Petro physical Evaluation and Reservoir Characterization of the Zubair Formation in Majnoon oil field, Southern Iraq.

The Barremian succession in the present study is represented by the Zubair Formation which the most significant sandstone reservoir in Iraq. The area of study is located in the Southern part of Iraq at Majnoon oil field, within the Mesopotamian basin. The thickness of the Zubair Formation is about 450 m in the studied area. It is divided into three lithofacies: The upper unit is composed mostly of shale layers, the middle unit is consisting of thick layers of sandstone rocks and the lower ones is consisting mainly of Shale with less sandstone layers. These units are characterized by three types of petrophysical features according to total porosity/effective porosity: High-moderate effective porosity rocks (type I), moderate effective porosity rocks (type II) and low-non pores rocks (type III). The upper unit of the Zubair Formation at Majnoon oil field is characterized by two horizons. The first is showing high resisitivity-high gamma ray which represent the upper part, while the lower part show low resisitivity-low gamma ray. There is a good reservoir horizon with high oil saturation (low water saturation) in this unit at the Majnoon oil field is appeared as a non-continuous horizon. The middle member is dominated by low resistivity-low gamma ray. The high percentage of water saturation in this unit caused the lack of clarity of the oil saturation, which appears in a narrow band. The lower member of Zubair Formation is distinguished by shale dominated rocks and poor sorted sandstone. This shows high resisitivity-high gamma ray. There are many sub horizons as bands within the lower horizon as high resistivity-low gamma ray. There is a good reservoir horizon with high oil saturation (low water saturation) in this unit.


INTRODUCTION
The Zubair Formation was introduced by Glynn Jones in 1948 from the Zubair oil field and amended by Nasr and Hudson in 1953 (Bellen et al., 1959). It is the most significant sandstone reservoir in Iraq, is composed of fluvio-deltaic, deltaic and marine sandstones. The study area is located in the Southern part of Iraq at the Mjnoon oil field, within the Mesopotamian basin at the stable shelf. The studied oil fields are located in Southern Iraq approximately 60 Km. Northwestern of Basra city, close to the Iranian border and extending North to Missan province " Fig. 1". Abeed, et. Al, 2013).

International Journal of Advanced Engineering Research and Science (IJAERS)
[ The Barremain succession represent a part of The Late Tithonian-Early Turonian Megasequence was deposited in a large intra-shelf basin contemporaneous with a new phase of ocean floor spreading in the Southern Neo-Tethys. Differential subsidence (and resultant thickness changes) occurred across transverse faults. The axis of the intra-shelf basin shifted towards the eastern Mesopotamian Zone into the Tigris Subzone from its previous position on the Salman Zone and western Mesopotamian Zone (Jassim and Goff, 2006). The progradational Zubair/Ratawi clastic shelf was FRYHUHG E\ WKH 6KX ¶DLED )RUPDWLRQ FDUERQDWHV IROORZLQJ backstopping of the Zubair and Ratawi Formations " Fig.  2". (Aqrawi, et al. 2010). Jassim and Guff (2006) suggested that the Zubair Formation depocenter was located at the eastern limit of the Salman Zone, as illustrated by the isopach of the Zubair Formation (Ali and Nasser, 1989 in Aqrawi et al. 2010) " Fig. 3". The upper contact of the formation with the Shuaiba Formation are mostly gradational and conformable. The lower boundary is, however unconformable with Ratawi Formation (Buday, 1980) and this unconformity is described by Douban and Al-Medhadi (1999).

II.
METHODOLOGY -Study of available well logs and relate the log response to facies and diagenetic changes for the studied succession intervals " Table 1". -Digitizing well logs using Didger software. -Using interactive petro physical software IP (V3.5) and petrel (V.14) for the environmental correction,lithology and mineralogy identification and logs interpretation. -Study of the well logs and relate the log response to facies and diagenetic changes. -Building petro physical models, Facies and structured maps reconstructed and the petro physical properties were distributed throughout well correlation in Zubair formation.   The upper member is composed mostly of Shale layers. The middle member consists of thick layers of sand rocks. The lower member consists mainly of layers Shale with less sandy layers. "

Lower member:
This zone is characterized by GR log values with many cycles as decreasing upward (bell shape) and grain size increasing. This unit also has (a funnel shape) in some positions. The thickness of this member is about (130m) in Majnoon oil field. " Fig. 5" shows the facies distribution through this member.

Middle member:
This zone is characterized by low gamma ray values with many cycles of fine to coarse sand (cylindrical shape of GR log) in the upper zone of this unit, and coarse upward (hour-glass shape) in some positions. Thickness of this zone is approximately (60m) Majnoon oil field. " Fig.  6" shows the facies distribution through this member.

Upper member:
This unit is characterized by high shale volume and the general GR log is almost forming a serrated shape, which refers to relatively increasing upward in gamma ray values. Thickness of the upper unit is approximately (10m) in the Majnoon oil field. " Fig. 7" illustrates the facies distribution through this unit.

IV.
Porosity distribution: The porosity of a rock is a measure of the amount of internal space that is capable of holding fluids. it is important because it represents a potential storage volume for hydrocarbons (Asquith and Gibson, 1982).

primary porosity:
This type is formed by syndepositional processes which contains visual and non-visual pores between the grains, so the spaces between the fragments of solid material deposited as sediment are the "primary" porosity. Porosity value can be obtained from the Sonic (Acoustic) log (Asquith and Gibson, 1982).

Secondary porosity:
This is formed after depositional processes. This type contains the pores which are created by dissolution processes and it be symmetrical formed by selective dissolution, which is called (moldic voids), or asymmetrical called (vuggy voids) or (channels) or (caverns). It may also be intercrystalline voids formed by dolomitization processes or fractures voids formed by different physical stresses (Asquith and Gibson, 1982).

Effective porosity:
The amount of internal space of void in a given volume of rock is a masseur of the amount of fluids a rock withholds and if this pore is interconnected and able to transmit fluids is called (Asquith and Gibson, 1982).

International Journal of Advanced Engineering Research and Science (IJAERS)
[    The Zubair sequence can be divided by the gamma ray and shale value into three zones (upper, middle and lower); therefore, we have an interpretation of the logs porosity and porosity evaluation according to these divisions. The porosity log (sonic log) in the studied boreholes showing an approximate matching with these zones. After the Petral application of sonic log drawing the porosity-depth relationship for each borehole rather than logs reading with depth. The sonic (porosity) log is studied to determination of the zones of void space that is interconnected and thus able to transmit fluids (effective porosity). These zones are divided in to three types of rocks according to total porosity " Fig. 10": 1. High-moderate active porosity rocks (type I). 2. Moderate active porosity rocks (type II). 3. Low-non pores rocks (type III).

Lower member:
The lower zone is represented the shale ± dominated member of Zubair Formation, with low to nonpores rocks (type III). This appeared in all studied wells " Fig. 11 & 12". There are many presences of the moderate porosity type (II) of rock within the sand bands lithofacies appears in these wells. This zone is containing two types of rocks according to total porosity, highmoderate ineffective porosity rocks (type II) and low-nonpores rocks (type III). The type (II) is appeared in the lower part of the upper unit at the Majnoon oil field. These features are matching with the sand rich rocks and low to moderate shale. The type (III) is represented the non-porous rocks which appeared in the lower part of this member. This type is distinguished the shale dominated rocks and poor sorted sandstone. This shows high resisitivity-high gamma ray. There are many sub horizons as bands within the lower horizon as high resistivity-low gamma ray. There is a good reservoir horizon with high oil saturation (low water saturation) in this unit.

International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-5, Issue-5, May-2018] https
This zone is characterized by two subzones in the southwestern part of Majnoon oil field. The upper part is characterized by high-moderate effective porosity (type I) while the lower part is characterized by moderate effective porosity (type II) because of presence a low volume of shale.
This zone is representing the middle part of Zubair Formation within the sand-dominated member. The middle unit is dominated by low resistivity-low gamma ray. The high percentage of water saturation in this unit caused the lack of clarity of the oil saturation, which appears in a narrow band.

Upper member:
This zone is containing high-moderate active porosity rocks (type I) because of well sorted coarse grains. This type is appeared through the upper unit at Majnoon oil field " Fig. 15 & 16". These features are matching with the sand rich rocks and low to moderate shale. There are limit presences for the high moderate effective porosity type (I) of rock within the sand bands lithofacies appears in these wells. The upper unit is characterized by alternative the high resistivity-low gamma ray horizon. There is good reservoir horizon with high oil saturation (low water saturation) in this unit.

VI. MODEL PROPERTIES (PROPERTY MODELING)
It is the process of filling the three-dimensional cells of the geological model with the characteristic readings of the logs (Sclumberger, 2013). The facies model is constructed according to the facies variations (horizontally and vertically) that interpreted from the well logs.
6.1 Preparing data: The first step in the Petrel software is the import of the available information to the software. The information included the coordinates of the wells of Majnoon oil field for Zubair Formation, the top lithofacies units of these wells and logs as well as the electrofacies in Petrel to build the model for Zubair formation.

Construction of three-dimensional lithofacies model:
The construction of the clamp (Pillar Gridding) is the basic process to build the 3D model, it depends on the depth map on which the model is built. This map is taken from the study of interpretations as in the present study. If it is not available, it can be drawn from the top formations provided that there are a good number of wells as shown in " Fig. 17".
The other important process is the scale up which convert the reading of the logs to the shape of the three dimensional cell to fit with the reading of the logs to the gap of the clamp to know the horizontal and vertical thicknesses of the cell three-dimensional cells.
Finally, the three dimensional model can be constructed with the lithofacies variations through three zones in Zubair Formation as shown in " Fig. 18".  Zubair sequence can be divided by the gamma ray and shale value into three zones (upper, middle and lower); therefore, we have an interpretation of the logs porosity and porosity evaluation according to these divisions. The porosity log (sonic log) in the studied boreholes showing an approximate matching with these zones. The structural and/or stratigraphic position have the greatest influence on the degree of diagenetic and developed of petrophysical properties. The lower member of Zubair Formation is distinguished by shale dominated rocks and poor sorted sandstone. It has a high gamma ray with very low porosity. There are many sub horizons as bands within the lower horizon as high porosity-low gamma ray within the sand membesr. There is a good reservoir horizon with high effective porosity (low shale volume) in this unit. The middle member is dominated by high porosity-low gamma ray. The high volume of effective porosity in this unit caused the lack of clarity of the oil show, which appears in a narrow band. The upper unit is characterized by alternative the high porosity-low gamma ray horizon. There is good reservoir horizon with high oil show (low water saturation) in this unit.