NMR Applications For The Determination Of Sedimentary Facies And Porosity Systems In Carbonatic Core Plug Samples

Abstract

Abstract The use of NMR techniques to determine petrophysical properties of reservoirs has been described in many previous works1,2. But, together with Gamma Ray logs, thin sections and capillary pressure curves, NMR can contribute further to the characterization of sedimentary environments and porosity systems. In this work the results of the above mentioned techniques are presented. Almost 40 plug samples form a cored well in the Formation of Cretaceous age in Southwest Venezuela have been measured and described according to four sedimentary facies. We have found that the shape of the NMR T2 distributions and thin fine section images of each samples show a pattern related to the sedimentary facies the samples belong to. Another contribution of this work is that, unlike other reservoirs in Venezuela, the shape of the T2 distributions for the sample set can have one, two, three or four maxima or peaks depending on the porosity's system type e.g. intracrystalline, intercrystalline, fractures, synsedimentary fractured and vugs. The correspondence between sedimentary facies and T2 distributions allows us to classify the rock quality as a function of the porosity, bound fluid and free fluid volumes expressed in the well known Timur-Coates permeability equation determined in each facies. The results contribute to reduce the uncertainty in the petrophysical understanding of the reservoir and to improve the application of NMR technique in the sedimentology. Specifically, the relationship between T2 distribution patterns, thin section images and depositional environments extend the applications of NMR techniques beyond the classical determinations of petrophysical variables and fluid properties.