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Many factors control accurate determination of water saturation (Sw). Cementation exponent (m) and tortuosity factor (a) are from those that have been focus of many studies. Log-log plot of porosity (φ) versus formation factor (F) is used to determine m and a. The cementation exponent is determined from the negative slope of the least square fit straight line of the plotted points, while the tortuosity factor is the intercept of the line where φ = 1. In heterogeneous carbonate reservoirs where pores and pore throat networks are complex due to various diagenetic processes, F and φ scatter significantly on the φ-F plot. This will cause a small coefficient of determination between F and φ and thus less reliable m and a. Although classification of data based on petrofacies and/or permeability may improve the correlation to some extent, but data still show significant scatter. Introducing current zone indicator (CZI) and electrical flow unit (EFU), this study has established a new approach to classify φ and F data. The approach enables one to obtain more accurate m and a and thus more realistic calculation of Sw. This study also shows forcing a to any fixed value, will lead to both optimistic and pessimistic estimation of Sw within a reservoir.

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The aim of geostatistical reservoir characterization is to utilize wide variety of data, in different scales and accuracies, to construct reservoir models which are able to represent geological heterogeneities and also quantifying uncertainties by producing numbers of equiprobable models. Since all geostatistical methods used in estimation of reservoir parameters are inaccurate, modeling of “estimation error” in form of uncertainty analysis is very important. In this paper, the definition of Sequential Gaussian Simulation has been reviewed and construction of stochastic models based on it has been discussed. Subsequently ranking and uncertainty quantification of those stochastically populated equiprobable models and sensitivity study of modeled properties have been presented. Consequently, the application of sensitivity analysis on stochastic models of reservoir horizons, petrophysical properties, and stochastic oil-water contacts, also their effect on reserve, clearly shows any alteration in the reservoir geometry has significant effect on the oil in place. The studied reservoir is located at carbonate sequences of Sarvak Formation, Zagros, Iran; it comprises three layers. The first one which is located beneath the cap rock contains the largest portion of the reserve and other layers just hold little oil. Simulations show that average porosity and water saturation of the reservoir is about 20% and 52%, respectively.