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.