Estimating the Petrophysical Properties Cutoff Values for Net Pay Determination: A Case Study of Khasib Formation, Southern Iraq

Abstract

In order to calculate pay thickness and net/gross ratio, a set of limiting values (called cutoffs) for petrophysical parameters like permeability, shale volume, porosity, and water saturation should be defined. This paper thoroughly explains the most useful and applicable methods for calculating the cutoff values, outlining their advantages, disadvantages, and required input data. These techniques include the traditional Best fit line, Quadrant, Worthington, and Cumulative hydrocarbon methods. Khasib formation in Amara oil field is taken as a study object; the cutoff values are estimated according to the proposed methods. Considering the accuracy, the data available, and the limitations of each method. Khasib formation is divided into four units the reservoir units identified as KH2 and KH4, The cutoffs for the petrophysical properties were calculated for each reservoir unit. It was determined that the suitable cut-off value for permeability changes from one unit to another because of the different rock types that characterize each unit. The permeability cut-off for the KH2 unit is 0.01 and the estimated porosity cutoff value is 0.09. In contrast, for the KH4 unit, the permeability cut-off is taken as 0.1 and the porosity cutoff is estimated as 0.1. Furthermore, the calculated cutoffs of the shale volume and the water saturation are similar for the two reservoir units (KH2 and KH4) and equal to 10 and 60% respectively. Finally, the determined cutoffs can be applied and the net pay thickness and ratio of net/gross can be calculated for each unit in the objective formation.