Structural Geology Study of Tuba Oilfield, Southern Iraq

Abstract


Introduction
Tuba oilfield or Tuba structure is located in southern Iraq about 40 km west of Basra city.It is located between two main oilfields; Rumaila oilfield from the west (North and South Rumaila oilfields) (2 km), while Zubair oilfield (5 km) from the east, and is separated from them by depressions.The main reservoir rocks of the Tuba oilfield are Mishrif and Zubair formations (Cretaceous Period) (Jalayh, 1979).The dimensions of the study area are 29 km in length and 9 km in width and the first exploration well (TU-001) was drilled in 1959.The second 2D seismic survey in 1977 showed the possibility of existing a second dome southern to the first one; therefore the well (TU-003) was drilled in 1978 in the southern part of the study area, and this well revealed there is no second dome (Exploration Oil Company, 1988).The current study aims to carry out a structural geology analysis (geometric and genetic) to define the structural description of the Tuba oilfield.The geometrical analysis employs the physical descriptions of the structure.The genetic analysis utilized the results of geophysical surveys and geometric analysis to determine the forming causes of the structure under study.

Materials and Methods
The current study used drilling data to obtain the depth, dip, and thickness parameters to utilize them in structural analysis.The stereographic projection used Stereonet software (v.11) to identify the structural attitudes of the Tuba oilfield (angle of interlimb, fold axis (hinge line), and axial plane).The calculation of dip and dip direction (clockwise) was obtained from the depth contour maps.The thickness variation between hinge area and limbs is obtained from thickness maps, which draw up by subtracting the tops of each two consecutive formations to measure the perpendicular thickness.There is no thickness map for the Ratawi formation because of no drilling data for the formation below it.

Study Area
Tuba oilfield is located in southern Iraq.Geologically, Iraq lies on the NE margin of the Arabian plate, nearby to the suture zone of the Arabian Plate-Eurasian plate collision (Abdulnaby, 2019).According to the tectonic divisions of Iraq (Fouad, 2010;Numan, 2000Numan, , 1997)), the study area is located in Mesopotamian Plain (or zone).The Mesopotamian Plain is a part of Mesopotamian Foredeep, which comprised the center and south part of Iraq (Fouad, 2010) (Fig. 1).(Fouad, 2010) The Mesopotamian Plain has been considered an unstable part of the Arabian Plate and this is attributed to salt structures, basement faults, and Alpine Orogenic movements (Al-Sakini, 1995;Buday and Jassim, 1987).These reasons form anticline subsurface structures in southern Iraq (Al-Sakini, 1995;Karim, 1993).While (Fouad and Sissakian, 2011) considered Mesopotamian Plain as part of the stable interior of the Arabian Platform.However, both (Al-Sakini, 1995;Fouad and Sissakian, 2011) agreed it has many active subsurface structures, indicating Neotectonic movements.The study area is located in Zubair Subzone (inside the Mesopotamian Plain), this zone is bounded by two basement faults; the Takhaded-Qurna fault from the north and the Basra-Zubair fault from the south (Jassim and Goff, 2006) (Fig. 2).
The negative and positive anomalies are associated with Zubair Subzone subsurface structures, the negative gravity may be related to the presence of deep-seated Infra-Cambrian Hormuz salt rocks (Jassim and Goff, 2006;Karim, 1993Karim, , 1989;;Karim et al., 2010), while positive residuals could be attributed to basement uplift (Karim et al., 2010).

Geometric analysis
The main role of geometric analysis is to determine the physical properties of the fold and its classification.Furthermore, it is to use these results in genetic analysis with available geophysical interpretations of the study area and surrounding structures to conclude the forming causes of the Tuba oilfield.Drilling data of the study area were used to obtain the Stereographic Projection results (Table 1) to classify the fold.The folds could classify into many kinds based on structural geometric parameters.The current study will use the main geometric description principles of a fold to determine the structural description of the study area.(Park, 1997) summarized the fold classifications to; facing of the fold, orientation of the fold (axial surface, fold axis or plunge of the hinge line, and fold's symmetry), and fold-shape in profile plane (interlimb angle and variation in thickness).The results of these classifications are shown in Table 1.The thickness variation of a fold considers a very significant parameter to classify the fold of the study area.There are four types of folds based on their thickness variation.These are S-Fold (Supratenuous Fold), when the thickness of the hinge is less than limbs, T-Fold (Thickened Fold), when the thickness of the hinge is more than limbs, P-Fold (Parallel Fold) when the thickness is the same as the hinge to limb, F-Fold (Flow Fold) when thickness variation has no certain distribution as previous types (Bhattacharya, 2005).The average values of the thickness show that the thickness difference between the hinge zone (or crest) and limbs of study area formations fluctuated from thinning crest to thickening crest, and this led to configure two major folding mechanisms responsible for forming the study area (Table 1).According to (Billings, 1972;Park, 1997;Bhattacharya, 2005;Fossen, 2016) these folding mechanisms are bending, (thinning hinge zone versus thickening limbs) and buckling (thickening hinge zone versus thinning limbs).Bending happens when the layer acts to a force across it (vertical force) while in buckling the force will be parallel to layers (Fossen, 2016) (Fig. 3).Arching and thinning of the folds are common features created by extensional tectonics, which is associated with the bending mechanism.while buckle mechanisms form thickening and shorting layers (Billings, 1972;Fossen, 2016;Van-der-pluijm and Marshak, 2004).The relationship between the fold mechanism role and tectonic events of the study area will be explained in the genetic analysis.The fold axis of the study area is NW-SE from deepest to shallowest formations with different inclination degrees for each formation.The shallower formations from Rus to Lower Fars (Palaeocene, Eocene, and Miocene) have no clear closure as deeper formations; therefore, the study mentions the direction (NW-SE) without specific value.Generally, the dips of formations' limbs are not equal, and the steepness varied between western and eastern limbs, and some formations have the same dips values.3. The relationship between fold mechanism and applied force direction, after (Fossen, 2016)

Genetic analysis
Three main forces formed the subsurface and relief structures in southern Iraq.These are a collision between Arabian and Eurasian Plates, basement faults, and salt structures (Al-Sakini, 1995;Karim, 1993Karim, , 1989;;Numan, 1997) (Fig. 4).The results of geophysical surveys of southern Iraq showed negative and positive gravities.Negative gravity might be a result of Infra-Cambrian salt beds, while the positive ones may be related to basement uplift (Jassim and Goff, 2006;Karim, 1993Karim, , 1989;;Karim et al., 2010).Almutury and Al-Asadi (2008) summarized the tectonic history of the Mesopotamian plain (included   A negative anomaly is associated with the Tuba structure and adjacent area (Samir, 2011).The negative gravity and positive magnetic residual anomalies were reported with other oilfields adjacent to the study area within the Zubair subzone (like Rumaila, Zubair, and Nahr Umr oilfields).The Negative gravity may be related to the salt structures of Hormuz salt, and positive magnetic residual could be attributed to basement uplift (Karim, 1993(Karim, , 1989;;Karim et al., 2010).These results are connected with the folding mechanisms (bending and buckle) to define the structural pictures of the other fields in southern Iraq (Al-Kubaisi and Lazim, 2016;Lazim andDawood, 2020, 2019).According to the thickness variation parameter of the study area, the majority of folding mechanisms are bending (S-Fold) and buckle (T-Fold).Consequently, the bending mechanism may be attributed to the influence of the salt activity of Hormuz salt and/or reactivated basement faults, while the buckling mechanism is due to the collision of the Arabian Plate and the Eurasian Plate.
Structurally, The Tuba oilfield is lowest than the Rumaila oilfield.This is the same for the Zubair oilfield, except the formations from Shiranish to recent are shallower than the Tuba oilfield (Fig. 6).The depression between Tuba and Zubair oilfields is deeper than the depression between Rumaila and Tuba oilfields but with the same direction NW-S.This may be due to the intensity of tectonic activities in the study area being less than in other fields.This could be noticed by the occurrence of normal faults in the Rumaila oilfield (Jaffar and Abdulnaby, 2018) and Zubair oilfield (Lazim et al., 2020b), while there is no fault in the study area according to 2D seismic interpretation (Samir, 2011).The NW-SE fold axis direction may be due to the counterclockwise motion of the Arabian plate and this direction is equivalent to surrounding fold axes of oilfields in southern Iraq, such as Rumaila oilfield (Jaffar and Abdulnaby, 2018), Zubair oilfield (Lazim and Dawood, 2020), West Qurna oilfield (Lazim et al., 2020a), and Nahr Umr oilfield (Al-Kubaisi and Lazim, 2016).

Discussion
The results of the structural analysis revealed that the study area is a structural trap controlled by tectonic activities.The tectonic activities may be related to Hormuz salt, reactivated basement faults, and collision between Arabian Plate and Eurasian Plate.Geometric analysis shows the main folding mechanisms affected in the study area are bending and buckle folding mechanisms.The bending folding mechanism; may be induced by the vertical force of the salt structure of Hormuz salt and/or reactivated basement faults, while the buckling folding mechanism; may be represented by the horizontal force of the collision between the Arabian Plate and the Eurasian Plate.Based on (Billings, 1972;Groshong, 2006;Van-der-pluijm and Marshak, 2004) these mechanisms have significant strain patterns on fold layers.This strain pattern is categorized by an outer arc and an inner arc separated by no strain neutral surface.The outer arc is characterized by extensional features, while those in the inner arc are compressed, with separate regions of extension from regions of contraction (Fig. 5a).The outer arc of the fold commonly forms extensional fractures like faults, joints, and veins (Groshong, 2006), (Fig. 5b).These fractures have an important role in oil migration, and the extensional fracture dies out down to the neutral surface which may be separate to systems of pressure, one belongs to the extension part and the other to the compressed part (Colman-Sadd, 1978).
The lower part of the Mishrif Formation of the study area is the most important in terms of oil saturation and petrophysical properties (porosity and permeability) (Samir, 2011).On the contrary, the upper part of the Mishrif formation in the Zubair oilfield is the main one, and this is compatible with the bending and buckle mechanisms' strain pattern (Lazim, et al., 2020;Lazim andDawood, 2019, 2020).In addition, the facies of Mishrif formation of Tuba oilfield are less quality than North Rumaila oilfield, although they are homogenous (Jalayh, 1979).The sedimentation cycles are the same for Tuba, Rumaila, and Zubair oilfields (Samir, 2011).This possibly could be attributed to the structural development of the Tuba oilfield being the same but the strength is less than Zubair and North Rumaila oilfields and this is reflected in its strain pattern by the absence of faults.Fractures (faults, joints, and veins) features associated with the extension of outer arc, after (Fossen, 2016).
The time maps referred to the Tuba oilfield include two domes, while the depth maps show only one dome.The structural analysis study couldn't confirm if the study area has two domes or not because the southern part contains one well (Tu-003) and this is not enough to use contour maps as evidence; therefore, the study uses the correlation section to compare structurally between northern and southern parts of the study area (Fig. 7a,b).This correlation section showed that the northern part is deeper than the southern part, but from the Sadi Formation, the southern part starts rising with no change in the center and northern parts.This may be referred to that the southern part (Tu-003) of the study area being subjected to tectonic activity unconnectedly with the central and northern parts, therefore the southern part could not belong to the Tuba oilfield.The fold axis of the study area is NW-SE and this is Wellmatched with nearby structures and oilfields.The shallower formations from Rus to Lower Fars (Palaeocene, Eocene, and Miocene) have no obvious closure (dome) as deeper formations.Accordingly, the current study doesn't mention a specific value for the fold axis in (Table 1) for these formations and referred to NW-SE as the fold axis direction.Regarding dip values of the same formations, it refers to the values of the western and eastern sides of the formations to show the effect of tectonic activity on the study area.This maybe supports the results of the current study, which referred to the structural growth of the Tuba oilfield as less than nearby oilfields.

Conclusions
Tuba oilfield is classified as asymmetrical structural trap (the length of the western limb is slightly less than the eastern limb), non-plunge, anticline, upright, and gentle fold.Regarding thickness parameter, there are two types of folds Thickened fold (Zubair, Mauddud, Rumaila, Khasib, Tanuma, Shiranish, Umm Er Radhuma, and Ghar formations) and Supratenuous fold (Shuaiba, Nahr Umr, Ahmadi, Mishrif, Sadi, Hartha, Tayarat, Rus, Dammam, and Lower Fars formations).The difference in thickness between the crest and limb of study area formations fluctuated between thickening and thinning from crest (hinge area) to limb.This may be attributed to the role of folding mechanisms, bending folding mechanism of vertical stress, which may be due to uplift of Hormuz salt and/or basement faults force, while buckle folding mechanism may be related to horizontal stress of Arabian and Eurasian Plates' collision.According to the strain pattern, the structural growth, and the clearness of the shallow formations (Rus, Dammam, Ghar, and Lower Fars formations), the structural development of the Tuba oilfield maybe is less than adjacent Rumaila and Zubair oilfields.
The general fold axis direction of the study area is NW-SE, with different inclination degrees (increase toward shallow successions).This direction may be affected by the counterclockwise rotation of the Arabian plate motion.However, this direction is analogous to surrounding fold axes' fields of southern Iraq.The results of geometric and genetic analyses revealed that the Tuba oilfield is probably controlled by tectonic activities (tectonic movements related to collision between Arabian Plate and the Eurasian plate, reactivated basement faults, and Hormuz salt) that worked together to form the Tuba structure.The Tuba oilfield has a homogenous structural shape from Lower Cretaceous formations to the Tanuma Formation.Then, the southern part (including TU-003) was rising individually from other parts (Fig. 7a).Consequently, the southern part possibly does not belong to the Tuba oilfield or is subjected to tectonic activity separately from the central and northern parts.

Fig. 2 .
Fig. 2.The location of the study area concerning the surrounding major structures at the depth of 4000 m, southern Iraq with some of the basement faults in the Mesopotamian Zone, after(Almutury and Al- Asadi, 2008) study area) into two phases: The Opening phase (Permian-Jurrasic), and the Closed phase (Upper Jurassic 'L.Tithonian' -Recent), which is an interesting phase for the current study.The closing phase involved compressional stress and reactivation of listric basement faults.The interesting geological periods for the current study are the Cretaceous period, Palaeogene (Paleocene and Eocene epochs), and Neogene (Miocene).

Fig. 5 .
Fig. 5. a) The strain distribution of Buckle and bending mechanisms with the neutral surface, and b)Fractures (faults, joints, and veins) features associated with the extension of outer arc, after(Fossen, 2016).

Table 1 .
Stereographic projection results of the study area.

Table 2 .
Geometric analysis of thickness variation Results of the study area