Assessing Cutoff Values and Evaluating Petrophysical Properties of a Shaly-Sand Reservoir with Heterogeneous Properties in a Southern Iraqi Oil Field

Abstract

The extensive approach to reservoir characterization provided by petrophysical characteristics emphasizes the relevance of reservoir quality. The presence of shale, especially in the sandstone formation, causes heterogeneity in petrophysical properties; thus, the estimation of these properties is sophisticated and leads to inaccurate production performance of oil reservoirs. The critical determinants of reservoir quality are shale volume, permeability, water saturation, and porosity. Petrophysical models are applied to the relationship between logging data and rock and fluid parameters. The contribution of the present study is to investigate hydrocarbon production and nonproductive layers using a petrophysical model by applying a cutoff approach in the Subba oil field located in the southern part of Iraq with the Shaly-Sand Nahr Umr formation. This investigation was conducted using well logs data that contained rock response of gamma-ray, density, neutron, resistivity, and cores data. The data was analyzed using Techlog Slb software and Microsoft Excel. After analyzing data and according to the petrophysical model and cutoff findings, four zones (S1, S21, S22, and S23) are potential productive hydrocarbon, while two zones (S24 and S3) are water-bearing; throughout these layers, there are seven barriers’ zones. In addition, the FZI approach gives a successful match for cored permeability. Furthermore, the porosity calculated from the density neutron also gives an acceptable match with cored data. Finally, the cutoff estimation revealed that the porosity cutoff has a value of 0.065 %, water saturation 0.7%, and shale volume 0.35%.