs. 8 and 9).Generally lower cretaceous is characterized by shallowing upward cycles, while the upper cretaceous represents deepening upward cycles, the stratigraphic setting of the Cretaceous period was illustrated and summarised by Razoyan (1995) (Fig. 10).

Results
According to the changes of the sedimentary environments in determining the extent of deepening and shallowing of the sedimentary basin and, since the variation of the sedimentary environments is important evidence in determining the accommodation space, It is a function for base-level transient cycle heterogeneity.This study has recognized: • There are eight maximum flooding surfaces (MFS).• The high system tract (HST) is represented by the Yamama, Ahmadi, Mishrif, Tanuma formations, the lower part of the Saadi, Hartha, and Tayarat formations.• The transgressive system tract (TST) is represented by the Ratawi, Shuaiba, Mauddud, Rumaila, and Khasib, the upper part of the Saadi formation and Shiranish formation.• The low stand system tract (LST) is represented by Zubair and Nahr Omr formations.
• The Cretaceous period is divided into one cycle from the third order, and seven and a half cycles from the fourth order.• The Cretaceous period includes seven genetic stratigraphic packages (GSS ) (Fig. 11).The concepts of plate tectonic theory give more realistic explanations of the stratigraphic situation in southern Iraq, especially when the structural nature of the sedimentary basin is taken into regard.The interpretation of the phenomenon of horizontal variation in sedimentation between marine, mixed and continental sediments is due to the tectonic position of the southern region of Iraq at the northeastern margin of the Arabian Plate, which was the passive margin, and characterized by the presence of subbasin result from many of listric faults.The effect of marine on the northeastern side of the passive margin of the Arabian plate is represented by the accumulation of high-thickness limestone sediments in the eastern regions of this edge, while the continental effect of the platform on the southwestern side deposited the clastic sediment in the western regions in the passive margin of the Arabian plate, which was not separated from the African plate at that time.This combined effect produced clastic sediment in the western desert and southern parts of the stable platform area in Iraq, which is represented by Zubair -Nahr Umr formations, while the platform areas far from the Western Desert toward the east were characterized by alternating clastic and limestone in Ratawi, Zubair, and Nahr Umr formations.The basin area was characterized by limestone sediments of the continental shelf in Sulaiy, Yamama, Ratawi, Shuaiba, Nahr Omar, and Mauddud formations.As a result, three sedimentary zones were formed; these zones are the marine limestone deposits, the mixed carbonate-clastic deposits, and the continental deposits.The boundaries between these zones are represented by relatively wide areas rather than sharp lines.The boundaries were characterized by their horizontal movement during the lower Cretaceous due to the dominated one of the marine or continental effect on the other (Fig. 12 and Table 3).

Stage and Substage Formations
The thickness of formation in the clastic zone in meters (Si-1, Dn-1) The dominance of the continental factor occurred firstly in the Young Kimmerian movement that led to the uplift of the passive margin as a result of the Arabian and Iranian plates convergence, then the marine regressive led to the dominance of the continental factor and shift the boundary between the three zones towards the east.Opposite of that, during the second tectonic movement (First Austrian Alpine movement) the marine factor is dominant because of the subjection of the ocean crust under the Iranian plate.Thus, the discharge of stresses resulting from the forces of pressure, this unloading of the strains led to subsidence in the passive margin and increased its depth in addition to the sea progress towards the stable platform, leading to a shift in the boundary between the three regions towards west land.It should be noted that the first movement caused alternation between the high thickness of the clastic and limestone formations.The second movement is the First Austrian Alpine movement that created alternation between the claystone and limestone with fewer thicknesses within the same formation (Nahr Omar), this movement ended with relative quiet in a short time and Mauddud Formation deposited.It can be concluded that the first movement was more active than the second one, in terms of the higher thickness in the first movement compared to the second.
The previous microfacies studies recognized micro interment within the same rock type, Benthonic lime mudstone microfacies concentrated within south and southwest from the platform, while Pelagic limestone and mudstone microfacies toward the northeast region.So is the sandstone in the southern and southwestern regions, which is represented by the dunes, while in the north-eastern areas, it becomes a river and deposits of the delta.also, within the Basra regions, it showed interference between clastic and limestone represented by a group of rocks, Limey Sandstone, Sandy limestone and conversely, Sandy shale, Limey shale, Shaly limestone, and Shaly sandstone.

Berrisian to Valanginian (144-132) Million Years
This period included the deposition of Sulaiy and Yamama formations in oilfield Si-1 and Dn-1 which are located in the region of the continental platform.The main structures (West Qurna, Rumaila, Zubair, etc.) in the unstable shelf province probably were growing during the Yamama deposition, leading to facies differentiation within the same structure.These formations have a relatively small thickness of about 35 meters for Sulaiy and 93 meters for Yamama.Moving to the wells in the east, which are located within the Basrah area, especially in the wells (Lu-12, WQ-115, Rt-3, Rt-4), the thickness of the two formations increases to several hundred meters, where the thickness of the Sulaiy is about 254 meters and Yamama is around 233 meters.Finally, the wells located within the north of Basra and Amara (HF-2, Am-2, Noor-1, Mj-3, Mj-4, WQ-13, Rf-1), Sulaiy Formation continue to have a high thickness of about 286 meters while Yamama Formation thickness is reduced to an about 171 meters.After this period the convergence occurred which led to the activation of the listric faults group responsible for the formation of the sub-basin, this reflects on the nature of the deposition of the Yamama Formation, which is deposited within a group of the secondary depositional basin (Al-Mohammed, 2002).The present study indicated a high thickness of Yamama formation within the Basrah area and that thickness decreases in Amara and Western desert, which indicates that the center of the basin is located within the Basrah oilfield (WQ-115).While Yamama Formation consists of limestone in the open sea area (Al-Bayati, 2001), these sediments shallowed to contain the remains of evaporators within Western desert.

Hauterivian to the Barremian (132-121) Million Years
During this period, a mix of lithofacies was presented by Ratawi and Zubair Formation deposits, where Ratawi is thinning in the wells (Si-1, Dn-1) with a thickness of 127 meters this thickness is increased to reach 384 meters in the Amara region.In contrast, Zubair Formation is thickening in wells (Si-1, Dn-1) about 440 meters, while disappearing completely in the east Amara Oilfields, reflecting the combined effect of the marine and continental factors.

Aptian to Albian (121-99) Million Years
The beginning of this period is still within the Young Kimmerian tectonic movement, was the sedimentation of the Shuaiba Formation, which decreased its thickness in a well (Dn-1) to 35 meters, while disappeared in the well (Si-1).In Basrah area wells the thickness is about to 86 meters, while in the east of Amara Oilfields the thickness reaches 178 meters.It should be noted that these overlap with the unity of the Halfaya (Patio) in the area of Basrah and towards the Amara area Al-Bayati (2001).This is attributed to the prevalence of marine influence in the fields of north Basra and the Amar area (Mj-3, Mj-4, WQ-13, Rf-1, HF-1, HF-2, Am2, Noor-1) then disappear towards the (Si-1) completely.Within this same period, disconformity occurred between the Aptian and Albian, which separates Shuaiba Formation from Nahr Omr Formation.Thus a new tectonic movement begins within the Lower Cretaceous is the First Austrian alpine movement, and here the effect of this movement is evident within the sediments of the Nahr Omar Formation itself, where it form clastic facies with a thickness of nearly 400 meters in a well (Si-1) and 190 meters in a well (Dn-1), while its thickness about 210 meters in the Basrah wells .The lower part is characterized by the deposition of sandstone interfering with Shale while the upper part is characterized by precipitation of limestone interfering with the Shale.It is noted in this region that the thickness of the lower part increases toward the wells (Si-1, Dn-1) versus decrease in thickness towards the wells north of Basra and the Amara region (HF-1, HF-2, Am-2, Noor-1, Mj-3, Mj-4, WQ-13, and Rf-1).The upper part, oppositely deposited, with increased thickness towards the Amara area, and decreased towards the wells (Si-1, Dn-1).After this, the marine influence (sea level rise) continues until the end of the Albian period, accompanied by the sedimentation of the lime Mauddud Formation, which ranges from 30 meters in a well (Si-1) to 226 meters in the north Basrah and Amara fields.The existence of a regional disconformity separates between the Aptian formation and alpine formation related to a wide decline in sea level that was followed by a rise in sea level that reached its peak at the end of Albian (Haq et al., 1988).Al-Fares et al. (1998) explained the pre-Albian disconformity because of the subsequent far-field stress after the opening of the center of the South Atlantic, this opening caused the uplift to raise the western part of the Arabian Craton, leading to transport of the deltaic sands and the transitional marine sediment from west and southwest to the east of the platform.

Cenomanian -Early Turonian (88-99.6) Million Years
This period included the deposition of Wara, Ahmadi, Rumaila, Kifl, and Mishrif Formations, the rate of sediment production was controlled by the tectonic factor more than the marine factor.Wara and Ahmadi Formations were deposited in the southwestern parts of the study area, particularly in well (Ks-1).Unconformity occurred between the sequences of the Mauddud and Wara Formations, this is indicated by the presence of glauconite mineral in the upper part of it, while this was not recognized between the Mauddud and Ahmadi Formations.These formations represented the intra-shelf basin development during the Cenomanian age by dominating shallow water of carbonate ramps that event was due to the growth of Oman-Zagros peripheral bulge (Al-Zaidy and Al Shwaliay, 2018).The beginning of the second Austrian tectonic movement had a massive effect on raising the passive margin, thus reducing the production of limestone deposits in the southwestern parts, while sedimentary shelf sediments were allowed to deposit (Wara Formation).Also, Ahmadi Formation was deposited in the same area, represented by restricted shallow marine sediments consisting of marl and shale, specifically within wells (Ks-1, AK-1, Gh-1, UmQ-1, Dn-1, Wk-1).In the middle of the study area wells (Rt-1, Rt-2, WQ-13, WQ-115, NNU-1, NNU-2, R-5), it is observed that Ahmadi Formation is represented by the shallow water lithofacies of gypsum in well WQ-13, and dolomite in the well NNU-1.It should be noted that the average formation thickness is 172.54 meters (Table. 4).This thickness is attributed to the increased growth of the Amara uplift area, which was working to trap the sediments, thus increasing the sedimentation rate.On the other hand, Ahmadi Formation was deposited in wells (Hf-1, Mj-3, Mj-4, Am-2, Snd-1, Rf-1, Noor-1) located east of the study area within the open marine environment represented by the detrital limestone facies.With the continuation of the second Austrian movement, the Amara bank was uplifting continuously as a result of the compression process increase towards the passive margin, which led to the formation of the sub-basin environments in the western regions and thus sedimentation of the Rumaila Formation consisting of marl, chalky limestone, and shale, with a large thickness about 129.5 meters, while it starts to shallow towards the wells of the central and eastern region of the study area to deposit in the form of a chalky limestone with limestone containing crystals.Mishrif Formation was characterized by a large thickness in all wells of the study area, where the highest thickness reached 408 meters in the well (Am-2).It had a succession of chalky limestone and shale, this high thickness expanded the eastern region towards the edge of the passive margin and prevented its lithofacies from spreading laterally, so it deposited as a form of rudeist limestone with shally limestone and limestone containing gypsum crystal, while the thickness of the formation is thinning towards the southwestern parts to disappear in wells (Dn-1, Gn-1) to deposit Kifl Formation instead of it with a small average thickness (30) meters from Marly limestone and anhydrite, which is considered a complement to the upper part of the Musharraf Formation, indicating the calm and shallow of the restricted water that allowed the deposition of thin layers of gypsum.This period ended with a regional discontinuity that occurred during the global regressive marine extended from Saudi Arabia, Kuwait, and southern Iraq to the northern Iraq regions (Mosul and Kirkuk) (Razoyan, 1995).It should be noted that the beginning of this discontinuity was the end of the second Austrian tectonic movement

The Middle Turonian -Middle Campanian (76.2-88) Million Years
The deposition of the sedimentary cycle (Khasib, Tanuma, and Saadi Formations), this period coincided with the beginning of the lrmidain tectonic movement activity, which was a compressive movement that worked to complete the uplift of the passive margin, that resulted from a regional inclination in the southwestern parts of the study area Thus, it prevented the sediments influx from the stable shelf area.With the deposition of the lower part of Khasib Formation (Late Turonian-Early Conacian), a relative calm occurred for the second Austrian tectonic movement with the continuation of the global marine regressive, which resulted in the deposition of relatively homogeneous sequences of formation in all wells of the study area, up to 57 meters in the eastern regions represented by chalky limestone and Shaly limestone.While the average thickness of the Formation was 53 meters in the central region wells, where it represented chalky limestone and marly limestone, as well as the presence of dolomite.the average thickness of the formation is about 57 meters within the wells of the eastern region, represented by the successions of limestone and shale.The diversity in the Khasib Formation lithofacies is believed to have resulted from the geochemical variation of marine waters due to the relative distance to the passive margin, as well as the variation of the local sea-level changes.
After that, the Armada tectonic movement became active and its effect appeared on the sedimentation of the upper part of the Tanuma Formation (Oolitic limestone facies), while its deposition within the quieter environments towards the stable shelf.The effect of this movement extended with the deposition of the lower part of the Saadi Formation represented by the open marine environment lithofacies towards the edge and the basinal environments in the middle and the sub basinal environments toward the west of the study, and therefore the effect of this movement on this system was shallowing upward.while the upper part of the Saadi Formation was deposited within the deepening upward system, which included sedimentation of chalky limestone of planktonic foraminifera,

Discussion
The comprehensive overview of all the available data on the Cretaceous period in southern Iraq has reached two basic definitions: • The tectonostratigraphic boundary (TSB) is defined as the boundary between phases or kinetic stages and represents the sediments that are deposited away from the impact of movement, as well as this limit may include part of a formation or several formations.It is obvious from Figure 8 that the boundaries are gradually extended through the area during the Cretaceous period and do not have a sharp limit.Additionally, these boundaries could disappear from some features during sedimentary periods, such as TSU 1D, which extends through the clastic and mixed zones, as well as with the two unites TSU 2A and TSU 2B, which are conjunction in the mixed zone.
Decreasing the movement forces with time leads to form the primary porosity of the upper TSU units and leads to moderate deposition rates of these units, which helped to accumulate the organic substance insufficient amount in them.The average sedimentation rate is considered a good condition to aggregate the organic substance.

Conclusions
• The Cretaceous period in southern Iraq was divided into four main categories of TSU units.Each category contains a set of subunits or secondary that is confined between five boundaries of TSB.• The lateral extension of TSU close to the passive margin from the northeast to the southwest part of the study area, which represents a transfer form of TSU units from a reservoir to a generator source and then to generator hydrocarbons.The transition from north to south represents the improvement of reservoir characteristics in the TSU units.• Vertically, the TSU units were characterized by improved reservoir properties with decreasing depth.
• Each TSU unit needs an isolated oil-producing model because it cannot be produced laterally or vertically from one layer or formation by concerned it in one system.• The lateral boundary between the TSU units is gradual limits rather than sharp limits.There is a relative convergence in their specifications for the central and south-western regions, compared with the north-eastern regions of the study area, which reflects the variation or similarity of the producing models of these units.• Tectonostratigraphic boundary (TSB) is considered as a generator source more than producing hydrocarbons.Therefore, it does not have a significant lateral variation compared to the TSU units.

Fig. 1 .
Fig.1.Location of the study area and the oil wells used in this study.

Fig. 2 .
Fig.2.The three longitudinal zones of sedimentations in the Arabian margin during the Early Cretaceous (A) the carbonates zone; (B) the mixed of clastic and carbonates zone, and (C) the clastic zone (Davies et al., 2002).

Fig. 5 .
Fig.5.The tectonic position of the Arabian plate during the Lower Cretaceous.(A) The location of the Arabian plate relative to the other nearby tectonic plates (Rich, 1996); (B) The cross-section (X-Y) that extends from the Arabian plate to the Iranian plate through the new Tithes Sea (Modified from Numan, 2000); (C) A detailed crosssection on the passive margin of the Arabic plate (Modified from Mutlak, 1999).

Fig. 6 .
Fig.6.The tectonic position of the Arabian plate during the Upper Cretaceous.(A) Tectonic position of the Arabian Plat during the Upper Cretaceous (Rich et al., 1996); (B) Cross-section (X-Y) shows the position of the Arabian passive margin during the Austrian movement (current study); (C) Cross-section (X-Y) shows the position of the Arabian passive margin during the Lramidic movement (current study).

Fig. 10 .
Fig.10.Represents the vertical and horizontal stratigraphic variation section with sedimentary environments of the Cretaceous Formations Razoyan (1995).

Fig. 12 .
Fig.12.The three depositional border zones of limestone, mixed, and clastic in the Lower Cretaceous.

Fig. 13 .
Fig.13.The boundaries between the three deposit regions of the Upper Cretaceous (Modified after Al Bayati, 2011).

•
The tectonostratigraphic unit (TSU) is a layer or set of layers that are deposited by the effect of motion.It represents a confined layer between two tectonic borders.According to the previous two definitions, the present study gives approximate results close to Al-Bayati et al. (2010) and Numan (2000 and 2011) (Fig.14).Fig.15.Summarizes the main results in the present study.The results show the presence of five stratigraphic boundaries of TSB that are represented by TSB1 to TSB5.The five TSBs are represented by Sulaiy, Shuaiba, Mauddud, Khsib, and the lower part of Tanuma-Shiranish formations.Each one of these TSBs boundaries has a set of four units TSU, which it has, in turn, secondary sub-units illustrate as follows: •TSU 1= (A -B -C -D) •TSU 2= (A -B) •TSU 3= (A -B -C) •TSU 4= (A -B -C)

Fig. 14 .
Fig.14 .A comparison between the Lower and Upper Cretaceous zones.

Fig. 15 .
Fig.15.Tectonic stratigraphic system of the Cretaceous period in Southern Iraq