Abstract
The Mishrif Formation is considered the main oil reservoir in the Buzurgan oilfield, southern Iraq. This study aims to characterize and evaluate the reservoir properties of the Mishrif Formation based on the interpretation of well logs data. The logs data for six wells have been analyzed and interpreted by using Techlog 2015.3 software. The lithology of the Mishrif Formation was determined by using the M-N cross plot method based on the interpretation of density, neutron, and sonic logs. The results showed that the Mishrif Formation is mainly composed of limestone. The shale volume in the Mishrif Formation has been estimated from the gamma-ray log. The results illustrated that the shale volume is about 20% of the bulk volume, and may increase to reach the highest value at the upper part of the MA unit of the formation. The porosity of the Mishrif Formation was calculated based on the interpretation of neutron, density, and sonic logs. To achieve accurate values of porosity, the log-derived porosity has been correlated with the core-derived porosity, and the comparison showed a good correlation between the two types of porosity. The results showed that the Mishrif Formation is characterized by low to medium porosity (about 5% to 18%). The secondary porosity of the formation is most dominant in the MB21 unit compared with the other stratigraphic units of the Mishrif Formations. This indicates that the MB21 unit was affected by the diagenesis processes. The MB21 unit of the Mishrif Formation represents the most dominant reservoir because it was delineated by high effective porosity and high oil saturation.
Highlights
The Mishrif Formation is considered one of the main oil carbonate reservoirs in the Mesopotamian Basin, and it contains about 30% of the proven oil reserves in Iraq (Al-Sakini, 1992)
The MB21 unit is considered the main oil-bearing reservoir in the Buzurgan oil field, and the main lithology of this unit is composed of bioclastic limestone and chalky limestone (Sang, et al, 2017)
The lithology of the Mishrif Formation was determined as limestone based on the interpretation of density, neutron, and sonic logs by using an M-N cross plot
Summary
The Mishrif Formation is considered one of the main oil carbonate reservoirs in the Mesopotamian Basin, and it contains about 30% of the proven oil reserves in Iraq (Al-Sakini, 1992). The Mishrif formation is described by an organic detrital limestone with algal, rudist, and coral-reef limestones (Bellen, et al, 1959). The formation was deposited in a carbonate platform ramp environment (Al-Ali, et al, 2019), and it is composed of bioclastic-detrital limestone with rudist, algal, and coral facies (Al-Ameri et al, 2009). The rudist facies of the Mishrif Formation is considered the best hydrocarbons reservoirs in southern Mesopotamian due to the presence of interconnected vugs in grain-dominated fabric (Mahdi, et al, 2013). In Halfaya oilfield, various types of grain shoal reservoirs that are dominated by grainstone and wackestone packstone characterize the formation (Jun, et al, 2016)
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