Structural geology and petrophysics analysis to Injection wells of Mishrif Formation in Shuaiba Dome – Zubair Oil Field

Mishrif formation is the one of the most important reservoir in Southern – Iraq and the injection water important to support the reservoir pressure. The current study combined the interpretations of PLT, structural geology and petro-physics to understand the distribution of the injection rate for nine injection wells to the Mishrif Formation in Shuaiba Dome (or culmination) Zubair Oil Field. PLT analysis calculated injection rate for Mishrif Formation, structural analysis included geometric and genetic analysis, whereas petrophysics analysis used open hole logs interpretation and core data for the injection wells to determine the petro-physics characteristics (especially the distribution of porosity and permeability). The current study concluded that the injection rate, porosity and permeability of Mishrif Formation distributed unequally across it. This variation is almost regular, whereas the Upper Mishrif more than Lower Mishrif, while Middle Mishrif get the least value. The thickness of Lower Mishrif more than Upper Mishrif. This may affected by a folding mechanism due to tectonic activity (reactivated basement faults and Hormuz salt structures).


Introduction
Mishrif Formation is one of the most important reservoirs in Iraq and divided into three main divisions Upper, Middle, and Lower Mishrif [22]. Study area lies within the Mesopotamian basin, Zubair Subzone [12]. Mishrif Formation is a carbonate Formation that deposited in the Middle Cretaceous (Cenomanian-Early Turonian) [2].
Rudist are the most important facies of Mishrif Formation, thus it considered as the most permeable zone [15, and 12].
Generally, the rudist facies deposited in the crestal areas of actively syn-sedimentary anticline structure southern Iraq and these facies affected by dissolution process [12] and [5]. The thinning, erosion and conglomerates of Misrif crest due to regional uplift during the late Cenomanian to Turonian period [6] and [9]. PLT analysis employed the highest injection rate to evaluate the distribution of water for perforated layers, structural All previous analyses connected together in trying to understand the geological situation and its effect on the injection performance of Mishrif Formation within Shuaiba Dome.

Geologic Setting:
Mishrif Formation is overlain unconformably with Khasib Formation and underlie conformably with Rumaila Formation. It is equivalent to Saravk Formation in Iran.
According to the tectonic division of [8], Zubair Oil Field lies in Mesopotamian Zone within an Unstable Shelf of Arabian Platform. While [17] and [18], the field lies in sagged basin within the Mesopotamian zone of the qusiplat form Foreland belt of the Arabian plate. Zubair Oil Field is located in the Zubair Subzone of the Mesopotamian Zone, whereas; the structures of this Subzone controlled by the basement structures and Infracam brain salt [12]. [4] stated that Zubair Oil Field belongs to the Unstable Shelf, and the factors of instability are basement faults, salt structures, and Alpine Orogenic Movements. These factors worked together to produce subsurface anticline structures southern Iraq. Zubair Subzone bounded by basement faults, which are Takhadid-Qurna Transversal fault from the north and Al Batin fault from the south [12] (Figure 1). The negative gravity anomaly of the primary Zubair Subzone structures confirmed the presence of deep-seated Infracam brain salt rocks [12, 13, 14 and 15].

3-2. Structural Geology analysis:
The current study used a geological model (static model) of the Zubair Oil Field to construct depth via Petrel v2013. 2 software with scale 1:60000. Steronet 9 software used for stereographic projection to determine the structural geological attitude

4.PLT analysis:
The PLT log is the one of the most important tool to evaluate the productivity and infectivity of specific zones [19]. The current study used the PLT data for nine selected injection wells to determine the injection rate of each perforated layers of Mishrif  The current study also noticed that the thickness of Upper Mishrif is less than Lower Mishrif in all injection wells of Shuaiba Dome and the thickness of left limb is greater than right limb. The reason and indications of thickness variation will clarify in genetic analysis because it is so important to understand the structural picture of Mishrif Formation and its reservoir implication.  744000  745000  746000  747000  748000  749000  750000  751000  752000  753000  754000  755000  756000  757000  758000  759000   744000  745000  746000  747000  748000  749000  750000  751000  752000  753000  754000  755000  756000  757000  758000  759000   0  1000  2000  3000  4000  5000m 1:60000

Symbol legend
Closed water injector Open water injector Oil water contact A-A Cross Section

5-2. Genetic Analysis:
Three combined main forces have been worked together to produce subsurface anticline structures in southern Iraq included Zubair Oil Field, these are tectonic Movements, reactivated basement faults, and Hormuz salt structures [4], [17], [12], [14] and [13] as shown in tectonic model (Figure 3). Geophysical surveys of southern Iraq indicated that the association of negative gravity could be as a result of deep-seated salt beds of Infra-Cambrian salt beds, while, the positive gravity referred to basement uplift [13] [14], [12], [15], and [16] referred to negative anomaly associated with north Zubair Arching and thinning of the layers originated by extensional tectonic are common features associated with salt structure and this leads to normal faults over the tops of the anticlines [20]. These faults are may be radial or parallel [7].

Petrophysics analysis:
The