Petrophysical Characteristics and Reservoir Modeling of Mishirf Formation at Noor Oil Field, South of Iraq

Petrophysical properties of the Mishrif reservoir at Noor oil field have been done. Based on the interpretation of the open hole data from wells (No-1, 2, 3, 4, and 5) .Which have been calculated total porosity, effective and secondary porosity, water and hydrocarbon saturation (moveable and residual hydrocarbon) in invaded and uninvaded zones. Depends on the calculated of petrophysical properties, Mishrif Formation can be divided into eight reservoir units (RU-1 to 8), separated by eight caped rock units (barrier) (Bar-1 to Bar8). Threedimensional reservoir model of oil saturation was constructed using the Petrel Software, (2009). Distribution of these petrophysical properties for each reservoir unit within the studied field has been done. The results showed that the best reservoir units are the second, fourth and first reservoir unit. It’s worth mentioned here that the heterogeneity of the thicknesses of these units and its individual direction. In addition, observed that the oil saturation increases towards the north of the field at the well (No-5) and the center of the field at the well (No-4). Introduction Mishrif Formation represents one of the important Cretaceous formations. As distinguished by its lithological and geographic spread makes it a good storage of hydrocarbons. It is the second oil storage after the Zubair Formation in southern Iraq (AlNaqib, 1967). As well as the importance it represents a unique lithological architectural derivative of multi-environments within a shallow shelf. Stratigraphic succession of Mishrif Formation at study wells shows that this formation deposits between the ages of (Late CenomanianEarly Turonian). Rabanit (1952) was first describing of this formation at type section in a well (Zb-3) in southern Iraq. Where the deposition of this formation gradually within environmental from the outer shelf to shallow No.13 Journal of Petroleum Research & Studies (JPR&S) E211 open shelf environments at the lower part of the formation, whereas, organic reefal complex deposited at the middle part, the upper part deposits under restricted lagoon environment condition (Razoian, 1995). The study area represented by five oil wells within the field (No-1, 2, 3, 4, and 5) note that the well (No-4) is not fully penetrated of drilling to study formation. The field is located in the province of Amara. Structural map on top of Mishrif Formation indicated that the dimensions of the field are about 20 km long and 7.5 km wide. The axis of structure extend to northwest – southeast direction, and it turned out that dip slope of the north-east flank approximately (2°), either southwest flank has reached the degree of inclination of about (1.5 °) (Figure 1). Mishrif Formation lays gradually contact with Rumaila Formation from the bottom and the unconformable contact with Khasib Formation at the top. The main goal of the study to determine petrophysical properties of Mishrif reservoir units in Noor oil field and detailed review of the most important petrophysical properties to clarify their changes vertically and laterally within the wells by using programs (Techlog and Petrel). Fig.(1) Structural contour map on top of Mishrif Formation at Noor Oil Field. No.13 Journal of Petroleum Research & Studies (JPR&S) E212 Research methods 1 Petrophysical characteristics have been calculated through the use of open hole logs, such as (Gamma Ray (GR), Neutron (NPHI), Density (RHOB), Sonic, Shallow and Deep Resistivity Logs (Rxo and Rt). 2 Mishrif rocks was divided into reservoir units and other unreservoir units dependent on the results of petrophysical properties account for (Computer Processes Interpretation (CPI)) using the program (Techlog). 3 Reservoir characteristics for reservoir units within the wells of the field were done by use the Petrel Software (2009). Petrophysical parameters 1Calculated of shale volume Gamma ray log is the best tool using to identify and calculate the volume of shale. Because of its so sensitive to the response of radioactive material. This tool concentrates on the appearance of shale and argillaceous carbonate rocks. Calculated of shale volume in the following equation: ................. (1) Where: IGR: Coefficient of gamma rays index. GRLog: read the gamma rays of the formation. GRmin: minimum gamma rays read opposite clean layers. GRmax: maximum gamma rays read opposite shale layers. It is then calculating the volume of shale (Vsh) using the following equation: No.13 Journal of Petroleum Research & Studies (JPR&S) E213 .................. (2) Depends on the percentage of the shale volume extracted from previous equation (2) for studied wells were identified free shale zones (clean zone), which is by the volume of shale less than (Vsh <% 10) and zones containing volume shale large from (Vsh ≥% 10) as shally zone (no clean zone). 2Calculation of porosity: Porosity can be divided into two classes depends on the time of their formation. The first class represents initial porosity (primary porosity) and the second represents a porous secondary (secondary porosity). There are several methods of calculation of porosity; it is possible to calculate the primary porosity of the sonic log, as in equation Wyllie et al. (1958) which are used in the depths of the shale-free (clean Zone): ........................ (3) Where: ØS: porosity calculated from sonic log. ∆t log: Full-wave interval of the formation and registration of the log is measured directly (μsec/ ft). ∆t ma: wave interval transit thought matrix (47.5 μsec/ ft to limestone rocks). ∆tf: wave interval transit thought the pore fluid (185 μsec/ ft to saline water). But in the depths that exceed Shale volume by about (10%), a zones bearing shale (Shally zone) are used equation (Dresser Atlas, 1979) to remove the effect of shale and correct, as in the following equation: ....................... (4) Where: ∆t sh : Full-wave interval of the adjacent shale. No.13 Journal of Petroleum Research & Studies (JPR&S) E214 To correct the effect of hydrocarbons are used equation Hilchie, (1978):


Introduction
Mishrif Formation represents one of the important Cretaceous formations. As distinguished by its lithological and geographic spread makes it a good storage of hydrocarbons. It is the second oil storage after the Zubair Formation in southern Iraq (Al-Naqib, 1967). As well as the importance it represents a unique lithological architectural derivative of multi-environments within a shallow shelf.
Stratigraphic succession of Mishrif Formation at study wells shows that this formation deposits between the ages of (Late Cenomanian-Early Turonian). Rabanit (1952) was first describing of this formation at type section in a well (Zb-3) in southern Iraq. Where the deposition of this formation gradually within environmental from the outer shelf to shallow E211 open shelf environments at the lower part of the formation, whereas, organic reefal complex deposited at the middle part, the upper part deposits under restricted lagoon environment condition (Razoian, 1995).

1-Calculated of shale volume
Gamma ray log is the best tool using to identify and calculate the volume of shale.
Because of its so sensitive to the response of radioactive material. This tool concentrates on the appearance of shale and argillaceous carbonate rocks. Calculated of shale volume in the following equation: Where: IGR: Coefficient of gamma rays index.
GRLog: read the gamma rays of the formation.
GRmin: minimum gamma rays read opposite clean layers.
GRmax: maximum gamma rays read opposite shale layers.
It is then calculating the volume of shale (Vsh) using the following equation:

……………… (2)
Depends on the percentage of the shale volume extracted from previous equation (2) for studied wells were identified free shale zones (clean zone), which is by the volume of shale less than (Vsh <% 10) and zones containing volume shale large from (Vsh ≥% 10) as shally zone (no clean zone).

2-Calculation of porosity:
Porosity can be divided into two classes depends on the time of their formation. The first class represents initial porosity (primary porosity) and the second represents a porous secondary (secondary porosity). There are several methods of calculation of porosity; it is possible to calculate the primary porosity of the sonic log, as in equation Wyllie et al. (1958) which are used in the depths of the shale-free (clean Zone):

E214
To correct the effect of hydrocarbons are used equation Hilchie, (1978): Where: Ø: Porosity calculated from the sonic log of the corrected effect of hydrocarbons.
Bhc: coefficient of hydrocarbon effect the impact of (0.7) for gas and oil (0.9).
It is possible to calculate the porosity of the Density log through the use of density as in equation Wyllie et al. (1958):
ρb: the density of the total composition.
ρf: density of the fluid (1.1 g / cm 3 for the saline water).
As for the interval containing shale can be used Dresser Atlas, (1979) equation to remove the effect of shale: Where ρsh represent total density of the adjacent shale.
While the Neutron log measures the porosity directly to the depths of the shale-free zones, but for the depths of the record containing shale can be used Tiab and Donaldson (1996) equation:
ØNc: porosity derived from neutron log corrected the impact of shale.
ØNsh: neutron porosity of the adjacent shale.

3-Calculate the total porosity (effective) and secondary porosity
Total porosity or the so-called influential porosity (effective porosity) is calculated through the use of Schlumberger, (1997) equation:

4-Calculate the formation temperature
The formation temperature (Tf) is an important factor in the analysis of borehole logs, because the drilling mud resistivity (Rm) and mud filtrate (Rmf) and formation water (Rw) changes with temperature, and we can determine the formation temperature from the equations (12 and13):

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Where: d: calculate the depth temperature (m).

G.G.: Geothermal Gradient (∆T/m).
BHT: the temperature of the bottom of the well (C °).

5-Calculation of Formation Factor ( F)
The formation factor is an important factor in the well log calculations , since this factor is associated with an inverse relationship between water resistivity (Rw), and a positive correlation with the resistivity formation saturated (100%) with water (Ro) as shown by Archie, (1944) equation:

…………….. (15)
The cementation factor (m) depends on the shape and distributed of the pores (pores geometry). Noted that the longer the road and held in front of the electric currents through the rock increased the value of (m), on the other hand, increase (m) increases the value of (F) too.
While (a) represents a tortuosity factor is based on the along the path that takes liquid or current to pass through the rock and it is usually given value (1) (Khyuikh, 1990).

6-Calculation of formation water resistivity
The water resistivity of Mishrif Formation (Rw) have been identified based on values that are installed at the heads of logs ( log header) , as well as the case for mud filtrate resistivity (Rmf), were then corrected (Rmf) for each depth by use the equation (16):

7-Water and Hydrocarbon saturation
Water saturation (Sw) is the ratio between the size of voids filled with water to a total It also we can be calculated hydrocarbon saturation by the following equation:

8-Calculate the total volume and movement of hydrocarbons
Can calculate the total volume of water in invaded zone (BVxo) and uninvaded zone (BVw) through the following equations: Whereas the value of the total calculated of water volume in the uninvaded zone at different depths constant, this indicates that these zones homogeneous and saturated by irreducible

B -Distribution tomography (UP Scale)
Using various mathematic methods in the distribution of reservoir properties .The purpose was to obtain a single value for each property suitable petrophysical properties in a single cell (one cell) per reservoir unit note that the dimensions of the cell is (500 * 500) meter.

Results and Discussions
Interpretation Noted that the increase of porosity around north of the field. This unit has highly oil saturation range (0.65-0.8), it's increased toward the wells located in the central and north of the field as in the wells (No-2, 4, and 5).

5-Third Barrier Unit (Bar 3):
This unit thickness ranges between (3) meters at the wells (No-4, and 5) caught the central and northern field. Thickness reached (21 meters) at the well (No-1) which locates at east of the field, and its average (9 meters) to all anther wells of the field. This unit is characterized by poor porosity does not exceed (0.1) in most parts of the field. Also is characterized by an oil saturation of between (0.1-0.3), as saturation increases toward the wells located in the north of the field as well (No-5).

6-Third Reservoir Unit (RU-3):
This unit thickness ranges between (15) meters at the well (No-1) which locates at east of the field. Thickness reached (61) meters at the well (No-2) which locates at the west of the field, whereas, reached thickness about (33 meters) for all wells of the field. This unit is characterized by porous medium (0.15-0.2), which increases to the north of the field at the well (No-5), while its oil saturation ranges between (0.4-0.6), with increasing saturation at the northern and southern sides of the field as in the wells (No-3, and 5).

7-Fourth Barrier Unit (Bar-4):
This unit thickness ranges between (3) meters at the well (No-2) which locates at west of the field and its thickness (13) meters at the well (No-1) which locates at east of the field. Average thickness of this unit reached about (7 meters) in all wells of the field. This unit is characterized by poor to moderate porosity ranging between (0.0 -0.15), with increasing porosity in the middle part of the field when the well (No-4). Also is characterized by an oil saturation of between (0.1-0.3), as varies saturation of one location to another within the wells of the field.

8-Fourth Reservoir Unit (RU-4):
This unit thickness ranges between (57) meters at the well (No-2) in the west of the field, and thickness (81) meters at the well  in the north of the field. Average thickness reached about (71) meters for all wells of the field. This unit is characterized by good to very good porosity ranging from (0.2-0.3), is also characterized by an oil saturation of between (0.4-0.7), with increasing porosity and saturation oil toward the wells located in the north of the field as well (No-5). As well as, noted that increasing oil saturation toward wells in the east and center of the field as in the wells (No-1, and 4), respectively.

13-Seventh Barrier Unit (Bar-7):
Ranges thickness of this unit between (9) meters at the well (No-2) which locates at west of the field. Note, that the increase of the thickness at the well (No-1) which locates at east of the field, its thickness reached about (31) meters.
Whereas, average thickness reached (18) meters for all wells field. Also, this unit characterized by porosity zero to a few so do not exceed (0.1) in most of the wells of the field.
Is also characterized by an oil saturation of between (0 -0.2), as oil saturation increases toward the wells located in the north of the field as well (No-5).

No.13 Journal of Petroleum Research & Studies (JPR&S)
E234 . Clear from the foregoing that the best wells in the oil saturations within the Noor field is the well (No-5) which locates at north of the field and the well (No-4) in the center of the field. While the best units in terms of oil saturation of reservoir unit is the second, fourth and first, respectively, taking into account the thickness variation of these units between wells of the field.