Basin Geohistory Analysis of the Mishrif Formation in Southern Iraq

Abstract


Introduction
The Mishrif Formation (Late Cenomanian -Early Turonian) is one of the most important geological formations containing oil in southern Iraq (Al-Mimar et al., 2018).The basin geohistory was analyzed through the Backstripping method, which enabled us to know the geological events that affected it and occurred during and after the deposition.Stratigraphic techniques have been utilized in creating geological burial curves, which use mathematical models developed by Harami et al. (1992); Allen and Allen (1993); Springer (1993); Jin (1994) Makhous et al. (1997) Milner (1998); Beicip-Franlab (2002).Mishrif Formation was described for the first time by Rabanet (1952) and Owen and Nasser (1958) from the typical section of well Zb-3 in the upper part that had previously been called the Khatiyah Formation in the Wasia Group, where Al-Kasim (1965) was divided the Mishrif Formation into neritic, coastal, closed coastal basin (Lagoon), and basin environment facies.The (Powers, 1968) divided the Khatiyah Formation into three formations (Ahmadi, Rumaila, and Mishrif).Refer (Buday, 1980) to the rocks of the Mishrif Formation were deposited on a main plateau, these deposits reflected two small sedimentary cycles separated by a relative recession, and (Sherwani, 1988) showed of this cycle ended with the deposition of the evaporative Kifle Formation.Where (Razoian, 2002) divided the Mishrif into three sedimentary units (Lower, Middle, and Upper) and ( 17) microfacies in Rumaila and West Qurna fields.A study by AL-Shahwan and AL-Najm (2021) indicated that the sedimentation cycle of the Mishrif Formation increased subsidence and sedimentation rates in the Tigris tectonic subzone.This is towards the direction of the basin center in fields (Noor and Halfaya), where the study of Boschetti et al. (2020) indicated that the Mishrif Formation is of marine origin.
This study notes that there are three regional unconformity surfaces.It indicated the extent of the activity of the depositional basin of Mishrif Formation and the formations above it.These tectonic events from the Cretaceous and Tertiary periods, as well as the burial history of the Mishrif Formation, demonstrate that sedimentation rates have a direct relationship with total subsidence, with an increase in the northeast for wells , indicating the center of the Mishrif Formation's sedimentary basin.

Tectonic and Geological Setting
The study area is located between the eastern lines (582400-749080) and the northern lines (3534 600-3357 197).Wells were chosen (Zb-114, R-270, WQ-17, Rt-5, Lu-2, Ns-5, Ri-1, No-2, and Hf-5) (Fig. 1) to covering the study area.These wells are distributed among three main tectonic subzones in the Mesopotamian zone, on the unstable platform of the Arabian plate (Fig. 2): 1. Zubair subzone in which the wells (Zb-114, R-270, WQ-17, Rt-5, and Lu-2); 2. Tigris subzone in which the wells (No-2 and Hf-5); 3. Euphrates subzone is where the wells (Ns-5 and Ri-1).The Mesopotamian zone was formed due to the secondary Hercynain Orogeny movement, which had a major impact on the unstable platform of the Arabian plate and was followed by the Late Turonaian movement, which worked to raise areas of the unstable shelf on the eastern side of the Arabian plate, also formed a large basin in the western side of the shelf at the end of Middle Ages (Ibrahim, 1979;AL-Kanaani, 2003).Pitman et al., 2004;Atfy et al., 2023).
The Mishrif Formation basin arose during and continued to decrease and consume the modern Tethys ocean due to the movement of the Arabian plate towards the north through a Cretaceous.This movement reached the peak at the end of the Cretaceous during the Laramide Orogeny in the Maastrichtian, which is the first phase of the Alpine Orogeny, resulted in the beginning of the collision of the Arabian plate with the Iranian and Turkish plates, and the consumption of the oceanic crust of the Tethys and the extrusion of Ophiolite in the of Zagros and Oman regions, its divided the basin into several internal basins separated by ridges, where the Mishrif Formation was deposited on a main plateau (Buday, 1980).

Materials and Methods
The history of the sedimentary basin of Mishrif Formation was analyzed by stratigraphic thickness as measured, a simple decompaction calculation based on porosity estimates that projects changes in thicknesses during burial, age of stratigraphic units (paleontologic and radiometric), and interpreted paleoenvironmental information that allows estimates of Paleobathymetry.That gives us a corrected subsidence profile, which was calculated using the Backstripping method.Using the (Microsoft Excel worksheet 2021) to make the tables and curves for wells , where calculations included:

Geological Time Scale
This study depended on the geological time scale of the Arabian Gulf region (Harland, 1990) to determine the geological age of the stratigraphic column.

Sea Level Changes
Sea level has always varied throughout geological history due to changes in the size of seas and oceans due to the melting of glaciers and other factors.So, this study relied on global sea level variation charts (Haq and Vail, 1987).

Paleobathymetry
The old water depth of the basin can be estimated based on facies studies, sedimentary structures, and the record of fine benthic fossils.There is also some geochemical evidence that indicates the depth of the paleowater depth in which the sediment units were deposited at the time of their deposition and since the global sea level has changed over the ages.So the depth bottom of the old sedimentary basin was restored based on the changes occurring in the level of the old bottom and its development through geological time (Ungerer, 1984), which must be combined with the variation in global sea level.

Sediment Compaction
The current thicknesses of the study formations are less than in the post-deposition stage due to the compaction resulting from the continuous deposition of the rock layers.Therefore, this study corrected and returned these thicknesses to the original thicknesses.The history of sediment burial and subsidence of the sedimentary basin of the Mishrif Formation and the formations above it were restored by calculating the changes occurring in its thicknesses.The process of calculating the compression coefficient depended on the extent of the evolution of porosity with depth, knowledge of the depth of the old basin, and the changes in the thicknesses of the rock units.This study can calculate the compaction constant correction by finding the initial porosity and relationship with depth by calculating:

Lithological Components Porosity
The porosity of the Mishrif Formation and the formations above it were calculated by summing the porosity of the rock components of these formations by (Beicip Franlap, 2002) equation: (1) Øz: Total porosity of the rock components of the formation at a given depth (%); Ci: Ratio of a certain part to its total volume (%); Øciz: Porosity of rock components (%).
The second equation by (Allen and Allen, 2005) is used for deep formations: L: Compression coefficient.

Restored of Original Thicknesses
The (Perrier and Quiblier, 1974) equation was used to calculate the original thicknesses of the Mishrif Formation and the formations above it: (4) Δhs: Original thickness of the formation (meter); Δh: Solid parts of the formation thickness (meter).
Where D was calculated by (Beicip Franlap, 2002) equation ( 6): S: Thickness of the sedimentary column (meter); ds: Average density of the sedimentary column (kg/m 3 ); dcomp: Density of the equivalent fluid used to calculate the deposition rate (zero for air and one for water) (kg/m 3 ); dm: Mantle density (kg/m 3 ); Pw: Depth of the Paleobathymetry (meter); dw: Water density (kg/m 3 ); SL: Global sea level (it is greater than zero if it is above sea level) (meter).
While calculating the subsidence rates by (Van Hint, 1978) equation: ) RT: Subsidence rate (cm./1000 year); A: Time of the formation deposition in millions years (Ma).
Sedimentation rates were also calculated relative to the estimated age of the sedimentary stratigraphic column units of the formations, extending from the Late Cenomanian to the Miocene through the (Van Hint, 1978) equation: (8) R S : Sedimentation rate (cm./1000 year); Tp: Total thickness of the formation (meter).

Regional Unconformity Surfaces
To calculate the eroded thicknesses at unconformity surfaces, the following two equations were used (Gudish and Yarzab, 1985): AGEE: Age of the erosion in millions years (Ma); R1& R2: Sedimentation rates for sedimentary layers above and below the unconformity surface (cm./1000 year); AGE1&AGE2: Ages of the sediments above and below the unconformity surface in millions years (Ma).

Discussion
The initial, restored thicknesses and sedimentation rates were calculated for the Mishrif Formation and the above formations.The total subsidence and burial history curves were drawn, and calculated of the sedimentation rates and eroded thicknesses of the unconformity surfaces were the results:

Original Thicknesses and Sedimentation Rates
The thicknesses were restored over the times of their deposition to their original thicknesses, where (Tables 1 to 9) show the present, restored, and initial thicknesses of formations, which indicated a decrease in the thicknesses due to the increase in the sediment load in geological time.Initial thickness (meter).

Initial thickness (meter).
The sedimentation rates showed three levels (high, moderate, and low).The high rates were founded in the formations (Tayarat, Umm Radhuma, and Injana ), which had a sedimentation rates greater than 6 cm/1000y (Fig. 3).As the formations (Mishrif, Sadi, Hartha, Dammam, Kirkuk Group, Fatha, and Dibdiba), they had moderate rates that between (2-6 cm/1000y) (Fig. 4).While the formations (Kifle, Khasib, Tanuma, Shiranish, Aliji, Jaddala, Rus, Ghar, and Jeribe) show the low rates less than 2 cm/1000y (Fig. 5).These figures showing an increase in the sedimentation rates of the formations towards the northeast of the study area when the center of the sedimentary basin.It was noticed an increase in the sedimentation rates of the Mishrif Formation for the wells (Ri-1, No-2, and Hf-5).

Regional Unconformity Surfaces
The stratigraphic column of the study wells shows three regional unconformity surfaces:

1st Regional unconformity surface
It resulted from the impact of the secondary Hercynian movement (Subhercynian Orogeny) (Buday, 1980), which had an effective impact on the study area, followed by the Late Turonian movement that is before (89-90 Ma).Worked to renew the movement activity of the basement rocks, causing interruptions deposition, a change in facies, and irregular distribution of sediments, which led to the process of uplifting the sedimentary basin and exposing the top of Mishrif Formation to eroded and sculpting processes, thus forming a regional unconformity surface (Fig. 6a).

2nd Regional unconformity surface
This surface occurred before (62.30-65Ma) due to the movement of the Arabian-African plate towards the north and the expansion of the Atlantic Ocean.Earth movements peaked at the end of the Cretaceous period, represented by the Laramide Orogeny movement during the Maastrichtian.Which is the first phase of the Alpine movement (Alpine Orogeny) resulted at the beginning of the impact of the Arabian plate with the Iranian and Turkish plates, after the consumption of the oceanic crust of the Tethys and the extrusion of Ophiolite in the regions of Zagros and Oman (Buday, 1980).This led to the rise and exposure of the top of the Tayarat Formation to eroded processes, forming the surface of a second regional unconformity surface (Fig. 6b).

3rd Regional unconformity surface
It was during the Late Eocene and Oligocene (23-36.50Ma) occurred as a result of a folding wave during this period that led to a comprehensive uplift in most of the unstable shelf (Buday, 1980), which caused the uplift of large parts of the Dammam Formation (Fig. 6c).
The sedimentation rates and eroded thicknesses at the surfaces of the three unconformities in the study wells were calculated (Table 10), which shows that the surface of the first unconformity has been exposure the Mishrif Formation to uplifting operations in the northeastern part to a greater than of the study area, the values of the eroded thicknesses in the wells (Ri-1, No-2, and Hf-5) (Fig. 6a) shows the largest among the other wells.The surface of the second unconformity was exposed to clear tectonic activity in the northern and southeastern parts, the top of the Tayarat Formation was exposed to a major uplifting and eroded process, especially at the wells (Zb-114, R-270, WQ-17, Ri-1) (Fig. 6b), while the third unconformity surface that the most parts of the study area have been exposed to uplift operations in the wells , confirms the top of the Dammam Formation is exposed to major erosion in general parts of the study area (Fig. 6c).Fig. 6.Eroded thicknesses of the regional unconformity surfaces of study wells, the maps drawn by Surfer 26.3 software with 5 meters contour interval: (a) Eroded thicknesses 1st regional unconformity surface; (b) Eroded thicknesses 2nd regional unconformity surface; (c) Eroded thicknesses 3rd regional unconformity surface.

Total Subsidence and Burial History of the Mishrif Formation
The process of calculating the total subsidence of the sedimentary basin began when the deepest rock unit of the Mishrif Formation was restored in the stratigraphic column and returned to its original thickness during the period of initial deposition of the layer after the load of the layers above it was gradually removed.The geological burial history curves (Fig. 7) show three levels of subsidence rates:
The subsidence rates of the Mishrif Formation and the formations under the study reflected a relationship with the depositional sedimentation of the basin, as this sedimentation helped contain the large quantities during the period of deposition of the above formations, resulting from the continuous variation in sea level during the different periods, while tectonic movements had a significant impact on the sedimentation of Mishrif Formation.Through the uplifting processes that occurred in the sedimentary basin, especially on the northeastern side of the study area (Fig. 6a), this effect was high in the wells (Ri-1, No-2, and Hf-5), and moderate in the wells .At the same time, it was less effective in the wells (Rt-5 and Lu-2), where a regional unconformity surface distinguishes the top of the Mishrif Formation.It resulted from an interruption in sedimentation for approximately (2-3 Ma), during which the rocks of the Mishrif Formation were exposed on the surface.
The geological burial history of the Mishrif Formation (Fig. 7) indicates the greatest total subduction in the eastern and northeastern Tigris tectonic subzone, the southern and southwestern sides show the least subsidence in the Euphrates subzone.The fields (Noor and Halfaya) represented the most significant burial depth of Mishrif Formation, while the fields (Ratawi and Luhais) had the most minor burial depth.This means that the increase in the thicknesses of the Mishrif Formation is towards the center of the sedimentation basin in the study area (Fig. 8).
It was noted that three regional unconformity surfaces occurred before (89-90 Ma) indicate the extent of the activity of the depositional basin of the Mishrif Formation and the formations above it.The effectiveness during the Cretaceous and Tertiary periods.The first regional unconformity surface resulted from uplifting in all parts of the Mishrif Formation, especially in the northeastern study area in wells (Ri-1, No-2, and Hf-5), showing the most considerable eroded thicknesses.The second unconformity surface occurred before (62.30-65Ma) as a result of the movement of the Arabian-African plate was exposed to evident tectonic activity in the northern and southeastern parts of the study area, where the top of the Tayarat Formation was exposed to a significant uplifting and eroded process in the wells (Zb-114, R-270, WQ-17, and Ri-1).The top of the Dammam Formation was exposed to major erosion in general parts, which created the third unconformity surface in wells (Zb-114, R-270, Rt-5, Ns-5, Ri-1, and Hf-5), It was during the Late Eocene and Oligocene (23-36.50Ma).

N meters
The burial history of Mishrif Formation was indicated to be the greatest of total subduction in the eastern and northeastern sides of the Tigris tectonic subzone, while the southern and southwestern sides show least subsidence.Mishrif Formation had the greatest burial depth among the fields (Rifai, Noor, and Halfaya), while the fields (Ratawi and Luhais) had the lowest.The thicknesses of the Mishrif Formation increases from the south and southwest towards the north and northeast of the study area, indicating that the formation was deposited on a slope with a relatively slight slope in the southern and southwestern parts and turned into the edge of the deep shelf in the northeastern, where it is expected that the porosity and permipelity for the Mishrif Formation improve in this part.

Fig. 1 .
Fig. 1.(a) Location map showing the study area (Map of oil fields in Iraq, 2019); (b) Shows the wells' locations.

Fig. 3 .
Fig. 3. Formations show high sedimentation rates in the study wells.

Fig. 4 .
Fig. 4. Formations show moderate sedimentation rates in the study wells.

Fig. 5 .
Fig. 5. Formations show low sedimentation rates in the study wells.

Fig. 8 .
Fig. 8. Present thicknesses of Mishrif Formation in the study wells, the map drawn by Surfer 26.3 software with 5 meters contour interval.

Table 1 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Zb-114.

Table 2 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well R-270.

Table 3 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well WQ-17.

Table 4 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Rt-5.

Table 5 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Lu-2.

Table 6 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Ns-5.

Table 7 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Ri-1.

Table 8 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well No-2.

Table 9 .
Present, restored, and initial thicknesses of the Mishrif and the formations above it for well Hf-5.

Table 10 .
Sedimentation rates and erosional thicknesses of the regional unconformity surfaces of study wells.