Geological Model for Khasib Formation of East Baghdad Field Southern Area

Geological model construction is an important phase of reservoir study as the production capacity of a reservoir depends on its structural and petrophysical characteristics. The economic benefit of the reservoir is evaluated by estimating the formation petrophysical properties and calculating the oil reserves. East Baghdad southern area field is a newly developing oil field in the middle region of Iraq, where Khasib formation is its main reservoir. The aim of this study is to estimate the petrophysical properties and determine the pay units of the formation under study and the initial oil in place. Sequential Gaussian Simulation was used here to distribute the petrophysical properties as the statistical method and volumetric method was used to calculate the oil in place. The results show that the main reservoir units of the formation are K2 and K3 units, and the estimated oil reserves equal to 2179 mmSTB (346.43 million cubic meters).


Introduction:
The construction of the reservoir geological model called static model, is probably the most important step of a reservoir study due to the large number of parameters involved, and its impact on the study results [1]. Geological model is a visualize representation of geological structure and geophysical properties distribution for a specific geological area, e.g. reservoir. In formation evolution studies, the geological model is used to assess the original oil in place and detect water oil contacts areas of the reservoir under study [2].
Geological model construction includes two main stages, structural modeling and Petrophysical modeling; Structural modeling is the interpretation of seismic and geophysical data to introduce the fault pattern and build a gridded three-dimensional framework to identify the model inner geometry. Structural modeling is subdivided into three processes; fault modeling, gridding and vertical layering. Petrophysical modeling is the process of distributing petrophysical log properties overall the reservoir, using geostatistical approaches [3].
In reservoir engineering, volume of hydrocarbon in a reservoir is called volume in place (oil and/or gas). Volume of hydrocarbon that can be commercially recovered is called "Reserves". Reserves must satisfy four criteria; discovered, recoverable, commercial and remaining based on the development method. For a new reservoir (no oil production history), volumetric methods are the most used method to estimate the oil in place of the reservoir. Estimating the oil in place is the most important factor for reservoir engineers to make a right decision if the discovered area is profitable or not [4].

The Study Area:
East Baghdad is a super-giant oil field that is located in Baghdad and Saladin governorates, 10 km east of Baghdad city. The contract area for east Baghdad field covers No.28-(9) 2020 Journal of Petroleum Research & Studies (JPRS) E23 the portion north-west of the Diyala River and is 65 kilometres long and 11 kilometres width. East Baghdad holds 8 billion barrels proven reserves. East Baghdad oil field is subdivided geographically into six areas from northwest to southeast; respectively North Extension, Al-Taji, Al-Rashdiya, Urban, South 2 and South 1 areas. East Baghdad southern area includes both south 2 and south 1 [5].
Khasib formation is the main reservoir of east Baghdad oil field, it is a carbonate  The objective of this study is to construct a 3D geological model of Khasib formation in east Baghdad southern area oil field, to identify the pay reservoir units and preferred perforation intervals of Khasib formation depending on the distribution of the petrophysical properties and estimate the quantity of oil in the reservoir, by volumetric method.

Methodology:
The geological model of Khasib reservoir had been constructed by Petrel E&P software platform 2016. data from different resources were employed in this process including: FWR, FGR well tops, CPI, structural map, seismic interpreted data, etc.

Petrophysical properties distribution:
After Upscaling the CPI data, Petrophysical properties have been distributed by petrophysical modeling, by which each cell in the model structure is assigned by porosity, permeability and water saturation value. Sequential Gaussian Simulation method was applied to predict Porosity, water saturation and permeability for every non-well cell in the reservoir. Sequential Gaussian simulation is a geostatistical method used to estimate formation characteristics between two points-. SGS is commonly used with continuous data due to its simplicity and flexibility and the ability of dealing with large amount of data [8].
Properties of each zone of the formation are shown in Table ( is saturated with water with a percentage exceeded 90%, while the west-south parts contain less water with a range of 55% to 90%. The water saturation in west-north sides ranged from 30% to 55%, which make it the best portions in the reservoir units as compared with water cut.

Perforation intervals:
The perforation intervals were determined for four of the wells under study. These Where, the net volume is obtained by multiplying the bulk volume (V ) by net to gross ratio: The (OOIP) is calculated by multiplying the pore volume by oil saturation divided by the oil formation volume factor (B ) for each grid cell [9].
The total stock tank original oil in place of the reservoir is the sum of each grid cell (OOIP) in the reservoir. It was estimated to be equal to 2179 mmSTB (346.43 million cubic meter) in Khasib formation of East Baghdad southern area oil field.

Conclusions:
The 3D geological model of Khasib formation in East Baghdad southern area oil field was constructed by presenting the structural model and distributing the petrophysical properties out of nine zones of the formation Khasib, the reservoir pay zones were K2 and K3 as the petrophysical properties; porosity, permeability and most importantly water saturation, were in the cutoff limits. Most of the oil quantity of the formation is in these zones. The Initial oil in place (OIIP) for Khasib formation of east Baghdad field southern area was estimated to be equal to 2179 mmSTB (346.43 million cubic meters).
The two regions of the field; south1 (S1) and south2 (S2), when compared to each other do not show much differences in many of geological and petrophysical properties.
The porosity and permeability ranged in the same domain for S1 and S2. yet the main difference, after faults types and distribution, where the water saturation, as S1 area is more saturated with water than S2, which obviously means that S2 has more quantity of oil than S1.