Characterization of Structural Geology of Faihaa Oilfield, Southern Iraq

Faihaa Oilfield is a new exploration Iraqi Oilfield located in southern Iraq and within Mesopotamian Plain, including the Block 9 exploration area, along the Iraqi-Iranian border. The study area included Faihaa Oilfield (in Iraq) and Yadavaran Oilfield (in Iran). They belong to one anticline (Dome) structure separated by the Iraqi-Iranian border, without a geological boundary between the fields. The current study aims to achieve structural geology analysis to the study area (Faihaa/Yadavaran structure). The structural analysis included geometric and genetic analyses of the study area. According to geometric and genetic analyses results, the Faihaa/Yadavaran structure is classified as an anticline, gentle, upright, non-plunge, and asymmetrical. According to thickness variation, there are two types of formations’ folds are recognized, Thickened and Supratenuous fold generated by two folding mechanisms bending and buckle mechanisms. Bending form Supratenuous fold, perhaps due to the vertical uplift of salt structure and/or basement faults, while buckle produces Thickened fold because of the parallel tectonic movement causes collision between Arabian Plate and Eurasian Plate. Consequently, a special strain pattern was formed and the reservoir quality in the crest of Mishrif and Yamama Formations was the best. The fold axis of the Faihaa/Yadavaran structure has a Boomerang shape, whereas it is almost straight from south to the center of the structure, then tends to the NW with different deviations over the study area formations. This direction may be due to the anticlockwise rotation of the Arabian Plate motion. The results of geometric and genetic analyses revealed that may be Faihaa/Yadavaran structure is one structural trap formed by tectonic activities; Hormuz salt structures, reactivated Basement faults, and Collision between Arabian and Eurasian Plates. The intensity of the tectonic activities of the study area maybe be less than surrounding Oilfields, therefore, the Faihaa Oilfield formation's depth was deeper than adjacent Oilfields. contour maps and thickness variation from drilling data of available formation data. The available image log interpretation of well Faihaa-1 (FH-1) was used to determine the stress distribution of Yamama and Mishrif Formations. The available porosity data from Faihaa static model utilized to constructed porosity contour maps for Mishrif and Yamama reservoirs to find if there is a relation between the structural picture and porosity of the study area.


Introduction
Most previous studies focused on petroleum and petrophysical properties of the Yamama Formation of Faihaa Oilfield, for example [1], [2]. Therefore, the current study based on the structural geology analysis integrated with available petrophysical analysis results to determine the structural picture of the study area in the Faihaa Oilfield. The study area lies in south Iraq, within the Block 9 exploration area, along the Iraqi-Iranian border [1]. According to [3] Faihaa Oilfield occupied the western-central part of the studied oilfield, while Yadavaran Oilfield (in Iran) the eastern-central part, and it separated by the official Iraqi-Iranian border and its main reservoirs were Yamama and Mishrif Formations (Cretaceous Period). Therefore, the current study will achieve structural analysis for both fields (Faihaa and Yadavaran) as one subsurface anticline structure and the structural analysis results will be for both fields. The structural analysis included geometrical and genetic analyses. The geometrical analysis is interested in the geometric properties, and genetic analysis employed the results of the geometric analysis with geophysics interpretations to determine the causes, type, and origin of the study area. The 3 current study will utilize the available petrophysical properties of the study area's reservoirs to find if there is a connection between the petrophysical properties and the structural picture of the study area.

Geologic Setting
Geologically, Iraq located within the NE margin of the Arabian plate, close to the suture zone of the Arabian -Eurasian plates collision [4]. According to the tectonic divisions of Iraq, the study area is located in Mesopotamian Plain (or zone) [5]- [8]. The Mesopotamian Plain is part of Mesopotamia Foredeep, which represents intern the terrestrial remnant of the Zagros Foreland Basin and it forms the central and the southern parts of Iraq. It contains several subsurface structures including faults, folds, and diapiric structures that are covered by Quaternary sediments, see Figure (1) [6]. The Mesopotamian Plain has been considered an unstable part of the Arabian Plate and the reasons for instability are basement faults, salt structures, and Alpine Orogenic Movements [9], [10] and these reasons form anticline subsurface structures in southern Iraq [9], [11]. The study area is located in Zubair Subzone, which is bounded by basement faults which are the Takhaded-Qurna Transversal fault from the north and Basra-Zubair fault from the south [12] as shown in Figure (2A). The study area is surrounded by other Oilfields (from north-west Azdagan and Majnoon Oilfields, from south-west Nahr Umr Oilfield, and south from Sindibad Oilfield), as shown in Figure (2B). The negative gravity of the Zubair Subzone structures may be due to the presence of deep-seated Infracambrian Hormuz salt rocks [11]- [14] while positive residuals attributed to basement uplift [13]. The study area included 17 geological formations within the Cretaceous, Paleogene, and Neogene, as shown in

Materials and Methods:
The current study used a geological model drilling data ( contour maps and thickness variation from drilling data of available formation data. The available image log interpretation of well Faihaa-1 (FH-1) was used to determine the stress distribution of Yamama and Mishrif Formations. The available porosity data from Faihaa static model utilized to constructed porosity contour maps for Mishrif and Yamama reservoirs to find if there is a relation between the structural picture and porosity of the study area.

Geometric Analysis:
The main role of geometric analysis is to determine the physical properties and classify the fold to use it in genetic analysis with available geophysical interpretations of the study area and surrounding structures to find the forming causes of the study area. Depth and dip contour maps of the Study area formations were used to obtain the Stereographic Projection results as shown in Table (2) to employ it in a fold classification.  There are several classifications of the folds and each one uses specific geometric parameters of the fold. The current study will use the main geometric description principles of a fold to determine the structural picture of the study area (Faihaa/Yadavaran Structure). [16] summarized the fold classifications to (a) Fold facing, (b) Fold orientation (an attitude of axial Plane, plunge of the hinge line (fold Axis), and symmetry of fold), and (c) Fold-shape in profile plane (interlimb angle and variation in thickness). Table (3) outlined the results of these classifications of the study area.
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 hinge is less than limbs, T-Fold (Thickened Fold), when the thickness of hinge is more than limb, P-Fold (Parallel Fold) when the thickness is same from hinge to limb, F-Fold (Flow Fold) when thickness variation has no certain distribution as previous types [17]. The thickness difference between hinge zone (or crest) and limbs of study area formations fluctuated from thinning crest to thickening crest, as shown in Table (3), and this led to configuring the folding mechanisms responsible for the form of the study area. According to [16], [18], [19] these folding mechanisms are bending, (thinning Bending happens when the layer acts to a force across it while buckling the force will be parallel to layers [19]. Arching and thinning of the layers originated by extensional tectonic are common features associated with folding form by bending mechanism, unlike buckle mechanism where the thickening and shorting of the layers [18]. The relationship between fold mechanism and tectonic events of the study area will be explained in the genetic analysis to clarify its implications to form situated case from fold within the studied area, and its influence on reservoir properties.   The fold axis of the study area has a Boomerang shape. It's almost straight from south to the center of the structure, then tends to the NW with different deviations over the study area formations. The image log and Sonic Scanner interpretations of Faihaa-1 well (FH-1) show the direction of maximum stress (σ 1 ) direction is NE-SW, while minimum stress (σ 3 ) direction is NW-SE of Mishrif Formation [20]. The σ 1 and fold axis directions are plotted as the rose diagram for Mishrif Formation and this exposed that the σ 1 is not perpendicular to the fold axis (as supposed to be), as shown in Figure (4).

Genetic Analysis:
The geometric analysis results referred to that the study area may be affected by tectonic activities. Three combined main forces worked together to form subsurface anticline structures in southern Iraq, these are tectonic Movements (collision between Arabian and Eurasian Plates), reactivated basement faults, and Hormuz salt structures [7], [9], [11], [13], [14] as shown in the tectonic model in Figure ( Infra-Cambrian salt beds, while the positive residual referred to basement uplift [11], [13], [14].  Table (4).

Fig. (5): Tectonic model for Oilfields in southern Iraq for Cretaceous Period, after [22].
The geometric analysis results of study area formations are similar, except the thickness variation parameter and based on the difference of thickness between the crest and limbs for each formation show the majority of two folding mechanisms; bend (S-Fold) and buckle (T-Fold). The bending mechanism acts due to vertical force, which may be related to the salt activity of deep-seated Hormuz salt as referred to by gravity negative anomalies associated with Zubair subzone and surrounding Oilfields to Faihaa\Yadavaran structure. The buckle mechanism acts because of parallel force, which is probably related to the collision between Arabian Plate and Eurasian Plate (specifically the Iranian Plate).  There is no interpretation for gravity and magnetic geophysical data of the study area, but depending on the results of adjacent Oilfields and structures within the Zubair subzone (like Nahr Umr and Zubair Oilfields) the salt structures of Hormuz salt and basement faults has been indicated [11], [13], [14]. These geophysical results are linked with the oilfields' structural pictures and folding mechanisms (bending and buckle) [23]- [25]. Therefore, the bending mechanism could be attributed to the influence of salt structure-activity of Hormuz salt and/or reactivated basement faults as vertical force, while buckling mechanism to the collision between Arabian Plate and Eurasian Plate as a parallel force. The depth of study area formations is deeper than adjacent oilfields formations, this may be due to the intensity of tectonic activities affected on the study area is less than other oilfields within Zubair Sub Zone. This could be confirmed by the occurrence of normal faults on the Nahr Umr field [22], while there is no fault on the study area and simplicity of its structural picture as confirmed by 3D seismic interpretation [26]. The fold axis of the study area trends to NW-SE and this direction may be attributed to counterclockwise rotation of the Arabian plate and this direction is similar to surrounding fold axes of Oilfields of southern Iraq, such as Nahr Umr Filed [22], Zubair confirm that they are one field or structure, belonging to the same subsurface anticline formed with the same structural events and history.

Results and Discussion:
Integration between geometric and genetic analysis results referred to that the study area may be controlled by tectonic activities. These results confirmed that the study area formed by same geological conditions of surrounding oilfields [22]- [25], [27], but with less intensity. The tectonic history of the study area passed through geological periods; Cretaceous, Paleogene, and Neogene Periods, and these have significant events, as shown in (Table 3). Each one could be triggered a particular folding mechanism with direct influence on the structural picture of the study area. These activities varied between the vertical force, which could be related to the salt structure of Hormuz salt and/or reactivated basement faults and parallel force may be due to compressional force of the collision between the Arabian plate and Eurasian Plate, and these are common geological causes responsible about the structural picture of southern Iraq oil fields [9], [11], [13], [14]. These forces are probably represented by two mechanisms, bending (vertical force) and buckling mechanism (horizontal force of compressional stress). Based on [18], [28], [29]

Conclusions:
1-Faihaa and Yadavaran Oilfields may be considered as one anticline structure (Faihaa/Yadavran structure) without a geological boundary separate between the fields.