Comparison between Rafidhiya and Shuaiba Domes within the properties of Mishrif Formation in Zubair Field; the implication of structural Geology and petrophysical analyses

The current study combined both concepts of structural geology and petrophysical to understand the structural feature of Mishrif Formation and its implication on the petrophysical characterization of the formation in Shuaiba and Rafidhiya Domes (or culminations) in Zubair Field. Shuaiba and Rafidhiya are adjacent domes and these domes belong to the same Field but the domes separated by saddle may related to Basra – Zubair basement fault. The domes have different petrophysical properties of Mishrif Formation; consequently, influenced in water and oil saturation. Therefore, the study tries to understand the structural and petrophysical position of Mishrif Formation of the domes. The structural analysis included geometric and genetic analysis, whereas petrophysical analysis used open hole logs interpretation to determine the petrophysical characteristics (especially the distribution of porosity, permeability, and water saturation. It was concluded that may a variation in porosity and permeability of Mishrif Formation for Shuaiba and Rafidhiya domes because each dome was formed by a different folding mechanism effected on the petrophysical properties. The structural geology analysis detects that may be Shuaiba dome formed by bending fold mechanism (vertical force of salt structure), while Rafidhiya dome by buckling fold mechanism (parallel force of collision of No.28(9) 2020 Journal of Petroleum Research & Studies (JPRS) E87 Arabian and Eurasian plate). These mechanisms may directly be affected in permeability distribution, and consequently on oil and water saturation of Mishrif Formation. Thus, Shuaiba Dome has thinning in hinge area and extensional force leads to create fractures and karst phenomena, and as a result, high permeability in upper Mishrif. On the contrary, Rafidhiya Dome has a thickening feature and there is no indication of karst phenomena and low permeability. Therefore, the Mishrif of Shuaiba dome permeable and oil-saturated, while, it flooded with water in Rafidhiya Dome. The disconnection in reservoir pressure confirmed by difference in initial reservoir pressure of Mishrif Formation of Shuaiba Dome and recent reservoir pressure of Mishrif Formation of Rafidhiya Dome.


Introduction:
The study area lies within the Mesopotamian basin, Zubair Subzone [9]. Mishrif Formation is a carbonate Formation that deposited in the Middle Cretaceous (Cenomanian-Early Turonian) [3]. It is divided into three main divisions Upper Mishrif, Middle, and Lower Mishrif [16] Rudist fancies are the most important fancies of Mishrif Formation, thus it's considered as the most permeable zone [1], [2], [6], and [9]. Generally, the rudist fancies deposited in the crestal areas of actively syn-sedimentary anticline structure southern Iraq and these fancies affected by the dissolution process [6] and [9]. There are no previous studies focused on the influence of the structural geology role of the Mishrif Formation in the study area on its petrophysical properties except only one study referred to the presence of oil just in Shuaiba and Hammar Domes, while Rafidhiya Dome classified as water aquifer.
This study supposed a southern boundary from Shuaiba dome categorized with the termination of good reservoir condition [14]. Thus, the current study tries to achieve a structural geology analysis included geometrical and genetic analyses. Geometrical analysis interpretations for selected wells in the current study area to determine the petrophysical characteristics of Mishrif Formation (especially the distribution of porosity and water saturation). All previous analyses connected to understand the structural geology picture to find its role in the petrophysical (especially oil/water-saturated) of Mishrif Formation within Rafidhiya and Shuaiba Domes.

Geologic Setting:
Mishrif Formation is overlain unconformably with Khasib Formation and underlie conformably with Rumaila Formation (Figure 1). According to the tectonic divisions of [15] and [16], the field lies in the sagged basin within the Mesopotamian zone of the qusiplatform Foreland belt of the Arabian plate. Zubair Field is located in the Zubair Subzone of the Mesopotamian Zone, whereas the structures of this Subzone controlled by the tectonic movements, basement structures, and Infracambrian salt [9]. [5] [9], [10], [11], [15], and [16]. (c) Fold shape in profile plane (interlimb angle [7] and variation in thickness [17]. The dips of Mishrif Formation of Shuaiba Dome are equal, while the western limb is slightly steeper than the eastern limb of Rafidhiya Dome (Tables 2 and 3).

Table (3) Results of Geometric Analysis for Mishrif Formation of Rafidhiya Dome
Regarding the thickness variation, the thickness map (

Results and Discussion:
According to the combination of the results of structural geology (geometric and genetic) and petrophysical analyses, the study suggests a scenario for Mishrif Formation in Shuaiba and Rafidhiya Domes. As for Shuaiba Dome, the salt structure induced by reactivated basement faults and differential density between the salt and overburden rocks to create the structural picture of Mishrif Formation and this confirmed by negative residual.
As result, Shuaiba Dome formed by a bending fold mechanism, which is recognized by thinning, extensional outer arc and may fracturing features with erosion surface represented by the unconformity of Mishrif -Khasib Formation and the dissolution processes create karst phenomena (high permeability zones) [13], and this may be the cause of Mishrif Formation of Shuaiba Dome saturated with oil because it included the best reservoir condition to accumulate the oil. Mishrif Formation in Rafidhiya Dome has a thickening feature may be due to buckling fold mechanism, because of a collision between Arabian and Eurasian plate, and may this the reason of association of positive residual with Rafidhiya Dome. Buckling force supposes to create extensional in an outer arc, in addition to shortening of layers, but shortening for parallel layers, which is occurs before folding may lead to a decrease or terminate the extensional of the outer part of a fold [8]. Mishrif Formation of Shuaiba Dome is thicker than Rafidhiya Dome. The thickness of the crestal area of Mishrif Formation is thinner than its limbs for Shuaiba Dome and this may be attributed to bending fold mechanism due to salt structure below it as referred by negative residual. This mechanism generates extensional outer arc (Upper Mishirf) and may be associated with fractures then influenced by meteoric water, which is dissolved the carbonate and make karst phenomena (high permeable zone). While, Rafidhiya Dome formed by a buckling fold mechanism, therefore, the crest area thicker than the area of the limb. it's maybe influenced by shortening for parallels layers before folding and this reduces