Structural and Stratigraphic Evolution of Mishrif and Ratawi Formations in Safawi Oil Field, Southern Iraq

Abstract


Introduction
The ultimate purpose of seismic interpretation is to comprehend better and precisely define the field's structural stratigraphic framework to enhance comprehension of hydrocarbon migration, trapping mechanisms, and fluid distribution patterns.A solid grasp of the basin's depositional environment, stratigraphy, and structural development lays the foundation for a reliable and comprehensive seismic interpretation.The structural analysis involves using attributes to improve the understanding of structural faults, stratigraphic patterns, and structural continuity, which are not usually visible on initial amplitude seismic data sets.Multi-trace seismic attributes can provide a better understanding of lateral fluctuations in seismic data, aiding in characterizing a field's structural framework (Onajite, 2013).In seismic reflection research, the diagnosis of precise subsurface rock layers, such as folds, faults, and stratigraphic attribute subsurface circumstances, data modeling, velocity, seismic selection, and time may be utilized to determine depositional environmental, stratigraphy traps, structural traps, facies, and direct hydrocarbon indicator (DHI) (Alsadi, 2017).Identifying a reflection with well logs involves linking a particular character of a reflection from one seismic line to another to ensure that the stratigraphic surface followed is the same over the area (Nanda, 2016).The velocity and density changes between rocks reflect the waves to the surface and how rapidly and firmly the waves are reflected shows what lies underneath, which means changes in acoustic impedance that always refer to the horizons (Nanda, 2021); This controls the amount of reflection energy (reflection coefficient) (Herron, 2011).As mentioned above, various tools can be used in seismic reflection procedures, with the seismic energy travel time being the most commonly utilized (Liner, 2012).
The fault plays a dominant role in reservoir parameters, e.g., fluid flow (hydrocarbons migrations), whoever the fault identification in seismic profile is governed by many criteria can listed below: Offset in stratigraphic markers: the best way to identify fault where the stratigraphic marker is displaced or slipped through seismic imaging.Termination of reflections: this can occur due to abrupt changes in facies when the older sequence erodes and later filled by modern sedimentary sequence.Also, this may occur for more than one reason, such as stacking problems.Abrupt changes in the dip: some parts of the fault may not be interpretable because the vertical separation component (throw) is still undetected due to seismic resolution.At the same location, the reflections became abruptly steepened dips.Almost invariably, there is an interpretable fault just below in the reflector's deeper section.Fault-plane reflections: the reflection of fault plane manifest because (i) fluid piling up over fault planes, (ii) differences in fluid pressure on each side of the fault (hanging wall and footwall) (Misra and Mukherjee, 2018) Associated folding or sag: sagging or folding of sediment is commonly related to faulting.The synforms and antiforms are related to thrusts that can appear in seismic sections.Fault shadow: diminishing seismic amplitude along fault plain due to severe lateral velocity variation on both sides of the fault causing ray curveting or by diffractions created by the fault plane that may not have collapsed during migration.Discontinuities: the discontinued seismic line may indicate joints, fractures, fissures, cracks, or fault plane presence.This evidence is used to trace Discontinuities in vertical sections (Davis et al., 2012;Rowland et al., 2021).
Many researchers have worked on interpreting 2D seismic data to recognize seismic stratigraphy and structural picture, including Khorshd and Almarsoomy (2015), Ali et al. (2019), Al-Dulaimy and Al-Banna (2022), Ahmed et al. (2022), andKhawaja andThabit (2022).The current study aims to track and pick the geological targets with promising hydrocarbons at the Ratawi and Mishrif formations and investigate the possibility of indicating the element of stratigraphic trapping and calculating the potential hydrocarbon resources.

Location of Study Area
The study area is located in the southwestern part of Iraq in the Western Desert region within the administrative boundary Line of Najaf governorate, where about 120 km away from the city center and bounded by the Iraqi-Saudi border from the southwestern part (Fig. 1).The study area was determined by coordinates according to the U.T.M. WGS, 1984 Zone 38N, as shown in Table 1.

Subsurface Geology and Structural Setting of Study Area
The information about the subsurface rock successions in the study area was based on data gathered from various sources.The Mishrif reservoir characterization and brine diagenesis was given by Boschetti et al. (2021).One of the key sources was the well Safawi-1, which was drilled in 1960 and reached a final depth of 1828 m, penetrating the Muhaiwir Formation (Fares et al., 2021) (Fig. 2).
Additionally, information from several other wells in the surrounding area, such as Samawa-1, Al-Salman-1, Glaisan-1 and Diwan-1, was also taken into account.Previous geological and geophysical studies, regional geological research, and studies in northern Iraq were also consulted.These studies revealed that a sedimentary cover overlays the crystalline base rocks (Fares et al., 2021).The Cretaceous sequence in the study area is composed of sixteen formations, including Sulaiy, Yamama, Ratawi, Zubair, Shu'aiba, Nahr Umr, Mauddud, Ahmadi, Rumaila, Mishrif, Khasib, Tanuma, Sa'di, Hartha, Qurna, and Tayarat.All these formations are made up of carbonate with clastic intermingled,  except for the Sa'di and Hartha formations, which have unconformable contacts (Roychoudhury and Nahar, 1980).The Ratawi Formation was initially reported according to (Bellen et al., 1950) from the Ratawi-1 well in southern Iraq by (Nasr, 1950).(Owen and Nasr, 1958), later provided an updated description.Several authors, however, ignored the original description, and the name Ratawi Formation was frequently used for units of varying age and lithology (Buday, 1980).The limestone intercalations in the lower section of the Ratawi Formation are likely correspond, in stratigraphic position, to the Garagu Formation found in northern and central Iraq.The upper section of the formation is represented in the wells of the Awasil area by coarse-grained sandstones, silts, and silty shales, identified as Zubair Formation.As a result, the upper Ratawi Formation is equal in age to the lower Zubair Formation of the Awasil, Fullujah, and Mileh Tharthar wells (Al-Naqib, 1967).The symbols were defined by Dr. Hassan K. Jasim located in lithological description (Fig. 2): (lstlimeston, pyr-pyrite, sh-shale, dol-dolomite, chk-chalk, arg-aragonite, bit-bitumen, foss-fossils, sstsiltstone, ool-oolite, mrl-marl, anhd-anhydrite, w-wackystone, grn-grainstone, fiss-fissile, glc-gluconite, bd-bedded, and md-mudstone).Mishrif Formation is Albian-Lower Turonian age and corresponds to tectonostratigraphic mega sequence AP8 (Aqrawi et al., 2010).Mishrif Formation is only a few feet thick at wells to the west (Samawa-1 and Safawi-1 and is absent at Ghalaisan well-1) (Bellen et al., 1959).Mishrif Formation, as in several western Desert subsurface strata, is commonly overlain by a thin evaporitic Lagoonal kifl deposit.In the far west, for example, at Safawi well -1, the Mishrif is an extremely complicated sequence originally characterized as a complex of Detrital limestones with occasional algal, rudist, and coral-reef limestones (Fares et al., 2021).Limonitic freshwater limestones form the cap.The Safawi area is on the stable shelf between the Rutba-Jezira and Salman zones.The tectonic map reveals that the general tendency of transversal fault networks is NE-SW (Fig. 3).Transversal fault systems arose during the Late Precambrian as a result of the Nabitah Orogeny and were repeatedly reactivated over the Phanerozoic (Jassim and Goff, 2006).It is flat due to almost horizontal beds, with a regional dip towards the east and northeast.The Euphrates Fault Zone, with NW-SE direction, is the most notable feature along the Southern Desert's external (northern) edge (Jassim and Goff, 2006).The proposed tectonic pattern is complicated, resulting from the interaction of differential movement on fault systems oriented primarily in NW-SE, NE-SW, and N-S directions (Fares et al., 2021).The accretion of the Arabian Peninsula during the Proterozoic (950-640 Ma) and the subsequent Late Proterozoic-Early Paleozoic (620-550 Ma) intercontinental failure and strike-slip faulting of Najd printed two major patterns of weakness within the continental basement.These are trended in N-S and NW-SE (Fouad, 2007).

Materials and Methods
Safawi oil field included more than 2D seismic survey projects (Qirnen, Ghlesan 1, Tkhaded) within Block 12, Which is located on the Iraqi-Saudi border.Symbolized by QN,1GN, and TI respectively.There are 45 seismic lines accomplished within the exploration area.This study comprises 38 seismic lines (Fig. 4).In addition to surveys, the current study adopted four wells (Samawa-1, Al-Salman-1, Glaisan-1, Diwan-1, Safawi-1).The Safawi-1 well was drilled in 1960, with the deepest penetrated to the 1828 m (Muhaiwir Formation).The total covered area length is 1786 km, and the total area of the current study is 3231 km 2 .A 2D seismic survey data was loaded on the interpretation program (Petrel) in SEG-Y format.The study involves three main tasks: extracting the structural setting, checking the DHI in seismic sections, and identifying seismic stratigraphic phenomena.Therefore, the following work steps can be summarized: Picking horizons of area interest (Mishrif and Ratawi formations).
Extracts TWT maps for Mishrif and Ratawi formations.Convert all seismic information from the time domain to the depth domain by using the velocity model and extracting depth maps.
Extract the average velocity map from the velocity model.Utilize instantaneous frequency attribute for the seismic section to check stratigraphic features and DHI.

Structural Setting and Faults Identification in Time Domain
The TWT maps for the Mishrif and Ratawi formations have been constructed with contour intervals of 5 ms.The map depicts the TWT between sea level, a reference surface, and the top of the Mishrif Formation.Fig. 5 illustrates that the lowest TWT value is in the NE (480 ms), while the highest time value is in the northeast of the map (650 ms).The general dip is towards the NE; three enclosures have been identified and highlighted (A, B, C), representing a folding axis extending along the west part in the NW-SE direction by the side of the Iraqi-Saudi border.There are two sets of faults determined in area of interest, it symbolizes them (F1 and F2).
The TWT map of the Ratawi Formation shows a general increase in SE with a maximum value of 920 ms (Fig. 6).The Time value also decreases in NW -SW with a minimum value of 680 ms.Also, three enclosures (A, B, C) have been identified in the Ratawi Formation but smaller than the Mishrif

Velocity Maps
Average velocity maps were utilized to convert time maps into depth maps and to determine the distribution of selected formation depths in the area of interest.The average velocity maps have been constructed with a contour interval of 10 m/s.The velocity maps show an increase in the average velocity values toward the SW direction while decreasing in the NE direction (Figs. 9 and 10).The average velocity values for Mishrif range from 220 to 2360 m/s while for Ratawi, range from 2320 to 2480 m/s.There is a general increase around SI-1 well in the southern part of the field from 2340 to 2470 m/s in both Mishrif and Ratawi formations, respectively.

Depth Maps
The depth maps have been constructed with contour intervals of 5 and 10 m for Mishrif and Ratawi formations, respectively, with the main sea level as a reference.The two maps show that the Safawi structure has trended in the NE direction.The depth map of the Mishrif Formation (Fig. 11) shows the general dip towards the NE.The minimum depth value 500 m is noticed at the W and SW and gradually increases toward the NE part of the area to reach 670 m, with 580 m around SI-1 well.The depth map of the Ratawi Formation (Fig. 12) shows the general dip towards the NE.The minimum depth value (740 m) is noticed at the west part and gradually increases toward the NE part of the area to reach (1080 m). with 940 m around the SI-1 well.The depth maps also show that the north and NE part of the study area represents a broad depression representing a syncline fold.

Seismic Stratigraphy and DHI
A fundamental principle of sequence stratigraphy is that seismic reflections are produced by contrasts in sonic velocity at chronostratigraphically significant strata surfaces and unconformities; therefore, they approximate timelines in the sedimentary record.Identifying terminations of these reflections is fundamental to defining systems tracts and key surfaces.Some reflection types terminate against an underlying surface.Onlap is defined by the termination of a reflection against a more steeply inclined underlying reflection, most commonly in a landwards direction (Selley et al., 2005).The bright spot is caused primarily by an increase in acoustic impedance contrast as a hydrocarbon (with a lower acoustic impedance) replaces the brine-saturated zone (with a higher acoustic impedance) that underlies a shale (with an even higher acoustic impedance), increasing the reflection coefficient.Hence the bright spot in Fig. 13 refers to hydrocarbon accumulations in Mishrif Formation.From a stratigraphic perspective, two stratigraphic phenomena have been identified within the study area.The first one, highlighted by (S1), represents a progradation facies pattern where the coarse-grained facies at the top of each Para sequence progressively shallower.The stacking pattern is interpreted as progradation (Figs. 13,14,15,16,17,and 18) in the Mishrif Formation.This phenomenon has been extremely extended along Tkhaded 16,18,20,22,24,26,28,30,32).These phenomena extend to the east part of the study area.Displays of instantaneous phase (Figs. 15,17,and 18) lead to assist in identifying weakly coherent events.Lateral discontinuities in the instantaneous phase lead make it easier to pick reflection terminations like those found at faults and pinchouts.At those same locations, instantaneous phase lead gives the true phase lead of a wavelet.The instantaneous phase attribute has been executed for TI (14, 22, 32) seismic lines.The second phenomena (Figs.19 and 20) represent progradation sediments highlighted by (S3) and at the western side of the first phenomena at a similar time level.This phenomenon was extended along QIRNEN QN (10,8,6,4,and 2).Because the sediment supply Mishrif Formation exceeds accommodation, sequences are accumulated by deposition in which beds are deposited basin-ward successively.As a result, the position of the shoreline migrates into the basin during progradation episodes, a process known as regression (Figs.21 and 22).In seismic data (Figs. 13,14,15,16,17,18,19,20,21,and 22), progradation is often represented by inclined seismic reflections that extend outward from the basin's margin.Due to the progradation process, these reflections result from depositing sedimentary layers with a shallower dip towards the basin's center.Prograding sedimentary units, known as clinoforms, are stacked packages of inclined strata that build out into the basin.These clinoforms result from sediment progradation and can be seen as distinct seismic reflections.Within each clinoform, sedimentary facies change vertically from finer-grained to coarser-grained as observed by core samples.This coarsening-upward sequence is a common characteristic of progradation.In a progradation facies pattern, seismic reflections and sedimentary layers may display an onlap configuration.Onlap occurs when younger sedimentary layers encroach upon the older layers in an inclined manner.Prograding seismic reflections often exhibit an asymmetric pattern, with one side of the reflection dipping more steeply than the other.This configuration is called the "progradation axis," indicating the progradation direction.

Conclusions
The time maps of the studied reflectors showed the existence of several phenomena as noses, these are generally in NW-SE direction, the time maps of the studied reflectors revealed two sets of faults, NW-SE and E-W.The first one corresponds to the major direction of the Najd fault system, while the second corresponds to the transversal faults system.The velocity maps show an increase in the average velocity values toward the SW direction while a decrease in the NE direction.Depth maps of Mishrif and Ratawi reflectors showed that the depth contours increase to the northeast and decrease towards the west.Safawi's structure has trended in the NE direction.Using seismic attribute techniques, including instantaneous phase, two stratigraphic phenomena appear (S1 and S3) within the Mishrif Formation.S1 and S3 represent progradation sediments facies.

Fig. 3 .
Fig.3.Structural division map of Iraq; the study area is located in the Mesopotamia basin block ( Jassimand Goff, 2006)

Fig. 4 .
Fig.4.Base map of the study area

Fig. 13 .
Fig. 13.The Seismic section TI-14 illustrates stratigraphic phenomena and DHI on the Mishrif Formation

Table 1 .
Coordinates of the study area