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

Abstract

Twenty oil wells were selected to study the tectonostratigraphic of the Cretaceous in southern Iraq, in order to develop a comprehensive description of the petroleum system in the region. That was conducted through an interpretation of the technical reports and the available information of the wells, which include sedimentary, stratigraphy, tectonic reports, and oil reservoir studies of the Cretaceous. Stratigraphically, a third order cycle was identified in the Cretaceous succession in southern Iraq, which also comprises seven and half cycles of the fourth order. Eight genetic stratigraphic sequences were also identified, as well as eight maximum flooding surfaces. The concept of the tectonostratigraphic boundary (TSB) and the tectonostratigraphic unit (TSU) has been adapted in this study. In the present study, the Cretaceous period in southern Iraq considers one tectonostratigraphic system (TSS) consisting of four main tectonostratigraphic categories. Each category consists of a set or group of secondary tectonostratigraphic units; these are TSU1A, TSU1B, TSU1C, TSU1D-TSU2A, TSU2B-TSU3A, TSU3B, TSU3C-TSU4A, TSU4B, and TSU4. These units are separated by five tectonostratigraphic boundaries presented from TSB1 to TSB5 by Sulaiy, Shuaiba, Mauddud, Khasib, lower part of Tanuma, and Shiranish. The lateral extensions of the TSUs that are close to the passive margin (northeast part of the study area) are hydrocarbon reservoirs. The lateral extensions TSUs that are far from the passive margin (southwest part of the study area) are hydrocarbon generator source. The intermediate unite is characterized as both a generator source and reservoir hydrocarbon. Vertically, the TSUs are characterized by improved reservoir properties with reduced depth due to the lack of compressional tectonic force, which leads to forming a good primary porosity. The transfer from north to south of the study area represents a trend of improvement in reservoir characteristics for the same reason as mentioned previously. Finally, the TSB represents a source generator hydrocarbon more than a reservoir.

Full text

Iraqi Geological Journal
Al-Muhamed et al.
2023, 56 (1A), 100-126
Iraqi Geological Journal
Journal homepage: https://www.igj-iraq.org
DOI:
100
Tectonostratigraphic Framework and Depositional History Pattern of the
Cretaceous Successions Period in Southern Iraq
Rafed A. Al-Muhamed1,*, Mazin Al Shaoosh 1 and Nagham A. Al Hawi 1
1
Department of Geology, College of Science, University of Basrah, Basrah, Iraq
*
Correspondence: rafed.ahmad@uobasrah.edu.iq
Abstract
Received:
20 June 2022
Twenty oil wells were selected to study the tectonostratigraphic of the Cretaceous in
southern Iraq, in order to develop a comprehensive description of the petroleum system in
the region. That was conducted through an interpretation of the technical reports and the
available information of the wells, which include sedimentary, stratigraphy, tectonic
reports, and oil reservoir studies of the Cretaceous. Stratigraphically, a third order cycle
was identified in the Cretaceous succession in southern Iraq, which also comprises seven
and half cycles of the fourth order. Eight genetic stratigraphic sequences were also
identified, as well as eight maximum flooding surfaces. The concept of the
tectonostratigraphic boundary (TSB) and the tectonostratigraphic unit (TSU) has been
adapted in this study. In the present study, the Cretaceous period in southern Iraq considers
one tectonostratigraphic system (TSS) consisting of four main tectonostratigraphic
categories. Each category consists of a set or group of secondary tectonostratigraphic units;
these are TSU1A, TSU1B, TSU1C, TSU1D-TSU2A, TSU2B-TSU3A, TSU3B, TSU3C-
TSU4A, TSU4B, and TSU4. These units are separated by five tectonostratigraphic
boundaries presented from TSB1 to TSB5 by Sulaiy, Shuaiba, Mauddud, Khasib, lower
part of Tanuma, and Shiranish. The lateral extensions of the TSUs that are close to the
passive margin (northeast part of the study area) are hydrocarbon reservoirs. The lateral
extensions TSUs that are far from the passive margin (southwest part of the study area) are
hydrocarbon generator source. The intermediate unite is characterized as both a generator
source and reservoir hydrocarbon. Vertically, the TSUs are characterized by improved
reservoir properties with reduced depth due to the lack of compressional tectonic force,
which leads to forming a good primary porosity. The transfer from north to south of the
study area represents a trend of improvement in reservoir characteristics for the same
reason as mentioned previously. Finally, the TSB represents a source generator
hydrocarbon more than a reservoir.
Accepted:
14 September 2022
Published:
31 January 2023
Keywords:
Cretaceous period; Southern Iraq; Tectonostratigraphic Unit; Petroleum
system.
1. 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-

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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).
Fig.1. Location of the study area and the oil wells used in this study.
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 and 2000), 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

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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).
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).
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

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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).
Fig.3. The tectonic divisions of Iraq after Fouad, (2015)

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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).
Fig.4. Two tectonic mega sequence phases (AP8, AP9) shown the Arabian plats position during the
Cretaceous period Sharland et al.(2000).
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.
Table 1. The tectonic movement phases in the Cretaceous period (Numan, 2000)
Episodes
Ages
Tectonic Movements
Late Cretaceous
End-Maastrichtian
Laramid
End-Cenomanian
Second Austrian Alpine
Early Cretaceous
End-Albian
First Austrian Alpine
Berriasian-Aptian
Young Kimmerian

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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 and 2000).
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

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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.
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 cross-
section on the passive margin of the Arabic plate (Modified from Mutlak, 1999).

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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).

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4. 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).
Fig.7. Illustrated the description of the lithology of the Cretaceous formations

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Table 2. The lithological descriptions of formations depend on the final oil well reports.
Lithology
Thickness(m)
Formation
Well
- Lower part (35m) limestone.
- Upper part (47m) marly limestone.
82
Shiranish
Noor-1
- Interbedded chalky and detrital limestone, in part(upper) Glauconite.
32
Hartha
-The lower part (28m) Oolitic detrital limestone.
- Upper part (103m) limestone, in part shaly and marly limestone.
131
Sa’adi
- Interbedded shale (limestone and shale) and limestone.
25
Tanuma
- Interbedded limestone and chalky limestone with shale in one bed (2m).
65
Khasib
- Limestone, in part chalky limestone(in the lower and middle) and in part
Rudist (in the upper).
383
Mishrif
-Interbedded limestone and detrital limestone, in part(middle)shaly limestone.
46.5
Rumaila
- Limy shale, in top beds of marl (2m), in the lower part bed of limestone
(2m).
16
Ahmadi
Limestone.
193
Mauddud
-The lower part(50m) interbedded Shaly sandstone and Sandstone
-Upper part(153) Limestone
203
Nhar Umr
- Interbedded Marley Limestone and Limestone and Dolomitic Limestone.
188.5
Shuaiba
None deposit.
Zero
Zubair
Interbedded Shale and Limestone and Shaly Limestone
321
Ratawi
- Interbedded Shale and limestone in last (25m) from upper part
confirms from shaly Limestone
78.5
Yamama
Interbedded shale and sandstone-
318
Sulaiy
-Lower part (50m) marly limestone, in part shale.
-The middle part (40m) shaly limestone, in part shale.
-Upper part (49m), in the lower interbedded marly and shaly limestone in
the upper pure limestone.
139
Shiranish
Mj- 4
-The lower part (32m) shaly limestone.
-The middle part (50m) interbedded detrital and chalky limestone in the
lower.in the upper interbedded dolomite and shale.
136
Hartha
-The lower part (15m) Oolitic limestone.
-The middle part (62m) interbedded marl and limestone.
-Upper part (37m) marly limestone and detrital limestone.
108
Sa’adi
Shale in part thinning bed of limestone.
29
Tanuma
Limestone, in part shale.
44
Khasib
-Lower part (115m) limestone.
-Upper part (129m) detrital limestone.
244
Mishrif
Shally limestone.
12
Rumaila
-The lower part (25m) interbedded shale and limestone.
-The middle part (45m) interbedded marly limestone and limestone
container chert.
-Upper part (101m) interbedded chalky and detrital limestone, in the top of
formation shale bed (10m).
171
Ahmadi
- Marley Limestone and Limestone and Shaly Limestone intermittent Shale.
180
Mauddud
- Limestone.
178
Nhar Umr
- The lower part(70m) interbedded Sandstone and Shale and Shaly
Sandstone and it contains one-bed Limestone.
196
Shuaiba
- None deposit
Zero
Zubair

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- Lower part(315m) Marly Limestone and Shaly Limestone and Shall
intermittent three beds of Sandstone.
- Upper part (162m) Limestone.
477
Ratawi
Interbedded Marl and Limestone and Shaly Limestone.
66*
Yamama
Limestone.
205
Sulaiy
-The lower part (-) shaly chalky limestone.
-Middle part (--) marl.
-Upper part (--) in the lower marly limestone, in the upper shaly limestone.
70
Shiranish
HF-2
- Chalky and detrital limestone, in part, thinning bed of shale.
34
Hartha
-The lower part (75m) in the lower interbedded shale and Oolitic chalky
limestone.in the upper interbedded limestone and chalky limestone.
-Middle part (36m) marl.
-Upper part(23.5m) chalky detrital limestone.
124.5
Sa’adi
-Lower part (7m) shale.
-Upper part (7.5m) interbedded shale and shaly limestone.
14.5
Tanuma
-The lower part (15m) chalky and detrital limestone.
-Upper part (69.5m) in the lower interbedded shale and shaly limestone and
in the upper interbedded detrital and shaly limestone.
84.5
Khasib
-Lower part (95m) chalky limestone, in part detrital limestone (15m) in the
lower. In part container Gypsum.
-Middle part (95m) interbedded chalky and shaly limestone.
-The upper part (214m) interbedded Rudest and detrital limestone.
404
Mishrif
-The lower part (23m) interbedded chalky and limestone, in part shaly
limestone.
-Upper part (26m) limestone.
49
Rumaila
Interbedded shale and chalky limestone, in the bottom, consisting of
limestone (5m).
18
Ahmadi
No information.
Mauddud
No information.
Nhar Umr
Interbedded Shaly Limestone and Limestone.
180
Shuaiba
None deposit.
Zero
Zubair
Interbedded Shaly Limestone. Upper part(12m) interbedded Shale and
Sandstone.
355
Ratawi
Interbedded Shaly Limestone and Limestone.
43
Yamama
Inter bedded Shaly Limestone and Shale
419
Sulaiy
-Lower part (160) dolomite.
-Middle part (24m) shaly limestone and 5m in the top is clay.
-Upper part (18m) dolomite.
202
Tayarat
Rt- 4
Shally marly Limestone.
173
Shiranish
-The lower part (35m) interbedded limestone and shaly limestone.
-The middle part (74m) consists of marly limestone in the bottom (55m) and
shaly limestone in the top (19m), in part chalky limestone.
-Upper part (111m) dolomite, in part the dolomite container shale. in the top
part consist of chalky limestone (16m).
220
Hartha
-The lower part (50m) interbedded limestone and shaly limestone and
chalky limestone.
-Middle part (30m) marly limestone.
-Upper part (114.7m) chalky limestone.
194.7
Sa’adi
Shale with one meter only limestone
56.8
Tanuma
-The lower part (22m) interbedded shale and shaly limestone in the bottom
and shale and limestone in the top part.
-Upper part (26.5m) limestone.
48.5
Khasib
Limestone, in the bottom, found Gypsum, in part shaly and marly limestone,
in the middle (thinning) tow to one bed.
136
Mishrif

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-Lower part (61m) limestone, in part shaly limestone.
-Upper part (38.5m) shaly limestone.
99.5
Rumaila
Interbedded thick beds Marl and chalky limestone, in part thin bed of shale
138.5
Ahmadi
Limestone.
126
Mauddud
The lower part(147m) Sandstone intermittent thin bed of Shale.
The upper part (92.5m) interbedded Shaly Limestone and Shaly Sandstone
and Shale.
239.5
Nhar Umr
Limestone and Shaly Limestone, rare dolomitic Limestone.
86.6
Shuaiba
Interbedded Shale and Sandstone, in the last 25m interbedded Limestone
and Shale.
439.4
Zubair
Interbedded Shaly Limestone and Limestone intermittent Shale.
365
Ratawi
Limestone.
170
Yamama
Marley Limestone and Shaly Limestone.
50*
Sulaiy
-Lower part (27m) limestone, in part thin bed of dolomite (4m).
-Upper part (103m) mostly marl, in part thinning beds of limestone.
130
Tayarat
WQ -13
-The lower part (20m) shaly limestone
-Upper part (91m) interbedded dolomite and limestone, in part thin beds of
shaly limestone and Glauconite at the top.
111.5
Shiranish
-Lower part (80m) limestone.
-Middle part (75m) dolomite.
-Upper part (21m) limestone.
176
Hartha
-Lower part (21m) limestone.
-Upper part (99m) interbedded shale and chalky limestone.
120
Sa’adi
-Lower part (20m) marl.
-Upper part (27m) interbedded shale and limestone.
47
Tanuma
Interbedded shale and shaly limestone, in part bed of shale (5m).
53.5
Khasib
-The lower part (150m) interbedded chalky limestone and limestone, in part
found Rudest.
-Upper part (90m) limestone.
240
Mishrif
Limestone.
30.2
Rumaila
-The lower part (15m) shaly and marly limestone.
-The middle part (127m) interbedded chalky limestone and limestone, in the
lower found Gypsum.
-Upper part (13.7m) shale with limestone.
155.7
Ahmadi
Limestone.
158.5
Mauddud
The lower part (112m) interbedded Shale and Sandstone with two beds of
Limestone.
The upper part (102m) interbedded Shale and Limestone.
214
Nhar Umr
Interbedded Limestone and dolomitic Limestone.
101
Shuaiba
None deposit.
Zero
Zubair
Interbedded Limestone and Shale. intermittent Limestone. The last 47m on
top represent by a limestone bed.
487
Ratawi
Limestone.
353
Yamama
Limestone.
15*
Sulaiy
Lower part limestone and chalk-
Middle part Marl-
Upper part marly limestone -
79
Shiranish
Am-2
-Interbedded shaly limestone and chalky limestone
56
Hartha
-Lower part detrital limestone and shale.
-Upper part interbedded marl and shale
limestone and chalky
136
Sa’adi
Shale and shaly limestone.
17
Tanuma
Interbedded chalky and detrital limestone.
78
Khasib
Interbedded chalky limestone with shaly limestone, in the lower part (5m)
there is one bed of shale
407
Mishrif

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-Limestone.
14
Rumaila
Interbedded shale with limestone.
25
Ahmadi
In lower part interbedded Shaly Limestone and Shale fallow by Limestone
in the upper part.
385.5
Mauddud
Interbedded Shale and thick bed of Sandstone.
71
Nhar Umr
Limestone.
171.5
Shuaiba
None deposit.
Zero
Zubair
The lower part (237m) interbedded Limestone and Shale. In the bottom
(5m) bed Sandstone.
Upper part (148m) interbedded Limestone and Shale, an intermittent thin
bed of Sandstone.
455
Ratawi
Limestone and Shaly Limestone.
117
Yamama
Limestone.
160
Sulaiy
Interbedded dolomite and dolomitic limestone, in part shale container fauna
110
Tayarat
Dn-1
-Shale and shaly limestone.
170
Shiranish
-Lower part is interbedded dolomitic chalky limestone and dolomite.
-Middle part dolomite with Anhydrite.
-Limestone with pyrite.
295
Hartha
Dolomitic chalky limestone and shale limestone in the upper.
205
Sa’adi
Shale in part shaly limestone with glauconite.
50
Tanuma
-Lower part shale.
-Upper part shaly limestone.
32
Khasib
Interbedded Limestone container anhydrite and shale.
26
Kifl
-Lower part dolomitic chalky limestone.
-Upper part limestone.
55
Rumaila
-Lower part interbedded shale and limestone.
-Upper part shale container fauna, in part, interbedded limestone and chalky
limestone.
135
Ahmadi
- Limestone and Shaly Limestone and Shale.
27
Mauddud
- Shale and Siltstone and Shaly Limestone and Sandstone.
190
Nhar Umr
- Dolomite contains Anhydrite.
35
Shuaiba
- Shale and Sandy Siltstone.
480
Zubair
- Limestone and Dolomite and thin bed of Shale, intermittent of a thin bed
of Limey Sandstone.
105
Ratawi
- Chalky limestone.
25
Yamama
- Chalky Limestone and arggelous Limestone and porous Limestone.
95
Sulaiy
Interbedded marl and marly limestone.
57
Shiranish
Rf-1
-The lower part (45m), in the lower (17m), interbedded marl and chalky
limestone, in the upper (28m) interbedded shale and chalky limestone.
-Upper part (94m) interbedded chalky and foraminiferal limestone.
139
Hartha
-The lower part (98m) interbedded shale and chalky limestone in part found
foraminifera limestone.
- Middle part (32m) marl with limestone.
-Upper part (19m) chalky and detrital limestone.
149
Saadi
-Lower part (18m) marl with one thin bed shaly limestone.
-The middle part (16m) shaly limestone.
-Upper part (18m) limly marl.
52
Tanuma
-The lower part (8m) chalky and Oolitic limestone.
-The middle part (18m) interbedded shale and Oolitic chalky limestone.
-Upper part (32.5m) interbedded chalky and Foraminifera limestone.
58.5
Khasib
-Interbedded chalky and Foraminifera limestone, in part in the lower found a
thin bed of marl and shaly limestone.
293.5
Mishrif
-The lower part (25m) chalky limestone.
-Middle part (15m) marly limestone.
56
Rumaila

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-Upper part (16m) pure limestone.
-Lower part (5m) limestone.
-Upper part (13m) shaly marl.
18
Ahmadi
Interbedded Shale and Limestone.
30
Mauddud
Sandstone.
70
Nahr Umr
Dolomitic Limestone.
130
Shuaiba
Sandstone and shale.
310
Zubair
Shaly Limestone and Shale.
160
Ratawi
No information.
Yamama
No information.
Sulaiy
-Shally limestone with glauconite.
100
Shiranish
WK-1
-Dolostone, in the middle part(14m), interbedded dolostone and Anhydrite.
375.5
Hartha
-Interbedded chalky limestone and Foraminifera limestone.
112
Sa’adi
-Interbedded shaly marl and limestone.
20
Tanuma
-The lower part (38m) interbedded Foraminfra limestone and shale chalky
limestone.
-Upper part (37.5m) limestone and Foraminifera limestone.
75.5
Khasib
-Interbedded limestone and Anhydrite.
28
Kifl
-The lower part (78m) chalky Foraminifera limestone with glauconite.
-The middle part (100m) interbedded chalky and limestone in the lower and
interbedded shale and chalky limestone in the upper.
-Upper part (70.5m), in the lower part (10m) marl, in the upper (60.5m)
interbedded limestone and dolostone.
248.5
Mishrif
Interbedded limestone and shaly limestone.
21
Rumaila
Marl
12
Ahmadi
Dolomitic limestone with chalky and shally limestone in some part.
294
Tayarat
Gn-1
Shally marl, in the middle part (28m) chalky limestone and (31m) shale.
460
Shiranish
-Lower part (280m) dolomitic limestone.
-The middle part (554m), in the upper (22om), interbedded Anhydrite and
dolomitic limestone. In the lower (110m) nearly Anhydrite only with a thin
bed of limestone.
-Upper part (377m) chalky limestone with (20m) shale in the middle.
1211
Hartha
-Lower part (41m) marly limestone,
-Upper part (35m) chalky dolomitic limestone.
395
Saadi
-Shale
15
Tanuma
-Chalky limestone.
77
Khasib
-Limestone
34
Kifl
The lower part (147m) interbedded chalky limestone and shale.
-Middle part (96m)limestone with marl in the upper.
-Upper part (243m) mixed chalky and dolomitic limestone.
486
Rumaila
and
Ahmadi
-Generally, consist of dolostone, in some part especially in the bottom found
Anhydrite and shaly chalky limestone, and in the top found beds of shale.
461
Tayarat
KS-1
Interbedded marly limestone and limestone.
82
Shiranish
-The lower part (75m) interbedded marl and chalky shaly limestone.
-Middle part (25m) dolostone,
-Upper part (96m) interbedded chalky limestone and limestone, in part thin
bed of shale.
196
Hartha

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-Lower part (85m) marl, in the bottom (5m) chalky limestone.
-Middle part (50m) marly limestone.
-Upper part (270m) interbedded limestone and chalky limestone.
405
Sa’adi
-Shale
60
Tanuma
-Interbedded shale and limestone, in the top (5m) chalky limestone.
31
Khasib
-The lower part (47m) interbedded chalky limestone shale.
-Middle part (40m) chalky limestone and in the bottom (5m) clay.
-Upper part (40m) pure limestone.
127
Mishrif
-Shally limestone, in part (8m) chalky limestone.
69
Rumaila
-The lower part (40m) interbedded thin bed of shale and tick bed of marl.
-The middle part (25m), (20m) shale, and (5m) clay.
-Upper part (15m) mixed shale and marl.
80
Ahmadi
-Lower part (18m) shale.
-Upper part (28m) sand with glauconite container in the middle (5m) marl.
46
Wara
Dolostone, in part thin beds of Evaporite.
264.8
Tayarat
UmQ-1
-Lower part (73m) Marl.
-Upper part (33.5m) marly limestone.
106.5
Shiranish
Limestone
121.5
Hartha
-The lower part (197.5m) interbedded marly limestone and chalky
limestone.
-Upper part (143.5m) chalky limestone.
340.5
Sa’adi
-Lower part (10m) shale.
-Upper part (9m) shaly limestone.
19
Tanuma
Interbedded shaly limestone and chalky limestone, in part thin bed of shale.
54.5
Khasib
Interbedded limestone and chalky limestone, in the upper beds of marly
limestone.
133.5
Mishrif
Interbedded chalky limestone and marly limestone and limestone.
91
Rumaila
-Lower part (10m) shale with marl.
-The middle part (105m) interbedded chalky limestone and limestone.
-Upper part (27.5m) shale with marl
142.5
Ahmadi

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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).
Fig.8. Stratigraphic correlation section for the Upper Cretaceous formations in oil wells (Dn-1, Rf-1, Amm-1, Hf-
1, Mj-1)
Fig.9. Stratigraphic correlation section for Lower Cretaceous Formations in oil wells (Gn-1, Dn-1, Rf-1, Amm-1,
Hf-1, Noor-1).

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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).
Fig.10. Represents the vertical and horizontal stratigraphic variation section with sedimentary environments of
the Cretaceous Formations Razoyan (1995).
5. 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).

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Fig.11. General sequence stratigraphic framework for Cretaceous southern Iraq
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 sub-
basin 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).

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Fig.12. The three depositional border zones of limestone, mixed, and clastic in the Lower Cretaceous.
Table 3. The high fluctuation of the formation thicknesses in the three zones of limestone, mixed, and
clastic in the Lower Cretaceous due to the combined effect of the marine and continental factors.
Tectonic
Movements
Stage and
Substage
Formations
The thickness of
formation in the
clastic zone in
meters
(Si-1, Dn-1)
The thickness of
formation in Mix
zone in
meters
(Lu1, WQ115,
Rt3, Rt4)
The thickness of
formation in the
carbonate zone in
meters
(WQ13, Am2,
Hf1, HF2, Mj3,
Mj4, Rf1, Noor1)
First
Austrian
Alpine
Late Albian
Mauddud
31
100.7
226.5
Early Albian
Nahr Umr
295
210
178.8
Young
Kimmerian
Late Aptian
Shuaiba
35
86.6
178.3
Barremian-Early
Aptian
Zubair
440
399.2
Zero
Hauterivian
Ratawi
127.5
268
384
Valanginian
Yamama
35.5
233
171
Berriasian
Sulaiy
93
254
286

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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.
5.1. 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.
5.2. 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

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(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.
5.3. 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 (Lu-12, Rt-3, Rt-4, WQ-115). 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.
5.4. 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

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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 gypsum
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
5. 5. 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,

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especially within the wells of the western study area where the formation average thickness is about 284
meters.
5. 6. Late Campanian - Maastrichtain (65.5-76.2) Million Years
During this period, three formations were deposited under the effect of the laramide tectonic
movement, these Formations are Hartha, Shiranish, and Tayarat. Hartha Formation that was deposited
by the uplifting process, whose effect is evident by the dominance of the deposition of the dolomitic
limestone, marly limestone, and the shale towards the middle and western wells of the study area,
represented by the Neritic facies’ environment at an average thickness 200 meters, while the chalky
limestone for open sea environments is deposited towards the wells of the study area close to the edge
of the passive margin, whereas Shiranish Formation was deposited with the end of the movement effect
within a basin environment represented by the dominance of marly limestone and shale with a system
of deepening upward and in conjunction with it deposition Tayarat Formation in the central parts and
the western part of the study area with a system of shallowing upward. As a result of this sedimentary
distribution in the passive margin during the Upper Cretaceous, three main sedimentary zones were
formed: The open marine environment zone towards the passive margin, the Shallow water zone in the
mid-region (Amara dome), and the Restricted water zone toward a stable shelf (Table.4). The thickness
and facies of the sediments of this zone are characterized by lateral and vertical heterogeneity depending
on the dominance of the intensity of the movement and this affected the variance of the thicknesses of
the Formations (Rumaila, Ahmadi, Kifl, Mishrif, Upper Tanuma, Saadi, Hartha, Shiranish, and Tayarat).
While the relative calm of the movement and the dominance of sea-level change affected precipitation
of Khasib and Lower Tanuma Formations.
Table 4. The average thickness to the Upper Cretaceous formations within each of the three regions
As a result of this sedimentary distribution in the passive margin during the Upper Cretaceous,
three main sedimentary zones were formed: The open marine environment zone towards the passive
margin, the Shallow water zone in the mid-region (Amara dome), and the Restricted water zone toward
a stable shelf (Fig.13).
Tectonic
Movement
The average
thickness of
Open marine
Zone Wells
(Am2, Noor1,
Hf1, MJ3,4,
Rf1, Snd1)
The average
thickness of
Shallow water
Zone Wells
(Rt3,4,
WQ13,115, R5,
NNu1,3)
The average
thickness of
Restricted
water Zone
Wells
(Wk1, UQ1,
KS1, AK1,
Gn1, Dn1)
Formation
Stage and Sub
stage
Period
Laramidian
Movement
Zero
193.41
293.56 m
Tayarat
Maastrichtian
Upper Cretaceous
93.75
173.928
174.08
Shiranish
Maastrichtian
97.142
216.357
379.833
Hartha
Late Campanian-
Maastrichtian
133.285
183.6
284.75
Saadi
Early Santonian-
Late Campanian-
56.857
40
31.08
Tanuma
Late Coniacian
57.42
73.54
56.33
Khasib
Late Turonian- Early
Coniacian
Second
Austrian
Alpin
Zero
Zero
30
Kifl
Late Cenomanian
300.21
308.17
134.5
Mishrif
Late Cenomanian
20.83
85.84
129.5
Rumaila
Early- Middle
Cenomanian
71.33
172.54
84.1
Ahmadi
Early Cenomanian
Zero
Zero
46
Wara
Early Cenomanian

Iraqi Geological Journal
Al-Muhamed et al.
2023, 56 (1A), 100-126
123
Fig.13. The boundaries between the three deposit regions of the Upper Cretaceous (Modified after Al Bayati,
2011).
6. 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.
• 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)
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.

Iraqi Geological Journal
Al-Muhamed et al.
2023, 56 (1A), 100-126
124
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.
Fig.14 . A comparison between the Lower and Upper Cretaceous zones.
Fig.15. Tectonic stratigraphic system of the Cretaceous period in Southern Iraq

Iraqi Geological Journal
Al-Muhamed et al.
2023, 56 (1A), 100-126
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7. 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.
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