Abstract
Net pay is one of the most important parameters used in determining initial oil in place of a reservoir. It can be delineated through the using of limiting values of the petrophysical properties of the reservoir. Those limiting values are named as the cutoff. This paper provides an insight into the application of regression line method in estimating porosity, clay volume and water saturation cutoff values in Mishrif reservoir/ Missan oil fields. The study included 29 wells distributed in seven oilfields of Halfaya, Buzurgan, Dujaila, Noor, Fauqi, Amara and Kumait.
 This study is carried out by applying two types of linear regressions: Least square and Reduce Major Axis Regression.
 The Mishrif formation was divided into three main units. They are MA, MB, and MC. The methods were applied to each unit of Mishrif formation individually and as one unit. The division of Mishrif formation into subunits led to a great improvement in the accuracy of the porosity-permeability correlations. The Results indicated that the regression lines method of defining cutoffs gives unrealistic values with the common assumption of permeability cutoff = 0.1 md.
 Another assumption for permeability cutoff = 1 is made and it was chosen due to lithology and hydrocarbon type which are limestone and oil respectively. This assumption led to more realistic and higher porosity cutoff and smaller water saturation and clay volume cutoff values using the two types of regression lines.
Highlights
Net pay can be defined as the portion of the reservoir that contains relatively good petrophysical properties and hydrocarbon accumulations that can be economically produced
The results that were concluded during this study are: 1. Making unit discretization of Mishrif formation as three units they are: MA, MB, and MC led to an increase in the coefficient R2 of the core porosity and permeability to values higher than that of considering Mishrif formation as one unit by 17%
Using the regression line method, the results of assuming kc =1md gives values of porosity cutoff that are high and water saturation and clay volume are small in comparison to the other assumption
Summary
Net pay can be defined as the portion of the reservoir that contains relatively good petrophysical properties and hydrocarbon accumulations that can be economically produced It is considered as one of the most important parameters that used in the estimation of initial hydrocarbon in place, the analysis of fluid injection, well test interpretations, unitization procedure, and reservoir engineering studies. A major use of net pay is to compute volumetric hydrocarbons in-place Another use of net pay is to determine the total energy of the reservoir i.e. both moveable and non-moveable hydrocarbons are taken into consideration. Net pay for this purpose may be much greater than that for volumetrics calculation, George and Stiles, 1978. Net pay and NGR are crucial to quantify the hydrocarbon reserves and have a significant impact on the economic viability of hydrocarbon reservoir production, Worthington and Cosentino, 2005
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