Reservoir Evaluation Model of the Khasib Formation in Halfaya Oil Field, Mesan Governorate, Southern Iraq.

Evaluating the reservoir characteristics of a formation or geological unit constitutes an essential element and an effective tool in leading the successful exploration project currently prevailing. The Khasib Formation, which belongs to the secondary depositional cycle (Turonian - Lower Campanian) in the study area within Mesan governorate southern Iraq, was chosen as a study site, as the Khasib is one of the main reservoirs within the Mesan oil fields. Seven wells were selected distributed over the field area, which structurally, is represented by two domes oriented from NW to SE, in addition to a thin dome with a length of 38000 m. and a width of 12000 m. The corrected well logs were used as input by creating an Excel sheet and then processing that data through the Petrel Software to prepare maps, sections and Three dimensional diagrams. The Cross-plot diagrams showed that the limestone (calcite) is a main lithology of Khasib with some dolomite. 3-dimensional petrophysical models were prepared, through which identified three reservoir units: (Khasib-A, Khasib-B and Khasib-C). The porosity model shows that there is a good percentage of porosity within the Khasib-A unit, with a low percentage in unit-B. As for unit-C, it was characterized by the best porosity percentages. The water saturation model showed a high percentage at Khasib-A unit, reaching 80%, and it increased at unit-B, reaching 90%, while unit-C was characterized by the lowest percentage of water saturation. The volume of shale model indicates low to moderate rates in the Khasib-A unit, and a slight increase in unit-B, while unit-C showed a low rate of shale volume within the three units with very low values in most of the study wells, which confirms the superior quality of this reservoir unit, which can be considered the main productive reservoir unit of the Khasib Formation in the Halfaya oil field.


Introduction
The building of the reservoir model represents an important stage in the study and evaluation of reservoir units, as it constitutes, along with the evaluation of the source rock using quantitative mathematical models, the most recent of which was presented by Al-kubaisi [1], an essential element in building the petroleum system for the field or oil region.The storage capacity and economic feasibility of the reservoir depend on the evaluation of its petrophysical properties.Halfaya oil field is one of the giant fields in southern Iraq, with an average of 16 bbl. of an-extracted oil reserves, of which 4 bbl.are recoverable, in addition to huge amount of associated gas.In addition to producing economic quantities in central Iraq, the results indicated that oil reserves in east Baghdad oil field are estimated equal to 2179 mm STB [2].There is a clear homogeneity within the vertical section of the formation within the productive subzones [3].The direction of the field is NW-SE, and it is located in the unstable shelf in the northern part of Arabian gulf basin, which is covered with a big thicknesses of strata [4].Khasib Formation constitutes the second pay in the field after Mishrif Formation, which is a carbonate sequence were deposited during the Cretaceous period in the secondary cycle (U.Turonian -L.Campanian) [5].The formation divided into two main units, the lower consists from shales, alternating with fine grained marly limestone, while the upper part consists only of grey, fine grained marly limestone [6].The Formation was deposited within the ramp setting, as several studies confirmed this, as in the study of the formation in the Amara oil field [7].The Formation is divided based on petrophysical properties into a several reservoir units that differ in number and characteristics from area to another.The latest study showed that there are five reservoir units of the Khasib Formation in the Amara oil field, the upper which is characterized by the highest petrophysical properties and gradually decreases towards the bottom until these characteristics are absent at the fifth lower unit [8].

Study area
The Halfaya oil field located in southern Iraq 30 km. south-east of Amara city (Figure 1).It's a gentle elongated anticline extending in the NW-SE direction.The length of the structure about 37.5 km. and its width is 12km.The seismic studies indicate that Halfaya structure consists of two domes extending in the NW-SE direction [5], it changes south at West Qurna field is the straight, horizontal, flat, moves way from the cylindrical, asymmetrical elongated dome [9].

Methodology
The present study aims to the reservoir evaluation of Khasib Formation and determine the trends to improve reservoir properties through the following stages: 1-Study of the geological final well reports (G.F.W.R.).2-Study and analysis the well logs (Master logs).These above tools provide an initial assessment through the first observation when drilling.

3-Processing and analysis of full set logs (open hole well logs):
This group includes (Resistivity, Density, Neutron, sonic, Spontaneous potential and Gama ray logs) 4-Well logs interpretation and petrophysical analysis of Khasib Formation made using Excel applications.5-Construction a static geological model (Petral 2013) software for building 3D petrophysical models.Through the model, the path and distribution of the reservoir characteristic was determined within the field through by correlated the petrophysical and facies properties of the study wells.

Tectonic and geologic Setting
Tectonically, Iraq is posited in the stable basin during the late M. Cretaceous -early U. cretaceous synchronous with the period of deposition of the Khasib Formation [5].The Halfaya oil field is posited on the unstable shelf in the northern part of the Arabian Gulf Basin, the field represented by a gentle elongated anticline extending in a NW-SE direction.This part of the basin is covered with large thicknesses of sediments [4].The structural of the basin reflects high thickness towards the east as a result of the water depth, but this structural is changes mainly in the N-S direction, as this change appears at unit B of the Formation.(Figure 2).[10].Khasib Formation is the base of Upper Turonian -lower Campanian cycle with Tanuma and Sa'di Formations above it.This succession consists of a siliciclasticcarbonate rocks refers to the middle shelf to sub-basinal depositional environment [11]; [12].Khasib reservoir is mainly composed of Limestone rocks, but the petrographic details refer the dark grey shales alternating with marly limestone at the lower part, while the upper part composed of marly limestone.
The configuration is bounded below by unconformable surface with the Mishrif Formation, but its upper boundary is gradual with the Sa'di Formation [6].The thickness of the formation reaches its maximum at the Tigris Subzone, southern Iraq, where it reaches about (225m.) at the Zubair oil field [13].The formation in the study area divided into two lithologic units, the thickness of the lower unit is 20m.and the upper is 30m.The thickness of the formation reaches a maximum value of about 60m., it increases rapidly in some directions and sometimes wedges out completely [14].

Reservoir Evaluation
The reservoir evaluation is a quantitative application to determine the hydrocarbon content and calculate the reservoir parameters, which includes three aspects: the position of hydrocarbon bearing units, estimating storage capacity, and evaluating the amount of hydrocarbons.The comprehensive evaluation of the reservoir includes calculating many of parameters based on well log data according to the steps of the flow chart (Figure 3), as follows: • Shale Volume (Vsh.):Calculating the Gama Ray index (IGR) is the main stage to deducing the value of the shale volume using a gamma ray log as in the following equations: ℎ = 0.083[2 2 *   − 1.0] (For unconsolidated rocks) Where:   : gamma ray index.  : gamma ray from log.   : gamma ray minimum (carbonate or clean sand),   : gamma ray maximum (shale),  ℎ : shale volume.
• Porosity: In reservoirs, the pore volume is the space available for storing hydrocarbons and water which is expressed as a percentage [15].Schlumberger, 1974 [16], suggested an equation to calculate the total porosity from neutron and density logs, as in the following equation: • Water and Hydrocarbon Saturation: According to (Schlumberger, 1998) [17] Archie submitted an equation by which the water saturation of shale-free units can be expressed in terms of their true resistivity value as follows: Where: Sw =Water Saturation, Rw =Resistivity of formation water.Rt =Formation resistivity.
The water saturation of flushed zone can also be expressed by the following equation: Where: Sxo = Invaded zone water saturation, Rmf = Mud filtrate resistivity.Rxo= Invaded zone resistivity.
• Bulk Volume of Water (BVW): It is the product of the water saturation multiplied by its porosity: Where: Sxo=Water Saturation, Φ = porosity.
• Residual Hydrocarbon: According to (Rider, 2000) [18] the residual hydrocarbon can be calculated from the equation: • Movable Hydrocarbon: According to (Seerra, 1986) [19] the movable hydrocarbon can be calculated from the equation:

Evaluation of petrophysical Model
The Petrophysical model aims to create a set of petrophysical Parameters through which the reservoir characteristics are distributed, which leads to an accurate evaluation of hydrocarbon reserve and an assessment of the economics of the oil field.To Simulate the distribution of petrophysical properties in the geological model processed by (Petrel software) Figure 3-8, deterministic and random out puts are provided [20].The geological model showed that the Khasib formation in Halfaya oil field includes three reservoir units: Khasib-A, Khasib-B and Khasib-C.The following is a description of the petrophysical properties of each unit: The thickness of this unit range between 15-40 m. and an average of 27m.The porosity is moderate to good and its percentage ranges between 10-20% and it's reach 35% in well H-4.The water saturation rate reached about 80% at well H-5.While it decreased significantly towards wells H-3 and H-4, reaching 10% at well H-4.As for the volume of shale, low to medium values were recorded in most of the study wells, ranging between 10-20 % with the exception of well H-4, where the volume of shale reached approximately 40%.
 Khasib-B unit: This unit considered the lowest in reservoir properties, as its thickness ranges between 38-60m., and an average of 50m.This unit is characterized by a low porosity rate (5-10%) in most of the studied wells, while it exceeds 25% in wells H-4 and H-2.On the contrary, water saturation recorded the highest rates at this unit, sometimes reaching 100% which the exception of well H-4, in which the water saturation rate was 40%.The volume of shale increases at this unit, especially at the H-1, H-2 and H-5 wells, ranging between 25-40% while these percentages decrease at the H-3 and H-4 wells, ranging between 15-20%.

 Khasib-C unit:
This unit is the main reservoir due to its high petrophysical properties that give it a distinctive reservoir potentiality.The unit thickness is set between 20-30m.The good percentage of porosity within the rocks of this unit, which reached 15-35%, was the most important strength in acquiring the reservoir character, in addition to the other characteristics, which were represented by a low percentages of water saturation ranging-between 15-30%.This unit is characterized by a low percentage of shale volume that does not exceed 10% in most wells except for well H-4, where it reaches approximately 20%.

Conclusions:
Petrophysical models for evaluating reservoir properties, based on corrected well logs data, showed that the Khasib formation in Halfaya oil field includes three reservoir units: (Khasib-A, Khasib-B, and Khasib-C).The unit-C recognized by the best petrophysical properties compared to the other two units, as it showed the best porosity rates, and the water saturation (Sw) for this unit was very low.The volume of shale model also showed that it was very low at unit-C less than what appeared at unit-A and unit-B, which recorded low to moderate value.According to the results of the reservoir properties model, unit-C had high reservoir specifications compared to the other two units, which earned it the status of the main productive reservoir unit of the Khasib Formation in the Halfaya oil field (Table 1).

Figure 1 .
Figure 1.Tectonic map of Iraq indicated by location map of the study area.Modified from [2].

Figure 8 .
Figure 8. Cross section showing water saturation distribution.

Figure 9 .
Figure 9. Cross section showing Volume of Shale distribution.