Petrophysical Evaluation and Reservoir Characterization of the Zubair Formation in the Luhais and Rachi oil fields, 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 the Luhaisand Rachi oil fields, 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 consists of thick layers of sandstone rocks and the lower ones is consists 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 theZubair Formation at the Luhais oil field is characterized by two horizons. The first is showing high resistivity-high gamma ray which represents the upper part; while the lower ones is shown low resistivity-low gamma ray. There are two good reservoir horizones with high oil saturation (low water saturation) in this unit at the Rachi oil field and Lu-3 borehole, while at the Lu-12 is appeared as one uncontaneoushorizon. The middle is characterized by two subzones in the southwestern part of Luhais oil field (Lu-12). The upper one characterized by high- moderate effective porosity while the lower one characterized by moderate effective porosity because of presence a low volume of shale. This unit is dominated by low resistivity-low gamma ray with three susbended bands of high resistivity-low gamma ray, these appeared in both studied oil fields. The high percentage of water saturation in this unit caused the lack of clarity of the oil saturation, which appears in narrow bands The lower unit is represented the shale – dominated member of Zubair Formation, with low to non-pores rocks. This appeared in all studied wells. There are limit presences for the high- moderate effective porosity of rock within the sand bands lithofacies appears in these wells. This unit is characterized by alternative the high resistivity-high gamma ray horizon with the low resistivity-low gamma ray horizon as four cycles. There are two good reservoir horizons with high oil saturation (low water saturation) in this unit with a distribution similar to that found in the upper part. From the reservoir geology which represent by property modelling, preparing data, construction of three-dimension clump and scale up for the studied oil fields. The Rachi oil field is appeared the better hydrocarbon saturation compared to Luhais oil field, although the Luhais oil field (Lu-3 and Lu-12) is the best of pertophysicalproperties. This indicates that the Rachi oil field was affected by the structural position as indicated by the model, while the model did not show any structural properties in the field of Lahis. Therefore, the structural and/or stratigraphic positions have the greatest influence on the hydrocarbon accumulation, regardless of petrophysical properties.

wells. This unit is characterized by alternative the high resistivity-high gamma ray horizon with the low resistivity-low gamma ray horizon as four cycles. There are two good reservoir horizons with high oil saturation (low water saturation) in this unit with a distribution similar to that found in the upper part. From the reservoir geology which represent by property modelling, preparing data, construction of threedimension clump and scale up for the studied oil fields. The Rachi oil field is appeared the better hydrocarbon saturation compared to Luhais oil field, although the Luhais oil field (Lu-3 and Lu-12) is the best of pertophysicalproperties. This indicates that the Rachi oil field was affected by the structural position as indicated by the model, while the model did not show any structural properties in the field of Lahis. Therefore, the structural and/or stratigraphic positions have the greatest influence on the hydrocarbon accumulation, regardless of petrophysical properties.

I. INTRODUCTION
I.
The Zubair Formation was introduced by Glynn Jones in 1948 from the Zubair oil field and amended by Nasr and Hudson in 1953 (Bellenet al., 1959).It is the most significant sandstone reservoir in Iraq, is composed of fluvio-deltaic, deltaic and marine sandstones. II.
The study area is located in the Southern part of Iraq at the Luhaisand Rachi oil fields, within the Mesopotamian basin at the stable shelf. The studied oil fields are located in the southern desert, about 90 km south-west of the city of Basra, which lies about of 50 km southwest of the Northern Rumaila oil field (Fig.1). III.
Rachioil field in southern Iraq, is located within the administrative border of the province of Basra, just 80 km south-west of the city of Basra southern Iraq. The progradational Zubair/Ratawiclastic shelf was covered by the Shu'aiba Formation carbonates following backstopping of the Zubair and Ratawi Formations (Fig. 2). Jassim and Buday (2006) suggested that the Zubair Formation depocentre 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 is mostly gradational and conformable. The lower boundary is, however unconformable with RatawiFormation (Buday, 1980), and this unconformity 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 petrophysical software IP (V3.5) and petrel (V.14) for the environmental correction, lithology and mineralogy identification and logs interpretati on.  Study of the well logs and relate the log response to facies and diagenetic changes.  Building petrophysical models, Faciesand structured maps wereconstructed andthe petrophysical properties weredistri buted throughout well correlation in Mauudud and NahrUmr formations.

Fig. 8: isopach map shows the upper unit thickness in Luhais and Rachi  Middle unit
This zone is This unit is characterized by low gamma ray values with two cycles of fine up-ward (Bell shape) in the lower zone of this part, and coarse upward (Funnel shape) in the upper zone divided the serrated shape into two cycle (8-22%) ( Fig.9), it appearsclearly in all studied boreholes. Thickness of this zone is approximately (256m) in Luhais oil field and (305m) in Rachi oil field (Fig.9).  IV. PETROPHYSICAL MODEL The petrophysical model is the determination of the values of the petrophysical properties such as porosity and water saturation on each cell of the triple clasp according to the many statistical methods found in the Petrel software to distribute the petrophysical qualities after the work of its scale up. 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 log porosity logs correlation among the studied boreholes showing an approximate matching with these zones. After the application of the wire log porosity procedures and drawing the porosity-depth relationship for each borehole rather than logs reading with depth. These relationships include neutron/density log (total porosity)depth, sonic porosity-depth and effective porosity-depth and the porosity evolution and reservoir characterization (Fig.11).

Fig. 11: North South cross section of porosity distributions of Zubair Formation in Luhais Oil Field
These types of porosity logs are studied for:-1. Determination of the zones of void space that is interconnected and thus able to transmit fluids (effective porosity). 2. Combination of all of these features and their relationships with the water or hydrocarbon give the porosity evolution and reservoir characterization. These zones are divided in to three types of rocks according to total porosity:-1. High-moderate effective porosity rocks (type I).
2. Moderate effective porosity rocks (type II). 3. Low-non pores rocks (type III).  Zone A This zone is containing two types of rocks according to total porosity, high-moderate ineffective porosity rocks (type II) and low-non pores rocks (type III). The type (II) is appeared in the lower part of the upper unit at the Luhais oil field, while its appeared at the upper part of this unit in the Rachi oil field (Figs. 12, 15 and 16). 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 other part of this unit. This type is distinguished the shale dominated rocks and poor sorted sandstone.
 Zone B This zone is characterized by two subzones in the southwestern part of Luhais oil field (Figs. 13, 15 and

International Journal of Advanced Engineering Research and Science (IJAERS)
[ Vol-4, Issue-12, Dec-2017]  https://dx.doi.org/10.22161/ijaers.4.12.13  ISSN: 2349-6495(P) | 2456-1908(O) 16). The upper one characterized by high-moderate effective porosity (type I) while the lower one characterized by moderate effective porosity (type II) because of presence a low volume of shale. These features are not clear to the east near the Lu-3 and to the Rachi oil field, where the moderate effective porosity (type II) is the common type at this zone. This zone is representing the middle part of Zubair Formation within the sand-dominated member.
 Zone C The zone (c) is represented the shaledominated member of Zubair Formation, with low to non-pores rocks (type III). This appeared in all studied wells ( Fig.  13.14, 15 and 16). There are limit presences for the high moderate effective porosity type (I) of rock within the sand bands lithofacies appears in these wells.  According to the relationship of resistivity-gamma ray and porosity there are three types of petrophysics features within the Zubair Formation: I. Low resistivity-high gamma ray II.
High resisitivity-high gamma ray III.
High resistivity-low gamma ray IV.
Lwo risistivity-low gamma ray The upper unit of the Zubair Formation at the Luhais oil field is characterized by two horizons. The first is showing high resisitivity-high gamma ray which represent the upper part, while the lower ones is shown low resisitivity-low gamma ray. There are many subhorizons as bands within the upper horizone as high rissistivity-low gamma ray (Figs.17,  18, and 21). The Rachi oil field is charactrized by defferent subdivions where appeared three horizones. The first is shown high resistivity-high gamma ray, the second is shown high resistivity-low gamma ray and the last is charachtrized by alternative of high resistivity-high gamma ray and lwo resistivity-low gamma ray (Figs.19, 20 and 21).   /dx.doi.org/10.22161/ijaers.4.12.13  ISSN: 2349-6495(P) | 2456-1908(O)  Fig. 21: Cross section shows the water saturation of Zubair Formation in Luhais and Rachi Oil fields. There are two good reservoir horizones with high oil saturation (low water saturation) in this unit at the Rachi oil field and Lu-3 borehole, while at the Lu-12 is appeared asone uncontaneous horizone. The middle unit is dominated by low resistivity-low gamma ray with three susbended bands of high resistivity-low gamma ray, these appeared in both studied oil fields (Figs.17, 18, and 21). The high percentage of water saturation in this unit caused the lack of clarity of the oil saturation, which appears in a narrow bands The lower unit is charactrized by alternative the high resistivity-high gamma ray horizon with the low resistivitylow gamma ray horizon as four cycles (Figs.17, 18, and 21). There are two good reservoir horizones with high oil saturation (low water saturation) in this unitwith a distribution similar to that found in the upper part (Figs.19,  20, and 21).

VI. RESERVOIR GEOLOGY
Deals with the study of the different reservoir characteristics and the relationship between these characteristics and the lithofacies. In addition, it evaluates the content of hydrocarbons when proven by different measurements and determining reservoir architecture. The assessment of the composition of Zubair formation in the current study was based on the available geological data in wells study, which includes data surface of the wells, geological reports and final drilling reports in addition to the previous studies in the database of the South Oil Company for an integrated understanding of the work Previous. Four wells were selected, representing two wells per field, namely Lu-3, Lu-12 in the Luhais field and Rc-1, Rc-2 in Rachioil field. The values of the GR log were used to determine the range of shale and Sand by determining the minimum and maximum values, which are the basis for calculating the volume of Shale.   porosity: -4. High-moderate effective porosity rocks (type I). 5. Moderate effective porosity rocks (type II). 6. Low-non pores rocks (type III).  Zone A This zone is containing two types of rocks according to total porosity, high-moderate ineffective porosity rocks (type II) and low-non-pores rocks (type III). The type (II) is appeared in the lower part of the upper unit at the Luhais oil field, while its appeared at the upper part of this unit in the Rachi 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 other part of this unit. This type is distinguished the shale dominated rocks and poor sorted sandstone. The upper unit of the Zubair Formation at the Luhais oil field is characterized by two horizons. The first is showing high resisitivity-high gamma ray which represent the upper part, while the lower ones is shown low resisitivity-low gamma ray. There are many subhorizons as bands within the upper horizone as high rissistivity-low gamma ray. The Rachi oil field is charactrized by defferent subdivions where appeared three horizones. The first is shown high resistivityhigh gamma ray, the second is shown high resistivity-low gamma ray and the last is charachtrized by alternative of high resistivity-high gamma ray and lwo resistivity-low gamma ray. There are two good reservoir horizones with high oil saturation (low water saturation) in this unit at the Rachi oil field and Lu-3 borehole, while at the Lu-12 is appeared asone uncontaneous horizone.

 Zone B
This zone is characterized by two subzones in the southwestern part of Luhais oil field (Lu-12). The upper one characterized by high-moderate effective porosity (type I) while the lower one characterized by moderate effective porosity (type II) because of presence a low volume of shale. These features are not clear to the east near the Lu-3 and to the Rachi oil field, where the moderate effective porosity (type II) is the common type at this zone. 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 with three susbended bands of high resistivity-low gamma ray, these appeared in both studied oil fields. The high percentage of water saturation in this unit caused the lack of clarity of the oil saturation, which appears in a narrow bands  Zone C The zone (c) is represented the shaledominated member of Zubair Formation, with low to non-pores rocks (type III). This appeared in all studied wells. There are limit presences for the high moderate effective porosity type (I) of rock within the sand bands lithofacies appears in these wells. The lower unit is charactrized by alternative the high resistivity-high gamma ray horizon with the low resistivitylow gamma ray horizon as four cycles. There are two good reservoir horizones with high oil saturation (low water saturation) in this unitwith a distribution similar to that found in the upper part. From the reservoir geology which represent by property modeling, preparing data,construction of three-dimension clump and scale up for the studied oil fields. The Rachi oil field is appeared the better hydrocarbone saturationcompared to Luhais oil field, although the Luhais oil field (Lu-3 and Lu-12) is the best of pertophysical probrties. This indicates that the Rachi oil field was affected by the structural position as indicated by the model, while the model did not show any structural properties in the field of Lahis. Therefore, the structural and/or stratigraphic position have the greatest influence on the hydrocarbon accummulation, regardless of petrophysical properties.